I'm pretty much a novice, with only 11 kits done. Early on, though, I bought a 5-pound bottle of CO2 and used it to purge the ullage of my 6-gallon carboy filled with ready-to-age wine that had all the CO2 sucked out of it at 75° F. I quickly plugged it with a silicone "cork" that has a kind of mushroom cap on it that allows gas to exit the carboy but not enter it. I had a question about a WinExpert kit and called their help line. The nice lady who answered was extremely upset when I told her I was using CO2 to exclude oxygen from the wine instead of topping up with either water or spare wine. She said, "You're trying to get *rid* of CO2; you shouldn't be adding it!" She mentioned that WinExpert recommends up to a quart of water can be added to reduce the ullage, but I didn't want to dilute the wine.
Later, I began to think about the natural inclination of CO2 to dissolve in water (wine), and I wondered what would replace it in the ullage if that happened. One thing is certain: it wouldn't be oxygen. So far, I haven't been able to answer that question because, as the wine cools to basement temperature from the 75° at which I sucked the CO2 out, there is a partial vacuum formed in the ullage. I can't distinguish *that* vacuum from a possible vacuum that would occur if the CO2 was, indeed, pulled into the wine at the cooler temperature. My guess is there is an equilibrium established between the CO2's attempt to mate up with the wine and the temperature-caused lowering of pressure in the ullage. There definitely is a partial vacuum when I pull the silicone cork to bottle the wine, but I don't know how much of it is attributable to temperature change and how much to the solution of CO2 into the wine. The small amount of CO2 I add has no effect on the taste of the wine, so far as I have been able to tell.
- Paul