Cream of tartar is an acid—specifically, tartaric acid. It’s a byproduct of wine production, the residue left on the barrels. Most commonly, cream of tartar is used as a leavener, because when it’s combined with baking soda, they produce carbon dioxide gas. That’s the same gas that’s produced by yeast in bread baking. When it’s added to egg whites, it boosts the strength of the individual air bubbles and slows down their natural tendency to deflate.