202 ELEMENTARY AGRICULTURAL CHEMISTRY 



globules tend to accumulate on the inner surface of the 

 revolving mass — i.e., near the centre. By providing suitable 

 outlets the skimmed milk can be directed into one channel, 

 and the cream into another, and by adjustment of the size of 

 one of these openings, thick or thin cream can be obtained 

 at will. 



Composition of Crea.m. — Cream varies enormously in com 

 position, the proportion of fat varying from as low as 10 per 

 cent, to as high as 60 or 70 per cent. By shallow setting a 

 product containing from 15 to 40 per cent, of fat is usually 

 obtained ; at low temperatures* about 20 per cent, of fat is 

 present. With the separator almost any desired proportion of 

 fat may be obtained. Usually the amount of the "solids not 

 fat " in the aqueous portion of the cream is slightly higher 

 than in milk, due probably to a slight loss of water by evapora- 

 tion during setting, though possibly also to the fat globules 

 holding around them by surface attraction a layer of liquid 

 slightly richer in casein, &c., than the rest of the aqueous 

 portion of the milk. 



In Devonshire " clotted cream," prepared as it is by heating 

 the milk during setting, the amount of evaporation of the 

 water is probably considerable. Such clotted cream usually 

 contains about 58 per cent, of fat, 34 per cent, of water, and 

 about 8 per cent, of solids not fat. 



Although the specific gravity cannot conveniently be deter- 

 mined directly if the cream contains more than 30 per cent, of 

 fat, yet, according to Richmond, the proportion of fat can in 

 most cases be calculated from the specific gravity, thus : 



F = 32-0 - 0-892^, 



where F = per cent, of fat, 



G = lactometer reading 



(i.e., specific gravity x 1000 - 1000), 

 D = true specific gravity. 



Separated cream is always thinner in consistency than 



