BACTERIA AND CHEESE MAKING.
Butter, such as that of the sweet-cream type that is highly esteemed
in many parts of the world, may be made without the aid of bacteria,
but no important kind of cheese can be made under commercial
conditions without them.
=Types of cheese.= Cheese consists of the fat and the precipitated
casein of milk, together with a large amount of water and the salts
found in milk. The numerous types of cheese may be divided into two
groups, depending on the manner in which the curdling of the milk is
brought about. Sour-milk cheese is made from curd, formed as a
result of the acid fermentation of the milk. Thus, at the very first
stage in the making of this type, the importance of bacteria is
apparent.
The second type is that made from curd, which is precipitated by the
addition of rennet to the milk. This type may also be divided into
two groups, depending upon their texture; the hard cheese, and the
soft cheese. The ordinary cheddar, the common American type, is the
most important example of the hard cheese; Limburger, of the soft
cheese. Cheese are designated as hard or soft, depending upon the
amount of whey that is retained in them during the making process.
The moisture content has an important influence on the type and
amount of life that develops on and in the curd mass, and as will be
seen, the ripening and flavor of the cheese are dependent upon these
biological factors.
The two groups of hard and soft cheese have no sharply defined
limits, but merge into each other. The extreme types of the hard
cheese are so dry and firm that they can be cut only with
difficulty. Such cheese are used primarily as condiments to impart a
flavor to certain dishes, as macaroni, and for this purpose are
grated. The extreme type of soft cheese is a soft, pasty mass and
can be easily spread with a knife.
Hard cheese, because the ripening process goes on uniformly
throughout the entire mass of cheese, may be made of any size which
permits of commercial handling. They can also be kept for long
periods and preserve their good qualities. Soft cheese are made in
small sizes, since on account of their consistency, they could not
otherwise be handled, and also because of the manner of ripening.
The ripening is due to the action of organisms developing on the
surface, the by-products of which diffuse into the curd. If the
cheese are too large, the outer layers become overripe, while the
interior remains more or less unchanged, or insufficiently changed.
Soft cheese mature much more rapidly than hard cheese; consequently
they are short lived.
Although made from the same substance, milk, it is noteworthy that
there are over four hundred varieties of cheese produced. Most of
these find only a local market where made. Less than a dozen
varieties are to be regarded as general articles of commerce.
=Quality of milk.= In the making of butter there are a number of
processes that the maker can use when he finds himself obliged to
utilize poor milk. The milk can be pasteurized and the harmful
bacteria thus destroyed; desirable kinds can then be added in the
form of a pure-culture starter. Pasteurization also drives off some
of the volatile by-products of the first acid fermentation. By the
use of these means, the maker can prepare a very good product from
poor material.
In the making of most kinds of cheese, especially those of the
greatest commercial importance, the cheese maker can call to his
help no such aids, but must use the milk as it is brought to him. It
is possible to prepare certain kinds of soft cheese from pasteurized
milk that differ in no essential point from the same cheese made
from raw milk. Hard cheese are also made from pasteurized milk, but
in most cases such cheese differ, especially in the degree of
flavor, from that made from unheated milk. It is quite probable
that, as the factors concerned in the ripening of cheese become
better known, methods will be evolved for the successful production
of many kinds of cheese from pasteurized milk.
It has been shown that the quality of milk is almost wholly
dependent upon the number and kinds of bacteria it contains. These
bacteria pass into the cheese, and there produce the same products
as they would have done in the milk itself. In butter making,
practically all processes are under the control of the maker, until
the product is ready for the market; but cheese, on the other hand,
passes through a complicated series of changes after it has left the
maker's control. During the manipulation of the milk and the curd in
the vat, he can exert some influence on the quality of the product,
but he is much more dependent on the quality of the milk than is the
case in butter making.
Every effort should therefore be made to furnish to the cheese maker
the quality of milk from which he can prepare fine cheese. In other
words, the milk should be produced under clean conditions and
carefully cooled and handled until delivered to the maker. Poor milk
from a single farm may have such an effect upon the cheese made from
the milk of twenty farms as to depreciate the selling value of the
entire product several cents per pound.
The tests that have been previously described (p. 105) have been
devised especially for testing the quality of the milk for cheese
making purposes, and are of the greatest service to the maker in
tracing the source of poor milk.
=Cheddar cheese.= The first step in the making of cheddar cheese is
the "ripening" of the milk, or the development of a small amount of
acid. In this fermentation, the development of acid is preceded by
an enormous increase in the number of acid-forming bacteria. Milk
for cheese making should show an acidity of about 0.2 per cent or
slightly more than in fresh milk. In other words, the maker wishes
the milk to be in such condition, bacteriologically, that if kept at
a temperature favorable for the growth of the acid-forming bacteria,
the acidity will increase rapidly.
