BACTERIA AND BUTTER-MAKING.
In making butter from the butter fat in milk, it is necessary to
concentrate the fat globules into cream, preliminary to the churning
process. The cream may be raised by the gravity process or separated
from the milk by centrifugal action. In either case the bacteria that
are normally present in the milk differentiate themselves in varying
numbers in the cream and the skim-milk. The cream always contains per
cc. a great many more than the skim-milk, the reason for this being that
the bacteria are caught and held in the masses of fat globules, which,
on account of their lighter specific gravity, move toward the surface of
the milk or toward the interior of the separator bowl. This filtering
action of the fat globules is similar to what happens in muddy water
upon standing. As the suspended particles fall to the bottom they carry
with them a large number of the organisms that are in the liquid.
~Various creaming methods.~ The creaming method has an important bearing
on the kind as well as the number of the bacteria that are to be found
in the cream. The difference in species is largely determined by the
difference in ripening temperature, while the varying number is governed
more by the age of the milk.
_1. Primitive gravity methods._ In the old shallow-pan process, the
temperature of the milk is relatively high, as the milk is allowed to
cool naturally. This comparatively high temperature favors especially
the development of those forms whose optimum growing-point is near the
air temperature. By this method the cream layer is exposed to the air
for a longer time than with any other, and consequently the
contamination from this source is greater. Usually cream obtained by the
shallow-pan process will contain a larger number of species and also
have a higher acid content.
_2. Modern gravity methods._ In the Cooley process, or any of the modern
gravity methods where cold water or ice is used to lower the
temperature, the conditions do not favor the growth of a large variety
of species. The number of bacteria in the cream will depend largely upon
the manner in which the milk is handled previous to setting. If care is
used in milking, and the milk is kept so as to exclude outside
contamination, the cream will be freer from bacteria than if
carelessness prevails in handling the milk. Only those forms will
develop in abundance that are able to grow at the low temperature at
which the milk is set. Cream raised by this method is less frequently
infected with undesirable forms than that which is creamed at a higher
temperature.
_3. Centrifugal method._ Separator cream should contain less germ-life
than that which is secured in the old way. It should contain only those
forms that have found their way into the milk during and subsequent to
the milking, for the cream is ordinarily separated so soon that there is
but little opportunity of infection, if care is taken in the handling.
As a consequence, the number of species found therein is smaller.
Where milk is separated, it is always prudent to cool the cream so as to
check growth, as the milk is generally heated before separating in order
to skim efficiently.
Although cream is numerically much richer in bacteria than milk, yet
the changes due to bacterial action are slower; hence milk sours more
rapidly than cream. For this same reason, cream will sour sooner when it
remains on the milk than it will if it is separated as soon as possible.
This fact indicates the necessity of early creaming, so as to increase
the keeping quality of the product, and is another argument in favor of
the separator process.
~Ripening of cream.~ If cream is allowed to remain at ordinary
temperatures, it undergoes a series of fermentation changes that are
exceedingly complex in character, the result of which is to produce in
butter made from the same the characteristic flavor and aroma that are
so well known in this article. We are so accustomed to the development
of these flavors in butter that they are not generally recognized as
being intimately associated with bacterial activity unless compared with
butter made from perfectly fresh cream. Sweet-cream butter lacks the
aromatic principle that is prominent in the ripened product, and while
the flavor is delicate, it is relatively unpronounced.
In the primitive method of butter-making, where the butter was made on
the farm, the ripening of cream became a necessity in order that
sufficient material might be accumulated to make a churning. The
ripening change occurred spontaneously without the exercise of any
especial control. With the development of the creamery system came the
necessity of exercising a control of this process, and therefore the
modern butter-maker must understand the principles which are involved in
this series of complex changes that largely give to his product its
commercial value.
In these ripening changes three different factors are to be taken into
consideration: the development of acid, flavor and aroma. Much confusion
in the past has arisen from a failure to discriminate between these
qualities. While all three are produced simultaneously in ordinary
ripening, it does not necessarily follow that they are produced by the
same cause. If the ripening changes are allowed to go too far,
undesirable rather than beneficial decomposition products are produced.
These greatly impair the value of butter, so that it becomes necessary
to know just to what extent this process should be carried.
In cream ripening there is a very marked bacterial growth, the extent of
which is determined mainly by the temperature of the cream. Conn and
Esten[153] find that the number of organisms may vary widely in
unripened cream, but that the germ content of the ripened product is
more uniform. When cream is ready for the churn, it often contains
500,000,000 organisms per cc., and frequently even a higher number. This
represents a germ content that has no parallel in any natural material.
