CFPS Off-Flavors and Quality Control

Question 1

What aroma is associated with acetaldehyde in beer, and what causes it?

Accepted Answer

Green apple or fresh-cut grass, caused by incomplete fermentation when yeast haven't fully reduced it to ethanol

Answer

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:

Accepted Answer

3-methylbut-2-ene-1-thiol (MBT)

Answer

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)?

Accepted Answer

A cooked or vegetal character from DMS, which forms from S-methylmethionine (SMM) during wort production

Answer

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:

Accepted Answer

Vinegar or nail polish remover

Answer

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?

Accepted Answer

Chlorophenols formed by wild yeast (especially Brettanomyces) or the reaction of chlorine sanitizers with phenolic compounds

Answer

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.

CFPS - Certified Cicerone Practice Test

CFPS - Certified Cicerone Practice Test

CFPS Off-Flavors and Quality Control