Excerpt from Chemical Testing of Milk and Cream
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 percent. The usual variation in mixed-herd milk is much less and is probably covered by 84 to 88 percent.
Fat. - The fat in milk - milkfat or butter fat is not in solution but exists as an emulsion of microscopic globules so small that a single drop of average milk contains more than one hundred millions of them. These globules, 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. Chemically 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 percent 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 produced by rennet, dilute acids, or certain other chemicals, when added to milk, is chiefly casein. Albumin is the flocculent precipitate 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 improperly, the term casein is applied to all the nitrogenous constituents in milk.
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