Oxidation

oxidation, the opposite of reduction, is the chemical reaction in which a chemical compound loses electrons. Although controlled and moderate oxidation can be beneficial and is essential for some wine styles (see below), the term usually refers to a wine fault resulting from excessive exposure to oxygen (as opposed to aeration, which is deliberate, controlled exposure to oxygen). Wines spoiled by oxidation are said to be oxidized. See also oxidative winemaking.

Oxidation is a threat as soon as the grape is crushed, which is why high-quality grapes are transported to the winery as fast as possible in shallow containers, and why field pressing stations sited as close as possible to the vineyard are increasingly common. When the grape is crushed, unless special precautions are taken to exclude oxygen, it immediately starts to react with the liberated juice compounds. The most obvious change is the browning of the juice resulting from the oxidation of phenolics catalysed by an enzyme (polyphenol oxidase) present in grapes and thus referred to as enzymatic oxidation or enzymatic browning. The presence on the grapes of moulds associated with rot introduces additional oxidative enzymes (laccases) which accelerate reactions with oxygen, especially those involved with browning. Small amounts of sulfur dioxide (5 g/hl) are therefore usually added to the must to inactivate enzymes and counter the oxidation of phenolics. See protective juice handling for the techniques involved in minimizing the risk of oxidation.