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winemaking and other varietal aromas. Oxidized wines also display distinct aroma and color changes. The sherry aroma attributed to oxidized wines is due to formation of aldehydes from their respective alcohols. The most abundant aldehydes found in oxidized wines are acetaldehyde, methional, (Z)-2-nonenal, (E)-2-octenal, furnaneol, dodencanal, (Z)-whiskey lactone, furfural, 5-hydroxymethylfurfural, phenylaldehyde and o-aminoacetophenone.2 As these aromas overwhelm the fruity aromas over time, the wine's freshness is lost and fruit complexity decreases. Oxidation reactions can also lead to formation of yellow and brown pigments, and these undesirable color changes are also associated with the term oxidized in reference to wines.31 For example, thiols that contribute varietal character to Sauvignon Blanc and many red varieties are sensitive to oxidation.3,20 Similarly, élevage is described as "reductive" or "oxidative," depending upon exposure of the wine to oxygen. Here it is assumed that wines with deliberate oxygen exposure during élevage will display some positive effects of that exposure in polymerization of tannins, softening of mouthfeel and stabilization of color. In reductive élevage, wine is protected from 68 p r acti c al w i ne ry & v i n e yard O CTO BER 20 13 oxygen. Restriction of oxygen may be needed to prevent unwanted microbial activity. Enzymatic oxidation in juice and wine Enzymes known as oxidases can catalyze oxidation/reduction reactions and transfer of electrons among substrates. Grape tyrosinase, more commonly called polyphenol oxidase, is active in grape juice upon crushing of the fruit. 21,25,29,36 Polyphenol oxidase (PPO) activity (Figure 5) is the primary enzyme responsible for oxygen consumption by juices, and this enzyme consumes O2 at a rapid rate, particularly in the absence of SO2.7 Although non-enzymatic oxidation is possible in juice and must, the affinity of PPO for molecular oxygen effectively prevents chemical oxidation. Addition of SO2 will block enzymatic oxidation but can also block some forms of non-enzymatic oxidation. As a result of polyphenol oxidase activity in the juice, oxygen can be consumed nearly as rapidly as it is introduced. In grape juice, polyphenol oxidase catalyzes the oxidation of caftaric and coutaric acid to caftaric acid o-quinone. Caftaric acid o-quinone then undergoes coupled oxidations with other compo- nents of the juice such that caftaric acid o-quinone is reduced back to caftaric acid (Figure 6). Thus PPO generates reactive intermediates that then participate in subsequent electron transfer and coupling reactions. Polyphenol oxidase is classified as a mono-oxygenase. Mono-oxygenases are a group of enzymes that, by definition, catalyze reactions in a way that one of the two oxygen molecules of O2 is incorporated into the organic substrate and the other is reduced to water (Figure 5). PPO is a complex mono-oxygenase present in many fruits and vegetables including wine grapes.21 This class of enzymes requires two substrates to serve as reductants of the two oxygen atoms of O2. The main substrate accepts one of the two oxygen atoms, and a co-substrate furnishes hydrogen atoms to reduce the other oxygen atom to water (Figure 5). Specifically, PPO consists of a dinuclear copper center, which is able to have an oxygen atom inserted in the ortho position to an existing hydroxyl group in an aromatic ring, followed by oxidation of diphenol to the corresponding quinone.21 Browning in both juice and wine is the result of quinone formation and formation of substituted polymers that are brown in color. In the case of juice, qui-