Oxidative/Reductive Reactions in Wine Enzymatic (biological) Oxidation –Polyphenol Oxidase (PPO;Tyrosinase) (plant) –Laccase (Botrytis & molds) Chemical Oxidation/Reduction –Cascade initiated by molecular oxygen –Electron rearrangements in absence of oxygen
Oxidative Taints Function of oxygen exposure and wine’s ability to consume oxygen Related to phenolic content Impacted by other factors such as pH Some oxidation reactions are desired; not all lead to defects = a delicate balance!
PPO versus Laccase PPO = tyrosinase/catecholase Laccase = p-phenoloxidase/diphenol oxidase Some overlap of substrates PPO mostly associated with off-colors; Laccase can give both off-colors and off- odors
Polyphenol Oxidase/Laccase OH O R OH R O O 2 H 2 O R
Laccase OH O-O- e-e- O2O2
PPO versus Laccase PPO is inhibited by sulfite PPO is inactivated by ethanol Laccase has a broader range of substrates than PPO –Broader range of off-color compounds formed –Can oxidize phenol-glutathione complexes Laccase is still active in wine post- fermentation
Control of Enzymatic Oxidation Use of sulfite to inhibit PPO (grape) Use of yeast to consume oxygen until ethanol inactivates PPO Laccase: Control mold in vineyard Laccase: use of HTST (high temperature short time) treatment to inactivate enzyme Bentonite fining of juice to remove enzymes
Control of Laccase Sulfite sensitivity: 150 ppm shows only 20% inhibition Ascorbic acid is a substrate of laccase More sensitive to heat than PPO
Is My Problem Laccase? Does it continue in presence of ppm SO 2 ? (wine in glass for hours) If wine is heated to inactivate enzymes (50°C+), does oxidation continue (is it chemical versus enzymatic?) Is syringaldazine oxidized? (need to remove other phenolics first with PVPP) Are there laccase-characteristic odor taints?
Redox Chemistry: Introduction Transfer of electrons: reactions in which a transfer of electrons occurs are known as oxidation-reduction (redox) reactions Oxidation involves the loss of electrons Reduction is the gain of electrons Redox potential refers to the tendency to gain or yield electrons of a specific atom, molecule or solution
Redox Chemistry of Wine Wine contains both oxidizing and reducing reagents Molecular oxygen is a good oxidizing agent (possessing an affinity for electrons) O 2 e O 2 - e O 2 2- e OH e OH - OH - + H + H 2 O
Redox Chemistry of Wine Phenolic compounds can be oxidized in the presence of oxygen Oxygen has limited reactivity towards phenolic compounds in its normal O 2 form Oxygen is “activated” by metal ion catalysts in the wine such as iron (Fe) Oxidation in wine is caused by the formation of reactive oxygen species (ROS) The hydroxyl radical ( OH) is the reactive agent
Redox Potential of Wine Dependent upon: –Oxygen concentration –Metals availability –Ethanol –Phenolic composition –Type of container –Stirring/agitation –pH (increasing pH decreases redox potential; oxidative reactions occur more readily)
Formation of Acetaldehyde Danilewicz 2007 Waterhouse and Laurie 2006
Chemical Bridging by Oxidized Compounds
Controlling Wine Oxidation Minimize oxygen exposure Use of antioxidant: SO 2 or ascorbate Monitor aldehyde levels
Oxygen in Wine From any transfer operation Pumping over or cap irrigation Centrifugation Filtration Mixing From headspace, penetrates only the first 10 to 20 cm of wine: stratification effects are observed Singleton: white wine 10 saturations; red wine 30 saturations
When Is Wine Damaged by Oxidation ? Oxidation reactions can be positive: –Stabilization of color –Loss of tannins due to polymerization –Loss of compounds that are perceived as negative when reduced Negative effects arise when: –Acetaldehyde or glyoxylic acid start to accumulate –Higher aldehydes start to accumulate –Loss of varietal character occurs
Factors Affecting Oxidation pH: hydrogen ions with a positive charge can quench oxidation cascades in the formation of water; oxidation 9 times faster at pH 4.0 than at pH 3.0 Amount of exposure to oxygen Type of closure: current practices optimized for natural cork? Antioxidants and Redox buffering capacity Time!