Yeast Biology & Fermentation Science Flashcards

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

Answer: Saccharomyces cerevisiae

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

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

Answer: Log (exponential) phase

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

What is the primary byproduct of yeast fermentation alongside ethanol?

Answer: Carbon dioxide

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

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

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.

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

Answer: Pitching

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

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

Answer: Flocculation

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

What does 'apparent attenuation' measure in a fermented beer?

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.

What is autolysis in brewing yeast, and what sensory impact does it have on beer?

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.

Which fermentation byproduct causes a buttery or butterscotch off-flavor in beer and must be reduced before packaging?

Answer: Diacetyl

Diacetyl is a vicinal diketone (VDK) produced during yeast growth that must be reabsorbed by healthy yeast at the end of fermentation.

What is the standard pitching rate recommended for an ale wort with an original gravity of 1.050?

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.

What is the primary distinguishing characteristic between top-fermenting ale yeast and bottom-fermenting lager yeast?

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.

What is the purpose of preparing a yeast starter before brew day?

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.

Which nutrient is considered most critical for yeast health and fermentation vigor in wort?

Answer: Free Amino Nitrogen (FAN)

Free Amino Nitrogen (FAN) provides the organic nitrogen yeast needs to synthesize proteins and enzymes essential for healthy fermentation.

What is the purpose of a diacetyl rest in lager production?

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.