⚙️ Pasteurizer

A pasteurizer is a unit that quickly heats milk to 72–76 °C, holds it there for 15–20 seconds and cools it right away. In dairies, it is usually a plate heat exchanger: a stack of thin, ridged stainless steel plates. Between the plates are narrow channels. Milk flows through some, and hot or cold water through the ones next to them. Heat passes through the wall, but the liquids do not mix.

A pasteurizer is made up of sections. In the regeneration (heat recovery) section, cold raw milk is warmed by hot, already pasteurized milk and cools it at the same time. This recovers up to 90–95% of the heat. Then the milk is heated the rest of the way with hot water and sent into the holding tube — a long pipe where it spends the required seconds. After that, the milk is cooled with ice-cold water.

If the temperature drops below the norm, a special valve sends the milk back to be heated again. A pump keeps the pressure on the finished-milk side higher than on the raw-milk side: if there is a leak, raw milk will not get into the pasteurized milk. Plate heat exchangers were patented in Germany in the late 19th century. They became the main type in the dairy industry from the 1920s.