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Milk is a suspension of fats and proteins in water. Spoiled milk involve the denaturation of the proteins, by the oxygen of the atmosphere and by the bacteria. Fats can also be oxydized (rancid) by oxygen when they are in saturated. Milk, due to presence of easily usable carbohydrates (lactose), fat, protein and high nutritional value, provides an ideal environment for microbial spoilage. Proteolysis and putrefaction are typical results of microbial spoilage of such high protein materials. Milk undergoes a predictable four-step succession during spoilage, acid production by Lactococcus lactis. Lactis is followed by additional acid production associated with the growth of the more acid tolerant organism such as Lactobacillus. At this point yeasts and molds become dominant and degrade the accumulated lactic acid and the acidity gradually decreases. Eventually protein-digesting bacteria become active, resulting in a putrid odor and bitter flavor. The milk, originally opaque, can eventually become clear.