Chapter 19 Quiz — Legumes, Nuts, and Seeds
15 multiple choice + 5 short answer. Answer key with explanations at the bottom.
Multiple Choice
1. Phytohaemagglutinin (PHA) is: a) A starch found in red kidney beans that takes longer to gelatinize than other bean starches b) A heat-labile lectin protein in raw kidney beans, capable of damaging the gut lining if not denatured by cooking c) A natural antioxidant in legumes that prevents oxidation of bean oils d) A polysaccharide that binds iron and other minerals in the human gut
2. The safest way to cook dried red kidney beans is: a) Slow cooker on LOW setting for 8 hours b) Slow cooker on HIGH setting for 4 hours, no pre-boil c) Soak, drain, then boil hard for at least 10 minutes before slow cooking or simmering d) Soak overnight, then simmer at 180°F (82°C) for 6 hours
3. The "don't salt beans early" rule: a) Has been proven correct by controlled experiments — salt does harden bean skins b) Was never tested before this century — Cook's Illustrated and others have shown the rule does not hold up at typical cooking-water salt concentrations c) Applies only to red kidney beans, not other beans d) Is correct only for beans cooked in soft water
4. The foam that rises when you boil dried beans is mostly: a) Phytic acid leaching out of the seed coat b) Lectins denaturing in solution c) Saponins (surface-active compounds) and proteins from broken cells d) Oligosaccharides reacting with the water
5. Aquafaba is: a) A traditional Mediterranean preparation of chickpeas in olive oil b) The thick, viscous liquid left after cooking dried chickpeas (or in canned chickpea liquid), capable of forming stable foams when whipped c) A Latin term for the bean cooking water that has long been a known ingredient in classical cuisine d) A specific cultivar of chickpea bred for its high protein content
6. The man who popularized aquafaba as an egg-white substitute and gave it its name was: a) A French pastry chef in Paris in 1923 b) Joël Roessel, a French chef who first noticed the foaming property c) Goose Wohlt, an Indiana software engineer who shared the technique on a vegan Facebook group in 2015 d) Both Roessel (who first observed foaming) and Wohlt (who systematized the egg-substitute use and named it)
7. Tofu is made by coagulating soy milk with: a) Heat alone, with no added agents b) A mineral salt (typically calcium sulfate or magnesium chloride / nigari) or an acid (such as glucono-delta-lactone) c) Rennet, the same enzyme used to make cheese d) Yeast, which converts sugars to acid and curdles the protein
8. Why do walnuts go rancid faster than almonds at the same temperature? a) Walnuts contain more protein, which oxidizes faster than fat b) Walnuts have a higher proportion of polyunsaturated fats (multiple double bonds), which are more vulnerable to oxidation than the monounsaturated fats that dominate in almonds c) Walnuts contain water, while almonds are essentially dry d) Walnuts lack the antioxidants found in almonds (almonds have higher tocopherol content), and this is the only reason
9. Toasting a nut develops new flavor compounds primarily through: a) Caramelization of the nut's natural sugars b) The Maillard reaction between amino acids in the nut's protein and reducing sugars c) Volatilization of the nut's oils into the air d) Denaturation of structural proteins
10. Mustard heat is produced when: a) Myrosinase enzyme cleaves glucosinolates to produce volatile isothiocyanates b) Capsaicin from the seed activates pain receptors on the tongue c) The seeds are heated, causing thermal decomposition of stable mustard oils d) Pressure of grinding releases pre-formed mustard heat compounds
11. Phytic acid in legumes and grains: a) Is a major calorie source for the seed embryo b) Is the seed's storage form of phosphorus and chelates divalent metal cations like iron and zinc, reducing their bioavailability c) Is a toxin that must be removed before eating d) Is a flavor compound that gives legumes their characteristic taste
12. The traditional processing techniques that reduce phytic acid include: a) Sourdough fermentation, sprouting, long soaking with mild acid, miso fermentation b) Freezing and thawing c) Cooking in alkaline (high-pH) water d) Drying in the sun for several days
13. Chia and flax seeds work as egg substitutes in baking because: a) Their proteins behave identically to egg-white proteins b) Their mucilaginous polysaccharide coats absorb water and form a hydrogel that performs the binding function of an egg c) They emulsify like egg yolks d) They contain natural lecithin in higher concentrations than eggs
14. Sesame is: a) Not considered an allergen in the United States b) A top-9 allergen in the United States as of 2023, with mandatory labeling required c) Only a regulated allergen in Canada and the EU, not the US d) A grain, not a seed
15. Hard water (high in calcium) makes beans tough because: a) The calcium ions form additional cross-links in the pectin of the seed coat, preventing softening b) Hard water has a lower boiling point and cannot cook beans fully c) The minerals neutralize the bean's natural softening compounds d) Hard water lacks the ions needed for protein denaturation
Short Answer
16. Explain why the slow-cooker LOW setting can be unsafe for dried red kidney beans. Address both the temperature reached and what that temperature does (or does not do) to phytohaemagglutinin.
