Heat-stable proteinases and lipases of psychrotrophic bacteria are not inactivated by pasteurization and can cause proteolysis of casein and lipolysis of milk lipids to produce flavor defects. However, under normal short-term refrigerated storage, these defects may not be enough to detect. In addition, psychrotrophic bacteria contaminating milk after pasteurization can also multiply during storage and cause spoilage, especially when the milk is either stored for a long time or temperature abused. In that event, it is difficult to differentiate the role of bacterial growth and heat-stable