Food Science

In 2027 Food Science is a dairy event. Everything you are scored on is about milk and the products made from it: you run experiments at home and record them in a notebook, you make ice cream, curds or butter at the tournament, and you take a written test whose topics are all dairy science. The teams that do well treat the three parts as one subject. The chemistry that explains why acid curdles milk (casein losing its charge), why shaking cream makes butter (fat globules breaking and clumping) and why salty ice freezes ice cream (freezing point depression) is the same chemistry the written test asks about. Learn the mechanisms once and you can use them in the lab, in the notebook and on the test.

Event format

Food Science is a Division B event for teams of up to two, and it runs about 50 minutes. It is a hybrid: part of your score is earned before the tournament (the notebook), part with your hands (the lab tasks) and part on paper (the written test).

The supervisor will not lend you the bags, the lidded container or the cheesecloth. A team that forgets them may be unable to do a lab task properly, so put them on your packing list next to the goggles.

The milk product for a lab is handed out in equal amounts to every team, somewhere between 120 and 150 mL depending on the task. You may not know in advance exactly what it is (the rules call it an "unknown" liquid milk product for two of the tasks), so your notebook experiments should cover several kinds of milk and cream.

What the rules cover

The 2027 rules (Division B rules manual, pages B27–B28) split the event into three graded parts.

The three possible lab tasks are ice cream formation (scored on the height of a self-standing scoop), curd mass (scored by comparing your curd mass with the supervisor's) and milk fat mass from cream (making butter, scored by comparing your butter mass with the supervisor's). You do not know in advance which task or tasks you will get, so you must prepare all three.

The written test is limited to dairy food science. The rules list these topics, which this guide groups into sections:

Written tests for this event have always mixed the dairy topics with the general food chemistry and food safety they depend on: what pH and water activity mean, how to test a food for sugar or protein, how bacteria grow, what each vitamin does, how a Nutrition Facts label works. This guide teaches that background too, because a dairy test cannot ask about yogurt without asking about lactic acid bacteria, or about pasteurization without asking about pathogens.

Food Science has no Division C version in 2027.

Scoring

The final score is the sum of the notebook, lab and test scores, minus any penalties. High score wins.

Ties are broken by preselected test questions.

Two things follow from the math. First, the written test is worth up to 50 points, more than everything else combined at a regional, so the test deserves at least half your preparation time. Second, the notebook is the easiest 18 points in the event: they are earned at home, with no time pressure, and lost only through incomplete or careless entries.

The notebook

Before the tournament you must do at least three experiments for each of the three lab tasks (at least nine in total), record each on the official Food Science Experiment Form from soinc.org, and put them together in the notebook. You may do more, but only the first three for each task are graded, so put your best, most complete entries first.

Each experiment must change only one independent variable. That is the rule that most often costs points. "We used cream instead of milk and also added more salt" is two variables, and the result cannot tell you which change mattered.

A good entry has a clear question, a hypothesis with a reason, the one variable you changed, the variables you held constant, a numbered procedure, a data table with units, a graph if it helps, and a conclusion that answers the question and says what you would change in the lab task. Repeat trials (three per condition is a sensible minimum) show whether a difference is real or just noise.

Use the same equipment at home that you will bring to the tournament. A different bag, container or cheesecloth changes heat transfer, shaking and draining, and your notebook results will not carry over.

Lab task 1: ice cream formation

You get the same amount of an unknown liquid milk product as every other team, plus the other ingredients. You make ice cream in bags, then shape it on a surface. It has to stand on its own for 5 seconds before the supervisor measures its height, and you may shape it by hand. You may use any amount of what is provided.

The procedure in the rules, summarized:

1. Mix up to 125 mL of milk, cream or half and half with up to 15 mL of sugar, and up to 1.2 mL of vanilla if you like.
2. Half fill the gallon bag with ice, add up to 90 mL of salt, seal it and shake to mix.
3. Seal the mix in the pint bag.
4. Put the pint bag inside the gallon bag and shake hard so the mix chills and stays mixed.
5. After roughly 5 to 10 minutes, take the ice cream out of the small bag.

Why salt and ice freeze ice cream: freezing point depression

Ice cream mix freezes a little below 0 °C, because dissolved sugar and lactose lower its freezing point. Plain ice at 0 °C cannot freeze it at all: heat only flows from warmer to colder, and the mix must be cooled below its own freezing point. Salt fixes that. When salt dissolves in the thin film of water on the ice, the brine has a lower freezing point than pure water, so ice keeps melting into it. Melting absorbs heat (the heat of fusion, about 334 J/g), and that heat comes from the brine itself, which cools well below 0 °C. The cold brine then pulls heat out of the pint bag through the plastic.

The size of the effect depends on the number of dissolved particles, not on what they are. That makes it a colligative property.

A NaCl brine cannot go below about −21 °C (its eutectic point), however much salt you add. More salt helps up to the point where the ice melts too fast and the bag runs out of ice.

Coarse rock salt is traditional for ice cream freezers because it is cheap and dissolves a little more slowly, but any salt works by the same colligative effect. Calcium chloride (CaCl₂) gives three ions per formula unit (i ≈ 3), so per mole it lowers the freezing point more than NaCl.

Boiling point elevation is the sister colligative property: dissolved particles also raise the boiling point, by ΔT_b = i · K_b · m with K_b = 0.512 °C·kg/mol for water. That is why a sugar syrup's boiling temperature climbs as water boils off and the syrup gets more concentrated, and why candy makers read concentration from a thermometer.

Supercooling. Water can be cooled a few degrees below 0 °C without freezing if nothing starts the first crystal: pure water, a smooth container and no shaking. Freezing needs nucleation, the formation of a tiny seed crystal, and the first one is the hardest to make. A scratch, a speck of dust, a seed crystal or a sharp tap triggers sudden freezing. Shaking the ice cream bag prevents supercooling and creates many nuclei at once.

What makes a tall, self-standing scoop

Ice cream is a frozen foam: ice crystals, fat globules and air bubbles held in a thick sugar syrup. Height depends on how stiff that structure is.

• Fat makes ice cream stand. During shaking, fat globules partly break and stick together (partial coalescence), forming a network that holds air bubbles and supports the shape. Cream and half and half have far more fat than milk, which is why they usually give firmer ice cream. Test this in your notebook rather than assuming it, because very high-fat cream can turn buttery if overshaken.
• Sugar softens ice cream. It lowers the freezing point, so at any given temperature more of the water stays liquid. Up to 15 mL is allowed; less sugar can mean a firmer scoop, though it also changes texture.
• Small ice crystals come from fast freezing with constant shaking. Many small crystals form when the mix is cooled quickly, and shaking breaks growing crystals. Large crystals give an icy, crumbly product.
• Temperature at measurement matters. Ice cream softens within minutes at room temperature. Shape quickly, keep your hands cold, and present it as soon as it is firm.

Ice cream composition, overrun and ice crystals

Commercial ice cream is a carefully balanced mix. A typical US ice cream mix is roughly 10–16% milkfat, 9–12% milk solids-not-fat (casein, whey proteins, lactose, minerals), 12–16% sugar, a fraction of a percent of stabilizers and emulsifiers, and about 60% water. The federal standard of identity requires at least 10% milkfat and 20% total milk solids, and at least 4.5 pounds per gallon, which caps how much air can be whipped in. Frozen custard (French ice cream) must also contain egg yolk solids; sherbet has only 1–2% milkfat; gelato has no federal standard and is usually denser, with less air.

Ice cream is a partly frozen foam: four phases at once.

The clustering and clumping of fat globules into a continuous network is called fat destabilization (or partial coalescence). Emulsifiers promote it; shaking or dashing drives it.

Overrun is the percentage increase in volume caused by the air:

Ice crystal size decides smoothness. Freezing has two steps: nucleation (new crystals form) and crystal growth (existing crystals get bigger). Fast freezing with constant agitation produces a huge number of nuclei, so the water is shared among many crystals and each stays small and smooth. Slow freezing makes few nuclei that grow large and icy. Liquid nitrogen (−196 °C) ice cream is extra smooth for exactly this reason. In storage, temperature swings (heat shock) melt the smallest crystals and refreeze that water onto larger ones, so ice cream that has been thawed and refrozen turns coarse. Stabilizers (guar, locust bean gum, carrageenan and other polysaccharides, including those from edible seaweeds) thicken the unfrozen serum and slow this growth.

Two other textural defects are tested: sandiness (lactose crystallizing because there is too much milk solids-not-fat or the product has been stored too long) and shrinkage (loss of air and volume in storage).

For comparison: ice cream, ordinary (sweet cream) butter and milkshakes are not fermented foods, while yogurt, cultured buttermilk, sour cream, kefir, cultured butter and cheese are (see Cultured dairy and probiotics).

Lab task 2: determining curd mass

You get the same amount of an unknown liquid milk product as everyone else. You curdle it with acid, drain the curds and weigh them. The supervisor does the same, and your score is the ratio of your mass to theirs.

The procedure, summarized: weigh 120 mL of milk in a beaker; make sure it is near room temperature; add drops of vinegar or lemon juice, swirl or stir for 30 seconds and let it stand about 5 minutes; pour it through cheesecloth tied over a beaker; let it drain for 15 minutes; weigh the curds.

Why acid makes curds

About 80% of the protein in cow's milk is casein. Casein molecules cluster into micelles, tiny spheres held together by calcium phosphate and covered with a "hairy" outer layer of κ-casein. At milk's normal pH of about 6.7, the micelles carry a net negative charge, so they repel each other and stay dispersed.

Adding acid lowers the pH. Hydrogen ions neutralize the negative charges and dissolve the calcium phosphate that binds the micelle together. As the pH approaches 4.6, casein's isoelectric point (the pH at which the protein has no net charge), the micelles stop repelling each other and clump into a gel that breaks into curds. The liquid left behind is whey, which holds the whey proteins, most of the lactose and many minerals. Whey proteins do not curdle at pH 4.6 at room temperature, so they drain away.

Cheesemakers also curdle milk with rennet, whose enzyme chymosin cuts off the hairy κ-casein layer. Rennet curds are firmer and springier than acid curds; the lab uses acid only.

Variables that change the curd mass

• Amount of acid. Too little and the pH never reaches 4.6, so some casein stays in the whey. Your notebook should find the number of drops that maximizes curd for each milk type.
• Temperature. The rules want milk near room temperature. Warmer milk curdles faster and makes firmer curds; cold milk curdles slowly.
• Milk type. Curds trap fat, so whole milk gives heavier curds than skim. Powdered milk reconstituted at the usual strength behaves much like fluid skim milk.
• Handling. Stir gently. Hard stirring breaks curds into fine pieces that pass through the cheesecloth. Scrape every curd off the cloth onto the balance.

Curd mass includes the water trapped in the curd, so a consistent draining time (15 minutes in the rules) is what makes results comparable.

Lab task 3: milk fat mass from cream

Everyone gets the same amount of cream and makes butter; your butter mass is compared with the supervisor's. The procedure, summarized: put 150 mL of heavy cream in your 500–600 mL plastic container and weigh it; the cream should be about 13 °C; cap the container and shake until butter grains clearly separate; pour off the liquid; weigh the butter.

Why shaking makes butter

Cream is an oil-in-water emulsion: fat droplets (globules) dispersed in watery serum, each globule wrapped in a thin milk fat globule membrane of phospholipids and proteins. Shaking whips air into the cream and the globules collect at the bubble surfaces. The collisions strip the membranes, and partly solid fat crystals poking out of one globule pierce the next, so they stick together. The clumps grow until the fat separates as butter grains, and the emulsion inverts: butter is a water-in-oil emulsion, with tiny water droplets inside a continuous fat phase. The liquid left behind is buttermilk.

The cream temperature is set at about 13 °C because churning needs some of the milk fat to be solid and some liquid. Very cold cream has fat that is too hard to fuse; warm cream gives soft, greasy butter that holds a lot of buttermilk and loses fat into the liquid.

Things to test in your notebook: cream temperature on either side of 13 °C, how full the container is (a half-empty container gives the cream room to slosh, which speeds churning), shaking style, and whether pressing extra buttermilk out of the butter changes the mass. Butter that still holds a lot of buttermilk weighs more, but follow the steps as written and ask the supervisor if a step is unclear.

Cream, whipped cream and butter

Cream is the fat-rich layer skimmed or centrifuged off milk. US standards of identity name the creams by milkfat:

If you shake or whip cream, it passes through stages: liquid → soft foam → whipped cream → grainy over-whipped cream → butter grains plus buttermilk. Whipped cream and butter come from the same process stopped at different points. In whipped cream (needs at least about 30% fat) the partly coalesced fat globules surround air bubbles and hold them in a stable foam; the continuous phase is still water. In butter, coalescence has gone so far that the fat becomes the continuous phase and the air and most of the water are expelled.

Traditional buttermilk is the liquid left after churning. The "buttermilk" sold in stores today is usually cultured buttermilk: low-fat milk fermented with lactic acid bacteria, so both are thin and slightly acidic, but only the second is deliberately fermented. Cultured butter is churned from cream that was fermented first, which gives it the buttery-smelling compound diacetyl.

Salting butter (usually 1–2% salt) adds flavor and controls bacterial growth. The salt dissolves only in butter's tiny water droplets, so each droplet becomes a strong brine with low water activity. Butter's water is also split into droplets too small for many bacteria to spread between, which is why butter keeps far longer than cream.

Ghee or clarified butter is butter heated until the water evaporates and the milk solids separate and are removed. With no water or protein, it keeps well and can be heated hotter before it smokes, because in whole butter the milk solids are the first thing to burn.

Milk composition

Cow's milk is about 87–88% water. The rest is the milk solids, which the rules' topics keep returning to:

Total protein is about 3.3–3.4%. One cup (240 mL) of milk, whatever its fat level, carries about 8 g of protein and about 12 g of lactose. Skim milk is not "watered down": removing fat leaves the protein, lactose and calcium.

When a test asks for the "four major components" of milk, the answer is water, fat, protein and lactose (minerals are the fifth, smaller one).

Milk is a complex mixture that is three kinds of mixture at once:

• a true solution of lactose, soluble salts and water-soluble vitamins;
• a colloidal dispersion of casein micelles (about 0.1 µm across), which is why skim milk is white rather than clear: the micelles scatter light;
• an emulsion of fat globules (about 1–10 µm in raw milk), which carry the fat-soluble vitamins and give whole milk its creamy color.

Fresh cow's milk has a pH of about 6.6–6.7, slightly acidic. Its density is about 1.03 g/mL and it freezes at about −0.53 °C. When bacteria ferment lactose to lactic acid the pH falls, which is why soured milk, yogurt, sour cream and buttermilk all have lower pH than fresh milk.

Sheep milk is richer in fat and protein (so it has less water) and makes more cheese per liter; human milk has much more lactose and much less protein than cow's milk, which is why infant formula is adjusted. Goat milk still contains lactose and casein, so it is not a safe swap for someone with milk allergy.

The most abundant amino acid in milk protein is glutamic acid (glutamate), and milk is rich in calcium phosphate, much of it bound inside the casein micelles.

Milk production, processing and testing

From cow to carton

Milk is made in the cow's mammary gland: secretory cells build lactose from blood glucose (lactose is made only there), assemble casein and whey proteins from amino acids, and release fat as membrane-wrapped globules. The first milk after calving, colostrum, is rich in antibodies (immunoglobulins) and protein. Breed affects composition:

A dairy plant handles milk in a standard order: receiving (testing and cooling the raw milk, which must be kept cold), clarification (a centrifuge removes sediment and cells), separation (a centrifugal separator splits cream from skim), standardization (cream is added back to reach exactly 3.25%, 2% or 1% fat), homogenization, pasteurization, rapid cooling, and packaging. Vitamins A and D are added before pasteurization. Homogenization usually comes just before or after pasteurization, while the milk is warm, because fat must be liquid to be broken up.

Color clues: skim milk looks slightly bluish because its casein micelles scatter blue light and there is no fat to whiten it; whey looks greenish-yellow because of riboflavin. Typical off-flavors: light-oxidized (cardboard or burnt-feather flavor from riboflavin-driven oxidation in clear containers), rancid (lipase), cooked (heat), and feed or weedy flavors from what the cow ate.

Standardization and the Pearson square

To make milk or a mix with a target fat percentage from two components, use the Pearson square:

Cheese yield follows the same mass-balance logic: cheddar yield is roughly 10 kg per 100 kg of milk, and it rises with the milk's casein and fat content (cheese yield formulas such as the Van Slyke formula are built from casein and fat percentages). That is also why whole milk gives more curd than skim in the lab task.

Milk quality tests

Under the Pasteurized Milk Ordinance, Grade "A" milk comes from farms and plants that meet sanitation, bacterial-count, somatic-cell and temperature standards. USDA grades butter as AA, A or B by flavor, body and color.

Dairy history milestones

Chemistry basics for food science

Every later section leans on a few chemistry ideas. Learn them here once.

