🏭 Milk processing at the dairy plant

Milk processing at the dairy plant covers what happens to raw milk between the tanker and the pack: cooling, testing, heating, homogenization, filling and labelling. At a dairy plant in the EU the incoming milk is quickly cooled to 6 °C or below if it is not processed within 4 hours of arrival.

The key step is heat treatment. In the EU, pasteurization means at least 72 °C for 15 seconds or 63 °C for 30 minutes; in practice homogenized milk is heated to 72–75 °C for 15–20 seconds. The laboratory checks that it worked: the alkaline phosphatase test has to be negative. According to FAO, pasteurization extends shelf life and cuts possible pathogens to a level that poses no significant danger.

For UHT treatment milk is heated in a continuous flow to at least 135 °C, in industry practice to 135–150 °C for 1–10 seconds. Afterwards, in an aseptically sealed pack, it has to stay stable for 15 days at 30 °C or 7 days at 55 °C.

A homogenizer breaks up the fat globules, on average from 3.5 µm to under 1 µm, so that the cream does not rise. The method was invented by Auguste Gaulin in 1899. Yet some countries leave milk unhomogenized, because a good cream line is seen as a sign of quality.

The milk is then standardized for fat (in the EU whole milk has at least 3.5% fat and skimmed milk no more than 0.5%), filled on the filling line and labelled. Part of it goes into cheese and fermented products, where the casein is set by rennet or by lactic acid bacteria from a starter culture.