⚙️ Cream separator

A separator is a centrifuge that splits milk into cream and skim milk. Inside, a drum with a stack of conical discs spins fast: the light fat gathers closer to the axis, the heavier skim part is flung to the edges, and the two streams leave through different outlets. Only 0.04–0.07% fat remains in the skim milk.

Fat globules in milk are up to 15 µm across. On their own they rise slowly: a 3 µm globule rises under gravity by about 0.6 mm per hour. Before centrifuges, fat was separated by gravity: fresh milk was left in a vessel until the globules floated up as a layer of cream, which was skimmed off by hand. In Tetra Pak’s example the drum makes 5,400 revolutions per minute, and the settling goes about 6,500 times faster.

Before separation the milk is usually warmed to 55–65 °C. Afterwards the cream is standardised to a pre-set fat content: in the handbook’s example, separating 100 kg of whole milk gives about 90 kg of skim milk with 0.05% fat and about 9.7 kg of cream with 40% fat. Hermetic bowls give cream with over 72% fat. Self-cleaning bowls discharge sludge every 30–60 minutes: about 1 kg per 10,000 litres of milk. A clarifier resembles a separator but has one outlet and only cleans the milk, while a separator has two outlets.

A German trade journal described a cream-separating drum in 1877, and in January 1879 a centrifugal separator was shown in Stockholm, where the Swedish engineer Gustaf de Laval had taken up the idea. From 1890 the separators built by de Laval had conical discs, under a patent granted in 1888 to the German Freiherr von Bechtolsheim and acquired in 1889 by the Swedish company AB Separator, of which de Laval was part-owner.

Why it matters to you. A separator lets a dairy mix cream and skim milk in the desired proportion: the handbook shows how skim milk and cream are blended to standardise market milk, for example to 3% fat. So the different fat contents on the carton are the result of such standardisation, not separate kinds of milk.