What is the primary mechanism by which pasteurization extends the shelf life of milk?

Pasteurization extends the shelf life of milk primarily by reducing the number of viable microorganisms, particularly spoilage bacteria and pathogens, through heat treatment. Viable microorganisms are those capable of growing and causing spoilage or illness. Pasteurization isn't sterilization; it doesn't eliminate all microorganisms. Instead, it reduces their population to a level where they are unlikely to cause spoilage or pose a health risk during the expected shelf life of the milk under refrigerated conditions. The heat denatures enzymes, biological catalysts, that contribute to spoilage and off-flavors, further increasing shelf life. For example, pasteurization targets Coxiella burnetii, a heat-resistant pathogen, ensuring its inactivation while minimizing changes to the milk's flavor and nutritional value. This reduction in microbial load and enzyme activity slows down spoilage processes like souring and off-flavor development, thus extending the milk's usability.