Lactic acid is primarily found in sour milk products, such as yogurt, kefir and some cottage cheeses. While 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

Lactic acid is found in many processed foods, usually either as a pH adjusting ingredient, or as a preservative. It is used to give flavor and tartness to carbonated juices and other beverages and to some desserts. Its acidity reduces spoilage in such foods as cheeses, olives, breads, butter, and candy. Lactic acid helps condition dough and stabilize wine as well. Lactic acid is also present in wheat beers, especially lambic, due to the activity of Pediococcus damnosus. It is also widely used for inhibiting pathogenic bacteria like E.coli, Salmonella, Campylobacter and Listeria on animal carcasses like beef, pork and poultry during the slaughtering process. to ferment carbohydrates from nondairy sources such as cornstarch, potatoes and molasses. Although it is commonly known as "milk acid", products claiming to be vegan do sometimes feature lactic acid as an ingredient.

Lactic acid is safe and on the GRAS list, but not used in infant formulas.

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