Beer off flavours, and what causes them

Why this is the most valuable skill in a brewery

Recipe design gets the attention. Quality control keeps the business alive.

A brewery that cannot reliably detect its own faults will ship inconsistent beer, and inconsistency loses customers faster than a beer being merely uninteresting. The brewers who become indispensable are the ones who can taste a tank, name the problem and point at the step that caused it.

Almost every fault below is preventable with process control rather than better ingredients.

Diacetyl: butter, butterscotch, slick

What you notice. Butter or butterscotch on the nose, and a slick, almost oily film across the tongue. At low levels it reads as a rounded sweetness rather than anything obviously wrong.

Where it comes from. Yeast produces a precursor during fermentation and then normally reabsorbs and reduces it. Diacetyl in finished beer usually means the yeast was not given the chance to finish that job, because fermentation was cooled too early, the yeast was unhealthy, or the beer was pulled off the yeast too soon. It can also come from a bacterial infection, which is the worse diagnosis.

The fix. A diacetyl rest, holding the beer a few degrees warmer near the end of fermentation, lets the yeast clean up. Do not crash cool until the beer tests clean. If it recurs across batches with good practice, investigate infection.

A small amount is acceptable, even traditional, in some English ales and Czech lagers. In a crisp modern lager it is a fault.

DMS: cooked sweetcorn, cabbage

What you notice. Cooked or tinned sweetcorn, sometimes cabbage or tomato.

Where it comes from. A malt derived precursor converts to dimethyl sulphide with heat, and DMS is volatile, so a vigorous open boil drives it off. It survives when the boil is too gentle, too short, or lidded, and it re-forms if hot wort sits around slowly cooling after the boil.

The fix. A strong rolling boil with the lid off, and rapid cooling afterwards. The gap between the end of the boil and pitching temperature is where a lot of DMS is created. Pale lagers show it most, because there is nothing to hide behind.

Acetaldehyde: green apple, emulsion paint

What you notice. Fresh green apple, sometimes a solvent edge like emulsion paint.

Where it comes from. It is an intermediate on the way to ethanol. Its presence generally means the beer is simply not finished, or the yeast was stressed and gave up early.

The fix. Patience, adequate yeast pitching rates, proper aeration of the wort before pitching, and stable fermentation temperature. Most acetaldehyde complaints are really a packaging schedule that ran ahead of the biology.

Oxidation: wet cardboard, sherry, stale

What you notice. Wet cardboard or paper in pale beers, a sherry or dried fruit note in stronger dark ones, and a general flattening of hop aroma.

Where it comes from. Oxygen picked up after fermentation, at transfer, filtration or packaging. Warm storage accelerates it dramatically.

The fix. This one cannot be corrected once it has happened, so it must be prevented. Purge tanks, kegs and cans, minimise splashing on transfer, fill without foaming, and control the cold chain from brewery to glass. In Indian conditions the cold chain is usually the weakest link, and a hoppy beer that sat warm in transit will arrive stale no matter how well it was brewed.

Phenolic, lightstruck and astringent

Phenolic. Clove, smoke, plastic or medicinal notes. Clove is correct and desirable in a hefeweizen, where it comes from the yeast strain. Plastic or medicinal character elsewhere usually points at chlorine in the brewing water reacting with phenols, or at wild yeast. Filter or treat your water and the problem generally disappears.

Lightstruck. A distinct skunky aroma, caused by light striking hop compounds. It develops in minutes in clear or green glass. Cans and brown bottles prevent it entirely, which is why so many careful brewers have moved to cans.

Astringency. A drying, puckering grip rather than a flavour. Caused by over milling the grain, sparging too hot or for too long, or pushing the mash pH too high, all of which extract tannin from the husk. Watch your sparge temperature and stop collecting wort earlier than you think.

Building a sensory programme that actually works

You do not need a laboratory. You need consistency.

Taste every batch at the same points in the process, with the same small panel of people, using the same vocabulary. Train that panel on spiked samples so everyone means the same thing by the same word. Record results, because the value appears in the trend rather than in any single tasting.

Then close the loop. A panel that identifies diacetyl but never changes the fermentation schedule is an expensive ritual. The point of tasting is to change what you do next.

Common questions

Is diacetyl always a fault?

No. A low level is traditional in some English ales and Czech lagers. In a clean modern lager or a hop forward beer it is a defect and usually indicates fermentation was cut short.

Why does my beer taste like wet cardboard?

Oxidation. Oxygen was picked up after fermentation, most often at transfer or packaging, and warm storage sped it up. It cannot be reversed, so the answer is prevention and a tighter cold chain.

Can off flavours be fixed after packaging?

Almost never. Diacetyl and acetaldehyde can sometimes be reduced before packaging if the yeast is still present and active. Oxidation, lightstruck character and infection cannot be undone.

What causes the skunky smell in some imported beers?

Light. Hop compounds react within minutes of exposure through clear or green glass. Brown glass helps and cans prevent it completely.