Chapter 16 — Quiz
18 questions: 14 multiple choice, 4 short-answer. Answer key with explanations at the bottom.
Multiple Choice
1. Cow's milk is approximately what percentage water by weight?
A. 60% B. 75% C. 87% D. 95%
2. The protein structure responsible for nearly all of milk's curdling and gelling behavior is:
A. Whey proteins (alpha-lactalbumin and beta-lactoglobulin) B. Casein micelles C. Lactoferrin D. Immunoglobulins
3. What molecule, present at the surface of casein micelles, stabilizes them in the water phase of milk?
A. Beta-casein B. Alpha-s1-casein C. Kappa-casein D. Lactoferrin
4. The chymosin enzyme used in rennet-coagulated cheese-making cleaves a specific peptide bond in kappa-casein. Which bond?
A. Lysine 105 – Arginine 106 B. Phenylalanine 105 – Methionine 106 C. Glycine 100 – Valine 101 D. Serine 110 – Threonine 111
5. Why does whipped cream require cold cream and a cold bowl?
A. Cold reduces the cream's viscosity, making whipping easier B. The fat must be partially crystallized to stabilize the air bubbles; at room temperature, the fat is too liquid C. Cold prevents bacterial growth during whipping D. Cold reduces the proteins' tendency to denature
6. Butter is structurally:
A. A water-in-fat emulsion (about 80% fat, 16% water, 4% milk solids) B. A fat-in-water emulsion (about 80% water, 16% fat, 4% milk solids) C. A pure-fat solid with no water content D. A foam stabilized by milk proteins
7. The flavor of cultured butter (and the "buttery" notes in microwave popcorn) come primarily from a single molecule produced by lactic acid bacteria. What molecule?
A. Lactic acid B. Diacetyl C. Acetaldehyde D. Methanethiol
8. A reader makes paneer at home by heating whole milk to about 85°C and adding lemon juice. The casein curdles. Which of the following best describes the underlying chemistry?
A. The acid lowered the pH below the casein isoelectric point (~4.6), neutralizing the negative charges on kappa-casein and removing the electrostatic repulsion between micelles B. The acid denatured the whey proteins, which formed a gel C. The heat alone caused the curdling; the acid was only for flavor D. The acid added calcium ions, which cross-linked the proteins
9. Which statement about lactose intolerance is most accurate?
A. Lactose intolerance is a digestive deficiency that affects a small minority of humans B. Lactose intolerance is the global majority condition; lactase persistence is the relatively rare evolutionary innovation C. Lactose intolerance can be cured with probiotics D. Lactose intolerance is the same as a milk allergy
10. Yogurt is typically lower in lactose than the milk it was made from because:
A. The yogurt-making process boils off the lactose B. Heat treatment destroys the lactose C. The bacteria consume some of the lactose during fermentation, converting it to lactic acid D. Lactose is removed by straining
11. Why does ghee have a higher smoke point than regular butter?
A. The fat in ghee is chemically different from the fat in butter B. Ghee has additional stabilizing additives C. The water and milk solids have been removed; the milk solids are what burn first in regular butter D. Ghee is fermented before clarification
12. A skin (lactoderm) sometimes forms on heated milk. Which protein is most responsible for this skin?
A. Casein B. Beta-lactoglobulin (a whey protein) C. Immunoglobulin G D. Lactoferrin
13. Which of the following is a climacteric dairy phenomenon — that is, a process driven by the autocatalytic production of a key compound that triggers further production of the same compound? (Trick question — answer carefully.)
A. Yogurt setting B. Cheese aging C. Butter making D. None of the above — climacteric ripening is a fruit phenomenon, not a dairy one
14. A cook adds heavy cream (35% fat) to a tomato sauce simmered with vinegar. The sauce stays smooth. The same cook substitutes half-and-half (12% fat) and the sauce curdles. Why?
A. The half-and-half contains more lactose B. Higher fat content provides more interfacial buffering against acid-induced casein aggregation C. The half-and-half is at a lower pH D. Cream is sterilized differently than half-and-half
Short Answer
15. In one paragraph, explain why a fresh cheese (paneer, queso fresco, ricotta-style) can be made at home with milk and lemon juice in 25 minutes, but a Parmigiano-Reggiano takes a year. What is happening chemically that distinguishes the two?
16. A teacher wants to demonstrate the casein-curdling phenomenon to a high school chemistry class on a tight budget. Describe a single 30-minute lab she could run with under $5 in materials, and identify the chemistry that the lab demonstrates. (Pat Hammond's actual demo will give you the answer; cover the materials, the procedure, and the molecular explanation.)
17. Cultured butter has a more complex flavor than sweet-cream butter, even though the starting cream is the same. Identify two distinct chemical contributors to cultured butter's flavor and explain where each one comes from.
18. A friend says, "I'm lactose intolerant — I can't eat dairy." Drawing on the chapter, write a short response (3–4 sentences) that distinguishes between the dairy products this person likely cannot tolerate and those they likely can.
Answer Key
1. C — 87%. Cow's milk is approximately 87% water, 3.5% fat, 3.3% protein, 4.7% lactose, and 0.7% minerals. The bulk medium is water; everything else is suspended or dissolved in it.
2. B — Casein micelles. Caseins make up about 80% of milk's protein, organized into spherical micelles roughly 100–400 nm across. Almost all dairy transformations (cheese, yogurt, paneer) involve casein. Whey proteins (option A) are dissolved free in the water phase and have a separate set of behaviors (they form lactoderms, ricotta, and play a role in heat-induced thickening, but they don't curdle the way caseins do).
3. C — Kappa-casein. Kappa-casein has a hydrophilic, negatively charged C-terminal end that projects outward from the micelle surface, forming the "hairy layer" that keeps micelles separated by electrostatic and steric repulsion.
4. B — Phenylalanine 105 – Methionine 106. Chymosin's specificity for the Phe105–Met106 peptide bond in kappa-casein is one of the most well-characterized examples of enzymatic specificity in food chemistry. The cleavage releases the hydrophilic C-terminal fragment (the caseinomacropeptide) into the whey, leaving the micelles without their protective hairy layer.
5. B — The fat must be partially crystallized. At cold temperatures, some of the milk fat is solid and some is liquid; the partial crystallization is what allows the fat to stabilize air bubbles at their interfaces. At room temperature, the fat is too liquid to stabilize the foam.
6. A — Water-in-fat emulsion. Butter is the inverted emulsion of cream. Cream is fat (small amount) dispersed in water (continuous phase). Butter is water (small amount) dispersed in fat (continuous phase). Mechanical churning is what inverts the emulsion.
7. B — Diacetyl. 2,3-butanedione (diacetyl) is the buttery-smelling four-carbon ketone produced by lactic acid bacteria fermenting lactose. It is the molecule that gives cultured butter, fermented dairy products, and microwave-popcorn-flavoring their characteristic buttery aroma.
8. A — The acid dropped the pH below the casein isoelectric point. Heat partially destabilized the kappa-casein hairy layer; acid brought the pH below the casein isoelectric point (~4.6), neutralizing electrostatic repulsion and allowing the micelles to aggregate.
9. B — Lactose intolerance is the global majority condition. Most adult humans worldwide are lactase non-persistent. Lactase persistence — the ability to digest lactose into adulthood — is a relatively recent evolutionary innovation found mainly in populations with histories of dairying.
10. C — The bacteria consume some of the lactose during fermentation. Yogurt-making bacteria ferment lactose into lactic acid; this is how the pH drops and the casein gel forms. A cup of yogurt typically contains 30–50% less lactose than a cup of milk.
11. C — Water and milk solids removed. The milk solids in butter (the 4% non-fat-non-water component, mostly proteins) are what scorch and smoke first when butter is heated. Ghee has been clarified to remove these solids, raising the smoke point dramatically (~250°C / 482°F vs. ~150°C / 302°F for regular butter).
