When you let milk go warm (allowing it to reach room temperature) and then put it in the fridge, several changes occur due to temperature fluctuations:
Bacterial growth: Warm temperatures create an ideal environment for bacterial growth in milk. Bacteria can multiply rapidly at room temperature, potentially leading to spoilage and the development of off-flavors.
Souring: As the milk warms up, the naturally occurring bacteria in the milk start to metabolize lactose (milk sugar) into lactic acid. This process contributes to the souring of milk.
Separation: The fat content in milk can separate when it is exposed to warm temperatures. This separation may lead to the formation of a cream layer on top of the milk.
Reversal of homogenization: Homogenized milk undergoes a process to disperse the fat globules evenly throughout the milk, preventing them from rising to the top. When the milk is warmed and then cooled again, this homogenization can be reversed, leading to the separation of the fat.
Texture changes: The proteins in milk can denature or change their structure when exposed to higher temperatures, which might affect the texture of the milk.
Cooling process: Once you put the warm milk back into the fridge, it will gradually cool down to refrigerator temperature. However, the changes that occurred while the milk was warm may not be entirely reversible, especially regarding bacterial growth and the development of sourness.
To maintain the quality and freshness of milk, it's best to store it in the refrigerator at a consistent temperature throughout its shelf life and avoid letting it warm up at room temperature for an extended period. If the milk has been left out at room temperature for more than a couple of hours, it's safer to discard it to prevent potential health risks.