Enology Notes
Enology Notes #95, November 4, 2004
To: Regional Wine Producers
From: Bruce Zoecklein, Head, Wine/Enology-Grape Chemistry Group, Virginia Tech
Subjects: Oxidation Reduction Potential; Upcoming Enology Extension Programs
- 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 (redox). 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 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, SO3) exhibit increasing tendencies to lose electrons, hence producing the oxidized form (e.g., sulfate ion, SO4).
The various components in wine exist as mixtures of their oxidized and reduced forms (called redox pairs). Wine is a complex system made up of many such redox pairs. 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 redox potentials, the position of this final equilibrium point in a wine is very dependent on the pH.
Oxidizing 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 higher the pH, the more negative the redox potential of many compounds, and therefore the better compounds act as reducing agents.
The rate of reaction for different reducing agents is variable. Ascorbic acid is rapid, SO2 is much slower, and phenols are even slower.
Redox and Volatile Compounds. 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. Therefore, any activity which impacts pH, including potassium bitartrate stabilization, may impact wine aroma.
One characteristic of the production of off sulfur-containing compounds is that they have particularly low redox potentials. Indeed, the presence of thio or sulfur-containing compounds in a wine, and the corresponding smell, require an abnormally low oxidation reduction potential -- values - 220 mV compared to wine values of +220 to +440.
Redox and Volatile Sulfur Compounds. The low redox potential in wines with off sulfur-containing compounds is why we refer to such wines as "reduced."
Redox potential is an important issue with regard to post-fermentation management of these compounds.
The concentrations of volatile sulfur compounds are not as great in barreled wines, due to the higher redox potential which is attained in wines stored in barrels vs tanks.
Frequent barrel stirring to put the lees in suspension, and limited oxidation across the staves, helps to inhibit the formation of post-fermentation volatile sulfur compound production.
During barrel aging, volatile sulfur compound concentrations decrease progressively due, in part, to an increase in the redox potential. This occurs at a more rapid rate in new wood, due to the greater oxygen dissolution and oxidizing effect of the new wood.
The goal of redox research is to understand how processing variations that impact oxygen (either in the incorporation or the buffering capacity) impact wine quality and longevity. Further understanding of the role of redox may help winemakers effectively manage wine aroma and aroma intensity.
- Juice and Wine Analysis Short Course.
The Wine/Enology-Grape Chemistry Group will offer a two-day juice and wine analysis short course January 11 and 12, 2005. This program will be a hands-on, practically oriented laboratory course. See Enology Notes # 93 for details.
Workshop, Juice and Wine Filtration. The Wine/Enology-Grape Chemistry Group, in conjunction with Pall Corporation, will offer a one-day juice and wine filtration workshop, February 10, 2005 at Horton Cellars from 10:00 am to 4:00 pm. This program will be a hands-on, practically oriented workshop, which will include the following features:
- Overview of Filtration
- Cross Flow Filtration
- Membrane Filtration
- Depth Filtration
- Sensory Evaluation
- Cross Flow Demonstration
This is an advanced program, which will involve technical discussions and sensory evaluations of wines filtered by various means.
Enrollment is Limited and Restricted: This workshop will be limited to a total of 30 participants, and is designed for Virginia wine producers. Registration info will be sent to each VA bonded winery.
Wine Fining. The Wine/Enology-Grape Chemistry Group, in conjunction with Scott Laboratory, will offer an afternoon workshop on juice and wine fining, February 28, 2005, location TBA, from 12:30 pm to 4:30 pm. Fining agents are added for the purpose of clarity, color, flavor and/or stability modification. This technically oriented, practically based workshop will include the following features:
- Summary of the various fining agents
- Review of the binding and adsorption capacity of fining agents
- Fining agent selection, preparation and addition
- Sensory evaluation of wines with different fining additions
- The program will highlight some of the newer agents/procedures, such as gums and yeast fining.
Enrollment is Limited and Restricted: This workshop will be limited to a total of 30 participants, and is designed for Virginia wine producers. Registration info will be sent to each VA bonded winery.
Subscription to Enology Notes. All past Enology Notes newsjournals are posted on the Enology-Grape Chemistry Group's web site at: http://www.vtwines.info/. Enology Notes are slightly different in content from the subscription based Vintner's Corner newsjournal.
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Dr. Bruce
Zoecklein
Professor and Enology Specialist Head Enology-Grape Chemistry Group
Department of Food Science and Technology, Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: http://www.vtwines.info/
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: