CFPS Off-Flavors and Quality Control 1 — Questions and Answers
Question 1: What aroma is associated with acetaldehyde in beer, and what causes it?
- Green apple or fresh-cut grass, caused by incomplete fermentation when yeast haven't fully reduced it to ethanol (Correct answer)
- Buttery butterscotch from bacterial contamination
- Vinegar sharpness from acetic acid production
- Rotten egg smell from hydrogen sulfide
Correct answer: Green apple or fresh-cut grass, caused by incomplete fermentation when yeast haven't fully reduced it to ethanol
Acetaldehyde is an intermediate in yeast fermentation; its green apple character indicates fermentation was cut short or yeast were stressed before completing the reduction to ethanol.
Question 2: The compound responsible for a 'skunky' or 'lightstruck' aroma in beer is:
- 3-methylbut-2-ene-1-thiol (MBT) (Correct answer)
- Diacetyl
- Dimethyl sulfide (DMS)
- Isoamyl acetate
Correct answer: 3-methylbut-2-ene-1-thiol (MBT)
MBT is produced when UV or blue-spectrum visible light breaks down iso-alpha acids from hops, generating a potent thiol compound detectable at concentrations as low as 4 parts per trillion.
Question 3: What off-flavor is described as 'cooked vegetables, creamed corn, or cabbage' and is caused by dimethyl sulfide (DMS)?
- A cooked or vegetal character from DMS, which forms from S-methylmethionine (SMM) during wort production (Correct answer)
- A roasty, acrid character from over-kilned malt
- A cheesy, goaty character from isovaleric acid
- A medicinal plastic character from chlorophenols
Correct answer: A cooked or vegetal character from DMS, which forms from S-methylmethionine (SMM) during wort production
DMS originates from SMM, a precursor present in pale malts that converts to DMS during wort boiling; a vigorous, uncovered boil drives off DMS, while poor boil vigor or slow chilling allows it to accumulate.
Question 4: Acetic acid off-flavor in beer smells and tastes like:
- Vinegar or nail polish remover (Correct answer)
- Rotten eggs
- Buttered popcorn
- Band-Aid or medicinal
Correct answer: Vinegar or nail polish remover
Acetic acid (vinegar) is produced by Acetobacter bacteria converting ethanol to acetate in the presence of oxygen, and is a common spoilage defect in inadequately protected beers.
Question 5: What is the cause of a 'Band-Aid' or 'medicinal' off-flavor in beer?
- Chlorophenols formed by wild yeast (especially Brettanomyces) or the reaction of chlorine sanitizers with phenolic compounds (Correct answer)
- Excessive lactic acid from LAB contamination
- High levels of carbonation creating an astringent texture
- Autolysis of dead yeast releasing sulfur compounds
Correct answer: Chlorophenols formed by wild yeast (especially Brettanomyces) or the reaction of chlorine sanitizers with phenolic compounds
4-chlorophenol and related compounds form when chlorine-based sanitizers react with phenolic precursors in wort/beer, or when Brettanomyces produces vinyl phenol compounds with a medicinal, Band-Aid character.
Question 6: Which sensory threshold concept explains why a trained taster can detect an off-flavor that most drinkers miss?
- Flavor threshold — trained tasters develop lower detection thresholds and can identify off-flavors at sub-threshold levels for average consumers (Correct answer)
- Flavor fatigue — constant exposure to off-flavors makes the average drinker immune
- Retronasal olfaction — trained tasters use a different sensory pathway
- Capsaicin suppression — spicy food masks off-flavor detection
Correct answer: Flavor threshold — trained tasters develop lower detection thresholds and can identify off-flavors at sub-threshold levels for average consumers
Regular sensory training lowers a taster's detection threshold for specific compounds through neural sensitization — trained Cicerones can identify off-flavors at concentrations undetectable to untrained palates.
CFPS Off-Flavors and Quality Control 1
What aroma is associated with acetaldehyde in beer, and what causes it?