Butter is one of the simplest-looking but physically most interesting products made from milk. What happens in production is the inversion of an emulsion.
What is phase inversion?
In milk and cream, fat exists as microscopic droplets dispersed in water.
A thin membrane surrounds those droplets and prevents them merging.
That structure is called an oil-in-water emulsion: water is the continuous phase, fat the dispersed one.
In butter the situation reverses. Fat becomes the continuous phase and water is dispersed within it as small droplets.
That transformation is called phase inversion.
Butter is roughly 82 percent fat, 16 percent water and 2 percent milk solids.
Butter is therefore not a mass of fat; it is a structure carrying water.
It is also the reason a pan of melting butter spits and gives off steam.
How is the cream prepared?
The process begins by separating the fat from the milk.
In the traditional method milk is left to stand and the fat rises by difference in density.
Industrial production uses a separator; centrifugal force separates the fat within minutes.
The resulting cream contains 35 to 40 percent fat.
The cream is pasteurised, which kills microorganisms and extends shelf life.
Then comes ripening. The cream rests in the cold for eight to twelve hours.
During that time fat crystals form, and those crystals are necessary for churning.
Cream without crystals produces no butter; the droplets do not merge.
How does churning work?
Churning applies mechanical force to the cream.
That force tears the membrane around the fat droplets.
Droplets with torn membranes begin sticking to one another.
Air enters the cream at the same time and foam forms. That is an intermediate stage; the cream first expands.
As churning continues the masses of fat grow into visible grains.
At that point phase inversion occurs: fat becomes the continuous phase and water is expelled.
The liquid expelled is buttermilk, containing around 0.5 percent fat and milk proteins.
Churning temperature is critical: 10 to 14 degrees. Warmer and the fat softens and grains do not form; colder and the process takes far too long.
| Stage | What happens | Duration |
|---|---|---|
| Separation | Fat separated from milk | Minutes |
| Pasteurisation | Microorganisms destroyed | 15-30 seconds |
| Ripening | Fat crystals form | 8-12 hours |
| Churning | Phase inversion | 20-45 minutes |
| Draining | Water phase removed | Minutes |
| Washing | Residual buttermilk cleared | Minutes |
| Working | Water distributed | 10-20 minutes |
Why are washing and working necessary?
After the buttermilk drains, some remains between the butter grains.
That buttermilk contains milk protein and lactose, both prone to spoilage.
Washing is done with cold water and removes what remains.
That step extends shelf life markedly. Unwashed butter sours within days.
Working brings the butter grains together and distributes the water evenly.
That step determines the texture. Well-worked butter is smooth and homogeneous; under-worked butter shows droplets of water.
Over-working is a problem too; the butter takes on a greasy, pasty texture.
Where salt is added, it goes in at this stage.
What separates sweet and cultured butter?
There are two production routes and their flavour profiles differ markedly.
Sweet cream butter is made directly from pasteurised cream. Its flavour is mild, milky and neutral.
That is the most common method in industrial production.
For cultured butter, lactic acid bacteria are added to the cream and fermentation applied.
The bacteria convert lactose into acid and produce a compound called diacetyl.
Diacetyl is the component giving butter its characteristic aroma.
Cultured butter is more intense and more aromatic; it is standard in parts of Europe.
Traditional churned butter belongs to this group too; the cream ferments naturally.
The characteristic flavour of village butter made in Turkey comes from that fermentation.
What does salt do?
The difference between salted and unsalted butter concerns shelf life as much as flavour.
Salt lowers water activity and slows microbial growth.
Salted butter keeps markedly longer than unsalted.
The salt level is generally 1.5 to 2 percent.
In patisserie unsalted butter is preferred; the salt has to be controlled within the recipe.
Salt also masks defects; in low-grade butter it can hide the taste of oxidation.
Unsalted versions are therefore a more reliable indicator when judging butter.
Unsalted butter keeps two to three weeks in a fridge, salted four to six.
How is clarified butter made?
Clarified butter is butter with the water and milk solids removed.
The butter is melted over low heat.
Three phases separate: foam at the surface, clear fat in the middle, milk solids at the bottom.
The foam is skimmed, the clear fat strained, and the solids left behind.
The result is a product containing over 99 percent fat.
With the milk solids gone the smoke point rises; where butter burns at 150 degrees, clarified butter withstands up to 250.
Shelf life extends too; with no water there is no microbiological risk.
It keeps months at room temperature.
Against that, the aroma contributed by the milk solids is lost; the flavour is flatter.
How is quality judged?
Five indicators determine the quality of butter.
The first is fat content. It must be at least 80 percent; below that a product cannot be called butter.
The second is colour. Natural butter changes colour with the season; butter from summer pasture-fed milk is yellower because of beta-carotene.
A constant colour may suggest added colouring.
The third is texture. Butter from the fridge should be firm but not brittle; brittle butter has been over-worked or has poor water distribution.
The fourth is smell. It should be clean and milky; a rancid or metallic note indicates oxidation.
The fifth is behaviour in the pan. Good butter foams little as it melts and leaves little residue at the bottom.
Heavy foaming and a lot of residue indicate a high proportion of water and milk solids.