The larger proportion of bacteria in cream as it is found in the
creamery belong to the acid-producing class, but in the process of
ripening, these forms seem to thrive still better, so that when it is
ready for churning the germ content of the cream is practically made up
of this type.
~Effect on churning.~ In fresh cream the fat globules which are suspended
in the milk serum are surrounded by a film of albuminous material which
prevents them from coalescing readily. During the ripening changes, this
enveloping substance is modified, probably by partial solution, so that
the globules cohere when agitated, as in churning. The result is that
ripened cream churns more easily, and as it is possible to cause a
larger number of the smaller fat-globules to cohere to the butter
granules, the yield is slightly larger--a point of considerable
economic importance where large quantities of butter are made.
~Development of acid.~ The result of this enormous bacterial
multiplication is that acid is produced in cream, lactic being the
principal acid so formed.
Other organic acids are undoubtedly formed as well as certain aromatic
products. While the production of acid as a result of fermentative
activity is usually accompanied with a development of flavor, the flavor
is not directly produced by the formation of acid. If cream is treated
in proper proportions with a commercial acid, as hydrochloric,[154] it
assumes the same churning properties as found in normally ripened cream,
but is devoid of the desired aromatic qualities. Lactic acid[155] has
also been used in a similar way but with no better results.
The amount of acidity that should be developed under natural conditions
so as to secure the optimum quality as to flavor and aroma is the most
important question in cream ripening. Concerning this there have been
two somewhat divergent views as to what is best in practice, some
holding that better results were obtained with cream ripened to a high
degree of acidity than where a less amount was developed.[156] The
present tendency seems to be to develop somewhat more than formerly, as
it is thought that this secures more of the "high, quick" flavor wanted
in the market. On the average, cream is ripened to about 0.5 to 0.65 per
cent. acidity, a higher percentage than this giving a strong-flavored
butter. In the determination of acidity, the most convenient method is
to employ the Farrington alkaline tablet, which permits of an accurate
and rapid estimation of the acidity in the ripening cream. The amount of
acidity to be produced must of necessity be governed by the amount of
butter-fat present, for the formation of acid is confined to the serum
of the cream; consequently, a rich cream would show less acid by
titration than a thinner cream, and still contain really as much acid as
the other. The importance of this factor is evident in gathered-cream
factories.
The rate of ripening is dependent upon the conditions that affect the
rate of growth of bacterial life, such as time and temperature, number
of organisms in cream and also the per cent of butter fat in the cream.
Some years ago it was customary to ripen cream at about 50 deg. to 60 deg.
F., but more recently better results have been obtained, it is claimed,
where the ripening temperature is increased and the period of ripening
lessened. As high a temperature as 70 deg. to 75 deg. F. has been
recommended. It should be said that this variation in practice may have
a valid scientific foundation, for the temperature of the ripening cream
is undoubtedly the most potent factor in determining what kind of bacteria
will develop most luxuriantly. It is well known that those forms that
are capable of producing bitter flavors are able to thrive better at a
lower temperature than some of the desirable ripening species.
The importance of this factor would be lessened where a pure culture was
used in pasteurized cream, because here practically the selected
organism alone controls the field.
It is frequently asserted that better results are obtained by stirring
the cream and so exposing it to the air as much as possible. Experiments
made at the Ontario Agricultural College, however, show practically no
difference in the quality of the butter made by these two methods. The
great majority of the bacteria in the cream belong to the facultative
class, and are able to grow under conditions where they are not in
direct contact with the air.
~Flavor and aroma.~ The basis for the peculiar flavor or taste which
ripened cream-butter possesses is due, in large part, to the formation
of certain decomposition products formed by various bacteria. Aroma is a
quality often confounded with flavor, but this is produced by volatile
products only, which appeal to the sense of smell rather than taste.
Generally a good flavor is accompanied by a desirable aroma, but the
origin of the two qualities is not necessarily dependent on the same
organisms. The quality of flavor and aroma in butter is, of course, also
affected by other conditions, as, for instance, the presence or absence
of salt, as well as the inherent qualities of the milk, that are
controlled, to some extent at least, by the character of the feed which
is consumed by the animal. The exact source of these desirable but
evanescent qualities in butter is not yet satisfactorily determined.
According to Storch,[157] flavors are produced by the decomposition of
the milk sugar and the absorption of the volatile flavors by the butter
fat. Conn[158] holds that the nitrogenous elements in cream serve as
food for bacteria, and in the decomposition of which the desired
aromatic substance is produced. The change is unquestionably a complex
one, and cannot be explained as a single fermentation.
