RELATION OF BACTERIA TO THE DAIRY INDUSTRY.
Dairying is one of the most primitive of our industries. From the
very earliest period, ever since man began to keep domestic
cattle, he has been familiar with dairying. During these many
centuries certain methods of procedure have been developed which
produce desired results. These methods, however, have been devised
simply from the accumulation of experience, with very little
knowledge as to the reasons underlying them. The methods of past
centuries are, however, ceasing to be satisfactory. The advance of
our civilization during the last half century has seen a marked
expansion in the extent of the dairy industry. With this expansion
has appeared the necessity for new methods, and dairymen have for
years been looking for them. The last few years have been teaching
us that the new methods are to be found along the line of the
application of the discoveries of modern bacteriology. We have
been learning that the dairyman is more closely related to
bacteria and their activities than almost any other class of
persons. Modern dairying, apart from the matter of keeping the
cow, consists largely in trying to prevent bacteria from growing
in milk or in stimulating their growth in cream, butter, and
cheese. These chief products of the dairy will be considered
separately.
SOURCES OF BACTERIA IN MILK.
The first fact that claims our attention is, that milk at the time
it is secreted from the udder of the healthy cow contains no
bacteria. Although bacteria are almost ubiquitous, they are not
found in the circulating fluids of healthy animals, and are not
secreted by their glands. Milk when first secreted by the milk
gland is therefore free from bacteria. It has taken a long time to
demonstrate this fact, but it has been finally satisfactorily
proved. Secondly, it has been demonstrated that practically all of
the normal changes which occur in milk after its secretion are
caused by the growth of bacteria. This, too, was long denied, and
for quite a number of years after putrefactions and fermentations
were generally acknowledged to be caused by the growth of micro-
organisms, the changes which occurred in milk were excepted from
the rule. The uniformity with which milk will sour, and the
difficulty, or seeming impossibility, of preventing this change,
led to the belief that the souring of milk was a normal change
characteristic of milk, just as clotting is characteristic of
blood. This was, however, eventually disproved, and it was finally
demonstrated that, beyond a few physical changes connected with
evaporation and a slight oxidation of the fat, milk, if kept free
from bacteria, will undergo no change. If bacteria are not
present, it will remain sweet indefinitely.
But it is impossible to draw milk from the cow in such a manner
that it will be free from bacteria except by the use of
precautions absolutely impracticable in ordinary dairying. As milk
is commonly drawn, it is sure to be contaminated by bacteria, and
by the time it has entered the milk pail it contains frequently as
many as half a million, or even a million, bacteria in every cubic
inch of the milk. This seems almost incredible, but it has been
demonstrated in many cases and is beyond question. Since these
bacteria are not in the secreted milk, they must come from some
external sources, and these sources are the following:
The first in importance is the cow herself; for while her milk
when secreted is sterile, and while there are no bacteria in her
blood, nevertheless the cow is the most prolific source of
bacterial contamination. In the first place, the milk ducts are
full of them. After each milking a little milk is always left in
the duct, and this furnishes an ideal place for bacteria to grow.
Some bacteria from the air or elsewhere are sure to get into these
ducts after the milking, and they begin at once to multiply
rapidly. By the next milking they become very abundant in the
ducts, and the first milk drawn washes most of them at once into
the milk pail, where they can continue their growth in the milk.
Again, the exterior of the cow's body contains them in abundance.
Every hair, every particle of dirt, every bit of dried manure, is
a lurking place for millions of bacteria. The hind quarters of a
cow are commonly in a condition of much filth, for the farmer
rarely grooms his cow, and during the milking, by her movements,
by the switching of her tail, and by the rubbing she gets from the
milker, no inconsiderable amount of this dirt and filth is brushed
off and falls into the milk pail The farmer understands this
source of dirt and usually feels it necessary to strain the milk
after the milking. But the straining it receives through a coarse
cloth, while it will remove the coarser particles of dirt, has no
effect upon the bacteria, for these pass through any strainer
unimpeded. Again, the milk vessels themselves contain bacteria, for
they are never washed absolutely clean. After the most thorough
washing which the milk pail receives from the kitchen, there will
always be left many bacteria clinging in the cracks of the tin or
in the wood, ready to begin to grow as soon as the milk once more
fills the pail The milker himself contributes to the supply, for
he goes to the milking with unclean hands, unclean clothes, and
not a few bacteria get from him to his milk pail. Lastly, we find
the air of the milking stall furnishing its quota of milk bacteria.
This source of bacteria is, how ever, not so great as was formerly
believed. That the air may contain many bacteria in its dust is
certain, and doubtless these fall in some quantity into the milk,
especially if the cattle are allowed to feed upon dusty hay before
and during the milking. But unless the air is thus full of dust
this source of bacteria is not very great, and compared with the
bacteria from the other sources the air bacteria are unimportant.
The milk thus gets filled with bacteria, and since it furnishes an
excellent food these bacteria begin at once to grow. The milk when
drawn is warm and at a temperature which especially stimulates
bacterial growth. They multiply with great rapidity, and in the
course of a few hours increase perhaps a thousandfold. The numbers
which may be found after twenty-four hours are sometimes
inconceivable; market milk may contain as many as five hundred
millions per cubic inch; and while this is a decidedly extreme
number, milk that is a day old will almost always contain many
millions in each cubic inch, the number depending upon the age of
the milk and its temperature. During this growth the bacteria
have, of course, not been without their effect. Recognising as we
do that bacteria are agents for chemical change, we are prepared
to see the milk undergoing some modifications during this rapid
multiplication of bacteria. The changes which these bacteria
produce in the milk and its products are numerous, and decidedly
affect its value. They are both advantageous and disadvantageous
to the dairyman. They are nuisances so far as concerns the milk
producer, but allies of the butter and cheese maker.
THE EFFECT OF BACTERIA ON MILK.
The first and most universal change effected in milk is its
SOURING. So universal is this phenomenon that it is generally
regarded as an inevitable change which can not be avoided, and, as
already pointed out, has in the past been regarded as a normal
property of milk. To-day, however, the phenomenon is well
understood. It is due to the action of certain of the milk
bacteria upon the milk sugar which converts it into lactic acid,
and this acid gives the sour taste and curdles the milk. After
this acid is produced in small quantity its presence proves
deleterious to the growth of the bacteria, and further bacterial
growth is checked. After souring, therefore, the milk for some
time does not ordinarily undergo any further changes.
Milk souring has been commonly regarded as a single phenomenon,
alike in all cases. When it was first studied by bacteriologists
it was thought to be due in all cases to a single species of
micro-organism which was discovered to be commonly present and
named Bacillus acidi lactici (Fig. 19). This bacterium has
certainly the power of souring milk rapidly, and is found to be
very common in dairies in Europe. As soon as bacteriologists
turned their attention more closely to the subject it was found
that the spontaneous souring of milk was not always caused by the
same species of bacterium. Instead of finding this Bacillus acidi
lactici always present, they found that quite a number of
different species of bacteria have the power of souring milk, and
are found in different specimens of soured milk. The number of
species of bacteria which have been found to sour milk has
increased until something over a hundred are known to have this
power. These different species do not affect the milk in the same
way. All produce some acid, but they differ in the kind and the
amount of acid, and especially in the other changes which are
effected at the same time that the milk is soured, so that the
resulting soured milk is quite variable. In spite of this variety,
however, the most recent work tends to show that the majority of
cases of spontaneous souring of milk are produced by bacteria
which, though somewhat variable, probably constitute a single
species, and are identical with the Bacillus acidi lactici (Fig.
19). This species, found common in the dairies of Europe,
according to recent investigations occurs in this country as well.
We may say, then, that while there are many species of bacteria
infesting the dairy which can sour the milk, there is one which is
more common and more universally found than others, and this is
the ordinary cause of milk souring.
When we study more carefully the effect upon the milk of the
different species of bacteria found in the dairy, we find that
there is a great variety of changes which they produce when they
are allowed to grow in milk. The dairyman experiences many
troubles with his milk. It sometimes curdles without becoming
acid. Sometimes it becomes bitter, or acquires an unpleasant
"tainted" taste, or, again, a "soapy" taste. Occasionally a
dairyman finds his milk becoming slimy, instead of souring and
curdling in the normal fashion. At such times, after a number of
hours, the milk becomes so slimy that it can be drawn into long
threads. Such an infection proves very troublesome, for many a
time it persists in spite of all attempts made to remedy it.
Again, in other cases the milk will turn blue, acquiring about the
time it becomes sour a beautiful sky-blue colour. Or it may become
red, or occasionally yellow. All of these troubles the dairyman
owes to the presence in his milk of unusual species of bacteria
which grow there abundantly.
