Chapter 13 Quiz

15 multiple-choice questions and 5 short-answer questions. Answer key with explanations at the bottom.

Multiple Choice

1. An enzyme is best described as: - a) A protein that gets consumed in a chemical reaction - b) A protein that catalyzes a reaction without being consumed - c) A carbohydrate that catalyzes a reaction - d) A type of bacteria found in food

2. Why can you make Jell-O with canned pineapple but not with fresh pineapple? - a) Canned pineapple contains preservatives that help gelatin set - b) Fresh pineapple is more acidic - c) Canned pineapple has been heated, which denatures the bromelain enzyme - d) Canned pineapple is sweeter

3. The "Q10 rule" for enzymes states that, in the operating temperature range: - a) Reaction rate doubles every 10°C increase in temperature - b) Reaction rate halves every 10°C increase in temperature - c) Enzymes work best at 10°C - d) An enzyme has 10 active sites

4. Which of the following is NOT a way to slow enzymatic browning of a cut apple? - a) Coating with lemon juice - b) Refrigerating the apple - c) Blanching in boiling water for 30 seconds - d) Storing in pure oxygen

5. The lock-and-key model of enzyme action was proposed by: - a) Daniel Koshland in 1958 - b) Emil Fischer in 1894 - c) Louis Pasteur in 1860 - d) Leonor Michaelis in 1913

6. Which enzyme is responsible for the curdling of milk in cheese-making? - a) Lactase - b) Bromelain - c) Chymosin (rennet) - d) Amylase

7. "Lactase non-persistence" describes: - a) A disease that causes lactose to build up in the body - b) The default mammalian state in which adult lactase production has stopped - c) An allergic reaction to lactose - d) A bacterial infection of the gut

8. Bromelain is most concentrated in which part of the pineapple? - a) The sweet flesh - b) The skin - c) The stem and core - d) The leaves

9. Which of the following is the active component of common commercial powdered meat tenderizer? - a) Bromelain (pineapple) - b) Papain (papaya) - c) Actinidin (kiwi) - d) Chymosin (calf stomach)

10. Why are vegetables blanched (briefly boiled) before freezing? - a) To kill all bacteria - b) To denature their own enzymes, which would otherwise produce off-flavors over time - c) To make them softer when cooked later - d) To improve their nutritional content

11. Polyphenol oxidase (PPO) requires which of the following to function? - a) Pure water and sugar - b) Oxygen and a phenolic substrate - c) Acid and salt - d) Vitamin C only

12. Why is pickling cucumbers NOT primarily an enzymatic process? - a) Because cucumbers don't contain enzymes - b) Because the salt instantly kills all enzymes - c) Because the chemistry is performed mainly by lactic acid bacteria, not cucumber enzymes - d) Because vinegar is the active ingredient

13. Vitamin C (ascorbic acid) helps prevent apple browning because it: - a) Coats the apple and prevents oxygen contact - b) Reduces the brown quinone products of PPO back toward colorless phenols - c) Reacts directly with bromelain - d) Has a high pH

14. Why do many adults of East Asian descent commonly experience digestive discomfort after drinking fresh milk? - a) They have an allergy to milk protein - b) Their immune system attacks dairy - c) They lost adult lactase production after weaning, like most adult mammals - d) Milk in their region is contaminated

15. Which of the following describes the "induced-fit" model of enzyme action? - a) The substrate is forced into a rigid active site - b) The enzyme molds itself slightly to match the substrate as it binds - c) Two substrates compete for the same active site - d) The enzyme breaks apart after each reaction

Short Answer

16. Explain in 3–4 sentences why a cook who uses fresh kiwi as a meat tenderizer must set a timer. What happens at 30 minutes that's desirable, and what happens at 4 hours that's undesirable?

17. A bread baker complains that her loaves come out with a fine crumb but a stubbornly pale crust even at 220°C / 425°F oven temperature. Suggest one likely cause involving an enzyme, and propose one way to fix it.

18. Trace through the three independent mechanisms by which a squeeze of lemon juice slows the browning of a cut apple. Be specific about what each mechanism does to which step of the chemistry.

19. Pat tells her students that dropping a piece of fresh liver into hydrogen peroxide produces dramatic foaming. Name the enzyme responsible and write a one-sentence summary of what reaction it is catalyzing.

20. Explain why "lactose intolerance" might be a misleading framing for the global majority of adults who do not produce adult lactase. What more accurate term is sometimes used? Give one example of a cuisine that has historically thrived without dairy as a default beverage.

Answer Key

1. b) Enzymes are biological catalysts — they speed reactions without being consumed. (Most are proteins, although a small class of catalytic RNAs exists.)

