Proceedings of the Texas Viticulture & Enology Research Symposium * June 2-3, 2009 * Granbury, Texas 38 Wine and Juice Oxidation Patricia Ann Howe Patricia Howe Wines, PO Box 10633 Napa, CA 94581 [email protected] Introduction The oxidative flaws of grape juices and wines are characterized by an increase in brown color and the development of the typical oxidative aromas of acetaldehyde, acetic acid, and ethyl acetate. Loss of pigmentation and tannins through precipitation can also occur. Oxidation is defined as the loss of electrons and relates to the status of the valance (outer shell) electrons; this definition has less utility for the average winemaker than the noticeable byproducts. Recent research in wine oxidation is demonstrating the importance of interrelated oxidative reactions and metallic cofactors in wine chemistry. Awareness of the chemistry and microbiology of these oxidative processes allow the winemaker to understand, prevent, control, or harness these reactions. chanisms of juice and wine oxidation: enzymic, chemical, and microbiological.llic cofactors. Like most r temperatures. They are also more active at higher pHs, and s are denatured by ethanol the mold laccases have continued activity in res s reatments used for grape tyrosinase reduction: fining, temperature control, and pH control. Sulfur dioxide (SO2) is less effective against laccase than rape tyrosinases. Chemical Oxidation Chemical (non-enzymic) oxidation is the process in which electrons are lost through stepwise reactions involving oxygen, phenols, and metal catalysts. Wine may be oxidized through simple exposure to air, which contains approximately 20% oxygen. Oxygen meters can be used to measure gross exposures There are three main me Enzymic Oxidation Enzymic oxidation from grape (tyrosinase) or mold (laccase) enzymes present in the grape juice causes browning and, in extreme cases, aldehyde production. These reactions require the presence of the enzymes, the required substrates (phenolic compounds), oxygen, and metaenzymes, they are more active at warmealthough the grape tyrosinasewine. These enzymic processes can be discouraged by judicious use of harvesting, crushing, and pressing parameters to minimize enzyme reaction rates and exposure to oxygen. Other control measurely mainly on removal of the enzyme (via fining, usually with bentonite), reducing exposure to oxygen, and judicious use of SO2. Some winemakers use juice “hyperoxidation” as a production method to reduce the phenolic and color compounds of their product. Typically this relies on several saturations of the must or juice with oxygen and can be monitored with oxygen probes. This is particularly useful in some heavier press fractions or insparkling wine bases; it has been used for reducing some varietal aromas and may be a factor in reducing “pinking” of white wine later but is also known to reduce some desirable aromas and could encourage microbial growth. Concentration of grape tyrosinases will be dependant on maturity and grape varietal. Farming practicecan affect the types and amounts of molds in the vineyard and is the first step in preventing oxidation from laccase. If laccase is already present (and can be tested for using a test kit and/or a botrytis “pregnancy test”) it can be controlled through similar tgProceedings of the Texas Viticulture & Enology Research Symposium * June 2-3, 2009 * Granbury, Texas 39 from wine transfers or bottling, but lt to measure as the oxygen may be below the level of the meters or oxygen level does not rise. Recent research has demonstrated the vit nd copper as part of these reactions, but juices and wines all contain adequate min se metals, and prevention of oxidation by removal of all metal is not a realistic approach. However, excess metals have been known to accelerate the reactions and thus co iron or copper containing metals are highly inadvisable as wine contact materi Phenolic compounds are required to start the reactions which result in the oxidation of other compounds which do not themselves react directly with oxygen. The numerous interactive steps (oxygen with phenols, production of peroxide, and reaction of peroxide with other susceptible compounds) occur at and speeds depending on concentration of reactants, temperature, pH, etc and help xplain the complexity of the oxidation and aging of wine. Alcohol is, second to water, the compound of f s poilage rather than wine “development”. vers the whole range of oxidative spoilage aromas and is frequently seen in onjunction with both enzymic and chemical oxidation as the causes and situations are mutually ) nd th; n ethyl phenol aromas are not addressed as oxidation in this summary, yet ese yeast are also able to produce the classic oxidative spoilage aromas. ice and alian wines which were sealed in such a fashion as to allow small amounts f oxidation in the headspace of a wine which contained adequate populations for spoilage. Because f ms and on the available carbon sources (sugars or ethanol). They are also managed y controlling oxygen and maintaining adequate sanitation and SO2. educing opportunity for any microbial spoilage involves: managing the populations of microbes through n low rates of exposure are more difficube consumed at such a rate that the al importance of iron aute concentration of thepper, iron, brass, bronze, and otherals. different rates ehighest concentration in wine and is thus the most likely compound to be affected. Classic symptoms owine oxidation are the conversion of ethanol to acetaldehyde and phenols to brown pigments. Judiciouuse of SO2 can be used as a substitute oxidative compound; ascorbic acid can also be used but depending on the circumstances can accelerate the oxidative cycle rather than stop it. Microoxidation (MOX) is the term used to manage wine aging and development by slow and controlled use of oxygen additions to wines. Reactions are as yet difficult to predict and this process must be monitored closely to prevent the occurrence of oxidation s Microbial Oxidation Microbial oxidation cocconducive. The most common yeast oxidation involve film yeasts (Candida) and wild yeast (Pichia and Kloeckerathat come from vineyard and winery sources, and under warm conditions ethyl acetate and/or aldehyde aromas can be produced within hours of harvesting. Some yeast strains are more tolerant to the