Milk is at once the most important and the most perishable of all food products. Fresh milk is the most perfect mechanical emulsion known and contains roughly proteids (albuminoid substances equivalent in nature to the white of the egg) fat, in the form of minute globules which are later represented as cream, milk sugar, certain mineral salts and water. There is more or less fluctuation in the proportion in which these substances appear in all milks, particularly the milk of cows, owing chiefly to the development (in some instances covering periods of many hundred years) of certain breeds of cattle, possessing, among other qualities, certain characteristics as milk producers. I say chiefly to the development of breeds, because repeated experiments with various breeds have shown that it is not possible to alter materially the proportion in which the various constituents of milk appear in the milk of any given cow by any process of feeding yet discovered.
Improved methods of feeding increase the total quantity of the output, but not materially the quality, and any attempt to force by feeding an increase in the percentage of any one of the ingredients in the milk (particularly the fat content) may increase slightly for a short time such content, but it soon drops to the normal for each cow, and the experimenter has run the risk of ruining the animal experimented upon.
A popular fallacy prevails which enshrines in the minds of the uninformed the belief that milk having a large percentage of fat is rich milk, and hence, the best milk. Milk rich in fat and the best milk from both a physical and a chemical view are not synonymous terms, either as a matter of domestic economy or as applied to its use for infants. While all good milk must possess fat, the consideration of the amount thereof from a nutritive standpoint is second to that of the proteid content except in a certain few selected cases, which rarely include babies or young children.
Dr. J. A. Gilbert, writing in The Medical Record (New York), takes the view that this devotion to "rich" milk has no logical basis. In our earnest search after a fat milk, he says, we have probably gone too far. To quote from an editorial in The Hospital (London) which notes Dr. Gilbert's opinion appreciatively:
Owing, then, to ignorance or personal interest, recent discussion of the milk problem throughout the land has revolved around the percentage of fat in milk and undue prominence has been given this phase of the question.
Protein is the most important nutritive content in all milks, and