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Chemical Issues During Aging
Linda F. Bisson
Department of Viticulture and Enology
University of California, Davis
Chemical Issues During Aging: Outline of Presentation
Character stability New character evolution Oxidative/reductive aging
Character Stability Volatilization Hydrolysis/Ethanolysis Chemical reactivity Microbial modification
New Character Evolution
Loss of masking characters New chemical species Modification of existing chemical species
Oxidative/Reductive Reactions in Wine
Enzymatic (biological) Oxidation– Tyrosinase (polyphenol oxidase) (plant)
– Laccase (Botrytis & molds) Chemical Oxidation/Reduction
– Cascade initiated by molecular oxygen
– Electron rearrangements in absence of oxygen
Enzymatic Oxidation
OH O
PPO
R OH R O
O2 H2O2
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
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: Introduction Oxidizing agents possess a strong affinity for
electrons, causing other substances to become oxidized by accepting electrons from them; the oxidizing agent itself becomes reduced and forces the other compound to be oxidized
Reducing agents readily give up electrons and thereby cause some other substance to be reduced; the reducing agent itself becomes oxidized
Redox Chemistry of Wine Wine contains both oxidizing and reducing
reagents Molecular oxygen is a good oxidizing agent
(possessing an affinity for electrons)
O2 e O2- e O2
2- e OH e OH-
OH- + H+ H2O
Redox Chemistry of Wine Phenolic compounds can be oxidized in the
presence of oxygen Oxygen has limited reactivity towards phenolic
compounds in its normal O2 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
Waterhouse and Laurie 2006
Chemical Bridging by Oxidized Compounds
1 2 3 4 5
Common Oxidation Reactions of Wine
Formation of polymerized pigment Tannin polymerization Alcohols to Aldehydes Organic Acids to Keto Acids Reaction with thiols and loss of varietal
character
Common Reductive Reactions in Wine Formation of “dried” characters
– Sun dried sheets
– Sun dried tomatoes
– Cedar chest Formation of/Return of S-characters
Controlling Wine Oxidation Minimize oxygen exposure Use of antioxidant: SO2 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
Oxidative Damage to Wine
Formation of off-colors (browning or pinking) – From oxidation of tartrate to glyoxylic acid
– Formation of reactive quinones Formation of oxidized flavors
– Aldehydic (sherry-like)
– Nutty
– Animal fur/cage
– Plaster
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 H2O2
– Dose relationship to normal aging?
– Dependent upon wine composition
Oxidative Taints Tasting
Glass 1: Control (Merlot) Glass 2: H2O2:
Glass 3: H2O2:
Glass 4: H2O2:
Glass 5: H2O2:
Glass 6: H2O2: