Off flavors in beer can create unpleasant experiences for beer drinkers and stray from a brewer’s desired flavor profile. In simple terms, a beer off flavor is any unintended flavor in the finished product. These off flavors can originate from raw ingredients, yeast and bacteria, or other contaminants in the brewing process. While some off flavors present themselves readily, others take time to develop in beer, which can make them harder to detect before they go to market. The good news is that they can often be prevented through proactive measures; good quality assurance practices can help catch them before they become a bigger problem.
The Brewers Association Quality Subcommittee understands that striking a careful balance between cost of goods, fermentation performance, production schedules, sales targets, and quality management can be quite the challenge. In order to make the process of detecting and managing beer off flavors a little easier, they have created convenient and quick fact sheets for a range of common beer off flavors.
Each fact sheet includes:
- The chemical makeup of each off flavor
- How it smells and tastes
- How it’s measured
- How it’s produced
- When it becomes an off flavor
- How it can be controlled
Acetaldehyde
Acetaldehyde is a naturally occurring compound in beer fermentation, but when the concentration goes above the sensory threshold, it becomes an off flavor reminiscent of green apples, pumpkin flesh, unripe avocado, and latex paint.
Acetic Acid
Acetic acid is an organic acid that is the product of various microorganisms’ metabolism. It is the primary acid in vinegar, giving it its distinctive aroma and assertive sour taste.
Butyric Acid
This compound is a product of the metabolism of long-chain fatty acids by some yeast and bacteria. It is found in foods such as butter and cheese. However, at high concentrations in beer, it is considered a particularly unpleasant off flavor, with an aroma of rancid butter or baby vomit.
Diacetyl
Vicinal diketones, or VDK, is the common umbrella name for the flavor compounds 2,3-butanedione, or diacetyl, which exhibits aromas of melted butter, buttered popcorn, margarine, or butterscotch, and 2,3-pentanedione, which is honey-like. Diacetyl production is inherent to all wort fermentations with yeast.
Dimethyl Sulfide (DMS)
Dimethyl sulfide (C2H6S) is commonly associated with a canned or creamed corn aroma when at high concentrations. It’s a naturally occurring flavor compound in beer that originates from a precursor found in barley. Ensuring a vigorous and sufficiently long boil is one way to prevent the DMS off flavor from manifesting in finished beer.
Hydrogen Sulfide
Hydrogen sulfide (H2S) is one of many sulfur compounds found in beer. At low levels it is part of the normal flavor and aroma profile of beer, especially lagers. Above its low sensory threshold, its distinct and unpleasant rotten egg aroma is dominant and considered an off flavor. Since H2S is a yeast-derived compound formed during fermentation, control of it hinges on yeast strain selection, wort composition, fermentation control, and yeast health.
Mercaptan
The term “mercaptan” is often associated with the unpleasant odor of garbage or sewage in beer. However, it also refers to a broader category of sulfur compounds, particularly thiols—an organic compound containing a sulfur atom bound to a hydrogen atom (R-SH). For this publication, the focus will be on methanethiol (CH₃SH), also known as methyl mercaptan.
Mousy
Tetrahydropyridines (THP) is the collective name for three organic compounds responsible for this mousy, or cereal-like, off flavor that is most commonly found in sour and mixed fermentation beer styles.
Onion/Garlic
Onion/garlic (O/G) is a general term used to describe the distinctive aromatic characteristic reminiscent of allium vegetables, which are naturally high in volatile organosulfur compounds. Organosulfur compounds are found in hops. Though O/G is not always undesirable depending on the beer style, there are a couple ways to avoid it as an off flavor.
Phenolic
This resource focuses on two major phenolic compounds originating from yeast metabolism, 4-vinylguaiacol (4VG) and 4-vinylphenol (4VP). 4VG exhibits clove, curry, spice, smoke, or bacon characteristics, and 4VP can be medicinal or spicy.
Sulfur Dioxide
Also known as “sulfites,” sulfur dioxide (SO2) is a byproduct of beer fermentation and therefore naturally occurring in beer. At high concentrations, however, it presents with a burnt or struck match aroma and may also smell like young white wine.
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