Can you name the acid found in sour milk products?

There are a lot of different milk products, and each of them has its own taste, consistence and health benefits. You probably like one of them: koumiss, laban, yogurt or, maybe, kefir? The thing is that all these products made of milk contain lactic acid. It curdles the casein in fermented milk. Sourdough breads tastes sour also because of the lactic acid.