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Why must pasteurized milk be refrigerated?

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Painting by Albert Edelfelt showing Louis Pasteur in his laboratory, holding a glass flask and examining it intently, surrounded by scientific equipment on a workbench. — Louis Pasteur In the 19th century Louis Pasteur's work established the germ theory of disease, proving that microorganisms cause fermentation and sickness, which led to the development of pasteurization, aseptic techniques in medicine, and effective vaccines for diseases such as rabies. (Painting by Albert Edelfelt, 1885.)

Pasteurized milk must be refrigerated because the pasteurization process, named for the French scientist Louis Pasteur, is not a sterilization method. While it effectively destroys pathogenic microorganisms and reduces the number of spoilage-causing microbes, it does not eliminate all bacteria.123

Specifically, conventional pasteurization does not kill thermoduric bacteria (those resistant to heat) or thermophilic bacteria (those that thrive at high temperatures). Because the milk is not sterile and still contains small numbers of harmless bacteria, it remains susceptible to microbial growth. Refrigeration is therefore essential to control this growth and extend the product's shelf life. While ultrahigh-temperature (UHT) milk can be stored without refrigeration due to more intensive heating and sterile, hermetically sealed packaging, conventionally pasteurized milk requires consistent cold storage to remain safe and fresh.2314