There is no longer much doubt but that both acid-forming and
casein-digesting species can take part in the production of proper
flavors as well as desirable aromas. The researches of Conn,[159] who
has studied this question most exhaustively, indicate that both of these
types of decomposition participate in the production of flavor and
aroma. He has shown that both flavor and aroma production are
independent of acid; that many good flavor-producing forms belong to
that class which renders milk alkaline, or do not change the reaction at
all. Some of these species liquefied gelatin and would therefore belong
to the casein-dissolving class. Those species that produced bad flavors
are also included in both fermentative types. Conn has found a number of
organisms that are favorable flavor-producers; in fact they were much
more numerous than desirable aroma-yielding species. None of the
favorable aroma forms according to his investigations were lactic-acid
species,--a view which is also shared by Weigmann.[160]
McDonnell[161] has found that the production of aroma in certain cases
varies at different temperatures, the most pronounced being evolved near
the optimum growing temperature, which, as a general rule, is too high
for cream ripening.
The majority of bacteria in ripening cream do not seem to exert any
marked influence in butter. A considerable number of species are
positively beneficial, inasmuch as they produce a good flavor or aroma.
A more limited number are concerned in the production of undesirable
ripening changes. This condition being true, it may seem strange that
butter is as good as it is, because so frequently the requisite care is
not given to the development of proper ripening. In all probability the
chief reason why this is so is that those bacteria that find milk and
cream pre-eminently suited to their development, e. g. the lactic-acid
class, are either neutral or beneficial in their effect on butter.
~Use of starters.~ Experience has amply demonstrated that it is possible
to control the nature of the fermentative changes that occur in ripening
cream to such an extent as to materially improve the quality of the
butter. This is frequently done by the addition of a "starter." While
starters have been employed for many years for the purpose mentioned, it
is only recently that their nature has been understood. A starter may be
selected from widely divergent sources, but in all cases it is sure to
contain a large number of bacteria, and the presumption is that they are
of such a nature as to produce desirable fermentative changes in the
cream.
In the selection of these so-called natural starters, it follows that
they must be chosen under such conditions as experience has shown to
give favorable results. For this purpose, whole milk from a single
animal is often used where the same is observed to sour with the
production of no gas or other undesirable taint. A skim-milk starter
from a mixed supply is recommended by many. Butter milk is frequently
employed, but in the opinion of butter experts is not as suitable as the
others mentioned.
It not infrequently happens that the practical operator may be misled in
selecting a starter that is not desirable, or by continuing its use
after it has become contaminated.
In 1890[162] a new system of cream ripening was introduced in Denmark by
Storch that possesses the merit of being a truly scientific and at the
same time practical method. This consisted in the use of pure cultures
of specific organisms that were selected on account of their ability to
produce a desirable ripening change in cream. The introduction of these
so-called culture starters has become universal in Denmark, and in parts
of Germany. Their use is also rapidly extending in this country,
Australia and New Zealand.
~Principles of pure-culture cream-ripening.~ In the proper use of pure
cultures for ripening cream, it is necessary first to eliminate as far
as possible the bacteria already present in cream before the culture
starter is added. This result is accomplished by heating the cream to a
temperature sufficiently high to destroy the vegetating organisms. The
addition of a properly selected starter will then give the chosen
organism such an impetus as will generally enable it to gain the
ascendency over any other bacteria and so control the character of the
ripening. The principle employed is quite like that practiced in raising
grain. The farmer prepares his soil by plowing, in this way killing the
weeds. Then he sows his selected grain, which is merely a pure culture,
and by the rapid growth of this, other forms are held in check.
The attempt has been made to use these culture starters in raw sweet
cream, but it can scarcely be expected that the most beneficial results
will be attained in this way. This method has been justified on the
basis of the following experiments. Where cream is pasteurized and no
starter is added, the spore-bearing forms frequently produce undesirable
flavors. These can almost always be controlled if a culture starter is
added, the obnoxious form being repressed by the presence of the added
starter. This condition is interpreted as indicating that the addition
of a starter to cream which already contains developing bacteria will
prevent those originally present in the cream from growing.[163] This
repressive action of one species on another is a well-known
bacteriological fact, but it must be remembered that such an explanation
is only applicable in those cases where the culture organism is better
able to develop than those forms that already exist in the cream.
If the culture organism is added to raw milk or cream which already
contains a flora that is well suited to develop in this medium, it is
quite doubtful whether it would gain the supremacy in the ripening
cream. The above method of adding a culture to raw cream renders
cream-ripening details less burdensome, but at the same time Danish
experience, which is entitled to most credence on this question, is
opposed to this method.
