in beer are the result of undesirable concentrations of flavor-active compounds. The expected flavor of an alcoholic beverage is a complex but fine balance of hundreds of different organic compounds created by raw materials, the action of yeast, and myriad other factors. More than 1,400 volatile compounds alone have been identified in beer. Many of these compounds may be normally present in beer in concentrations below or just above “taste threshold,” the point where they can be tasted or smelled by the human tongue or olfactory sense. When the concentration of one or more of these compounds increases significantly, nuances in taste and aroma differing from the norm may occur, causing flavor faults, or off-flavors, in the beer.

Beers, like other foods and beverages, are consumed with a certain expectation, based on a brand’s known flavor and reputation. Up until the point of consumption they should have a consistent flavor, remain stable, and be microbiologically sound. If this expectation is not met, the commercial implications can be significant. Off-flavors in beer made at home will be disappointing to the homebrewer, but off-flavors in commercial beer can spell disaster for a commercial brewery.

Whether a flavor is regarded as undesirable or an off-flavor will depend on several factors: (i) the beverage type or style, (ii) the sensitivity of the taster, and (iii) the consumer expectation. Social and ethnic practices will also influence whether a flavor is regarded as acceptable. Some flavor compounds are regarded as positive under certain circumstances but negative under others, whereas others such as “metallic” or chlorophenol are always considered unacceptable. The “clovelike” spiciness that is characteristic of a Bavarian weissbier would be considered a wild yeast contamination if found in another beer style such as a North American mass market pilsner.

The varying flavor components in beer differ in their importance as well in their concentration. Concentration alone, however, will not be the deciding factor as to the importance of the compounds because the individual thresholds of each compound vary so much. One compound, for example, may greatly influence beer flavor in concentrations below 1 part per billion (ppb), whereas others may be present in several hundreds of parts per million without being of any special importance. The concentration at which an individual compound becomes unacceptable depends very much on the background flavor of the product. An off-flavor will become apparent at lower concentrations in more lightly flavored beers. Diacetyl, a buttery-tasting compound, is a good example of a flavor that is acceptable at different levels in products. In lager beer it is generally regarded as an off-flavor when present at levels greater than 45 ppb, but when present at low levels in an English-style bitter it is considered acceptable. See diacetyl.

The human palate and nose are extremely sensitive, capable of detecting some undesirable notes such as the compound associated with “skunky” flavor in beer at the parts per trillion level. This aroma is referred to as “lightstruck” character because it is the result of photochemical reactions during improper storage, especially in green glass bottles, which offer little protection. However, the flavor the consumer associates with a particular beer may not be that which the brewer had originally intended. If the sales volume of beers sold in green glass bottles is any indication, some consumers would appear not to find the lightstruck flavor objectionable, although most brewery quality control departments would reject a beer showing such a flavor. See lightstruck.

Although much is known and well documented about the off-flavors associated with beer, research is ongoing and will continue to discover new off-flavors and their origins, together with new pathways and causes of previously identified off-flavors.

Morton Meilgaard, the famous brewing sensory scientist, attempted to classify beer flavors into five categories: desirable, desirable in small amounts, desirable in specialty beers, indifferent unless in excess, and undesirable. Those flavors in the fifth category contain defects that are undesirable at any concentration and beer containing these compounds above the taste threshold would certainly be considered off-flavored. They include moldy, metallic, worty, grainy, strawlike, woody, bready, papery, chlorophenol, rancid, oily, skunky, catty, and stale. In reviewing this list, one can identify many of the above-mentioned compounds as those that are associated with oxidation or staling. See oxidation.

The flavor scientist S. J. E. Bennett divided possible sources of off-flavors into six groups: (i) raw materials, (ii) a shortfall in process control, (iii) microbiological spoilage, (iv) packaging, (v) storage, and (vi) accidental contamination. The task of categorizing off-flavors themselves is not as simple as it might seem. An undesirable note may have several potential causes. Diacetyl, for example, may be the result of the individual yeast strain utilized by a brewery, insufficient beer maturation time, or, worse, microbial contamination. The good brewer, therefore, is constantly vigilant and should have a well-formed idea of exactly how the brewery’s beer should taste and smell.

See also faults in beer.