Part 11

Although we object in this country to slimy milk, in Holland it is in special request for the production of a certain cheese known under the name of Edam. In Norway this kind of milk forms a popular drink called Taettemjolk, and to produce it artificially they put the leaves of the common butter-wort (Pinguicula vulgaris) into milk. Professor Weigmann has discovered a micro-organism which frequents the leaves of this plant endowed with particular powers of producing slimy milk, and doubtless the credit of furnishing Taettemjolk is really due to this microbe, and not to the innocent butter-wort. "Soapy" milk, again, has been traced to a specific germ discovered in large numbers in straw used for bedding, whilst it was also detected in the hay that served for fodder. During milking these sources had supplied the infection, and the peculiar fermentation was distinctly shown to be microbial in origin. So-called red and blue milk, and those various hues ranging from bright lemon to orange and amber, are also now known to be directly attributable to bacterial activity.

But of even greater significance than all these bacterial dairy troubles is the risk of spreading disease which is furnished by milk contaminated with pathogenic micro-organisms.

"There can be no shadow of doubt," said the Lancet now many years ago, "that the contagia of typhoid and scarlet fever are disseminated by milk, and that boiled milk enjoys a much greater immunity from the chance of conveying disease."

This was written at a time when the study of bacteria was yet in its infancy, and before any direct experimental evidence had been obtained on the behaviour of microbes in milk or concerning the part played by them in the dissemination of disease. The writer evidently did not venture to cast further aspersions on the character of milk, or he might have included diphtheria amongst the diseases which can be spread by its means; but there is another omission which still more conclusively indicates the remote age in the history of bacterial science at which this correspondent to the Lancet wrote, and that is the absence of all reference to the tubercle bacillus in relation to milk. At the present day hardly a bacteriological journal is published which does not contain some reference to the question of tuberculosis and milk, and the transmissibility of this disease when present in cattle to man.

As regards the dissemination of various zymotic diseases by milk, the evidence which has been collected points very conclusively to the responsible part which may be played by milk in this connection. Many instances have been cited, also, of the culpability of milk in distributing typhoid germs. A striking case which occurs to me, and which may be mentioned in passing, is one which occurred in a city in America a few years ago, in which an outbreak of this disease was traced to a dairy in which the vessels had been washed out with typhoidal-polluted water. No less than 386 cases of typhoid declared themselves in six weeks, and of this number over 97 per cent. occurred amongst families obtaining their milk from the same dairy. A careful inspection revealed the fact that the milk-cans had been rinsed out with water from a shallow well contaminated with typhoid dejecta.

Diphtheria is also justly associated with infected milk, and if we take into consideration the now established fact that diphtheria bacilli thrive and multiply with particular facility in milk, even more so than in ordinary broth cultures; that they have been found in air in a vital and virulent condition, and may be scattered far and wide attached to dust particles; and if we remember the numerous opportunities offered for the infection of milk by persons handling it, who either themselves are suffering from this disease or are in diphtheria surroundings--then indeed we can readily understand how milk becomes a diphtheria-carrier of the first order.

Tuberculosis in cattle, and how this disease may affect the character of dairy produce, is, as already pointed out, a subject which is attracting the attention of a large number of investigators.

The general public is perhaps hardly aware of how widespread this disease is amongst cattle, and it is only of late years that very careful inquiries have elicited the fact that it is not only very extensively distributed, but may be present in animals to all outward appearance in perfect health.

In Germany it was asserted a few years ago that every fifth cow was tuberculous, and even this was regarded as a moderate estimate. The distinguished Danish pathologist, Professor Bang, is responsible for the announcement that during the years 1891-3 17·7 per cent. of the animals slaughtered in Copenhagen were infected with tuberculosis. In Paris we have been told that, of every thirteen samples of milk sold, one was infected with tubercle bacilli, whilst in Washington one in every nineteen samples of milk was stated to be similarly tainted.

The existence of tubercular disease in cows, and its transmission to other animals fed with their milk, has been brought out in a striking manner in investigations published by the Massachusetts Society for the Promotion of Agriculture. In one case as many as over 33 per cent. of the calves fed with milk from tuberculous cows succumbed to the same disease. According to Hirschberger, 10 per cent. of the cows living in the neighbourhood of towns where the conditions of their environment are not generally the most satisfactory or conducive to health suffer from tuberculosis, and 50 per cent. of these animals yield milk containing tubercle bacilli.