Bacteriologists have been able to make out satisfactorily the
connection of all these infections with different species of the
bacteria. A large number of species have been found to curdle milk
without rendering it acid, several render it bitter, and a number
produce a "tainted" and one a "soapy" taste. A score or more have
been found which have the power of rendering the milk slimy. Two
different species at least have the power of turning the milk to
sky-blue colour; two or three produce red pigments (Fig. 20), and
one or two have been found which produce a yellow colour. In
short, it has been determined beyond question that all these
infections, which are more or less troublesome to dairymen, are
due to the growth of unusual bacteria in the milk.
These various infections are all troublesome, and indeed it may be
said that, so far as concerns the milk producer and the milk
consumer, bacteria are from beginning to end a source of trouble.
It is the desire of the milk producer to avoid them as far as
possible--a desire which is shared also by everyone who has
anything to do with milk as milk. Having recognised that the
various troubles, which occasionally occur even in the better
class of dairies, are due to bacteria, the dairyman is, at least
in a measure, prepared to avoid them. The avoiding of these
troubles is moderately easy as soon as dairymen recognise the
source from which the infectious organisms come, and also the fact
that low temperatures will in all cases remedy the evil to a large
extent. With this knowledge in hand the avoidance of all these
troubles is only a question of care in handling the dairy. It must
be recognised that most of these troublesome bacteria come from
some unusual sources of infection. By unusual sources are meant
those which the exercise of care will avoid. It is true that the
souring bacteria appear to be so universally distributed that they
can not be avoided by any ordinary means. But all other
troublesome bacteria appear to be within control. The milkman must
remember that the sources of the troubles which are liable to
arise in his milk are in some form of filth: either filth on the
cow, or dust in the hay which is scattered through the barn, or
dirt on cows' udders, or some other unusual and avoidable source.
These sources, from what we have already noticed, will always
furnish the milk with bacteria; but under common conditions, and
when the cow is kept in conditions of ordinary cleanliness, and
frequently even when not cleanly, will only furnish bacteria that
produce the universal souring. Recognising this, the dairyman at
once learns that his remedies for the troublesome infections are
cleanliness and low temperatures. If he is careful to keep his
milk vessels scrupulously clean; if he will keep his cow as
cleanly as he does his horse; and if he will use care in and
around the barn and dairy, and then apply low temperatures to the
milk, he need never be disturbed by slimy or tainted milk, or any
of these other troubles; or he can remove such infections speedily
should they once appear. Pure sweet milk is only a question of
sufficient care. But care means labour and expense. As long as we
demand cheap milk, so long will we be supplied with milk procured
under conditions of filth. But when we learn that cheap milk is
poor milk, and when we are willing to pay a little more for it,
then only may we expect the use of greater care in the handling of
the milk, resulting in a purer product.
Bacteriology has therefore taught us that the whole question of
the milk supply in our communities is one of avoiding the too
rapid growth of bacteria. These organisms are uniformly a nuisance
to the milkman. To avoid their evil influence have been designed
all the methods of caring for the dairy and the barn, all the
methods of distributing milk in ice cars. Moreover, all the
special devices connected with the great industry of milk supply
have for their foundation the attempt to avoid, in the first
place, the presence of too great a number of bacteria, and. in the
second place, the growth of these bacteria.
BACTERIA IN BUTTER MAKING.
CREAM RIPENING.--Passing from milk to butter, we find a somewhat
different story, inasmuch as here bacteria are direct allies to
the dairyman rather than his enemies. Without being aware of it,
butter makers have for years been making use of bacteria in their
butter making and have been profiting by the products which the
bacteria have furnished them. Cream, as it is obtained from milk,
will always contain bacteria in large quantity, and these bacteria
will grow as readily in the cream as they will in the milk. The
butter maker seldom churns his cream when it is freshly obtained
from the milk. There are, it is true, some places where sweet
cream butter is made and is in demand, but in the majority of
butter-consuming countries a different quality of butter is
desired, and the cream is subjected to a process known as
"ripening" or "souring" before it is churned. In ripening, the
cream is simply allowed to stand in a vat for a period varying
from twelve hours to two or three days, according to
circumstances. During this period certain changes take place
therein. The bacteria which were in the cream originally, get an
opportunity to grow, and by the time the ripening is complete they
become extremely numerous. As a result, the character of the cream
changes just as the milk is changed under similar circumstances.
It becomes somewhat soured; it becomes slightly curdled, and
acquires a peculiarly pleasant taste and an aroma which was not
present in the original fresh cream. After this ripening the cream
is churned. It is during the ripening that the bacteria produce
their effect, for after the churning they are of less importance.
