How gravity-skimmed butter is made

Fresh milk is strained through a cloth into wide, shallow pans while it is still warm. About 4% of it is fat, spread through the milk as billions of separate droplets.

No machine takes part in what comes next. The fat is lighter than the rest of the milk, and given enough time it floats to the top on its own.

Milk fat is a little lighter than the watery skim around it: the Tetra Pak handbook's worked example uses 980 kg/m³ against 1,028 kg/m³. That small difference is all that lifts a globule.

How fast it rises depends on its size. By Stokes' law the speed grows with the square of the diameter, so large globules overtake small ones. In that example a typical 3 µm globule climbs only 0.6 mm in an hour.

The oldest answer is to make the milk shallow. It was set 2 to 4 inches (5–10 cm) deep in wide pans on a cool shelf, at about 60 °F (16 °C), and left alone for a day and a half.

By then a layer of cream lies on top with skim milk underneath. The method is simple but wasteful: an 1897 dairy textbook reckoned that a fifth of the fat stayed behind in the skim milk.

Chilling raw milk changes the picture. In the cold, a protein called agglutinin makes the fat globules stick to one another, a process known as cold agglutination.

A cluster behaves like one big globule, so it rises far faster than its members could alone and sweeps up smaller globules on the way. In cold raw milk a cream layer forms within 20 to 30 minutes. Heating destroys the agglutinin and homogenizing prevents the creaming, which is why shop milk never forms a cream line.

That is the idea behind deep cold setting, the ice method J. G. Swartz introduced in Sweden in 1864. Milk went into tall, narrow cans, about 8 inches across and 20 deep, standing in a trough of ice water.

Held at about 40 °F (4 °C) for 12 to 24 hours, the milk gave up its cream sooner and more completely than in pans, leaving about 0.2% fat in the skim milk.

Once the cream has risen it is lifted off with a skimmer, a shallow perforated dish that lets skim milk drain back into the pan, and gathered in a crock.

Gravity cream is thin: even from deep cans it seldom held more than 18 to 20% fat. And gravity never finishes the job. The centrifugal separator, invented in 1878, leaves only 0.04–0.07% fat in the skim milk, which is why it swept the old methods away.

A farm rarely had enough cream to churn after one milking, so skimmings were saved over several days. By churning day the cream had begun to ferment on its own, and until well into the 19th century all butter was made from cream soured this way.

The souring was steered rather than left to chance. A 1916 farm bulletin has the cream brought slowly up to 65–70 °F (18–21 °C) a day or so before churning. Ripened cream is what gives cultured butter its tang.

The simplest churn is a tall wooden tub with a staff through its lid. Working the staff up and down drives a perforated dasher through the cream.

Temperature matters more than effort. The same bulletin gives 55–60 °F (13–16 °C) for gravity cream in summer and a little warmer in winter. Cream that is too cold takes very long to churn; too warm, and the butter comes soft. Done right, it comes in 25 to 40 minutes.

The churn is filled only about a third full, because the cream needs room. Each stroke forces cream and air through the holes in the dasher and beats the cream into a foam.

The foam is where the work happens. Fat globules are drawn to the surfaces of the air bubbles, where the membranes that kept them apart are stretched and torn.

With their membranes damaged, globules stick together instead of bouncing apart. The foam collapses and the fat gathers into butter grains, at first too small to see and then larger and larger.

The liquid left behind is buttermilk. The emulsion has turned inside out: cream was fat dispersed in water, and butter is water dispersed in fat.

The buttermilk is drained off and the grains are washed in cold water, at 45–55 °F (7–13 °C). Then the butter is worked: pressed and folded with wooden paddles to squeeze out water and knit the grains into one mass. Salt, typically 1–3%, is worked in for flavour and keeping.

Finally it is shaped and stamped with a carved wooden print. What comes off the table meets the same standard as butter from a modern dairy: at least 80% milk fat and no more than 16% water.