Beer Flavor and Evaluation: Compounds, Faults and the Scoresheet
Beer Flavor and Evaluation
This domain tests whether you can name what you smell, decide whether it belongs in the style, and write it up in the language of a scoresheet. Advanced-level questions link a compound to its descriptor, its cause and the process fix - not just 'this is off'.
Compound-to-descriptor map
- Diacetyl (2,3-butanedione): butter, butterscotch, slick mouthfeel. Caused by packaging before yeast reabsorbed alpha-acetolactate, low pitch rate, or Pediococcus.
- Acetaldehyde: green apple, cut pumpkin. Incomplete fermentation or oxygen pickup.
- DMS: cooked corn, tomato. From S-methyl methionine in malt; driven off by a vigorous, open boil.
- trans-2-Nonenal: cardboard, papery. Lipid oxidation during storage.
- 3-Methyl-2-butene-1-thiol: skunky. Lightstruck, from iso-alpha-acids.
- Isovaleric acid: cheesy, sweaty. Oxidised or aged hops.
- 4-Vinyl guaiacol: clove, phenolic. Normal in weizen and some Belgian styles; a fault elsewhere.
- Chlorophenol: band-aid, medicinal. Chlorinated brewing water or sanitizer residue.
- Isoamyl acetate and ethyl acetate: banana and solvent. Esters from warm fermentation and low pitch rates.
- Hydrogen sulfide: rotten egg. Yeast stress; usually clears with lagering.
How to evaluate
- Work in order: appearance, aroma, flavor, mouthfeel, overall impression. Note intensity and whether the character is appropriate to the declared style.
- Warm the sample: volatiles release as temperature rises, so faults hide in cold beer.
- Calibrate with spiked samples; individual thresholds vary widely.
Worked example
A lager is clean when fresh but smells of buttered popcorn a week after kegging, with a slick finish. The fault appears after packaging, so it is not a contamination from the tap line. This is diacetyl re-forming from alpha-acetolactate that was still present when the beer was chilled or filtered - yeast was removed before it could reabsorb the precursor. The fix is a diacetyl rest at 1-2 C above fermentation temperature, held until a forced test shows clean, before crashing.
Traps that cost marks
- Calling DMS a corn-adjunct fault; the precursor comes from malt, and adjunct use only deepens it.
- Using 'skunky' for oxidised beer; those are different compounds and different causes.
- Penalising style-appropriate character - clove in a weizen, or diacetyl's creamy note in some English ales.
- Reporting a fault without stating its cause or the process correction.