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A Guide to Identifying Common Beer Faults

A beer fault, also known as a beer flaw or beer defect, refers to an undesirable characteristic or quality found in a beer that deviates from the expected or desired flavour, aroma, appearance, or mouthfeel. These faults can occur at various stages of the brewing process, from the ingredients used to fermentation, packaging, and storage. Beer faults can significantly impact the overall drinking experience and may render a beer undrinkable or unenjoyable.

Recognizing these faults can help you identify and avoid them. Here are five common beer faults and how to recognize them:


Recognize: Oxidized beer often has a cardboard-like or stale flavour and aroma. It may also exhibit a noticeable loss of hop aroma and flavour.

Causes: Exposure to oxygen during brewing, packaging, or storage.


Recognize: Diacetyl produces a buttery or butterscotch-like aroma and flavour. It can also give beer a slick or oily mouthfeel.

Causes: Yeast byproducts or bacterial contamination, often a result of incomplete fermentation or poor yeast health.


Recognize: Acetaldehyde imparts a green apple or freshly cut pumpkin aroma and flavour to beer.

Causes: Incomplete fermentation or premature removal of yeast from the beer, or bacterial contamination.

Light Struck (Skunky Beer):

Recognize: Light-struck beer develops a strong skunky odour and flavour when exposed to ultraviolet (UV) light, similar to the smell of a skunk.

Causes: The reaction between hops and UV light, typically from direct sunlight or fluorescent light, results in this off-flavor.

Acetic Acid (Infected Beer):

Recognize: Infected beer can display a wide range of off-flavours, including sourness, funkiness, or vinegar-like qualities. It may also develop off-putting aromas.

Causes: Bacterial or wild yeast contamination, often due to improper sanitation practices.

To recognize these faults, you can follow these general steps:

Visual Inspection: Check the beer’s appearance for clarity, colour, and any visible sediment or particles.

Aroma: Swirl the beer gently in the glass and take a sniff. Note any unusual or off-putting aromas.

Taste: Take a sip and let it cover your palate. Pay attention to any unusual or undesirable flavours.

Mouthfeel: Assess the beer’s texture and mouthfeel. Some faults may affect the beer’s body or carbonation.

Finish: Note the aftertaste and how long any off-flavours linger on your palate.

Identifying and understanding beer faults is essential for brewers to produce high-quality beer, and it’s also important for consumers to identify and avoid beers with faulty beers to ensure an enjoyable drinking experience. Homebrewers and commercial brewers take measures to prevent these faults through proper brewing practices, quality control, and careful handling of ingredients and equipment.