Food Science Practice Tests

dairy food science for the 2027 season: milk composition, dairy carbohydrates, lipids, and proteins, food preservation, and cultured dairy products.

Event focus 2027 (dairy)

• Division B event; written test topics are limited to dairy food science
• Lab tasks: ice cream formation (freezing point depression), curd mass (acid coagulation of casein), butter from cream (emulsion inversion)
• Everything tested centers on milk and products made from it

Milk composition (key numbers)

• Whole cow's milk: ~87-88% water, ~3.25% fat, ~4.8% lactose, ~3.3% total protein, ~0.7% minerals
• Four major components: water, fat, protein, lactose (minerals are the smaller fifth)
• Protein split: ~80% casein (2.6%), ~20% whey (0.6%)
• pH 6.6-6.7 (slightly acidic), density ~1.03 g/mL, freezing point ~-0.53 °C
• Milk is three mixtures at once: true solution (lactose, salts, water-soluble vitamins), colloidal dispersion (casein micelles ~0.1 µm, scatter light → white), emulsion (fat globules 1-10 µm)
• One cup (240 mL) of milk at any fat level: ~8 g protein, ~12 g lactose, ~300 mg calcium
• Sheep milk: highest fat/protein (best cheese yield per liter); human milk: more lactose, less protein than cow's

Carbohydrates in dairy

• Lactose = glucose + galactose joined by a glycosidic bond (disaccharide)
• Lactose is synthesized only in the mammary gland; least sweet of common sugars
• Lactase enzyme hydrolyzes lactose (hydrolysis = add water to split; opposite of dehydration synthesis)
• Lactose crystallization causes "sandy" defect in ice cream

Lipids

• Triglyceride = glycerol + 3 fatty acids joined by ester bonds (esterification = dehydration synthesis)
• Saturated fats: no C=C double bonds, straight chains, solid at room temperature (palmitic, stearic, myristic)
• Unsaturated fats: one or more C=C, kinked chains, liquid at room temperature (oleic = most common unsaturated in milk fat)
• Most abundant fatty acid in milk fat: PALMITIC (~30%); characteristic/unique fatty acid: BUTYRIC (~3-4%, gives rancid butter its smell)
• Essential fatty acids (body cannot make): linoleic (omega-6) and alpha-linolenic (omega-3)
• Hydrogenation adds H to double bonds → semisolid fats, creates trans fats
• Fat = 9 kcal/g (more than double carbs/protein at 4 kcal/g)
• Milk fat globule membrane: phospholipid + protein wrapper that keeps fat emulsified

Proteins - casein and whey

• Casein (~80% of milk protein): forms micelles held together by calcium phosphate, with a "hairy" κ-casein layer keeping micelles apart
• At pH ~6.7 casein micelles are negatively charged and repel; adding acid neutralizes charge
• CASEIN ISOELECTRIC POINT = pH 4.6: no net charge → micelles clump into a gel → CURDS; leftover liquid = WHEY
• Whey proteins (lactalbumin, lactoglobulin) stay soluble at pH 4.6 but denature with heat
• Rennet enzyme (chymosin) cuts off κ-casein → firmer, springier curds (used in cheesemaking)
• Nitrogen is the element in proteins but not carbs/fats; peptide bonds join amino acids (4 kcal/g)
• Kjeldahl method measures protein via nitrogen (N × 6.38 for dairy)

Freezing point depression (ice cream)

• Colligative property: depends on NUMBER of dissolved particles, not their identity
• ΔTf = i × Kf × m, where Kf(water) = 1.86 °C·kg/mol, m = molality, i = van 't Hoff factor
• i values: sucrose = 1, NaCl ≈ 2, CaCl₂ ≈ 3
• Salt + ice forms brine below 0 °C (melting ice absorbs heat of fusion ~334 J/g); NaCl brine eutectic ≈ -21 °C
• Dissolved sugar/lactose lower the mix's freezing point (~-1 °C), so plain 0 °C ice cannot freeze it
• Fast freezing + shaking → many small ice crystals (smooth); slow freezing → few large crystals (icy)
• Ice cream = frozen foam: ice crystals + air cells + partially coalesced fat network + unfrozen syrup
• Overrun = (V_icecream - V_mix)/V_mix × 100%; standard ice cream ~100% overrun, premium 25-50%

Emulsification (butter and cream)

• Milk and cream = oil-in-water (O/W) emulsions; butter = water-in-oil (W/O) - churning INVERTS the emulsion
• Churning/shaking: air whipped in → fat globules collide at bubble surfaces → membranes strip → partial coalescence → butter grains + buttermilk
• Cream whips best at ~13 °C (some fat solid, some liquid); stages: liquid → soft foam → whipped cream → over-whipped → butter + buttermilk
• US cream standards: half-and-half 10.5-18% fat, light cream 18-30%, light whipping 30-36%, heavy cream ≥36%
• Butter: ≥80% milkfat, ~16-18% water; salting (1-2%) lowers water activity in droplets
• Emulsifiers (lecithin) stabilize emulsions; phospholipids have polar + nonpolar ends
• Ghee/clarified butter: water and milk solids removed → higher smoke point, longer keeping

