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Quiz: Sourdough and Wild Fermentation

Test your understanding of sourdough starters, wild yeast, lactic acid bacteria, and fermentation control with these questions.

  1. Whole-grain flour has more starch, giving yeast more sugar to ferment quickly
  2. The bran of whole grains harbors large populations of wild yeast and lactic acid bacteria
  3. Whole-grain flour creates a stiffer dough that traps CO₂ more effectively
  4. Refined white flour contains preservatives that kill beneficial microorganisms
Show Answer

The correct answer is B. The outer bran layer of whole grains harbors dense populations of wild yeast and lactic acid bacteria. Including whole-wheat or rye flour inoculates the starter with these naturally occurring microorganisms, speeding up the establishment of the microbial community. Option A is incorrect — starch content is not the main advantage. Option C is not why whole-grain flour is recommended. Option D is not accurate — commercially available white flour does not typically contain antimicrobial preservatives.

Concept Tested: Wild Yeast Capture

2. A mature sourdough starter contains a stable community of which two microorganism types?

  1. Mold and bacteria
  2. Wild yeast and lactic acid bacteria
  3. Commercial Saccharomyces cerevisiae and Acetobacter
  4. Homofermentative bacteria and baking powder
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The correct answer is B. A healthy sourdough starter ecosystem contains wild yeast (primarily Saccharomyces cerevisiae and related species) and heterofermentative lactic acid bacteria (from genera like Lactobacillus). The yeast produce CO₂ for leavening; the bacteria produce lactic and acetic acids for flavor and preservation. Option A includes mold, which is not part of a healthy starter. Option C is incorrect — sourdough uses wild, not commercial, yeast, and Acetobacter is not a core sourdough organism. Option D is nonsensical — baking powder is not a microorganism.

Concept Tested: Sourdough Starter Ecosystem

3. In the 1:1:1 sourdough feeding ratio, what do the three numbers represent?

  1. One cup of water, one cup of flour, one tablespoon of salt
  2. Equal parts (by weight) of existing starter, fresh flour, and fresh water
  3. One feeding per day, for one week, using one type of flour
  4. One hour of fermentation, one rest period, one shaping session
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The correct answer is B. The 1:1:1 feeding ratio means equal weights of existing starter, fresh flour, and fresh water. For example: 50g starter + 50g flour + 50g water. This maintains microbial health by providing fresh nutrients while controlling acid accumulation through partial discarding of old starter. Options A, C, and D describe entirely different concepts and do not relate to the standard sourdough feeding protocol.

Concept Tested: Sourdough Feeding Ratio

4. The sourdough float test works because a starter at peak activity

  1. Has a higher sugar content that makes it less dense than water
  2. Is filled with CO₂ gas bubbles that reduce its density below that of water
  3. Has absorbed so much water that it becomes gelatinous and floats
  4. Contains acid compounds that react with water to produce buoyancy
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The correct answer is B. When a sourdough starter is at peak activity, it is filled with carbon dioxide gas bubbles produced by yeast fermentation. These bubbles make the starter less dense than water, so it floats. A starter that sinks has not yet produced enough CO₂ and is not ready to leaven bread effectively. Options A, C, and D describe incorrect mechanisms unrelated to the float test.

Concept Tested: Sourdough Starter Float Test

5. A baker wants a mild, creamy-sour loaf rather than a sharp, vinegary one. Which fermentation conditions should the baker choose?

  1. Cool temperature (55–65°F) with stiff, low-hydration dough
  2. Warm temperature (75–80°F) with a wet, high-hydration dough
  3. Overnight retarding in the refrigerator (38°F)
  4. Very fast fermentation at 90°F for maximum acid production
Show Answer

The correct answer is B. Warm fermentation temperatures (75–80°F) with a high-hydration (wet) dough favor lactic acid production, which creates a mild, creamy sourness similar to yogurt. Cool temperatures and stiff doughs (option A) favor acetic acid production, creating a sharper, vinegary flavor. Overnight refrigeration (option C) also produces more acetic acid. Very fast fermentation at 90°F (option D) would produce excessive acid quickly and reduce flavor complexity.

