Chapter 32 Quiz
Mix of recall, application, and "explain why." Answers and explanations follow.
Multiple Choice
1. The pH at which milk's casein protein loses its net charge and aggregates into a gel is approximately: - A) 6.7 - B) 5.2 - C) 4.6 - D) 3.0
2. Which two organisms together define the legal standard for "yogurt" in many countries? - A) Lactobacillus plantarum and Saccharomyces cerevisiae - B) Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus - C) Lactococcus lactis and Penicillium camemberti - D) Brevibacterium linens and Acetobacter
3. The enzyme chymosin (the active component of rennet) cleaves which specific peptide bond? - A) Lysine-arginine bond on alpha-casein - B) Phenylalanine 105-Methionine 106 bond on kappa-casein - C) Glycine-glycine bond on beta-casein - D) Cysteine-cysteine disulfide bond on whey proteins
4. Why is whole milk usually pre-heated to 85°C / 185°F for several minutes before making yogurt? - A) To sterilize it (this is the only reason) - B) To increase the lactose content available to the bacteria - C) To denature whey proteins so they form a smoother gel network with casein - D) To raise the pH
5. The pH at which mozzarella curd becomes elastic and stretchy when heated is approximately: - A) 6.7 - B) 5.2 - C) 4.6 - D) 3.5
6. "Lactic acid bacteria" (LAB) is best described as: - A) A single species, Lactobacillus acidophilus - B) A taxonomic family within bacteria - C) A functional grouping of bacteria that ferment carbohydrates and produce lactic acid as the major waste product - D) A grouping of probiotic bacteria found only in the human intestine
7. Aged cheeses such as cheddar and Parmigiano-Reggiano typically contain very little lactose because: - A) Lactose is removed during pasteurization - B) Most lactose drains away in the whey, and any remaining is metabolized by lactic-acid bacteria during early aging - C) The molds in aged cheese specifically destroy lactose - D) The salt added to cheese inhibits lactose
8. Penicillium roqueforti is the mold responsible for which type of cheese? - A) Brie and Camembert - B) Cheddar and Gouda - C) Roquefort, Stilton, and Gorgonzola (blue cheeses) - D) Mozzarella and provolone
9. Which bacterium is responsible for the strong aroma of washed-rind cheeses such as Limburger and Époisses? - A) Lactobacillus bulgaricus - B) Penicillium camemberti - C) Brevibacterium linens - D) Saccharomyces cerevisiae
10. The white crystals that you can see and crunch in a well-aged Parmigiano-Reggiano are mostly: - A) Salt - B) Calcium carbonate - C) Tyrosine, a free amino acid that has reached saturation and crystallized - D) Frozen ice crystals
11. Which category of cheese is typically made using direct acid (lemon juice, vinegar, or citric acid) rather than rennet? - A) Cheddar - B) Parmigiano-Reggiano - C) Paneer, ricotta, queso fresco - D) Roquefort
12. A SCOBY in fermentation jargon stands for: - A) Sterilized Culture Of Bacterial Yeast - B) Sour Curd Of Buttermilk Yogurt - C) Symbiotic Culture Of Bacteria and Yeast - D) Saccharomyces Caseinase Of Bacterial Yield
13. Kombucha's signature acidity comes primarily from: - A) Lactic acid produced by Lactobacillus - B) Acetic acid produced by Acetobacter (and related genera) oxidizing ethanol - C) Citric acid added during brewing - D) Phosphoric acid produced by yeast
14. Cultured buttermilk's distinctive buttery flavor is largely due to diacetyl, which is produced primarily by: - A) Streptococcus thermophilus - B) Penicillium roqueforti - C) Leuconostoc mesenteroides and certain Lactococcus strains - D) Brevibacterium linens
15. A mozzarella that comes out rubbery rather than stretchy is most likely: - A) Made with the wrong milk fat content - B) Stretched at too low a temperature (under 60°C) - C) At the wrong pH — specifically still above 5.4, not yet acidified enough - D) Over-salted
16. The role of salt in early cheese-making (immediately after curd formation) includes all of the following EXCEPT: - A) Drawing out additional moisture by osmosis - B) Slowing the lactic-acid bacteria so the cheese doesn't over-acidify - C) Inhibiting unwanted spoilage organisms - D) Catalyzing the chymosin reaction
Short Answer
17. Explain in 3–5 sentences why milk has been "designed" by evolution as a beverage (rather than a gel), and what specifically the casein micelle's hairy layer contributes to that design. Then explain how lactic-acid bacteria undermine that design.
18. Many cheese-making traditions arose independently in regions where most adults could not digest fresh milk efficiently. How does fermentation make milk's nutrition more accessible to lactose-malabsorbing populations? Give two distinct mechanisms.
19. Compare an acid-coagulated cheese (e.g., paneer) and a rennet-coagulated cheese (e.g., young cheddar) at the molecular level. What is happening to the casein micelle in each case? Why do the textures differ?
20. Probiotic claims about yogurt often go beyond what evidence supports. List one specific health claim that is supported by evidence and one common claim that is not well-supported.
Answer Key with Explanations
1. C — pH 4.6. This is the isoelectric point of casein, where the protein has zero net charge. With no electrostatic repulsion between micelles, they aggregate into a gel.
2. B — Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. The yogurt symbiotic duo. Each provides nutrients the other needs; together they drive faster acidification and produce the characteristic acetaldehyde aroma.
3. B — Phenylalanine 105-Methionine 106 on kappa-casein. Chymosin's specificity is extraordinary: it cuts essentially this one bond out of millions of peptide bonds in milk. The cut detaches the kappa-casein hairy layer; the micelles aggregate.
4. C — to denature whey proteins. Heat-denatured beta-lactoglobulin coats the casein micelles and forms disulfide cross-links that produce a smoother, firmer yogurt gel that releases less whey on standing.
5. B — pH 5.2. At pH 5.2 with heat and mechanical work, the calcium-depleted casein chains can slide and align into fibers. This is the structural basis of mozzarella, provolone, queso Oaxaca, and other stretched-curd cheeses globally.
6. C — a functional grouping that ferments carbohydrates and produces lactic acid as the major waste product. Major culinary genera include Lactobacillus, Lactococcus, Streptococcus, and Leuconostoc. They are not a single taxonomic family; they are grouped by what they do.
7. B — Most lactose drains away in the whey, and any remaining is metabolized by LAB during early aging. A standard 30 g serving of aged cheddar contains less than 0.1 g of lactose; aged parmesan, less than 0.05 g.
8. C — Roquefort, Stilton, Gorgonzola, and other blues. P. roqueforti tolerates low oxygen and grows in the air veins through the cheese, producing the characteristic short-chain fatty acids and methyl ketones (2-pentanone, 2-heptanone) that give blue cheeses their pungency.
9. C — Brevibacterium linens. B. linens produces volatile sulfur compounds (methanethiol, methional, dimethyl sulfide) that give washed-rind cheeses their characteristic pungency. It is a salt-tolerant, alkali-tolerant bacterium and a close relative of skin bacteria.
10. C — Tyrosine, a free amino acid that has reached saturation and crystallized. As long aging proteolyzes casein, free tyrosine accumulates. At sufficient concentrations it crystallizes out, often associated with calcium lactate. Some Parmesan crystals are calcium lactate; the famous gritty crystals you bite are mostly tyrosine.
11. C — Paneer, ricotta, queso fresco. These fresh cheeses are made by heating milk and adding direct acid (lemon juice, vinegar, citric acid). The acid drops the pH past 4.6; the casein crashes; you press and use within days.
12. C — Symbiotic Culture Of Bacteria and Yeast. Used to describe kefir grains, kombucha mothers, and other consortia where multiple microbes work together.
13. B — Acetic acid produced by Acetobacter (and related genera) oxidizing ethanol. Yeasts in the SCOBY first produce ethanol from sugar; acetic acid bacteria then oxidize the ethanol to acetic acid.
14. C — Leuconostoc mesenteroides and certain Lactococcus strains. Diacetyl is the buttery-aroma compound responsible for much of the "cultured" flavor of buttermilk and sour cream.
15. C — At the wrong pH; still above 5.4, not yet acidified enough. Mozzarella's stretch window is narrow (roughly pH 4.9–5.4). Above 5.4 the curd is rubbery and won't stretch; below 4.9 it crumbles.
16. D — Catalyzing the chymosin reaction. Salt is added after curd formation, when chymosin has already done its work. Salt does not catalyze chymosin. The other three roles of salt (osmosis, slowing LAB, inhibiting spoilage) are all real and important.
17. Milk's casein proteins (about 80% of milk protein) are mostly hydrophobic and would, on their own, clump and fall out of solution. Kappa-casein has a hydrophilic, negatively-charged C-terminal tail that projects outward from the casein micelle, forming the hairy layer that keeps the micelles dispersed by both electrostatic repulsion (negative charges repelling each other) and steric repulsion (the physical bristle of the tails). As long as the hairy layer is intact, milk is a stable beverage and the calf can drink it. Lactic-acid bacteria undermine this design by excreting lactic acid into the milk, which protonates (neutralizes) the negative charges on the kappa-casein tails. Once the charges are neutralized, the micelles can drift close enough to aggregate, and the milk gels. (Bonus: rennet bypasses the charge mechanism by enzymatically cutting the kappa-casein tails off entirely.)
18. Two mechanisms: (1) Aged cheese contains very little lactose. Most lactose drains away in the whey during cheese-making, and any remaining is metabolized by lactic-acid bacteria in the first weeks of aging. By the time a cheese has aged six months or more, lactose is essentially gone. So aged cheese is a way to access milk's nutrition (calcium, protein, fat) without the lactose. (2) Yogurt's live cultures help digest residual lactose. The bacteria in live yogurt continue producing lactase enzyme during digestion in the host's small intestine, helping to break down both their own remaining lactose and some of what would otherwise be unprocessed. This is why many lactose-malabsorbing adults tolerate yogurt better than fresh milk even though yogurt's lactose content per gram is similar.
19. Acid coagulation (paneer): Lactic acid (from added lemon juice, vinegar, or culture) drops the milk's pH past casein's isoelectric point (~4.6). The kappa-casein hairy layer is charge-neutralized but not removed; the negatively charged carboxylate groups become protonated. With charge gone, the micelles aggregate. The resulting curd is somewhat granular and crumbly because the micelles are simply charge-precipitating in a relatively unorganized way, often at lower pH and higher temperature. Rennet coagulation (cheddar): Chymosin enzymatically cuts the kappa-casein hairy layer off (Phe105-Met106 cleavage). The cut is made at the milk's natural pH (about 6.5–6.7), with no acid involvement. The hydrophobic body of the casein is uniformly exposed; calcium ions bridge between adjacent micelles via the still-bound phosphoseryl residues. The result is a smoother, denser, more elastic gel because the micelles are gathering together in an organized fashion through specific calcium-mediated linkages, with the protein structure largely intact at neutral pH.
20. Supported claim: Live yogurt cultures help adults with lactose malabsorption digest lactose during a yogurt-containing meal. This is repeatedly demonstrated and well-mechanized (the bacteria's beta-galactosidase enzyme works in the small intestine alongside the host's reduced lactase activity). Not well-supported claim: That eating yogurt or commercial probiotic supplements "boosts immunity" or "prevents illness" in healthy adults. When tested in well-designed randomized trials, such claims generally do not hold up consistently. Specific probiotic strains have been shown to help in specific clinical situations (e.g., shortening antibiotic-associated diarrhea), but broad-spectrum "immunity boost" claims are not what the evidence supports.