The demand which is being made by municipal authorities to be invested with the power of inspecting the country farms from whence their cities are supplied with milk and other agricultural produce could not have received stronger support than was recently supplied by a case tried in Edinburgh, and as this is only a sample of what is doubtless a daily, although undetected occurrence in many municipalities, it will not be out of place to quote the following from the published report of the proceedings:--

"A cow was brought into the city for sale as food, and the evidence showed it to be in the last stages of tubercular disease. 'Its head was hanging down; it breathed with difficulty, and it had frequent fits of coughing; while its udder was swollen with the disease.' All the organs were diseased, and the milk teemed with bacilli. Yet, it seemed, the milk from this animal had been regularly sent into Edinburgh for sale. In face of facts like these, it is difficult to see on what grounds the claim of towns to inspect country dairies doing a town business can be resisted. At least the towns should have the power to refuse admission to milk from sources not open to inspection. It is not enough for the county authorities to say that they inspect the dairies in their own areas. In this case the condition of the animal was only found out when it was brought into the town to be sold for food."

Further comment is unnecessary!

Some German investigators have discovered the interesting fact that the centrifugal method of separating milk not only has a remarkable effect upon its bacterial contents, but also upon tubercle bacilli when present. On examining the so-called "separator slime," it is found to contain not only large quantities of solid matters, but also masses of bacteria which have been thrown out during the operation. This method of treating milk has, curiously, a particular effect upon tubercle bacilli present, for Professor Scheurlen has found that they are nearly all left in the slime. Naturally his observation was not slow in being tested by other investigators; but Professor Bang has quite independently confirmed Scheurlen's discovery, and, still more recently, Moore purposely infected milk with these bacilli, and found that they were deposited in the slime to a most remarkable extent. Coupled, however, with this peculiar behaviour of tubercle bacilli in separated milk is the fact called attention to by Ostertag, that tuberculosis is much more prevalent among swine in Denmark and North Germany, where the centrifugal process in creaming is extensively used, and where, until recently, this slime was given to the animals in its raw, uncooked condition.

Before leaving this subject of separated milk, reference may be made to a danger, which has recently been publicly called attention to, surrounding the use which is made of skim milk. By an arrangement with the farmers who supply the milk, those clients who principally use it for producing butter return the skim milk to them after it has been through the separator, when it is employed for stock-feeding purposes. The milk in large dairies derived from different farmers is mixed, and hence the skim milk which is returned is also mixed. Thus, in the event of the milk from one farm being infected, not only is the whole milk-supply of a particular dairy infected, but, in returning the mixed skim milk likewise infected in its proper proportion to the different farmers, the virus is distributed over several farms. So real is this danger, and such unfortunate results have followed this practice of returning mixed infected skim milk, that since 1894 the Prussian Government has issued special orders for its disinfection by means of heat, in the hope of coping with this difficulty.

The longevity of the tubercle bacillus and its remarkable vitality under all kinds of untoward circumstances have not unnaturally added fresh significance to this frequent discovery of its presence in milk; moreover, laboratory experiments have shown that these germs can live for upwards of one hundred and twenty days in butter, and from sixty to seventy days in cheese. It is not surprising, therefore, to find a Royal Commission appointed in 1890 with the express object of inquiring and reporting upon "What is the effect, if any, of food derived from tuberculous animals on human health?"

In the summary appended to the report we read: "Tuberculous matter in milk is exceptionally active in its operation upon animals fed either with milk or with dairy produce derived from it. No doubt the largest part of the tuberculosis which man obtains through his food is by means of milk containing tuberculous matter."

That the Commissioners were alive to the great importance of this means of spreading disease is further shown by the following significant paragraph: "In regard to milk, we are aware of the preference by English people for drinking cow's milk raw, a practice attended by danger on account of possible contamination by pathogenic organisms."

