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

by H. L. Russell

Fig. 9.

Showing the bacterial contamination arising from hair. These hairs were allowed to fall on a sterile gelatin surface. The adherent bacteria developed readily in this medium, and the number of bacteria thus introduced into the milk from these hairs can be estimated by the number of developing colonies.]

The animal herself contributes materially to the quota of germ life finding its way into the milk through the dislodgment of dust and filth particles adhering to its hairy coat. The nature of this coat is such as to favor the retention of these particles. Unless care is taken the flanks and udder become polluted with fecal matter, which upon drying is displaced with every movement of the animal. Every hair or dirt particle so dislodged and finding its way into the milk-pail adds its quota of organisms to the liquid. This can be readily demonstrated by placing cow's hairs collected with care on the moist surface of gelatin culture plates. Almost invariably, bacteria will be found in considerable numbers adhering to such hairs as is indicated in Fig. 9. Dirt particles are even richer in germ life. Not only is there the dislodgment of hairs, epithelial scales and masses of dirt and filth, but during the milking process, as at all other times, every motion of the animal is accompanied by a shower of invisible particles more or less teeming with bacterial life.

The amount of actual impurities found in milk is often considerable and when it is remembered that about one-half of fresh manure dissolves in milk,[24] and thus does not appear as sediment, the presence of this undissolved residue bespeaks filthy conditions as to milking. From actual tests made, it is computed that the city of Berlin, Germany consumes about 300 pounds of such dirt and filth daily. Renk has laid down the following rule with reference to this insoluble dirt: If a sample of milk shows any evidence of impurity settling on a transparent bottom within two hours, it should be regarded as too dirty for use.

While the number of organisms here introduced is at all times large, the character of the species is of even greater import. Derived primarily from dirt and fecal matter, it is no wonder that such forms are able to produce very undesirable fermentative changes.

Influence of milker. The condition of the milker is not to be ignored in determining all possible factors of infection, for when clothed in the dust-laden garments that have been worn all day, a favorable opportunity for direct contamination is possible. The filthy practice of wetting the hands with milk just before milking is to be condemned. The milker's hands should be washed immediately before milking in clean water and dried. A pinch of vaseline on hands is sometimes used to obtain a firmer grasp and prevents the ready dislodgment of scales.[25] It must also be borne in mind that the milker may spread disease through the milk. In typhoid fever and diphtheria, the germs often remain in the system for weeks and thus make infection possible. Stocking[26] has shown that the individual milker exerts a potent influence on the total germ content of milk, even where the procedure is quite the same. In sanitary dairies milkers are usually clad in white duck suits.

Milking by machinery. Several mechanical devices have been invented for milking, some of which have been tested bacteriologically as to their efficiency. Harrison[27] has examined the "Thistle" machine but found a much higher germ content than with hand-drawn milk. The recent introduction of the Burrel-Lawrence-Kennedy machine has led to numerous tests in which very satisfactory results have been obtained. If the rubber parts of the milker are thoroughly cleaned and kept in lime water solution, they remain nearly sterile. When milk is properly handled, the germ content may be greatly reduced.

Reduction in dirt and adherent bacteria. No factor of contamination is so susceptible of improvement as that which relates to the reduction in filth and dirt which gains access during and immediately subsequent to the milking. The care which is taken to keep the coat of the animal clean by grooming lessens very much the grosser portion of such contamination, but with a dry, hairy coat, fine scales and dust particles must of necessity be dislodged.[28] Ordinarily the patron thinks all evidence of such dirt is removed if the milk is strained, but this process only lessens the difficulty; it does not overcome it. Various methods are in use, the effectiveness of which is subject to considerable variation. Some of these look to the elimination of the bacteria after they are once introduced into the milk; others to the prevention of infection in the first place.

1. Straining the milk. Most of the visible, solid particles of filth, such as hairs, dirt particles, etc., can be removed by simple straining, the time-honored process of purification. As ordinarily carried out, this process often contributes to instead of diminishing the germ life in milk. The strainer cloths unless washed and thoroughly sterilized by boiling harbor multitudes of organisms from day to day and may thus actually add to the organisms present. Various methods have been suggested for this simple process, but the most practical and efficient strainer is that made of fine wire gauze to which is added 3-4 layers of cheese cloth, the whole to set over the storage milk can.