Why does this happen to red wine?

Factual Questions
1

My husband and I had a couple of glasses of red wine yesterday and set the glasses in the sink overnight because we didn’t feel like washing them. This morning, when we woke up, I decided it was time to do the dishes, so I poured a little water in one of the glasses. The liquid was still a dark red, but lighter than the red wine would have been because of the addition of the water. I swirled it around a little, then put in a drop of Dawn dishwashing liquid. After a moment of swirling, the liquid in the glass turned bright blue. Does anyone know what’s going on to cause the color change?

I have noticed this several times before, and seems to be caused by the addition of the dishwashing liquid (as in it doesn’t happen with water alone). Any idea what chemicals could be reacting to cause this?

2

The red colour (anthocyanins, I think) in wine is like litmus dye; it changes colour depending on the PH of the mixture; red in the presence of acid (such as those present in wine), blue in the presence of alkali (such as those present in detergents.

Other fruit pigments also do this; blueberries, blackberries etc…

3

Cool, I didn’t know this. So I decided to try it out: I just poured a little shiraz in a glass, watered it down, and dumped in a pinch of baking soda. Instant blue-gray!

4

Here’s a list of some of the chemicals found in wine.

I’m no chemist, but I’d guess Mangetout’s answer is a correct one, something to do with acid and PH.

Pretty neat, I’d never noticed it myself.

5

Science!

6

Yeah, purple cabbage juice does it too, and seems to be sorta traditional in science classes for youngsters. Based on my own experiments as a kid, virtually any purplish vegetable liquid worked - beet juice, any kind of purple fruit juice, etc . . .

And based on my experiments in high school chemistry class, coffee becomes a lighter brown in the presence of concentrated hydrochloric or nitric acids. Never did tell the teacher about that . . .

7

I expect he noticed anyway, when he drank it.

8

Yup, Mangetout nailed it.
This process has been known for ages, and the romans apparently had recipes for converting red wine to ‘white’, by adding alkaline substances such as eggwhites and ashes. I can’t imagine what it might have tasted like though…

(According to Harold McGee in on Food and Cooking)

9

Don’t worry, I wouldn’t have poisoned him. It was actually an old cup of coffee (my own) and I rinsed it thoroughly when I was done. Not like the more hazardous things like watching as my gunpowder caused shards of hot glassware to come shooting out of the fume hood, or spilling nitric acid everywhere when making guncotton . . . took some real cleanup, it did.

10

Thanks for the explanation, Mangetout! After I noticed the color change, my husband had to physically restrain me from pouring other things into it just to see what happens. He kept telling me, “No! Those are some of the glasses we got from the wedding! At least use a paper cup!!” I finally just washed everything and put it away to keep it all out of harm’s way.

11

Actually, the question is incompletely answered at the moment, because all we’ve done is to say that it does change colour with the PH, without explaining why; there’s a very good (if somewhat technical) explanation of the process here.