Chapter 30. Quiz
Eighteen questions. Mix of multiple choice, true/false, and short answer. Answer key with explanations at the bottom. Try the quiz before checking the key.
Multiple Choice
1. In strict biochemical terms, fermentation refers to:
a. Any controlled microbial transformation of food. b. The breakdown of sugars by microbes in the absence of oxygen as a final electron acceptor. c. Any process producing alcohol or acid. d. The aging of food at room temperature.
2. The dominant yeast species in bread, beer, wine, and sake is:
a. Lactobacillus plantarum b. Aspergillus oryzae c. Saccharomyces cerevisiae d. Acetobacter aceti
3. A successful lacto-fermentation typically reaches a final pH of:
a. ~7.0 b. ~5.5 c. Below 4.0 d. Below 2.0
4. Vinegar is produced by:
a. Yeast running anaerobic fermentation of sugar. b. Acetobacter running aerobic oxidation of ethanol. c. Lactobacillus running anaerobic acidification of lactose. d. Mold-driven hydrolysis of starches.
5. Japan's officially designated "national microbe" is:
a. Saccharomyces cerevisiae b. Lactobacillus delbrueckii subsp. bulgaricus c. Aspergillus oryzae (koji) d. Penicillium camemberti
6. Clostridium botulinum, the pathogen most associated with the safety concern in canning and certain other low-acid preserved foods, is inhibited at pH:
a. Below 6.0 b. Below 5.5 c. Below 4.6 d. Below 2.0
7. Glycolysis converts glucose into:
a. Two molecules of pyruvate b. One molecule of pyruvate c. Lactic acid directly d. Ethanol directly
8. In yeast alcoholic fermentation, after glycolysis, pyruvate is first decarboxylated to acetaldehyde, which is then reduced to:
a. Lactate b. Ethanol c. Acetic acid d. Acetyl-CoA
9. The Crabtree effect describes:
a. The tendency of S. cerevisiae to ferment sugar even when oxygen is plentiful, given high sugar concentrations. b. The ability of LAB to outcompete spoilage bacteria. c. The pH drop in a sourdough starter. d. The crystallization of fat in chocolate.
10. A thin, white film on the surface of a vegetable ferment is most likely:
a. A dangerous mold; discard the entire batch. b. Kahm yeast; usually safe but contributes off-flavor; skim and continue. c. The early sign of botulism. d. Salt crystals.
11. In a closed-population microbial culture (such as a sealed ferment), the four phases of growth are, in order:
a. Death, lag, log, stationary b. Lag, log, stationary, death c. Log, lag, stationary, death d. Stationary, log, lag, death
12. Aspergillus oryzae (koji) functions in soy sauce, miso, and sake fermentation primarily by:
a. Directly producing alcohol from rice starches. b. Producing acidic compounds that lower pH. c. Producing amylase and protease enzymes that break starches and proteins into sugars and amino acids that other organisms then ferment. d. Killing competing bacteria with antibiotic-like compounds.
13. Free amino acids, including glutamate, contribute strongly to the umami taste of fermented foods because:
a. Glutamate is added during fermentation as a flavor enhancer. b. Microbial proteases break down proteins into peptides and free amino acids; glutamate is one of the most abundant. c. Glutamate is produced from glucose by fermentation. d. The fermentation removes glutamate from the food.
14. In kimchi fermentation, late-stage dominance is typically by:
a. Saccharomyces cerevisiae b. Acetobacter aceti c. Aspergillus oryzae d. Lactobacillus plantarum (now Lactiplantibacillus plantarum)
15. Modern food microbiology generally lists how many overlapping mechanisms by which fermentation preserves food (as enumerated in the chapter)?
a. Two b. Three c. Five d. Ten
True / False
16. All commercially available fermented foods contain live microbial cultures at the time of consumption. (T / F)
17. The "fermentation" of black tea is technically an enzyme-driven oxidation rather than a microbial fermentation in the strict sense. (T / F)
Short Answer
18. A first-time fermenter friend has set up a jar of sauerkraut at room temperature. After 6 days, they tell you that the brine has gone cloudy and there are some small bubbles when they press the cabbage. They are worried this means the kraut has gone bad. Write a 4–5 sentence reply that addresses what is happening, whether to be concerned, and what to look for as actual signs of trouble.
