Pasteurization 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 pasteurization 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.[1] Milk simply labeled "pasteurization " is usually treated with the HTST method, whereas milk labeled "ultra-pasteurization " or simply "UHT" has been treated with the UHT method
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