Beer Faults and Off-Flavors Flashcards
6 cards from real CFPS practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Beer Faults and Off-Flavors flashcards as text
What causes the 'skunky' or lightstruck off-flavor in beer, and which packaging type is most susceptible?
Answer: UV/visible light converting iso-alpha acids into 3-methyl-2-butene-1-thiol (MBT); clear and green bottles are most susceptible
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.
Which off-flavor, described as 'butterscotch' or 'movie theater popcorn,' indicates incomplete fermentation or inadequate yeast health?
Answer: Diacetyl from vicinal diketone accumulation not reabsorbed by yeast
Diacetyl (a vicinal diketone) is a normal fermentation byproduct that healthy yeast reabsorb during conditioning; residual diacetyl indicates rushed or stressed fermentation.
What is the primary chemical compound responsible for the 'wet cardboard' or 'papery' oxidation off-flavor in stale beer?
Answer: Trans-2-nonenal formed from lipid oxidation during aging
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.
Which compound produces the 'green apple' or 'fresh-cut apple' off-flavor found most commonly in young or under-attenuated beer?
Answer: Acetaldehyde
Acetaldehyde is an intermediate fermentation product between sugar and ethanol that yeast normally convert fully; elevated levels indicate incomplete fermentation.
What causes DMS (dimethyl sulfide) in beer, creating an aroma reminiscent of cooked corn or vegetables?
Answer: Conversion of S-methylmethionine (SMM) from malt, not driven off during an incomplete or insufficiently vigorous boil
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.
Which sensory descriptor indicates contamination by Brettanomyces wild yeast in an unintentionally infected beer?
Answer: Barnyard, horse blanket, or 'wet leather' funky phenolic character
Brettanomyces produces 4-ethylphenol and 4-ethylguaiacol, which create distinctive barnyard, leathery, and funky aromas that are a fault in most styles but desirable in intentionally wild-fermented beers.