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: