1. Chemical Issues During Aging Linda F. Bisson Department of Viticulture and Enology University of California, Davis

2. Chemical Issues During Aging: Outline of Presentation  Character stability  New character evolution  Oxidative/reductive aging

3. Character Stability  Volatilization  Hydrolysis/Ethanolysis  Chemical reactivity  Microbial modification

4. New Character Evolution  Loss of masking characters  New chemical species  Modification of existing chemical species

5. 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

6. Enzymatic Oxidation OH O PPO R OH R O O 2 H 2 O 2

7. 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

8. Chemical Oxidation/Reduction

9. 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

10. 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

11. 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

12. 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

13. 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)

14. Formation of Acetaldehyde Danilewicz 2007 Waterhouse and Laurie 2006

15. Chemical Bridging by Oxidized Compounds 1 2 3 4 5

16. 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

17. Common Reductive Reactions in Wine  Formation of “dried” characters –Sun dried sheets –Sun dried tomatoes –Cedar chest  Formation of/Return of S-characters

18. Controlling Wine Oxidation  Minimize oxygen exposure  Use of antioxidant: SO 2 or ascorbate  Monitor aldehyde levels

19. 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

20. 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

21. 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

22. 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!

23. Predicting Oxygen Impact  Termination of aging  Closure decision  Market shelf-life assessment

24. 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

25. Oxidative Taints Tasting  Glass 1: Control (Merlot)  Glass 2: H 2 O 2 :  Glass 3: H 2 O 2 :  Glass 4: H 2 O 2 :  Glass 5: H 2 O 2 :  Glass 6: H 2 O 2 :