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winemaking O OXYGEN IN WINEMAKING Effect of aeration treatments prior to/during fermentation 2 BY Linda F. Bisson, Lauren A. Schwartzburg, Andrew L. Waterhouse, Department of Viticulture & Enology, UC Davis present and not available for chemical reactions or enzymatic oxidation. The use of sulfur dioxide to inhibit grape polyphenol oxidases further olecular oxygen and oxidative reduces the competitiveness of enzyconditions influence the commatic oxidation for oxygen introduced position and quality of a wine prior to and during fermentation. Oxyand therefore play important gen will be consumed by organisms in roles during the entire winemaking the juice and must even in the absence of process.16,26 direct inoculation. Oxygen can participate directly in Many winemakers try to introduce chemical redox reactions, function as a oxygen in the early stages of red wine fersubstrate for grape and microbial oximentation on the skins with the hope that dases, stimulate phenolic compound tannin formation and structure will be polymerization and influence microbial modified, but most of the oxygen will be metabolism, survivability and end prodconsumed biologically. Micro-oxygenuct formation. ation during wine aging, assuming the From a quality perspective, oxygen can wine is not held on biologically active have both positive and negative impacts yeast lees, may in contrast lead to chemion juice and wine composition. Some oxical changes in the wine. dation reactions add complexity to the In white wine production, introduction wine, others lead to off-character or offof oxygen early in processing can lead to color formation. Similarly, the presence off-color formation in the juice and resultof oxygen stimulates the metabolic activiing wine. Hyper-oxygenation may be ties of both desirable and undesirable performed with the goal of converting microbes. Oxygen exposure may occur at browning potential into brown products any point in wine production, but the that can then be removed from the wine, consequences of that exposure differ. but this is not always successful. If popuOxygen exposure occurs during normal lations of microorganisms are high, the winemaking operations such as press-toadded oxygen may be consumed biologitank and tank-to-tank transfers. Oxygen cally. may also be introduced with cap manageAlthough the impact of oxygen in wine Figure 2: Oxygen Reduction Steps ment and extraction procedures such as has been studied intensively,6,12,13,14,18,26,29,33 the role of oxygen treatments prior to pump over, cap irrigation or délestage. élevage have been less well delineated. However, if microbial activity is occurThere are three primary fates of oxygen ring, the oxygen introduced will most introduced to the winemaking process: likely be consumed by the organisms non-enzymatic oxidation, enzy– – – +e +e +e +e– matic oxidation O2 2OH – O2.– OH O2 and microbial conPeroxide Hydroxyl Superoxide sumption. radical The most active +H+ +H+ +H+ consumption of oxygen in fresh H2O2 HO2. OH 2 At wine pH juice with a low microbial bioload Hydroperoxyl Hydrogen peroxide Water is via the phenol radical oxidases. In must, given the higher Figure 2: Oxygen reduction steps. microbial popula- M Relative Values Oxygen Consumption Comparison Juice Enzymatic PPO Wine Must Yeast/Microbial Metabolism Non-enzymatic chemical oxidation Figure 1: Fates of oxygen during winemaking. tions, competition of microbial activity with enzymatic oxidation occurs. Once microbial populations build in size, the most active consumption switches to microbial metabolism. In finished wine, grape oxidases are made inactive by the ethanol produced by the microorganisms, and the microbes are no longer active so the principal consumption of oxygen is non-enzymatic (Figure 1). Mold-produced laccase is a phenol oxidase that retains activity in the presence of ethanol, so mold-infected fruit will also display active enzymatic oxidation in the wine. Enzymatic oxidation is generally more of a concern in juices and musts than in wines. In addition, wine and juice harbor different microbiota, so the specific microbial responses to addition of oxygen will vary for wine and juice/must. Processing decisions can also impact wine and juice oxidation. Barrel fermentation and/or élevage contribute oak wood ellagitannins to wine. These compounds are able to participate in oxidation/reduction reactions.32 During fermentation, oxygen plays an important role as a yeast nutrient, and actively fermenting cultures consume oxygen efficiently blocking both enzymatic and chemical oxidation.8 Both biological and chemical impacts of oxygen are due to its ability to function as an oxidant in oxidation/reduction reactions. Here we will address oxidation reactions and outcomes of enzymatically catalyzed oxidation. Oxygen and non-enzymatic or chemical oxidation Oxidation/reduction or redox reactions pr actica l win ery & vin eya r d O CTO B ER 20 13 65