Until recent years, it was supposed that the composition of milk could be decidedly changed by the feed which the cow consumed, but later Investigation has established the fact that the feed has but slight, if any, influence on the relative amounts of the various constituents. Certain feeds, however, may affect the quality of the milk fat; for example, linseed or gluten meal makes an oily, soft fat, while cotton seed meal and wheat bran make a hard fat. The fat exists in milk in the form of minute globules which are held in their spherical form by the surface tension of the liquid fat and the viscous nature of the milk serum. Since the value of milk is largely proportional to its per cent of fat, it is often necessary to determine its fat content. This may be done by chemical analysis but this method involves considerable time and expense and the method generally used in com mercial work in this country is that devised by Dr. S. M. Babcock, of the University of Wis consin, and known as the 'Babcock Test' for milk-fat. This test is based on the principle of liberating the fat globules by dissolving the casein with strong sulphuric acid and the sep aration of the fat by centrifugal force—the fat being lighter than the other elements in the milk. Briefly, the method of making the Babcock Test is as follows: (1) Secure an accurate sample of the milk to be tested; (2) measure out 17.6 cubic centimeters, and place it in the test bottle; (3) add 17.5 cubic centimeters of sulphuric acid, and mix thor oughly by rotating the test bottle; (4) place bottles in centrifuge and whirl at proper speed for five minutes; (5) add hot water to bottom of neck and whirl two minutes; (6) add hot water to bring fat up to graduated scale and whirl one minute; (7) place test bottles in water bath at F. for five minutes; (8) read amount of fat column, including meniscus both at top and bottom of column.
Milk is heavier 'than water, the average specific gravity of normal milk being 1.032, with variations between 1.029 1.035. The specific gravity of milk may be determined by an instru ment palled a “lactometer.° Taken in connec tion with the Babcock Test, lactometer reading makes it possible to determine both the total solids and the solids-not-fat jh. any given sample of milk. There are a number a formulae which may be used for this purpose. modified form is as follows: + 2F ± .14= S. n. F.
in which L stands for the lactometer reading and F for the per cent of fat. The total solids may be obtained by adding the solids-not-fat to the fat reading.
Milk is a delicate substance and very sus ceptible to undesirable flavors and odors. These may be either of two types: (a) those which are absorbed from feed eaten by the cow or from outside sources, or (b) those resulting from the action of microorganisms, especially bacteria, in the milk itself. If it is exposed to strong odors, such as foul stable air, cabbage, gasoline, etc., they will be absorbed very If placed in an ice-chest with such foods as pineapple or strawberries, it will be quickly tainted and have a disagreeable flavor. Dis agreeable as these absorbed troubles may some times be, they are not so serious as those result ing from the growth of bacteria. The former decrease after removal from the source of con tamination, while those resulting from bacterial action continue to increase in intensity as the milk grows older. It is highly desirable that
milk should reach the consumer in as nearly as possible, the condition in which it leaves the udder of the healthy cow. This means that much care must be exercised in its production and handling to prevent contamination. Bac teria gets into milk chiefly from the following sources: the interior of the cow's udder, the milk utensils, the surface of the cow's body, the atmosphere of the stable and milk house, and the hands and clothing of the milker. If the udder is free from disease, it.is not a serious source of contamination, since its germ content rarely exceeds a few hundred per cubic centi metre. If, however, the udder is diseased, it may be a very serious source of infection. To secure milk with a low bacteria content those parts of the cow's body around the udder should be as clean as possible at milking time. The stable air should be free from dust, the milker's hands should be dean, and he should wear a clean suit. In many cases the utensils are the worst source of trouble, and great care should be taken to see that these are prop. erly constructed and thoroughly washed and well sterilized either with boiling water or steam.
As soon as produced, milk should be cooled to a temperature which will prevent the rapid development of the bacteria in it. Bacteria grow very rapidly at temperatures above 70° F., but more and more slowly as the temperature is reduced below that point; at 50° or beloW, growth is relatively slow, hence for best results, milk should be held at these low temperatures. During recent years, the "pasteurization)) of milk for the purpose of reducing the bacteria content and destroying any disease germs which may be present has become a common practice. Pasteurization consists of heating the milk to a temperature sufficient to kill all pathogenic organisms and a large percentage of the other forms, and then immediate cooling to prevent the rapid growth of those that remain. A tem perature of 145° F. maintained for 30 minutes is believed to be sufficient to destroy all disease organisms.
The average per capita consumption of milk is about one-half pint per day. Large quantities therefore, are required to supply the needs of the large cities, New York city alone consuming about 2,000,000 quarts daily. The problem this vast amount of milk to the consumer in the city is a serious one as the cities have increased in size, it has been neces sary to go farther away for the necessary sup ply of nulk. At present, New. York city obtains part of its milk supply from farms 300 to 400 miles away, which means railroad transportation of 10 to 12 hours, after which it must be taken to the city plant, pasteurized, cooled and bottled before ready for delivery. As a result this milk is frequently 24 to 36 hours old when delivered• to the consumer. Because of the great rapidity with which bacteria grow, the injection of this long period of time between production and consumption has greatly increased the problem of a satisfactory city milk supply and has changed many of the methods of production and handling on the farm, during transportation and in the city.