The Commissioners spared no pains in endeavouring to throw light upon the important question they were appointed to report upon, and five years elapsed before they published the results of their inquiries. A decade ago the opinions expressed by them represented the current opinions of the leading bacteriological authorities in scientific circles at home and abroad, and these opinions were gradually filtering down to the general public, which is so conservative in clinging to traditions and popular delusions, when, like a flash out of the blue, the bacteriological Jove, Professor Robert Koch, hurled his thunderbolt into the arena, and at the British Congress on Consumption, held in London in the summer of 1901, declared his belief that bovine and human tuberculosis were distinct diseases. The significance of such a challenge to current scientific opinion, and its far-reaching influence if proved to be correct, was quickly appreciated by the distinguished audience who had gathered to hear what so great an authority as Dr. Robert Koch had to say on consumption and its distribution. The vital question raised by the original discoverer of the tubercle bacillus is still the subject of discussion, experimental inquiry, and much controversy, and we cannot here attempt to discuss the pros and cons for the acceptance or rejection of this new theory concerning the character of tuberculosis. It would, however, be regrettable in the extreme if the publication of this opinion were to encourage dairy authorities to relax in the slightest the efforts now so tardily being made by them to protect their dairy produce and ensure its safety for food-supply.

Before leaving this branch of the subject reference must be made to some very important researches recently published by Professor Ostertag, of Berlin, on the presence of tubercle bacilli in the milk derived from cows which, whilst reacting to the tuberculin test, exhibit no clinical symptoms of tuberculosis. The importance of this investigation to farmers and all breeders of stock is evident, for it has not infrequently been urged that all the milk from such tuberculin-reacting cows should be discarded for dietetic purposes. Professor Ostertag, at the request of the German Government, has carried out a most elaborate and very extensive series of investigations to determine the question as to whether such milk is dangerous to health. I cannot do better than quote the conclusions appended to the original memoir, in which Professor Ostertag expresses himself as follows: "The milk of cows which only react to tuberculin does not contain tubercle bacilli; calves and pigs can be fed during weeks and months with milk derived from such cows without contracting tuberculosis."

A very important rider, however, is added, in which it is pointed out that inasmuch as no doubt exists as to the highly infectious character of the milk derived from cows the udders of which are tuberculous, and from animals in which the disease is clinically recognisable, the weeding out of all such animals must be regarded as the most important measure for the prevention of the dissemination of tuberculosis through milk.

We must now pass on to a consideration of some of the methods which are available for obtaining germ-free milk, some of which are, however, attended with too great labour and inconvenience to admit of practical application. Thus, wishing to prepare some sterile milk without altering its chemical composition to feed certain microbes with, I had to patiently heat it for from one to two hours on five successive days, watching the while that the temperature remained between 58° and 65° centigrade. The milk was sterile, and I kept it for months, but such a process, of course, is impossible for domestic purposes.

The addition of chemicals to milk is both undesirable and ineffectual; amongst such substances boracic acid, borax, and salicylic acid are employed, but whilst the two former have been found to produce but little effect upon disease germs present in milk, salicylic acid hinders curdling more than other substances, and even if added in the small proportion of twelve grains per quart is said to impart a taste to the milk, and is, moreover, incapable of destroying typhoid bacilli if present.

Authorities are, moreover, not agreed as to the harmlessness of this ingredient, and in France the employment of salicylic acid in the preservation of food is strenuously opposed by doctors, who consider its habitual use injurious to health.

A Departmental Committee of the Local Government Board was appointed in this country to inquire into the use of preservatives in foods. In their report they state that from 42 up to 126 grains of boracic acid were detected in milk offered for sale, and that on one occasion no less than 80 grains of this material were present in a pint of milk sold to their inspector. It is pointed out that as long as preservatives are permitted there is no guarantee against the addition of excessive amounts to milk, and that evidence has been obtained pointing to an injurious effect of boracised milk upon the health of young children. The Committee report that in Denmark the use of preservatives is strictly prohibited, and the prohibition is strongly enforced; neither are preservatives permitted in Belgium.

The application of heat to milk is, in fact, the only advisable and reliable method for rendering it free from germs, but a great deal depends upon the manner in which the heat is applied and the cleanly condition or otherwise of the milk employed.

The difficulties which have to be overcome in producing efficiently sterilised milk are due, in the first place, to the remarkable power of resisting heat which characterises not only some disease germs, but also some of the microbes which are particularly partial to milk; secondly, to the sensitiveness of milk to heat, as exhibited by its alteration in taste and other respects through exposure to high temperatures.

To overcome these difficulties many ingenious pieces of apparatus have been devised, based upon a process originally introduced by Pasteur for preventing certain defects in wine and beer, and which consists in the application of a temperature of about 60° Centigrade. This process is known as Pasteurisation, after its renowned initiator.