Outlines of dairy bacteriology, 10th edition / A concise manual for the use of students in dairying

by H. L. Russell; E. G. Hastings

Fig. 7.—Sectional View of Udder.
Teat with milk duct connecting the exterior with the milk cistern. Milk sinuses which conduct the milk from the secreting tissue to the milk cistern. (After Moore & Ward.)

Number of bacteria in fore-milk. If a bacteriological examination is made of the milk drawn from each teat at different periods during the milking process, it will be found that the fore-milk, i.e., the first few streams, contains, as a rule, many more organisms per cubic centimeter than that removed later. Not infrequently thousands of organisms per cubic centimeter may be found in the first streams while the middle milk, or strippings, will contain much smaller numbers.

Distribution and nature of bacteria in udder. If the udder itself is carefully examined as to its bacterial content, it appears that the majority of organisms found is confined to the lower portion of this organ, in the teat, milk-cistern and large milk-ducts; while bacteria occur in contact with the secreting tissue, they are relatively less abundant. This would seem to indicate that the more probable mode of infection is through the open teat.

While there is no constant type of bacteria found in the fore-milk, yet it is noteworthy that nearly all observers agree that the organisms most commonly found are not usually the acid-producing, or gas-generating type, so abundant on the skin or hairy coat of the udder and which predominate in ordinary milks. Coccus forms, belonging to both liquefying and non-liquefying types are most generally present. Many of these produce acid slowly and in small quantities.

The bacteria coming from the interior of the udder are of small practical significance since they do not grow rapidly at the temperatures at which milk is stored. If the milk is protected from contamination from other sources, the bacteria from the udder will ultimately cause it to spoil, but under ordinary conditions other forms are present in such greater numbers, and grow so much more rapidly in milk, that the udder forms have small opportunity to exert any effect.

It is interesting to note that the bacteria found in the udder are similar to those that seem to be most abundant in such glandular tissues as the liver and spleen. This fact increases the probability that these comparatively inert coccus forms of the udder may originate directly from the blood stream. The organisms that normally are found in the udder exert no harmful effects on the gland. It might be thought that due to the presence of abundant food and a favorable temperature that growth would be abundant, but such is not the case. At times the udder may be invaded by forms that are not held in check by the natural factors and an inflammation of the udder is likely to result.

Germicidal property of milk. It has been claimed that freshly drawn milk, like other body fluids, possesses germicidal properties, i.e., the power of destroying bacteria with which it may be brought in contact. If milk is carefully examined bacteriologically, hour by hour, after it is withdrawn from the udder, it will generally be found that there is at first not only no increase in number of organisms during a longer or shorter period when it is kept at temperatures varying from 40° to 70° F., but that an actual reduction not infrequently takes place. When cultures of bacteria, such as B. prodigiosus, a red organism, lactic acid organisms, and even the yellow, liquefying coccus, so commonly found in the fore-milk, are artificially introduced into the udder, it has been found that no growth occurs and that in the course of a few days the introduced organisms actually disappear. Whether this failure to colonize can be regarded as evidence of a germicidal property or not is questionable. In fact, this question is a matter of but little practical importance in the handling of milk since, under the best of conditions, the keeping quality of the milk is not materially enhanced. It may be of importance in inhibiting growth in the udder.

Rejection of fore-milk. The fact that the fore-milk contains per cubic centimeter so much more germ life than the remainder of the milk has led some to advocate its rejection when a sanitary milk supply is under consideration. While from a purely quantitative point of view, this custom may be considered advantageous, in practice, however, it is hardly worth while since it is not at all certain that the rejection will have any effect on the keeping quality or healthfulness of milk. This is especially true if the ends of the teats are thoroughly cleaned before milking. It is true that the fore-milk is relatively deficient in fat so that the loss of butter fat occasioned by the rejection of the first few streams is comparatively slight.

Contamination from utensils. One of the most important phases of contamination is that which comes from the utensils used to hold the milk from the time it is drawn until it is utilized. Not only is this important because it is a leading factor in the infection of milk, but because much improvement can be secured with but little trouble, and it is especially necessary that the dairy student should be made familiar with the various conditions that obtain. Pails and cans used to hold milk may be apparently clean to the eye, and yet contribute materially to the germ content of the milk placed in them. Not only does much depend upon their condition, but it is equally important to take into consideration their manner of construction. Dairy utensils should be simple in construction, rather than complex. They should be made so that they can be readily and easily cleaned, or otherwise the cleaning process is apt to be neglected.