Why does pasteurization extend the shelf life of milk?

Pasteurization extends the shelf life of milk primarily by reducing the total number of microorganisms present in the product. While the process is primarily designed as a biological safeguard to destroy pathogenic bacteria, the heat treatment also kills most of the microorganisms responsible for food spoilage.123

By lowering the initial microbial load, pasteurization slows the rate at which milk spoils. However, because pasteurization is not a sterilization process—meaning it does not kill all bacteria, such as heat-resistant thermoduric or thermophilic strains—the milk remains perishable. Consequently, pasteurized milk must be kept under refrigeration to inhibit the growth of any remaining microbes and maintain its quality.2134

The specific shelf life depends on the intensity of the heat treatment and the packaging method:
* Standard HTST pasteurization: Typically lasts about two weeks when properly refrigerated.24
* Ultrapasteurization (UP): Uses higher temperatures to extend shelf life to 60–90 days, provided it remains refrigerated.14
* Ultrahigh-temperature (UHT) pasteurization: When combined with sterile, hermetically sealed packaging, this allows the product to remain shelf-stable without refrigeration for months.124