A Powder in Your Pantry

• Most of the acidic ingredients commonly available to home cooks are liquids. If you need to lend a bracing tang to your cooking or baking you can turn to buttermilk, fruit juices, wine, Worcestershire sauce or hot sauces, or hundreds of different kinds of vinegar. Finding acidic ingredients that are naturally dry is much harder. Dried lemon zest adds flavor but little acidity, and the same is true of the vividly red sumac powder used as a spice in Middle Eastern cooking. One of the very few commonly available acidic ingredients that's dry is cream of tartar.

Bring on the Wine

• Cream of tartar is the retail version of tartaric acid, a chemical salt that occurs naturally in some kinds of fruit. In its commercial form, it's a by-product of wine making. While the grape juice slowly turns into wine in its large vats, a number of chemical processes take place. One of them causes crystals of tartaric acid to form in the wine, and slowly settle to the bottom of the vat along with other impurities. After the wine is filtered away for bottling, the tartaric acid in the sediment is separated and refined.

Silky Soft

• The purified crystals are a very pure white, but rough and grainy to the touch. To make them quicker-dissolving and therefore more useful in the kitchen, manufacturers grind them to a very fine white powder. That's the form of tartaric acid you'll find in the supermarket, labeled as cream of tartar. The "cream" in its name is a reference to its whiteness, and to the remarkably soft, smooth texture it gains from being ground so very finely.

Just a Pinch

• Cream of tartar is sold in relatively small containers, because even the busiest of bakers home bakers usually use it in small quantities. Most often, a small pinch of tartar is added to egg whites before they're beaten into meringue for pies, souffles or sponge cakes. The acidity helps the proteins in the egg whites bond together into a long-lasting foam, without adding a distinctive flavor as vinegar or lemon juice would. It's also an ingredient in most baking powders, where it provides a shelf-stable acid that will react reliably with sodium bicarbonate, a dry, powdered base also used in baking powder, once the baking powder is moistened.