Lactic acid is an organic compound. In its solid state, it is white and water-soluble. In its liquid state, it is colorless. It is produced both naturally and synthetically.
Lactic acid is found primarily in sour milk products, such as koumiss, laban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor of sourdough bread.