The curdling of the milk to precipitate the cheese solids is
produced by the addition of rennet, which is obtained by extracting
the fourth stomach of the young calf with a solution of common salt.
In the past the maker prepared his own rennet solution from the
dried stomachs ("rennets"), but at present, the extract is prepared
commercially, in a much more uniform manner. The rapidity of the
curdling is dependent upon the acidity of the milk. In order to
secure proper rennet action, a slight increase of acid over that
found in fresh milk is usually necessary; thus at the very beginning
of the process of making cheddar cheese, the bacteria are of
importance.
As the milk curdles, the bacteria are enclosed in the curd as are
the fat globules. The curd is cut into small fragments by means of
a curd knife, and as the mass is warmed, the acid develops, causing
the curd particles to shrink, thus expressing the whey. Within a
short time, the volume of the curd is not more than one-eighth that
of the milk, but in the curd are held over 75 per cent of the
bacteria of the milk. To secure rapid curdling in the vat, the milk
is warmed to 85 deg. to 90 deg. F., a temperature that is most favorable
for the growth of the lactic bacteria. Since there is a large number of
bacteria concentrated in a small volume, and the temperature, as
well as all other conditions, is favorable to growth, multiplication
of the bacteria goes on rapidly, and as a consequence, acid is
formed in large amounts, as is shown by the following figures given
by Publow for the manufacture of the export type of cheddar cheese:
Acidity of milk before adding rennet .2 to .21 per cent
Acidity of whey before heating curd .14 to .145 "
Acidity of whey before removing from curd .16 to .18 "
Acidity of whey coming from the curd after
removal of whey and curd is packed .24 to .30 "
Acidity of whey coming from curd before milling .65 to .75 "
Acidity of whey coming from curd before salting .90 to 1.10 "
If the milk had been kept at the same temperature as the curd, the
acidity would have increased much more slowly since the acid would
have been distributed through a larger volume. In the cheese curd
the same amount of acid is probably formed, as would have been
produced in the total amount of milk during the same interval.
The acid produced by this bacterial activity has a most marked
effect on the curd. At first the curd masses are tough and firm,
the particles showing no tendency to adhere to each other. As the
acid increases in amount, the curd becomes plastic, the outer
surface of the particles adhering or "matting," as the maker
expresses it. The result is a solid coalescent mass of curd, which
is cut into small pieces, _i.e._, "milled," before it is put to
press. The acid allows the blending of the pieces under the
influence of the pressure so that a cheese is one single mass. Under
certain abnormal conditions, the development of acid may be
interfered with and the particles of curd fail to mat, in which
case, the cheese will be crumbly when it is cut. The determination
of the proper time for pressing is made by the application of what
is known as the hot iron test. This is made by determining the
length of the "strings" or "threads" which can be drawn from a mass
of curd when it is brought in contact with a hot iron at a cherry
red heat, the length of the curd threads being a measure of the
amount of acid that has been formed in the curd.
The rate of acid formation within the curd particles is also
measured by determining the acidity of the whey as it comes from the
curd at different stages in the making. This test, which is often
used in place of the "hot iron" test is carried out in the same
manner, as in determining the acidity of milk or cream. The quality
of the cheese, both as to texture and flavor, is dependent to a
great degree upon the amount of acid that is formed during the
various stages in making; hence, the successful maker must follow
closely by some means the acid formation in the curd until it is put
to press.
It is very necessary that the milk shall contain a sufficient number
of acid-forming bacteria to produce the required amount of acid. If
a sufficient number of bacteria are not present in the milk as it is
received, as is the case with very sweet milk, they must be added
by the maker in the form of a starter, or the process of making will
be much prolonged.
[Illustration: Fig. 35.--Bacteria in Cheese.
A photomicrograph of curd just after curdling has taken place. Note
the few lactic acid bacteria embedded in the curd.]
=Starters in cheese making.= The starters used in cheese making, are
identical with those employed in butter making and the same
precautions should be observed in their propagation. It is important
that the starters should not be such as to form a hard curd that
cannot be mixed uniformly with the milk, since the curd particles
would appear as white specks in the cheese. The starter should be
added to the milk through a hair sieve, and well mixed with the
milk, so as to distribute the bacteria uniformly. Amounts varying
from 0.5 to 2 per cent are used. In butter making, it is essential
that the bacteria of the starter be able to form not only acid, but
sufficient flavor-forming substances to impart to the butter a
desirable flavor. In cheese making it is not probable that this
latter characteristic is of any particular importance.
