Thermisation is a sub-pasteurisation process sometimes used to extend the period of refrigerated storage of milk prior to further processing and/or further heat treatment. The heating conditions vary but are in the range 57–68°C for 20 - 5 s and usually 63-65°C for 15-20 s. Thermisation extends milk shelf-life for 3–4 days before further processing. It is also used for the manufacture of some semi-hard cheeses e.g. in Switzerland and other countries in the European Alps and when applied to these products, heating conditions for the cheese milk are 57°C/30 min, 60°C/5 min or 65°C/15 s. Such cheese needs to be aged for 60 day or longer in the US and many other countries.. In principle, every heat treatment between > 40°C and < 72°C counts as thermisation, but no legal definition exits in most countries. Thermisation greatly reduces the numbers of psychrotrophic bacteria and their production of proteases and lipases, many of which are heat-resistant and can cause flavour defects in products such as Extended Shelf-Life (ESL) and Ultra-High Temperature (UHT) milk. Thermisation reduces the numbers of most pathogenic bacteria; it can be effective in reducing the numbers of Campylobacter spp., E. coli and Salmonella. Surviving microorganisms are heat-stressed and become more vulnerable to subsequent microbiological hurdles. However, because the effect is not sufficient to kill all pathogens likely to be present in raw milk, it is not suitable for the production of milk for human consumption. Cheeses made from thermised milk, compared to the ones from pasteurised milk, show more proteolysis and lipolysis, which contributes to flavour development.
