With some wines, you can recork a 1/2 finished bottle, put it in the fridge, take it out 2 days later, bring it back to serving temperature, and it still tastes fine.

Other wines, like the 2003 Muscadet I tried last weekend, fall apart after a day. You uncork the 1/2 bottle and it smells and tastes tart, like much of the fruit has turned to vinegar.

My question is, what exactly is going on in the wine? Is it a subtractive or additive process (meaning, is it a matter of the wine losing positive aromas/flavors, or gaining negative ones)?

Also, what components in the wine make it more, or less, susceptible to turning vinegary with exposure to air?

I only drink a 1/2 bottle at at time. The 1st half of a bottle of Toad Hollow Rose I recently had was good like a still blanc d noir,but with a little spritze. the next day it was too sour for my taste.It wasn't vineger or volotile acidity, just sour grapes.I call it the Dr. Jekel and Mr. Hyde syndrome.

Hi Vinyrd Skynyrd.

As soon as you pull the cork on a bottle, you’re exposing the wine to oxygen. Oxygen and warm temperatures are spoilage ingredients for all foods, including wine (which is why we usually cover that leftover quiche in plastic wrap and pop it into the fridge).

When I have a leftover half bottle, I usually pour it into a clean beer bottle and cork it or float some wine preserving gas over the wine before I pop the wine into the fridge.

With air exposure, wine aromas and flavors decrease. This degradation is most quickly noticed in delicate whites. Robust reds, which have a lot of aromas and flavors to begin with, last longer.

As for vinegar, that’s caused by acetobacter. In the presence of oxygen, this bacteria converts alcohol into acetic acid. In the winery, sanitation and SO2 usually keep acetobacter from rearing its ugly head.

And Walt’s sour rose? As the ripe fruit aromas and flavors decreased, the wine’s acidity became more obvious.

Are sulfites used to help prevent or slow oxidation?

Sulfites (SO2, or sulfur dioxide) are used in the vineyard and in the winery to minimize oxidation (long-term exposure to oxygen).

SO2 binds oxygen molecules so they don't affect the wine.

Using SO2 is always a balancing act. Most winemakers use as little as possible, just enough to protect the grapes/wines. If you use too much, it marks the wine and leaves a nasty odor of sulfur or burnt matchtips.

How much SO2 is actually in wine? Miniscule amounts, less than was in the salad bar I grazed for lunch yesterday.