2. c) Fresh pineapple contains active bromelain, a protease that cleaves the gelatin protein in Jell-O. The canning process heats pineapple to temperatures that denature the bromelain.

3. a) The Q10 ≈ 2 rule of thumb: in the temperature range below denaturation, the rate of an enzyme-catalyzed reaction roughly doubles for every 10°C increase. This breaks down once the enzyme starts denaturing at higher temperatures.

4. d) Pure oxygen would speed enzymatic browning because PPO requires oxygen as a substrate. Lemon juice (low pH + antioxidant), refrigeration (slows the rate), and blanching (denatures PPO) all slow browning.

5. b) Emil Fischer proposed the lock-and-key model in 1894. Koshland refined it to the induced-fit model in 1958. Both are correct — the induced-fit model is the more accurate refinement.

6. c) Chymosin (rennet) cleaves a single specific bond on κ-casein, destabilizing the milk's protein micelles and producing curds.

7. b) Lactase non-persistence is the default mammalian state — most mammals stop producing adult lactase after weaning. Lactase persistence (continued adult production) is a relatively recent genetic adaptation found mainly in populations with long histories of dairy farming.

8. c) Bromelain is most concentrated in the stem and core of pineapple. Industrial bromelain is extracted from pineapple stem, the part typically discarded in food preparation.

9. b) Papain, from papaya, is the most common active ingredient in commercial powdered meat tenderizer.

10. b) Blanching denatures the vegetable's own enzymes (PPO, peroxidases, lipoxygenases, and others) that would otherwise continue working slowly during freezer storage and produce off-flavors and color loss over time. It does kill some bacteria but that is not the primary purpose.

11. b) PPO requires both oxygen (as an electron acceptor) and a phenolic substrate (the molecule it oxidizes). The product is a quinone, which then polymerizes to form brown melanin pigments.

12. c) Pickling is primarily a microbial process. The lactic acid bacteria on the cucumber skin colonize the salty brine and convert sugars in the cucumber to lactic acid. The cucumber's own enzymes contribute very little.

13. b) Ascorbic acid is a reducing agent. PPO's product, the brown quinone, can be reduced back to the colorless phenol by ascorbic acid. This actively reverses the browning rather than merely slowing it.

14. c) Most adult East Asian populations have lactase non-persistence — the default mammalian condition. They are not allergic to milk protein (which is a separate condition); their bodies simply stopped producing lactase after weaning, like most other mammals.

15. b) The induced-fit model: the active site is somewhat flexible and shifts shape as the substrate binds, "embracing" it. This is more accurate than the rigid lock-and-key model but slightly more complex to teach.

16. Sample answer: Kiwi contains actinidin, a protease that cleaves protein bonds. At 30 minutes of contact, the enzyme has cleaved enough connective-tissue collagen to produce a tenderizing effect on the meat surface. At 4 hours, the enzyme has continued working past the connective tissue and into the muscle proteins themselves, producing mushy, mealy meat. The enzyme does not know to stop at "tender"; the cook must remove the meat from contact at the right moment.

17. Sample answer: The flour may not contain malted barley flour, leaving the dough with insufficient amylase activity. With less amylase, less starch is converted to maltose during fermentation, and the crust ends up with fewer reducing sugars to drive Maillard browning. Solutions include adding 1 tablespoon of diastatic malt powder per loaf, or switching to a flour that has malted barley flour listed on the ingredient label.

18. Sample answer: First, lemon juice has a pH around 2, well below the optimum pH (~6–7) for polyphenol oxidase. This denatures and inhibits the enzyme. Second, lemon juice contains ascorbic acid (vitamin C), an antioxidant that reduces the brown quinone product of PPO back toward the original colorless phenols, effectively reversing the chemistry. Third, the acid coating on the apple surface forms a thin barrier that slows oxygen diffusion to the cut surface, starving the enzyme of one of its substrates.

19. Sample answer: Catalase. Catalase catalyzes the decomposition of hydrogen peroxide (H₂O₂) into water and oxygen gas; the foaming is the released oxygen.

20. Sample answer: "Lactose intolerance" implies that the inability to digest lactose is an abnormal or pathological condition, when in fact it is the default mammalian state — most adult humans, like most adult mammals, do not produce lactase. The more accurate term is "lactase non-persistence." Cuisines that have historically thrived without dairy as a default beverage include most East Asian cuisines (which built around soy as a protein and broth medium), most Mesoamerican and Andean cuisines (which built around corn and beans, often using nixtamalization), and many West and Central African cuisines (which built around grains, tubers, and fermented grain drinks like kunu or ogi).

End of quiz.