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Quiz: Food Microbiology — Microbes, Fermentation, and Cultured Foods

Test your understanding of bacteria, yeast, mold, and fermentation with these questions.

1. What makes bacteria prokaryotes?

  1. They are too small to see without a microscope
  2. They reproduce by budding rather than binary fission
  3. Their DNA floats freely in the cytoplasm with no membrane-bound nucleus
  4. They can only survive in acidic environments
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The correct answer is C. The defining feature of prokaryotes is the absence of a membrane-bound nucleus — their DNA floats freely in the cytoplasm as a single circular chromosome. Option A describes all microorganisms, not just bacteria. Option B describes yeast reproduction (budding), while bacteria use binary fission. Option D is false — bacteria thrive across a wide range of pH values.

Concept Tested: Bacteria Cell Structure

2. During which phase of the bacterial growth curve do bacteria divide at the fastest possible rate?

  1. Lag phase
  2. Log (exponential) phase
  3. Stationary phase
  4. Death phase
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The correct answer is B. During the log (exponential) phase, bacteria divide as rapidly as possible — doubling every 20–30 minutes under ideal conditions. The lag phase is a period of adjustment before reproduction accelerates. The stationary phase is when growth equals death rate. The death phase is when the population declines as nutrients run out.

Concept Tested: Log Phase of Growth

3. In alcoholic fermentation, yeast convert glucose into which two main products?

  1. Lactic acid and water
  2. Oxygen and glucose
  3. Ethanol and carbon dioxide
  4. Acetic acid and hydrogen
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The correct answer is C. In alcoholic fermentation, yeast convert glucose (C₆H₁₂O₆) into ethanol (C₂H₅OH) and carbon dioxide (CO₂). The CO₂ is what makes bread rise and beer fizz; the ethanol evaporates during baking. Option A describes lactic acid fermentation. Option B is the reverse of fermentation. Option D describes acetic acid fermentation produced by Acetobacter bacteria.

Concept Tested: Alcoholic Fermentation

4. Lactic acid fermentation is responsible for which of the following foods?

  1. Apple cider vinegar
  2. Yogurt and kimchi
  3. Wine and beer
  4. Soy sauce and miso
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The correct answer is B. Yogurt and kimchi are both made through lactic acid fermentation, where Lactobacillus bacteria convert sugars to lactic acid, creating a tangy, acidic product. Apple cider vinegar (option A) is made through acetic acid fermentation. Wine and beer (option C) are made through alcoholic fermentation by yeast. Soy sauce and miso (option D) are made primarily through mold-based fermentation with Aspergillus oryzae (koji mold).

Concept Tested: Lactic Acid Fermentation

5. Why are biofilms on food preparation surfaces particularly dangerous?

  1. They produce visible mold that contaminates food directly
  2. Bacteria in biofilms are protected by an extracellular matrix, making them harder to remove
  3. They raise the temperature on cutting boards, accelerating spoilage
  4. Biofilms only form on metal surfaces and contaminate knives
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The correct answer is B. Biofilms are communities of bacteria embedded in a self-produced extracellular matrix (a slime layer) that shields them from cleaning agents and sanitizers. This makes them much more difficult to eliminate than free-floating bacteria. Option A describes mold colonies, not biofilms. Option C is not how biofilms work. Option D is false — biofilms form on a wide variety of surfaces including cutting boards and counters.

Concept Tested: Biofilm Formation

6. What is the primary reason that yogurt thickens during production?

  1. Lactic acid bacteria produce starch molecules that gel the milk
  2. Lactic acid lowers pH to about 4.0–4.5, causing casein proteins to coagulate
  3. Heat treatment causes whey to evaporate, concentrating the remaining liquid
  4. Added rennet enzyme splits lactose, releasing sugars that solidify the milk
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The correct answer is B. As lactic acid bacteria ferment lactose, they produce lactic acid that lowers the milk's pH from about 6.7 to 4.0–4.5. At this pH, casein proteins coagulate and form a gel — the characteristic thick texture of yogurt. Option A is incorrect — bacteria don't produce starch. Option C is not accurate. Option D confuses yogurt production with cheesemaking (rennet is used in cheese, not yogurt).

Concept Tested: Yogurt Production Science

7. How does the Norovirus differ from bacteria as a food safety threat?

  1. Norovirus produces toxins that survive cooking temperatures
  2. Norovirus multiplies rapidly in high-protein foods like meat
  3. Norovirus doesn't grow in food — it only needs to survive long enough to be consumed
  4. Norovirus is only found in produce, not in processed foods
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The correct answer is C. Unlike bacteria, viruses cannot multiply in food — they are not cells and have no metabolic machinery. Norovirus only needs to survive on surfaces or in food long enough to be ingested, where it can then infect intestinal cells. This is why infected food handlers are such a serious risk even when food looks and smells normal. Options A, B, and D are all incorrect descriptions of Norovirus behavior.

Concept Tested: Viruses in Food

8. What role does mold play in the production of cheeses like Roquefort and Brie?

  1. Mold is an unwanted contaminant that must be removed before serving
  2. Specific mold species are deliberately cultivated to develop flavor and texture
  3. Mold converts lactose to lactic acid, acidifying the curd
  4. Mold produces rennet, which is necessary for curd coagulation
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The correct answer is B. Specific beneficial mold species are intentionally introduced into these cheeses: Penicillium roqueforti creates the blue veins in Roquefort, and Penicillium camemberti forms the white rind on Brie. These molds develop the complex flavors and textures that define these cheeses. Option A is wrong — the mold is intentional. Option C describes bacterial (lactic acid fermentation) activity. Option D is incorrect — rennet comes from animal or microbial sources, not mold.

Concept Tested: Mold in Food

9. In cheese making, what happens when rennet is added to acidified milk?

  1. Lactose is fermented into acetic acid, creating a vinegar flavor
  2. The enzyme chymosin cleaves casein proteins, forming a solid curd
  3. Bacteria multiply rapidly, producing carbon dioxide that aerates the cheese
  4. The fat molecules separate and rise to form the rind on the cheese surface
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The correct answer is B. Rennet contains the enzyme chymosin (and other proteases), which cleaves κ-casein proteins. This disrupts the structure that keeps casein micelles suspended, causing them to aggregate into a solid gel called curd. Option A describes fermentation, which happens before rennet is added. Option C describes yeast activity in bread, not cheese. Option D is not how rinds form.

Concept Tested: Cheese Making Microbiology

10. A kombucha SCOBY is best described as

  1. A single species of mold that grows on the surface of sweet tea
  2. A symbiotic culture of bacteria and yeast that ferments tea into a tangy beverage
  3. A bacterial starter culture used exclusively for dairy fermentation
  4. A chemical catalyst added to tea to produce carbon dioxide
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The correct answer is B. SCOBY stands for Symbiotic Culture of Bacteria and Yeast. It contains wild yeast (which perform alcoholic fermentation) and acetic acid bacteria (which convert ethanol to acids), working together to ferment sweetened tea into kombucha. Option A describes a mold colony, not a SCOBY. Option C is a dairy starter culture, not a SCOBY. Option D describes a chemical process; SCOBYs are living communities, not catalysts.

Concept Tested: Kombucha Science