Raw milk contains many types of microorganisms coming from different sources. The average composition of cow’s milk is protein 3.2%, carbohydrates 4.8%, lipids 3.9%, and minerals 0.9%. Besides casein and lactalbumin, it has free amino acids, which can provide a good N source (and some C source, if necessary). As the main carbohydrate is lactose, those microorganisms with lactose-hydrolyzing enzymes (phospho-b-galactosidase or b-galactosidase) have an advantage over those unable to metabolize lactose. Milk fat can be hydrolyzed by microbial lipases, with the release of small-molecular volatile fatty acids (butyric, capric, and caproic acids).