The milk house should be separate from the stable and so located as to be free from dust and odors. It should be used for no other purpose and should be light, clean, and well screened.

All utensils which come in contact with the milk should be of metal with smoothly soldered joints. In addition to being thoroughly washed the utensils should be sterilized by means of steam or boiling water and then kept either closed or inverted in a clean place free from dust until used. By furnishing the farmers with sterilized utensils for each milking and insisting upon the use of covered milking pails, Dr. North greatly reduced the numbers of bacteria shown by the milk.

Milking is performed sometimes by machine, sometimes by hand. Cleanliness of the milker and his clothing are essential to cleanliness of the milk. On well-conducted milk farms the milker puts on a special washable suit for milking and washes and dries his hands immediately before commencing to milk each cow. The cows having previously been cleaned, the udders and flanks should be wiped with a moist cloth preparatory to milking. As a further precaution against the falling of dust and bacteria into the milk a covered or hooded milking pail should be used.

Machines by means of which one man may milk several cows at a time are now on the market. These have the advantage of reducing the number of employees required in milking and diminishing the opportunities for contamination of the milk through contact with the air of the stable or the hands of the milker; among the disadvantages are the cost of the equipment necessary for machine milking and the care needed in preventing the rubber parts of the mechanism from becoming a breeding place for bacteria. In 1912, an extended study at the New York State Agricultural Experiment Station led to the conclusion that machine milking compared favorably with ordinary hand milking in its effect upon the milk flow and upon the germ con

1 Bulletin No. 353, November, 1912.

tent of the milk; and that machine milking had proven practicable. Labor conditions have led to a rather rapid increase of machine milking during the past few years.

Handling the milk. The milk is removed from the stable to the milk room as quickly as possible and (after clarifying or straining through sterile cotton or cloth if deemed necessary) is promptly cooled to prevent the growth and multiplication of such bacteria as it may contain. The multiplication of bacteria in milk does not begin as soon as the milk is drawn, but is preceded by a short period in which there is an apparent decrease in the number of bacteria. This is attributed to a "bactericidal property " of freshly drawn milk. The importance of early and thorough cooling was well shown in an experiment by Conn, in which it was found that the multiplication of bacteria in 24 hours in milk kept at 50° F. (10° C.) was only fivefold, while at 70° F. (21° C.) it was seven hundred and fifty fold.

Usually the milk is first poured into a mixing tank, then run over a metal cooler, and the cold milk filled into cans or bottles and kept cold both in storage and during transportation. In many localities it is required by law that milk be held at a temperature not above 50° or 55° F. until delivered to the consumer. Preferably the milk is bottled in the country, the bottles packed in cracked ice and kept so until delivered to the con

sumer.

General sanitation. In recent years much attention has been given to improvements in the sanitary conditions surrounding the production and handling of milk, largely because it is realized that contaminated milk may undergo such deterioration as to become unwholesome, or may be the means of transmitting specific infectious diseases. As an aid to dairy farmers the Dairy Division of the United States Department of Agriculture has published for free distribution Dairy Suggestions with Special Reference to Sanitation, which are printed on cloth in poster form suitable for posting in barns and milk rooms. The rules

for the production and handling of certified milk, which may be taken as representing ideal conditions, are given in full at the back of this book (Appendix C).

As an example of the influence of sanitary precautions upon the keeping qualities of milk, it may be noted that three American dairy farms exhibited raw milk at the Paris Exposition of 1900, one of them sending weekly shipments throughout the summer, each of which was kept on exhibition in the raw state without spoilage until the next shipment arrived. It was difficult to convince the jury of European experts of the fact that "cleanliness and cold" were the only preservatives needed to accomplish the keeping of raw milk in a fresh, sweet condition for two to four weeks in midsummer.

Score cards are largely used in connection with the sanitary inspection of dairy farms. Their primary purpose is said to be educational, but the scores shown are sometimes used also as a factor in the grading of the milk which the farm produces. Some sanitarians believe that milk should be graded chiefly according to the sanitary score of the producing farm; others that the chief basis for judgment should be the bacteriological examination of milk as it is offered for sale.

In its Bulletin 642, the United States Department of Agriculture emphasizes the following as "the four essentials in the production of milk of low bacterial content ": (1) Use of "smalltop" or "covered" milking pails; (2) Sterilization of utensils; (3) Careful cleaning of cows; (4) Keeping milk cold. By observance of these four precautions it is believed that "the average dairyman on the average farm without expensive barns and equipment" can produce milk of high sanitary quality.

Certified milk. This term is properly applied only to milk produced under sanitary conditions of exceptional excellence, by the most painstaking methods and under the constant supervision and inspection of a Medical Milk Commission. It is understood as meaning that the milk is certified as to its quality

and wholesomeness by a properly constituted medical milk commission. The medical profession was led to engage in the certification of milk in order that there might be made available for infant feeding at least a limited supply of milk of exceptional excellence which should be as nearly as possible absolutely safe. The requirements placed upon the producer and handler are such as to make the cost of certified milk about twice that of ordinarily good bottled milk. Although less than 1 per cent of the market milk of commerce is of this grade, the certified milk movement has had great influence in improving dairy practice and raising the sanitary quality of the general milk supply. The detailed requirements for the production and handling of certified milk are given in Appendix C at the back of this book.

The North system of sanitary milk production differs from the system in which certified milk is produced in that much of the responsibility which the certified milk system imposes upon the farmer is by the North system transferred to the receiving station. The farmer must keep only healthy cows and must clean them before milking, and the milk must be drawn by a clean milker into covered pails, transferred without straining to the milk cans and kept in ice water until sent to the receiving station. At the receiving station all pails and cans are thoroughly washed, sterilized with live steam, dried with a blast of hot air, covered and delivered back to the farmer, who must keep them unopened until the next milking time. Other features of the North system are daily laboratory tests of each farmer's milk at the receiving station and payment according to the quality of the milk as shown by these tests.

Pasteurization of milk by heating it to a temperature of 60° to 63° C. (140° to 145° F.) and holding at this temperature for 20 to 30 minutes serves to destroy any bacteria of diseases regarded as transmissible by milk. The use of higher temperatures in order to shorten the time required for pasteurization is often permitted, but seems undesirable because higher heating

kills an undue proportion of the lactic acid bacteria while the spores of certain bacteria which decompose the proteins of the milk are not destroyed and, if the milk be kept too long, may render it unwholesome before it becomes sour. As a precaution against subsequent contamination it is desirable that the milk be pasteurized in the sealed bottles in which it is to be delivered to the consumer, or that it be filled into sterilized bottles while still hot. There is a growing tendency on the part of public health authorities to require the pasteurization of all market milk which is not obtained under good sanitary conditions from tuberculin-tested cows.

General Composition of Milk

The qualitative composition was concisely stated by Richmond as follows: "It is essentially an aqueous solution of milk sugar, albumin, and certain salts, holding in suspension globules of fat, and in a state of semi-solution, casein together with

88

FIG. 6.

- Fat globules in milk magnified 300 diameters.

mineral matter. Small quantities of other substances are also found." Books on colloid chemistry may be consulted for fuller descriptions in more technical terms of the state of dispersion of the proteins, particularly caseinogen, in milk. Under the microscope the fat globules are readily seen floating in the fluid portion or serum of the milk. (See Fig. 6.) These globules

1 For this protein as it exists in milk the term "caseinogen" is perhaps preferable, the term "casein" being more strictly applicable to the coagulated protein as it exists in curd or cheese.