Outlines of Dairy Bacteriology, 8th edition / A Concise Manual for the Use of Students in Dairying

by H. L. Russell

Fig. 13.

Bacterial content of milk drawn with care. Diminished germ content is shown by smaller number of colonies (330 bacteria per cc.). Compare this culture with that shown in Fig. 12.

"Sanitary" or "certified" milk. Within recent years there has been more or less generally introduced into many cities, the custom of supplying high grade milk that has been handled in a way so as to diminish its germ content as much as possible. Milk of this character is frequently known as "sanitary," "hygienic" or "certified," the last term being used in connection with a certification from veterinary authorities or boards of health as to the freedom of animals from contagious disease. Frequently a numerical bacterial standard is exacted as a pre-requisite to the recommendation of the board of examining physicians. Thus, the Pediatric Society of Philadelphia requires all children's milk that receives its recommendation to have not more than 10,000 bacteria per cc. Such a standard has its value in the scrupulous cleanliness that must prevail in order to secure these results. This in itself is practically a guarantee of the absence of those bacteria liable to produce trouble in children. The number of organisms found in such milks is surprisingly low when compared with ordinary milk. Naturally, there is considerable fluctuation from day to day, and occasionally the germ content is increased to a high figure without any apparent reason. The average results though, show a greatly reduced number of organisms. De Schweinitz[39] found in a Washington dairy in 113 examinations extending throughout a year, an average of 6,485 bacteria per cc. The daily analyses made of the Walker-Gordon supply sold in Philadelphia for an entire year, showed that the milk almost always contained less than 5,000 bacteria per cc. and on 120 days out of the year the germ content was 1,000 organisms per cc. or less.

From a practical point of view, the improvement in quality of sanitary milk, in comparison with the ordinary product is seen in the enhanced keeping quality. During the Paris Exposition in 1900, milk and cream from several such dairies in the United States were shipped to Paris, arriving in good condition after 15 to 18 days transit. When milk has been handled in such a way, it is evident that it is much better suited to serve as a food supply than where it has undergone the fermentative changes incident to the development of myriads of organisms.

Application of foregoing precautions to all milk producers. Milk is so susceptible to bacterial changes that it is necessary to protect it from invasion, if its original purity is to be maintained, and yet, from a practical point of view, the use to which it is destined has much to do with the care necessary to take in handling. The effect of the bacterial contamination of milk depends largely upon the way in which the product is used. To the milk-man engaged in the distribution of milk for direct consumption, all bacterial life is more or less of a detriment, while to the butter-maker and cheese-maker some forms are a direct necessity. It is unnecessary and impracticable to require the same degree of care in handling milk destined to be worked up into factory products as is done, for instance, in sanitary milk supplies, but this fact should not be interpreted to mean that the care of milk for factories is a matter of small consequence. In fact no more important dairy problem exists, and the purer and better the quality of the raw material the better the product will be. Particularly is this true with reference to cheese-making.

Dairymen have learned many lessons in the severe school of experience, but it is earnestly to be hoped that future conditions will not be summed up in the words of the eminent German dairy scientist, Prof. Fleischmann, when he says that "all the results of scientific investigation which have found such great practical application in the treatment of disease, in disinfection, and in the preservation of various products, are almost entirely ignored in milking."

Growth of bacteria in milk. Milk is so well suited as a medium for the development of germ life that it might be expected that all microörganisms would develop rapidly therein, and yet, as a matter of fact, growth does not begin at once, even though the milk may be richly seeded. At ordinary temperatures, such as 70° F., no appreciable increase is to be noted for a period of 6-9 hours; at lower temperatures (54° F.) this period is prolonged to 30-40 hours or even longer. After this period has elapsed, active growth begins and continues more or less rapidly until after curdling.

The cause of this suspended development is attributed to the germicidal properties inherent to the milk.[40]

Milk is of course seeded with a considerable variety of organisms at first. The liquefying and inert species are the most abundant, the distinctively lactic acid class occurring sparsely, if at all. As milk increases in age, germ growth begins to occur. More or less development of all types go on, but soon the lactic species gain the ascendency, owing to their being better suited to this environment; they soon outstrip all other species, with the result that normal curdling generally supervenes. The growth of this type is largely conditioned by the presence of the milk sugar. If the sugar is removed from milk by dialysis, the liquid undergoes putrefactive changes due to the fact that the putrefactive bacteria are able to grow if no acid is produced.