There are various processes for preserving milk.
In pasteurisation, the milk is heated to at least 72 °C and held at this temperature for approx. 15 seconds before being cooled instantly. This makes it almost sterile and ready to drink. Pasteurised milk keeps in the fridge for several days.
In microfiltration, the milk is filtered before pasteurisation, allowing a refrigerated shelf life of three weeks to be achieved without further heating. The shelf life can be extended by two weeks, because the filtration step uses very fine screens to filter out spores and germs that cannot be killed by pasteurisation.
UHT stands for ultra-high-temperature. In this process, the milk is heated to approx. 135 °C and then cooled very quickly. Long-life milk is germ-free and can be kept unopened for several months.
Generally, milk is also homogenised. This means that the milk fat is broken up and distributed finely throughout the milk. The fat can no longer rise to the surface. The addition of preservatives to extend the shelf life of milk for drinking is generally prohibited.