Staling, a complex set of organic chemical changes that occur in beer over time, transforming its flavor and causing it to diverge from the desired and expected flavor and appearance. Staling, it should be noted, is a different (if related) phenomenon than “aging,” which describes intentional and desirable flavor changes over time. See aging of beer. Although the word “stale” is nowadays always pejorative, this was not always so. In 19th-century England, the word “stale” was often synonymous with “old,” and the flavor of older beer, sometimes including acidity, was widely considered superior. For most of history, brewers have largely been concerned with the prevention of beer spoilage by microorganisms. Now that microbiological concerns have largely been conquered by modern technology and sanitation, brewers focus on shelf life, which can be defined as stability of flavor, aroma, and appearance.

Many consumers may not recognize the flavor of stale beer. However, most do recognize the normal flavor of their favorite beers and those flavors are usually the flavors of fresh beer. As beer ages, it develops new flavors and the fresh flavors diminish. Most of the changes that occur as fresh beer becomes stale involve oxygen, which is one reason why brewers attempt to avoid oxidation throughout the brewing process. Among the first and most prominent attributes of beer staling is the development of cardboardlike, sherrylike, and/or black currantlike flavors. Caramel flavors may develop in pale beers, even those that did not include any caramelized malts in the brewing process. As beer stales, it becomes darker in color, a change that will be particularly notable in pale beers such as pilsner. Hazes or precipitates will also eventually tend to develop in filtered beers. See haze.

Much attention has been concentrated on the development of the oxidative compound (E)-2-nonenal, which has a strong damp cardboard aroma. However, many other compounds contribute to the flavor of stale beer, particularly a large range of aldehydes. See aldehydes and (e)-2-nonenal. Maillard reaction products result in sweet, bready, toffeeish, and winelike flavors in stale beers. Controversially, it has been suggested that slight staleness may be partly responsible for the desirable depth of malt character seen in certain beer styles, particularly German doppelbock. Many consumers associate skunklike flavors with beer staling, but this is technically a form of damage by light and will not occur in beer kept under dark conditions. See lightstruck. This is an understandable misconception, especially because beer that has become stale may also have been exposed to light for long periods of time. In beers with strong hop aromatics, for example, India pale ales, the hop aromatics eventually degrade from bright, fresh floral and citrus aromas to duller tealike aromas.

Beer staling is slowed by colder temperatures and accelerated by warmer temperatures. The nature of the staling is itself temperature dependent; beer that is quickly staled at warm temperatures tastes different than beer that slowly stales at cold temperatures. This means that laboratory “forcing” tests, in which beer is stored at elevated temperatures to mimic the effects of age, can give an indication of a beer’s shelf life, but not necessarily how the beer will taste when it becomes stale.

The consumer’s best protection against buying or serving stale beer is the information hopefully provided by the brewer in the form of a bottling date or, better yet, a “best-before” date. The former will give the consumer an idea of whether the beer can be expected to be fresh, and the latter will involve the brewer’s best determination as to how long the beer will look and taste as it should.

See also faults in beer, off-flavors, and oxidation.