Part of them collect in the butter, part of them are washed off
from the butter in the buttermilk and the subsequent processes.
Most of the bacteria that are left in the butter soon die, not
finding there a favourable condition for growth; some of them,
however, live and grow for some time and are prominent agents in
the changes by which butter becomes rancid. The butter maker is
concerned with the ripening rather than with later processes.
The object of the ripening of cream is to render it in a better
condition for butter making. The butter maker has learned by long-
experience that ripened cream churns more rapidly than sweet
cream, and that he obtains a larger yield of butter therefrom. The
great object of the ripening, however, is to develop in the butter
the peculiar flavour and aroma which is characteristic of the
highest product. Sweet cream butter lacks flavour and aroma,
having indeed a taste almost identically the same as cream.
Butter, however, that is made from ripened cream has a peculiar
delicate flavour and aroma which is well known to lovers of
butter, and which is developed during the ripening process.
Bacteriologists have been able to explain with a considerable
degree of accuracy the object of this ripening. The process is
really a fermentation comparable to the fermentation that takes
place in a brewer's malt. The growth of bacteria during the
ripening produces chemical changes of a somewhat complicated
character, and concerns each of the ingredients of the milk. The
lactic-acid organisms affect the milk sugar and produce lactic
acid; others act upon the fat, producing slight changes therein;
while others act upon the casein and the albumens of the milk. As
a result, various biproducts of decomposition arise, and it is
these biproducts of decomposition that make the difference between
the ripened and the unripened cream. They render it sour and
curdle it, and they also produce the flavours and aromas that
characterize it. Products of decomposition are generally looked
upon as undesirable for food, and this is equally true of these
products that arise in cream if the decomposition is allowed to
continue long enough. If the ripening, instead of being stopped at
the end of a day or two, is allowed to continue several days, the
cream becomes decayed and the butter made therefrom is decidedly
offensive. But under the conditions of ordinary ripening, when the
process is stopped at the right moment, the decomposition products
are pleasant rather than unpleasant, and the flavours and aromas
which they impart to the cream and to the subsequent butter are
those that are desired. It is these decomposition products that
give the peculiar character to a high quality of butter, and this
peculiar quality is a matter that determines the price which the
butter maker can obtain for his product.
But, unfortunately, the butter maker is not always able to depend
upon the ripening. While commonly it progresses in a satisfactory
manner, sometimes, for no reason that he can assign, the ripening
does not progress normally. Instead of developing the pleasant
aroma and flavour of the properly ripened cream, the cream develops
unpleasant tastes. It may be bitter or somewhat tainted, and just
as sure as these flavours develop in the cream, so sure does the
quality of the butter suffer. Moreover, it has been learned by
experience that some creameries are incapable of obtaining an
equally good ripening of their cream. While some of them will
obtain favourable results, others, with equal care, will obtain a
far less favourable flavour and aroma in their butter. The reason
for all this has been explained by modern bacteriology. In the
milk, and consequently in the cream, there are always found many
bacteria, but these are not always of the same kinds. There are
scores, and probably hundreds, of species of bacteria common in and
around our barns and dairies, and the bacteria that are abundant
and that grow in different lots of cream will not be always the
same. It makes a decided difference in the character of the
ripening, and in the consequent flavours and aromas, whether one or
another species of bacteria has been growing in the cream. Some
species are found to produce good results with desired flavours,
while others, under identical conditions, produce decidedly poor
results with undesired flavours. If the butter maker obtains cream
which is filled with a large number of bacteria capable of
producing good flavours, then the ripening of his cream will be
satisfactory and his butter will be of high quality. If, however,
it chances that his cream contains only the species which produce
unpleasant flavours, then the character of the ripening will be
decidedly inferior and the butter will be of a poorer grade.
Fortunately the majority of the kinds of bacteria liable to get
into the cream from ordinary sources are such as produce either
good effects upon the cream or do not materially influence the
flavour or aroma. Hence it is that the ripening of cream will
commonly produce good results. Bacteriologists have learned that
there are some species of bacteria more or less common around our
barns which produce undesirable effects upon flavour, and should
these become especially abundant in the cream, then the character
of the ripening and the quality of the subsequent butter will
suffer. These malign species of bacteria, however, are not very
common in properly kept barns and dairies. Hence the process that
is so widely used, of simply allowing cream to ripen under the
influence of any bacteria that happen to be in it, ordinarily
produces good results. But our butter makers sometimes find, at the
times when the cattle change from winter to summer or from summer
to winter feed, that the ripening is abnormal. The reason appears
to be that the cream has become infested with an abundance of
malign species. The ripening that they produce is therefore an
undesirable one, and the quality of the butter is sure to suffer.