[Illustration: Fig. 36.--Bacteria in Cheese.
A photomicrograph of curd at the time the salt is added. The lactic
acid bacteria have increased materially in numbers.]
It is desirable that the process of cheese making shall conform as
closely as possible to that which experience has shown to give the
best results. The rate at which acid is developed in the curd and
the rapidity with which the whey is expelled therefrom should bear a
certain ratio to each other. If the milk has too high a degree of
acidity, _i.e._, is overripe, the acidity developed in the curd will
be too high before the curd is sufficiently firm; with a very sweet
milk, the reverse may be true. It is desirable for the cheesemaker
to obtain as good an idea as possible of the condition of the milk
with reference to its bacterial content, since this will determine
the rate at which acid will be formed in the curd. If the milk is
too sweet, _i.e._, too low in acid-forming bacteria, a starter
should be added. The only methods by which this information can be
obtained by the maker is by determining the acidity by the usual
method or better by the use of the rennet test by which is
ascertained the time required for a given amount of rennet to curdle
a definite quantity of milk at a standard temperature. The varying
factor in the test will be the acidity of the milk. Very slight
differences influence profoundly the time of curdling. If, working
under standard conditions, it is found that the time of curdling of
one sample is 10 seconds and of another sample, 20 seconds, it is
proof that the acidity of the first is higher than that of the
second, that its bacterial content is greater and that acidity will
develop in the curd more rapidly. The first may need a small amount
of starter, the second a larger quantity. Working with milk from the
same source, the maker, from his experience, will know how much
starter should be added to milk that has given a certain result with
the rennet test in order that the acid shall be developed in the
curd at a desired rate.
=Ripening of cheese.= The curd at the time it is put to press is tough
and rubbery, and has none of the characteristic flavor of cheddar
cheese; it is also quite insoluble and indigestible. Before the
cheese is fit to eat it must pass through a complex series of
changes which are collectively known as _ripening_. In these changes
there is not only a breaking down of the casein into soluble
compounds, which process makes the cheese soft and plastic under
pressure, but the characteristic flavor is developed in greater or
less degree. A very considerable part of the cheese thus becomes
soluble in water, and it is much more easily digested than in an
unripened condition.
The different factors that are operative in the ripening changes are
not yet fully known, but in recent years as a result of scientific
study, material progress in the study of the changes has been made.
=Rennet.= The commercial rennet extract when in condition for use
contains very few bacteria. A preservative, boric acid, is added by
the manufacturer to restrain the bacteria, otherwise the extract
would soon be unfit for use. The bacteria in the commercial rennet
extract are too few to be of any importance whatever in the ripening
process.
Rennet extract contains an enzyme, rennin, that causes the milk to
curdle; also another enzyme, pepsin, that exerts a digestive action
on the curdled casein. Pepsin is always found in the stomach juices
of all animals, but no digestive action takes place, unless the
reaction is distinctly acid, as is the ease under normal conditions,
since hydrochloric acid is excreted by the walls of the stomach.
Outside of the stomach, the same conditions must obtain with
reference to the presence of acid, if pepsin is to exert a digestive
effect. In the cheese curd, the milk sugar is rapidly changed into
lactic acid by the action of the bacteria. This gives the proper
chemical reaction for peptic action, and the enzyme is then able to
act on the paracasein, the nitrogenous part of the cheese. If milk
contains no acid-forming bacteria, conditions will not permit of
peptic action, and as a consequence, the ripening processes do not
take place. If the sugar is fermented by some organism that does
not form acid, as the lactose-fermenting yeasts, the cheese does not
ripen. The lactic bacteria are therefore an essential factor in
inaugurating the ripening changes in all types of rennet cheese.
=Preservative action of acid.= In a previous chapter it was shown that
raw milk does not undergo putrefaction because of the restraining
effect of the acid formed by the lactic bacteria on the putrefactive
organisms. This same phenomenon is noted in cheese. Milk always
contains putrefactive bacteria which pass into the cheese, but they
cannot grow therein because of the high acidity. In the absence of
the acid-forming organisms in the cheese, the cheese may remain
tough and rubbery, on account of the lack of suitable conditions for
the action of the pepsin of the rennet extract, or when the milk
contains large numbers of digesting organisms, the cheese may
develop a putrefactive condition, as noted by the offensive odor and
soft pasty texture.
