How are cream and butter made

Mechanical agitation of the cream - a process called whipping - creates a metastable foam (i.e. it contains much air). Further whipping causes this foam to collapse some water separates out, and the major product is yellow butter. Incidentally, butter is a different form of colloid from milk, since its dispersed medium is water droplets and its dispersal phase is oil (milk is an oil-in-water colloid). Forming butter from milk is a simple example of emulsion inversion. 

Similarly, the protein in milk is very rich in colloidal chemistry. Most of the protein is bound within aggregates called casein micelles (see p. 512). The colloids in milk are essentially stable even at elevated temperatures, so a cup of milky tea, for 

Such separation occurs naturally, since cream is less dense than milk, but such separation is too slow for commercial cream production. 

In the dairy industry, milk is warmed to kill potentially dangerous bacteria such as E. coli, lysteria, and salmonella - a process known as pasteurization after Louis Pasteur (1822-1895) who discovered the effect in 1871. 

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