2 CONSTITUENTS OF MILK 



definite relation has been established between the breed of 

 cattle and the size and number of globules there are a number 

 of results which indicate that during interrupted milking the 

 size of the globules increa^s with the fat content and also that 

 as the lactation period proceeds the globules decrease in size 

 and increase in number (see p. 43). 



The origin and method of formation of milk fat have not 

 been discovered although many hypotheses have been proposed. 

 The normal process seems to be the formation of milk fat, 

 directly or indirectly, from nutritive fat, but when this source 

 is eliminated the formation of milk fat proceeds, though dimin- 

 ished in activity, by drawing upon the body fat. Even when 

 the body fat is exhausted, milk fat can be formed: this is attrib- 

 uted to proteids acting as the source of fat. 



The various analytical and physical constants of milk fat 



are: 



37 8 



Specific gravity ^ 0.9094-0.9140 



o7.o 



Refractive index, 35° C 1 . 4550-1 . 4586 



Melting-point 28° C.-36° C. 



Solidifying point 21° C.-27° C. 



Reichert-Wollny value 25-27 



Iodine absorption 31-35 



The calorific value of butter fat, according to Stohmann, 

 is 9.231 calories per gram and according to At water, from 9.320 

 to 9.362 calories. A value of 9.3 is usually employed in cal- 

 culating the calorific value of milk fat. The molecular weight 

 of fat, as calculated from the amount of alkali required for 

 saponification and assuming that all the acids present are mono- 

 basic, is from 720-740, whilst the direct determination by the 

 cryoscopic method points to values from 696-716. The 

 presence of dibasic acids would harmonise these two sets of 

 results, but such acids have not been isolated from butter fat. 



Lactose. Although there is some evidence of the presence 

 of traces of a monosaccharide in milk, the carbohydrate secreted 



. 