=Other factors concerned in cheese ripening.= There are other factors
that are also concerned in the complex series of ripening changes
noted in cheddar cheese. All animal fluids and tissues, if kept
under perfectly sterile conditions at ordinary temperatures, will
undergo a certain amount of decomposition, due apparently to their
content in enzymes that have a digestive action. Meat kept in
storage becomes more tender due to the softening of the connective
tissue. Milk, derived as it is from actively secreting cell tissue,
gives certain reactions that are common to living material. If
chloroform, which restrains the action of bacteria, but does not
prevent the activity of enzymes, is added to it, it will curdle in
the course of a few weeks and will become partially digested. This
digesting ferment found in milk is known as _galactase_. Compounds
are formed in milk thus preserved that are similar to those found in
a ripe cheddar cheese. Many experiments have been made with
pasteurized milk, but it has not been possible to produce typical,
normal cheese from thoroughly pasteurized milk. Such cheese are
markedly deficient in the typical flavor of cheddar cheese. From
this fact it is believed that the inherent enzymes of milk are a
factor of some importance in the ripening of this type of cheese at
least, if not of all types.
In the past, other factors have been thought to be of importance.
Duclaux, a French bacteriologist, considered that the enzymes formed
by the digesting bacteria are responsible for the ripening. It is
now known that they can have but little if any part in the process,
since they are not present in all cheese in sufficient numbers to
have any marked effect, and since the acidity of the cheese mass
will not permit of their development.
Other types of bacteria have been considered by bacteriologists to
be of importance in the ripening process, but it is certain that the
purely digestive change in the mass of the cheese can be accounted
for through the action of the factors already noted.
=Flavor production.= The flavor of any type of cheese is the most
important characteristic, just as it is in butter, for it is largely
the flavor that determines the selling value of the product, and is
the most difficult thing to control. It has been thought that the
flavor-producing substances were derived from the paracasein of the
curd and were produced by the factors that are concerned in the
digestion of the paracasein. It has been shown that a cheese may be
thoroughly ripened as far as its physical properties are concerned;
that it may contain the end products of casein digestion, and yet
be low in flavor. From recent researches it seems probable that the
production of flavor is connected with the change that the sugar
undergoes in the acid fermentation, as volatile acids, acetic,
formic, etc., as well as alcohols and esters are formed in
increasing amounts as the ripening progresses. These may have come
from the decomposition of the milk sugar, or from a secondary change
in the products of the lactic fermentation. There are organisms in
both milk and cheese that do not grow on the ordinary culture media
used by the bacteriologist, and it may well be that some of these
are of importance in flavor production. Their destruction in
pasteurization is likely to be one of the reasons for the failure of
cheese made from pasteurized milk to develop typical flavor.
=Effect of temperature on ripening.= The temperature at which the
ripening cheese is kept has been found to be of the greatest
importance in determining the quality of the product. If the cheese
is kept at high temperatures, the ripening proceeds rapidly; the
cheese is short lived, and has a sharp, strong flavor, and generally
a more or less open texture. Unless the cheese is made from the best
quality of milk, it is likely to undergo undesirable fermentations
when ripened at high temperatures.
Within recent years it has been found possible to ripen cheese at
temperatures that were previously thought to be certain to spoil the
product. Much of the cheese is now ripened at temperatures below 50 deg.
F. The ripening goes on more slowly than at higher temperatures, but
the flavor of the cheese is clean and entirely devoid of the sharp
undesirable tang that is so frequently noted in old cheese, and the
texture is solid and meaty. Ripening at low temperatures, when the
milk is not of the best quality, is certain to result in a much
better product than when higher temperatures are employed.
=Abnormal fermentations in cheese.= As has been previously shown, it
is necessary to have an abundant supply of acid-forming bacteria in
the milk from which cheese is to be made. If these bacteria are
supplanted by other kinds, the product will be more or less abnormal
either in texture or in flavor, or possibly in both. Many of these
abnormal fermentations have been studied and the organisms concerned
in the changes found.
If the milk is handled carelessly, it will contain many bacteria
able to form acid and gas. As noted previously, these organisms form
products in milk that have an offensive odor and a disagreeable
taste. In cheese the gases cause the formation of holes, more or
less numerous, depending on the number of the gas-forming bacteria
in the milk. Where these bacteria are abundant, gas may appear while
the curd is in the vat, causing it to float in the whey, when it is
known as a "floater." Again, the gas may not become evident until
the cheese is in the press or on the curing shelf, when it becomes
apparent by the swelling or bulging of the cheese. Such cheese is
termed "huffed" or "swelled." The internal pressure may be so great
as to cause the cheese to crack and to force out some of the curd.
The presence of gas holes is indicative of a poor cheese, because
the formation of gas is always accompanied by the presence of other
undesirable compounds.
