Master Brewer Certification Yeast Biology & Fermentation Science 3

Question 1

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

Accepted Answer

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

Answer

Yeast's ability to survive and ferment at high alcohol concentrations above 10% ABV

Answer

The inhibition of yeast flocculation caused by hop iso-alpha acids

Answer

The tendency of yeast to produce more esters at higher fermentation temperatures

Question 2

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

Accepted Answer

Methylene blue staining combined with hemocytometer cell counting

Answer

Refractometer reading of the yeast slurry

Answer

Specific gravity measurement of the yeast suspension

Answer

pH measurement of the harvested yeast slurry

Question 3

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

Accepted 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

Answer

Trub is hop residue that provides essential bitterness nutrients to yeast during fermentation

Answer

Trub is a natural clarifying agent that improves beer clarity and head retention

Answer

Trub contains natural antimicrobial compounds that protect beer from contamination

Question 4

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

Accepted Answer

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

Answer

Oxygen directly converts maltose to simple sugars that yeast can ferment more easily

Answer

Oxygen activates the alpha-amylase enzymes that continue converting dextrins during fermentation

Answer

Oxygen is only necessary during the death phase to prevent excessive autolysis

Question 5

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

Accepted Answer

Acetaldehyde

Answer

Diacetyl

Answer

Dimethyl sulfide (DMS)

Answer

Butyric acid

Question 6

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

Accepted 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

Answer

Real attenuation is measured before fermentation; apparent attenuation is measured after final packaging

Answer

Real attenuation uses a calibrated refractometer; apparent attenuation uses a standard hydrometer

Answer

Real attenuation measures CO2 volume produced; apparent attenuation measures ethanol yield

Question 7

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

Accepted 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

Answer

A dry-hopping technique where hops are added to actively fermenting beer to maximize aroma extraction

Answer

A clarification method using isinglass or gelatin added during secondary fermentation

Answer

The process of adding a portion of unfermented wort to increase gravity after primary fermentation