MILK, BLOOD AND URINE 211 



The fat of milk is held in suspension in the form of an emul- 

 sion, and is one of the constituents to which the characteristic 

 opaque white color is due. Owing to the fact that its specific 

 gravity is less than that of water it rises to the surface on 

 standing in the form of cream, or it may be separated by sub- 

 jecting the milk to a strong centrifugal action as in the ordinary 

 milk separator. The size of the fat globules present in the 

 emulsion varies with the particular breed of cow and with the 

 period at which the milk is obtained. Barthel gives the size 

 as o.oooi to 0.022 mm. in diameter with an average of 

 0.003 nim. 



The particular physical and chemical properties of milk fat 

 which serve as a basis of analysis will be referred to under 

 butter. The simple determination of the amount of fat present 

 in milk is carried out in the laboratory by absorbing the milk 

 into a porous block of filter paper and then subjecting this, after 

 drying, to extraction with ether or light petroleum oils (benzine). 

 After evaporation of the solvent the fat is left practically pure. 

 In the dairy the determination is made by means of the well- 

 known Babcock method. The principle upon which this 

 method is based is that, by the addition of a certain amount of 

 sulphuric acid, the specific gravity of the milk liquid is raised 

 considerably so that the fat separates very completely from its 

 emulsion state. The separation is assisted by whirling the 

 mixture in a centrifugal machine after first placing it in a specially 

 constructed bottle with a narrow graduated neck. After cen- 

 trifuging, the fat stands completely separated as a clear layer 

 in the graduated neck and is read directly in per cent of fat in 

 the milk. 



Proteins. — The protein constituents of milk are three, viz. 

 caseinogen, a phosphoprotein ; an albumin, lactalbumin; and a 

 globulin, lacto globulin. Caseinogen is in much greater amount 

 than either of the other two, 85 per cent of the total protein 

 being in this form. The lactalbumin constitutes most of the 

 remaining 15 per cent, while the lactoglobulin is present only in 

 traces. 