12. B — Beta-lactoglobulin. Whey proteins, especially beta-lactoglobulin, denature at the air-water interface of heated milk and aggregate into a thin protein film. This film traps fat globules and casein micelles within it. The chapter notes that the same physics underlies yuba (Japanese soy-milk skin) and malai (Indian milk skin in rabri).
13. D — None of the above; climacteric ripening is a fruit phenomenon. "Climacteric" is the term used in plant biology for fruits that ripen via the autocatalytic ethylene-mediated process (apples, bananas, tomatoes — see Chapter 18). It is not used for dairy; the question is a trick to check that you understand the term in its actual scientific context.
14. B — Higher fat content provides more interfacial buffering. More fat means more surface area covered by fat-stabilizing emulsifiers and more separation between casein micelles, giving the cream more stability against acid-induced aggregation. Crème fraîche (even higher fat, slightly acidic) is even more heat-and-acid stable than heavy cream.
15. Sample answer. Both paneer and Parmigiano-Reggiano start with the same chemistry: casein micelles that aggregate when their stabilizing hairy layer is disrupted. The difference is in what happens after the curd forms. Paneer is curdled with acid (which produces a softer, more crumbly gel), drained, lightly pressed, and eaten within days. Parmigiano-Reggiano is curdled with rennet (which produces a firmer, more elastic gel), aggressively pressed (driving out most water), salted, and aged for at least one year — typically 2–3 years. During that aging, microbial cultures and residual enzymes slowly transform the proteins, fats, and trace sugars into hundreds of flavor compounds (ketones, aldehydes, free fatty acids, free amino acids, sulfur compounds, and many others). The aging is essentially controlled microbiology and biochemistry over time. Paneer is curdling chemistry alone; Parmigiano is curdling chemistry plus a year-long microbial-and-enzymatic transformation.
16. Sample answer. Materials: 1 L (1 quart) whole milk ($1.50), 2 tablespoons distilled white vinegar ($0.10), pinch of salt, paper coffee filter or cheesecloth ($0.50). Procedure: Heat the milk in a saucepan to a simmer, just below boiling. Remove from heat. Add the vinegar; stir gently 2–3 times. Within seconds, white curds visibly separate from clear pale-yellow whey. Pour through a strainer lined with cheesecloth; rinse briefly; salt to taste. Total time: 20 minutes. The chemistry: heat partially destabilizes the casein's protective hairy layer (kappa-casein); vinegar's acidity drops the pH below 4.6, the casein isoelectric point, neutralizing the negative charges on kappa-casein and allowing the casein micelles to aggregate into a curd. The students see, in real time, the role of pH in protein folding and aggregation — a classroom demonstration of one of the foundational chemistries of food.
17. Sample answer. Two contributors: (1) Lactic acid — produced by the cultured lactic acid bacteria as they ferment lactose during the cream's fermentation; lactic acid registers as a slight tang on the tongue. (2) Diacetyl — also produced by the lactic acid bacteria, specifically as a byproduct of citric-acid metabolism (in the lactic-acid pathway, citrate is converted to acetate plus diacetyl plus other intermediates); diacetyl is volatile and aromatic, registering on the nose as the characteristic "buttery" smell. Other contributors include acetoin, acetaldehyde, and small amounts of other ketones and aldehydes — but lactic acid and diacetyl are the two most prominent.
18. Sample answer. Lactose-intolerant people often retain more dairy options than they realize. The dairy products most likely to cause problems are fluid milk and fresh cheeses (ricotta, fresh mozzarella, cottage cheese), where lactose content is high. Fermented dairy products — yogurt, kefir, sour cream — typically contain 30–50% less lactose than the milk they were made from, because the bacteria have eaten some of it during fermentation; many lactose-intolerant people tolerate yogurt fine. Aged cheeses (cheddar, Parmigiano, gruyère, manchego) contain very little lactose because most went out with the whey during cheese-making and the rest was consumed during aging; a serving of well-aged cheese is essentially lactose-free. Lactase pills (over-the-counter) and lactose-free milk (pre-treated with lactase enzyme) are also widely available options.