Saturday, November 28, 2009

More on the Redox Potential of Wine


I continue to be interested in the change in wine as it is exposed to the air.

One way to measure that is with the redox potential, which is measured like pH, but with a platinum electrode instead of a glass electrode.

Anyway, I got an article by Tomlinson and Kilmartin from New Zealand, (J Appl Electrochem 29:1:9 (1997) 1125-1137), and they explain how there are a multitude of redox couples active in a bottle of wine.

The electrochemical potential is a thermodynamic property, but in wine there are multiple equilibria, and we typically don't want the wine to degrade before we measure it at equilibrium. People often want to test it in as it is stored in the bottle, and after several months or years - it must be fairly stable. My interest is in the rapid change in tannin, fruit and other flavors as the wine breathes. It strikes me that there must be a lot of chemistry happening all at once.

Tomlinson & Kilmartin also point out the pH generally correlates with reduction potential -- perhaps because H+ ion is involved in a lot of the reactions. This is good for me since I am unlikely to buy a platinum electrode for my wine shop.

Zoecklein from Virginia Tech prepared the chart above showing different kinds of equilibria in wine. Most notable is the oxidation of ethanol to acetaldehyde or ethanal. Bad because acetaldehyde is poisonous, and because there is 13% alcohol for the reaction to work with. I am also interested in trying to control the redox potential with ascorbate rather than sulfite to avoid the flavor problem.