Because cream of tartar is a relatively uncommon ingredient, it can sometimes mystify cooks who do not have any on hand. This white powder is a by-product of winemaking that is formed when tartaric acid, found in grape skin, combines with potassium hydroxide to make potassium hydrogen tartrate. However, you can use some more common ingredients as substitutes for cream of tartar.
Consequences of leaving out
Leaving cream of tartar out of a recipe without an adequate substitution can significantly alter the results as cream of tartar is an acid that reacts with other ingredients in the recipe. Leaving cream of tartar out when a recipe calls for it is especially disastrous in baking recipes that include cream of tartar and baking soda. This is because the baking soda will not activate without an acid, which is provided by the cream of tartar. Meringue recipes that call for cream of tartar will usually work out, but you need to beat the eggs for an extra period of time to ensure that they are stiff because the cream of tartar helps to stabilise them. Icing recipes sometimes include cream of tartar to create a smoother texture, but the frosting should turn out usable if you leave out the cream of tartar and use the icing right away.
Use baking powder as a substitute for the baking soda and cream of tartar in a recipe. This works because some types of baking powder are made entirely out of cream of tartar and baking soda. For every 1/4 tsp baking soda and 5/8 tsp cream of tartar the recipe calls for, use 1 tsp baking powder instead.
Vinegar or lemon juice
Because vinegar and lemon juice are strong acids, you can sometimes use them instead of cream of tartar. In general, use two to three times as much white vinegar or lemon juice as you would cream of tartar. Therefore, if a recipe calls for 1 tsp cream of tartar, use 2 to 3 tsp white vinegar or lemon juice.
Substitutions for cream of tartar slightly change the chemistry, and potentially the flavour, of the recipe. For example, if you use lemon juice or white vinegar instead of cream of tartar, these substitutions will add a sour flavour. Therefore, you might need to add extra sugar to the recipe to partially counteract the tartness of the vinegar or lemon juice. On the other hand, lemon juice works especially well as a substitute for cream of tartar in the meringue for lemon meringue pie because the lemon flavour is already present.