Oxidation in technical terms refers to a large number of chemical reactions, including many involved in the brewing of fermented beverages. In brewing, however, it is most commonly used in brewing to refer to the creation of aldehydes through the oxidation of compounds in the wort, usually due to the introduction of oxygen at temperatures over about 80 degrees F, known as hot side aeration (HSA).
Oxidation results in musty or stale aromas and flavors often described as resembling cardboard or wet cardboard, paper, stale bread, or sometimes sherry.
 Oxidation in Beer
Cardboard-like oxidation flavors and aromas are not considered appropriate in any style. However, sherry-like oxidation character appears in most beers after long aging and is considered desirable in some stronger, aged beer styles such as Barleywine and Old Ale.
 Causes of Oxidation
The most significant cause of oxidation is hot-side aeration, which occurs when air is mixed into the wort at temperatures above about 80 degrees F. At that temperature, aldehydes are formed in the wort which are not degraded by later boiling and which remain in the finished beer. These aldehydes will gradually result in an oxidative character as the beer is stored.
Because some degree of oxidation is inevitable in any beer, beers aged for long periods will inevitably result in some oxidative character; in those beers, however, which are usually higher in alcohol, oxidation generally perceived as a "sherry" rather than a "cardboard" character and is not considered a flaw in many aged beer styles.
 Preventing Oxidation
The easiest way to avoid oxidation is to avoid hot-side aeration, by not splashing the wort while the temperature is above 80 degrees, during mashing, lautering, transfer, boiling, or cooling.
However, the degree of caution required is a matter of debate. Some have suggested that significant oxidation only occurs at commercial levels of production or with unusual amounts of splashing during home production.
 Creating MBT
A sherry-like oxidation character can be created in beer for tasting or judging testing or calibration with the addition of a small amount of dry sherry to the beer.