Pasteurisation is a term most people understand, even if they are unfamiliar with the details. Simply defined, pasteurisation is making foods hot enough, for long enough, to kill most of the microorganisms in the food and reduce the risk of food-borne illness. A side benefit of pasteurisation is that it stops the activity of natural enzymes that cause spoilage. It is less well-known that repeating the pasteurisation process can extend the shelf life of milk.
Pour the milk into a heavy-bottomed saucepan and heat it gently, stirring frequently to prevent the bottom from sticking and scorching.
Sterilise an instant-read or candy thermometer, and use it to monitor the temperature of the milk. As the temperature of the milk rises, slightly turn down the heat to reduce the risk of scorching.
Bring the milk to a temperature of 71.1 degrees Celsius, and hold it there for no less than 30 seconds. After 30 seconds, remove the milk from the heat, and chill it quickly. Refrigerate as soon as it nears room temperature.
Bring the milk to a boil, if you do not have access to a thermometer. Boil for the same 30-second interval, then cool and refrigerate the milk as previously directed. Boiling is equally effective, but it does alter the flavour of the milk. Whenever possible, use a thermometer and the lower temperature.
Milk will keep for another seven to 10 days after being processed. Instead of the hob method, the milk can be heated in a microwave oven until it reaches a temperature in excess of 71.1 degrees Cor at least 30 seconds. The milk frothing wand of an espresso machine also works well, but home machines might not contain enough water to adequately heat a large quantity of milk.
Milk that has already begun to sour cannot be revived by this method.