So long as butter was made only in private dairies it was a matter
of comparatively little importance if there was an occasional
falling off in quality of this sort. When it was made a few pounds
at a time, and only once or twice a week, it was not a very
serious matter if a few churnings of butter did suffer in quality.
But to-day the butter-making industries are becoming more and more
concentrated into large creameries, and it is a matter of a good
deal more importance to discover some means by which a uniformly
high quality can be insured. If a creamery which makes five
hundred pounds of butter per day suffers from such an injurious
ripening, the quality of its butter will fall off to such an
extent as to command a lower price, and the creamery suffers
materially. Perhaps the continuation of such a trouble for two or
three weeks would make a difference between financial success and
failure in the creamery. With our concentration of the butter-
making industries it is becoming thus desirable to discover some
means of regulating this process more accurately.
The remedy of these occasional ill effects in cream ripening has
not been within the reach of the butter maker. The butter maker
must make butter with the cream that is furnished him, and if that
cream is already impregnated with malign species of bacteria he is
helpless. It is true that much can be done to remedy these
difficulties by the exercise of especial care in the barns of the
patrons of the creamery. If the barns, the cows, the dairies, the
milk vessels, etc., are all kept in condition of strict
cleanliness, if especial care is taken particularly at the seasons
of the year when trouble is likely to arise, and if some attention
is paid to the kind of food which the cattle eat, as a rule the
cream will not become infected with injurious bacteria. It may be
taken as a demonstrated fact that these malign bacteria come from
sources of filth, and the careful avoidance of all such sources of
filth will in a very large measure prevent their occurrence in the
cream. Such measures as these have been found to be practicable in
many creameries. Creameries which make the highest priced and the
most uniform quality of butter are those in which the greatest
care is taken in the barns and dairies to insure cleanliness and
in the handling of the milk and cream. With such attention a large
portion of the trouble which arises in the creameries from malign
bacteria may be avoided.
But these methods furnish no sure remedy against evils of improper
species of bacteria in cream ripening, and do not furnish any sure
means of obtaining uniform flavour in butter. Even under the very
best conditions the flavour of the butter will vary with the
season of the year. Butter made in the winter is inferior to that
made in the summer months; and while this is doubtless due in part
to the different food which the cattle have and to the character
of the cream resulting therefrom, these differences in the flavour
of the butter are also in part dependent upon the different
species of bacteria which are present in the ripening of cream at
different seasons. The species of bacteria in June cream are
different from those that are commonly present in January cream,
and this is certainly a factor in determining the difference
between winter and summer butter.
USE OF ARTIFICIAL BACTERIA CULTURES FOR CREAM RIPENING.
Bacteriologists have been for some time endeavouring to aid butter
makers in this direction by furnishing them with the bacteria
needful for the best results in cream ripening. The method of
doing this is extremely simple in principle, but proves to be
somewhat difficult in practice. It is only necessary to obtain the
species of bacteria that produce the highest results, and then to
furnish these in pure culture and in large quantity to the butter
makers, to enable them to inoculate their cream with the species
of bacteria which will produce the results that they desire. For
this purpose bacteriologists have been for several years searching
for the proper species of bacteria to produce the best results,
and there have been put upon the market for sale several distinct
"pure cultures" for this purpose. These have been obtained by
different bacteriologists and dairymen in the northern European
countries and also in the United States. These pure cultures are
furnished to the dairymen in various forms, but they always
consist of great quantities of certain kinds of bacteria which
experience has found to be advantageous for the purpose of cream
ripening.
There have hitherto appeared a number of difficulties in the way
of reaching complete success in these directions. The most
prominent arises in devising a method of using pure cultures in
the creamery. The cream which the butter makers desire to ripen
is, as we have seen, already impregnated with bacteria, and would
ripen in a fashion of its own even if no pure culture of bacteria
were added thereto. Pure cultures can not therefore be used as
simply as can yeast in bread dough. It is plain that the simple
addition of a pure culture to a mass of cream would not produce
the desired effects, because the cream would be ripened then, not
by the pure culture alone, but by the pure culture plus all of the
bacteria that were originally present. It would, of course, be
something of a question as to whether under these conditions the
results would be favourable, and it would seem that this method
would not furnish any means of getting rid of bad tastes and
flavours which have come from the presence of malign species of
bacteria. It is plainly desirable to get rid of the cream bacteria
before the pure culture is added. This can be readily done by
heating it to a temperature of 69 degrees C. (155 degrees F.) for
a short time, this temperature being sufficient to destroy most of
the bacteria. The subsequent addition of the pure culture of
cream-ripening bacteria will cause the cream to ripen under the
influence of the added culture alone. This method proves to be
successful, and in the butter making countries in Europe it is
becoming rapidly adopted.
