Just exactly what is cream of tartar that makes it so magical? Technically, it’s an acid—specifically, tartaric acid. It’s a byproduct of wine production, the residue left on the barrels, actually. Most commonly, cream of tartar is used as a leavener, because when it’s combined with baking soda, together 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. And when added to simple syrup, it prevents sugar’s natural tendency to re-bond and form crystals.
But if you don’t have it in your pantry and your recipe calls for it, just substitute fresh lemon juice or white vinegar for the cream of tartar. For every 1/2 teaspoon of baking soda in the recipe, use 1 teaspoon lemon juice or white vinegar. As an example, if your cookie recipe calls for 1 teaspoon baking soda, add 2 teaspoons lemon juice instead of the cream of tartar. If your simple syrup recipe asks for 1/8 teaspoon cream of tartar, use 3-4 drops of lemon juice. And for the whipping egg whites? Add 1/2 teaspoon lemon juice per egg white.
The results will be so close, you probably won’t notice the substitution.