17. Maya's mother's egusi soup uses ground melon seeds to thickener and enrich a stew. West African peanut stew uses peanuts the same way. What is the chemical and structural commonality between these two seeds that allows them to perform the same culinary function?
18. A reader complains that their hummus tastes "grassy" and grainy. Diagnose two likely causes and propose a fix for each.
19. Aquafaba and egg white both produce stable foams when whipped, but they have different protein compositions. What physical/chemical principle is shared between the two, and what does this convergence tell us about foam formation more generally? (3–5 sentences)
20. A friend tells you they soak their dried beans for 24 hours with a tablespoon of yogurt added to the water before cooking. Explain what the yogurt is doing chemically, and decide whether this is reasonable practice or unnecessary.
Answer Key
1. b) PHA is a heat-labile lectin protein. It is concentrated in raw red kidney beans and denatures with sufficient cooking. It is not a starch (a), an antioxidant (c), or a polysaccharide (d).
2. c) Soak, drain, boil hard for 10+ minutes, then continue cooking. The boiling step is what fully denatures the PHA. Slow-cooker-only methods (a, b) do not reach reliable lectin-denaturing temperatures. Long simmers below 180°F (d) may not fully inactivate PHA either.
3. b) The "don't salt early" rule was a folk theory that became received wisdom. Controlled experiments by Cook's Illustrated, Kenji López-Alt, and others have shown that typical cooking-water salt concentrations (~6 g/L) do not measurably harden bean skins or slow softening, and they produce better-flavored beans because the salt has the entire cooking time to penetrate.
4. c) The foam consists primarily of saponins (natural surfactants from the seed coat — saponin literally means "soap-like") and proteins released from broken cells. Saponins are mildly bitter, hence the tradition of skimming.
5. b) Aquafaba is the cooking liquid from chickpeas (or canned chickpea liquid), containing chickpea proteins, saponins, and oligosaccharides. The proteins denature at the air-water interface during whipping, and the saponins act as surfactants stabilizing the resulting foam.
6. d) The discovery is shared. Roessel first noticed the foaming property; Wohlt systematized the egg-white substitution and named it aquafaba in 2015 via a vegan Facebook group post.
7. b) Tofu is made by coagulating soy milk with mineral salts (calcium sulfate, magnesium chloride/nigari) or acid (glucono-delta-lactone). The choice of coagulant affects texture: nigari produces a slightly coarser, more complex tofu; calcium sulfate produces a softer, smoother set.
8. b) Walnuts are higher in polyunsaturated fats (especially alpha-linolenic acid, an omega-3 with three double bonds), which are far more vulnerable to oxidative attack than the monounsaturated fats (one double bond) that dominate in almonds. Almonds also have higher antioxidant tocopherol content, which is a contributing factor (d is partly true but not the primary mechanism — the polyunsaturation difference is the main story).
9. b) Maillard reaction between amino acids and reducing sugars produces hundreds of volatile aromatic compounds. Caramelization (a) plays a smaller role since the sugar content of nuts is modest. Volatilization (c) and denaturation (d) are not the flavor-builders.
