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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
O2
R
H2O2
O
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-
OH- + H+
e
O22-
e
H2O
OH
e
OH-
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: