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If you are beating eggs whites and don't have cream of tartar, you can substitute white vinegar (in the same ratio as cream of tartar, generally 1/8 teaspoon per egg white). It is a little more problematic to find a substitute for cream of tartar in baking projects. White vinegar or lemon juice, in the ratio of 3 times the amount of cream of tartar called for, will provide the right amount of acid for most recipes. But that amount of liquid may cause other problems in the recipe, and bakers have found that cakes made with vinegar or lemon juice have a coarser grain and are more prone to shrinking than those made with cream of tartar.