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winemaking Figure 7: Hypothesized anthocyanin degradation by PPO. of these reactions can be reversible. Yeast metabolic activity leads to a more reduced juice environment and can alter the oxidation state of molecules in the juice. Reductive conditions formed by yeast metabolism can lead to a dissolution of insoluble complexes, so it is important to rack the hyper-oxidized juice off of the brown sediment prior to initiation of fermentation. Poor removal of brown sediment is thought to explain variable experiences with hyper-oxidation in white wine production.27 In some cases the juice may need to be filtered to remove brown pigments, if they are still in solution and have not precipitated. Aeration in red musts is often associated with loss of color.29 Enzymatic oxidation and production of quinones leads to a disappearance of anthocyanin mediated by the quinones.7,19,35 The proposed mechanism of color bleaching is thought to involve formation of unstable colorless phenolic acid and aldehyde derivatives of the anthocyanins catalyzed by quinones,19 and the loss of color is irreversible (Figure 7). Thus there are fundamental differences in the impact on both color and polymerization reactions involving anthocyanins, comparing juices with active polyphenol oxidase activity to wines lacking oxidase activity. In wine, the formation of hydrogen peroxide and subsequent oxygen radicals leads to the presence of acetaldehyde which plays a key role in polymerization, color stability and changes in tannin composition. In juices, acetaldehyde is not formed in appreciable concentrations because ethanol has not yet been produced and because hydrogen peroxide is not a product of PPO-mediated activity. However, even if PPO activity was effectively eliminated via SO2 addition, there would be no available ethanol to yield acetaldehyde. Polymerization, if it occurs via oxidation in juice, seems to lead to colorless or brown products. If laccase is present in the finished wine, it will generate quinones in the wine that will also lead to loss of color, rather than polymerized pigment formation and color stabilization. Hyper-oxidation can be achieved by tank-to-tank or within-tank transfers using an air diffuser, or if a technique such as flotation is used for juice clarification, air can be introduced instead of nitrogen. It is important to monitor and limit these operations to avoid growth of undesired microbes in the juice. Immediate inoculation with yeast can prevent unwanted microbial activity, as will SO2 addition following clarification. If done with sufficient speed, hyper-oxidation does not result in microbial deterioration of the juice or wine. PPO is able to compete for oxygen under these conditions and limits the availability of oxygen for grape microbes and acetobacter. Electrochemical micro-oxidation, which involves passing a current into a wine to add electrons, and which enables a greater control of the nature of the compounds that will be reduced/oxidized, has been explored as an alternative to micro-oxygenation in wine,17 but has not been investigated for use as an alternative to juice hyper-oxidation. Sensory analysis of wines made from hyper-oxidized juices has yielded varying results. In some studies hyper-oxidation was beneficial to wine quality, in others it was neutral or detrimental.27 The impact of hyper-oxidation obviously depends upon the composition of the juice and presence of sufficient reactants to obtain efficient polymerization and avoid damage to varietal aromas. Often it is difficult to quantify how much oxygen has been consumed by a juice during aeration treatments. Oxygen levels are easily measured in finished wine in the form of dissolved oxygen, when there is no enzymatic activity consuming oxygen, and oxygen levels change slowly. However, in juice, measurement of dissolved oxygen is difficult. The juice released from grapes at crushing will initially be saturated with oxygen from the air. The mixing of oxidases and their substrates in the juice will generally result in complete consumption of this dissolved oxygen in a matter of minutes under natural conditions.5 The difficulty of measuring dissolved oxygen in juice makes it complicated to define precise starting conditions with a quantified amount of dissolved oxygen. However, color change is significant when oxidative conditions have been altered. PPO activity consumes O 2 so quickly that it often cannot be detected in Fine Wine Begins With Mueller ® Synchronizing Wine with Wood Paul Mueller Company designs and manufactures equipment for all aspects of winemaking. Superb craftsmanship and strict attention to detail make our products the most desired among today's prominent winemakers. Call us at 1-800-MUELLER or email sales@paulmueller.com for all your winemaking needs! ©2013 Paul Mueller Company 72 p racti c al w i ne ry & v i ne yard O CTO BER 20 13 380-9