Answer Key
1. b. Strict biochemistry distinguishes fermentation (anaerobic) from respiration (aerobic). The chapter notes that the culinary definition is broader, including aerobic processes like vinegar production.
2. c. Saccharomyces cerevisiae, "the sugar-fungus of beer," underlies bread, beer, wine, and sake.
3. c. Successful lacto-fermentation typically lands below pH 4.0, often around 3.4–3.8 for sauerkraut and kimchi at full ferment.
4. b. Acetobacter converts ethanol to acetic acid using oxygen — aerobic, not strictly fermentation in the biochemist's sense, but conventionally called "acetic fermentation."
5. c. Japan formally designated Aspergillus oryzae (koji) the national microbe (kokkin) in 2006.
6. c. Below pH 4.6, C. botulinum cannot grow. This is the central threshold for fermentation safety and home canning of low-acid foods.
7. a. Glycolysis converts one glucose (six carbons) into two pyruvate (three carbons each), with a net gain of 2 ATP and 2 NADH.
8. b. Yeast alcoholic fermentation: pyruvate → acetaldehyde + CO₂ (decarboxylation step), then acetaldehyde + NADH → ethanol + NAD⁺ (reduction step).
9. a. The Crabtree effect: S. cerevisiae in high-glucose environments will ferment even when oxygen is available. Strategically valuable because the ethanol produced suppresses competitors.
10. b. Kahm yeast — a wild yeast common on the surface of acidic, low-oxygen ferments. Not dangerous; can contribute off-flavors. Skim and continue, or restart if flavor is unacceptable.
11. b. Lag (adjustment) → log (exponential growth) → stationary (resource limitation) → death (decline and lysis).
12. c. Koji is the enzyme factory — its amylases and proteases pre-digest the substrate, freeing sugars and amino acids that other organisms (LAB, yeasts) then ferment.
13. b. Microbial proteases liberate amino acids during fermentation. Glutamate is among the most abundant free amino acids in long-aged ferments and is the headline umami taste compound.
14. d. Lactobacillus plantarum (now Lactiplantibacillus plantarum) is the most acid-tolerant of the dominant LAB and dominates the late stage of kimchi, sauerkraut, and many other vegetable ferments.
15. c. The chapter enumerates five: pH drop, alcohol, microbial competitive exclusion, salt, and low water activity.
16. False. Many commercially available fermented foods have been pasteurized or otherwise treated, killing the live microbes. Soy sauce, beer, baked sourdough, miso added to soup, vinegar, and many shelf-stable kimchi and kraut products may not contain live cultures at consumption. Read labels — "raw" and "live cultures" claims indicate live microbes.
17. True. Black tea oxidation is performed by the tea leaf's own polyphenol oxidase enzymes acting on the leaf's polyphenols after the leaves are bruised — not microbial. The word "fermentation" stuck for cultural and historical reasons; we'll discuss this in Chapter 34.
18. Sample answer. Your friend's kraut is doing exactly what it should be doing — cloudy brine and small bubbles at day 6 are the visible signs of vigorous lactic-acid fermentation, and they mean Lactobacillus species are at work. The cloudiness is yeast and bacterial cells along with material released from the broken cabbage cells. The bubbles are CO₂ from heterofermentative LAB activity. Tell them to keep going. The actual signs of trouble would be fuzzy colored mold (green, blue, black, pink) on the surface, a slimy or stringy texture, or a putrid or rotten smell — none of which they're describing. Encourage them to dip a pH strip; the brine should be approaching pH 4.0 by now if all is well.