In this country, however, this process has not as yet become very
popular, inasmuch as the heating of the cream is a matter of
considerable expense and trouble, and our butter makers have not
been very ready to adopt it. For this reason, and also for the
purpose of familiarizing butter makers with the use of pure
cultures, it has been attempted to produce somewhat similar though
less uniform results by the use of pure cultures in cream without
previous healing. In the use of pure cultures in this way, the
butter maker is directed to add to his cream a large amount of a
prepared culture of certain species of bacteria, upon the
principle that the addition of such a large number of bacteria to
the cream, even though the cream is already inoculated with
certain bacteria, will produce a ripening of the cream chiefly
influenced by the artificially added culture. The culture thus
added, being present in very much greater quantity than the other
"wild" species, will have a much greater effect than any of them.
This method, of course, cannot insure uniformity. While it may
work satisfactorily in many cases, it is very evident that in
others, when the cream is already filled with a large number of
malign species of bacteria, such an artificial culture would not
produce the desired results. This appears to be not only the
theoretical but the actual experience. The addition of such pure
cultures in many cases produces favourable results, but it does
not always do so, and the result is not uniform. While the use of
pure cultures in this way is an advantage over the method of
simply allowing the cream to ripen normally without such
additions, it is a method that is decidedly inferior to that which
first pasteurizes the cream and subsequently adds a starter.
There is still another method of adding bacteria to cream to
insure a more advantageous ripening, which is frequently used,
and, being simpler, is in many cases a decided advantage. This
method is by the use of what is called a natural starter. A
natural starter consists simply of a lot of cream which has been
taken from the most favourable source possible--that is, from the
cleanest and best dairy, or from the herd producing the best
quality of cream--and allowing this cream to stand in a warm place
for a couple of days until it becomes sour. The cream will by that
time be filled with large numbers of bacteria, and this is then
put as a starter into the vat of cream to be ripened. Of course,
in the use of this method the butter maker has no control over the
kinds of bacteria that will grow in the starter, but it is found,
practically, that if the cream is taken from a good source the
results are extremely favourable, and there is produced in this
way almost always an improvement in the butter.
The use of pure cultures is still quite new, particularly in this
country. In the European butter-making countries they have been
used for a longer period and have become very much better known.
What the future may develop along this line it is difficult to
say; but it seems at least probable that as the difficulties in
the details are mastered the time will come when starters will be
used by our butter makers for their cream ripening, just as yeast
is used by housewives for raising bread, or by brewers for
fermenting malt. These starters will probably in time be furnished
by bacteriologists. Bacteriology, in other words, is offering in
the near future to our butter makers a method of controlling the
ripening of the cream in such a way as to insure the obtaining of
a high and uniform quality of butter, so far, at least, as
concerns flavour and aroma.
BACTERIA IN CHEESE.
Cheese ripening.--The third great product of the dairy industry is
cheese, and in connection with this product the dairyman is even
more dependent upon bacteria than he is in the production of
butter. In the manufacture of cheese the casein of the milk is
separated from the other products by the use of rennet, and is
collected in large masses and pressed, forming the fresh cheese.
This cheese is then set aside for several weeks, and sometimes for
months, to undergo a process that is known as ripening. During the
ripening there are developed in the cheese the peculiar flavours
which are characteristic of the completed product. The taste of
freshly made cheese is extremely unlike that of the ripened
product. While butter made from unripened cream has a pleasant
flavour, and one which is in many places particularly enjoyed,
there is nowhere a demand for unripened cheese, for the freshly
made cheese has a taste that scarce any one regards as pleasant.
Indeed, the whole value of the cheese is dependent upon the
flavour of the product, and this flavour is developed during the
ripening.
The cheese maker finds in the ripening of his cheese the most
difficult part of his manufacture. It is indeed a process over
which he has very little control. Even when all conditions seem to
be correct, when cheese is made in the most careful manner, it not
infrequently occurs that the ripening takes place in a manner that
is entirely abnormal, and the resulting cheese becomes worthless.
The cheese maker has been at an entire loss to understand these
irregularities, nor has he possessed any means of removing them.
The abnormal ripening that occurs takes on various types.
Sometimes the cheese will become extraordinarily porous, filled
with large holes which cause the cheese to swell out of proper
shape and become worthless. At other times various spots of red or
blue appear in the manufactured cheese; while again unpleasant
tastes and flavours develop which render the product of no value.
Sometimes a considerable portion of the product of the cheese
factory undergoes such irregular ripening, and the product for a
long time will thus be worthless. If some means could be
discovered of removing these irregularities it would be a great
boon to the cheese manufacturer; and very many attempts have been
made in one way or another to furnish the cheese maker with some
details in the manufacture which will enable him in a measure to
control the ripening.
The ripening of the cheese has been subjected to a large amount of
study on the part of bacteriologists who have been interested in
dairy products. That the ripening of cheese is the result of
bacterial growth therein appears to be probable from a priori
grounds. Like the ripening of cream, it is a process that occurs
somewhat slowly. It is a chemical change which is accompanied by
the destruction of proteid matter; it takes place best at certain
temperatures, and temperatures which we know are favourable to the
growth of micro-organisms, all of which phenomena suggest to us
the action of bacteria. Moreover, the flavours and the tastes that
arise have a decided resemblance in many cases to the
decomposition products of bacteria, strikingly so in Limburger
cheese. When we come to study the matter of cheese ripening
carefully we learn beyond question that this a priori conclusion
is correct. The ripening of any cheese is dependent upon several
different factors. The method of preparation, the amount of water
left in the curd, the temperature of ripening, and other
miscellaneous factors connected with the mechanical process of
cheese manufacture, affect its character. But, in addition to all
these factors, there is undoubtedly another one, and that is the
number and the character of the bacteria that chance to be in the
curd when the cheese is made. While it is found that cheeses which
are treated by different processes will ripen in a different
manner, it is also found that two cheeses which have been made
under similar conditions and treated in identically the same way
may also ripen in a different manner, so that the resulting
flavour will vary. The variations between cheeses thus made may be
slight or they may be considerable, but variations certainly do
occur. Every one knows the great difference in flavours of
different cheeses, and these flavours are due in considerable
measure to factors other than the simple mechanical process of
making the cheese. The general similarity of the whole process to
a bacterial fermentation leads us to believe at the outset that
some of the differences in character are due to different kinds of
bacteria that multiply in the cheese and produce decomposition
therein.
When the matter comes to be studied by bacteriology, the
demonstration of this position becomes easy. That the ripening of
cheese is due to growth of bacteria is very easily proved by
manufacturing cheeses from milk which is deprived of bacteria. For
instance, cheeses have been made from milk that has been either
sterilized or pasteurized--which processes destroy most of the
bacteria therein--and, treated otherwise in a normal manner, are
set aside to ripen. These cheeses do NOT ripen, but remain for
months with practically the same taste that they had originally.
In other experiments the cheese has been treated with a small
amount of disinfective, which is sufficient to prevent bacteria
from growing, and again ripening is found to be absolutely
prevented. Furthermore, if the cheese under ordinary conditions is
studied during the ripening process, it is found that bacteria are
growing during the whole time. These facts all taken together
plainly prove that the ripening of cheese is a fermentation due to
bacteria. It will be noticed, however, that the conditions in the
cheese are not favourable for very rapid bacterial growth. It is
true that there is plenty of food in the cheese for bacterial
life, but the cheese is not very moist; it is extremely dense,
being subjected in all cases to more or less pressure. The
penetration of oxygen into the centre of the mass must be
extremely slight. The density, the lack of a great amount of
moisture, and the lack of oxygen furnish conditions in which
bacteria will not grow very rapidly. The conditions are far less
favourable than those of ripening cream, and the bacteria do not
grow with anything like the rapidity that they grow in cream.
Indeed, the growth of these organisms during the ripening is
extremely slow compared to the possibilities of bacterial growth
that we have already noticed. Nevertheless, the bacteria do
multiply in the cheese, and as the ripening goes on they become
more and more abundant, although the number fluctuates, rising and
falling under different conditions.
When the attempt is made to determine the relation of the
different kinds of ripening to different kinds of bacteria, it has
thus far met with extremely little success. That different
flavours are due to the ripening produced by different kinds of
bacteria would appear to be almost certain when we remember, as we
have already noticed, the different kinds of decomposition
produced by different species of bacteria. It would seem,
moreover, that it ought not to be very difficult to separate from
the ripened cheese the bacteria which are present, and thus obtain
the kind of bacteria necessary to produce the desired ripening.
