Butter is a dairy product made by churning fresh cream. It consists of an emulsion of water and milk proteins in a matrix of fat, with over 80% being fat. It is used as a condiment and for cooking in much the same ways as vegetable oils or lard.
It is solid but soft at room temperature, and melts easily. Its color is generally pale yellow, but can vary from deep yellow to nearly white (butter is typically paler in the winter, for example, when dairy cattle feed on stored hay rather than fresh grass).
Butter sold in American markets is typically salted. Visitors from Europe watch out: anything labeled "BUTTER" is salted unless specified otherwise. Flavorings, colorings, and preservatives may also be added.
Butter has, at various times, been used as a food, a cosmetic and a medicine. In antiquity, the process was simply to let a pan of milk sit until the cream floated to the top; the cream could then be skimmed off. The cream was allowed to sour (lactose converted to lactic acid by means of natural bacteria) and "set" (thicken) into a form called "clabber cream". Various techniques were then used to shake up this clabber cream in order to separate the cream from the emulsifier lecithin[?], which would ordinarily prevent it from solidifying. The liquid (buttermilk) and the solid unemulsified fat (butter) can then be separated.
The process of shaking up the cream is known as "churning", and various forms of churn[?] have been used for the purpose. In Europe from the Renaissance until the Industrial Revolution, this was generally as simple as a barrel on rockers, which was rocked by hand. Afterward, mechanical means of churning were usually substituted.
The Household Cyclopedia of 1881 instructs: