Master Brewer Certification Yeast Biology & Fermentation Science — Questions and Answers

Question 1: What is the scientific name of the yeast species most commonly used in ale brewing?

Saccharomyces pastorianus
Saccharomyces cerevisiae (Correct answer)
Brettanomyces bruxellensis
Kluyveromyces marxianus

Correct answer: Saccharomyces cerevisiae

Saccharomyces cerevisiae is the top-fermenting yeast responsible for producing ales at warmer temperatures.

Question 2: During which phase of the yeast growth curve does the most vigorous fermentation activity occur?

Lag phase
Log (exponential) phase (Correct answer)
Stationary phase
Death phase

Correct answer: Log (exponential) phase

The log phase is when yeast cells multiply rapidly and sugar consumption and CO2 production are at their peak.

Question 3: What is the primary byproduct of yeast fermentation alongside ethanol?

Lactic acid
Acetic acid
Carbon dioxide (Correct answer)
Hydrogen sulfide

Correct answer: Carbon dioxide

Yeast converts sugars into ethanol and carbon dioxide through glycolysis and alcoholic fermentation.

Question 4: What is the typical optimal fermentation temperature range for lager yeast (Saccharomyces pastorianus)?

50–55°F (10–13°C) (Correct answer)
65–70°F (18–21°C)
75–80°F (24–27°C)
35–40°F (2–4°C)

Correct answer: 50–55°F (10–13°C)

Lager yeasts are cold-tolerant bottom-fermenters that work best in the 50–55°F range, producing clean flavor profiles.

Question 5: What is the brewing term for the act of adding yeast to cooled wort to begin fermentation?

Pitching (Correct answer)
Sparging
Mashing
Krausening

Correct answer: Pitching

Pitching refers specifically to the introduction of yeast into wort to initiate the fermentation process.

Question 6: Which yeast property describes how readily yeast cells clump together and settle out of suspension after fermentation?

Attenuation
Flocculation (Correct answer)
Esterification
Autolysis

Correct answer: Flocculation

Flocculation is the tendency of yeast cells to aggregate and drop out of suspension, affecting beer clarity and yeast harvesting.

Question 7: What does 'apparent attenuation' measure in a fermented beer?

The alcohol content as measured by gas chromatography
The percentage of original extract converted to alcohol and CO2, calculated from gravity readings (Correct answer)
The pH drop during fermentation
The number of viable yeast cells per milliliter

Correct answer: The percentage of original extract converted to alcohol and CO2, calculated from gravity readings

Apparent attenuation is calculated from the drop in specific gravity and reflects the degree to which yeast has fermented available sugars, though it overstates true attenuation due to alcohol's lower density.

Question 8: 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 9: 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 10: 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 11: 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 12: 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 13: 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 14: 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.

Question 15: What is the Crabtree effect as it applies to Saccharomyces cerevisiae in a brewing context?

Yeast's ability to survive and ferment at high alcohol concentrations above 10% ABV
The phenomenon where yeast preferentially ferments glucose via anaerobic glycolysis even in the presence of oxygen when sugar concentrations are high (Correct answer)
The inhibition of yeast flocculation caused by hop iso-alpha acids
The tendency of yeast to produce more esters at higher fermentation temperatures

Correct answer: The phenomenon where yeast preferentially ferments glucose via anaerobic glycolysis even in the presence of oxygen when sugar concentrations are high

The Crabtree effect means S. cerevisiae chooses alcoholic fermentation over respiration in high-sugar environments regardless of oxygen availability, explaining why pitching oxygenated wort still results in fermentation.

Question 16: Which laboratory method is most commonly used to measure yeast cell count and viability before pitching?

Refractometer reading of the yeast slurry
Methylene blue staining combined with hemocytometer cell counting (Correct answer)
Specific gravity measurement of the yeast suspension
pH measurement of the harvested yeast slurry

Correct answer: Methylene blue staining combined with hemocytometer cell counting

Methylene blue dye is excluded by living cells (they appear clear) but stains dead cells blue, allowing a hemocytometer to count viable versus non-viable cells under a microscope.

Question 17: What is trub, and why is excessive trub contact during fermentation a concern?

Trub is hop residue that provides essential bitterness nutrients to yeast during fermentation
Trub is hot and cold break material — proteins and lipids — that can cause off-flavors, harsh bitterness, and increased yeast stress if fermented on heavily (Correct answer)
Trub is a natural clarifying agent that improves beer clarity and head retention
Trub contains natural antimicrobial compounds that protect beer from contamination

Correct answer: Trub is hot and cold break material — proteins and lipids — that can cause off-flavors, harsh bitterness, and increased yeast stress if fermented on heavily

Excessive trub can contribute lipid oxidation products, harsh tannins, and increased yeast stress, leading to off-flavors and reduced fermentation quality.

Question 18: Why is oxygenating wort important for yeast health at the start of fermentation?

Oxygen directly converts maltose to simple sugars that yeast can ferment more easily
Yeast requires oxygen to synthesize sterols and unsaturated fatty acids needed for healthy, permeable cell membranes (Correct answer)
Oxygen activates the alpha-amylase enzymes that continue converting dextrins during fermentation
Oxygen is only necessary during the death phase to prevent excessive autolysis

Correct answer: Yeast requires oxygen to synthesize sterols and unsaturated fatty acids needed for healthy, permeable cell membranes

During aerobic respiration early in fermentation, yeast uses oxygen to build ergosterol and unsaturated fatty acids that maintain cell membrane integrity and alcohol tolerance.

Question 19: Which off-flavor compound is primarily responsible for the 'green apple' aroma associated with underfermented or 'green' beer?

Diacetyl
Acetaldehyde (Correct answer)
Dimethyl sulfide (DMS)
Butyric acid

Correct answer: Acetaldehyde

Acetaldehyde is an intermediate in ethanol synthesis; healthy yeast fully converts it to ethanol, but stressed or prematurely removed yeast leaves residual acetaldehyde with its characteristic green apple character.

Question 20: What is the key difference between 'real attenuation' and 'apparent attenuation' in fermentation measurement?

Real attenuation corrects for alcohol's lower density compared to water, giving a truer measure of extract consumed; apparent attenuation overstates fermentation because alcohol registers as lower gravity than it actually is (Correct answer)
Real attenuation is measured before fermentation; apparent attenuation is measured after final packaging
Real attenuation uses a calibrated refractometer; apparent attenuation uses a standard hydrometer
Real attenuation measures CO2 volume produced; apparent attenuation measures ethanol yield

Correct answer: Real attenuation corrects for alcohol's lower density compared to water, giving a truer measure of extract consumed; apparent attenuation overstates fermentation because alcohol registers as lower gravity than it actually is

Because alcohol is less dense than water, hydrometer readings underestimate residual extract, making apparent attenuation higher than real attenuation; real attenuation accounts for this by using corrected extract values.

Question 21: What is krausening, and what brewing benefit does it provide?

A dry-hopping technique where hops are added to actively fermenting beer to maximize aroma extraction
The addition of actively fermenting young beer (at high krausen) to a finished beer to provide natural carbonation and promote cleanup of diacetyl and acetaldehyde (Correct answer)
A clarification method using isinglass or gelatin added during secondary fermentation
The process of adding a portion of unfermented wort to increase gravity after primary fermentation

Correct answer: The addition of actively fermenting young beer (at high krausen) to a finished beer to provide natural carbonation and promote cleanup of diacetyl and acetaldehyde

Krausening introduces fresh, vigorously fermenting beer containing active yeast that both naturally carbonates and metabolically cleans up residual off-flavor precursors in the conditioned beer.

Master Brewer Certification Yeast Biology & Fermentation Science

What is the scientific name of the yeast species most commonly used in ale brewing?

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