Pure culture starters are often used to overcome gassy
fermentations. In cheese a certain amount of acid can be produced by
the acid-forming bacteria. When the pure lactic bacteria alone are
present, the cheese is very likely to be of good quality. If the
sugar is fermented by gas-forming organisms, the curd will be full
of holes and the flavor poor, while if the sugar is fermented by a
mixture of the desirable and undesirable bacteria, the quality of
the product will depend on the relation of the two types. If through
the addition of a pure-culture starter, the proportion of desirable
bacteria is increased, the gas will be lessened in amount and the
cheese improved. It was formerly supposed that the lactic bacteria
had an injurious effect on the gas-forming organisms. There is no
good reason to believe that this is the case, but that both grow in
the milk and cheese, but since only a certain amount of acid can be
produced, it is important to have as much of it formed by the lactic
bacteria as possible, since the amount of injurious products in the
cheese will thus be limited.
[Illustration: Fig. 37.--Gassy Cheese.
Such a cheese is worthless on account of its poor flavor. The
irregular holes are mechanical. The crack on the upper side is due
to the pressure of the gas which has caused the cheese to bulge at
this point.]
The gas formed in the curd before the cheese is put to press can be
gotten rid of by proper manipulation of the curd. While this
treatment may improve the appearance of the cheese, it does not
eliminate the substances that impart to the cheese undesirable
qualities.
Gassy curds have also been treated by washing the curd with cold
water. Care must be taken in applying this method for the removal of
too much of the sugar and acid from the curd by the washing will
permit the growth of injurious forms of bacteria. The addition of
salt or of saltpeter has also been made to the milk in order to
overcome gassy conditions in the milk. In the handling of gassy
milk, the usual practice has been to develop a larger amount of acid
before drawing the whey than in the case of good milk. This was done
with the idea that acid suppressed gas formation. It has been shown
previously that this is not the case. It has also been shown by
Doane that the development of too much acid before drawing the whey
is likely to result in undesirable flavors, producing what is known
as "high-acid" or "sour" cheese.
The gas-forming bacteria grow best at high temperatures; hence,
cheese kept under these conditions are more likely to be affected by
this trouble than are those kept at lower temperatures.
The most successful method of preventing trouble with gassy milk in
cheese making is to eliminate undesirable milk by frequent testing
of the supply of the different patrons by means of the Wisconsin
curd test.
Not only gas-forming bacteria may be the cause of gassy cheese, but
the lactose-fermenting yeasts may cause similar trouble. If these
are abundant in the milk, a considerable part of the sugar may be
fermented by them, in which case, carbon dioxide gas is abundantly
formed. The cheese thus rendered gassy will present the same
appearance to the eye as where the gas is formed by bacteria, but
will have a different flavor. The odor of alcohol may be evident,
and if most of the sugar has been fermented by the yeast, the
acidity of the cheese may not be sufficient for the pepsin to exert
its digestive action.
Milk containing many gas-forming bacteria occurs most frequently in
summer. It is claimed by some that the milk of cattle pastured on
low lands is more likely to contain the gas-forming organisms than
that from cattle running on higher lands. If this is true, it must
be due to the bacterial content of the soil; the udders of the
animals become soiled as they lie on the ground, and during the
milking, the dust finds its way into the pail. Many cheese makers
think that the milk from an animal suffering from a garget may be
the cause of the huffing of cheese. This belief is undoubtedly well
founded, as some of the bacteria known to be the cause of garget are
gas-forming.
=Bitter cheese.= In a previous chapter the bitter fermentation of milk
has been discussed. If milk containing large numbers of such
organisms is made into cheese, the bitterness is very likely to be
noted in it. Cheese made from milk containing few or no lactic
bacteria is likely to develop a bitter taste, due to the growth of
the digestive bacteria that are able to grow through the lack of
acid in the cheese.
If the milk contains considerable numbers of yeasts, a sweet or
fruity flavor is apt to develop, due to the products of the
fermentation of the sugar by the yeast. This flavor resembles that
of fermented fruit, or the bouquet of certain kinds of wine.
=Putrid cheese.= In the absence of acid-forming bacteria, the cheese
may develop a putrid or rotten odor, due to the growth of some types
of putrefactive or digesting bacteria. This trouble is very
infrequent in cheddar cheese, since this is made from ripened milk,
but occurs more frequently in those types in which no acid is
developed.
Bacteria develop in the cheese in colonies or masses, just as they
do in the plate cultures of the bacteriologist, made with
transparent media, such as gelatin. Cheese is opaque; therefore, the
growing colonies cannot be readily discovered, but when
pigment-forming bacteria grow in the cheese, their presence is
likely to be noted, because of the colored spots that are formed.
