Pollution at the Time of Milking arises from the animal, the milker, or unclean methods of milking. It is now well known that in tuberculosis of the cow affecting the udder the milk itself shows the presence of the bacillus of tubercle. In a precisely similar manner all bacterial diseases of the cow which affect the milk-secreting apparatus must inevitably add their quota of bacteria to the milk. To this matter we shall have occasion to refer again. There is a further contamination from the animal when it is kept unclean, for it happens that the unclean coat of a cow will more materially influence the number of micro-organisms in the milk than the popularly supposed fermenting food which the animal may eat. It is from this external source rather than from the diet that organisms occur in the milk. The hairy coat offers many facilities for harbouring dust and dirt. The mud and filth of every kind that may be habitually seen on the hinder quarters of cattle all contribute largely to polluted milk. Nor is this surprising. Such filth at or near the temperature of the blood is an almost perfect environment for many of the putrefactive bacteria.
The milker is also a source of risk. His hands, as well as the clothes he is wearing, can and do readily convey both innocent and pathogenic germs to the milk. Clothed in dust-laden garments, and frequently characterised by dirty hands, the milker may easily act as an excellent purveyor of germs. Not a few cases are also on record where it appears that milkers have conveyed germs of disease from some case of infectious disease, such as scarlet fever, in their homes. But under the more efficient registration of such disease which has recently characterised many dairy companies, the danger of infection from this source has been reduced to a minimum. The habit of moistening the hands with a few drops of milk previous to milking is one to be strongly deprecated.
Professor Russell recounts a simple experiment which clearly demonstrates these simple but effective sources of pollution:
"A cow that had been pastured in a meadow was taken for the experiment, and the milking done out of doors, to eliminate as much as possible the influence of germs in the barn air. Without any special precaution being taken the cow was partially milked, and during the operation a covered glass dish, containing a thin layer of sterile gelatine, was exposed for sixty seconds underneath the belly of the cow in close proximity to the milk-pail. The udder, flank, and legs of the cow were then thoroughly cleaned with water, and all of the precautions referred to before were carried out, and the milking then resumed. A second plate was then exposed in the same place for an equal length of time, a control also being exposed at the same time at a distance of ten feet from the animal and six feet from the ground to ascertain the germ contents of the surrounding air. From this experiment the following instructive data were gathered. Where the animal was milked without any special precautions being taken there were 3250 bacterial germs per minute deposited on an area equal to the exposed top of a ten-inch milk-pail. Where the cow received the precautionary treatment as suggested above, there were only 115 germs per minute deposited on the same area. In the plate that was exposed to the surrounding air at some distance from the cow there were 65 bacteria. This indicates that a large number of organisms from the dry coat of the animal can be kept out of milk if such simple precautions as these are carried out."[54]
The influence of the barn air, and the cleanliness or otherwise of the barn, is obviously great in this matter. As we have seen, moist surfaces retain any bacteria lodged upon them; but in a dry barn, where molecular disturbance is the rule rather than the exception, it is not surprising that the air is heavily laden with microbic life. Here again many improvements have been made by sanitary cleanliness in various well-known dairies. Still there is much more to be done in this direction to ensure that the drawn milk is not polluted by a microbe-impregnated atmosphere.
The risks in transit differ according to many circumstances. Probably the commonest source of contamination is in the use of unclean utensils and milk-cans. Any unnecessary delay in transit affords increased opportunity for multiplication; particularly is this the case in the summer months, for at such times all the conditions are favourable to an enormous increase of any extraneous germs which may have gained admittance at the time of milking. Thus we have (1) the milk itself affording an excellent medium and supplying ideal pabulum for bacteria, (2) a more or less lengthened railway journey or period of transit giving ample time for multiplication, (3) the favourable temperature of summer heat. We shall refer again to the rate of multiplication of germs in milk.
Lastly, many are the advantages given to bacteria when milk has reached its commercial destination. In milk-shops and in the home there are not a few risks to be added on to the already imposing category. Water is occasionally, if not frequently, added to milk to increase its volume. Such water of itself will make its own contribution to the flora of the milk, unless indeed, which is unlikely, the water has been recently and thoroughly boiled before addition to the milk. Again, it is impossible to suppose that in small homes—perhaps of only one room—where the milk stands for several hours, pollution is avoidable. From a hundred different sources such milk runs the risk of being polluted.
Before proceeding, a word must be said respecting the first milk which flows from the udder in the process of milking, and which is known as the fore-milk. This portion of the milk is always rich in bacterial life on account of the fact that it has remained in the milk-ducts since the last milking. However thorough the manipulation, there will always be a residue remaining in the ducts, which will, and does, afford a suitable nidus and incubator for organisms. The latter obtain their entrance through the imperfectly closed teat of the udder, and pass readily into the milk-duct, sometimes even reaching the udder itself and setting up inflammation (mastitis). Professor Russell states that he has found 2800 germs in the fore-milk in a sample of which the average was only 330 per cc. Schultz found 83,000 micro-organisms per cc. in the fore-milk, and only 9000 in the mid-milk. As a matter of fact, most of this large number belong to the lactic-acid fermentation group, and the fore-milk rarely contains more than two or three species, and still more rarely any disease-producing bacteria. Still, they occur in such enormous numbers that their addition to the ordinary milk very materially alters its quality. Bolley and Hall, of North Dakota, report sixteen species of bacteria in the fore-milk, twelve of which produced an acid reaction. Dr. Veranus Moore, of the United States Department of Agriculture,[55] concludes from a large mass of data that freshly drawn fore-milk contains a variable but generally enormous number of bacteria, but only several species, the last milk containing, as compared with the fore-milk, very few micro-organisms. The bacteria which become localised in the milk-ducts, and which are necessarily carried into the milk, are for the greater part rapidly acid-producing organisms, i. e., they ferment milk-sugar, forming acids. They do not produce gas. Still their presence renders it necessary to "pasteurise" as soon as possible. Dr. Moore holds that much of the intestinal trouble occurring in infants fed with ordinarily "pasteurised" milk arises from acids produced by these bacteria between the drawing of the milk and the pasteurisation.
The Number of Bacteria in Milk. From all that has been said respecting the sources of pollution and the favourable nidus which milk affords for bacteria, it is not surprising that a very large number of germs are almost always present in milk. The quantitative estimation of milk appears more alarming than the qualitative. It is true some diseases are conveyed by bacteria in milk, but on the whole most of the species are non-pathogenic. Nor need the numbers, though serious, too greatly alarm us, for, as we shall see at a later stage, disease is a complicated condition, and due to other agencies and conditions than merely the bacteria, which may be the vera causa. In addition to the fact that the high numbers have but a limited significance, we must also remember that there is no uniformity whatever in these numbers. The conditions which chiefly control them are (1) temperature, (2) time.
The Influence of Temperature. We have already noticed, when considering the general conditions affecting bacteria, how potent an agent in their growth is the surrounding temperature. Generally speaking, temperature at or about blood-heat favours bacterial growth. Freudenreich has drawn up the following table which graphically sets forth the effect of temperature upon bacteria in milk: