Pasteurization is the process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food. Unlike sterilization, pasteurization is not intended to kill all microorganisms in the food. Instead, pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated and consumed before its expiration date). Pasteurization is typically associated with milk. It is the main reason for milk’s extended shelf life. High temperature–short time (HTST) pasteurized milk typically has a refrigerated shelf life of two to three weeks. There are two main types of pasteurization used today: HTST and extended shelf life (ESL) treatment. In the HTST process, milk is forced between metal plates or through pipes heated on the outside by hot water, and is heated to 71.7 degrees C (161 degrees F) for 15–20 seconds. ESL milk has a microbial filtration step and lower temperatures than ultrahigh temperature (UHT).
Microorganism is the term applied to all microscopically small living organisms. We tend to associate microorganisms with disease. Microorganisms that cause disease are called pathogens. However, few microorganisms are pathogens and microorganisms play a crucial part in the life of our planet. For example, they provide food for fish; they occur in soil where they provide nutrients for plants; and they play an important role in ruminant digestion. In dairying, some microorganisms are harmful (for example, spoilage organisms, pathogens) while others are beneficial (cheese and yoghurt starters, yeasts and moulds used in controlled fermentations in milk processing). The microorganisms principally encountered in the dairy industry are bacteria, yeasts, moulds and viruses.