=Rusty spot.= The "rusty spot" that has been encountered in New York
and Canada is due to one of the colored bacteria which produces an
orange or yellowish-red pigment. Various other pigment-forming
organisms have been met in cheese, each producing its colored colony
which differentiates itself from the mass of the cheese. If the
pigment is produced in considerable quantities, and is soluble in
any of the constituents of the cheese, the color will not appear in
spots but will be more diffuse, or may impart a color to the entire
mass.
Cases of acute poisoning arising from the ingestion of cheese are
not infrequently reported; similar instances result from the use of
ice cream. In both cases it is believed that poisonous products have
been formed by bacteria, probably by some of the putrefactive forms.
From what has been said with reference to the abnormal fermentations
of cheese, it will be seen that they are always due to the lack of
acid-forming bacteria, or to their partial replacement by other
types. In order to prevent such troubles, it is necessary to insure
that the milk has been produced under clean conditions, from healthy
cows, and has been handled in such a manner as to reach the maker in
as sweet and fresh condition as possible. The maker can, by the use
of proper starters, control the kinds of bacteria essential for the
ripening process. A well trained maker should be able to prepare
from such milk a uniform product of the highest quality. The effort
of cheese makers at the present time is to handle milk of more or
less objectionable quality so as to secure from it as good cheese as
is possible. But cheese is so sensitive as to character of milk used
that greater effort should be spent in securing an improved supply.
=Moldy cheese.= In the case of the cheddar cheese and other types of
hard cheese, it is essential that their surfaces be kept clean, and
not discolored by the growth of molds, which find favorable
conditions for growth on the surface of the cheese in the moist
atmosphere of the curing room. The molding of cheddar cheese can be
prevented by covering the cheese with a layer of paraffin which
stops the development of the mold spores, by shutting off the
necessary supply of oxygen. For this purpose the cheese are dipped
in melted paraffin when a few days old.
In the case of types of cheese which are salted by applying the salt
to the surface, or with soft cheese which ripen from the outside,
other methods of mold prevention are employed, such as rubbing and
washing the cheese. The curing room itself may be freed from the
mold spores by the use of such standard disinfectants as formalin or
sulphur.
=Swiss cheese.= One of the most important kinds of hard cheese, is the
Swiss or Emmenthaler, so named, from the country and valley in which
the cheese was first made. In America, this type was introduced by
Swiss immigrants, and is being made in constantly increasing
quantities in Ohio and Wisconsin.
Swiss cheese is a hard firm type, appearing in the markets in the
form of the flat circular "drum" cheese, two to three feet in
diameter, and six to eight inches thick, or in the smaller "block"
form. In this country the cheese is prepared twice a day, since it
is necessary to work up the milk while it is perfectly sweet.
Indeed, the milk is received at the factories while it is still
warm, and within five or six hours after it is drawn from the cow
the cheese is on the press. If the attempt is made to prepare Swiss
cheese from the kind of milk that is best suited for cheddar
purposes, _i.e._, milk in which the acidity has increased to some
extent, the flavor of the resulting product is likely to approximate
a cheddar cheese rather than that of a Swiss.
In the salting process, the salt is not mixed with the curd before
it is pressed, but is applied by immersing the cheese for a few days
in a saturated brine, and then rubbing salt over the surface of the
cheese. In this way the salt gradually diffuses quite uniformly
through the cheese. The method of salting has apparently a marked
influence on the ripening process, since if the salt is added in the
same way, and in amounts used in the cheddar process, the flavor
will not be that of a Swiss cheese but will resemble a cheddar.
In cheddar cheese, the whey is expelled from the curd by means of
the acid which is developed in the curd, and by heating the curd to
a temperature of 95 deg. F. to 100 deg. F. In Swiss cheese the development
of acid during the making process is prevented, because of the
smaller number of acid-forming bacteria in the milk; other factors
must therefore contribute to the expulsion of the whey to secure a
firm curd. This is accomplished by cutting the curd into very small
pieces and by briskly stirring it during the making, heating it
during this process for a period of 20 to 30 minutes at 130 deg. to
140 deg. F. It might be thought that this high temperature, which is
approximately that used in pasteurization would destroy the
acid-forming bacteria, but these are apparently protected as they
are within the curd. During the time the cheese is being pressed,
the contained bacteria begin to grow and the whey coming from the
cheese toward the end of the pressing shows a high acidity. If it
does not show such a development of acid, the maker has reason to
believe that the cheese may never ripen in a typical manner.
