Chapter 9 — Quiz

Multiple Choice (12)

1. Which of the following is a monosaccharide?

a) Sucrose b) Lactose c) Glucose d) Maltose

2. Sucrose, the sugar in your sugar bowl, is composed of which two monosaccharides linked by a glycosidic bond?

a) Glucose + galactose b) Glucose + fructose c) Fructose + galactose d) Glucose + glucose

3. Starch is built from two polymers of glucose. Which of the following correctly describes them?

a) Amylose is branched; amylopectin is linear b) Amylose is linear; amylopectin is branched c) Both are linear d) Both are branched

4. The temperature at which potato starch typically begins gelatinizing is approximately:

a) 35–40°C b) 50–55°C c) 58–65°C d) 75–80°C

5. What is "retrogradation"?

a) The decomposition of starch by heat into smaller sugars b) The slow recrystallization of cooled gelatinized starch, with amylose chains finding each other and re-forming hydrogen bonds c) The chemical conversion of starch into cellulose d) The browning of starch via the Maillard reaction

6. Why is a flour-based roux a different thickening tool than a cornstarch slurry?

a) The roux uses fat to pre-coat starch granules, preventing them from clumping when they hit hot liquid; this is necessary because flour contains gluten that would otherwise clump b) Roux is hotter than slurry c) Roux uses different starch granules d) The two methods produce identical results; the choice is purely cosmetic

7. Why does starch generally not turn deep blue with iodine if salivary amylase has acted on it for a minute or two?

a) Amylase converts starch into protein b) Amylase chops the long amylose chains into pieces too short to form helices long enough to host iodine c) Iodine is destroyed by amylase d) Saliva has the wrong pH for the reaction

8. Sticky (glutinous) rice contains essentially no amylose — almost 100% amylopectin. Based on the chemistry of this chapter, which of the following best predicts its cooking behavior?

a) Cooks up dry and individual, like long-grain basmati b) Cooks up cohesive and sticky; resists retrogradation; doesn't go hard in the fridge as quickly c) Won't gelatinize at normal cooking temperatures d) Is sweeter than ordinary rice because of the missing amylose

9. A waxy cornstarch and a regular ("dent") cornstarch are placed side by side in identical sauces. After cooling and refrigerating overnight, which sauce is most likely to retrograde firmer and turn gritty?

a) The waxy cornstarch sauce b) The regular dent cornstarch sauce c) Both retrograde identically d) Neither retrogrades at refrigerator temperature

10. Cellulose is a polysaccharide of glucose, just like starch — but humans cannot digest it. Why?

a) Cellulose contains different atoms than starch b) Cellulose's glycosidic bonds are oriented in the β configuration; humans lack the enzyme to break this bond, while we have enzymes for the α-bonds in starch c) Cellulose is too small for our digestive enzymes to grab d) Cellulose is a protein, not a carbohydrate

11. Which of the following best explains why bread refrigerated stales faster than bread left on the counter?

a) Refrigerator temperatures dry the bread out faster b) Refrigerator temperatures (~4°C) sit in the optimum kinetic zone for amylose recrystallization c) Bacteria in the fridge eat the starch d) Refrigeration causes the gluten network to shrink

12. A sauce thickened with cornstarch is brought to a boil and held there, with vigorous whisking, for 15 minutes. What's the most likely outcome?

a) The sauce thickens further and becomes pasty b) The sauce thins out as the granules rupture and the starch chains fragment c) The sauce becomes sweet because heat converts starch to sugar d) Nothing changes

Short Answer (5)

13. Explain in 3–4 sentences the difference between the Maillard reaction (Chapter 8) and gelatinization (this chapter), and why both can be happening simultaneously at the bottom of Maya's jollof rice pot.

14. A friend microwaves leftover rice the next morning and complains that it's hard. They didn't add water. Diagnose what happened in molecular terms, and explain why a tablespoon of water and a covered microwave dish would have helped.

15. Predict what happens when you drop iodine onto an extremely overripe (heavily brown-spotted) banana versus a green banana. Explain the chemistry behind the prediction.

16. A fruit pie thickened with cornstarch is pulled from the oven the moment its crust looks done. The next morning the filling is watery. Explain in chemical terms why the timing of "done" matters for starch-thickened pie fillings.

17. Suppose you're developing a frozen lasagna for sale. Native cornstarch in the white sauce thins out and turns grainy after the freeze-thaw cycle. Reason from this chapter's chemistry to recommend two strategies a food scientist would use to fix the problem.

Answer Key

1. c) Glucose. Sucrose, lactose, and maltose are all disaccharides (two sugar units joined). Glucose is a single sugar ring — a monosaccharide.

2. b) Glucose + fructose. Sucrose is glucose linked to fructose by a glycosidic bond. (Lactose is glucose + galactose; maltose is glucose + glucose.)