Macromolecules, monomers and the two key reactions

Foods are built from four classes of biological molecules. Three are polymers made of repeating monomers; lipids are not true polymers.

Nitrogen is the element found in every amino acid but not in sugars or fats. Minerals and water are nutrients too, but they are inorganic (they do not contain carbon–hydrogen frameworks). Every organic compound contains carbon.

Two reactions build and break all of these:

• Dehydration synthesis (condensation): two molecules join and release one water molecule. Glycosidic, peptide and ester bonds all form this way. Building large molecules from small ones uses energy, so it is an anabolic process.
• Hydrolysis: water is added to split a bond. Digestion is hydrolysis: lactase hydrolyzes lactose, proteases hydrolyze peptide bonds, lipase hydrolyzes ester bonds. Breaking molecules down is catabolic.

Hydrolysis and condensation are opposites.

Functional groups

A functional group is a cluster of atoms that gives a molecule its typical reactions. Tests ask you to spot them in structures.

A carboxylic acid and its ionized form have paired names: lactic acid / lactate, citric acid / citrate, acetic acid / acetate. Acetic acid (CH₃COOH) is the acid in vinegar; its sodium salt, sodium acetate, is a preservative.

Isomers

Isomers have the same molecular formula but different structures. Glucose, galactose and fructose are all C₆H₁₂O₆:

• Structural (constitutional) isomers have atoms connected differently. Glucose (an aldose) and fructose (a ketose) are structural isomers.
• Stereoisomers have the same connections but a different 3D arrangement. Glucose and galactose are stereoisomers that differ only at carbon 4.
• D and L forms are mirror-image configurations. Nearly all natural sugars are D sugars and nearly all amino acids in proteins are L. Enzymes are shaped for one form, which is why the other form is not digested the same way.
• α and β are the two ring forms (anomers) of a sugar: in a Haworth drawing the −OH on carbon 1 points down in α and up in β. Starch is built from α-glucose and cellulose from β-glucose, which is why we digest one and not the other.

Cis and trans describe groups on the same or opposite sides of a double bond (see Lipids).

Acids, bases and pH

pH measures how acidic a solution is: it is the negative logarithm of the hydrogen-ion (H⁺) concentration in mol/L. The scale usually runs 0–14; 7 is neutral at 25 °C, below 7 is acidic and above 7 is basic (alkaline). Because the scale is logarithmic, each step of 1 pH unit is a tenfold change in [H⁺]: pH 4 is ten times as acidic as pH 5 and a hundred times as acidic as pH 6.

Indicators change color with pH. Litmus paper turns red in acid and blue in base. A pH meter gives a numerical reading. A food with pH above 7 (such as pH 8) is basic. Acetic and lactic acids are weak acids: in water only a small fraction ionizes, in an equilibrium such as CH₃COOH(aq) + H₂O(l) ⇌ H₃O⁺(aq) + CH₃COO⁻(aq). The hydronium ion (H₃O⁺) is what "H⁺" means in the pH formula; strong acids such as HCl ionize completely.

In food safety, the key number is pH 4.6. Foods at or below it are "acid" or "acidified" foods, in which the botulism bacterium cannot grow (see Pasteurization and dairy preservation). Most bacteria grow best near neutral pH (about 6.5–7.5); molds and yeasts tolerate much more acid.

Polarity, solubility and water

Water is a polar molecule: oxygen pulls electrons more strongly than hydrogen, so the oxygen end is slightly negative and the hydrogen ends slightly positive. Water molecules attract each other through hydrogen bonds, which gives water its high boiling point, high specific heat (4.18 J/g·°C) and ability to dissolve so many polar and ionic substances that it is called the universal solvent.

"Like dissolves like": polar and ionic substances (sugars, salt, many vitamins) are hydrophilic ("water-loving") and dissolve in water; nonpolar substances (fats, oils, fat-soluble vitamins) are hydrophobic ("water-fearing") and do not. Liquids that do not mix, such as oil and water, are immiscible; they separate unless an emulsifier holds one dispersed in the other. Osmosis is the movement of water across a semipermeable membrane from where solute is less concentrated to where it is more concentrated; it stops when the concentrations (more strictly, the water potentials) on both sides are equal. Salt and sugar preserve food partly by pulling water out of microbial cells by osmosis.

The same rule explains why milk, not water, soothes the burn of chili peppers: capsaicin is nonpolar and fat-soluble, so it dissolves in milk fat (and is also bound by casein), while water just spreads it around. Even skim milk works well in taste tests, so milk's protein matters, not just its fat.

Carbohydrates: lactose, sugars and starches

Carbohydrates are built from carbon, hydrogen and oxygen in roughly a CH₂O ratio. The simplest are monosaccharides (glucose, galactose, fructose). Two monosaccharides joined by a glycosidic bond form a disaccharide. Long chains form polysaccharides such as starch.

Lactose, the sugar of milk, is a disaccharide of galactose and glucose joined by a β(1→4) glycosidic bond. Milk is nearly the only natural source of it. Lactose is a reducing sugar (its glucose end can open into a free aldehyde), which matters for browning when milk is heated. It is only about one-fifth to two-fifths as sweet as sucrose, and it dissolves poorly in cold water. In ice cream stored too long, lactose can crystallize into gritty "sandy" crystals.

Classifying sugars

Carbohydrates are classified by how many sugar units they contain:

The empirical formula of the common simple sugars (glucose, galactose, fructose, ribose) is CH₂O; deoxy sugars such as deoxyribose are the exception. Glucose, galactose and fructose are all C₆H₁₂O₆; lactose and sucrose are both C₁₂H₂₂O₁₁. "Saccharide" is the chemical name for any sugar unit, so "monosaccharide" means "one sugar".

Monosaccharides are named by their carbonyl group and carbon count. An aldose has an aldehyde (−CHO) at carbon 1; a ketose has a ketone (C=O) at carbon 2. Glucose and galactose are aldohexoses (aldehyde, six carbons); fructose is a ketohexose. Ribose (C₅H₁₀O₅, in RNA) is an aldopentose; Mannose is another aldohexose (C₆H₁₂O₆, differing from glucose only at carbon 2), a reducing sugar found in fruits and in plant gums; xylose is a five-carbon aldose (aldopentose), so it is not a hexose; deoxyribose (C₅H₁₀O₄, in DNA) is ribose missing the oxygen on carbon 2 ("deoxy" means "without oxygen"). In water, these sugars exist mostly as rings, in equilibrium with a small amount of the open chain. An aldohexose such as glucose can therefore be drawn several ways: as an open (straight) chain (Fischer projection, aldehyde at carbon 1, −OH groups on carbons 2–6), as a six-membered pyranose ring (five carbons and one oxygen; carbon 1 bonds to the oxygen on carbon 5) or, less often, a five-membered furanose ring; each ring has α and β anomers, and each sugar exists as D and L enantiomers (mirror images). Fructose is usually drawn as a five-membered furanose ring (its form in sucrose), although free fructose in water is mostly the six-membered pyranose. In a Haworth drawing of β-D-glucose, every −OH alternates up and down around the ring with the −CH₂OH group (carbon 6) above it; sucrose is an α-glucose ring joined through its carbon 1 to carbon 2 of a β-fructose ring.

A reducing sugar has a free aldehyde or ketone group (or a ring that can open to one), so it can donate electrons and reduce a mild oxidizing agent such as the Cu²⁺ in Benedict's reagent; the sugar's aldehyde is oxidized to a carboxylic acid in the process. All monosaccharides and most disaccharides (lactose, maltose) are reducing sugars. Sucrose is not, because its glycosidic bond joins the carbonyl carbons of both glucose and fructose, so neither ring can open. Trehalose is also non-reducing.

Common sugars compared

Invert sugar is sucrose split (by acid or the enzyme invertase) into an equal mix of glucose and fructose. It is sweeter than sucrose, holds moisture and resists crystallizing, which is why it is used in candies, syrups and baked goods. High-fructose corn syrup (HFCS) is made from corn starch: amylases break the starch into glucose, and the enzyme glucose isomerase converts part of the glucose to fructose. The common grades are HFCS-42 (42% fructose) and HFCS-55 (55% fructose, about 42% glucose, used in soft drinks). Honey is mostly fructose and glucose and contains no lactose.

Sweetness and the glycemic index

Sweetness values vary with temperature and concentration, so sources differ; the order fructose > sucrose > glucose > lactose is what tests want.

The glycemic index (GI) ranks how fast a food raises blood glucose compared with pure glucose (GI 100). Glucose is absorbed directly, so it is the reference; fructose must first be converted in the liver, so its GI is low. Milk and yogurt have low GIs (roughly 30–40) because lactose is half galactose and because protein and fat slow digestion. The tendency of a food to raise blood sugar sharply is its glycemic effect.

Starch, glycogen, cellulose and fiber

Starches are polysaccharides of glucose: amylose (straight chains) and amylopectin (branched). Milk itself has no starch, but starches and other polysaccharides (guar gum, carrageenan, cornstarch) are added to puddings, yogurts and commercial ice cream as stabilizers. They soak up water and thicken the liquid phase, which slows the growth of ice crystals and keeps the texture smooth.

Humans make amylase to digest the α bonds of starch and glycogen, but no enzyme for cellulose's β(1→4) bonds. Cows and other ruminants, and termites, digest cellulose only with the help of microbes in their guts. Cellulose is still valuable to us as dietary fiber. Starch and glycogen are the storage polysaccharides; cellulose and chitin are structural. Glycogen is not a simple sugar, and structural polysaccharides are not an energy source for humans.

Dextrins are the short glucose chains made when starch is partly broken down by heat (toasting), acid or amylase. They are classed with the polysaccharides (short ones overlap with oligosaccharides); maltodextrin, made by partial acid or enzyme hydrolysis of starch (usually corn), is a common bland filler and thickener in powdered foods. Ordinary dextrins are digestible, so they count as available carbohydrate, not fiber (specially processed "resistant dextrins" are the exception and count as fiber).

Simple versus complex carbohydrates. Simple carbohydrates (monosaccharides and disaccharides) are digested and absorbed quickly and give quick energy, raising blood glucose fast. Complex carbohydrates (starch) must first be broken down by amylase, so they release glucose more slowly and give longer-lasting energy, especially when eaten with fiber, fat or protein. In the end, digestible carbohydrates are converted to glucose, the body's main fuel and the brain's preferred one; extra glucose is stored as glycogen and, beyond that, as fat. Glycogen, the body's short-term carbohydrate store in liver and muscle, fuels exercise (especially hard exercise); body fat is the much larger long-term energy store.

Dietary fiber is carbohydrate that human enzymes cannot digest in the small intestine. Soluble fiber (pectin, gums, oat β-glucan, inulin) dissolves into a gel that slows digestion, blunts blood-sugar spikes and lowers blood cholesterol. Insoluble fiber (cellulose, hemicellulose, and the non-carbohydrate lignin) adds bulk and keeps food moving through the gut. Fiber is listed under Total Carbohydrate on the label; the Daily Value is 28 g. On a label, total carbohydrate minus fiber and sugars leaves mostly starch (and sugar alcohols, if any).

Starch in cooking: gelatinization and retrogradation

When starch is heated in water, its granules absorb water and swell, the amylose leaks out, and the mixture becomes thick, viscous and more translucent. This is gelatinization, and it is how cornstarch thickens a pudding or a starch-stabilized yogurt. As a starch gel cools and ages, the starch chains line up again and squeeze out water (amylose within hours, amylopectin's branches over days): retrogradation. It firms puddings, makes bread stale and causes syneresis (liquid "weeping" out of a gel). Cooking can also break polysaccharides down, by acid hydrolysis, by enzymes or by heat.

Carbohydrate digestion

Digestion of starch begins in the mouth with salivary amylase, which breaks starch into maltose and short chains, and continues in the small intestine with pancreatic amylase. Enzymes on the lining of the small intestine finish the job: maltase (maltose → 2 glucose), sucrase (sucrose → glucose + fructose) and lactase (lactose → glucose + galactose). The monosaccharides are absorbed mostly in the small intestine, and the liver converts galactose and fructose to glucose. Extra glucose is stored as glycogen in the liver and muscles; when glycogen runs out, the body breaks down fat. A missing enzyme causes a specific intolerance: lactase deficiency causes lactose intolerance, and congenital sucrase-isomaltase deficiency causes sucrose intolerance.

Sweeteners and sugar substitutes

Nutritive sweeteners supply energy: sugars (4 kcal/g), honey, syrups, and the sugar alcohols (polyols such as sorbitol, xylitol, erythritol, maltitol), which supply fewer calories (about 2 kcal/g on average, from about 0.2 for erythritol to about 3). Sugar alcohols are made by hydrogenating sugars (glucose → sorbitol), occur naturally in some fruits, do not cause tooth decay, and in large amounts have a laxative effect.

Non-nutritive (high-intensity) sweeteners are so many times sweeter than sucrose that the tiny amount used adds almost no Calories. They taste sweet because they bind the tongue's sweet-taste receptors, but the body gets little or no energy from them. They do not supply sugar's bulk, browning, moisture retention or crystal structure, so they cannot simply replace sugar in baking or ice cream.

The FDA has approved six high-intensity sweeteners as food additives (saccharin, aspartame, acesulfame potassium, sucralose, neotame and advantame) and accepts purified stevia and monk fruit extracts as GRAS ("generally recognized as safe"). For each of the six approved additives the FDA sets an acceptable daily intake (ADI), the amount considered safe to eat every day over a lifetime (the stevia ADI comes from the WHO/FAO expert committee, JECFA, and monk fruit's is "not specified").

Sugar crystallization

A sugar crystal forms from a supersaturated solution, one holding more dissolved sugar than it normally could at that temperature (made by dissolving sugar hot and cooling it). Crystallization then goes through nucleation (the first stable cluster of molecules forms), crystal growth (molecules add to existing crystals) and ripening (small crystals dissolve and large ones grow). Stirring, seed crystals, dust and fast cooling while supersaturated promote nucleation. Interfering agents (corn syrup, invert sugar, fat, protein, or acid such as cream of tartar or lemon juice that inverts some sucrose) get in the way and keep crystals small or prevent them. A sugar that cools too fast to crystallize sets as a clear, amorphous glass, with no ordered crystal structure.

For dairy, lactose is the sugar that crystallizes. It is not very soluble, so it can form gritty crystals in ice cream ("sandiness") and in sweetened condensed milk. The crunchy crystals in aged cheeses are not lactose, which is mostly fermented away: in aged Gouda they are mostly the amino acid tyrosine, and in cheddar often calcium lactate. Spray-dried milk powder contains lactose in an amorphous, glassy state, which is why it absorbs moisture and cakes if left open.

Carbohydrate's role in dairy foods is mostly energy (4 kcal/g), sweetness and texture, plus food for bacteria: lactic acid bacteria ferment lactose to lactic acid in yogurt, cultured buttermilk and cheese.

Food tests for nutrients

Classic color tests show which nutrients a food contains. They are a favorite on written tests ("what does a positive Biuret test look like?") and are an easy way to design a milk experiment: milk is positive for protein, reducing sugar and fat, and negative for starch.

Why Benedict's works. In the alkaline, hot solution, the free aldehyde (or ketone) group of a reducing sugar reduces blue copper(II) ions (Cu²⁺) to copper(I), which precipitates as red copper(I) oxide (Cu₂O); the sugar is oxidized. The more sugar, the more precipitate, so the color shifts from green toward brick red. Four traps:

• Sucrose is non-reducing, so table sugar or beet sugar gives a negative (blue) result. If you first boil the sucrose with dilute acid (hydrolysis) and neutralize it, the freed glucose and fructose make it positive.
• Fructose is a ketose but still tests positive, because in hot alkaline solution it rearranges (through an enediol) into aldose forms that reduce copper.
• Starch and other complex carbohydrates are negative, because only the chain ends are reducing.
• Starchy foods such as bread, bagels and crackers usually give only a weak green to yellow result: their starch is negative, but they contain small amounts of free glucose, maltose (from amylase acting on starch during dough-making) and sometimes added sugar. Milk and fruit juice give stronger (orange-red) results.

Benedict's reagent is preferred over Fehling's because it is a single, stable solution that keeps on the shelf and is less caustic (it uses sodium carbonate instead of sodium hydroxide). Benedict's test was historically used to detect glucose in urine as a sign of diabetes.

Why Biuret works. In alkaline solution, copper(II) ions bind to the nitrogen atoms of peptide bonds, forming a violet complex that absorbs light most strongly at about 540 nm. It needs a chain with at least two peptide bonds, so free amino acids (such as glycine) and dipeptides do not give a positive result; proteins and longer peptides do. The more protein, the deeper the purple, which is the basis of a protein assay. The test needs no heat. The sodium hydroxide makes the solution alkaline; the copper sulfate supplies the Cu²⁺.

Why iodine works. Amylose coils into a helix about six glucose units per turn, and iodine–iodide ions (I₃⁻ and longer polyiodide chains) slide into the core of the helix. The trapped chain absorbs light strongly and looks blue-black. Branched amylopectin forms only short helices and gives a weaker reddish-purple, glycogen a red-brown, and cellulose or chitin no color at all. Heat uncoils the helix, so the test is read at room temperature.

