How centrifugal butter is made

Whole milk reaches the dairy chilled and is held in an insulated stainless tank. About 4% of it is fat, yet none is visible: the fat is spread through the milk as billions of separate droplets.

Everything that follows is about getting those droplets out of the milk and then persuading them to stick together.

Zoom in far enough and milk is a watery liquid, the skim, carrying fat globules mostly between 1 and 10 µm across. Each globule is wrapped in a thin membrane of phospholipids and proteins that keeps it from merging with its neighbours.

Fat is lighter than skim, so globules slowly drift upward. For centuries cream was made exactly that way: milk stood in shallow pans for a day or more and the cream was skimmed off the top.

Before separation the milk runs through a plate heat exchanger: a pack of thin corrugated steel plates with milk on one side of each plate and hot water on the other.

Warm milk is less viscous and its fat is fully liquid, so globules move through it more easily. The speed a globule drifts at is inversely proportional to the viscosity of the liquid around it, which is why separators run warm.

A cream separator does the skimming pan's job in seconds by swapping gravity for centrifugal force. Its bowl spins thousands of times a minute; in the Tetra Pak handbook's worked example, 5,400 rpm at a radius of 0.2 m makes globules separate about 6,500 times faster than they would under gravity.

Gustaf de Laval built the first continuously working separator: milk flows in, and cream and skim milk flow out of separate spouts without ever stopping the machine. It was demonstrated in Stockholm in 1879.

Cut the bowl open and it is packed with dozens of thin conical discs, stacked with narrow gaps between them. Milk comes down the centre, is flung outward, and rises through holes in the discs into those gaps.

In every gap the lighter cream moves inward, toward the axis, and leaves through a central outlet. The denser skim milk moves outward past the edge of the stack and up to a separate, outer outlet. The conical discs were patented in 1888 by the German baron von Bechtolsheim and fitted to de Laval's separators from 1890.

The discs turn one deep pool into many shallow channels. A globule no longer has to travel the whole radius of the bowl, only across a single thin gap before it meets a disc surface and slides inward along it.

That short path is what makes separation both fast and thorough: skim milk leaves a good separator with only 0.04–0.07% fat, and the cream that collects at the centre is about ten percent of the milk that went in.

The cream is pasteurized hotter than drinking milk, usually at 85 °C for 15 seconds or more, then cooled into a jacketed ripening tank.

Ripening usually takes 12–15 hours. A controlled programme of cooling and gentle warming makes part of the fat crystallize in a deliberate way, which sets how firm or spreadable the butter will be. For cultured butter, lactic acid bacteria are added at this stage to sour the cream and build flavour.

In a batch churn the drum is only filled to 40–50%, leaving room for foam. As it turns, the cream is beaten violently and fills with air.

The globule membranes are drawn to the surfaces of the air bubbles and some of them tear. Bare, partly crystalline fat sticks to other fat, the foam grows denser and collapses, and the fat gathers into butter grains: invisible at first, then larger and larger.

Seen up close, churning is a cascade of collisions. Globules that have lost their membranes clump together around air bubbles, and the clumps join into grains.

The liquid left behind is buttermilk: the water, protein and sugar of the cream with only a little fat. The emulsion has flipped. Cream was fat dispersed in water; butter is water dispersed in fat.

The buttermilk is drained away. The grains may be washed, and salt can be added, before the butter is worked: kneaded until the fat forms one continuous phase with the remaining water broken into a fine dispersion of tiny droplets.

The result has to meet a legal definition. In the United States butter must contain at least 80% milk fat; the international Codex standard also caps water at 16% and other milk solids at 2%.