CFPS Off-Flavors and Quality Control 2 — Questions and Answers
Question 1: What is oxidation in beer and what flavor does it typically produce?
- Reaction of beer compounds with oxygen producing stale cardboard, sherry-like, or papery flavors, particularly in hop-forward beers (Correct answer)
- Reaction between CO2 and water reducing hop bitterness
- Fermentation byproduct producing butterscotch flavors
- Acid-base reaction between hops and water increasing pH
Correct answer: Reaction of beer compounds with oxygen producing stale cardboard, sherry-like, or papery flavors, particularly in hop-forward beers
Oxygen reacts with melanoidins, alcohols, and other beer compounds (especially trans-2-nonenal) to produce stale flavors ranging from cardboard/papery to sherry-like depending on the degree and timing of exposure.
Question 2: A 'catty' or 'black currant' aroma in beer is most commonly caused by:
- 4-mercapto-4-methylpentan-2-one (4MMP), a thiol from certain hop varieties or oxidative stress (Correct answer)
- Geraniol from dry hopping with New Zealand hops
- Ethyl acetate from high fermentation temperatures
- Phenylethanol from Belgian yeast producing rose-like notes
Correct answer: 4-mercapto-4-methylpentan-2-one (4MMP), a thiol from certain hop varieties or oxidative stress
4MMP is a potent thiol compound naturally present in some hop varieties (e.g., Nelson Sauvin) or formed through oxidative degradation, detectable at extremely low concentrations with a cat urine or black currant character.
Question 3: Polyphenol (tannin) astringency in beer is best described as:
- A drying, puckering, gripping sensation in the mouth caused by polyphenols binding to salivary proteins (Correct answer)
- A sharp acidic bite from lactic or acetic acid
- A harsh bitter taste from over-bittered hop addition
- A sulfury, eggy smell from yeast stress
Correct answer: A drying, puckering, gripping sensation in the mouth caused by polyphenols binding to salivary proteins
Tannins and polyphenols from grain husks or hops precipitate salivary proteins, reducing lubrication and causing the characteristic drying, gripping mouthfeel associated with over-sparging or overboiling.
Question 4: What causes 'gushing' in a bottled beer when it is opened?
- Overcarbonation from residual fermentable sugars, infection with wild yeast/bacteria, or fungal-derived proteins acting as nucleation sites (Correct answer)
- Undercarbonation causing CO2 to escape rapidly
- Excessive hop oils rising to the surface
- Autolysis of yeast creating gas pockets
Correct answer: Overcarbonation from residual fermentable sugars, infection with wild yeast/bacteria, or fungal-derived proteins acting as nucleation sites
Gushing occurs when excess CO2 comes out of solution rapidly due to overcarbonation, nucleation sites from infection, or hydrophobic proteins from Fusarium malt fungi that trigger foam formation.
Question 5: The off-flavor 'isoamyl acetate' at high levels smells like:
- Banana candy or nail polish remover (Correct answer)
- Coconut and vanilla
- Sulfur and rotten eggs
- Vinegar and acetic acid
Correct answer: Banana candy or nail polish remover
Isoamyl acetate is a fruity ester naturally produced by ale yeasts; at appropriate levels it contributes pleasant banana notes (desired in Hefeweizen), but excessive amounts smell like artificial banana candy or solvent.
Question 6: A 'cheesy', 'goat-like', or 'rancid' off-flavor in beer is typically caused by:
- Isovaleric acid, produced by degradation of hop compounds or bacterial fermentation of leucine (Correct answer)
- Sulfur compounds from yeast autolysis
- Excess isoamyl acetate from high fermentation temperature
- Residual DMS from slow wort chilling
Correct answer: Isovaleric acid, produced by degradation of hop compounds or bacterial fermentation of leucine
Isovaleric acid is produced when hops are poorly stored and the acids degrade, or when certain bacteria ferment the amino acid leucine — both produce the characteristic cheesy, sweaty feet, or rancid aroma.
CFPS Off-Flavors and Quality Control 2
What is oxidation in beer and what flavor does it typically produce?