Sources of infection. The bacterial life that finds its way into milk while it is yet on the farm may be traced to several sources, which may be grouped under the following heads: unclean dairy utensils, fore milk, coat of animal, and general atmospheric surroundings. The relative importance of these various factors fluctuates in each individual instance.
Dairy utensils. Of first importance are the vessels that are used during milking, and also all storage cans and other dairy utensils that come in contact with the milk after it is drawn. By unclean utensils, actually visible dirt need not always be considered, although such material is often present in cracks and angles of pails and cans. Unless cleansed with especial care, these are apt to be filled with foul and decomposing material that suffices to seed thoroughly the milk. Tin utensils are best. Where made with joints, they should be well flushed with solder so as to be easily cleaned (see Fig. 6). In much of the cheaper tin ware on the market, the soldering of joints and seams is very imperfect, affording a place of refuge for bacteria and dirt.
Cans are often used when they are in a condition wholly unsuitable for sanitary handling of milk. When the tin coating becomes broken and the can is rusty, the quality of the milk is often profoundly affected. Olson[1] of the Wisconsin Station has shown that the action of rennet is greatly impaired where milk comes in contact with a rusty iron surface.
Fig. 6.
With the introduction of the form or hand separator a new milk utensil has been added to those previously in use and one which is very frequently not well cleaned. Where water is run through the machine to rinse out the milk particles, gross bacterial contamination occurs and the use of the machine much increases the germ content of the milk. Every time the separator is used it should be taken apart and thoroughly cleaned and dried before reassembling.[2]
Use of milk-cans for transporting factory by-products. The general custom of using the milk-cans to carry back to the farm the factory by-products (skim-milk or whey) has much in it that is to be deprecated. These by-products are generally rich in bacterial life, more especially where the closest scrutiny is not given to the daily cleaning of the vats and tanks. Too frequently the cans are not cleaned immediately upon arrival at the farm, so that the conditions are favorable for rapid fermentation. Many of the taints that bother factories are directly traceable to such a cause. A few dirty patrons will thus seriously infect the whole supply. The responsibility for this defect should, however, not be laid entirely upon the shoulders of the producer. The factory operator should see that the refuse material does not accumulate in the waste vats from day to day and is not transformed into a more or less putrid mass. A dirty whey tank is not an especially good object lesson to the patron to keep his cans clean.
It is possible that abnormal fermentations or even contagious diseases may thus be disseminated.
Suppose there appears in a dairy an infectious milk trouble, such as bitter milk. This milk is taken to the factory and passes unnoticed into the general milk-supply. The skim-milk from the separator is of course infected with the germ, and if conditions favor its growth, the whole lot soon becomes tainted. If this waste product is returned to the different patrons in the same cans that are used for the fresh milk, the probabilities are strongly in favor of some of the cans being contaminated and thus infecting the milk supply of the patrons. If the organism is endowed with spores so that it can withstand unfavorable conditions, this taint may be spread from patron to patron simply through the infection of the vessels that are used in the transportation of the by-products. Connell has reported just such a case in a Canadian cheese factory where an outbreak of slimy milk was traced to infected whey vats. Typhoid fever among people, foot and mouth disease and tuberculosis among stock are not infrequently spread in this way. In Denmark, portions of Germany and some states in America, compulsory heating of factory by-products is practiced to eliminate this danger.[3]
Pollution of cans from whey tanks. The danger is greater in cheese factories than in creameries, for whey usually represents a more advanced stage of fermentation than skim-milk. The higher temperature at which it is drawn facilitates more rapid bacterial growth, and the conditions under which it is stored in many factories contribute to the ease with which fermentative changes can go on in it. Often this by-product is stored in wooden cisterns or tanks, situated below ground, where it becomes impossible to clean them out thoroughly. A custom that is almost universally followed in the Swiss cheese factories, here in this country, as in Switzerland, is fully as reprehensible as any dairy custom could well be. In Fig. 7 the arrangement in vogue for the disposal of the whey is shown. The hot whey is run out through the trough from the factory into the large trough that is placed over the row of barrels, as seen in the foreground. Each patron thus has allotted to him in his individual barrel his portion of the whey, which he is supposed to remove day by day. No attempt is made to clean out these receptacles, and the inevitable result is that they become filled with a foul, malodorous liquid, especially in summer. When such material is taken home in the same set of cans that is used to bring the fresh milk (twice a day as is the usual custom in Swiss factories), it is no wonder that this industry is seriously handicapped by "gassy" fermentations that injure materially the quality of the product. Not only is the above danger a very considerable one, but the quality of the factory by-product for feeding purposes, whether it is skim-milk or whey, is impaired through the development of fermentative changes.