Since the questions of production, handling, storage and age, closely affect the microbial content of milk, it is necessary to consider their influences.
(r) Production and Handling.—The amount of fresh milk consumed daily in this country is enormous. Practically all the milk of this supply is furnished by small producers, the majority of whom at present have no special knowledge of the conditions necessary for the production of a hygienic milk, or the facilities to apply such knowledge. The milk supply of the large centers of population is secured from the small pro ducers within a radius of from ioo to 200 miles and, owing to transportation difficulties, may be thirty six hours old when it reaches the consumer. On the other hand with the smaller cities and towns the radius of production is narrower and the consumer seldom gets milk 24 hours of age and usually less.
The principal sources from which micro-organisms gain introduction are the following: r. The udder and teats of the cow, as well as their flanks and bellies (Fig. 48).
2. The hands of the milker.
3. Dirt and dust of the stables (Figs. 9o, 4. Cow manure (Fig. 48).
5. Milking utensils.
Of these the first, fourth and fifth are probably of greatest importance. In general it may be said that the presence of micro-organisms is closely associated with the introduction of dirt, particularly cow manure, which commonly gains introduc tion from the teats, udder and flanks of the cow by dropping into the milking pail (Fig. 49). It is therefore apparent that a milking pail with a small top will prove of material assistance in reducing the amount of dirt that gains entrance (Fig. 59). In addition the thorough brushing of the cows with brush and curry comb (Fig. 52) followed by a wiping with a damp cloth, largely eliminates gross dirt from the milk pail. The character of the milk pails and utensils is of importance from another standpoint. If their interior is rusty or their seams rough, their cleaning is made difficult. Milk remains in the recesses in which bacteria multiply and which serve as cultures to seed the fresh milk when the utensils are again in use Hence in addition to possessing small tops, the milk pail should have a smooth interior and be subjected to sterilization after clean sing. For this purpose streaming steam is very effective. (Fig
57)• The temperature at which the milk is tored between the in terval of its production and its consumption is of great impor tance. If kept at air temperature, bacterial multiplication will be rapid and the milk will soon spoil. The rapid cooling (Fig. 55) of the milk after its drawing and its maintenance at a temperature of io°C. (5o°F.) or lower will largely inhibit bacterial multiplication and hence prevent decomposition. Practically sterile milk in commercial quantities cannot be produced, but on the other hand, care in the production and handling of milk will keep the count very low.
4. Classification of a hygienic standpoint we may consider either of two classifications of milk, first: (a) Raw milk, and (b) Pasteurized milk.
Raw milk includes all milk that has not been heated to a temperature sufficient to destroy most bacteria, and which at the same time will not alter the nutritional properties of the milk. Hygienically, raw milk may be of any grade of the second classification. Potentially it is dangerous, since with the greatest of care it is impossible to prevent the introduction of infective agents into some lots.
Pasteurization is a process of sterilizing or partially steri lizing organic solutions without altering their chemical proper ties, devised by Pasteur and first applied to beer and wines (Fig. 58). As applied to milk or cream three methods are in use. They are the following: (a) The flash method which from a hygienic standpoint is the poorest. It is used chiefly in those dairies run as adjuncts to creameries, where the milk or cream is heated to destroy undesirable lactic fermentation. These lactic organisms are destroyed at temperatures lower than the pathogenic organisms likely to be present, hence the latter are not affected. The milk is momentarily heated but the temperature is not main fained.