The science behind making butter with cream.
Heavy cream is what is called an “emulsion”. An emulsion exists when tiny droplets
of one type of liquid are floating around in another type of liquid that does not like to
mix with the first. In the case of heavy cream, tiny globules of fat are suspended in
mostly water. See picture (a).
Cream emulsion.
Shaking cream in a jar.
By shaking the heavy cream in the jar, you are forcing the fat globules to slam into
one another. If they hit each other with enough force, they will simply stick together,
the fat collection becoming bigger and bigger with each extra globule. After enough
shaking, the fat globules form a chunk of butter.
Whipping cream.
When you first begin whipping cream, millions of tiny air bubbles get whisked into the liquid.
The cream becomes frothy and lightens. If you stopped whisking now, the bubbles would
eventually work their way out and the cream would become a uniform liquid again.
But if you keep whipping, something more happens. Bubbles are still getting whisked into
the liquid, but now the action of the whisk also starts stripping away the protective outer
membranes on the fat globules. This allows the fat to join and gradually form protective
bubbles around the tiny pockets of air.
Technically this is an emulsion - air suspended in liquid and held stable by fat - and is also
why you need cream with a relatively high fat content (at least 30%) in order to whip cream.
The less fat, the more those globules are stretched thin and the harder it becomes to make a
stable emulsion.
Changes in the cream as air is being whipped into the liquid.
Air added to
yellow fat
globules.
Protective outer
membrane on fat
beginning to be
stripped away.
Protective outer
membrane on fat
beginning to be
stripped away
further.
Protective outer
membrane on fat has
been stripped away
allowing the fat
globules to merge
together forming a
protective bubble
against the pockets of
air.