10. a) Myrosinase cleaves glucosinolates (sulfur-containing precursors) to release isothiocyanates, the volatile sharp-tasting compounds. The reaction requires water and is inhibited by heat (denatures the enzyme) or low pH (inactivates it).
11. b) Phytic acid (myo-inositol hexaphosphate, IP6) is the seed's phosphorus reserve. Its phosphate groups chelate divalent cations (Fe²⁺, Zn²⁺, Ca²⁺, Mg²⁺), reducing their bioavailability when consumed.
12. a) Long fermentation, sprouting, long soaking with mild acid, and miso/tempeh fermentation all activate endogenous or microbial phytase enzymes that degrade phytic acid. Effects can be 50–90% reduction in long ferments.
13. b) The mucilage in chia and flax (a polysaccharide layer on the seed) absorbs water and forms a hydrogel — a polymer-water mesh — that performs the binding function of an egg in many baking applications. It does not replicate egg's protein-foaming or coagulation functions, so it doesn't work for meringue or custard.
14. b) Sesame became the ninth recognized major allergen in the US under the FASTER Act (effective January 2023), requiring mandatory labeling. It has been recognized as an allergen earlier in Canada, the EU, Japan, and elsewhere.
15. a) Calcium ions in hard water form additional cross-links in the pectin of the bean seed coat and cell walls, locking the structure in place and preventing softening — even with extended cooking. The fix is to use soft/filtered water or a small pinch of baking soda.
16. PHA (phytohaemagglutinin) requires temperatures at or above boiling (around 100°C / 212°F) to denature in a reasonable time. Slow cookers on the LOW setting typically reach 175–200°F (80–95°C), below the denaturation threshold. Worse, this intermediate temperature can release lectins from cells without inactivating them, potentially increasing the active PHA dose compared to raw beans. The fix is straightforward: pre-boil dried kidney beans on the stove for at least 10 minutes before transferring to the slow cooker. Pressure cookers reach above 240°F (115°C) and handle kidney beans safely without pre-boiling.
17. Both are seeds with high fat content (~50% in peanuts; ~50% in egusi melon seeds) and meaningful protein content (~25%). When ground and stewed, the fat emulsifies into the watery stew and the protein binds the system into a thickened, cohesive sauce. The principle is general: a seed that is roughly half fat and a quarter protein will, when ground and slowly cooked into a liquid, produce a thick, savory sauce that performs structurally as a stew base. Different seeds produce different flavors, but the texture chemistry is shared.
18. Grassy: the chickpeas may have been undercooked. Cook them to the point of falling apart — much softer than al dente. Fix: simmer longer, possibly with a small pinch of baking soda to accelerate skin softening. Grainy: processing time too short, or chickpea skins not removed. Fix: process longer in a food processor (ideally 5–8 minutes for silky hummus); add ice water by the tablespoon to keep the temperature low; for the silkiest result, remove the skins from each chickpea by squeezing them between thumb and forefinger before processing.
19. Both rely on protein denaturation at the air-water interface. As the foam forms, proteins unfold (their hydrophobic regions exposed), align at the interface between air bubbles and the surrounding water, and form an interconnected network that stabilizes the foam. Different proteins (egg-white ovalbumin, chickpea albumins/globulins) accomplish this with different efficiencies and stabilities, but the underlying principle is shared. The convergence tells us that foam formation is about interfacial protein behavior, not about any specific magic of egg whites — any sufficiently soluble protein with the right amphipathic structure can foam.
20. The yogurt adds lactic acid and small amounts of live bacteria. The acid lowers the pH of the soaking water into the range where the bean's endogenous phytase enzyme is most active (around pH 5–6), accelerating the breakdown of phytic acid and freeing more iron and zinc for absorption. The lactic bacteria may also contribute to mild fermentation if the soak is long enough. This is reasonable practice with a real biochemical basis. Whether it's necessary depends on how heavily your friend relies on legumes — for someone eating beans daily as a primary protein, it can meaningfully improve mineral absorption. For someone eating beans occasionally, the effect is small. It's a traditional practice (you'll find versions of it in Indian, North African, and Italian cooking) that has real chemistry behind it.