Hygiene and Safety Aspects

Milk, in its raw state, does not stay fresh for very long and is an ideal medium for bacteria to grow. Various types of heat treatment are used to improve the keeping quality of milk and to kill any harmful bacteria present. These techniques are pasteurization, sterilization, and ultra heat treatment, and they are also used for cream.

Pasteurization is named after the French scientist Louis Pasteur and entails the heating of milk to at least 71.7 °C for a minimum of 15 s. After heating, the milk is quickly cooled to less than 10 °C. In the UK, pasteurization accounts for over 90% of all heat-treated milk and has little effect on the taste and nutritional value of milk.

Sterilization is a more severe process, in which milk is heated to a temperature of 115-130 °C for 10-30 min and then poured into sterile plastic or glass bottles. Unopened, sterilized milk will keep for 2-3 months without refrigeration, although once opened it has to be treated in the same way as pasteurized milk and will only keep for 4-5 days in a refrigerator. Sterilized milk has a slight caramel taste because the heat 'cooks' the lactose present in milk. Sterilization also reduces the levels of the heat-labile vitamins (see above).

Ultra heat treatment is a milder form of sterilization in which the milk is held at a temperature of not less than 135 °C for at least 1 s, and is then packed into sterile cartons. Such milk will keep unrefrigerated for many months, but once opened it needs to be refrigerated and used in 4-5 days.

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