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Pasteurisation (or Pasteurization), is a process which retards microbial growth in foods. The process was named after its creator, French chemist and microbiologist Louis Pasteur. The first pasteurisation test was completed by Louis Pasteur and Claude Bernard on April 20, 1862.
Unlike sterilisation, pasteurisation is not intended to kill all pathogenic microorganisms in the food or liquid. Instead, pasteurisation aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurisation product is refrigerated and consumed before its expiration date). Commercial-scale sterilisation of food is not common because it adversely affects the taste and quality of the product.
Pasteurisation typically uses temperatures below boiling since at temperatures above the boiling point for milk, casein micelles will irreversibly aggregate (or "curdle"). There are two main types of pasteurisation used today: High Temperature/Short Time (HTST) and Extended Shelf Life (ESL) treatment. Ultra-high temperature (UHT or ultra-heat treated) is also used for milk 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 °C (161 °F) for 15-20 seconds. UHT processing holds the milk at a temperature of 138 °C (250 °F) for a fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST. Milk simply labeled "pasteurisation " is usually treated with the HTST method, whereas milk labeled "ultra-pasteurisation " or simply "UHT" has been treated with the UHT method.