Funny you mentioned SO2. We just did an interesting lab excercise on SO2 in my Wine Production class. Here's some excerpts:
In grape wine sulfur dioxide can block chemical oxidation of phenolic compounds by binding to oxygen radicals. Its reaction rate with molecular oxygen is so slow under wine production conditions that it will not directly block generation of oxygen radicals in the first place. It also has a perhaps more important role in inhibiting enzymatic oxidation reactions.
Grapes possess an enzyme known as polyphenol oxidase, PPO. PPO reacts with molecular oxygen, water and phenolic compounds to produce an oxidized phenolic compound and hydrogen peroxide. The hydrogen peroxide can then react with other components in the juice or wine. PPO functions in defense against microbial attack and is one of the most important enzymes of the vine chemical warfare program. Hydrogen peroxide is the goal of the catalysis as this compound has strong antimicrobial effects.
It is not clear if the oxidized phenolic compounds are also inhibitory. PPO is released to the must or juice upon crushing of the berries, so is present and active in fermentation. Many oxidized phenolic compounds are undesirable as is the production of hydrogen peroxide. Yeast possess an enzyme called catalase which degrades hydrogen peroxide so are not inhibited by the levels of this compound produced in the must or juice. However its effects on the chemical composition of the wine, and inducement of aldehyde and off-color (brown, orange, pink) formation are undesirable.
Molecular oxygen serves as a micronutrient for many organisms, required for the biosynthesis or degradation of many compounds. PPO successfully competes with the microbes present in wine for O2. Oxygen is required by the yeast for optimal ethanol tolerance (this is why we aerate in order to get O2 into the must for the yeast so they can synthesize sterols to strengthen the cell walls and have a higher ETOH tolerance), if PPO activity is unchecked, the yeast may be in a nutrient deficient situation (in this case O2 as a nutrient). This is an additional reason to add SO2 in grape musts. It is not clear at the molecular level exactly how SO2 is able to inhibit PPO activity.
It is important to note that sulfur dioxide levels are affected by both pH and the fact that it is bound by many different substances in wine including tannins, thiamin, and certain oxidative enzymes such as polyphenol oxidase. Sulfur dioxide binds with other substances such as aldehydes and ketones, and will reduce over time due to chemical volatility. It is this affinity to bind and react with other substances in wine that necessitates close monitoring of the levels of free (unbound) sulfur dioxide before, during and after fermentation has been completed. Also consider that free-run juice such as used in making white wines where the grapes are pressed and only the juice and no pulp is used will have less suspended material for the SO2 to bind with, than in red wine where there will be considerable material from the pulp, seeds, skin, etc. So that is also a factor to consider in both the influence of pH on the bound and unbound (free) levels of SO2.
I have a couple of tables that I'll see about posting that help determine where to be with your dosages in red and white wine. Other factors such as temperature, containers (stainless, barrel, glass etc), and the turbidity of the juice all play a role.
Hope that helps,
Oskaar
Sources:
Texts:
Bisson, L., 2005. VEN 124 Wine Production for Distance Learners, Lesson 5: Juice and Must Treatments and Additions Topic 5.2: Juice Additions
Boulton, R., V. Singleton, L. Bisson, and R. Kunkee. 1996. Principles and Practice of Winemaking. Chapman and Hall. New York
Peynaud, Emile, 1984. Knowing and Making Wine, (English Translation) John Wiley and Sons, Inc. USA
Journal Articles:
Baldwin, G., Basic Effects of Sulfur Dioxide on Yeast Growth. American Journal of Enology & Viticulture Volume 2: 45–53. 1951
Cocolin, L., and D.A. Mills Wine Yeast Inhibition by Sulfur Dioxide: A Comparison of Culture-Dependent and Independent Methods. Am. J. Enol. Vit. 54: 125 – 130. 2003
Other Articles:
Delteil, D., Enological yeast effect on the sulfur dioxide content and management in wines. Institut Cooperatif du Vin, 1 – 4 1992