free from danger of disease. If at the bottom of the bottle of milk there is a distinct sediment, the purchasing public will uniformly reject the milk as being of poor quality. The public is justly desirous of having a clean food supply, and there is probably no food product regarding which it is more sensitive than milk. The extreme sensitiveness of the public in this matter is due in part to the fact that milk naturally lends itself to careful inspection. The white milk forms a natural background against which any foreign matter stands out with startling distinctness. As a result of these physical conditions the unaided eye is able to detect the presence of foreign matter in milk when it is present in such minute quantities as to practically defy detection by analytical methods. The sensitiveness of this inspection is shown by the fact that it is possible thus to find traces of foreign matter in practically any quart of milk which is critically examined, regardless of the care exercised in its production. In the certified milk from the cleanest dairies in the country which is annually brought together in competition at the National Dairy Show, such foreign matter is evident to the eye in over 80 per ct. of the bottles. On the other hand, the amount of this foreign matter is so slight in all certified milk, and in practically all commercial milk, as to be upon the very margin of detection by analytical methods. Taking advantage of the sensitiveness of the eye to differences in color, a method called the sediment test9 has been devised for determining the cleanliness of milk. In applying this sediment test, measured quantities of milk are passed thru cotton and the dirt is observed as a residue upon the white cotton. This test has been quite widely applied in commercial work. While in rare cases the presence of considerable amounts of dirt has been demonstrated, in practically all instances the amount of dirt found in the milk has been slight. When attention has been directed to the presence of any considerable quantities of dirt, the conditions of milk production have been promptly modified so as to bring the milk to a uniformly high standard of cleanliness. Milk as it is now generally produced and handled is one of our cleanest foods. 9 Weld, I. C. A plan for Improving the Quality of Milk and Cream Furnished to New Hampshire Creameries. N. H. Agr. Exp. Sta. Bul. 132. 1907. KEEPING QUALITY. In order that a milk shall be justly entitled to be called good milk, it is not sufficient that it be high in food value, that it be free from danger of carrying disease, and that it be clean, because if it is sour when delivered to the consumer or sours promptly thereafter, it is unsatisfactory. In the northern states, at least, the delivery of milk once a day is expected to supply the needs of the family for the succeeding twenty-four hours. Accordingly, good milk must remain sweet during that period, and preferably during a longer period, in order to justly entitle it to be called good. It is possible to estimate the condition of the milk with regard to food value, healthfulness, and cleanliness, by comparatively simple methods. The situation with regard to keeping quality is more complex. Souring is induced by the growth in the milk of minute forms of plant life-bacteria. This plant life attacks the sugar of the milk, using it as a food, and producing acid as a by-product. When the accumulation of this acid amounts to approximately 0.3 per ct., the milk begins to taste sour; and when the accumulation of acid has reached approximately 0.7 per ct., the milk curdles. The problem of maintaining a satisfactory keeping quality is essentially a problem of restricting the development of germ life. It is possible to meet this problem either by preventing the entrance of germs, by destroying them after they enter, or by holding the milk under conditions which will prevent the activity of the germs after they enter. While the problem of the keeping quality of milk can thus be stated in simple terms, the actual restriction of contamination and of development of germ life is a complex matter. There is still a lack of knowledge regarding the relative importance of the various avenues thru which bacteria gain access to the milk, and this results in a lack of knowledge regarding the most practicable means of preventing their entrance. In attempting to control keeping quality, various cities have made regulations establishing a maximum number of germs permissible in their milk supplies. 10 These regulations did not attain the desired results, and in many cases the cities further stipulated various conditions which must accompany milk production. The establishment of 10 Bacteriological Standards for Milk. U. S. Public Health Reports 29: 1218-1221. 1914. bacterial standards placed upon the milk producer and the milk dealer the responsibility of translating these standards into terms of dairy processes, while the detailed recommendations formulated by the health authorities are an attempt on their part to make this translation. In practice both these attempts have failed to accomplish the desired end. As a measure of the keeping quality there are many advantages in a direct 11 determination of the germ life, but this is a technical process not readily available to the dairymen and accordingly has certain limitations. The true measure of the keeping quality of milk is the time which elapses before it actually sours. This is the measure employed by the consumer, but manifestly it cannot be applied in advance at any earlier stage in the commercial life of the milk. A modification of this is possible in that samples of the milk under consideration may be held at high temperatures and the interval before curdling noted. From a comparative study of the effect of a temperature on germ growth, it would then be possible to translate this interval into the time which would elapse before the original milk would sour at the lower temperature at which it would normally be held. This procedure involves some time and