Tuesday, October 27, 2009

Oxygen and the Flavor of Wine


I opened up the 2008 Merlot Wine now that it is one year old. I have not tasted this wine in six months, and when I took the first sip, it tasted thin, and while alcoholic, I thought it lacked flavor.

I decanted the wine into gallons and only decanted a few bottles for drinking. Later when I tasted it again, it was better, fruiter, and more flavorful. I tasted the same bottle the next evening, and it was much more fruity. Now it tasted like a young wine that needed more aging.

I just think it is fascinating that the flavor changes simply due to reactions with the air. It is so tangible & taste-able, and also I worry since I don't want a whole batch of wine that tastes bad.  Maybe no one else thinks this is interesting, but I do.

What is so interesting to me is how rapidly the flavor changes. It seems that oxygen rapidly reacts with the chemicals in the wine.  Some enologists/wine chemists (Cacho et al) say that iron and manganese are important to how the flavor changes, presumably because they catalyze the oxidation reaction. Peuch (et al) say that the oak heartwood from barrel aging is critically important to the flavor changes. This turns the light on oak tannins -- which are a whole class of phenol/sugar compounds.

The large compound above is an elligitannin. You can see there are lots of different oxidation pathways for it -- reaction of the aromatic hydroxyls to form double bonded oxygen, for example.

The interaction of oxygen in wine, or elsewhere, is measured in terms of the redox-potential, which is a simplified pH electrode, where one electrode is simply a platinum wire. You can measure it with a regular pH meter, and the special electrode.

Wine chemicals can undergo a continuous array of oxidations and reduction reactions, and the equilibrium between them can be measured by the redox potential. An aerated red wine has a potential of 400-450 mV, but an non-aerated wine has a potential of 200-250 mV. In this sense a higher number indicates more oxygen exposure (and paradoxically that the wine has a greater potential TO oxidize.)

One way this impacts the flavor is the behavior of thiols, which are --S-H groups. These smelly compounds affect the flavor of the wine. As shown in the figure, exposure to oxygen couples up the thiols into disulfide bonds, which have 24 times less odor. This makes the wine taste better.

One group of researchers (Tomlinson in New Zealand) found that redox potential correlates with pH for 24 different wines. This is not surprising for young wines which are saturated with carbon dioxide, which would contribute reducing power, but also would drop the pH. This will save me money since I only have a pH electrode at home.

Several people have noted that the bottom of the barrel has more reducing power presumably because strongly reduced materials have floated down to a muck at the bottom, and because subsequent exposure to air at the top of the barrel/tank would remain at the top. This introduces the idea of creating a wine battery with the potential difference between the top and the bottom. It might be a few tenths of a volt, probably not enough to power the household, but enough to light an LED perhaps, or win a bet at the bar.