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Beer faults - oxidation, DMS, diacetyl, autolysis

Even good beer can be faulty - and it is not a matter of taste, but of specific, recognisable faults with known chemical causes. Cardboard and staleness, cooked corn, butter, rubbery yeastiness - each of these notes is a signal that something went wrong at the stage of brewing, fermentation or storage. The ability to recognise beer faults is a mark of a mature palate and a key to conscious tasting. The four classic faults are oxidation, DMS, diacetyl and autolysis. Interestingly, some of them in small amounts can be acceptable in certain styles. Here is a guide to beer faults: where oxidation, DMS, diacetyl and autolysis come from, how to recognise them and when they are a real fault and when not.

Why know beer faults

Recognising faults is a higher level of tasting. It is not about spoiling your pleasure, but about understanding what you sense and why. A fault is not a matter of subjective „I do not like it”, but a specific, described defect with a known cause - chemical and process-related. When you learn to name them, you start to understand what went wrong: whether the beer is old, badly brewed or badly stored. It is knowledge that makes you a conscious taster and helps you choose better beers. It also lets you appreciate when a beer is clean and faultless. Understanding that a fault is a described defect, not a matter of taste, is the starting point. It is the language of beer quality. It is a tool of conscious drinking. We cover the fault from light more in skunky beer.

Oxidation - cardboard and staleness

The first great fault is oxidation. It forms when beer comes into contact with oxygen - during production, packaging or simply with age and storage in warmth. Oxygen reacts with compounds in the beer, giving characteristic notes: wet cardboard, paper, staleness, and in stronger beers sometimes notes of sherry or caramel. The beer loses freshness, the hop aroma fades, and the flavour becomes flat and stale. Oxidation is above all a fault of age - the longer and warmer the beer sits, the worse (with the exception of the few beers meant for ageing). Understanding that oxidation gives cardboard and staleness from oxygen and age is the key to this fault. It is the smell of old beer. It is a signal that freshness has passed.

DMS - cooked corn

The second fault is DMS, that is dimethyl sulfide. It gives a characteristic smell of cooked corn, cooked vegetables, and sometimes tinned tomato. DMS forms from the malt (from a precursor present especially in pale malt) and should be removed during a vigorous boil of the wort - it evaporates with the steam. If the boil was too short or the wort was cooled too slowly (leaving time for DMS to form again), the fault stays in the beer. Importantly, in some pale lagers a trace of DMS is considered normal, even typical. But clear cooked corn is a fault. Understanding that DMS is a note of corn from the malt and an error in the boil lets you recognise it. It is the smell of a vegetable can. It is a signal of a problem with the wort.

A table: four faults

Let us gather them in one place:

The table shows the four classic beer faults, their characteristic notes and sources. Each has a different cause and a different smell, which lets you tell them apart.

Diacetyl - butter and toffee

The third fault is diacetyl. It gives a clear smell and taste of butter, toffee or butterscotch, as well as a slick, oily mouthfeel. Diacetyl forms naturally during fermentation as a by-product of the yeast’s work. Normally the yeast then removes it itself, if given time - that is the so-called diacetyl rest. If the beer was racked too early, the yeast did not have time to clean up and the buttery note stays. It is a common fault of young, unfinished beer, especially lagers. Interestingly, in some British ales a trace of diacetyl is accepted as an element of the style. Understanding that diacetyl is butter from unfinished fermentation lets you recognise it. It is a buttery signal of haste. It is a fault the yeast could have prevented.

Autolysis - dead yeast

The fourth fault is autolysis. It is a rarer but very unpleasant fault, giving rubbery, yeasty, meaty notes, and in extreme cases almost like soy sauce or rotten egg. It forms when beer sits too long on the sediment of dead yeast, which begins to break down (autolysis is literally the self-digestion of the cells). Unpleasant compounds are then released. It is mainly a fault of home brewing or badly managed production - in commercial beer it is rare, because beer is racked off the sediment in time. Understanding that autolysis is a rubbery, meaty note from decomposing yeast lets you recognise it. It is the smell of spoiled yeast. It is a signal that the beer sat too long on the sediment.

When a fault, and when not

A key subtlety: not every one of these notes is always a fault. Much depends on the style and the intensity. A trace of DMS can be typical in pale lagers like a pilsner. Light diacetyl can be accepted, even desired, in some British ales or Czech lagers. Even delicate notes of oxidation can be part of the character of aged strong beers, like barley wine. A fault begins where the note is clear, out of place in the given style and breaks the balance. That is why assessment requires knowing the style. Understanding that the same note can be a fault or a feature depending on the style and intensity is a mark of a taster’s maturity. It is context over a rigid rule. It is the difference between a defect and a character.

How to practise recognising them

How to learn to catch faults? First, taste consciously and smell carefully - many faults give themselves away above all in the smell. Second, learn by contrasts: compare fresh beer with old (oxidation), or look for diacetyl in young, unfinished beer. Third, know the typical notes: cardboard, corn, butter, rubber - these are your four signposts. Fourth, note what you sense, to build sensory memory. In time, catching faults will become a reflex. It is not about spoiling beer for yourself, but about understanding it more deeply. Understanding that recognising faults can be learned by practice makes it accessible to everyone. It is training for the palate. We cover the durability and ageing of beer more in whether beer expires.

Other faults worth knowing

Beyond the big four there are other faults worth knowing. Acetaldehyde gives a note of green apple or fresh paint and latex - it is a sign of young, unfinished beer (the yeast has not yet turned it into alcohol). Bacterial infection gives an undesirable sourness, a vinegary or lactic note, when wild microbes got into the beer (unless it is a deliberate sour style). A metallic, bloody or inky note betrays a problem with the equipment or water. Astringency (drying, like after strong tea) can be the result of errors in the mash. Finally a musty, cellar-like note points to contamination. Knowing these faults broadens your tasting vocabulary beyond the four main ones. Understanding that there are more faults than the four classic ones makes you a more complete taster. It is a further map of beer defects. It is a step towards mastery in assessment.

The essentials in brief

Let us gather it up. The four classic beer faults are oxidation, DMS, diacetyl and autolysis. Oxidation (cardboard, staleness) comes from contact with oxygen, age and warmth. DMS (cooked corn) forms from the malt with too short a boil or slow cooling of the wort. Diacetyl (butter, toffee) is a product of fermentation that stays when the beer was racked too early, without a diacetyl rest. Autolysis (rubbery, meaty yeastiness) forms when beer sits too long on dead yeast. Crucially: some of these notes in small amounts can be typical or accepted in certain styles, so assessment requires knowing the style. Recognising faults can be learned by practice. Now you know the four main beer faults.

Note every beer in GustoNote - including the faults you sense and their intensity. In time you will learn to recognise oxidation, DMS, diacetyl and autolysis and to tell a fault from a feature of the style yourself.