Off-Flavours: Each One Names a Mistake

FUSEL ALCOHOLS — hot, solventy, like nail varnish, and a headache afterwards. Fermented too warm, or under-pitched so the yeast grew too hard. Permanent. ESTERS — fruity, banana, pear drops. Warm fermentation again, and also under-pitching. Desirable in some styles and a fault in others, which is the clearest example of temperature as a dial rather than a mistake. PHENOLS — clove, smoke, medicinal, plastic. Some yeasts make them deliberately (Belgian, wheat beer strains); in a strain that should not, they mean wild yeast contamination or chlorine in the water reacting with yeast compounds. The chlorine version is fixed by using filtered water or a campden tablet, and it is far commoner than people expect.

DIACETYL — butter, butterscotch, a slick feel. It is produced by every fermentation and reabsorbed by healthy yeast afterwards. Tasting it in a finished beer means the beer was taken off the yeast too early, or chilled before the yeast had finished. ACETALDEHYDE — green apple, fresh-cut pumpkin. The intermediate between sugar and alcohol, also reabsorbed. Same cause, same cure: leave it on the yeast, warm, for a few more days. Both are the argument for a diacetyl rest and for patience at the end. They are also the two faults most likely to be tasted in an over-hurried batch, and the only two on this list that time reliably fixes.

SOUR, tart, lactic — lactobacillus or pediococcus. Wanted in a sour beer, a contamination anywhere else, and it usually comes from a scratched plastic vessel or a hose that was not replaced. VINEGAR — acetic acid bacteria, which need oxygen. That points straight at the oxygen rung: a slack lid, a long half-empty vessel, or splashing during racking. A RING of growth at the liquid surface, or a white film, is a wild yeast or a pellicle. Some are harmless and some are not, but either way the vessel and every hose that touched the batch need replacing rather than cleaning, because plastic does not come clean once it is scratched.

Learning to name these takes reference points. Commercial off-flavour kits exist and are worth doing once; failing that, tasting a known-good commercial example of the style alongside your own is a control in the experimental sense. Taste at serving temperature, in a glass that concentrates the aroma, and smell before you sip. Most of these are aromas rather than tastes, and a cold sample in a straight glass hides nearly all of them. Write it down against the batch's log — its temperature record, its pitching rate, its timings. A named off-flavour next to the number that caused it is the whole point, and it is how the next batch is better rather than just different.