How Does Cream Turn Into Butter? Fat Droplets Forced Together

Cream is a suspension of tiny fat droplets in water, each droplet wrapped in a membrane that keeps it separate from its neighbours. Agitation forces those droplets to collide repeatedly and damages the membranes, and once damaged the fat surfaces touch directly and stick. Clumps grow, incorporating air and more droplets, until they become large enough to be visible as grains and then to come together into a solid mass, at which point the remaining liquid separates out as buttermilk. The process is entirely physical, with no fermentation, chemical change or cooking involved, which is why it works with nothing more than a container and persistence.

Several conditions matter and getting them wrong prevents it entirely:

• •Fat content, since cream below about thirty per cent will not come together
• •Temperature, with a narrow window around twelve to sixteen degrees working best
• •Too cold and the fat stays hard and the droplets do not stick
• •Too warm and the result is greasy and difficult to work
• •A container filled about half full, since air space is needed for agitation
• •Cream aged a day, which produces a better yield than very fresh cream

The sequence of designs is a good illustration of incremental improvement. The earliest arrangement was a skin or pot rocked or shaken, which is documented in Mesopotamian art from around four thousand years ago and survived in use into recent times. The plunge design, an upright barrel with a stick worked up and down through a hole in the lid, is the form most people picture and was the European standard for centuries, requiring around half an hour of hard work. Rotating barrel and paddle designs from the nineteenth century reduced the effort substantially. Industrial production now uses continuous machines that take in cream at one end and deliver finished butter at the other without stopping.

The liquid drained off is a product rather than a waste, and its character depends on how the cream was treated. Traditional buttermilk is what remains after churning cream that has been left to sour naturally, so it carries the acidity and the flavour produced by the bacteria that developed in the cream, and it is thin, tangy and useful in baking because its acid reacts with bicarbonate of soda to raise a batter. That is the liquid behind soda bread, scones and a great many pancakes. Modern cultured buttermilk sold in shops is not a churning product at all, being milk deliberately fermented to imitate the traditional liquid, which works for baking and is a different thing.

The work is not finished when the butter forms, and the remaining steps determine how long it keeps. The buttermilk must be drained and the mass washed in cold water repeatedly until the water runs clear, because any buttermilk remaining contains sugars and proteins that spoil quickly and make the butter rancid within days. The mass is then worked with a paddle or by hand to force out the remaining droplets of water, which is what gives butter its solid texture rather than a crumbly one. Salt is added at this stage, both for flavour and because it inhibits spoilage substantially. Properly washed, worked and salted butter keeps for weeks without refrigeration.

Agitation breaks the membranes around fat droplets so they stick to each other, clumping into a mass that separates from the buttermilk, which is a purely physical process. Fat content and temperature determine whether it works at all. Designs run from rocked skins four thousand years ago through the plunge barrel to continuous machines. Washing out the buttermilk and working out the water are what make the result keep.