~Reputed advantages of culture starters.~ _1. Flavor and aroma._ Naturally
the flavor produced by pure-culture ferments depends upon the character
of the organism used. Those which are most extensively used are able to
produce a perfectly clean but mild flavor, and a delicate but not
pronounced aroma. The "high, quick" flavor and aroma that is so much
desired in the American market is not readily obtained by the use of
cultures. It is quite problematical whether the use of any single
species will give any more marked aroma than normally occurs in natural
ripening.
_2. Uniformity of product._ Culture starters produce a more uniform
product because the type of fermentation is under more complete control,
and herein is the greatest advantage to be derived from their use. Even
the best butter-maker at times will fail to secure uniform results if
his starter is not perfectly satisfactory.
_3. Keeping quality of product._ Butter made from pasteurized cream to
which a pure-culture starter has been added will keep much better than
the ordinary product, because the diversity of the bacterial flora is
less and the milk is therefore not so likely to contain those organisms
that produce an "off" condition.
_4. Elimination of taints._ Many defective conditions in butter are
attributable to the growth of undesirable bacteria in the cream that
result in the formation of "off" flavors and taints. If cream is
pasteurized, thereby destroying these organisms, then ripened with pure
ferments, it is generally possible to eliminate the abnormal
conditions.[164] Taints may also be present in cream due to direct
absorption from the cow or through exposure to foul odors.[165] Troubles
of this sort may thus be carried over to the butter. This is
particularly true in regions where leeks and wild onions abound, as in
some of the Atlantic States. The heating of the cream tends to expel
these volatile taints, so that a fairly good article of butter can be
made from what would otherwise be a relatively worthless product.
~Characteristics desired in culture starters.~ Certain conditions as the
following are desirable in starters made from pure cultures:
1. Vigorous growth in milk at ordinary ripening temperatures.
2. Ability to form acid so as to facilitate churning and increase the
yield of butter.
3. Able to produce a clean flavor and desirable aroma.
4. Impart a good keeping quality to butter.
5. Not easily modified in its flavor-producing qualities by artificial
cultivation.
These different conditions are difficult to attain, for the reason that
some of them seem to be in part incompatible. Weigmann[166] found that a
good aroma was generally an evanescent property, and therefore opposed
to good keeping quality. Conn has shown that the functions of
acid-formation, flavor and aroma production are not necessarily related,
and therefore the chances of finding a single organism that possesses
all the desirable attributes are not very good.
In all probability no one germ possesses all of these desirable
qualities, but natural ripening is the resultant of the action of
several forms.[167] This idea has led to the attempt at mixing selected
organisms that have been chosen on account of certain favorable
characteristics which they might possess. The difficulty of maintaining
such a composite culture in its correct proportions when it is
propagated in the creamery is seemingly well nigh insuperable, as one
organism is very apt to develop more or less rapidly than the other.
A very satisfactory way in which these cultures are marketed is to mix
the bacterial growth with some sterile, inert, dry substance. This is
the method used in most of the Danish cultures. In this country, some of
the more prominent cultures employed are marketed in a liquid form.
~Culture vs. home-made starters.~ One great advantage which has accrued
from the use of culture or commercial starters has been that in
emphasizing the need of closer control of the ripening process, greater
attention has been paid to the carrying out of the details. In the
hands of the better operators, the differences in flavor of butter made
with a culture or a natural starter are not marked,[168] but in the
hands of those who fail to make a good product under ordinary
conditions, an improvement is often secured where a commercial culture
is used.
~Pasteurization as applied to butter-making.~ This process, as applied to
butter making, is often confounded with the treatment of milk and cream
for direct consumption. It is unfortunate that the same term is used in
connection with the two methods, for they have but little in common
except in the use of heat to destroy the germ life of the milk. In
pasteurizing cream for butter-making, it is not necessary to observe the
stringent precautions that are to be noted in the preservation of milk;
for the addition of a rapidly developing starter controls at once the
fermentative changes that subsequently occur. Then again, the physical
requirement as to the production of a cooked taste is not so stringent
in butter-making. While a cooked taste is imparted to milk or even cream
at about 158 deg. F., it is possible to make butter that shows no
permanent cooked taste from cream that has been raised as high as 185
deg. or even 195 deg. F. This is due to the fact that the fat does not
readily take up those substances that give to scalded milk its peculiar
flavor.
Unless care is taken in the manipulation of the heated cream, the grain
or body of the butter may be injured. This tendency can be overcome if
the ripened cream is chilled to 48 deg. F. for about two hours before
churning. It is also essential that the heated cream should be quickly
and thoroughly chilled after being pasteurized.
