270 THE CHEMICAL CONTROL OF THE DAIEY. 



Roughly speaking, the ratio of solids not fat to water is double 

 the average ratio in milk. 



The ratio of ash to solids not fat is very nearly the same in 

 clotted cream as in milk ; it is, however, slightly lower. This 

 is partly, if not entirely, due to the fact that on heating mUk 

 certain salts of calcium, probably chiefly citrate, are deposited, 

 leaving a smaller proportion in the milk and also in the cream 

 derived from it. 



The Thickness of Cream. — The thickness is the factor by 

 which cream is usually judged when used for direct consumption. 

 This can be quantitatively estimated by the method generally 

 employed for the determination of " viscosity " — i.e., noting the 

 time taken for a given volume of cream to flow through a tube 

 of constant size. The viscosity of a liquid depends on the 

 internal friction — i.e., the friction of molecules passing each 

 •other ; the viscosity or internal friction of cream is not quite of 

 the same order as that of a homogeneous liquid ; in the latter 

 ■case, the molecules are of equal size (or nearly so), and very 

 small in comparison with the diameter of the tube through 

 which the liquid passes. The viscosity of cream depends on two 

 factors — the internal friction of the very small molecules of the 

 milk serum, and the friction between the comparatively large 

 fat globules. 



As the fat globules_ have an appreciable size compared to the 

 size of the tube, we cannot expect the laws to be of the same 

 kind as those governing the viscosity of a liquid composed of 

 molecules of infinitely small size. The actual and relative size 

 ■of the globules will also have considerable influence ; thus if we 

 have two creams identical in chemical composition, in one of 

 which the relative size of fat globules is much larger than in the 

 ■other, the " viscosities " will differ. 



It is not possible to compare the thickness of cream by making 

 a determination of the percentage of fat in a sample. It is 

 possible, however, to make a comparison of cream which contain 

 globules of relatively the same size. For instance, if cream be 

 diluted with the separated milk, which is practically free from 

 fat, the thickness can be deduced by making determinations of 

 iat. 



The law connecting thickness or viscosity and amount of fat 

 is expressed by the following empirical formula — 



V = lO^^s 

 ^where V = the viscosity, 



r, = the volume of fat in 100 volumes of cream, 

 and x = a. factor dependent on the units in which the viscosity is ex- 

 pressed, and on the relative size of fat globules. 



