After reading up on the proper books, I started adding some acid when beating meringues. At home, I use cream of tartar, but when I am baking at somebody else's kitchen, I am lucky if they have at least some citric acid; cream of tartar is a huge rarity here.
The trouble is that the books don't specify a ratio. They speak of throwing in "a pinch", suggesting that the exact amount is not that important. In my experience, this is not true.
I have had meringues fail despite cream of tartar. They went from a runny mess to an overtightened stiff layer floating on water without a noticeable sweet spot inbetween. I admit that I may have used the wrong whipping speed (this happens especially often when I whip by hand), but 1) I can't attribute it to speed alone, and 2) this is exactly the kind of problem I hope the acid works against.
Then again, I have whipped beautiful meringues with acid. Cream of tartar helps there too, but citric acid frequently gives me a great texture from a single pinch (maybe 1.5 g) per 2-3 eggwhites. The meringue gets smooth and glossy, with robust little bubbles, and holds without deflating when folded into other ingredients, even during a not-so-gentle macaronage. The problem: at this concentration, the sour taste is already very noticeable. In some very sweet applications, this still works, but most times it adulterates the taste.
So, what is the minimal ratio at which acid works (given a sensible technique - forget hand whipping for now?) And what is the maximum ratio at which acid is still undetectable in taste? I am interested in both cream of tartar and citric acid here.