Acids: lactic acid comes from sour milk, yogurt, cheese, and other dairy foods, while malic, tartaric, and citric acids are derived from fruits. Malic acid gives green apples their natural tartness; tartaric acid is found in grapes. These acids are used both as seasonings and for their special properties. Lactic acid, for example, can coagulate protein gels, while tartaric acid is an antioxidant and sometimes can prevent crystallization of some sugar syrups. All of these ingredients can be purchased online, but citric acid can also be found in many grocery stores, and malic acid is sold by some brewing-supply stores.