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winemaking cell biomass due to the ability to synthesize an array of cellular building blocks that depend upon oxygen as a catalyst.5 In a recent experiment, we examined the impact of oxygen addition in juices that were supplemented with ample nutrients. The goal of this study was to determine if oxygen would have an impact during juice fermentation if the cellular components requiring oxygen for their biosynthesis were plentiful in the environment.28 Five different yeast strains were examined. Oxygen exposure showed little to no impact on fermentation behavior in all trials,28 and confirmed that the primary benefit of oxygen during fermentation is the enabling of biosynthesis of growth requirements. In addition to direct consumption of molecular oxygen, yeast also creates a reductive environment in the fermenting juice. This means that end products of metabolism are good electron donors and, once in the fermenting juice, can participate as reducing agents. For example, inorganic sulfur from a vineyard will be reduced to hydrogen sulfide due to reductive conditions created by yeast. Addition of large amounts 74 p racti c al w i ne ry & v i ne yard O CTO BER 20 13 of oxygen during fermentation can "reverse" these reductive products by overwhelming the yeast's ability to absorb oxygen, and inducing chemical oxidation. The resulting quinones will then quickly react with H2S that is present, removing "reduced" aromas. Lactic acid bacteria can also impact the fate of oxygen in wine. Like yeast, the bacteria will use molecular oxygen for biosynthesis, and introduction of oxygen will stimulate both growth and metabolism of the organism. Micro-oxygenation of wine has been shown to increase activity of malolactic bacteria and lead to color stabilization,24 suggesting that the bacteria are not as competitive in oxygen consumption as the yeast are during alcoholic fermentation, and non-enzymatic oxidation will occur simultaneously with malolactic (ML) fermentation. In addition, non-enzymatic reaction products produced will differ if microoxygenation occurs before or after ML fermentation, 15 suggesting that metabolic activities of malolactic bacteria can contribute or consume compounds participating in oxidation/reduction reactions. Conclusions Oxygen is an important component in winemaking. Oxygen can stimulate the growth and metabolic activities of microorganisms impacting microbial contribution to wine aroma and flavor. A better understanding of how oxygen affects these metabolic activities and metabolites of sensory importance is needed. Oxygen can also participate in both enzymatic and non-enzymatic oxidation reactions with differing outcomes for the wine. Enzymatic oxidation leads to destabilization of wine color, while non-enzymatic oxidation can lead to color stabilization due to formation of different reactants and ultimate end-products. The timing of aeration and therefore oxygen exposure is critical. In juices, PPO consumption of oxygen dominates unless the enzyme is inhibited via SO2 addition, heat or fining treatments. During active fermentation, yeast consumption is the dominant form of oxygen utilization. Under élevage conditions, nonenzymatic oxidation becomes more prevalent in the wine. The optimal amount of oxygen exposure will vary by grape variety and wine style being produced. PWV