282 MILK, BUTTER, AND CHEESE [chap. 



pans, deep metal vessels jacketed with ice were some- 

 times employed, by which means a very effective 

 separation of cream was brought about, but an even 

 larger quantity of fat, over i per cent, was left in the 

 milk. The only real gain in this process was that the 

 cream was protected from the influx of the harmful 

 bacteria. Where any large quantity of butter is made, 

 the mechanical separator is now invariably used. 

 These instruments depend for their action upon 

 bringing the milk into a violent whirling motion by 

 making it flow into a bowl revolving at the rate of 

 about 6000 revolutions per minute. Under these 

 conditions the force of gravity, which tends to separate 

 the lighter fat globules from the denser milk serum, is 

 exchanged for a centrifugal force many times greater, 

 which draws the heavier serum to the outside of the 

 bowl and leaves the lighter cream at the centre, suitable 

 ducts being arranged to take away the two liquids which 

 have thus been separated. The cream which is obtained 

 from the separator may contain anything from 15 to 50 

 per cent, of fat, according to the velocity at which the 

 separator is run. The separated milk, which is the 

 name given to the liquid from which the cream has 

 been removed, contains as little as 0-15 per cent, of fat 

 when the separator is working properly ; the rest of the 

 serum possesses the same composition as it does in 

 uncreamed milk. After the cream has been separated 

 it may be churned at once, in which case the product 

 is known as sweet-cream butter, but the flavour is 

 then somewhat inferior and the yield of butter is lower. 

 It is customary to put aside the cream for at least forty- 

 eight hours in order to develop a certain amount of 

 acidity before churning. While the cream is thus 

 ripening it should stand in a warm place, as nearly as 

 possible 65° F. and should be stirred from time to time 



