Oxidative/Reductive Taints Linda F. Bisson Department of Viticulture and Enology University of California, Davis

Oxidative Taints  Off-colors: –pink –brown  Off-flavors: –aldehyde (nutty) –rancid –“hamster fur”/ animal characters –chemical notes

Reductive Taints  Sulfur Compounds  Sun dried characters  Wood characters

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

Laccase Substrates: SubstrateRelative Activity (%) 4-Methylcatechol100 Catechol104 Protocatechuic acid119 Caffeic acid132 (+) Catechin100 Gallic acid109 Phloroglucinol143 p-Coumaric acid90 Ferulic acid109 Anthocyanins97 Leucoanthocyanins84

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?

Chemical Oxidation/Reduction

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!

Predicting Oxygen Impact  Termination of aging  Closure decision  Market shelf-life assessment

Predicting Oxygen Impact  Exposure to air: hard to separate microbial and chemical effects  Spiking with H 2 O 2 –Dose relationship to normal aging? –Dependent upon wine composition

Oxidized Character Observations with Hydrogen Peroxide Spiking:  Acetaldehyde: chemical taint (rotten apple)  Higher aldehydes –Nutty (sherry) –Rancid  Mustiness  Fur (hamster not mousy)

Oxidative Taints Tasting  Glass 1: Control (French Colombard)  Glass 2: French Colombard H 2 O 2 : 10ppm  Glass 3: French Colombard H 2 O 2 : 25ppm  Glass 4: Chardonnay with Laccase  Glass 5: Chardonnay with Laccase H 2 O 2 : 50 ppm  Glass 6: Commercial Wine