The Danes, who were the first to employ pasteurization in butter-making,
used, in the beginning, a temperature ranging from 158 deg. to 167 deg.
F., but owing to the prevalence of such diseases as tuberculosis and
foot-and-mouth disease, it became necessary to treat all of the skim
milk that was returned from the creameries. For this purpose the skim
milk is heated to a temperature of 176 deg. F., it having been more
recently determined that this degree of heat is sufficient to destroy the
seeds of disease. With the use of this higher temperature the capacity of
the pasteurizing apparatus is considerably reduced, but the higher
temperature is rendered necessary by the prevailing conditions as to
disease.
When the system was first introduced in Denmark, two methods of
procedure were followed: the whole milk was heated to a sufficiently
high temperature to thoroughly pasteurize it before it was separated, or
it was separated first, and the cream pasteurized afterwards. In the
latter case, it is necessary to heat the skim milk after separation to
destroy the disease organisms, but this can be quickly done by the use
of steam directly. Much more care must be used in heating the cream in
order to prevent injury to the grain of the butter. In spite of the
extra trouble of heating the cream and skim milk separately, this method
has practically supplanted the single heating. With the continual spread
of tuberculosis in America the heating of skim milk separately is
beginning to be introduced.[169]
~Use of starters in pasteurized and unpasteurized cream.~ In order to
secure the beneficial results presumably attributable to the use of a
starter, natural as well as a pure culture, it should be employed in
cream in which the bacteria have first been killed out by
pasteurization. This is certainly the most logical and scientific method
and is the way in which the process has been developed in Denmark.
Here in this country, the use of pure cultures has been quite rapidly
extended, but the system of heating the cream has been used in only a
slight measure. The increased labor and expense incurred in pasteurizing
the cream has naturally militated somewhat against the wide-spread use
of the process, but doubtless the main factor has been the inability to
secure as high a flavor where the cream was heated as in the unheated
product. As the demands of the market change from a high, quick flavor
to one that is somewhat milder but of better keeping quality, doubtless
pasteurization of the cream will become more and more popular. That such
a change is gradually occurring is already evident, although as yet only
a small proportion of butter made in this country is now made in this
way. Where the cream is unheated, a considerable number of species will
be found, and even the addition of a pure culture, if that culture is of
the lactic acid-producing species, will to some extent control the type
of fermentation that occurs. Such would not be the case with a culture
composed of the casein-digesting type of bacteria. Only those forms
could thus be used which are especially well suited to development in
raw cream. For this reason the pure culture ferments that are generally
employed in creamery practice are organisms of the lactic acid type,
able to grow rapidly in cream and produce a pure cream flavor in the
butter.
~Purity of commercial starters.~ Naturally the butter maker is forced to
rely on the laboratory for his commercial starter, and the question will
often arise as to the purity and vigor of the various ferments employed.
As there is no way for the factory operator to ascertain the actual
condition of the starter, except by using the same, the greatest care
should be taken by the manufacturer to insure the absolute purity of the
seed used.
A bacteriological examination of the various cultures which have been
placed on the market not infrequently reveals an impure condition. In
several cases the writer has found a not inconsiderable number of
liquefying bacteria mixed with the selected organism. Molds not
infrequently are found in cultures put up in the dry form. Doubtless the
effect of these accidental contaminations is considerably less in the
case of a starter composed of a distinctively lactic acid-producing
organism than with a form which is less capable of thriving vigorously
in milk, and it should be said that these impurities can frequently be
eliminated by continued propagation.
The virility and vigor of the starter is also a fluctuating factor,
dependent in part at least, upon the conditions under which the organism
is grown. In some cases the germ is cultivated in solutions in which
acid cannot be formed in abundance. Where the conditions permit of the
formation of acid, as would be the case if sugar was present with a
lactic acid-producing species, the vitality of the culture is often
impaired by the action of the gradually accumulating acid. Some
manufacturers attempt to minimize this deleterious condition by adding
carbonate of lime which unites with the acid that is formed.
~Propagation of starters for cream-ripening.~ The preparation and
propagation of a starter for cream-ripening is a process involving
considerable bacteriological knowledge, whether the starter is of
domestic origin or prepared from a pure-culture ferment. In any event,
it is necessary that the starter should be handled in a way so as to
prevent the introduction of foreign bacteria as far as possible. It
should be remembered at all times that the starter is a live thing and
must be handled throughout its entire history in a way so as to retain
its vitality and vigor unimpaired. The following points should be taken
into consideration in growing the starter and transferring it from day
to day:
1. If a commercial starter is used, see that it is fresh and that the
seal has not been broken. If the culture is too old, the larger part of
the organisms may have died out before it is transferred, in which case
the effect of its addition to the sterilized milk would be of little
value.