It has been mentioned that the milk contains but few acid-forming
bacteria. The maker, however, attempts to insure the presence of a
sufficient number by the use of "home-made" rennet. This is prepared
by placing a piece of dried rennet, _i.e._, the stomach of the calf,
in whey, keeping the same in a warm place for twenty-four to
thirty-six hours. As the rennet contains acid-forming organisms,
these grow rapidly in the warm whey, so that by adding this sour
whey to the milk, the maker is not only adding rennet, that is to
curdle the milk, but also a small starter of lactic bacteria. If the
rennet thus prepared contains no harmful bacteria and the milk is of
good quality, the cheese is likely to ripen in a normal manner. The
rennet should be prepared with due regard to bacteriological
principles, a condition that is rarely met in Swiss factories in
this country.
Swiss cheese has two striking characteristics, the flavor and the
presence of holes or "eyes." The flavor is sweetish rather than the
sharp and pungent flavor of cheddar cheese. The bacteria concerned
in its production are not known, but it is certain that specific
organisms play some role, since if the flora of the cheese is
changed by salting the curd or by the use of milk containing large
numbers of lactic bacteria, the flavor will also be changed. This
role of the acid-forming bacteria in Swiss is the same as in
cheddar, _i.e._, through the acid, conditions are established for
peptic action, the curd being partially digested while at the same
time the curd mass is protected from putrefactive processes.
In Swiss cheese during the ripening process, holes about the size of
a large cherry develop which should be quite uniformly distributed
throughout the cheese. The inner surface of the hole is glistening
and, in a well-ripened cheese, a small quantity of clear brine,
_i.e._, "tears" may be noted. These holes or "eyes" may be called
the trade mark of the Swiss cheese, since without them the product
has a lessened commercial value, even if it possesses the typical
flavor. The "eyes" are caused by bacteria that ferment the lactic
acid produced by the lactic bacteria, forming from it propionic
acid and carbon dioxide, the latter gas being the cause of the hole
or "eye."
[Illustration: Fig. 38.--Swiss Cheese.
Normal development of "eyes" in a Swiss cheese. The eyes are
generally as large as a cherry.]
The "eye"-forming organisms cannot grow in the presence of any
amount of salt, hence, if salt is added directly to the curd, the
cheese is likely to be "blind" or free from holes. The eyes are
formed not at the time gas holes are produced in a cheddar cheese,
_i.e._, early in the ripening process, but after a lapse of three or
four weeks. They are most abundant in the middle of the cheese since
the manner of salting is such as to inhibit their formation near the
surface. The eye-forming bacteria may have some effect on the flavor
of the cheese.
The Swiss maker encounters the same troubles as does the cheddar
maker. Gassy cheese is more prevalent in the Swiss than in the
cheddar industry, since the maker cannot call to his aid the methods
used by the cheddar maker, viz., the addition of a heavy starter,
the washing of the curd, etc. It is especially important that the
quality of the milk be first class in every respect, and yet
customs prevail in the Swiss industry that are directly inimical to
the production of good milk. The grossest carelessness prevails at
the factories in the matter of handling the whey. It is often kept
in individual barrels for each patron. (See Fig. 8.) These are not
kept thoroughly clean and the result is that the whey taken back to
the farm in the cans that are used to bring the fresh milk is often
in an advanced stage of fermentation.
There are many other kinds of hard cheese; but in each, so far as is
known, the role of the acid-forming bacteria is identical with that
noted in cheddar and Swiss cheese, viz, in preparing conditions
favorable for peptic action, and preventing the development of
putrefactive bacteria present in the curd.
=Roquefort cheese.= Among the more important foreign types of cheese
that are characterized by the development of mold is Roquefort, so
named from the district in France in which it is made. This cheese
is made from sheep's milk, in much the same manner as cheddar. The
characteristic process in its preparation is the inoculation of the
curd, at the time it is put to press, with the spores of a
particular kind of mold, a type closely related to the ordinary
green mold of bread and cheese. The mold for inoculation is grown on
bread, the whole mass being dried so that it can be powdered; then
the ground-up material is sprinkled on the curd as it is placed in
the press hoops. The first stage in the ripening of Roquefort is
probably identical with that of the types of hard cheeses already
considered, the breaking-down of the curd being due to the pepsin of
the rennet used, which action is made possible by the acid formed by
the bacteria.
