🔄 Separation

Separation is the splitting of whole milk into cream and skim milk in a centrifugal separator. Fat is lighter than the rest of the milk, so in a fast-spinning bowl fat globules move to the centre and heavier skim milk to the edge. This works thousands of times faster than standing. ¹

How it used to be done. For centuries fresh milk was left in a vessel: the fat globules clustered, rose, and the cream was skimmed off by hand. Globules can be up to 15 micrometres, and a 3-micrometre globule rises only 0.6 mm an hour, so gravity creaming is slow and incomplete. ¹

Where the separator came from. In 1877 a German dairy journal described a rotating drum that separated cream, and the young Swede Gustaf de Laval set out to show that centrifugal force works in Sweden too. ¹ Together with Oscar Lamm he patented a separator in 1878. ² In January 1879 a continuous separator was demonstrated daily in Stockholm, and from 1890 the bowls were fitted with conical discs patented in Germany in 1888. ¹

How the bowl works. Inside is a stack of conical discs with narrow gaps between them. Milk enters through holes in the discs; the cream moves along the gaps towards the axis, the skim milk towards the edge. At 20 cm from the axis and 5,400 rpm, a fat globule moves about 6,500 times faster than under gravity. Milk is usually heated to 55–65 °C, and for cold separation it is first held cold for 10–12 hours. ¹

What comes out. The smallest globules, under one micrometre, have no time to rise and leave with the skim milk, which keeps 0.04–0.07% fat. Cream is usually about 10% of the flow, and its fat content is set with valves: for example, 20,000 litres an hour of 4% milk give 2,000 litres of 40% cream. ¹

Sediment and self-cleaning. Heavy impurities are thrown to the bowl wall: straw, hairs, udder cells, white blood cells and bacteria, about 1 kg per 10,000 litres of milk. Modern separators eject the sediment themselves every 30–60 minutes. A clarifier has one outlet, a separator two; similar centrifuges called bactofuges remove bacteria and spores from milk at 50–55 °C. ¹

Standardisation. After separation, cream and skim milk are often mixed back to the required fat content. In the EU, whole drinking milk has at least 3.5% fat, semi-skimmed 1.5–1.8% and skimmed no more than 0.5%. ³

Separation or standardisation? Separation only splits whole milk into cream and skim milk; standardisation comes after, when cream and skim milk are mixed back to the fat content the pack states. A clarifier has one outlet and a separator two, while bactofuges are similar centrifuges that remove bacteria and spores. Gravity creaming, the old way of skimming cream by hand after the milk has stood, is slow and incomplete: a 3-micrometre globule rises only 0.6 mm an hour, whereas in a separator it moves thousands of times faster.

The words on the pack tell you what separation and standardisation did: whole, semi-skimmed or skimmed. In the EU these terms are tied to fat content, so whole has at least 3.5%, semi-skimmed 1.5–1.8% and skimmed no more than 0.5%. You will find them in the chilled dairy aisle of a supermarket or at a dairy shop. Cream is sold nearby, and its fat content is set by the valves of the separator, for example 40% cream.

Why it matters to you. Skim milk is not watered down: centrifugal force removed only the fat, while the proteins, lactose and calcium remain. ¹ The fat figure on the pack is the result of separation and standardisation, not a natural property of the milk. ³ Cream, butter, sour cream and ice cream all start at the separator; at home cream also rises on standing whole milk, but slowly and incompletely. ¹