3. b) Amylose is linear; amylopectin is branched. Both are made of glucose units joined by α(1→4) glycosidic bonds, but amylopectin has additional α(1→6) branch points roughly every 20–25 glucose units.

4. c) 58–65°C. Potato starch gelatinizes earliest among common starches because of its larger granules and characteristic phosphorylated amylopectin. Cornstarch is closer to 62–72°C; rice starch around 68–78°C.

5. b) The slow recrystallization of cooled gelatinized starch. Specifically, amylose chains drift back toward each other below ~60°C and re-form hydrogen bonds, releasing bound water and stiffening the gel. This is why bread stales and refrigerated rice gets hard.

6. a) The roux uses fat to pre-coat starch granules. Flour contains gluten-forming proteins; if dumped into hot liquid without dispersion, the proteins hydrate and clump before the starch can swell. The fat in a roux separates each granule. Cornstarch is pure starch (no protein) and can be slurried directly in cold water without this issue.

7. b) Amylase chops the long amylose chains into pieces too short to form helices. Iodine produces its blue color by intercalating into the central tunnel of an amylose helix; if the chains are short, they cannot form sustained helices, and iodine has no place to bind.

8. b) Cooks up cohesive and sticky; resists retrogradation. Without amylose, there's nothing to retrograde at refrigerator temperatures (the linear amylose chains are the main culprits in retrogradation). Sticky rice stays soft for longer in the fridge and is more cohesive when freshly cooked.

9. b) The regular dent cornstarch sauce. Retrogradation is driven primarily by amylose. Dent cornstarch is ~25% amylose; waxy cornstarch is essentially 0% amylose, hence cannot retrograde via the amylose mechanism.

10. b) Cellulose's glycosidic bonds are β; we lack the enzyme. Same monomer (glucose), different bond orientation. Humans produce α-glucosidase but not cellulase. Ruminants and termites host gut bacteria that supply the missing enzyme.

11. b) Refrigerator temperatures sit in the optimum kinetic zone for amylose recrystallization. Below ~0°C the water can't move and retrogradation is paused; above ~60°C the chains can't settle. The fastest retrogradation kinetics fall right in the 0–8°C range — refrigerator temperature.

12. b) The sauce thins out. Prolonged hard boiling and vigorous shear cause the swollen starch granules to rupture and the released starch chains to fragment further. The thickening structure collapses; the sauce thins back out. This is why over-cooked or over-stirred starch sauces become disappointingly thin.

13. The Maillard reaction is a chemical browning that requires both an amino acid (from a protein) and a reducing sugar, plus heat — it produces the brown color and complex savory flavor of a seared crust. Gelatinization is a physical (not chemical) process: starch granules absorb water, swell, and burst at characteristic temperatures, but the molecules themselves don't fundamentally change identity. Both can happen simultaneously at the pot's bottom because the conditions overlap: the surface is hot enough for Maillard (amino acids in tomato/onion + residual sugars + 180-200°C metal contact) while the rice grains immediately above are still gelatinizing in the moisture above the metal interface.

14. The starch in cooked rice gelatinized when first cooked. Refrigeration overnight at ~4°C is the optimum kinetic zone for retrogradation: amylose chains find each other and re-crystallize, locking up water and stiffening each grain. Microwaving without water provides heat (which can reverse retrogradation) but no extra moisture for the starch to re-hydrate; result is hot but still firm rice. Adding a tablespoon of water and covering the dish creates a steam environment that re-supplies the moisture needed for the starch to re-soften — and most of the original texture returns.

15. Drop iodine on a green banana: deep royal-blue color appears as iodine slips into amylose helices throughout the still-starchy fruit. Drop iodine on a heavily overripe banana: little to no color change, because the banana's own ripening enzymes (amylase, plus others) have already chopped most of the starch into free sugars and short chains, and there are no remaining amylose helices for iodine to bind to. The two-banana experiment is essentially a visualization of how much starch-to-sugar conversion has occurred during ripening.

16. Cornstarch doesn't fully thicken until the filling actually boils — the granules need to reach roughly 80°C+ throughout to fully gelatinize and rupture, and below the surface a partially-baked pie may not have reached this. A pie pulled from the oven when only the crust is golden may have a center that hasn't reached the gelatinization endpoint. The center of the pie should visibly bubble through any vent holes for at least a minute before pulling — that's the signal that the cornstarch has fully hydrated and the filling will set as it cools.

17. Two strategies: (1) Use a cross-linked starch — chemically modified so the granules resist rupture from freezing damage and shear during reheating. (2) Use a substituted starch (acetylated or hydroxypropylated) — chemically blocked from retrogradation, so the freeze-thaw cycle doesn't allow amylose chains to find each other and crystallize. A combined cross-linked, hydroxypropylated tapioca or waxy maize starch is a common food-industry choice for frozen savory sauces; the modifications are precisely targeted at the chemistry described in this chapter.