When the commercial ferment is received, it should be stored in the
refrigerator pending its use so as to retard as much as possible the
changes that naturally go on in the culture liquid. Be careful that the
bottle is not exposed to the influence of direct sunlight for in a
transparent medium the organisms may be readily killed by the
disinfecting action of the sun's rays.
2. If a home-made starter is employed, use the greatest possible care in
selecting the milk that is to be used as a basis for the starter.
3. For the propagation and perpetuation of the starter from day to day,
it is necessary that the same should be grown in milk that is as
germ-free as it is possible to secure it. For this purpose sterilize
some fresh skim-milk in a covered can that has previously been well
steamed. This can be done easily by setting cans containing skim-milk in
a vat filled with water and heating the same to 180 deg. F. or above for
one-half hour or more. Steam should not be introduced directly. This
process destroys all but a few of the most resistant spore-bearing
organisms. This will give a cooked flavor to the milk, but will not
affect the cream to which the starter is added. Dairy supply houses are
now introducing the use of starter cans that are specially made for this
purpose.
4. After the heated milk is cooled down to about 70 deg. or 80 deg. F.,
it can be inoculated with the desired culture. Sometimes it is desirable
to "build up" the starter by propagating it first in a smaller volume of
milk, and then after this has developed, adding it to a larger amount.
This method is of particular value where a large amount of starter is
needed for the cream-ripening.
5. After the milk has been inoculated, it should be kept at a
temperature that is suitable for the rapid development of the contained
bacteria, 65 deg.-75 deg. F., which temperature should be kept as uniform
as possible.
This can best be done by setting the covered can in a vat filled with
warm water. The starter cans are often arranged so that temperature can
be controlled by circulating water.
6. The starter should not be too thoroughly curdled when it is needed
for use, but should be well soured and only partially curdled for it is
difficult to break up thoroughly the curd particles if the starter is
completely curdled. If these curd masses are added to ripening cream,
white specks may appear in the butter.
7. The vigor of the starter is in all probability stronger when the milk
is on the point of curdling than it is after the curd has been formed
some time. The continued formation of lactic acid kills many of the
bacteria and thus weakens the fermentative action. It is therefore
highly important that the acidity of the starter should be closely
watched.
8. Do not refrigerate the starter when it has reached the proper stage
of development, as this retards the bacterial growth in the same manner
as cold weather checks the growth of grain. It is preferable to dilute
the starter, if it cannot be used when ready, with sufficient freshly
sterilized sweet milk to hold the acidity at the proper point and thus
keep the bacteria in the starter in a condition which will favor
vigorous growth.
9. The starter should be propagated from day to day by adding a small
quantity to a new lot of freshly prepared milk. For this purpose two
propagating cans should be provided so that one starter may be in use
while the other is being prepared.
~How long should a starter be propagated?~ No hard-and-fast rule can be
given for this, for it depends largely upon how carefully the starter is
handled during its propagation. If the starter is grown in sterilized
milk kept in steamed vessels and is handled with sterile dippers, it is
possible to maintain it in a state of relative purity for a considerable
period of time; if, however, no especial care is given, it will soon
become infected by the air, and the retention of its purity will depend
more upon the ability of the contained organism to choke out foreign
growths than upon any other factor. Experience seems to indicate that
pure-culture starters "run out" sooner than domestic starters. While it
is possible, by bacteriological methods, to determine with accuracy the
actual condition of a starter as to its germ content, still such methods
are inapplicable in creamery practice. Here the maker must rely largely
upon the general appearance of the starter as determined by taste and
smell. The supply houses that deal in cultures of this class generally
expect to supply a new culture at least every month.
~Bacteria in butter.~ As ripened cream is necessarily rich in bacteria, it
follows that butter will also contain germ life in varying amounts, but
as butter-fat is not well adapted for bacterial food, the number of
germs in butter is usually less than in ripened cream.
Sweet-cream butter is naturally poorer in germ life than that made from
ripened cream. Grotenfelt reports in sweet-cream butter, the so-called
"Paris butter," only a few bacteria while in acid cream butter the germ
content runs from scores to hundreds of thousands.
~Effect of bacteria in wash water.~ An important factor in contamination
may be the wash water that is used. Much carelessness often prevails
regarding the location and drainage of the creamery well, and if same
becomes polluted with organic matter, bacterial growth goes on apace.
