Contamination of milk should be strictly guarded against and a thorough inspection should determine not only the fault mentioned above, but should also detect such grosser contamination as the presence of barn manure or hair. These evidences of filth and uncleanliness get access during the milking process usually falling from the udder into the milk pails. Such gross contaminations are, in the light of modern dairying, absolutely without excuse. In a well-conducted dairy, not only are all the cows in a condition of good health, but they are kept in clean stalls. The dairyman washes the udder of the cow, brushes her sides, and also washes his own hands before he begins to milk. Not infrequently the further precaution is observed of straining the milk through a gauze top placed over the milk pail. This catches any hairs that may be accidentally dislodged from the udder. After milking, the milk is very carefully strained so as to remove larger dust particles and is at once subjected to a chilling process that lowers the temperature to about 50° F., at which temperature it ought to be maintained until used.

The object of thus reducing the temperature is to decrease the bacterial multiplication. All milk contains a certain number of bacteria. These are usually nonpathogenic. The principal species is the lactic acid bacteria. If the milk can be kept cool, these bacteria are held so completely in abeyance that they will not cause a souring of the milk for several days. If the temperature, however, is allowed to rise much above fifty degrees the bacteria will begin a rapid multiplication, and as these bacteria consume milk sugar, and throw out as a waste product lactic acid, they will cause the milk to sour. This souring of the milk would take place within a few hours after the milk is drawn if it were kept at body temperature.

A thorough inspection of the milk is not complete until the number of bacteria in a unit volume of the milk is determined. When this number passes a certain standard minimum it is accepted as evidence that the milk has not been properly cared for. In this connection, it may be stated that among other elements in the care of milk should be the thorough cleaning of all receptacles and containers. Not only should milk pails, milk cans, milk bottles, and other utensils that come in contact with milk be thoroughly washed in hot soapsuds, but they should be thoroughly scalded in hot water after the washing. Only by taking all of these precautions can the milk be kept pure.

The reason for all these precautions is that milk is a specially fine culture medium for bacteria in general. Many forms of bacteria thrive in milk, not only those forms already mentioned, but, unfortunately, certain disease germs also. Sanitarians have traced the spread of such diseases as typhoid fever, diphtheria, tuberculosis, and scarlet fever to contaminated milk. It is not uncommon, for example, for milk to become contaminated with typhoid-fever germs in the following manner: There is a case of typhoid fever in the family of a dairyman. The excreta from the patient, instead of being thoroughly destroyed or disinfected, are emptied into a vault or outdoor water-closet. Seepage from this closet bears typhoid germs to a near-by barn well. The dairyman rinses his milk cans from the water in this barn well, the cans become infected, the germs propagate rapidly in the milk, and may be distributed over a wide district. Perhaps a more common way of distributing typhoid-fever germs from the sick room to the milk cans is through the agency of flies. If a fly walks over any of the excreta from the patient either in the sick room or at any time before it is disinfected he may carry away on his hirsute surface thousands of bacteria. If he lights upon the rim of a just filled milk can to take a meal from a drop of milk that has accidentally gathered there, he may infect the drop which remains to dry upon the rim of the can, and infect the contents of the can as the milk is poured out. Tubercular contamination of milk may be direct from tuberculous cows, or from dust bearing tuberculous germs. Without doubt, the dissemination of disease was very general and widespread through the milk supply before the general interest in sanitation. At the present time, however, inspection of milk and of dairies and the general dissemination of information among dairymen and milk dealers as to sources of contamination and methods of avoiding such contamination have resulted in a very great improvement of conditions, so that one has not much to fear in the use of the milk from the reputable dealers.

Incident to the souring of the milk, the casein is coagulated into a delicately gelatinous mass similar in consistency to an egg-milk custard. Milk in this condition is called clabber. After a few hours there seems to be a moderate contraction of the coagu-lum because there is a division of the coagulum from the yellowish whey. If the clabber is heated slightly this contraction of the coagulum becomes very readily apparent, and there is a fairly complete separation of whey from curds. In this separation the cream and most of the lactalbumin are entangled with and held with the curd, while the milk sugar and milk salts, together with a possible trace of the albumin, are separated out into whey. Cheese is made from the curd. Whey is usually fed to the pigs. The simplest cheese is the cottage cheese, or schmier-kase, made by every housewife. The varieties of cheese found in the market represent various processes of pressing and curing, the almost innumerable variety, owing their differences in part to the proportion of casein and fat, and in part to the conditions under which the cheese is cured; the finest grades of cheese possessing the most delicate flavor being cured in cooler climates and higher altitudes.

The coagulation or thickening of milk, as described above, is typical of that produced by lactic acid. Casein of the milk may, however, be coagulated by any vegetable, fruit, or mineral acid. Hydrochloric acid, for example, causes a very rapid coagulation even when that acid is so dilute as a tenth of one per cent. In this strength the hydrochloric acid of the stomach causes a coagulation of the milk. The curds formed through the influence of the stronger acids, as hydrochloric acid, for example, are much tougher and more leathery than are the curds formed through the influence of lactic acid. This has a direct bearing upon the use of milk in the sick room, because the curds produced by hydrochloric acid are not easily digested by a delicate and sensitive stomach. For that reason it is common to modify the milk used by invalids. The simplest modification is the skimming of the milk that removes a large part of the fat and makes the milk somewhat more easily digested. Boiling also renders the milk more easily digestible, partly through the removal of the lact-albumin which is separated out on boiling, and rises on the surface of the milk in a thin crinkly membrane. The casein is also somewhat modified by boiling, so that it coagulates in the stomach in small flakes instead of large curdy masses. Whole milk may be diluted with limewater, which modification is brought about by adding to milk about an eighth to a sixth of its volume of limewater. Barley water and oatmeal water are also sometimes added, which additions seem to prevent the formation of leathery curds, and thus increase the digestibility of the milk.

In the digestion of milk in the stomach of the infant, in common with all mammalian young, the milk is curdled not through the agency of acids, but through a milk-curdling ferment called rennin. Rennin or rennet extracted from the stomach of young mammals, for example, from the stomachs of the veal and young pigs slaughtered for market, is used for curdling milk in the cheese-making process. The great advantage of using this agent for separating the curds from the whey, is that the resulting curd is not sour, as is the case when the coagulation is caused by acid. Another use of rennet is in the junket tablets which are used in the preparation of the dainty milk custard called junket, which is nothing more than coagulated whole milk flavored with some such flavor as vanilla.

Condensed milk is a preparation made by evaporating the water of the milk in a vacuum. The milk thus reduced to about one fourth of its original bulk and sterilized incident to the process, placed in hermetically sealed cans, may be kept indefinitely without decomposition taking place. In this form milk is convenient for use in camps, on journeys or voyages, or in any situation where fresh milk is not available. Undiluted, it serves as a palatable substitute for fresh cream in coffee. Diluted with three or four volumes of water it may be used in cooking the same as fresh milk. As a beverage it is not palatable to most people because of a modification of its flavor.

Mention was made above of the wide use of eggs in composite preparations, such as custards, cakes, muffins, etc. Even more widely used than eggs in composite foods is milk. It is almost universally used wherever eggs are used with such foods, and in many places where eggs are not used, as, for example, in the liquid element of bread sponge already mentioned above. This prevalent use of milk in composite foods is to be heartily recommended, because milk is in itself a perfect and complete food, and naturally increases the nourishing properties of all composite foods of which it is an ingredient.