But for some reason this does not prove to be so easy in practice
as it seems to be in theory. Many different species of bacteria
have been separated from cheeses. One bacteriologist, studying
several cheeses, separated about eighty different species
therefrom, and others have found perhaps as many more from
different sources. Moreover, experiments have been made with a
considerable number of these different kinds of bacteria to
determine whether they are capable of producing normal ripening.
These experiments consist of making cheese out of milk that has
been deprived of its bacteria, and which has been inoculated with
large quantities of the species in question. Hitherto these
experiments have not been very satisfactory. In some cases the
cheese appears to ripen scarcely at all; in other cases the
ripening occurs, but the resulting cheese is of a peculiar
character, entirely unlike the cheese that it is desired to
imitate. There have been one or two experiments in recent times
that give a little more promise of success than the earlier ones,
for a few species of bacteria have been used in ripening with what
the authors have thought to be promising success. The cheese made
from the milk artificially inoculated with these species ripens in
a satisfactory manner and gives some of the character desired,
though up to the present time in no case has the typical normal
ripening been produced in any of these experiments.
But these experiments have demonstrated beyond question that the
abnormal ripening which is common in cheese factories is due to
the presence of undesirable species of bacteria in the milk. Many
of the experiments in making cheeses by means of artificial
cultures of bacteria have resulted in decidedly abnormal cheeses.
Many of the cheeses thus manufactured have shown imperfections in
ripening which are identical with those actually occurring in the
cheese factory. Several different species of bacteria have been
found which, when artificially used thus for ripening cheese, will
give rise to the porosity and the abnormal swelling of the cheese
already referred to (Fig. 24). Others produced bad tastes and
flavours, and enough has been done in this line to demonstrate
beyond peradventure that the abnormal ripening of cheese is due
primarily to the growth of improper species therein. Quite a long
list of species of bacteria which produce abnormal ripening have
been isolated from cheeses, and have been studied and experimented
with by bacteriologists. As a result of this study of abnormal
ripening, there has been suggested a method of partially
controlling these--remedying them. The method consists simply in
testing the fermenting qualities of the milk used. A small sample
of milk from different dairies is allowed to stand in the cheese
factory by itself until it undergoes its normal souring. If the
fermentation or souring that thus occurs is of a normal character,
the milk is regarded as proper for cheese making. But if the
fermentation that occurs in any particular sample of milk is
unusual; if an extraordinary amount of gas bubbles are produced,
or if unpleasant smells and tastes arise, the sample is regarded
as unfavourable for cheese making, and as likely to produce
abnormal ripening in the cheeses. Milk from this source would
therefore be excluded from the milk that is to be used in cheese
making. This, of course, is a tentative and an unsatisfactory
method of controlling the ripening, and yet it is one of some
practical value to cheese makers. It is the only method that has
yet been suggested of controlling the ripening.
Our bacteriologists, of course, are quite confident that in the
future more practical results will be obtained along this line
than in the past. If it is true that cheeses are ripened by
bacteria; if it is true that different qualities in the cheese are
due to the growth of different species of bacteria during the
ripening, it would seem to be possible to obtain the proper kind
of bacteria and to furnish them to the cheese maker for
artificially inoculating his cheese, just as it has been possible
to furnish artificially cultivated yeasts to the brewer, and as it
has become possible to furnish artificially cultivated bacteria to
the butter maker. We must, however, recognise this to be a matter
for the future. Up to the present time no practical results along
the lines of bacteria have been obtained which our cheese
manufacturers can make use of in the way of controlling with any
accuracy this process of cheese ripening.
Thus it will be seen that in this last dairy product bacteria play
even a more important part than in any of the others. The food
value of cheese is dependent upon the casein which is present. The
market price, however, is controlled entirely by the flavour, and
this flavour is a product of bacterial growth. Upon the action of
bacteria, then, the cheese maker is absolutely dependent; and when
our bacteriologists are able in the future to investigate this
matter further, it seems to be at least possible that they may
obtain some means of enabling the cheese maker to control the
ripening accurately. Not only so, but recognising the great
variety in the flavours of cheese, and recognising that different
kinds of bacteria undoubtedly produce different kinds of
decomposition products, it seems to be at least possible that a
time will come when the cheese maker will be able to produce at--
will any particularly desired flavour in his cheese by the
addition to it of particular species of bacteria, or particular
mixtures of species of bacteria which have been discovered to
produce the desired effects.