Melick[170] has made some interesting studies on using pasteurized and
sterilized well waters for washing. He found a direct relation to exist
between the bacterial content of the wash water and the keeping quality
of the butter. Some creameries have tried filtered water but under
ordinary conditions a filter, unless it is tended to with great
regularity, becomes a source of infection rather than otherwise.
~Changes in germ content.~ The bacteria that are incorporated with the
butter as it first "comes" undergo a slight increase for the first few
days. The duration of this period of increase is dependent largely upon
the condition of the butter. If the buttermilk is well worked out of the
butter, the increase is slight and lasts for a few days only, while the
presence of so nutritious a medium as buttermilk affords conditions much
more favorable for the continued growth of the organisms.
While there may be many varieties in butter when it is fresh, they are
very soon reduced in kind as well as number. The lactic acid group of
organisms disappear quite rapidly; the spore-bearing species remaining
for a somewhat longer time. Butter examined after it is several months
old is often found to be almost free from germs.
In the manufacture of butter there is much that is dependent upon the
mechanical processes of churning, washing, salting and working the
product. These processes do not involve any bacteriological principles
other than those that are incident to cleanliness. The cream, if ripened
properly, will contain such enormous numbers of favorable forms that the
access of the few organisms that are derived from the churn, the air, or
the water in washing will have little effect, unless the conditions are
abnormal.
BACTERIAL DEFECTS IN BUTTER.
~Rancid change in butter.~ Fresh butter has a peculiar aroma that is very
desirable and one that enhances the market price, if it can be retained;
but this delicate flavor is more or less evanescent, soon disappearing,
even in the best makes. While a good butter loses with age some of the
peculiar aroma that it possesses when first made, yet a gilt-edged
product should retain its good keeping qualities for some length of
time. All butters, however, sooner or later undergo a change that
renders them worthless for table use. This change is usually a rancidity
that is observed in all stale products of this class. The cause of this
rancid condition in butter was at first attributed to the formation of
butyric acid, but it is now recognized that other changes also enter
in.[171] Light and especially air also exert a marked effect on the
flavor of butter. Where butter is kept in small packages it is much more
prone to develop off flavors than when packed in large tubs. From the
carefully executed experiments of Jensen it appears that some of the
molds as well as certain species of bacteria are able to incite these
changes. These organisms are common in the air and water and it
therefore readily follows that inoculation occurs.
Practically, rancidity is held in check by storing butter at low
temperatures where germ growth is quite suspended.
~Lack of flavor.~ Often this may be due to improper handling of the cream
in not allowing it to ripen far enough, but sometimes it is impossible
to produce a high flavor. The lack of flavor in this case is due to the
absence of the proper flavor-producing organisms. This condition can
usually be overcome by the addition of a proper starter.
~Putrid butter.~ This specific butter trouble has been observed in
Denmark, where it has been studied by Jensen.[172] Butter affected by it
rapidly acquires a peculiar putrid odor that ruins it for table use.
Sometimes, this flavor may be developed in the cream previous to
churning.
Jensen found the trouble to be due to several different putrefactive
bacteria. One form which he called _Bacillus foetidus lactis_, a close
ally of the common feces bacillus, produced this rotten odor and taste
in milk in a very short time. Fortunately, this organism was easily
killed by a comparatively low heat, so that pasteurization of the cream
and use of a culture starter quickly eliminated the trouble, where it
was tried.
~Turnip-flavored butter.~ Butter sometimes acquires a peculiar flavor
recalling the order of turnips, rutabagas, and other root crops. Often
this trouble is due to feeding, there being in several of these crops,
aromatic substances that pass directly into the milk, but in some
instances the trouble arises from bacteria that are able to produce
decomposition products,[173] the odor and taste of which strongly
recalls these vegetables.
~"Cowy" butter.~ Frequently there is to be noted in milk a peculiar odor
that resembles that of the cow stable. Usually this defect in milk has
been ascribed to the absorption of impure gases by the milk as it cools,
although the gases and odors naturally present in fresh milk have this
peculiar property that is demonstrable by certain methods of aeration.
Occasionally it is transmitted to butter, and recently Pammel[174] has
isolated from butter a bacillus that produced in milk the same peculiar
odor so commonly present in stables.
~Lardy and tallowy butter.~ The presence of this unpleasant taste in
butter may be due to a variety of causes. In some instances, improper
food seems to be the source of the trouble; then again, butter exposed
to direct sunlight bleaches in color and develops a lardy flavor.[175]
In addition to these, cases have been found in which the defect has been
traced to the action of bacteria. Storch[176] has described a
lactic-acid form in a sample of tallowy butter that was able to produce
this disagreeable odor.