technical apparatus which is not often available. The commercial milk men have long employed the acid test, as well as their trained sense of taste and smell, in estimating the probable keeping quality of milk as delivered at their plants. By these means they have been able to anticipate somewhat the time at which milk will be no longer acceptable to the whole-milk trade, but it is only as milk approaches this limit that its condition is determinable by these means. During the past few years there have been suggested a number of technical milk tests more or less closely related to keeping quality, such as the reductase test, the Schardinger reaction, the alcohol test, the catalase test, and the hydrogen ion concentration. In general the availability of these tests seems limited because they are mainly useful only in the later stages of the commercial life of milk. In view of this unsatisfactory condition of knowledge regarding the measurement and control of the keeping quality of milk, the 11 Breed, R. S., and Brew, J. D. Counting Bacteria by Means of the Microscope. N. Y. (Geneva) Agr. Exp. Sta. Tech. Bul. 49. 1916. New York 12 and Illinois 13 Agricultural Experiment Stations have undertaken extended and detailed study of the various factors affecting the entrance and growth of germ life in milk. COMPLEXITY OF THE PROBLEM. The above outline of the various phases of milk quality brings out the fact that at various times the students of the milk question have been interested first in one, then in another element of milk quality, and that in connection with each such attempt they have succeeded in devising a more or less successful index of quality with regard to the particular point under observation. This publication is designed to emphasize the fact that the quality of city milk is not a simple matter to be adequately expressed after a consideration of any one factor, but that it is a complex matter which can be expressed only after an adequate consideration and evaluation of each of these four essential factors; namely, food value, healthfulness, cleanliness, and keeping quality. While the percentage of fat in milk is not a perfect measure of the food value, it is an easily determined index of food value. While medical supervision of the health of the cows and the men or proper pasteurization are not absolutely self-sufficient guarantees of the healthfulness of milk, they are the most practicable and easily applied indices of healthfulness. The sediment test, while open to some objections, is a simple and easily applied index of milk cleanliness. The problem of a satisfactory index for keeping quality is not so simply solved. Among the many available tests, that one must be selected which will best suit the purpose in hand. 12 Harding, H. A., Wilson, J. K., and Smith, G. A. of Method of Handling on the Germ Content of Milk. Sta. Bul. 317. 1909. Milking Machines: Effect Harding, H. A., Wilson, J. K., and Smith, G. A. The Modern Milk Pail. N. Y. (Geneva) Agr. Exp. Sta. Bui. 326. 1910. Harding, H. A., Ruehle, G. L., Wilson, J. K., and Smith, G. A. The Effect of Certain Dairy Operations upon the Germ Content of Milk. N. Y. (Geneva) Agr. Exp. Sta. Bul. 365, pp. 198-233. 1913. Harding, H. A., and Wilson, J. K. A Study of the Udder Flora of Cows. N. Y. (Geneva) Agr. Exp. Sta. Tech. Bul. 27. 1913. Ruehle, G. L. A., and Kulp, W. L. Germ Content of Stable Air and Its Effect upon the Germ Content of Milk. N. Y. (Geneva) Agr. Exp. Sta. Bul. 409, pp. 418 471. 1915. 13 Prucha, M. J., and Weeter, H. M. Germ Content of Milk: I. As Influenced by Factors at the Barn. Ill. Agr. Exp. Sta. Bul. 199. 1917. Much of the confusion in the public mind regarding milk quality has been due to a failure to discriminate properly between germ content and healthfulness, on the one hand, and germ content and cleanliness, on the other. The introduction of the public to the subject of germ life came. thru the attention which was early given to germs as producers of diseases such as tuberculosis 14 and typhoid fever. To the public, bacteria and disease became practically synonymous words. Later the attention of the public was directed to germ life in milk 15 at about the same time that its attention was directed to the possibility of germs of tuberculosis 16 being present in milk. Therefore, it is not at all strange that in public thought germ life and unhealthfulness of milk should have seemed identical. Early in the present century Metchnikoff 17 and other writers began to lay stress upon the health-giving qualities connected with certain germs in milk, as those of the Bulgaricus group. More recently extensive commercial use has been made, not only of cultured milks of various kinds, but also of vast quantities of buttermilk containing the ordinary sour-milk organisms with or without the addition of cultures of the Bulgaricus forms. There is a continued satisfactory use of these sour-milk drinks which contain many millions or billions of bacteria per cubic centimeter, not only of these special organisms with foreign names, but also of the organisms present in our sour milk of commerce. These experiences are gradually bringing home to the public an appreciation of the fact that there is very little connection between the amount of germ life in milk and the healthfulness of milk. The confusion in the public thought between the presence of germ life and cleanliness arises from the fact that it was originally believed that the seeding of milk with bacteria came about primarily as a result of a large quantity of bacteria being carried into the milk upon various forms of foreign matter, such as dirt and dust. Each particle 14 Koch, R. Die Aetiologie der Tuberkulose. Mitt. aus dem Kaiserl. Gesundheitsamte 2: 1-88. 1884. 15 Sedgwick and Bacthelder. A Bacteriological Examination of the Boston Milk Supply. Boston Med. and Surg. Jour. 12: 25-. 1892. 16 Russell, H: L. The Infectiousness of Milk from Tuberculous Cows. Wis. Agr. Exp. Sta. Ann. Rpt. 11, pp. 196-200. 1895. 17 Metchnikoff, E. The Prolongation of Life. 1908. |