Wine Wizard replies: If you’re looking for a way to boost acid without adding extra sugar, stick to acid blend. Using grape-juice concentrate as an additional fermenting agent in fruit wines, on the other hand, is a good way to add extra acid, sugar, flavor and yeast nutrients.
Acid blends are often comprised of citric, tartaric and malic acids. They add tartness to your wine. Acid blends come in white powder form, which you dissolve in a little water or juice and then add to your fermentation.
Grape-juice concentrates are just that — juice concentrates. They pack a heck of a lot more than tartness and include water, carbohydrates of various stripes, amino acids, nitrogen, tartaric acid, citric acid, malic acid … lots of stuff that grapes naturally possess. If you want to add sugar and grapey flavors to your wine, use the juice concentrate. To boost tartness without adding anything else, go with the acid blend.