From the time the milk is secreted from the cow, it undergoes a series of cooling and heating steps. Milk straight from the cow is about 101 degrees F, but it is quickly cooled to 40 degrees F and is maintained at that temperature until it goes through processing steps.

Pasteurization of milk is done to kill bacterial contaminants while not substantially altering the milk characteristics.

• Heating at 74 C for 15 seconds (low pasteurization) results in killing most organisms and inactivates some enzymes, but does not otherwise alter the milk.
• Heating at 90 C for 15 seconds (high pasteurization) results in killing all vegetative microorganisms, inactivating most enzymes, but also rendering some whey proteins insoluble.
• Heating at 118 C for 20 seconds sterilizes the milk, killing all microorganisms, including spores, inactivating all enzyme activities, but also causes changes to the milk such as the browning reaction involving the proteins and sugars.
• Ultra-high temperature (UHT) pasteurization of milk at 145 C for a few seconds sterilizes milk while minimizing the chemical changes. UHT milk which is properly stored has a long shelf life, even at room temperature.