🔄 Homogenization

Homogenization is the breaking up of fat globules in milk so the cream does not rise to the top. In fresh milk, fat floats in globules averaging 3.5 micrometres; a homogenizer forces the milk under pressure through a narrow gap, and the globules shrink to less than 1 micrometre. These tiny globules are spread evenly throughout the carton.

According to the Tetra Pak handbook, the pressure is usually 10–25 MPa (100–250 bar) and the milk is heated to 55–80 °C: below 40 °C the fat is partly solid and cannot be broken up properly. The second stage works at about 3 MPa. Every 4 MPa of pressure drop warms the milk by roughly 1 °C. In a dairy plant, the homogenizer sits in the pasteurisation line and treats milk that is already warm.

After homogenization, the surface area of the fat globules increases 4–6 times. The milk becomes whiter and more appealing, the cream does not separate, and the fat oxidises less. The downside is sensitivity to light: a “sunlight” off-flavour can develop. The method itself was patented by the Frenchman Auguste Gaulin in 1899.

Homogenization or pasteurisation? They are two different steps. Pasteurisation heats milk to destroy disease-causing microbes, while homogenization only breaks up fat mechanically and kills none, which is why a dairy plant puts the homogenizer into the pasteurisation line and treats milk that is already warm. Homogenization or simply skimming? Skimming changes the fat content; homogenization leaves it as it was and only keeps the fat evenly spread, so the milk looks whiter and no cream layer forms.

The pack of milk that has been through this step says «homogenised» (homogenized in US spelling), and you will see no cream layer on top. The fat class is the legal name: whole milk from 3.5% fat, semi-skimmed 1.5–1.8%, skimmed up to 0.5%. Homogenization is no reason to expect a different fat content. You will find these on the dairy shelf of any supermarket or small grocery.

Why it matters to you. The 1970s hypothesis that homogenized milk harms the heart through the enzyme xanthine oxidase was examined in the American Journal of Clinical Nutrition in 1983: absorption of this enzyme from food has not been proven, and the data did not support the hypothesis. Homogenization does not remove microbes — that is the job of pasteurisation. Nor does it change the fat content on the pack: in the EU, whole milk has from 3.5% fat, semi-skimmed 1.5–1.8% and skimmed up to 0.5%.