9,000 bacteria had in one case doubled, and in another more than
trebled themselves. It will be seen that the temperature most
favourable to the multiplication of these bacteria was 25 degrees
Centigrade.
If a sample of milk containing originally such a comparatively small
number of bacteria--for a figure under 10,000 per cubic centimetre
sinks into utter insignificance when we read of samples containing
2,500,000--if such relatively bacterially pure samples may support
such prodigious numbers of these Lilliputians, what the microbial
population of less satisfactory samples may amount to well-nigh
baffles our powers of calculation. Professor Russell writes: "If we
compare the bacterial flora of milk with that of sewage, a fluid that
is popularly, and rightly, supposed to be teeming with germ life, it
will almost always be observed that milk when it is consumed is richer
in bacteria by far than the sewage of our large cities. Sedgwick, in
his Report to the Massachusetts Board of Health for 1890, found that
the sewage of the city of Lawrence contained at the lowest 100,000
germs, whilst the maximum number was less than 4,000,000 per cubic
centimetre.[7] This range in numbers is much less than is usually
found in the milk-supply of our large cities."
[7] American sewage, it must be noted, is usually weaker and
poorer in bacterial life than that of our country, by reason
of the greater amount of water with which it is diluted.
Numerous researches have been carried out during the last half-dozen
years to try and localise the origin of some of the principal dairy
troubles, with a view to their possible extinction, or at least
control. In the course of these investigations quite a number of the
bacteria found in milk have been successfully hunted down, and their
offences brought home to them.
Thus, from so-called "bitter" milk a bacillus has been isolated by
Professor Weigmann, and found responsible for this particular change.
Another microbe was discovered in bitter cream whose office apparently
consisted in rendering milk strongly acid and extremely bitter. Again,
that objectionable condition of milk known as slimy, ropy, or stringy,
is brought about by certain bacteria which render it viscous; whilst
another crop of microbes are occupied in conferring upon it the power
of sticking to everything that touches it, making it capable of being
drawn out into threads from several inches to several feet in length.
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.
So-called "Pasteurised" milk has become during the last year or so
increasingly popular in this country, whilst on the Continent it has
been largely dealt in for several years past, and has commercially
proved a great success. Indeed, so strong is the prejudice amongst our
neighbours across the Channel against using unboiled milk that in
Leipzig and other cities in Germany endeavours have been made by
charitable and other societies to encourage the use of sterile milk
amongst the poorer classes, whilst it has been stated that the
introduction of Pasteurised milk among the poor of New York City,
through the philanthropic efforts of Mr. Nathan Straus, has done much
to reduce the high rate of mortality amongst infants during the hot
summer months. In France, _i.e._ in Paris and Grenoble, in order to
reduce if possible the lamentable mortality amongst infants from
diarrhoea in the summer months, which was largely attributed to the
use of unboiled milk, sterilised milk was distributed to the poor at
the cost of the community in general. In Grenoble, according to
statistics collected by Berlioz during the years 1894-6, the
death-rate of infants under a year old in the months of July, August,
and September fed on raw milk reached 69·3 per 1,000, whilst amongst
those supplied with sterilised milk it was reduced to 27·9 per 1,000.
Just, however, as all is not gold that glitters, so all sterilised
milk so-called is not necessarily free from bacteria. Indeed,
according to a recent German authority, "the complete and certain
sterilisation of milk is not yet to hand."
Dr. Weber examined the sterilised milk as supplied by various
companies in the city of Berlin. As many as 150 bottles were tested
from eight different sources, with the result that not one of these
eight companies was found to be supplying milk free from bacteria, or,
in other words, what it professed to be--sterile. True, the percentage
of sterile bottles varied from 5 per cent. in some of the supplies to
86 per cent. in others.
Thus it may be realised how, as has been already pointed out,
difficult a matter it is to devise an efficient apparatus for the
reliable sterilisation of milk. So far it appears that the best
results have been obtained with an apparatus devised by Flaack, a
director of the Brunswick Sterilising Milk Company, and known as the
Flaack apparatus. Exhaustive examinations made during the course of a
whole year in the Hygienic Institute at Würzburg never once showed a
failure, all the samples tested being germ-free.
Some supervision is, therefore, necessary in the case of these
milk-sterilising companies to ensure that the public is obtaining what
it is paying for, as it has been shown by Professor Flügge, a
world-renowned authority on the subject of milk and its sterilisation,
that the bacteria left over in these so-called sterilised milk samples
are by no means invariably a harmless residue, but, on the contrary,
may consist of individuals which he has gathered together in a class
under the heading of poisonous peptonising bacteria, and which owe
this unfortunate designation to the rapidity and energy with which
they can engender the putrefaction of albumen. As indicating how
essential it is that every detail in the sterilisation of milk should
be adequately assessed, I may mention a paper recently published by H.
L. Russell and E. G. Hastings, of the Wisconsin Agricultural
Experiment Station in the United States, on the importance of
Pasteurising milk in closed rather than in open vessels, bacteria
having been found more resistant in milk when heated in contact with
the air than in closed vessels, this variation being attributed to the
formation of a surface pellicle, which readily forms on milk when
heated in open vessels to a temperature of about 60° Centigrade or
above. Experiments showed that organisms present in this pellicle or
skin were capable of retaining their vitality when exposed to a
temperature _six degrees higher_ than that of the milk beneath the
membrane in which they were destroyed.