Lipids: milk fat, esterification and fat bonding

Milk fat is about 98% triglycerides. A triglyceride is a glycerol backbone (a three-carbon alcohol with three −OH groups) with three fatty acids attached.

Esterification

The bond between a fatty acid and glycerol forms by esterification: a carboxylic acid (−COOH) reacts with an alcohol (−OH) to give an ester (−COO−) and a molecule of water. Because water is released, it is a condensation (dehydration) reaction. Three esterifications make one triglyceride and release three water molecules. The reverse reaction, splitting an ester by adding water, is hydrolysis.

Hydrolysis matters in dairy. The enzyme lipase hydrolyzes milk fat and frees fatty acids. Milk fat is unusual because it contains short-chain fatty acids, especially butyric acid (four carbons), and freed butyric acid smells and tastes rancid. This hydrolytic rancidity is why raw milk that is shaken or pumped roughly can develop off flavors, and why pasteurization (which inactivates lipase) helps shelf life. Some cheeses rely on controlled lipase activity for their sharp flavor.

Saturated and unsaturated fats

A saturated fatty acid has only single carbon–carbon bonds, so every carbon holds as many hydrogens as it can. Its chain is straight. An unsaturated fatty acid has one (monounsaturated) or more (polyunsaturated) carbon–carbon double bonds. A natural cis double bond puts a kink in the chain.

Straight chains pack closely and attract each other through many weak London dispersion forces, so saturated fats are solid at room temperature. London dispersion forces (temporary dipoles from shifting electrons) act between all molecules, polar or not, and are the only attraction between nonpolar molecules such as fat chains; they grow with molecular size and contact area. Kinked chains cannot pack tightly, so unsaturated fats are usually liquid oils. Trans fats have double bonds that keep the chain fairly straight, so they behave more like saturated fats. Milk fat naturally contains a small amount of trans fats (such as vaccenic acid) made by bacteria in the cow's rumen.

Roughly two-thirds of milk fat is saturated. That mix of saturated (higher melting) and unsaturated (lower melting) fats is why butter is solid in the fridge but spreads when warm, and why cream at 13 °C is partly crystalline, the right state for churning.

Essential fats

The body can make most fatty acids, but not linoleic acid (omega-6) or alpha-linolenic acid (omega-3), because human cells cannot put double bonds at the positions those molecules need. These are the essential fatty acids, and they must come from food. Milk supplies small amounts of both, plus conjugated linoleic acid (CLA). Fat also carries the fat-soluble vitamins A, D, E and K, which is why vitamin A is added back to low-fat and fat-free milk.

The "omega" name tells you where the first double bond counted from the methyl (−CH₃, "omega") end of the chain sits. In an omega-3 fatty acid the first double bond is three carbons from the methyl end; in an omega-6, six. (Chemists also number from the carboxyl end, which gives the Δ or "C-1" numbering.) A fatty acid is a hydrocarbon chain with a carboxyl group at one end and a methyl group at the other.

Conditionally essential means the body can make the compound (EPA and DHA from ALA) but not always fast enough, for example in infancy or illness, so a dietary supply helps. Human conversion of ALA to EPA and DHA is small. DHA is concentrated in the brain and retina and is important for brain development before and after birth. There is no official RDA for EPA plus DHA; the US Adequate Intake is set only for ALA (1.6 g/day for men, 1.1 g/day for women), and many health organizations suggest roughly 250–500 mg/day of EPA plus DHA combined, about two servings of fatty fish a week. Omega-3 intake is linked with lower triglycerides and heart health.

Mammals cannot make LA or ALA because they lack the desaturase enzymes that put double bonds beyond carbon 9 of the chain (counted from the carboxyl end). Signs of essential fatty acid deficiency, rare in people eating normal diets, include dry, scaly skin, hair loss, poor wound healing and poor growth in children.

Role of fat in dairy foods

Fat gives richness and mouthfeel, carries flavor compounds, supplies energy (9 kcal/g, more than twice protein or carbohydrate) and builds structure: the partly coalesced fat network in ice cream and whipped cream, and the continuous fat phase in butter. In curds, fat globules are trapped inside the casein network, which is why whole milk gives a heavier curd than skim. Fat also transfers heat in cooking and keeps food from sticking and burning, because fats melt easily but do not break down until they are much hotter than boiling water.

Lipid classes: triglycerides, phospholipids, sterols and waxes

Lipids are the diverse group of biological molecules that do not dissolve in water. They are the one major class that does not form polymers.

A phospholipid is amphipathic: its phosphate head is polar and its two tails are not. That is why phospholipids form cell membranes (heads facing water, tails facing each other) and why lecithin from egg yolk makes mayonnaise an emulsion. In membranes, saturated tails pack tightly and make the membrane stiffer; unsaturated, kinked tails keep it fluid.

Cholesterol is made by the body and found only in animal foods (milk, butter, eggs, meat); plants contain none. Plants make their own sterols instead (phytosterols such as β-sitosterol, stigmasterol and campesterol), which lower cholesterol absorption and are added to some spreads; fungi and yeasts make ergosterol, which UV light converts to vitamin D₂ (as in UV-treated mushrooms). Sunlight converts a cholesterol derivative in the skin (7-dehydrocholesterol) into vitamin D. Cholesterol travels in the blood in lipoproteins: LDL ("bad" cholesterol) delivers cholesterol to tissues and, in excess, deposits it in artery walls (atherosclerosis), raising the risk of heart attack and stroke; HDL ("good" cholesterol) carries cholesterol back to the liver. For adults an LDL below 100 mg/dL is considered optimal. Diets high in saturated and trans fats raise LDL; replacing them with unsaturated fats lowers it, which is why unsaturated fats are called the healthier choice.

Melting point and fatty acid structure

Three rules predict which fats are solid:

1. Longer chains melt higher (more surface for London dispersion forces). Butyric acid (4 C) is liquid; stearic acid (18 C) melts near 70 °C.
2. More double bonds melt lower. Each cis double bond adds a kink. Stearic (18:0) > oleic (18:1, melts around 13–16 °C) > linoleic (18:2, below 0 °C).
3. Trans melts higher than cis. Elaidic acid, the trans form of oleic acid, melts around 45 °C, much higher than oleic, because its chain is nearly straight.

So for 18-carbon acids: stearic > elaidic (trans) > oleic (cis) > linoleic. Fats rich in saturated fatty acids have higher melting points (so they are solid) and, with fewer double bonds for oxygen to attack, better oxidative stability than oils. Coconut oil (about 90% saturated, much of it the 12-carbon lauric acid) is solid at about 25 °C or below. Lard, butter and coconut oil are high in saturated fat; olive, canola, peanut and corn oils, nuts, seeds, avocados and fatty fish are rich in unsaturated fat.

Hydrogenation and trans fats

Hydrogenation adds hydrogen (H₂) across carbon–carbon double bonds, turning unsaturated fatty acids into saturated ones. It is run with a metal catalyst (usually nickel; also palladium or platinum), heat and pressure. The catalyst gives the reaction a lower-energy pathway, so it runs at practical temperatures and speeds. Hydrogenation turns liquid vegetable oil into a solid or semi-solid fat that is spreadable, has a higher melting point and resists rancidity: margarine and vegetable shortening are hydrogenated products. Sugars can be hydrogenated too: the carbonyl group is reduced to an alcohol, which is how sugar alcohols such as sorbitol are made.

Full hydrogenation saturates every double bond. Partial hydrogenation leaves some double bonds, and during the reaction many of them flip from the natural cis form to the trans form. Partially hydrogenated oils (PHOs) were therefore the main source of artificial trans fats in shortening, stick margarine, baked goods, crackers and fried foods.

Why trans fats are harmful: they raise LDL cholesterol and lower HDL cholesterol, a worse combination than saturated fat, and increase the risk of heart disease. The FDA required trans fat on the Nutrition Facts label from 2006, determined in 2015 that PHOs are no longer "generally recognized as safe", and set 2018 as the main compliance date for removing them from foods. A label may say 0 g trans fat if a serving has less than 0.5 g. Small amounts of natural trans fats (vaccenic acid, CLA) occur in milk and beef fat, made by bacteria in the cow's rumen. Historically most dietary trans fat came from PHOs, not animal foods; since the PHO ban, the small natural amounts in dairy and meat make up a larger share.

Rancidity, antioxidants and the smoke point

Fats spoil by rancidity, which produces off odors and flavors:

Oxidation is speeded by light, heat and traces of metals such as copper and iron, which is one reason milk is sold in opaque containers. The chain reaction (autoxidation) runs in three steps. Initiation: light, heat or a metal ion pulls a hydrogen atom from a carbon next to a double bond, leaving a free radical. Propagation: the radical reacts with oxygen to form a peroxyl radical, which steals a hydrogen from another fatty acid, making a hydroperoxide and a new radical, so the chain keeps going. Termination: two radicals combine, or an antioxidant donates a hydrogen and forms a stable radical, ending the chain. The hydroperoxides break down into the aldehydes and ketones that smell rancid. Defenses:

• Antioxidants stop the free-radical chain: BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), TBHQ, vitamin E (tocopherols, the natural antioxidant in vegetable oils) and vitamin C (ascorbic acid).
• Chelators (sequestrants) such as EDTA and citric acid capture metal ions so they cannot catalyze oxidation.
• Nitrogen flushing or vacuum packaging removes oxygen; potato-chip bags are filled with nitrogen for this reason.

The smoke point is the temperature at which a heated fat starts to break down and give off visible smoke. Triglycerides split into free fatty acids and glycerol, and glycerol breaks down into acrolein, an irritating, acrid compound. Fats high in free fatty acids smoke at lower temperatures, so frying oil that is reused again and again darkens, foams, thickens and smokes sooner. Whole butter smokes and burns at a fairly low temperature because its milk proteins and lactose scorch; clarified butter does not have that problem.

Saponification

Heating a fat with a strong base such as sodium hydroxide (lye) or potassium hydroxide hydrolyzes its ester bonds and gives glycerol plus soap, the sodium or potassium salts of the fatty acids. This is saponification. A soap molecule, like a phospholipid, has a charged, water-loving head and a long fat-loving tail, which is how it lifts grease into water. Lye is also used in food processing (peeling fruits, curing olives, making pretzels' brown crust), always rinsed or neutralized afterward.

Lipase enzymes catalyze the hydrolysis of triglycerides into glycerol and fatty acids, and they work at the oil–water interface, which is why fat must be emulsified (by bile in the gut) before it is digested efficiently.

Proteins: casein and whey

Proteins are chains of amino acids joined by peptide bonds. Each amino acid has an amino group (−NH₂), a carboxyl group (−COOH), a hydrogen and a side chain on one central carbon. A peptide bond forms when the carboxyl group of one amino acid joins the amino group of the next, and like esterification it is a condensation reaction that releases water. A protein's sequence (primary structure) folds into helices and sheets (secondary), a 3D shape (tertiary) and sometimes multi-chain assemblies (quaternary).

Milk protein is about 80% casein and 20% whey protein, and the two behave very differently:

Of the 20 amino acids in proteins, nine are essential: the body cannot make them, so they must be eaten. They are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. Milk protein supplies all nine in useful amounts, so it is a complete protein. Whey protein is especially rich in leucine, which is why it is popular for muscle building.

Denaturation is the unfolding of a protein's shape without breaking its peptide bonds. Heat, acid, salt and mechanical stress (whipping) can denature proteins. Casein curdling in the lab is a different process from denaturation: the micelles aggregate because their charge is neutralized, while the individual proteins stay largely intact.

Amino acids and the peptide bond

Every amino acid has the same backbone around a central (alpha) carbon: an amino group (−NH₂), a carboxyl group (−COOH), a hydrogen atom and a variable side chain, the R group. Only the R group differs among the 20 amino acids, and its size, charge and polarity decide how the protein folds. Amino acids take their name from the amino group. In water near neutral pH the amino group carries H⁺ (−NH₃⁺) and the carboxyl group loses it (−COO⁻), so the molecule is a zwitterion, positive and negative at once.

A peptide bond (an amide bond) joins the carboxyl carbon of one amino acid to the amino nitrogen of the next, releasing water (dehydration synthesis). A chain of amino acids is a polypeptide; a protein is one or more polypeptides folded into a working shape. Most peptide bonds are in the more stable trans arrangement.

Levels of protein structure

A protein's function depends on how it folds. Denaturation destroys the secondary, tertiary and quaternary structure but leaves the primary structure (the peptide bonds) intact. Agents that denature proteins: heat, acid or base (pH change), salts and heavy metals, alcohol and other organic solvents, freezing, and mechanical stress such as whipping or beating. In food, denatured proteins often bond to one another and form a solid network, which is coagulation: egg white turning white and firm when boiled, a beaten egg white foam setting, scrambled eggs, cheese curd made with heat and acid, and steak turning opaque as its muscle proteins (actin and myosin) denature and tighten. Stomach acid (HCl) denatures dietary proteins so that the enzyme pepsin can reach the peptide bonds. Raw egg white contains avidin, a protein that binds the vitamin biotin and blocks its absorption, so eating many raw egg whites can cause biotin deficiency; cooking denatures avidin and removes the problem. Not every kitchen change is protein denaturation: a cake rising as it bakes is caused by expanding gas.

Essential amino acids and protein quality

Mnemonic for the nine essential amino acids: "PVT TIM HaLL": phenylalanine, valine, threonine, tryptophan, isoleucine, methionine, histidine, leucine, lysine.

Essential versus nonessential is about whether the body can make it, not about whether the body needs it: the body needs all 20. Tyrosine is made from phenylalanine, so people with phenylketonuria (PKU), who lack the enzyme phenylalanine hydroxylase, must limit phenylalanine (including the phenylalanine in aspartame) and get tyrosine from food.

A complete protein supplies all nine essential amino acids in useful amounts: milk, yogurt, cheese, eggs, meat, fish, soy and quinoa. Most other plant proteins are incomplete, low in one limiting amino acid: grains are low in lysine, legumes in methionine. Eating complementary proteins (rice and beans, bread and peanut butter) together, or over the day, supplies a complete set; this is how strict vegetarians avoid shortages. Tryptophan is in nearly all food proteins, including turkey, milk, kefir, cheese and eggs.

The RDA for protein is 0.8 g per kg of body weight per day for adults (about 0.95 g/kg for ages 4–13 and 0.85 g/kg for ages 14–18). Protein builds and maintains muscles, organs, enzymes, hormones, antibodies and connective tissue; it can also be burned for energy (4 kcal/g).

Protein–energy malnutrition comes in two classic forms. Marasmus is severe shortage of total energy and protein: extreme wasting of fat and muscle ("skin and bones"). Kwashiorkor is protein shortage with somewhat adequate energy (often after weaning onto starchy foods): swelling (edema), starting in the feet and legs, a swollen belly, an enlarged fatty liver, skin and hair changes, and poor growth. Milk powders and other protein-rich foods are used to treat both.

Isoelectric point and charge

Each protein's isoelectric point (pI) is the pH at which its positive and negative charges balance, so its net charge is zero. At the pI, proteins repel each other least and are least soluble, so they clump and precipitate: casein's pI of about 4.6 is why milk curdles there.

• At a pH below the pI, extra H⁺ makes the protein net positive; in an electric field it moves toward the negative electrode (cathode).
• At a pH above the pI it is net negative and moves toward the positive electrode (anode).

Milk at pH 6.6 is above casein's pI, so casein carries a negative charge there. Isoelectric focusing separates proteins by running them through a pH gradient in an electric field: each protein stops where the pH equals its pI. Ordinary gel electrophoresis separates proteins by size and charge instead. Casein is the major milk protein that is insoluble at its pI; the whey proteins stay soluble. Within the micelle, the hydrophilic, "hairy" κ-casein coats the surface, while the αs- and β-caseins, which are rich in phosphate groups that bind calcium, sit inside.

Enzymes

Enzymes are biological catalysts: they speed up chemical reactions in living things without being used up, so one enzyme molecule can act again and again. Most enzymes are proteins (a few, called ribozymes, are RNA, so "all enzymes are proteins" is false). Their names usually end in -ase, after diastase (an amylase), the first enzyme isolated, in 1833. The word "enzyme" was coined by Wilhelm Kühne in 1877.

How they work. An enzyme lowers a reaction's activation energy, the energy barrier reactants must cross. The reactant, the substrate, binds at the enzyme's active site to form an enzyme–substrate complex. The active site's shape makes each enzyme specific to one substrate or one type of reaction. Two models describe the fit: lock and key (the substrate fits a rigid site exactly) and induced fit (the site changes shape slightly as the substrate binds, then returns to its original shape after the products leave; the enzyme is not permanently changed). Enzymes speed the forward and reverse reactions equally, so they help a reaction reach equilibrium faster but do not change the equilibrium or the overall energy released.

