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Pasteurization is not intended to destroy all pathogenic micro-organisms in the food or liquid. Instead, pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming pasteurization product is stored as indicated 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.

Pasteurization is typically associated with milk, first suggested by Franz von Soxhlet in 1886.

Pasteurization typically uses temperatures below boiling since at very high temperatures, 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 135 °C (275 °F) for a fraction of a second. ESL milk has a microbial filtration step and lower temperatures than HTST.[4] Milk simply labeled "pasteurised" is usually treated with the HTST method, whereas milk labeled "ultra-pasteurised" or simply "UHT" has been treated with the UHT method.

The HTST pasteurization standard was designed to achieve a 5-log reduction, killing 99.999% of the number of viable micro-organisms in milk. This is considered adequate for destroying almost all yeasts, mold, and common spoilage bacteria and also to ensure adequate destruction of common pathogenic heat-resistant organisms (though, not including Mycobacterium tuberculosis, which causes tuberculosis) and Coxiella burnetii, which causes Q fever. HTST pasteurization processes must be designed so that the milk is heated evenly, and no part of the milk is subject to a shorter time or a lower temperature.

Effectiveness of pasteurization

Milk pasteurization has been subject to increasing scrutiny in recent years, due to the discovery of pathogens that are both widespread and heat resistant (able to survive pasteurization in significant numbers). One of these pathogens, Mycobacterium avium subsp. paratuberculosis (MAP), is linked to Crohn's Disease.

http://en.wikipedia.org/wiki/Pasteurization

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