What causes the blue tinge when I clean red wine out of my wineglass?

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Dear Dr. Vinny,

Sometimes when I wash my wineglasses after drinking a red, the water takes on a blue tinge (as opposed to red). Is this a result of some kind of coloring agent?

—Thomas, Boca Raton, Fla.

Dear Thomas,

It’s not a coloring agent; it's science! Red wines take their color from grape skin pigments called anthocyanins, the same things that give berries, plums and cherries their stain-inducing colors. Some grapes and types of wines have more pigments than others, and how ripe a grape gets can affect how much color is released from its skins.

Anthocyanins are also nature’s litmus test: Their color can be reflective of the pH of whatever they come into contact with. When anthocyanins are in an acidic environment (like wine), they're red, but if you introduce them to an alkaline solution (like water with a slightly elevated pH, or if cleaning products are added into the mix) they turn blue.

You might also notice this phenomenon if you are trying to clean a wine spill out of your carpet or shirt: The stain might turn blue before (or instead of) disappearing.

—Dr. Vinny

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