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Lactic acid in food
Lactic acid is primarily found in sour milk products, such as: koumiss, leban, yogurt, kefir and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid.
Although it can be fermented from lactose (milk sugar), most commercially used lactic acid is derived by using bacteria such as Bacillus acidilacti, Lactobacillus delbueckii or Lactobacillus bulgaricus to ferment carbohydrates from nondairy sources such as cornstarch, potatoes and molasses. Thus, although it is commonly known as "milk acid", products claiming to be vegan do sometimes feature lactic acid as an ingredient.
Lactic acid may also be found in various processed foods, usually either as a pH adjusting ingredient, or as a preservative (either as antioxidant or for control of pathogenic micro-organisms). It may also be used as a fermentation booster in rye and sourdough breads.