Used to boost the total acidity of wine, generally 0.1% per tsp per gallon. Preserves color in white wine and meads. Also worth trying if you're experimenting with a batch of sour/lambic beer, in conjunction with a blend of other acids (lactic, tartaric, malic, acetic, etc).
Dissolve in water before adding. Usage varies, but add sparingly as it will add a very tart flavor in greater quantities.