Chapter 7 Quiz — Proteins
Multiple Choice (Questions 1–14)
1. A protein is a chain of which kind of small molecule? - a) Sugars - b) Fatty acids - c) Amino acids - d) Nucleotides
2. How many different amino acids commonly appear in the proteins of living things? - a) 4 - b) 12 - c) 20 - d) 64
3. What is denaturation? - a) The breaking of peptide bonds in a protein chain - b) The unfolding of a protein from its specific 3D shape into a more open chain - c) The cooking of an egg - d) The chemical reaction between a protein and a sugar
4. Which of the following is NOT a common cause of protein denaturation in cooking? - a) Heat - b) Acid - c) Mechanical agitation (whisking) - d) Refrigeration
5. What temperature is approximately the threshold at which egg-yolk proteins begin to denature significantly? - a) 40°C / 104°F - b) 65°C / 149°F - c) 85°C / 185°F - d) 100°C / 212°F
6. What temperature is approximately the threshold at which beef tenderloin reaches medium-rare doneness? - a) 40°C / 104°F - b) 54°C / 129°F - c) 75°C / 167°F - d) 95°C / 203°F
7. Coagulation is best described as: - a) The unfolding of individual protein chains - b) The breaking of peptide bonds - c) The linking of denatured proteins into a network - d) The conversion of starch to sugar
8. Why is it impossible (in practice) to "uncook" an egg? - a) The peptide bonds have been broken by heat - b) The denatured proteins have linked into a coagulated network that cannot be undone - c) The amino acids have been destroyed - d) The water has evaporated
9. Which of the following denatures proteins primarily by changing the charge on amino-acid side chains? - a) Heat - b) Mechanical agitation - c) Acid - d) Sound waves
10. The four levels of protein structure (in order) are: - a) Primary, secondary, tertiary, quaternary - b) Atomic, molecular, cellular, organismal - c) Linear, helical, sheet, ball - d) Hot, warm, cool, cold
11. Why does brining work to keep meat juicier? - a) The salt evaporates during cooking, taking water with it - b) Salt partially denatures muscle proteins, changing their water-holding capacity - c) Salt reacts chemically with collagen - d) Salt cools the meat from the inside
12. The green-gray ring around an overcooked egg yolk is caused by: - a) Mold - b) A reaction between sulfur compounds (from cysteine) and iron in the yolk - c) Air pockets in the egg - d) Chemical preservatives
13. Which of the following is a fully covalent bond involved in stabilizing some folded proteins? - a) Hydrogen bond - b) Hydrophobic interaction - c) Disulfide bridge - d) Salt bridge
14. Why don't typical marinades penetrate deeply into meat? - a) Because they are heavier than water - b) Because the molecules are mostly too large to diffuse through dense muscle tissue - c) Because they evaporate too quickly - d) Because they contain chemicals that meat repels
Short Answer (Questions 15–18)
15. Explain in 2–3 sentences why a soft-boiled egg has a fully-set white but a still-runny yolk, even though they cooked at the same time and temperature.
16. Maya wants to make a custard sauce that doesn't curdle. Using protein chemistry, explain to her why the sauce must be stirred constantly and held below boiling temperature.
17. A cook tells you they "tenderized their steak" by marinating it in vinegar for an hour. Using what you know about denaturation and diffusion, evaluate this claim. Was the steak tenderized? If so, where?
18. Pat's classroom demo uses three slow cookers at three temperatures (60°C, 70°C, 80°C). Why is it important that the temperatures be held precisely, rather than approximating them with stovetop burners?
Answer Key
1. c — Amino acids. Proteins are chains of amino acids linked by peptide bonds.
2. c — 20. There are 20 amino acids that commonly appear in the proteins of living organisms (a few rare ones exist, but 20 is the standard set).
3. b — The unfolding of a protein from its specific 3D shape into a more open chain. Denaturation does NOT break peptide bonds; the chain itself remains intact.
4. d — Refrigeration. Cold can slow protein activity, but it does not denature proteins. Heat, acid, mechanical agitation, salt, and alcohol are all denaturants.
5. b — 65°C / 149°F. Egg yolk proteins begin denaturing meaningfully around 65°C and finish around 70°C.
6. b — 54°C / 129°F. Medium-rare beef hits the protein-denaturation threshold where myosin is denatured but myoglobin still gives a deep red color.
7. c — The linking of denatured proteins into a network. Once unfolded, the sticky parts of the protein link to neighbors, forming a 3D mesh.
8. b — The denatured proteins have linked into a coagulated network. The peptide bonds are intact, the amino acids are still there, but undoing the tangled network of weak bonds at scale is essentially impossible.
9. c — Acid. Acid changes the charge state of certain amino-acid side chains, disrupting salt bridges and causing the protein to unfold. (This is why ceviche "cooks" fish in lime juice.)
10. a — Primary, secondary, tertiary, quaternary. These are the standard four levels of protein structural organization.
11. b — Salt partially denatures muscle proteins, changing their water-holding capacity. Sodium and chloride ions modify the protein structure in ways that allow more water to be retained when the meat cooks.
12. b — A reaction between sulfur compounds (from cysteine) and iron in the yolk. Hydrogen sulfide gas, released from cysteine when overheated, reacts with iron in the yolk to form ferrous sulfide.
13. c — Disulfide bridge. Disulfide bridges between cysteine residues are real covalent (sulfur-sulfur) bonds. Hydrogen bonds, hydrophobic interactions, and salt bridges are all weaker, non-covalent.
14. b — Because the molecules are mostly too large to diffuse through dense muscle tissue. Aromatic compounds, sugars, and most of the molecules in marinades cannot diffuse far into meat in any reasonable time. Salt is the major exception (small ions diffuse fairly deeply).
Short Answer Explanations
15. The egg-white proteins (mainly ovalbumin) finish denaturing at about 80°C, while egg-yolk proteins denature and set at about 65–68°C. Because heat penetrates the egg from the outside in, the white reaches its setting temperature throughout while the center of the yolk is still below 65°C. So the white is fully coagulated and the yolk's center is still mostly raw.
16. A custard is a dilute system of egg proteins suspended in milk, sugar, and (sometimes) cream. If the temperature exceeds the denaturation/coagulation threshold for the proteins, they will unfold and coagulate locally — forming visible curds (the curdled scramble texture) instead of a smooth gel. Constant stirring distributes the heat evenly so no part of the custard exceeds the threshold faster than the rest. Holding below boiling (typically below 85°C for crème anglaise) gives time for the proteins to unfold and link gradually into a smooth mesh, instead of crashing together into curds.
17. Marinating in vinegar for an hour will denature the surface proteins of the steak through acid-driven denaturation, and may slightly tenderize that surface layer (a few millimeters at most). The interior of the steak is essentially unchanged because acetic acid molecules cannot diffuse meaningfully through dense muscle tissue in an hour. A "tenderized" steak from a vinegar marinade is, in practice, a steak with a slightly softer outer rim. The interior is still the same as before.
18. Because the chapter's whole point — that small temperature differences produce dramatically different denaturation states — depends on knowing the actual temperature. A stovetop burner can fluctuate by 20°C or more around a target as the burner cycles, and water that is "just below boiling" by visual inspection might be anywhere from 85°C to 99°C. Pat's demo only works as a pedagogical tool if the three baths are reliably at 60°C, 70°C, and 80°C — otherwise students cannot draw the conclusion that the same egg in different temperature baths produces different textures, because the same egg in different burner-cycling baths produces results that are within experimental noise.