CFPS Beer Faults and Off-Flavors

Question 1

What causes the 'skunky' or lightstruck off-flavor in beer, and which packaging type is most susceptible?

Accepted Answer

UV/visible light converting iso-alpha acids into 3-methyl-2-butene-1-thiol (MBT); clear and green bottles are most susceptible

Answer

Light breaks down iso-alpha acids in the presence of sulfur compounds, creating MBT — the same mercaptan found in skunk spray; brown glass provides much better UV protection.

Question 2

Which off-flavor, described as 'butterscotch' or 'movie theater popcorn,' indicates incomplete fermentation or inadequate yeast health?

Accepted Answer

Diacetyl from vicinal diketone accumulation not reabsorbed by yeast

Answer

Diacetyl (a vicinal diketone) is a normal fermentation byproduct that healthy yeast reabsorb during conditioning; residual diacetyl indicates rushed or stressed fermentation.

Question 3

What is the primary chemical compound responsible for the 'wet cardboard' or 'papery' oxidation off-flavor in stale beer?

Accepted Answer

Trans-2-nonenal formed from lipid oxidation during aging

Answer

Trans-2-nonenal is an aldehyde produced when unsaturated fatty acids oxidize; it has a very low flavor threshold and is the signature compound of stale, oxidized beer.

Question 4

Which compound produces the 'green apple' or 'fresh-cut apple' off-flavor found most commonly in young or under-attenuated beer?

Accepted Answer

Acetaldehyde

Answer

Acetaldehyde is an intermediate fermentation product between sugar and ethanol that yeast normally convert fully; elevated levels indicate incomplete fermentation.

Question 5

What causes DMS (dimethyl sulfide) in beer, creating an aroma reminiscent of cooked corn or vegetables?

Accepted Answer

Conversion of S-methylmethionine (SMM) from malt, not driven off during an incomplete or insufficiently vigorous boil

Answer

SMM in malt converts to DMS throughout the brewing process; a vigorous, uncovered boil and rapid wort chilling are essential to drive off and prevent DMS formation.

CFPS - Certified Cicerone Practice Test

CFPS - Certified Cicerone Practice Test

CFPS Beer Faults and Off-Flavors