be classified* in to are w well-marked groups, as follows: (1) 

 Water, (2) fat, (3) nitrogenous constituents, (4) sugar, and 

 (5) ash. The components other than water are collectively 

 known as total solids or milk solids, and the solids other 

 than fat as solids not fat. Milk serum, or more properly 

 milk plasma, is the term used to denote the milk minus 

 the fat; hence the terms serum solids and plasma solids 

 are synonymous with solids not fat. 



Water. The water in milk varies from 82 to 90 per cent. 

 The usual variation in mixed-herd milk is much less and 

 is probably covered by 84 to 88 per cent. 



Fat. The fat in milk milkfat or butterfat is not in 

 solution but exists as an emulsion of microscopic glob- 

 ules so small that a single drop of average milk contains 

 more than one hundred millions of them. These glob- 

 ules, even in milk from one cow, are not all of the same 

 size. Some may be two or three times the size of others, 

 the average size depending upon several factors, the 

 principal one of which is the breed of the animal. Chem- 

 ically the fat is not a single compound but a mixture of 

 several compounds known as glycerids. Some of these 

 glycerids are common to all fats, while others are peculiar 

 to butter. This fact is made use of in detecting oleo- 

 margarin. 



Cow's milk usually contains from 3 to 6 per cent of fat, 

 depending very largely upon the breed of the animal. 



Nitrogenous constituents. These are principally casein 

 and albumin, with traces of less important nitrogenous 

 compounds. The coagulum, or curd, produced when 

 rennet, dilute acids, or certain other chemicals, are added 

 to milk, is chiefly casein. Albumin is the flaky precipi- 

 tate produced by heating whey or skimmed milk from 

 which the casein has been removed. In constitution and 

 behavior it closely resembles white of egg. Casein is not 

 really in solution in the milk, but exists in an extremely 

 fine colloidal condition in combination with some of the 

 ash constituents. With an appropriate filter of clay it is 

 possible to separate it from the water. Albumin is in true 

 solution in the water of the milk. Frequently, but im- 

 properly, the term casein is applied to all the nitrogenous 

 constituents in milk. Sometimes the term total proteins 



