Lactic acid consists lactic acid bacteria. These lactic acid bacteria cause the cheese to acidify. Acidification of the cheese takes place through the formation of lactic acid from the milk sugars in the cheese milk. The formation of acid is important for the shelf life, firmness and taste of the cheese. The lowering of the pH (= acidity) also limits the growth of micro-organisms.
Slightly soured milk curdles better and is especially firmer. Acid formation also stimulates the exit of whey from the curd during curd preparation.