I had a red wine with a head of foam. What's that about?!

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Dear Dr. Vinny,

At a restaurant, I recently encountered a red wine with a head of foam, and it also tasted extremely sweet. Could that explain the foam?

—Annamaria, Honolulu

Dear Annamaria,

Fermenting wine is known to bubble up, but I suspect you’d describe the wine as being yeasty and effervescent if that was the case. I’ll also rule out the theory that the decanter had some residual soap—I imagine you’d taste or smell that, and soap would bring on a bitter note, not a sweet one.

It’s not unusual for a red wine to foam or froth when it's agitated, by being shaken, or aggressively poured from one vessel to another. I see it sometimes when rinsing out my wineglasses or decanters—a little bit of leftover wine can create a surprising amount of foam.

Science tells us that bubbles for more readily when a liquid's surface tension is decreased. That’s what detergent does to make your shampoo and dish soap bubbly. In wine, it’s the tannins and ethanol (alcohol) that reduce the surface tension of the wine. Tannic, high-alcohol red wines are likely to produce a fair amount of foam.

But sugar has the opposite effect, actually increasing the surface tension of water, so that doesn’t explain your foamy wine. What you describe is certainly unusual, and between that and the fact that you found the wine overly sweet, I think it would have been OK to say something to your server. Next time just tell them that the wine isn’t to your liking because of the sweetness, and the foaminess is off-putting.

—Dr. Vinny

Ask Dr. Vinny

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