Factors that change the rate:

• Temperature: rate rises with temperature up to an optimum (about 37 °C for human enzymes), then falls sharply as the enzyme denatures. Cold slows enzymes (refrigeration); high heat destroys them (blanching, pasteurization).
• pH: each enzyme has an optimum pH: pepsin in the stomach works near pH 2, trypsin in the small intestine near pH 8. Far from the optimum, the active site loses its shape.
• Substrate concentration: rate rises until every enzyme is busy (saturation).
• Enzyme concentration: more enzyme, faster rate, if substrate is plentiful.
• Inhibitors: a competitive inhibitor resembles the substrate and blocks the active site (overcome by adding more substrate); a noncompetitive inhibitor binds elsewhere and changes the enzyme's shape. In feedback inhibition the end product of a pathway inhibits an early enzyme.
• Cofactors: many enzymes need a helper. Inorganic cofactors are metal ions (Mg²⁺, Zn²⁺, Fe²⁺); organic coenzymes are often made from vitamins (NAD⁺ from niacin, FAD from riboflavin). A cofactor bound tightly and permanently, such as the heme in catalase, is a prosthetic group.

Some enzymes are made as inactive precursors called zymogens (pepsinogen, trypsinogen) and activated only where needed, so they do not digest the cells that make them. Enzymes with different structures that catalyze the same reaction are isoenzymes.

Proteases are also used to "pre-digest" proteins in infant formulas and medical foods (protein hydrolysates). Fruits, vegetables and milk all contain active enzymes, which is why raw foods change in storage and why vegetables are blanched (briefly scalded) before freezing.

Calories and nutrition math

A calorie (small c, cal) is the heat needed to raise the temperature of 1 g of water by 1 °C; it equals 4.184 joules. The food Calorie (capital C, Cal) is a kilocalorie (kcal), 1,000 calories: the heat to raise 1 kg of water by 1 °C. The "Calories" on a Nutrition Facts label are kilocalories. The calorie and the joule both measure energy. Watch the capital letter: "the heat to raise 1 g of water by 1 °C" defines the small calorie, not the food Calorie.

Nutrients are grouped into six classes: carbohydrates, proteins, lipids, vitamins, minerals and water. Only the first three (plus alcohol) supply energy. Calories themselves are not a nutrient. The energy in food is chemical potential energy stored in the bonds of carbohydrate, fat and protein molecules; cellular respiration releases it as the molecules are oxidized to CO₂ and water. Fat holds more energy per gram because its carbon chains are more reduced (more C−H bonds, little oxygen).

So fat is the most concentrated energy source: the ratio of fat to carbohydrate energy is 9 : 4. These are the Atwater factors, averages used for labels.

Dietary recommendations for energy sources (US Dietary Reference Intakes, adults): carbohydrate 45–65% of Calories, fat 20–35% (children 4–18: 25–35%), protein 10–35%. Added sugars and saturated fat should each be under 10% of Calories. On a 2,000-Calorie diet, 10% is 200 kcal, or 50 g of added sugar, which is the Daily Value for added sugars.

A typical adult needs roughly 1,600–3,000 Calories a day depending on size, sex and activity, not 5,000. Labels use 2,000 Calories as a reference, not a recommendation for everyone.

A food is calorie-dense, or energy-dense, when it packs many Calories into a small weight or volume, usually because it is high in fat and low in water: butter, oil, nuts, chips. Water and fiber lower energy density. Nutrient density compares nutrients to Calories: milk has high nutrient density because one cup supplies about a quarter of the daily calcium and a sixth of the protein for about 80–150 Calories. The nutritional value of a food depends both on its nutrient content and on its bioavailability, how much the body can actually absorb and use (the calcium in milk is well absorbed; that in spinach is mostly bound by oxalate).

Vitamins and minerals in dairy

Dairy foods are the main source of calcium in the American diet, and important sources of vitamin D (from fortification), phosphorus, potassium, riboflavin (vitamin B2), vitamin B12, vitamin A and iodine.

The Recommended Dietary Allowance (RDA) is the daily intake that meets the needs of nearly all (97–98%) healthy people of a given age and sex. (The rules write it as "recommended daily allowance".) When there is not enough evidence to set an RDA, an Adequate Intake (AI) is used instead, as for potassium. Values for ages 9–13, from the NIH Office of Dietary Supplements:

Riboflavin is destroyed by light, which is one reason milk is often sold in opaque containers. Vitamin B12 is found almost only in animal foods, so dairy is an important source for vegetarians.

The 13 vitamins

Vitamins are organic compounds needed in small amounts that the body cannot make in sufficient quantity, so they must come from food. There are 13. They split into two groups by solubility, and that split predicts how they behave:

• Fat-soluble (A, D, E, K) are absorbed with dietary fat, stored in the liver and fatty tissue, and can build up to toxic levels with large supplement doses. Healthy people rarely need supplements of them. They are lost when fat is removed from milk, which is why vitamin A (and D) is added back.
• Water-soluble (the eight B vitamins and C) dissolve in water, are not stored for long (except B12, stored in the liver), and excess is excreted in urine, so they are needed regularly and toxicity is rare. They leach into cooking water. Most B vitamins work as coenzymes in energy metabolism.

The body can make a few: vitamin D in skin exposed to sunlight (the "sunshine vitamin"), vitamin K and some B vitamins by gut bacteria, and niacin from the amino acid tryptophan.

ReferenceVitamin reference· 13 rows

F = fat-soluble; W = water-soluble. "Dairy" marks vitamins for which milk products are an important source.

13 rows

Vitamin	Name(s)	F/W	Main roles	Deficiency	Good sources
A	Retinol; beta-carotene is a provitamin	F	Vision in dim light (part of rhodopsin), healthy skin and linings, immunity, growth	Night blindness, dry eyes (xerophthalmia), infections	Liver, eggs, butter, fortified milk (dairy); orange and dark-green vegetables (carrots, sweet potatoes, spinach, kale) as beta-carotene; golden rice
D	Calciferol (D3, cholecalciferol)	F	Absorption of calcium and phosphorus; bone and tooth formation	Rickets (children), osteomalacia (adults); osteoporosis risk	Sunlight on skin, fortified milk (dairy), fatty fish, egg yolk
E	Tocopherol	F	Antioxidant protecting cell membranes; used as an antioxidant in oils	Rare: nerve and muscle damage (loss of feeling in arms and legs, poor coordination), weakened immunity	Vegetable oils, nuts, seeds
K	Phylloquinone (K1, plants), menaquinones (K2, bacteria)	F	Blood clotting; bone proteins	Bleeding, easy bruising; newborns get a vitamin K shot	Leafy greens (kale, spinach), gut bacteria; K2 in cheese and fermented foods. Intake should stay steady for people on the blood thinner warfarin (Coumadin)
B1	Thiamin	W	Coenzyme in carbohydrate metabolism; nerves	Beriberi (nerve and heart damage); seen in diets of polished white rice	Whole and enriched grains, pork, legumes
B2	Riboflavin	W	Coenzymes FAD and FMN in energy metabolism	Cheilosis (cracks at the mouth corners), sore throat, inflamed tongue	Milk and dairy (a top source), eggs, meat; destroyed by light
B3	Niacin	W	Coenzymes NAD and NADP	Pellagra: dermatitis, diarrhea, dementia (and death)	Meat, fish, poultry, enriched grains; made from tryptophan
B5	Pantothenic acid	W	Part of coenzyme A	Very rare	Found in almost all foods
B6	Pyridoxine	W	Amino acid metabolism; red blood cells	Anemia, dermatitis, confusion	Poultry, fish, potatoes, bananas
B7	Biotin	W	Carboxylation reactions in fat and sugar metabolism	Hair loss, rash; caused by eating many raw egg whites (avidin)	Eggs (cooked), nuts; gut bacteria
B9	Folate (folic acid in supplements)	W	DNA synthesis and cell division	Megaloblastic anemia; neural tube defects such as spina bifida in early pregnancy	Leafy greens, legumes, liver, fortified grains. Large folic acid intakes can mask B12 deficiency
B12	Cobalamin (contains cobalt)	W	Red blood cell formation, nerve myelin, DNA synthesis	Pernicious (megaloblastic) anemia with large red cells, fatigue, nerve damage, depression	Animal foods only: milk and dairy, meat, fish, eggs; vegans need fortified foods or supplements
C	Ascorbic acid, C₆H₈O₆	W	Collagen synthesis (blood vessels, skin, wound healing), antioxidant, helps iron absorption	Scurvy: bleeding gums, bruising, poor wound healing	Citrus fruits, strawberries, peppers, broccoli; milk is a poor source

Deficiency pairs tests love: A–night blindness, D–rickets, K–bleeding, B1–beriberi, B2–cheilosis, B3–pellagra, B9–spina bifida, B12–pernicious anemia, C–scurvy. Vitamin C (ascorbic acid) is not a B vitamin, and adenine (once called "vitamin B4") is not a vitamin at all.

Fermenting microbes make several vitamins: lactic acid bacteria in cultured dairy can produce riboflavin (B2) and folate (and some strains vitamin K2), while vitamin B12 is made mainly by propionibacteria (as in Swiss cheese) and only a few lactic acid bacterial strains; they do not make vitamin E. Both lactic acid fermentations (yogurt, cheese, sauerkraut) and yeast fermentations (bread, kefir, beer) can raise the content of some B vitamins (such as folate and riboflavin), because the microbes synthesize them, but the effect depends on the strain: some cultures use up vitamins instead (yogurt usually has less B12 than the milk it was made from), and cooking or baking afterward destroys some.

Minerals

Minerals are inorganic elements. Major minerals are needed in amounts of 100 mg or more a day; trace minerals (iron, zinc, copper, iodine, selenium, manganese, fluoride, chromium, molybdenum) in smaller amounts. Calcium, phosphorus and magnesium together keep bones and teeth hard.

Salt hides in many foods. If a cake recipe contains only a little added table salt but the label shows much more sodium, the rest comes from other ingredients: baking soda (sodium bicarbonate) and baking powder, salted butter, cheese, and sodium-containing additives such as sodium caseinate or sodium benzoate.

Fortification, enrichment and biofortification

• Fortification adds nutrients that were not there, or not in useful amounts: vitamin D in milk, calcium in orange juice and plant milks, iodine in salt.
• Enrichment puts back nutrients lost in processing: refined (white) flour is enriched with thiamin, riboflavin, niacin and iron (and fortified with folic acid).
• Biofortification raises nutrient levels in the crop itself by breeding or genetic engineering, such as beta-carotene-rich golden rice.

Adding riboflavin to a ready-to-eat breakfast cereal is usually called fortification; putting it back into white flour is enrichment. A food that provides 20% or more of the Daily Value for a nutrient per serving may be called an "excellent source" (or "high in"); 10–19% is a "good source".

Cooking and dairy chemistry

Heat changes milk in several ways at once, and tests ask you to connect a visible change to its chemistry:

• Whey protein denaturation. Above about 70 °C, β-lactoglobulin unfolds and exposes sulfur-containing groups, which give boiled or UHT milk its "cooked" flavor. Denatured whey proteins also stick to casein micelles.
• Skin formation. When milk is heated uncovered, water evaporates at the surface, and denatured whey proteins and fat concentrate there into a film. Stirring or covering the pan prevents it.
• Scorching and fouling. Proteins and minerals deposit on hot pan surfaces and burn, which is why milk is heated gently.
• Maillard browning. Lactose (a reducing sugar) reacts with amino groups, especially those on the amino acid lysine, when milk is heated for a long time. The products are brown pigments and caramel-like flavors, as in dulce de leche and sweetened condensed milk. The reaction also slightly lowers protein quality because the lysine bound into browning products is no longer available.
• Caramelization. Sugar heated on its own to high temperature breaks down into brown, bitter-sweet compounds. Unlike Maillard browning, it does not need protein.
• Curdling. Heat plus acid (tomato soup with milk, milk added to hot coffee with lemon) makes casein clump at a higher pH than it would in the cold. Paneer and ricotta are made deliberately this way.
• Melting and oiling off. Cheese fat melts and the casein network softens when cheese is heated. Overheated cheese squeezes out fat and turns rubbery as its proteins tighten.

Browning reactions compared

Foods brown in three fundamentally different ways. Two are non-enzymatic (Maillard and caramelization); one is enzymatic.

As temperature rises, the reactions come in the order Maillard → caramelization → pyrolysis (charring, above roughly 200 °C, when sugars and proteins decompose to carbon). The Maillard reaction is named after Louis-Camille Maillard, who described it in 1912. It is not one reaction but a network: a reducing sugar's carbonyl reacts with an amino group, the product rearranges into Amadori products, and these break down into hundreds of color and flavor compounds. A side product, acrylamide, forms from the amino acid asparagine plus reducing sugars in starchy foods fried or baked at high temperature (fries, chips, toast); it is classed as a probable human carcinogen (IARC Group 2A). Advanced Maillard products can also lower nutritional value by tying up lysine.

Caramelization is the breakdown of sugar by heat alone: the sugar melts, loses water, and fragments and recombines into hundreds of compounds, so less and less of the original sugar remains. A "wet" caramel (sugar plus water) takes longer than a "dry" one because the water must boil off first; stirring a hot syrup can make it crystallize instead of caramelizing smoothly. Caramelization is not enzymatic, and caramelizing pure sugar is not a Maillard reaction because there is no protein. Caramel's aroma comes from compounds such as diacetyl (buttery, butterscotch), furans such as hydroxymethylfurfural (nutty, sweet), maltol (toasty, cooked-sugar) and acids, while caramelans, caramelens and caramelins are its brown polymers. Chemically, caramelization is a pyrolysis (thermal decomposition) of sugar; it is slowest near neutral pH and faster in acidic or alkaline conditions. Charring at still higher temperatures is pyrolysis carried to completion.

Enzymatic browning is prevented by acid (lemon juice), ascorbic acid, sulfites, blanching (which destroys the enzyme), cold or keeping out oxygen.

Other effects of heat and cooking

• Physical versus chemical changes. Melting butter, freezing ice cream and dissolving sugar are physical changes (no new substances). Browning, curdling with acid and heat, rancidity and fermentation are chemical changes.
• Nutrient losses. Water-soluble vitamins (C and the B vitamins) dissolve into cooking water and are destroyed by heat and oxygen. Steaming, microwaving and quick cooking in little water preserve more nutrients than long boiling. Fat-soluble vitamins (A, D, E, K) behave the opposite way: they do not leach into water, but they can dissolve into frying oil and are damaged by the high heat and oxygen of deep-frying, so fried foods lose some of them. Pasteurization changes milk's nutrition only slightly.
• Boiling point and altitude. At higher elevation the air pressure is lower, so water boils below 100 °C and foods cook more slowly in boiling water.
• Heating milk for yogurt to about 85 °C denatures the whey proteins on purpose: they unfold and bind to the casein micelles, which gives a firmer, smoother yogurt that weeps less whey.

Emulsions

An emulsion is a dispersion of droplets of one liquid in another liquid it does not mix with. Milk and cream are oil-in-water emulsions; butter is water-in-oil. An emulsifier has a water-loving (hydrophilic) end and a fat-loving (hydrophobic) end, so it settles at the oil–water boundary and keeps droplets from merging. In milk, the phospholipids and proteins of the fat globule membrane do this; casein and whey proteins are also good emulsifiers.

Homogenization forces milk through a tiny gap at high pressure, breaking fat globules from a few micrometers across to well under one micrometer. Small globules rise so slowly that the cream no longer separates. Proteins coat the new globule surfaces and keep them stable. Unhomogenized milk forms a cream layer at the top.

Commercial ice cream adds emulsifiers such as mono- and diglycerides or the lecithin in egg yolk. Oddly, these emulsifiers do not stabilize the fat in ice cream. They displace proteins from the globule surface, which makes the globules partly coalesce during freezing, creating the fat network that gives ice cream its dry, stiff texture. Stabilizers (polysaccharide gums) thicken the water phase instead.

Emulsion types and emulsifiers

Food emulsifiers come from several chemical classes: proteins (casein, whey proteins, egg white and egg yolk proteins), phospholipids (lecithin from egg yolk or soy), mono- and diglycerides and other synthetic lipid emulsifiers (polysorbates), and some polysaccharides such as gum arabic. Thickening polysaccharides (starches, xanthan gum, guar gum, carrageenan, pectin, cellulose gums) are mostly stabilizers: they raise the viscosity of the water phase so droplets cannot move and merge. Proteins and polysaccharides are the two main kinds of colloid that keep food emulsions stable. Gum arabic (acacia gum) is both: an emulsifier and stabilizer in soft drinks and candy coatings. If a test asks you to classify ingredients: sucrose is a carbohydrate (disaccharide), corn starch a polysaccharide, egg white mainly protein, and egg yolk a source of lipids, including the phospholipid lecithin.

An emulsion breaks down in steps: creaming (droplets rise, as cream rises on raw milk), flocculation (droplets clump without merging), coalescence (droplets merge into bigger ones) and finally separation into two layers. Phase inversion turns one type into the other, as when cream (O/W) is churned into butter (W/O).