~Oily butter.~ Jensen has isolated one of the causes of the dreaded oily
butter that is reported quite frequently in Denmark. The specific
organism that he found belongs to the sour-milk bacteria. In twenty-four
hours it curdles milk, the curd being solid like that of ordinary sour
milk. There is produced, however, in addition to this, an unpleasant
odor and taste resembling that of machine oil, a peculiarity that is
transmitted directly to butter made from affected cream.
~Bitter butter.~ Now and then butter develops a bitter taste that may be
due to a variety of different bacterial forms. In most cases, the bitter
flavor in the butter is derived primarily from the bacteria present in
the cream or milk. Several of the fermentations of this character in
milk are also to be found in butter. In addition to these defects
produced by a biological cause, bitter flavors in butter are sometimes
produced by the milk being impregnated with volatile, bitter substances
derived from weeds.
~Moldy butter.~ This defect is perhaps the most serious because most
common. It is produced by the development of a number of different
varieties of molds. The trouble appears most frequently in packed butter
on the outside of the mass of butter in contact with the tub. Mold
spores are so widely disseminated that if proper conditions are given
for their germination, they are almost sure to develop. In some cases
the mold is due to the growth of the ordinary bread mold, _Penicillium
glaucum_; in other cases a black mold develops, due often to
_Cladosporium butyri_. Not infrequently trouble of this character is
associated with the use of parchment wrappers. The difficulty can easily
be held in check by soaking the parchment linings and the tubs in a
strong brine, or paraffining the inside of the tub.
~Fishy butter.~ Considerable trouble has been experienced in Australian
butter exported to Europe in which a fishy flavor developed. It was
noted that the production of this defect seemed to be dependent upon the
storage temperature at which the butter was kept. When the butter was
refrigerated at 15 deg. F. no further difficulty was experienced. It is
claimed that the cause of this condition is due to the formation of
trimethylamine (herring brine odor) due to the growth of the mold fungus
_Oidium lactis_, developing in combination with the lactic-acid
bacteria.
A fishy taste is sometimes noted in canned butter. Rogers[177] has
determined that this flavor is caused by yeasts (_Torula_) which produce
fat-splitting enzyms capable of producing this undesirable change.
FOOTNOTES:
[153] Conn and Esten, Cent. f. Bakt., II Abt., 1901, 7:746.
[154] Tiemann, Milch Zeit., 23:701.
[155] Milch Zeit., 1889, p. 7; 1894, p. 624; 1895, p. 383.
[156] Dean, Ont. Agr. Coll., 1897, p. 66.
[157] Storch, Nogle, Unders. over Floed. Syrning, 1890.
[158] Conn, 6 Storrs Expt. Stat., 1893, p. 66.
[159] Conn, 9 Storrs Expt. Stat., 1896, p. 17.
[160] Weigmann, Milch Zeit., 1891, p. 793
[161] McDonnell, ue. Milchsaeure Bakterien (Diss. Kiel, 1899), p. 43.
[162] Storch, Milch Zeit., 1890, p. 304.
[163] Conn, 9 Storrs Expt. Stat., 1896, p. 25.
[164] Milch Zeit., 1891, p. 122; 1894, p. 284; 1895, p. 56; 1896, p.
163.
[165] McKay, Bull. 32, Iowa Expt. Stat., p. 47
[166] Weigmann, Landw. Woch. f. Schl. Hol., No. 2, 1890.
[167] Weigmann, Cent. f. Bakt., II Abt., 3:497, 1897.
[168] At the National Creamery Buttermakers' Association for 1901, 193
out of 240 exhibitors used starters. Of those that employed starters,
nearly one-half used commercial cultures. There was practically no
difference in the average score of the two classes of starters, but
those using starters ranked nearly two points higher in flavor than
those that did not.
[169] Russell, Bull. 143, Wis. Expt. Stat., Feb. 1907.
[170] Melick, Bull. 138, Kansas Expt. Stat., June 1906.
[171] Reinmann, Cent. f. Bakt., 1900, 6:131; Jensen, Landw. Jahr. d.
Schweiz, 1901.
[172] Jensen, Cent. f. Bakt., 1891, 11:409.
[173] Jensen, Milch Zeit., 1892, 6, Nos. 5 and 6.
[174] Pammel, Bull. 21, Iowa Expt. Stat., p. 803.
[175] Fischer, Hyg. Rund., 5:573.
[176] Storch, 18 Rept. Danish Agric. Expt. Stat., 1890.
[177] Rogers Bull. 57, B. A. I. U. S. Dept Agric., 1904.