The milk starts on the left and enters the piping with working enzymes which, when heated, lose their structure. This stops the enzymes from working, and so stops pathogen growth. The cooling process prevents further changes. The pasteurization process can heat the cells to the point that they burst from pressure build-up

Pasteurization (or pasteurisation) is a process of heat processing a liquid or a food to kill pathogenic bacteria to make the food safe to eat and to preserve longer. It involves heating the food to kill most harmful microorganisms. Producers pasteurize dairy and other foods to make them safe to eat. The process is named after Louis Pasteur. He was the first person to learn how to do it successfully. The first pasteurization was done by Louis Pasteur and Claude Bernard on 20 April 1862.