Vol.17, No. 6 November - December, 2002
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
E-mail: bzoeckle@vt.edu
Web site: http://www.vtwines.info/

I. Redox

Most winemakers are aware of the negative effects of oxygen on wine, and most also appreciate the positive influences under the proper conditions.

One of the areas of renewed interest in enology is oxidation - reduction potential. The redox potential is what determines the effects of important processing variation, such as microoxygenation and sur lie storage, on wine quality and longevity. In order to understand how oxygen impacts wine, it is important to understand redox potential.

The oxidation-reduction (redox, pronounced REE-dox) potential is a measure of the tendency of the molecules, or ions, to gain or lose electrons. A compound with a large positive reduction potential (e.g., oxygen) will readily accept electrons, producing the reduced form of that compound (e.g., water). Conversely, molecules (ions) with a lower (negative) redox potential (e.g., SO₂ or sulfite ion, SO^2-₃) exhibit increasing tendencies to lose electrons, hence producing the oxidized form (e.g., sulfate ion, SO^2-₄ ).

The various components in wine exist as mixtures of their oxidized and reduced forms (called a redox pair). Wine is a complex system made up of many such redox pairs (see Table 1). Thus reduction of one component causes oxidation of another, until a final equilibrium point is reached, and net reduction equals net oxidation. Because pH affects the values of some redox potentials, the position of this final equilibrium point in a wine is very dependent on the pH. The higher the pH, the more negative the redox potential of many compounds, and therefore the better compounds act as reducing agents.

AOxidizing agents@ are compounds (such as oxygen) that cause other compounds to be oxidized (that is, lose some of their electrons). A reducing agent, on the other hand, is a molecule or ion that causes other components to be reduced. The main reducing (or antioxidizing) agents found in wines are SO₂, ascorbic acid, and phenols. These compounds can react or bind with oxygen, and lower the overall redox potential of the system (i.e., a lower oxygen level yields a lower redox potential for the system). Again, pH influences the reducing power of these agents.

The rate of reaction for different reducing agents is variable. Ascorbic acid is rapid, SO₂ is much slower, and phenols are even slower.

It is the molecular form of a compound (e.g., acetic acid) that is volatile and responsible for aroma. Ionized forms (e.g., acetate), themselves, are nonvolatile. Thus, shifts in the redox potential of a wine may produce the more volatile form of a number of compounds, causing a change or increase in the aroma. The opposite may also occur, with aromas being masked or eliminated. Because pH affects the redox potential, it also determines the equilibrium state of a wine and the relative volatility of some aroma compounds.

The goal of redox research is to understand how processing variations that impact oxygen (either the incorporation or the buffering capacity) impact wine quality and longevity. For example, is oxygen at bottling a good thing? How much and for what wine?

Table 1. Typical Redox Pairs of Compounds Found in Wine^a

^aStandard conditions are unit activity for all components with the exception of H⁺, which is 3.2 x 10^-4M or 10^-7M; the gases are at 1 atm pressure.

II. Upcoming VVA Winter Meeting

The winter meeting of the Virginia Vineyards Association is scheduled for February 13 - 15 at the Omni Hotel in Charlottesville, Va.

This year, we will have a full day devoted to enology. The enology program is titled "The Synergy of Wine Structure and Flavor." This will include technical presentations and sensory evaluations. Topics include:

• Wine structure and flavor components
• Contrasting short vs. long vatted wines
• Contrasting Delestage vs. Pressage produced wines
• Aroma/flavor capture and return research
• Wine production in Uruguay
• Tannat production in Uruguay
• The impact of alcohol on aroma/flavor

Detailed information to follow, and will be available on the VVA website, and the Enology-Grape Chemistry Group website.

III. France Trip

Twenty Virginia winemakers and growers participated in a technical study tour from December 4 - 17th. We visited Vinitech, then on to Pauillac, Saint Ernilion, Sauternes, Madiran, Juracon, Cahors, Gaillac, Fronton, Minernois, and then up the Rhone Valley and into Beaujolais.

This was enjoyable and educational. At each stop, two participants were assigned to take detailed notes. I am in the process of editing these, and will post them on the Enology-Grape Chemistry Group website. Additionally, an overview of the trip will be given at the VVA Winter Meeting in February (see above).

If you would like to have your name added or removed from our list serve for future Enology Notes, please send a blank e-mail to bzoeckle@vt.edu with the word "Add" or "Remove" in the subject line.