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

Bacterial contamination; cans are most susceptible

Answer

High fermentation temperatures; kegs are most susceptible

Answer

Oxidation during filling; bottles without oxygen absorbing caps

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

Acetaldehyde from immature fermentation

Answer

Acetic acid from Acetobacter infection

Answer

DMS from inadequate wort boiling

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

Acetic acid formed by Acetobacter bacteria

Answer

DMS formed from S-methylmethionine in malt

Answer

Isoamyl acetate formed by yeast ester production

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

Diacetyl

Answer

DMS (dimethyl sulfide)

Answer

Isoamyl acetate

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

Lactic acid bacterial infection during primary fermentation

Answer

Excessive sulfate additions to brewing water

Answer

Extended contact with dry hops during conditioning

Question 6

Which sensory descriptor indicates contamination by Brettanomyces wild yeast in an unintentionally infected beer?

Accepted Answer

Barnyard, horse blanket, or 'wet leather' funky phenolic character

Answer

Sharp astringency and tannin puckering

Answer

Medicinal chlorophenol Band-Aid aroma

Answer

Metallic mineral taste on the finish