Lactic acid is a compound found in foods, most notably dairy products. Natural fermentation forms lactic acid. The human body produces lactase to process lactic acid. People with lower levels of lactase production can have issues digesting lactic acid and become lactose intolerant. In addition to dairy products, a number of other food items contain high amounts of lactic acid.
Dairy products, including milk, cheese and yogurt, contain high levels of lactic acid. Milk contains some of the highest levels of lactic acid, with condensed milk and evaporated milk having higher levels. Cheese varities including Swiss, cheddar, American and blue all contain high levels of lactic acid. Other dairy products with high lactic acid levels include ice cream, cream cheese and sour cream.
Lactic acid helps prevent spoilage in pickled goods. You'll often find it in items such as pickled olives, onions and other vegetables. Lactic acid is most commonly found in items jarred in brine.
Lactic acid is a sourdough acid produced in sourdough through natural fermentation. The unique taste of sourdough bread is a result of the presence of lactic acid. Sourdough bread loafs, biscuits, pancakes and even cookies contain lactic acid.