PROTEINS 235 



Fat . .. . .... . . 3.90 



Milk sugar . .. ... 4.75 



Mineral matter . . . .75 



Freshly drawn milk also contains a small quantity of 

 carbon dioxide gas and takes up oxygen and nitrogen of 

 the air during the process of milking. The fat and some 

 of proteins are suspended in the water, the rest being in 

 solution. The term milk-serum is applied to the 

 liquid portion of the milk in which the fat and solid 

 materials float. It will be useful to deal with each of 

 these constituents separately. 



(i) Proteins. These are highly complex organic 

 substances, containing carbon, hydrogen, oxygen, and 

 nitrogen with a small amount of phosphorus and 

 sulphur. There are several kinds present in milk, the 

 following being the chief: Caseinogen, lact-albumin, 

 with traces of lacto-globulin, and certain enzymes, the 

 nature of which is imperfectly known. More than 80 

 per cent, of the proteins in milk is in the form of a complex 

 phosphoprotein, which we shall speak of as caseinogen. 

 It is not present in the blood of the cow, but is a 

 product manufactured in and by the secreting cells of 

 the mammary glands : it is in combination with calcium 

 as explained below. 



Unfortunately the term casein has been and is still 

 used for this compound as well as for the two curds 

 thrown down when acids or rennet are added respectively 

 to milk, although these curdy substances are quite 

 different from each other and from the original protein 

 in milk. 



While it is not possible in the present state of know- 

 ledge to define exactly the relationship of these " caseins " 

 to each other, to prevent needless confusion it is 



