CFPS Beer Faults and Off-Flavors 1 — Questions and Answers
Question 1: What causes the 'skunky' or lightstruck off-flavor in beer, and which packaging type is most susceptible?
- Bacterial contamination; cans are most susceptible
- UV/visible light converting iso-alpha acids into 3-methyl-2-butene-1-thiol (MBT); clear and green bottles are most susceptible (Correct answer)
- High fermentation temperatures; kegs are most susceptible
- Oxidation during filling; bottles without oxygen absorbing caps
Correct 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.
Question 2: Which off-flavor, described as 'butterscotch' or 'movie theater popcorn,' indicates incomplete fermentation or inadequate yeast health?
- Acetaldehyde from immature fermentation
- Acetic acid from Acetobacter infection
- Diacetyl from vicinal diketone accumulation not reabsorbed by yeast (Correct answer)
- DMS from inadequate wort boiling
Correct 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.
Question 3: What is the primary chemical compound responsible for the 'wet cardboard' or 'papery' oxidation off-flavor in stale beer?
- Trans-2-nonenal formed from lipid oxidation during aging (Correct answer)
- Acetic acid formed by Acetobacter bacteria
- DMS formed from S-methylmethionine in malt
- Isoamyl acetate formed by yeast ester production
Correct 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.
Question 4: Which compound produces the 'green apple' or 'fresh-cut apple' off-flavor found most commonly in young or under-attenuated beer?
- Diacetyl
- DMS (dimethyl sulfide)
- Isoamyl acetate
- Acetaldehyde (Correct answer)
Correct answer: Acetaldehyde
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?
- Lactic acid bacterial infection during primary fermentation
- Conversion of S-methylmethionine (SMM) from malt, not driven off during an incomplete or insufficiently vigorous boil (Correct answer)
- Excessive sulfate additions to brewing water
- Extended contact with dry hops during conditioning
Correct 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.
Question 6: Which sensory descriptor indicates contamination by Brettanomyces wild yeast in an unintentionally infected beer?
- Sharp astringency and tannin puckering
- Medicinal chlorophenol Band-Aid aroma
- Barnyard, horse blanket, or 'wet leather' funky phenolic character (Correct answer)
- Metallic mineral taste on the finish
Correct 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.
CFPS Beer Faults and Off-Flavors 1
What causes the 'skunky' or lightstruck off-flavor in beer, and which packaging type is most susceptible?