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 offodors
Polyphenol Oxidase/Laccase
OH
R
R
O
OH
O2
R
H 2O
O
Laccase
O-
OH
eO2
OH
OH
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 postfermentation
Laccase Substrates:
Substrate
Relative Activity (%)
4-Methylcatechol
100
Catechol
104
Protocatechuic acid
119
Caffeic acid
132
(+) Catechin
100
Gallic acid
109
Phloroglucinol
143
p-Coumaric acid
90
Ferulic acid
109
Anthocyanins
97
Leucoanthocyanins
84
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 50-75 ppm
SO2? (wine in glass for 12-24 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)
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
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
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
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 H2O2: 10ppm
 Glass 3: French Colombard H2O2: 25ppm
 Glass 4: Chardonnay with Laccase
 Glass 5: Chardonnay with Laccase H2O2:
50 ppm
 Glass 6: Commercial Wine