Colloids and foams

A colloid is a mixture in which tiny particles (roughly 1 nm to 1 µm) of one substance are dispersed through another without settling out. Emulsions are one kind of colloid.

Foams are colloidal dispersions of gas bubbles in a liquid (or solid). A stable foam needs a liquid viscous enough to hold the air, a stabilizer at the bubble surfaces (proteins or fat), and low surface tension. Whipped cream is a stable foam held by partly coalesced fat. Sugar adds viscosity and makes egg-white and cream foams more stable (though it slows how fast they form); temperature matters, since cold cream whips well and warm cream does not; and a trace of fat or egg yolk ruins an egg-white foam. Beating sugar into egg whites dissolves the sugar into the protein film around the bubbles and gives a glossy, stable meringue. Cream of tartar (an acid) also stabilizes egg-white foams.

Pasteurization and dairy preservation

Pasteurization heats milk enough to kill disease-causing microbes (such as Salmonella, Listeria, E. coli O157:H7 and Mycobacterium bovis) while keeping flavor and nutrients largely intact. It does not kill every microbe, so pasteurized milk still needs refrigeration. The common US methods:

Pasteurization also inactivates the enzyme alkaline phosphatase, which is slightly more heat-resistant than the toughest milk pathogens, so if it is gone the pathogens are too. Testing milk for leftover phosphatase activity is how processors confirm pasteurization worked.

Other dairy preservation methods use the same ideas as all food preservation: remove what microbes need or make conditions hostile.

• Refrigeration (at or below 40 °F, 4 °C; the FDA Food Code allows 41 °F, 5 °C) slows bacterial growth but does not stop it.
• Fermentation lowers pH. Lactic acid bacteria turn lactose into lactic acid in yogurt, buttermilk and cheese, and the acid inhibits many spoilage organisms and pathogens.
• Removing water lowers the water available to microbes: evaporated milk (concentrated, then canned and sterilized), sweetened condensed milk (sugar binds the remaining water), milk powder and hard aged cheeses.
• Salt in cheese and butter draws water and slows growth.
• Freezing stops growth in ice cream and frozen butter.

Pasteurization, sterilization and thermal processing

Louis Pasteur developed pasteurization in the 1860s to stop wine and beer from spoiling; it was applied to milk later in the 19th century. Pasteurization is a comparatively mild heat treatment: it destroys the vegetative (non-spore) pathogens that matter in the product and most spoilage organisms, but not all spoilage organisms and not bacterial spores (including spores of pathogens such as C. botulinum and B. cereus). It also inactivates enzymes such as lipase. It works by heat (not by cold), it does not require temperatures above 100 °C, and it changes milk's nutrient content only slightly.

The US Pasteurized Milk Ordinance lists equivalent time–temperature pairs: the hotter the treatment, the shorter it needs to be.

Products with more fat or added sweeteners (cream, chocolate milk, ice cream mix) are heated a few degrees higher, because fat and sugar protect bacteria from heat. The HTST standard was set high enough to destroy Coxiella burnetii (the cause of Q fever), the most heat-resistant pathogen that does not form spores. A product heated to 72 °C (345 K) for 15 s is pasteurized.

Sterilization destroys all microorganisms, including spores. Food is "commercially sterile" when nothing left in it can grow under normal storage. Canning seals food in a container and then heats it: high-acid foods (pH 4.6 or below) need only a boiling-water bath (100 °C), but low-acid foods must be heated under pressure to about 121 °C (250 °F) to destroy spores of Clostridium botulinum. UHT milk is heated to about 135–150 °C for a few seconds and then filled into sterile cartons in a sterile environment, aseptic processing, so it keeps unopened at room temperature for months. Once any can, bottle or carton is opened, microbes from the air and utensils get in, so the food can spoil like fresh food and must be refrigerated.

Three numbers describe how fast heat kills microbes:

• D-value: the time at a given temperature to kill 90% of a population (a one-log, tenfold reduction).
• z-value: the temperature increase needed to cut the D-value tenfold.
• F-value: the total lethality of a process, expressed as the number of minutes at a reference temperature (121.1 °C, 250 °F) needed to destroy a specified number of microbes. Low-acid canning targets a "12D" process for C. botulinum: enough heating to reduce its spores by a factor of 10¹².

Other preservation methods

Food is preserved to extend shelf life, prevent spoilage and foodborne illness, and keep nutritional value, flavor and texture. Most methods work by inhibiting the growth or activity of microbes rather than killing every one: they take away what microbes need (water, warmth, neutral pH, oxygen) or make conditions hostile. Cooking or baking a food, wrapping it in plastic wrap, or putting it in a moist environment are not preservation methods in this sense.

Historic milestones: salting, drying and fermenting are prehistoric; Nicolas Appert invented canning (in glass jars) around 1809; John Landis Mason patented his home-canning jar in 1858; Pasteur's heat treatment dates from the 1860s; the first US and British patents for killing bacteria in food with ionizing radiation were issued in 1905; Clarence Birdseye's quick-freezing machines came in the 1920s.

Preservatives and other additives

Food additives are substances added for a technical purpose: preservation, color, flavor, texture, nutrition. Preservatives must be listed in the ingredient list by their common name with their function ("sorbic acid, to preserve freshness").

Water activity

Water activity (a_w) measures how much water in a food is free to dissolve substances, take part in reactions and support microbes. It is defined as the ratio of the vapor pressure of water in the food to the vapor pressure of pure water at the same temperature.

Water activity is not the same as moisture content. Moisture content is the total amount of water, free plus bound, usually as a percentage of the food's mass (measured by oven drying or Karl Fischer titration). Water activity measures only the available water. Water bound to salt, sugar, proteins and starch is still in the food but unavailable to microbes. Jam and fresh bread can have similar moisture contents but very different water activities. Moisture content answers "how much water?" (a quantity question for yield and labeling); water activity answers "how safe and stable?" (a quality and safety question). Water content is not directly proportional to water activity.

What lowers water activity: drying (removes water), adding solutes such as salt and sugar (they bind water), and freezing (liquid water becomes ice). Water moves from high a_w to low a_w: dry cereal in milk absorbs water and goes soggy as its a_w rises, crackers left open in humid air go stale, and meat at a_w 0.89 exposed to drier air (a_w 0.63) loses water and dries out. For most foods, at a fixed moisture content, water activity rises somewhat as temperature rises; the exact effect depends on the food. Water activity is measured with hygrometers: chilled-mirror dew-point instruments, capacitance hygrometers and resistive electrolytic hygrometers. Moisture content is measured differently: by oven drying (loss on drying), Karl Fischer titration (a chemical titration specific to water), or moisture analyzers and meters. A titrator measures moisture content, not water activity.

Gram-negative bacteria need the most water, then most bacteria, then yeasts, then molds. Halophiles tolerate high salt, osmophiles high sugar, and xerophiles dry conditions.

Intermediate-moisture foods (a_w about 0.60–0.85, such as dried fruit, jams and soft jerky) are shelf-stable without refrigeration even though they contain noticeable moisture. A food at a_w 0.36, such as crackers, does not need refrigeration for safety, because no microbe can grow; refrigeration would only slow staling.

Acid, low-acid and acidified foods

US canning rules sort foods by pH and water activity, because Clostridium botulinum cannot grow or make toxin at pH 4.6 or below or at a_w 0.85 or below (the regulatory a_w line includes a safety margin: proteolytic C. botulinum actually stops growing near 0.93–0.94).

So "acid food" and "acidified food" are not synonyms: acid foods are naturally acidic, acidified foods are made acidic. The acidified-food rules do not cover carbonated drinks, jams and jellies, fermented foods, foods stored and sold refrigerated, or acid foods with a little low-acid food added whose final pH is not significantly different from that of the main acid ingredient (for example applesauce from apples at pH 3.9 with a few figs, if the final pH stays near 3.9). Cultured dairy foods, which are fermented and refrigerated, are outside these canning rules.

Acidified foods are thermally processed after acidification to kill vegetative pathogens and spoilage organisms and inactivate enzymes, then sealed airtight so nothing can recontaminate them. Acidic foods are packed in glass or lined cans, because acid corrodes bare metal, dissolving metal ions into the food. If an acidified or canned food is processed incorrectly, C. botulinum can survive and produce botulinum toxin; the FDA's concern for these foods is the botulism toxin.

Food microbiology and food safety

Milk is close to a perfect growth medium: water, sugar, protein, vitamins and near-neutral pH. That is why dairy science is so concerned with microbes, both the useful ones (starter cultures) and the dangerous ones (pathogens). Fermented dairy foods are "riskier" than many foods because if the acidification fails, pathogens in the milk can grow.

Microbes in food

The three main groups of microorganisms in food are bacteria, fungi (molds and yeasts) and viruses; parasites (such as Giardia, Toxoplasma, Trichinella) also cause foodborne disease. Of these, bacteria are the greatest threat to food safety overall, while norovirus causes the most cases.

Not all bacteria are harmful: lactic acid bacteria make yogurt and cheese, and gut bacteria help digestion. Many bacteria can swim using flagella.

Bacterial shapes and the Gram stain

Bacteria come in three basic shapes: cocci (spheres; Streptococcus in chains, Staphylococcus in grape-like clusters, Lactococcus), bacilli (rods; Lactobacillus, Listeria, Clostridium, E. coli) and spirilla (spirals; Campylobacter is curved-spiral).

The Gram stain (Hans Christian Gram, 1884) sorts bacteria by cell wall:

1. Stain with crystal violet (purple).
2. Add iodine (the mordant), which locks the dye into a crystal violet–iodine complex.
3. Rinse with alcohol or acetone (decolorizer).
4. Counterstain with safranin (red).

Gram-positive bacteria have a thick peptidoglycan wall that traps the dye complex, so they stay purple/blue. Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane; alcohol washes the purple out and they take up the safranin, appearing pink/red.

Clostridium and Bacillus form endospores, tough dormant forms that survive boiling, pasteurization and drying and germinate when conditions improve. Lactic acid bacteria do not form spores. Bacteria can be identified by microscopy (shape, Gram reaction), culture on selective media, biochemical tests, and DNA methods such as PCR. Selective media contain ingredients that stop unwanted microbes from growing; differential media make different microbes look different. MacConkey agar is both: bile salts and crystal violet inhibit Gram-positive bacteria (selective), and lactose plus a pH indicator turn lactose-fermenting Gram-negatives such as E. coli pink, while non-fermenters such as Salmonella and Shigella stay colorless (differential). Hektoen enteric and deoxycholate citrate agars are other selective media used to recover Salmonella and Shigella.

Oxygen and temperature needs

From coldest to warmest: psychrophiles, mesophiles, thermophiles. Most pathogenic bacteria are mesophiles, because human body temperature (37 °C) is in their range. That is also why foodborne illness from Salmonella is more common in summer: it multiplies faster at warm temperatures.

The bacterial growth curve

A culture of bacteria in a fixed amount of food passes through four phases:

In the log phase, bacteria divide by binary fission at a constant generation (doubling) time, so the count grows exponentially.

FAT TOM and the danger zone

The six conditions that let foodborne bacteria grow are remembered as FAT TOM:

Refrigerate at 40 °F (4 °C) or below (the FDA Food Code uses 41 °F); below this, many pathogens cannot multiply well, though Listeria still can, slowly. Freeze at 0 °F (−18 °C). Milk should be refrigerated at or below 40 °F.

Foods that need time and temperature control to stay safe are called TCS foods (time/temperature control for safety), formerly potentially hazardous foods (PHF); the FDA defines them in its Food Code. They include milk and dairy products, meat, poultry, fish, eggs, cooked rice and vegetables, cut melons, cut tomatoes, cut leafy greens, sprouts and garlic-in-oil. A food that does not support rapid growth of pathogens, because of its pH, water activity or both, is "non-TCS".

Food safety hazards fall into three categories: biological (microbes, toxins, parasites), chemical (cleaning agents, pesticides, allergens, natural toxins) and physical (glass, metal, bone). The system the food industry uses to control them is HACCP, Hazard Analysis and Critical Control Points, developed in the 1960s for NASA's space food. Its seven principles: (1) conduct a hazard analysis; (2) identify critical control points (such as the pasteurizer); (3) set critical limits (72 °C for 15 s); (4) monitor; (5) set corrective actions; (6) verify; (7) keep records.

Foodborne illness and the pathogens that matter for dairy

Common symptoms of foodborne illness are nausea, vomiting, diarrhea, abdominal cramps and fever, appearing from 30 minutes to several days after eating. Most cases resolve, but anyone with bloody diarrhea, a high fever, signs of dehydration, or symptoms lasting more than a few days should seek medical care, as should pregnant women, young children, older adults and people with weak immune systems. Foodborne illness is caused by bacteria, viruses, parasites, toxins (bacterial or fungal) and chemicals.

ReferenceFoodborne pathogens reference· 11 rows

G+ = Gram-positive; G− = Gram-negative.

11 rows

Pathogen	Type	Key facts	Typical foods
Norovirus	Virus	Causes the most foodborne illness in the US (about half of outbreaks); spread by infected food handlers; vomiting and diarrhea	Ready-to-eat foods, shellfish
Salmonella	G− rod, facultative	Leading cause of US foodborne deaths and a major cause of hospitalizations (CDC's 2025 estimates rank norovirus first for illnesses and hospitalizations); grows faster when warm	Poultry, eggs, raw milk, produce
Listeria monocytogenes	G+ rod, facultative, psychrotroph	Grows at refrigerator temperatures (slowly, even near 0 °C), tolerates salt and fairly acidic conditions (grows down to about pH 4.4 and survives even lower), which makes it a hazard in poorly acidified or recontaminated foods; dangerous to pregnant women (miscarriage), newborns, older adults and the immunocompromised	Soft cheeses (especially raw-milk or queso fresco), deli meats, hot dogs, raw milk, smoked seafood
E. coli O157:H7 (Shiga toxin-producing)	G− rod	Bloody diarrhea; can cause kidney failure (hemolytic uremic syndrome)	Undercooked ground beef, raw milk, leafy greens
Campylobacter jejuni	G− curved/spiral, microaerophile	A leading bacterial cause of diarrhea	Raw milk, undercooked poultry
Staphylococcus aureus	G+ cocci in clusters	Makes a heat-stable toxin in food; rapid vomiting 30 min–8 h after eating; tolerates salt and low a_w (0.86)	Foods handled by people: cream-filled pastries, ham, cheese, salads
Clostridium botulinum	G+ rod, obligate anaerobe, spore-former	Botulinum neurotoxin causes paralysis (botulism); spores survive boiling and pasteurization, but the toxin is destroyed by boiling; controlled by pH ≤ 4.6, a_w ≤ 0.85–0.93, refrigeration, nitrite and 121 °C canning	Improperly canned low-acid foods (bulging cans), reduced-oxygen packages, garlic in oil, honey (infant botulism)
Clostridium perfringens	G+ rod, spore-former	Grows in large batches of cooked food cooled slowly	Gravies, stews, roasts
Bacillus cereus	G+ rod, spore-former	Vomiting or diarrheal toxins	Cooked rice left warm
Brucella, Mycobacterium bovis, Coxiella burnetii	Bacteria	Historic raw-milk diseases (brucellosis, bovine tuberculosis, Q fever), largely eliminated by pasteurization	Raw milk
Cronobacter	G− rod	Dangerous to infants	Powdered infant formula

Raw (unpasteurized) milk can carry Salmonella, Listeria, E. coli O157:H7, Campylobacter and Brucella. US law bans interstate sale of raw milk for drinking, and cheese made from raw milk must be aged at least 60 days.

Cross-contamination is the transfer of microbes from one food or surface to another (raw chicken juices on a cutting board). Cross-contact is the transfer of an allergen (a trace of peanut on a shared scoop); cooking kills microbes but does not remove allergens. "Best by" and "sell by" dates are about quality, not safety (infant formula is the federal exception), so a food past its best-by date is not automatically hazardous; you cannot tell whether food is unsafe just by smelling it, because pathogens usually do not change odor.

Food spoilage is any change that makes food unacceptable: microbial growth (souring, slime, mold, off odors), enzymes, chemical reactions such as oxidation, physical damage (freezer burn, moisture loss) and pests. Spoilage organisms are often harmless but make food inedible; pathogens often cause no visible change.

Cultured dairy and probiotics

Cultured dairy foods are made by adding starter cultures of lactic acid bacteria. Yogurt uses Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, which grow together. Cheese and cultured buttermilk use Lactococcus and other lactic acid bacteria. Kefir uses a mix of bacteria and yeasts. The bacteria ferment lactose to lactic acid, which lowers the pH toward 4.6, sets the casein into a gel (that is why yogurt is thick) and gives the tangy flavor.

Probiotics are live microorganisms that give a health benefit when eaten in adequate amounts. Strains often added to yogurt and kefir include Lactobacillus acidophilus, Lacticaseibacillus rhamnosus GG and species of Bifidobacterium. Proposed benefits include reducing some kinds of diarrhea and helping lactose digestion, since yogurt bacteria carry their own lactase. Effects are specific to the strain, and not every yogurt contains live cultures in useful numbers; heat-treated yogurt does not. Prebiotics are different: they are food ingredients (usually fibers such as inulin) that feed beneficial gut bacteria.

Fermentation basics

Fermentation is the breakdown of sugars by microorganisms without using oxygen, producing acids, gases or alcohol. It is anaerobic and releases a little energy (it is exothermic): only 2 ATP per glucose, compared with about 30 or more from aerobic respiration, so a respiring organism grows much faster than a fermenting one. Cells ferment because, without oxygen, they must recycle the carrier NADH back to NAD⁺ so that glycolysis can keep making ATP; pyruvate (or something made from it) accepts the electrons. Fermentation happens in the cytoplasm of bacterial cells. Its end products (lactic acid, ethanol) still contain most of the glucose's energy, but without oxygen as a final electron acceptor the cell cannot oxidize them further, so it releases them as waste; that leftover energy is why ethanol and lactate can later be burned or respired. The study of fermentation is zymology. Louis Pasteur showed in the 1850s that fermentation is carried out by living microorganisms (including the lactic fermentation of milk), and Eduard Buchner showed in 1897 that yeast extract with no living cells could still ferment sugar, proving enzymes do the work (Nobel Prize, 1907).

Human muscle cells also carry out homolactic fermentation during hard exercise, when oxygen runs short. Kefir is best classified as a lactic acid fermentation of milk (by its lactic acid bacteria, several of them heterofermentative) combined with a mild alcoholic fermentation by its yeasts; yogurt is a purely (homo)lactic fermentation.

Most lactic acid bacteria grow and make acid fastest at about pH 5.5–6.5, slowing as their own acid drives the pH below about 4.5; mesophilic strains work best at about 20–30 °C and thermophilic strains at about 37–45 °C. The critical control factors for a safe fermentation are acid development (pH and how fast it falls), temperature and time, plus salt concentration where salt is used (cheese, and vegetable ferments, where about 2–3% salt favors lactic acid bacteria over spoilers). Too little acid, too warm or too long lets pathogens and spoilage organisms win. Lactic acid bacteria break lactose down with the enzyme β-galactosidase (lactase) before fermenting the sugars, which is why fermented milks contain less lactose than milk. The two lactic types use different pathways: homofermenters run glucose through ordinary glycolysis (the Embden–Meyerhof–Parnas pathway) and reduce both pyruvates to lactate (2 ATP per glucose); heterofermenters lack the glycolytic enzyme aldolase and use the phosphoketolase pathway, which splits a five-carbon sugar phosphate into a three-carbon part that becomes lactate and a two-carbon part that becomes ethanol or acetate, releasing CO₂ (about 1 ATP per glucose). Many lactic fermentations run at moderate temperatures, around 30–35 °C for many mesophilic dairy and vegetable cultures (cheddar starters near 30–32 °C), and around 40–45 °C for thermophilic yogurt cultures.

Lactic acid bacteria

Lactic acid bacteria (LAB) are the workhorses of cultured dairy. They are Gram-positive, non-spore-forming rods or cocci that make lactic acid as their main product, lack the enzyme catalase, and grow with or without oxygen (they are aerotolerant). Important genera: Lactobacillus (rods), Lactococcus and Streptococcus (cocci), Leuconostoc, Pediococcus (mostly meat and vegetable fermentations) and Enterococcus. Bifidobacterium is often grouped with them as a probiotic. In 2020 the old genus Lactobacillus was split into 25 genera, so you may see new names such as Lacticaseibacillus rhamnosus and Lactiplantibacillus plantarum.

LAB make fermented foods safe in several ways: lactic acid lowers the pH, they use up sugar and nutrients that pathogens would need, and some produce hydrogen peroxide and bacteriocins (natural antimicrobial peptides such as nisin). A starter culture is a preparation of known microorganisms, chosen strains in a growth medium, added at the start so that they dominate the fermentation quickly and reliably, which makes the product safer and more consistent than relying on wild microbes. Most dairy fermentations are pasteurized-milk fermentations seeded with starters. Properly made fermented and acidified foods have an excellent safety record over thousands of years of use, because acid, competition and bacteriocins keep pathogens out; the few outbreaks come from failed fermentations (acid too slow), contamination after processing (Listeria in soft cheese), or unpasteurized ingredients.

Cultured dairy products

Making yogurt

1. Heat the milk to about 85 °C for about 30 minutes (or 90–95 °C for a few minutes). This kills competing microbes and denatures the whey proteins, which then bind to casein and give a firmer gel with less whey separation.
2. Cool to about 43 °C (108–112 °F), the best temperature for the two cultures together.
3. Inoculate with the starter culture.
4. Incubate for about 4–6 hours until the pH falls to about 4.6 or below and the milk sets.
5. Refrigerate to slow the bacteria. Cold does not kill them, but it slows acid production, so yogurt stays only mildly sour for weeks. Without refrigeration it keeps souring.

The two yogurt bacteria help each other (protocooperation). S. thermophilus grows first and fast, lowers the pH and makes formic acid and CO₂ that stimulate L. bulgaricus; L. bulgaricus breaks casein into peptides and amino acids that feed S. thermophilus and makes acetaldehyde, the main flavor compound of yogurt. The US standard of identity for yogurt (revised in 2021) requires both cultures and a pH of 4.6 or lower, measured on the finished product within 24 hours of filling; the old minimum titratable-acidity requirement was dropped. Under the standard, a label may say "contains live and active cultures" only if the yogurt has at least 10⁷ CFU/g at manufacture (and is expected to keep 10⁶ CFU/g through its shelf life). A "Live & Active Cultures" seal means the yogurt contained at least 100 million cultures per gram at manufacture; yogurt heat-treated after culturing has none.

Probiotics, prebiotics and fermented-food benefits

Probiotics act mainly in the gut. Benefits supported by evidence include fewer cases of antibiotic-associated diarrhea, some relief from irritable bowel syndrome (IBS) symptoms, help with lactose digestion, and support for a balanced gut microbiome and immune function; effects depend on the strain. Health claims for probiotics and the gut microbiome (including the gut–brain axis) are an active and controversial research area, and US regulators have not approved disease claims for them. In breast-fed infants, milk oligosaccharides (and lactose) encourage Bifidobacterium in the gut.

Benefits of fermented foods in general: preservation and longer shelf life; better digestibility (lactose and proteins partly broken down); more vitamins (B vitamins, vitamin K2) and better mineral bioavailability; bioactive peptides released from casein, some of which have antioxidant or blood-pressure-lowering effects; and new flavors, aromas and textures. Not every fermented food contains live cultures (bread, beer, pasteurized sauerkraut and heat-treated yogurt do not). Foods that are not fermented include fresh milk, ice cream, milkshakes, ordinary sweet-cream butter, applesauce, fresh fruit juice, tofu, nori and canned tuna; fermented ones include yogurt, kefir, cheese, cultured buttermilk, sour cream, sauerkraut, kimchi, pickles made by brining, vinegar, soy sauce, miso, tempeh, salami, chocolate (the cacao beans are fermented), bread and beer.

Side effects and cautions: the most common is temporary gas and bloating; aged cheeses and other fermented foods can contain histamine or tyramine that trigger headaches in sensitive people; many are salty; and people with compromised immune systems should ask a doctor before taking live probiotics. A fermented food that smells putrid, grows fuzzy mold of the wrong kind, turns slimy or develops off colors has spoiled.

Cheese and cheesemaking

Cheese is concentrated milk curd: mostly casein and fat, with the whey drained away. About 10 kg of milk makes 1 kg of cheddar. Thousands of varieties come from the same few steps, varied in milk, culture, coagulant, cutting, cooking, salting and aging.

The two essential steps in making any cheese are acidification (lactic acid from starter bacteria, or added acid) and coagulation (by rennet, acid, or acid plus heat); everything else (cutting, cooking, salting, pressing, aging) shapes moisture, texture and flavor.

1. Prepare the milk. Standardize its fat and usually pasteurize it. (US cheese from raw milk must be aged at least 60 days.) Plant extracts such as annatto may be added to color cheddar orange.
2. Acidify with a starter culture: lactic acid bacteria convert lactose into lactic acid.
3. Coagulate with rennet: the enzyme chymosin cuts κ-casein, the micelles lose their protective hairy layer and clump, with calcium bridging them, into a gel. Rennet traditionally comes from the fourth stomach (abomasum) of young calves and other ruminants; today most US cheese uses chymosin made by fermentation with engineered microbes, or microbial coagulants from molds.
4. Cut the curd. Smaller pieces lose more whey and make a drier, harder cheese.
5. Cook and stir to expel more whey (syneresis), then drain it. For cheddar, slabs of curd are stacked and turned (cheddaring).
6. Salt: salt adds flavor, draws moisture out of the curd, slows unwanted microbes (preserving the cheese) and controls ripening.
7. Mold and press into shape.
8. Ripen (age): enzymes and microbes break proteins (proteolysis) and fats (lipolysis) into flavor compounds and soften or harden the texture. The longer the aging, the stronger the flavor: mild cheddar is aged about 2–3 months, sharp about 6–9 months and extra-sharp a year or more.

Most of the lactose leaves with the whey, and the bacteria ferment what remains, so aged hard cheeses such as cheddar, Swiss and Parmesan contain almost no lactose, which makes them a good calcium source for people with lactose intolerance. Acid-set fresh cheeses (cottage cheese) lose more calcium into the whey than rennet cheeses, because acid dissolves the calcium phosphate in the micelles.

Unripened (fresh) cheeses are eaten soon after they are made, without an aging period: cottage cheese, cream cheese, ricotta, fresh mozzarella. Higher moisture means a softer cheese; a cheese with 55% water is soft. Ricotta ("recooked") is made by heating acidified whey, which coagulates the whey proteins that acid alone leaves in solution. Paneer and queso blanco are curdled with acid and heat. The simplest cheese of all is fresh curd: milk curdled with acid (as in the lab) or rennet, then drained of whey.

The mold in blue cheese is added on purpose and is safe to eat. Cheddar's starter (Lactococcus) makes only lactic acid; Swiss cheese's nutty flavor and eyes come from an extra propionic fermentation.

Processed cheese is natural cheese melted with emulsifying salts (sodium citrate, sodium phosphates). The salts pull calcium out of the casein network so the proteins become better emulsifiers, and the cheese melts smoothly without oiling off. In natural cheese the casein exists as calcium paracaseinate (casein after rennet, cross-linked by calcium), which is a poor emulsifier; the sodium salts swap sodium for calcium (ion exchange), turning it into more soluble sodium paracaseinate that coats fat droplets and emulsifies them. Whey, the watery liquid left after the curd forms, contains the whey proteins, most of the lactose, and water-soluble vitamins and minerals; sweet whey comes from rennet cheeses and acid whey from Greek yogurt and cottage cheese. It is dried into whey protein concentrates and isolates. In the lab, curds are separated from whey by pouring through cheesecloth.

The bean-curd analogue, tofu, is made by coagulating soy milk proteins with calcium sulfate or magnesium chloride (nigari): the divalent Ca²⁺ and Mg²⁺ ions bridge negatively charged proteins, just as calcium helps rennet curd form.

Milk types: low-fat, lactose-free, organic and non-dairy

US federal standards define each fluid milk by its fat content:

Protein, lactose and calcium stay roughly the same across the four. Because vitamin A is dissolved in the fat, removing fat removes vitamin A, so low-fat and fat-free milk must have vitamin A added back. Vitamin D is added to most milk.

Lactose-free milk is regular milk treated with the enzyme lactase, which splits lactose into glucose and galactose (some brands also filter out part of the lactose). It tastes sweeter than regular milk, even though no sugar is added, because glucose and galactose together are sweeter than lactose. Its protein, calcium and vitamins are the same as regular milk.

Organic milk in the US must meet the USDA National Organic Program rules: cows eat organic feed, graze on pasture for at least 120 days a year and get at least 30% of their dry matter from pasture during the grazing season, and they are not given antibiotics or synthetic growth hormones. Organic milk is often ultra-pasteurized for a longer shelf life. Its basic nutrient content is very similar to conventional milk; studies report somewhat more omega-3 fatty acids from the grass-heavy diet.

Non-dairy "milks" are plant-based drinks made by grinding and filtering soybeans, nuts, oats, rice or peas with water, then adding emulsifiers and (usually) calcium and vitamins. They contain no lactose and no milk protein, which makes them choices for people with lactose intolerance or milk allergy, and for vegans. Their nutrients vary widely: unsweetened soy and pea drinks have roughly as much protein as cow's milk (about 7–8 g per cup), oat drinks about 3 g and almond drinks about 1 g. FDA guidance on plant-based milk alternatives (first issued in draft in 2023) lets them use names like "soy milk" and recommends a voluntary statement comparing their nutrients with cow's milk.

Most are fortified with calcium and vitamins D, A and B12, and need emulsifiers or gums (lecithin, gellan gum) to stay smooth. They contain no cholesterol.

Concentrated, dried and specialty milks

Removing fat by centrifuging gives cream and skim milk. Skim (nonfat) milk must have less than 0.5 g of fat per 1-cup serving, which is under about 0.2% fat; older federal standards defined skim milk as less than 0.5% milkfat, so "skim milk has no more than 0.5% fat" is true either way.

rBST (recombinant bovine somatotropin) is a hormone that raises a cow's milk yield. Organic rules forbid it, and labels claiming "from cows not treated with rBST" generally carry a statement that no significant difference has been shown between milk from treated and untreated cows.

Lactose intolerance and milk allergy

These are different conditions, and tests often check that you can tell them apart.

Lactose intolerance is a digestive problem. The small intestine makes too little lactase, so lactose passes into the colon. Colon bacteria ferment it, producing gas (hydrogen, CO₂, methane) and acids, and the undigested sugar draws water into the gut by osmosis. The results are bloating, gas, cramps and diarrhea from 30 minutes to two hours after eating dairy. Most mammals lose lactase after weaning, and so do about two-thirds of people worldwide; lactase that persists into adulthood is common mainly in populations with a long history of dairy farming. It is diagnosed with a hydrogen breath test and managed by limiting lactose, choosing lactose-free milk, yogurt or hard aged cheeses (most lactose leaves with the whey or is fermented), or taking lactase pills.

Galactosemia is a rare inherited disorder (often caught by newborn screening) in which an enzyme needed to convert galactose to glucose is missing. Galactose builds up and damages the liver, brain and eyes (cataracts), so affected babies cannot have breast milk, regular formula, milk or even lactose-free milk (which still contains galactose); they use soy-based formula. Lactase persistence, by contrast, is a genetic trait: variants in a control region (in the MCM6 gene) near the lactase gene LCT keep lactase switched on in adults, and they spread in populations with a long history of dairying.

Milk allergy is an immune reaction to milk proteins, casein, whey proteins or both. In the usual (IgE-mediated) form, antibodies trigger hives, vomiting, wheezing and, in severe cases, anaphylaxis, within minutes. Even a trace of milk protein can cause a reaction, so lactose-free milk is not safe for someone with milk allergy. Milk allergy is one of the most common food allergies in young children, and many outgrow it.

Milk is one of the nine major food allergens that US law requires labels to declare: milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat, soybeans and sesame. Labels must name the allergen's source, for example "whey (milk)" in the ingredient list or a "Contains: Milk" statement.

How an allergic reaction works

Almost all food allergens are proteins. In the common IgE-mediated allergy (a type I hypersensitivity reaction):

1. Sensitization. On first exposure, the immune system wrongly treats a food protein as a threat and makes IgE antibodies against it. The IgE attaches to mast cells (in skin, gut and airways) and basophils.
2. Reaction. On a later exposure, the allergen binds and cross-links the IgE on those cells, and they release histamine and other chemicals within minutes.
3. Effects of histamine: blood vessels widen and leak (redness, hives, swelling of lips, tongue and throat), smooth muscle in the airways contracts (wheezing, shortness of breath), and the gut cramps (vomiting, diarrhea). Itching, coughing and dizziness are also common.

Anaphylaxis is the most severe reaction: rapid, affecting several body systems, with swelling that can close the airway and a sudden drop in blood pressure (shock) that can cause loss of consciousness and death. It is treated immediately with an epinephrine auto-injector, then emergency care; antihistamines alone are for mild reactions. Even a trace of the allergen can trigger a reaction, which is why cross-contact matters.

Allergies are diagnosed with a skin-prick test, a blood test for food-specific IgE, and, as the definitive test, a supervised oral food challenge. Some reactions are non-IgE (such as food protein-induced enterocolitis in infants), with slower, mostly digestive symptoms.

Among US children the most common food allergies are milk, egg and peanut (milk is the most common in infants and toddlers); peanut is the most common in US children overall, and shellfish in adults. Allergies to milk, egg, soy and wheat are the ones most likely to be outgrown. A food allergy can develop at any age, even to a food eaten often for years.

Major allergens and allergen law

The Food Allergen Labeling and Consumer Protection Act (FALCPA) of 2004 named eight major food allergens, the "Big 8", which together caused about 90% of food allergic reactions: milk, eggs, fish, crustacean shellfish, tree nuts, peanuts, wheat and soybeans. The FASTER Act of 2021 added sesame as the ninth, effective January 1, 2023. Foods like citrus fruit, strawberries, pork, honey or pollen are not major allergens under the law.

A packaged food must identify the food source of every major allergen, in one of two ways:

1. In the ingredient list, with the source in parentheses after the ingredient's name: "whey (milk)", "lecithin (soy)", "flour (wheat)"; or
2. In a "Contains" statement right after or next to the ingredient list: "Contains: Milk, Soy".

For tree nuts, fish and shellfish the specific kind must be named (almond, cod, shrimp). A separate allergen line is not always present: with the parenthetical method, the allergens appear only inside the ingredient list. Advisory statements such as "may contain milk" or "produced in a facility that also processes peanuts" are voluntary, not required by FALCPA, and address possible cross-contact. These labeling rules protect people with food allergies and the parents and caregivers who shop for allergic children.

Milk-derived ingredients that must be flagged as milk include casein, caseinates (sodium caseinate), whey, lactalbumin, curds, butter, ghee and milk solids. Beware the word "non-dairy": non-dairy creamers and whipped toppings can legally contain sodium caseinate, a milk protein, so they are not safe for milk allergy.

Other reactions to food

An allergy is an immune reaction that affects only sensitized people. A toxic reaction (to a pesticide residue, a mycotoxin from mold, a bacterial toxin, or natural toxins in some fish) affects anyone who eats enough of the substance, does not involve the immune system, and depends on dose.

Labels and standards

Dairy labels are controlled by the FDA (and USDA for grading and organic claims).

• Standards of identity define what a product must contain to use its name. Milk must have at least 3.25% milkfat and 8.25% milk solids other than fat; heavy cream at least 36% milkfat; butter at least 80% milkfat; ice cream at least 10% milkfat and a minimum weight per gallon, which limits how much air it can contain. A frozen dessert with less fat must use another name ("frozen dairy dessert") unless it qualifies as a modified ice cream with a nutrient content claim, such as "reduced fat" or "light" ice cream.
• Nutrition Facts panels list the serving size, Calories, nutrients per serving and a % Daily Value column. Since the 2016 revision, the required micronutrients are vitamin D, calcium, iron and potassium. Vitamins A and C became optional.
• Nutrient content claims have defined meanings: "fat free" is less than 0.5 g fat per serving; "low fat" is 3 g or less; "reduced" means at least 25% less than the regular product. That is why 2% milk is labeled "reduced fat".
• Allergen labeling requires milk to be declared, as described above.

Who regulates food labels

Key laws: the Federal Food, Drug, and Cosmetic Act (1938); the Nutrition Labeling and Education Act (NLEA, 1990), which amended it to require Nutrition Facts on most packaged foods (in effect from 1994, including the sodium declaration) and set rules for nutrient content and health claims; FALCPA (2004) for allergens; trans fat labeling from 2006 (first proposed in 1999); and the FASTER Act (2021) for sesame. The current Nutrition Facts design comes from a 2016 FDA rule; manufacturers with $10 million or more in annual food sales had to comply by January 1, 2020, and smaller manufacturers by January 1, 2021.

The package panels

• The principal display panel (PDP) is the part of the label most likely to be seen by a shopper. It must carry the statement of identity (the product's common name, such as "Cheddar Cheese") and the net quantity of contents (in both US and metric units, in the bottom 30% of the panel).
• The information panel is the panel immediately to the right of the PDP as the shopper faces it. It carries the Nutrition Facts, the ingredient list (with allergen information) and the name and place of business of the manufacturer, packer or distributor. These items may also go on the PDP. On a package too small for both, the Nutrition Facts can go on any panel the shopper can see.

So the five mandatory label elements are: statement of identity, net quantity, Nutrition Facts, ingredient list (with allergens), and manufacturer's name and address.

Ingredient lists run in descending order of predominance by weight: the ingredient present in the largest amount comes first, the smallest last. Ingredients appear by their common or usual name (the label says "sugar", not "sucrose"), sub-ingredients go in parentheses, and chemical preservatives must state their function. A substitute that resembles a traditional food but is nutritionally inferior must be labeled "imitation".

The Nutrition Facts panel, line by line

From top to bottom on the current label:

1. Servings per container and serving size (serving size in bold, larger type). Serving sizes are not recommendations of how much to eat; they are based on the amounts people customarily eat (the FDA's "reference amounts customarily consumed"), and some were enlarged in 2016 (ice cream went from ½ cup to ⅔ cup). Servings per container may be a decimal (about 3.5) and are not always rounded to a whole number.
2. Calories, in large bold type.
3. % Daily Value column heading.
4. Total Fat (indented: Saturated Fat, Trans Fat), Cholesterol, Sodium, Total Carbohydrate (indented: Dietary Fiber, Total Sugars, and "Includes X g Added Sugars"), Protein.
5. Vitamin D, calcium, iron and potassium, with amounts and %DV.
6. The footnote: the %DV tells you how much a nutrient in a serving contributes to a daily diet; 2,000 Calories a day is used for general nutrition advice.

The panel's design follows a "limit these, get enough of these" logic. Limit: saturated fat, trans fat, sodium and added sugars (and cholesterol). Get enough: dietary fiber, vitamin D, calcium, iron and potassium on the current label. On the older (1993) label the "get enough" nutrients were dietary fiber, vitamin A, vitamin C, calcium and iron.

Total Sugars includes both naturally occurring sugars (such as lactose in milk) and added sugars; the "Added Sugars" line shows only the sugars and syrups added during processing (plus sugar from concentrated juice beyond what the juice would contain). Lines with no %DV: trans fat and total sugars (no Daily Value exists), and usually protein (a %DV for protein is required only if a protein claim is made or the food is for children under 4).

Rounding rules explain some odd-looking labels: fats under 0.5 g per serving are declared as 0 g (so "0 g trans fat" can mean up to 0.49 g); Calories under 5 are declared as 0; above 5 they are rounded to the nearest 5, and above 50 to the nearest 10.

The label normally appears as a boxed panel set in an easy-to-read type style; FDA's sample label uses Helvetica, but no specific font is required (minimum type sizes are required, for example at least 8-point for most text and larger type for Calories and serving size), the box does not have to be a particular shape on a round package, and small packages may use a tabular or linear (one-line) format instead. Dual-column labels (per serving and per package) are required for certain packages that could be eaten in one or several sittings (those holding two to three times the reference serving), so shoppers can see the total for the whole package. A package holding less than twice the reference amount, such as a 20-oz soda, must be labeled as a single serving.

What changed in 2016

• Calories and serving size are larger and bolder.
• Serving sizes updated to reflect what people actually eat.
• Added Sugars line (in grams and %DV) added.
• Vitamin D and potassium became mandatory (actual amounts and %DV); calcium and iron stayed mandatory.
• Vitamins A and C became optional, because deficiencies of them are now rare in the US.
• "Calories from Fat" was removed, because research shows the type of fat matters more than the amount.
• Daily Values were updated (below), and the footnote was rewritten.
• Dual-column labels for some multi-serving packages.

Daily Values

Older tests (written before the new label) use the old values, so read the question's context. Daily Values are for adults and children 4 and older on a 2,000-Calorie reference diet, not a 2,500-Calorie one.

Claims on labels

A label that says a product diagnoses, cures, mitigates, treats or prevents a disease is making a drug claim, which can make the product legally a drug.

Health benefits of dairy foods

Dairy foods deliver a lot of nutrients per Calorie: high-quality complete protein, calcium and phosphorus for bone, vitamin D (fortified), potassium, riboflavin and B12. Milk drinking during childhood and adolescence, when most bone mass is built, helps reach a higher peak bone mass, which lowers the risk of osteoporosis later. Potassium and calcium in dairy are linked with healthy blood pressure. Fermented dairy foods can supply live cultures, and yogurt is often well tolerated by people with lactose intolerance. Research is mixed on full-fat dairy and heart disease. Earlier US guidelines (through 2020–2025) recommended mostly low-fat and fat-free choices; the 2025–2030 Dietary Guidelines (released January 2026) instead say to include full-fat dairy with no added sugars.

Dairy is not required for health: people who avoid it can get the same nutrients from fortified plant drinks, leafy greens, canned fish with bones and other foods. A balanced answer on a test names both the benefits and the alternatives.

The 2020–2025 US dietary guidelines recommended 2 cup-equivalents of dairy a day for ages 2–3, 2½ for ages 4–8, and 3 for ages 9 and older; the 2025–2030 guidelines keep 3 servings a day on a 2,000-Calorie pattern. One cup of milk or yogurt, 1½ oz of natural cheese, or 1 cup of fortified soy milk counts as one cup-equivalent.

Possible downsides to know: whole-milk products and cheese add saturated fat and Calories, many cheeses are high in sodium, flavored milks and yogurts can contain a lot of added sugar, and lactose intolerance and milk allergy limit what some people can eat. Too much salt (sodium) in the diet raises blood pressure and the risk of heart disease and stroke.

Strategy

• Split the written test by topic. One partner takes chemistry (carbohydrates, lipids and esterification, proteins, cooking changes, emulsions, freezing point depression). The other takes nutrition and regulation (vitamins and RDAs, allergies and intolerance, milk types, labeling, pasteurization, probiotics, health benefits). Swap at the end to check each other.
• Plan the lab in advance. Decide who measures, who mixes and who shakes for each task. Ice cream and butter both need hard, continuous shaking; trade off every minute so the shaking does not slow down.
• Do the lab first if you can. Time on lab tasks is fixed by the procedure (draining takes 15 minutes, ice cream 5–10). Start the curd draining, then work on the test while it drains.
• Use both note sheets well. One sheet can hold the lab data from your notebook (best salt amount, acid drops for each milk, cream temperature), the scoring formulas and the freezing point depression equation. The other can hold the RDA table, the casein/whey table, milk standards and the allergy versus intolerance comparison.
• Pack a checklist. Goggles, lab coat or apron, the two bags, the lidded container, cheesecloth, whatever you want to use from the Chemistry Events Lab Equipment List, a thermometer if you prefer your own, and two calculators.
• Clean up completely. The cleanup penalty can be up to 10% of the total possible score, which is close to the full value of a curd or butter task.

Common pitfalls

How tournaments test this

Tests written to the 2027 dairy rules are not yet public, so the best guide to what will be asked is the archive of earlier Food Science tests, which mixed dairy topics with general food chemistry, nutrition and food safety. Our question bank holds about 6,000 Food Science questions. About 1,000 of them are AI-written dairy practice questions (labeled "Scio.ly Original"), not tournament questions. The other 5,000 or so come from real tournament tests: about 1,400 are exported from or match the 1,667 questions in our public test archive, and most of the rest come from older invitational tests the archive does not have. About 2,900 of the tournament questions fall within the 2027 dairy topics and the food chemistry and safety background they rest on. The rest are about yeast and alcoholic fermentation, cellular respiration (glycolysis, the Krebs cycle), bread, pickles, kimchi, kombucha, candy stages and older density lab tasks, which are outside the 2027 rules.

• Format. Of the in-scope questions, about 43% are free response, 41% multiple choice, 9% true/false and 7% select-all. Free-response "explain why" items are where most points are lost.
• What gets asked most. By share of in-scope questions: carbohydrates (sugar classes, lactose, starch versus cellulose, reducing sugars, sweeteners) about 22%; food preservation, water activity and food safety about 17%; cultured dairy, lactic acid bacteria and probiotics about 9%; lipids and fats about 8%; vitamins and minerals about 8%; chemistry basics, pH and enzymes about 7%; proteins and casein about 5%; Calories and nutrition math about 4%; cheese and curds about 4%; labeling, allergens and food tests about 3% each; then cooking reactions, pasteurization, butter, emulsions and ice cream.
• Favorite question types. Classify a sugar (mono-, di-, oligo- or polysaccharide); name the units of lactose and sucrose; predict Benedict's, Biuret and iodine results; define water activity and give microbe a_w limits; define acid and acidified foods (pH 4.6, a_w 0.85); name the yogurt bacteria; list the major allergens and the 2016 label changes; match vitamins to deficiency diseases; distinguish saturated, unsaturated and trans fats; explain hydrogenation; contrast Maillard browning and caramelization.
• Calculations. Calories from grams of macronutrients (and grams from Calories), %DV, label serving arithmetic, pH from [H⁺], freezing point depression, water activity from vapor pressures, moisture content, bacterial doubling, overrun, percent curd and the lab scores. Every one has a worked example in this guide.
• Lab-linked questions. Expect questions that tie to the lab tasks: why salt is added to ice, why acid makes curds, why cream turns into butter, and how to design a controlled experiment.

Test yourself

These questions come from public tournament tests in our archive and were chosen because they fall inside the 2027 dairy topics. Click an answer to check it; each links to its full test. The explanations are ours.

Test yourselfSugars, fats and milk proteins

0 / 0 correct

1. 1

   Fructose is an example of aldose sugar.

   Question 1 options

   ATrueBFalse

   From: BirdSO MiniSO Invitational, 2020-21, Division B

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Test yourselfIce cream, cultures and labels

0 / 0 correct

1. 1

   What is the main purpose of adding polysaccharides when making ice cream?

   Question 1 options

   ATo complete the syneresis processBTo increase the viscosityCTo destabilize the fat globulesDTo soften the final product

   From: High Desert Mini Invitational 1, 2021-22, Division B

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Test yourselfMolecules, tests and nutrients

0 / 0 correct

1. 1

   Which of the following is NOT a monosaccharide?

   Question 1 options

   ASucroseBGlucoseCFructoseDGalactose

   From: South Forsyth Science Olympiad Invitational 2020, Division B

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Test yourselfMilk processing, safety and allergens

0 / 0 correct

1. 1

   What process is used to decrease the size of the fat globules in milk so that there is no separation of the fat from the milk serum?

   Question 1 options

   AHomogenizationBPasteurizationCChemical synthesisDChemical hydrolysisESeparation

   From: Holt Invitational 2015-16, Division B

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Breadth reference

Tests in this event reward wide, shallow recall: a term, a name, a date, a single number. These filterable tables collect the one-line facts that the chapters above use or mention, so you can look one up fast during study. Type in a table's filter box to find a term. Each row is one fact; follow the chapter for the explanation.

ReferenceGlossary of food science terms· 113 rows

113 rows

Term	Meaning
Acid food	Food with a natural pH of 4.6 or below
Acidified food	Low-acid food made pH ≤ 4.6 by adding acid, with a_w > 0.85 (21 CFR 114)
Activation energy	Energy barrier a reaction must cross; enzymes and catalysts lower it
Additive	Substance added to food for a technical effect (preservation, color, texture, flavor, nutrition)
Aerobe	Organism that uses oxygen
Aldose / ketose	Sugar with an aldehyde (−CHO) group / with a ketone (C=O) group
Amorphous	Solid without an ordered crystal structure (hard candy glass, spray-dried lactose)
Amphipathic	Having a water-loving and a fat-loving part (phospholipids, soaps, emulsifiers)
Anabolic / catabolic	Building larger molecules (uses energy) / breaking them down (releases energy); together, metabolism
Anaerobe	Organism that grows without oxygen
Anaphylaxis	Severe, rapid, whole-body allergic reaction; airway swelling and blood-pressure drop; treat with epinephrine
Anomers	The α and β ring forms of a sugar, differing at carbon 1
Antioxidant	Compound that slows oxidation (vitamin E, vitamin C, BHA, BHT)
Aseptic processing	Sterilizing product and package separately and filling in a sterile environment (UHT milk cartons)
Bacteriocin	Antimicrobial peptide made by bacteria (nisin from Lactococcus lactis)
Binary fission	Bacterial reproduction by splitting in two
Bioavailability	Share of a nutrient the body can absorb and use
Blanching	Brief scalding in boiling water or steam to inactivate enzymes before freezing or drying
Bound water	Water held by solutes and food molecules, unavailable to microbes
Brine	Salt solution (for ice cream freezing, cheese salting, pickling)
Buttermilk	Liquid left after churning butter; or cultured low-fat milk
Casein	About 80% of milk protein; forms micelles; precipitates at pH 4.6
Catalyst	Substance that speeds a reaction without being consumed
Chelator (sequestrant)	Compound that binds metal ions (EDTA, citric acid)
Coagulation	Clumping of denatured or destabilized proteins into a solid network
Coalescence	Droplets or fat globules merging into larger ones
Colligative property	Property depending only on the number of dissolved particles (freezing point depression, boiling point elevation, osmotic pressure)
Colloid	Particles about 1 nm–1 µm dispersed without settling (sol, gel, emulsion, foam)
Commercial sterility	No microbes able to grow in the food under normal non-refrigerated storage, and no viable pathogens
Complete protein	Supplies all nine essential amino acids
Condensation (dehydration synthesis)	Joining two molecules with loss of water
Cross-contact	Transfer of an allergen from one food to another
Cross-contamination	Transfer of microbes from one food or surface to another
Curd	Coagulated casein (and trapped fat) separated from whey
Curing	Preserving with salt (often nitrite) and drying; also adds flavor, firm texture and, with nitrite, a pink color
D-value	Minutes at a given temperature to kill 90% of a microbial population
Danger zone	40–140 °F (4–60 °C), where bacteria multiply fastest
Denaturation	Unfolding of a protein's 3D shape without breaking peptide bonds
Dextrose	Another name for glucose
Diacetyl	Buttery-flavored compound made by some lactic acid bacteria
Emulsifier	Molecule with polar and nonpolar parts that stabilizes droplets of one liquid in another
Emulsion	Dispersion of droplets of one liquid in another it does not mix with
Endospore	Heat-resistant dormant form of Clostridium and Bacillus
Enrichment	Restoring nutrients lost in processing (thiamin, riboflavin, niacin, iron and added folic acid in white flour)
Enzyme	Biological catalyst, usually a protein
Essential nutrient	One the body cannot make in sufficient amounts
Eutectic point	Lowest freezing temperature of a salt–water mixture (about −21 °C for NaCl)
F-value	Process lethality, in minutes at 121.1 °C (250 °F)
Facultative anaerobe	Grows with or without oxygen
Fat destabilization	Partial coalescence of fat globules into a network (ice cream, whipped cream)
Fermentation	Anaerobic breakdown of sugars by microbes into acids, gases or alcohol
Foam	Gas bubbles dispersed in a liquid or solid
Fortification	Adding nutrients not originally present or in larger amounts (vitamin D in milk)
Free water	Water available to microbes and reactions; measured by water activity
Functional food	Food with a health benefit beyond basic nutrition (probiotic yogurt)
Gel	Liquid held in a solid network (yogurt, cheese curd)
Gelatinization	Starch granules swelling with water when heated, thickening a liquid
Generation time	Time for a bacterial population to double
GRAS	"Generally recognized as safe", an FDA status for food substances
HACCP	Hazard Analysis and Critical Control Points, a preventive food-safety system
Halophile	Salt-loving microbe
Homogenization	Forcing milk through a small gap under pressure to shrink fat globules so cream does not rise
Hydrogenation	Adding hydrogen to C=C double bonds (metal catalyst)
Hydrolysis	Splitting a bond by adding water
Hygroscopic	Readily absorbs moisture from air (fructose, honey, milk powder)
Immiscible	Liquids that do not mix (oil and water)
Inoculation	Adding a starter culture or microbe to a food or medium
Interfering agent	Ingredient that blocks sugar crystallization (corn syrup, fat, acid)
Invert sugar	Equal mix of glucose and fructose from hydrolyzed sucrose
Isoelectric point	pH at which a protein has no net charge and is least soluble (casein about 4.6)
Isomers	Same molecular formula, different structure
Lactase persistence	Keeping lactase activity into adulthood
Low-acid food	pH > 4.6 and a_w > 0.85 (21 CFR 113)
Lyophilization	Freeze-drying: freezing, then removing ice by sublimation under vacuum
Macronutrient / micronutrient	Needed in grams (carbohydrate, protein, fat, water) / in milligrams or micrograms (vitamins, minerals)
Mesophile	Microbe growing best at moderate temperatures (about 20–45 °C)
Micelle (casein)	Spherical cluster of casein molecules held by calcium phosphate, coated with κ-casein
Moisture content	Total water in a food, as a percentage of mass
Mordant	Fixing agent in staining (iodine in the Gram stain)
Nucleation	Formation of the first stable crystal seed
Nutrient density	Nutrients per Calorie
Organoleptic	Sensed by the senses: flavor, aroma, texture, color
Osmophile	Sugar-tolerant microbe (osmophilic yeasts)
Osmosis	Water movement across a semipermeable membrane toward higher solute concentration
Overrun	Percentage increase in volume of ice cream from whipped-in air
Pasteurization	Mild heat treatment that kills pathogens but not all microbes or spores
Pathogen	Disease-causing microorganism
Peptide bond	Amide bond linking amino acids
pH	−log[H⁺]; 7 neutral, below 7 acidic
Phase inversion	An emulsion switching type (cream O/W → butter W/O)
Prebiotic	Nondigestible substance that feeds beneficial gut microbes
Probiotic	Live microorganisms that give a health benefit in adequate amounts
Protease	Enzyme that breaks down proteins
Psychrotroph	Microbe that grows at refrigerator temperatures (Listeria, Pseudomonas)
Rancidity	Off-flavors from fat hydrolysis or oxidation
Reducing sugar	Sugar with a free aldehyde or ketone group; Benedict's positive
Rennet	Enzyme preparation (chymosin) that coagulates milk for cheese
Retrogradation	Starch molecules reassociating as a gel cools or ages (staling, weeping)
Saponification	Hydrolysis of fat with a strong base, giving soap and glycerol
Starter culture	Known microorganisms added to start a fermentation
Sterilization	Destroying all microorganisms, including spores
Supercooling	Liquid cooled below its freezing point without freezing
Supersaturated	Holding more dissolved solute than normally possible at that temperature
Syneresis	Liquid weeping out of a gel (whey on yogurt, water from a starch gel)
TCS food	Time/temperature control for safety food (formerly potentially hazardous food)
Thermophile	Heat-loving microbe (about 45–70 °C or more)
UHT	Ultra-high-temperature processing (about 135–150 °C for seconds)
Water activity (a_w)	Vapor pressure of water in a food ÷ that of pure water
Whey	Liquid left after curd forms; carries whey proteins, lactose, minerals
Xerophile	Microbe tolerating dry conditions (xerophilic molds)
z-value	Temperature rise that cuts the D-value tenfold
Zwitterion	Molecule with both a positive and a negative charge (amino acids in water)
Zymology	The study of fermentation

ReferenceNamed microorganisms in food· 25 rows

25 rows

Organism	Type	Role
Streptococcus thermophilus	Thermophilic LAB, coccus	Yogurt, mozzarella, Swiss cheese starter
Lactobacillus delbrueckii subsp. bulgaricus	Thermophilic LAB, rod	Yogurt (acetaldehyde flavor)
Lactococcus lactis (formerly Streptococcus lactis)	Mesophilic LAB, coccus	Cheddar, cultured buttermilk, sour cream; makes nisin
Lactobacillus acidophilus	LAB, rod	Probiotic; acidophilus milk
Lacticaseibacillus rhamnosus GG	LAB	Probiotic
Lactobacillus helveticus	Thermophilic LAB	Swiss and Italian cheeses
Leuconostoc species	Heterofermentative LAB	Diacetyl aroma in buttermilk; kefir
Bifidobacterium species	Gram-positive gut bacteria	Probiotics; favored by milk oligosaccharides in infants
Propionibacterium freudenreichii	Propionic acid bacterium	Swiss cheese eyes and flavor; makes vitamin B12
Penicillium roqueforti	Mold	Blue cheeses (Roquefort, Gorgonzola, Stilton)
Penicillium camemberti	Mold	Brie, Camembert rinds
Brevibacterium linens	Bacterium	Smear-ripened cheeses (Limburger)
Kefir grains	Bacteria + yeasts in kefiran	Kefir
Saccharomyces cerevisiae	Yeast	Bread and beer (alcoholic fermentation)
Saccharomyces boulardii	Yeast	Probiotic
Acetobacter	Gram-negative acetic acid bacterium	Vinegar (oxidizes ethanol to acetic acid)
Pseudomonas	Gram-negative psychrotroph	Spoilage of refrigerated milk
Listeria monocytogenes	Gram-positive psychrotroph pathogen	Soft cheese, deli meat, raw milk
Clostridium botulinum	Gram-positive spore-forming anaerobe	Botulism; canned low-acid foods
Staphylococcus aureus	Gram-positive coccus	Heat-stable toxin; handled foods
Salmonella	Gram-negative rod	Poultry, eggs, raw milk
E. coli O157:H7	Gram-negative rod	Beef, raw milk, produce; kidney damage
Campylobacter jejuni	Gram-negative microaerophile	Raw milk, poultry
Bacillus cereus	Gram-positive spore-former	Cooked rice
Norovirus	Virus	Most foodborne illness; food handlers

ReferenceNamed compounds and ingredients· 32 rows

32 rows

Compound	What it is	Where it matters
Lactic acid (C₃H₆O₃)	Product of lactic acid bacteria	Sours yogurt, cheese, buttermilk; lowers pH
Acetic acid (CH₃COOH)	Acid of vinegar (about 4–8%)	Acid curdling in the lab; pickling
Citric acid	Acid of lemon juice; chelator	Curdling; prevents oxidation
Propionic acid	Three-carbon fatty acid	Swiss cheese flavor; mold inhibitor (calcium propionate)
Butyric acid	Four-carbon fatty acid	Characteristic of milk fat; rancid-butter smell
Acetaldehyde	Flavor compound	Main yogurt flavor
Diacetyl	Flavor compound	Buttery aroma of cultured butter and buttermilk
Lactose (C₁₂H₂₂O₁₁)	Galactose + glucose	Milk sugar
Galacto-oligosaccharides	Short galactose chains (usually ending in glucose), made from lactose	Prebiotic
Lecithin	Phospholipid	Emulsifier in egg yolk and soy
Mono- and diglycerides	Partial glycerides	Emulsifiers in ice cream
Carrageenan, alginate, agar	Seaweed polysaccharides	Stabilizers and thickeners in dairy desserts
Guar gum, locust bean gum, xanthan gum	Plant and microbial polysaccharides	Stabilizers
Gum arabic	Acacia tree gum	Emulsifier and stabilizer
Annatto	Seed extract of Bixa orellana	Orange color of cheddar
Sodium caseinate	Milk protein salt	"Non-dairy" creamers; must be declared as milk
Sodium citrate, sodium phosphates	Emulsifying salts	Processed cheese
Calcium chloride	Salt	Added to pasteurized milk to firm rennet curd; i ≈ 3 for freezing point depression
Chymosin	Protease	Rennet
β-lactoglobulin, α-lactalbumin	Whey proteins	Denature on heating; milk allergens
Potassium sorbate	Preservative	Mold and yeast inhibitor on cheese
Sodium benzoate	Preservative	Acidic foods
Nisin	Bacteriocin	Processed cheese
BHA, BHT, TBHQ	Synthetic antioxidants	Prevent rancidity
Tocopherols	Vitamin E	Natural antioxidants in oils
Ascorbic acid	Vitamin C	Antioxidant; stops browning; prevents color and flavor loss
Sodium bicarbonate	Baking soda	Base; a hidden sodium source
Monosodium glutamate (MSG)	Sodium salt of glutamic acid	Umami flavor enhancer; GRAS
Acrylamide	Maillard by-product	Fried starchy foods; probable carcinogen
Melanoidins	Brown Maillard polymers	Browned foods
Copper(I) oxide (Cu₂O)	Red precipitate	Positive Benedict's test
Copper(II) sulfate	Blue reagent	Benedict's, Fehling's and Biuret tests

ReferencePeople and dates· 21 rows

21 rows

Person or event	Year	Contribution
Antonie van Leeuwenhoek	1670s	First to observe bacteria with a microscope
Nicolas Appert	about 1809	Invented canning (heat-sealed glass jars)
Anselme Payen and Jean-François Persoz	1833	Isolated diastase (amylase), the first enzyme
Louis Pasteur	1857–1860s	Showed fermentation is done by living microbes (lactic fermentation of milk); developed pasteurization (1864, patented 1865)
John Landis Mason	1858	Patented the screw-top home-canning jar
Wilhelm Kühne	1877	Coined the word "enzyme"
Hans Christian Gram	1884	Gram stain
Eduard Buchner	1897 (Nobel 1907)	Cell-free fermentation by yeast extract, proving enzymes do it
Louis-Camille Maillard	1912	Described the Maillard reaction
Clarence Birdseye	1920s	Quick (flash) freezing
Otto Wüst	1930	French patent for preserving food by irradiation (earliest US/UK patents on irradiating food: 1905)
Arthur Harden and Hans von Euler-Chelpin	Nobel 1929	Sugar fermentation and its enzymes
Federal Food, Drug, and Cosmetic Act	1938	Basic US food law
HACCP	1960s	Developed for NASA space food
Nutrition Labeling and Education Act	1990 (labels 1994)	Mandatory Nutrition Facts
FALCPA	2004 (effective 2006)	Eight major allergens labeled
Trans fat labeling	2006	Required on Nutrition Facts
Food Safety Modernization Act	2011	Preventive controls for food facilities
Nutrition Facts redesign	2016 (compliance 2020–2021)	Added sugars, vitamin D, potassium
Partially hydrogenated oils removed	2018	No longer GRAS
FASTER Act	2021 (effective 2023)	Sesame becomes the ninth major allergen

ReferenceConstants and key numbers· 17 rows

17 rows

Quantity	Value
K_f of water	1.86 °C·kg/mol
K_b of water	0.512 °C·kg/mol
Heat of fusion of ice	334 J/g
Specific heat of water	4.18 J/g·°C (1 cal/g·°C)
1 food Calorie	1 kcal = 4.184 kJ
Molar mass: glucose / sucrose and lactose / NaCl	180.16 / 342.30 / 58.44 g/mol
NaCl eutectic	about −21 °C at about 23% salt
Milk density / freezing point / pH	1.03 g/mL / −0.53 °C / 6.6–6.7
Casein isoelectric point	pH 4.6
Botulism pH limit	pH 4.6
Lowest a_w for any microbial growth	about 0.60
Danger zone	40–140 °F (4–60 °C)
HTST pasteurization	72 °C (161 °F), 15 s
Canning low-acid foods	121 °C (250 °F) under pressure
Atwater factors	4 / 4 / 9 / 7 kcal/g (carbohydrate / protein / fat / alcohol)
Protein RDA (adults)	0.8 g/kg body weight
Calcium RDA ages 9–18	1,300 mg

ReferenceProtein in common foods· 14 rows

Approximate grams of protein per typical serving (USDA FoodData Central values, rounded).

14 rows

Food	Serving	Protein
Chicken breast, cooked	3 oz (85 g)	about 26 g
Beef, cooked	3 oz	about 22–25 g
Fish (salmon), cooked	3 oz	about 19–22 g
Greek yogurt, plain	1 cup	about 20–23 g
Cottage cheese	½ cup	about 12–14 g
Lentils, cooked	½ cup	about 9 g
Milk (any fat level)	1 cup	about 8 g
Soy milk	1 cup	about 7–8 g
Cheddar cheese	1 oz	about 7 g
Peanut butter	2 tbsp	about 7 g
Egg	1 large	about 6 g
Whole-wheat bread	1 slice	about 3–4 g
Rice, cooked	½ cup	about 2 g
Apple	1 medium	under 1 g

Animal foods, dairy and legumes are the richest protein sources per serving; fruits and most vegetables supply very little. Per 100 g, cooked chicken (about 30 g) and hard cheeses (about 25 g) beat milk (about 3.3 g), because milk is mostly water.

Condensed facts

References and further reading

• Goff, H. D., and Hartel, R. W., Ice Cream (7th ed., Springer, 2013): ice cream structure, partial coalescence, freezing point depression.
• University of Guelph, Dairy Science and Technology Education (H. D. Goff): milk composition, casein micelles, pasteurization, butter and ice cream: https://www.uoguelph.ca/foodscience/book-page/dairy-science-and-technology-ebook
• US FDA, Grade "A" Pasteurized Milk Ordinance (pasteurization times and temperatures): https://www.fda.gov/food/milk-guidance-documents-regulatory-information/national-conference-interstate-milk-shipments-ncims-model-documents
• Electronic Code of Federal Regulations, 21 CFR Part 131 (milk and cream standards) and Part 135 (frozen desserts): https://www.ecfr.gov/current/title-21/chapter-I/subchapter-B/part-131
• US FDA, Food Allergies (the nine major allergens and labeling): https://www.fda.gov/food/food-labeling-nutrition/food-allergies
• US FDA, The Nutrition Facts Label and Daily Values: https://www.fda.gov/food/nutrition-facts-label/daily-value-nutrition-facts-and-supplement-facts-labels
• NIH Office of Dietary Supplements, fact sheets for calcium, vitamin D, riboflavin, vitamin B12 and potassium (RDAs and deficiency): https://ods.od.nih.gov/factsheets/list-all/
• NIH MedlinePlus Genetics, Lactose intolerance: https://medlineplus.gov/genetics/condition/lactose-intolerance/
• USDA Agricultural Marketing Service, National Organic Program (pasture rule): https://www.ams.usda.gov/about-ams/programs-offices/national-organic-program
• OpenStax, Chemistry 2e, section 11.4 (colligative properties, freezing point depression): https://openstax.org/books/chemistry-2e/pages/11-4-colligative-properties
• SciOly Wiki, Food Science (another summary of the event): https://scioly.org/wiki/index.php/Food_Science
• Practice more: the Scio.ly test archive has public Food Science tests you can filter by subtopic.