Master Brewer Certification Yeast Biology & Fermentation Science 2 — Questions and Answers
Question 1: What is autolysis in brewing yeast, and what sensory impact does it have on beer?
- Yeast reproduction in high-gravity wort, producing more alcohol
- The self-destruction of stressed or aged yeast cells, releasing meaty, rubbery, or soapy off-flavors (Correct answer)
- The conversion of maltose to glucose by yeast enzymes during fermentation
- Yeast adaptation to high-alcohol environments through cell wall thickening
Correct answer: The self-destruction of stressed or aged yeast cells, releasing meaty, rubbery, or soapy off-flavors
Autolysis occurs when yeast enzymes digest their own cell contents, releasing unpleasant compounds like fatty acids and proteases into the beer.
Question 2: Which fermentation byproduct causes a buttery or butterscotch off-flavor in beer and must be reduced before packaging?
- Acetaldehyde
- Dimethyl sulfide (DMS)
- Diacetyl (Correct answer)
- Isoamyl acetate
Correct answer: Diacetyl
Diacetyl is a vicinal diketone (VDK) produced during yeast growth that must be reabsorbed by healthy yeast at the end of fermentation.
Question 3: What is the standard pitching rate recommended for an ale wort with an original gravity of 1.050?
- 0.5 million cells/mL/°Plato
- 1 million cells/mL/°Plato (Correct answer)
- 5 million cells/mL/°Plato
- 10 million cells/mL/°Plato
Correct answer: 1 million cells/mL/°Plato
The widely accepted standard pitching rate for ales is 1 million viable cells per mL per degree Plato of original gravity.
Question 4: What is the primary distinguishing characteristic between top-fermenting ale yeast and bottom-fermenting lager yeast?
- Top-fermenting yeasts produce more alcohol than bottom-fermenting yeasts
- Ale yeasts ferment at warmer temperatures and rise to the surface; lager yeasts ferment cooler and settle to the bottom (Correct answer)
- Bottom-fermenting yeasts produce more esters and fruity flavors
- Top-fermenting yeasts cannot ferment maltotriose
Correct answer: Ale yeasts ferment at warmer temperatures and rise to the surface; lager yeasts ferment cooler and settle to the bottom
The key difference is fermentation temperature and flocculation behavior — ale yeasts (S. cerevisiae) are warm-tolerant top-croppers; lager yeasts (S. pastorianus) are cold-adapted bottom settlers.
Question 5: What is the purpose of preparing a yeast starter before brew day?
- To initiate mashing enzymes at a controlled temperature
- To propagate and increase the quantity of viable yeast cells to achieve a proper pitching rate (Correct answer)
- To naturally pre-carbonate the wort before fermentation
- To measure wort gravity and fermentability in advance
Correct answer: To propagate and increase the quantity of viable yeast cells to achieve a proper pitching rate
A starter grows a larger, healthier yeast population from a smaller initial culture, ensuring enough cells for a healthy pitch without underpitching.
Question 6: Which nutrient is considered most critical for yeast health and fermentation vigor in wort?
- Magnesium
- Zinc
- Calcium
- Free Amino Nitrogen (FAN) (Correct answer)
Correct answer: Free Amino Nitrogen (FAN)
Free Amino Nitrogen (FAN) provides the organic nitrogen yeast needs to synthesize proteins and enzymes essential for healthy fermentation.
Question 7: What is the purpose of a diacetyl rest in lager production?
- To halt fermentation at a target final gravity
- To raise the fermenting beer's temperature near the end of fermentation so yeast can reabsorb diacetyl (Correct answer)
- To clarify the beer by encouraging yeast to flocculate and drop
- To add flavor adjuncts to the fermenter during active fermentation
Correct answer: To raise the fermenting beer's temperature near the end of fermentation so yeast can reabsorb diacetyl
Raising the temperature to 60–65°F (15–18°C) near the end of lager fermentation reactivates yeast metabolism to reabsorb diacetyl and its precursors.
Master Brewer Certification Yeast Biology & Fermentation Science 2
What is autolysis in brewing yeast, and what sensory impact does it have on beer?