Concept Tested: Sourdough Temperature Effects

6. Why does sourdough bread stay mold-free longer than commercial yeast bread?

  1. Sourdough bread is baked at higher temperatures that create a thicker, antimicrobial crust
  2. Lactic and acetic acids produced during fermentation lower the bread's pH, inhibiting mold growth
  3. Wild yeast in sourdough produce natural antifungal compounds during fermentation
  4. The long fermentation time dehydrates the dough, resulting in a lower water activity
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The correct answer is B. During sourdough fermentation, lactic acid bacteria produce lactic and acetic acids that lower the final loaf's internal pH to approximately 3.5–4.0. At this pH, mold growth is severely inhibited, extending shelf life to 4–7 days at room temperature compared to 2–3 days for commercial yeast bread. Option A is not the primary mechanism. Option C is not a documented primary mechanism. Option D is not accurate — hydration levels are similar between the two bread types.

Concept Tested: Lactic Acid Production

7. What is "bulk fermentation" in sourdough bread making?

  1. The initial drying phase when flour absorbs all available water before yeast activity begins
  2. The first rise of the mixed dough before shaping, when yeast and bacteria develop flavor and gas
  3. The final proofing of the shaped loaf in the refrigerator overnight
  4. The commercial-scale process of fermenting large batches of dough simultaneously
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The correct answer is B. Bulk fermentation is the first rise of sourdough bread dough — after mixing flour, water, salt, and starter, but before shaping. During this phase, wild yeast produce CO₂ to inflate the dough, and lactic acid bacteria produce flavor acids. It typically takes 4–6 hours at room temperature. Option A describes hydration, not fermentation. Option C describes cold proofing/retarding, not bulk fermentation. Option D is unrelated to the term as used in artisan bread making.

Concept Tested: Bulk Fermentation

8. If a sourdough starter develops pink or orange streaks, what should a baker do?

  1. Add more salt to suppress the colored bacteria and continue feeding normally
  2. Refrigerate the starter immediately to slow all microbial activity
  3. Discard the starter and begin a new one — colored streaks indicate contamination
  4. Add a tablespoon of vinegar to lower pH and eliminate the unwanted organisms
Show Answer

The correct answer is C. Pink or orange discoloration in a sourdough starter indicates contamination by unwanted bacteria that can be harmful. The correct response is to discard the contaminated starter and start fresh with clean equipment and fresh flour and water. Attempting to save a contaminated starter by adding salt or vinegar (options A and D) or refrigerating it (option B) will not reliably eliminate the contamination.

Concept Tested: Wild Yeast Capture

9. The relationship between wild yeast and lactic acid bacteria in a sourdough starter is best described as

  1. Competitive — yeast and bacteria fight for the same nutrients and one eventually wins
  2. Parasitic — bacteria consume products made by yeast without contributing anything in return
  3. Mutualistic — both organisms benefit from each other's activities and coexist stably
  4. Neutral — yeast and bacteria occupy completely separate ecological niches and do not interact
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The correct answer is C. The relationship is mutualistic. Wild yeast produce CO₂ and consume oxygen, creating the anaerobic conditions that favor lactic acid bacteria. The bacteria produce acids that lower pH, which select for acid-tolerant wild yeast and exclude competing bacteria. Both organisms benefit from the partnership and the community remains stable over time with regular feeding. Options A, B, and D do not accurately describe this cooperative microbial relationship.

Concept Tested: Sourdough Starter Ecosystem

10. "Hooch" forming on top of a sourdough starter means the starter

  1. Has been contaminated and should be discarded immediately
  2. Has too much mold growth and needs to be refrigerated
  3. Is overdue for feeding — the yeast have exhausted available sugars and produced alcohol
  4. Has reached its peak activity and is ready to use for baking
Show Answer

The correct answer is C. "Hooch" is a grayish or brownish liquid layer of alcohol (ethanol) and water that forms on top of a hungry starter when yeast have fermented all available sugars. It signals that the starter needs to be fed. The baker can stir it back in or pour it off before refreshing. Option A is incorrect — hooch is a normal sign of a hungry starter, not contamination. Option B is wrong — hooch is not mold. Option D is the opposite — hooch appears when the starter has collapsed, not at peak activity.

Concept Tested: Sourdough Feeding Ratio