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Naturally occurring enzymes that react with molecular oxygen as hydrogen acceptors and can transfer two or four hydrogen electrons to oxygen. The phenol oxidases in particular are responsible for the discolouration or browning of the must in the event of undesired oxidation during winemaking. The grape's own species tyrosinase is largely removed during clarification. The species laccase, which occurs in wet weather and grape rot as well as in botrytis, remains in larger quantities and is also resistant to sulphur dioxide. This can cause the wine defect brown break. Oxidases are very sensitive to heat and are destroyed when the mash is heated. See also under reduction.

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