Exactly how churning works is still unknown. Current theory runs along these lines: just as happens in whipped cream, some air is incorporated into the liquid, bubbles form, and the fat globules collect in the bubble walls. But where whipping cream is kept cold, and the agitation stopped when a a stable, airy foam is produced, churned cream is warmed to the point that the globules soften and to some degree liquify. The ideal temperature range is said to be 55° to 65°F (12° to 18°C). Persistent agitation knocks the softened globules into each other enough to break through the protective membrane, and liquid fat cements the exposed droplets together. The foam structure is broken both by the free fat and the released membrane materials, which include emulsifiers like lecithin. These materials disrupt thin water layers and so burst bubble walls, and once enough of them have been freed in the process of whipping or churning cream, the foam will never be stable again. As churning continues, then, the foam gradually subsides, and the butter granules are worked together into larger and larger masses.
Paddles slowly agitate the cream causing it to thicken and separate into butter grains and buttermilk. Cold water at 10°C is then added and then it is agitated again. Added water is necessary to help the cream to 'break' but the water should not exceed 25% of the total volume of cream. Churning continues until the butter granules are about the size of wheat grains.
The fat globule
Fat globules vary from 0.1 - 10 micon in diameter. The fat globule membrane is comprised of surface active materials: phospholipids and lipoproteins.
Fat globules typically aggregate in three ways:
- partial coalescence
The process of butter making can be described as an inversion of the original cream emulsion. The system of fat droplets dispersed in water is converted into a continuous phase of fat that contains water droplets. The final product is about 80% milk fat, 18% water, and 2% milk solids, mainly proteins and salts carried in the water. The physical structure of butter is, however, a bit more complicated. The continuous, amorphous phase of solid fat surrounds not only the water droplets, but also air bubbles, intact fat globules, and highly ordered crystals of milk fat that have grown during the cooling process. The proportion of continuous or "free" fat can vary from 50% of the total to nearly 100%, and it has a direct influence on the behavior of butter. The more fat there is in discrete globules or crystals, the harder and more crumbly the butter, even to the point of brittleness. A preponderance of free fat, on the other hand, makes for a malleable butter that softens readily and may even weep some liquid fat in the process. The difference is a matter of both large-scale and molecular arrangements. In a mass where the free fat merely fills the small interstices between globules and crystals, the texture will be largely that of the separate particles. And it takes more energy to separate the molecules ordered in a crystal than it does to disrupt an already disordered phase of the same molecules. Mostly crystalline butter, then, will be relatively stiff and not as smooth as mostly amorphous butter. The ideal, of course, lies somewhere between the two extremes, and is attained by manipulating the cooling process (much as one controls the texture of candy).