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Dear Dr. Vinny,

I always run my red wines through an aerator and into a decanter before drinking. During the process, some wines foam a little, and some foam a lot. What causes this variation?

—Charles C., Stow, Ohio

Dear Charles,

Not only might you notice foamy wine in a decanter, but you might also notice that sometimes when you are rinsing out wineglasses, that little bit of leftover wine can create a surprising amount of foam.

Foam, and bubbles in general, are caused when the surface tension of water is decreased, which is how soap creates so many bubbles. Let’s assume that there isn’t soap in your decanter (but you should be sure you are rinsing them out thoroughly, just to be certain). That means there is something in the wine itself that is affecting the surface tension of the wine or the rinsing water.

There are two sources of surface-tension reduction in wine. The first is a protein that’s present in tannins. I tend to notice the foam phenomenon much more often in red wines than in whites, and there are more tannins in reds. Alcohol (ethanol) also has an effect on surface tension. So, you can expect a tannic, high-alcohol wine to yield the most foam among still wines.

—Dr. Vinny

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