Pasteurization and preservation

• Pasteurization kills pathogens: HTST = 72 °C for 15 s; batch/LTLT = 63 °C for 30 min; UHT = 135 °C for 2-5 s (sterile, shelf-stable)
• Alkaline phosphatase test verifies pasteurization (enzyme destroyed by heat)
• Homogenization: pressure breaks fat globules into smaller ones (done warm so fat is liquid) → no cream layer
• Processing order: receiving/cooling → clarification → separation → standardization (Pearson square) → homogenization → pasteurization → cooling → packaging
• Vitamins A and D added before pasteurization (D fortification began 1930s, ended rickets)
• Preservation methods: refrigeration, drying (powdered milk), condensing with sugar (Borden, 1856), fermentation, salting, freezing
• FAT TOM (bacterial growth): Food, Acidity, Temperature, Time, Oxygen, Moisture; danger zone 4-60 °C (40-140 °F)
• Water activity (aw): lower aw = less microbial growth

Cultured dairy and probiotics

• Yogurt cultures: Streptococcus thermophilus + Lactobacillus delbrueckii subsp. bulgaricus (grow symbiotically)
• Lactic acid bacteria ferment lactose → lactic acid → pH drops toward 4.6 → casein gels (why yogurt is thick and tangy)
• Kefir: bacteria + yeasts; cultured buttermilk/sour cream: Lactococcus species
• Fermented dairy: yogurt, kefir, cheese, cultured buttermilk, sour cream, cultured butter (diacetyl = buttery aroma)
• NOT fermented: ice cream, sweet-cream butter, milkshakes
• Probiotics = live beneficial microorganisms; prebiotics = fibers that feed them (e.g., inulin)
• Typical pH values: yogurt 4.0-4.6, buttermilk/sour cream ~4.4-4.8, cheddar ~5.0-5.5, fresh milk 6.6-6.7 MILK TYPES, INTOLERANCE, AND ALLERGIES:
• Fat levels: whole 3.25%, reduced-fat 2%, low-fat 1%, fat-free/skim ~0%
• Lactose-free milk: lactase enzyme added (hydrolyzes lactose into glucose + galactose; tastes sweeter)
• LACTOSE INTOLERANCE = enzyme deficiency (no/low lactase): gas, cramps; NOT an allergy; most adults worldwide have some degree
• MILK ALLERGY = immune (IgE) reaction to milk proteins (casein); goat milk still has lactose and casein → not safe
• Non-dairy milks (soy, almond, oat): fortified but not nutritionally equivalent to cow's milk
• Milk is a major allergen (FDA requires declaration); lactose-free milk still contains milk protein

Labels and regulation

• FDA regulates US food labels (Nutrition Facts panel, ingredient list, allergen declaration)
• Fortification = adding nutrients; enrichment = adding back nutrients lost in processing
• Daily Values (DV) based on a 2,000 kcal diet; milk's nutrients of concern: calcium, potassium, vitamin D, riboflavin (B2)

Milk quality tests (commonly asked)

• Babcock (1890) and Gerber tests: measure milk fat % (sulfuric acid + centrifuge)
• Lactometer: specific gravity (detects added water); cryoscope: freezing point (added water raises toward 0 °C)
• Somatic cell count: high = mastitis (udder infection); standard plate count: bacteria/sanitation
• Pearson square: standardize fat % by mixing two ingredients (parts = diagonal differences)

Lab task essentials (2027)

• Ice cream: milk/cream + sugar in small bag, ice + salt in large bag; scored by height of self-standing scoop
• Curds: acid (vinegar/lemon juice) drops → stir → stand → drain 15 min through cheesecloth → weigh; % curd = m_curds/m_milk × 100
• Butter: shake ~150 mL cream at ~13 °C until grains separate, pour off buttermilk, weigh; score = (your mass ÷ supervisor's mass) × 10, capped at 10
• Each experiment in the notebook must change only ONE independent variable

Common mistakes

• Confusing lactose intolerance (enzyme problem) with milk allergy (immune response)
• Mixing up butyric acid (characteristic of milk fat) vs palmitic acid (most abundant)
• Thinking whey proteins curdle at pH 4.6 (only casein does; whey denatures with heat)
• Forgetting freezing point depression depends on particle count, not salt type
• Getting cream fat percentages out of order (half-and-half < light cream < light whipping < heavy)
• Assuming goat milk is safe for milk allergy (it still contains casein and lactose)