The second stage in ripening, and one in which the characteristic
flavor of the cheese is developed, is due to the growth of the mold
with which the cheese is seeded. Molds can grow only in the presence
of air, and in order to provide this condition, the cheese are run
through a machine having a series of needle-like projections which
fills the cheese with fine holes. This allows the air to penetrate
the cheese and the mold to grow, the fruiting of which develops the
characteristic flavor. The changes produced by the mold are not well
understood, but the flavor is evidently connected with its
development since in the absence of mold, it does not appear. The
cheese must be cured under carefully controlled conditions, as to
temperature and moisture; in France these are secured by curing the
cheese in limestone caves that are highly saturated with moisture.
Attempts have been made to make Roquefort cheese in other parts of
the world, but they have never been successful, due undoubtedly to
the fact that the proper environment and conditions for the
development of the various types of organisms necessary in the
ripening process have not been met. This cheese is sold for 50 to 75
cents per pound in the markets of the world.
There are two other kinds of cheese that are closely related to
Roquefort, as to the manner of ripening, viz., the Gorgonzola of
Italy and the Stilton of England, both of which possess their
characteristic flavors by reason of the development of molds. In
Stilton cheese the mold is not intentionally added, the maker
relying on the contamination that comes from the factory for the
usual seeding. If this does not develop, it is sometimes inoculated
by exchanging plugs with a well-ripened Stilton. This method is not
so certain as in the inoculation of Roquefort.
=Camembert cheese.= A typical example of soft cheese is one of the
French types, known as Camembert. This cheese is prepared from cow's
milk which is curdled by rennet. The curd is not cut but is dipped
into the forms, which condition, taken with the absence of pressure
in forming the cheese, accounts for the large quantity of whey in
it. The finished cheese are about one inch in thickness and three
inches in diameter. In the ripening, the moisture and temperature of
the curing room are very carefully regulated.
The first stage in the ripening is due to the rennet and the lactic
bacteria. Later there appears on the surface of the moist cheese, a
moldy growth. In this, there are at least two kinds of molds, the
ordinary mold that appears on sour milk, _Oidium lactis_, and
another that is related to the bread mold but which has a white
instead of a green fruiting stage. These molds are confined to the
surface of the cheese but the enzymes which they produce diffuse
into the substance, changing the color from a dull, opaque white to
a translucent yellow. The acid that has been formed by the lactic
bacteria is gradually used up by the growth of the mold, and
conditions then become favorable for the growth of putrefactive
bacteria which digest the curd. The cheese is ready for use when the
action of the mold has penetrated to the center of the cheese, and
before any pronounced putrefaction has taken place. The production
of the typical flavor is dependent upon there being a definite
relation between the growth of the molds and bacteria. This relation
is dependent largely upon the moisture and temperature of the curing
room. These cannot always be regulated with exactness; and hence,
much of this type of cheese is not of first quality, and must be
sold for a low price. While such fancy cheeses, as Camembert, bring
fifty cents and upward per pound, and the yield from the milk is
much greater than with the hard type of cheese, yet the difficulties
of successful manufacture are such as to make success less easily
attained than with the other types.
There are many other kinds of soft cheese that depend for their
ripening upon factors similar to those concerned in the ripening of
Camembert; most of them are, however, of small importance from a
commercial standpoint.
=Limburger cheese.= A very famous cheese is one originally made in
Germany to which the name Limburger is given. It is classed as a
soft cheese although it is much firmer than Camembert. This cheese
is made from cow's milk and is pressed very lightly or not at all,
which condition accounts for its high per cent (50 per cent) of
moisture. The surface is kept moist by repeated washing of the
cheese, and by keeping the air of the curing room very moist. A
yellowish, slimy, bacterial layer soon develops on the surface under
these conditions. The enzymes produced by this external growth
gradually diffuse to the center of the cheese, when it is regarded
as ripe. The odor of the matured product is somewhat putrefactive,
but is not so offensive as is usually supposed.
Definite knowledge concerning the types of organisms concerned in
the surface layer is very limited. It is not certain whether the
same kinds of organisms must always be present. Limburger is much
easier to make than Camembert, due possibly to the fact that there
are not needed definite forms of life and that the balance between
them is not so delicate.
A cheese known as brick is closely related to Limburger in its
method of making and of ripening but is less pronounced in flavor.
In the manufacture of all of these types of cheese, troubles are
likely to develop, due to an abnormal bacterial condition of the
milk.
It will be seen from what has been said that the bacteria are
essential factors in cheese ripening, and that the cheese industry,
like the butter industry, may be called a true fermentation
industry. Close co-operation must exist between the milk producer,
and the maker so that the type of fermentation that goes on in the
milk can be controlled. A recognition of the fundamental principles
governing these fermentations, both normal as well as abnormal, is
now regarded as an essential part of the training of the dairy
manufacturer of today.