BACTERIA IN CHEESE.
The art of cheese-making, like all other phases of dairying, has been
developed mainly as a result of empirical methods. Within the last
decade or so, the subject has received more attention from the
scientific point of view and the underlying causes determined to some
extent. Since the subject has been investigated from the bacteriological
point of view, much light has been thrown on the cause of many changes
that were heretofore inexplicable. Our knowledge, as yet, is quite
meager, but enough has already been determined to indicate that the
whole industry is largely based on the phenomena of ferment action, and
that the application of bacteriological principles and ideas is sure to
yield more than ordinary results, in explaining, in a rational way, the
reasons underlying many of the processes to be observed in this
industry.
The problem of good milk is a vital one in any phase of dairy activity,
but it is pre-eminently so in cheese-making, for the ability to make a
first-class product depends to a large extent on the quality of the raw
material. Cheese contains so large a proportion of nitrogenous
constituents that it is admirably suited, as a food medium, to the
development of bacteria; much better, in fact, than butter.
INFLUENCE OF BACTERIA IN NORMAL CHEESE PROCESSES.
In the manufacture of cheddar cheese bacteria exert a marked influence
in the initial stages of the process. To produce the proper texture that
characterizes cheddar cheese, it is necessary to develop a certain
amount of acid which acts upon the casein. This acidity is measured by
the development of the lactic-acid bacteria that normally abound in the
milk; or, as the cheese-maker expresses it, the milk is "ripened" to the
proper point. The action of the rennet, which is added to precipitate
the casein of the milk, is markedly affected by the amount of acid
present, as well as the temperature. Hence it is desirable to have a
standard amount of acidity as well as a standard temperature for
coagulation, so as to unify conditions. It frequently happens that the
milk is abnormal with reference to its bacterial content, on account of
the absence of the proper lactic bacteria, or the presence of forms
capable of producing fermentative changes of an undesirable character.
In such cases the maker attempts to overcome the effect of the unwelcome
bacteria by adding a "starter;" or he must vary his method of
manufacture to some extent to meet these new conditions.
~Use of starters.~ A starter may be employed to hasten the ripening of
milk that is extremely sweet, so as to curtail the time necessary to get
the cheese to press; or it may be used to overcome the effect of
abnormal conditions.
The starter that is employed is generally one of domestic origin, and is
usually taken from skim milk that has been allowed to ferment and sour
under carefully controlled conditions. Of course much depends upon the
quality of the starter, and in a natural starter there is always the
possibility that it may not be perfectly pure.
Within recent years the attempt has been made to control the effect of
the starter more thoroughly by using pure cultures of some desirable
lactic-acid form.[178] This has rendered the making of cheese not only
more uniform, but has aided in repressing abnormal fermentations
particularly those that are characterized by the production of gas.
Recently, pure cultures of Adametz's _B. nobilis_, a digesting organism
that is claimed to be the cause of the breaking down of the casein and
also of the peculiar aroma of Emmenthaler cheese, has been placed on the
market under the name _Tyrogen_. It is claimed that the use of this
starter, which is added directly to the milk and also rubbed on the
surface of the cheese, results in the improvement of the curds, assists
in the development of the proper holes, imparts a favorable aroma and
hastens ripening.[179]
Campbell[180] states that the discoloration of cheese in England, which
is due to the formation of white spots that are produced by the
bleaching of the coloring matter in the cheese, may be overcome by the
use of lactic-acid starters.
The use of stringy or slimy whey has been advocated in Holland for some
years as a means of overcoming the tendency toward gas formation in Edam
cheese which is made from practically sweet milk. This fermentation, the
essential feature of which is produced by a culture of _Streptococcus
Hollandicus_,[181] develops acid in a marked degree, thereby inhibiting
the production of gas.
The use of masses of moldy bread in directing the fermentation of
Roquefort cheese is another illustration of the empirical development of
starters, although in this instance it is added after the curds have
been prepared for the press.
~Pasteurizing milk for cheese-making.~ If it were possible to use properly
pasteurized milk in cheese-making, then practically all abnormal
conditions could be controlled by the use of properly selected starters.
Numerous attempts have been made to perfect this system with reference
to cheddar cheese, but so far they have been attended with imperfect
success. The reason for this is that in pasteurizing milk, the soluble
lime salts are precipitated by the action of heat, and under these
conditions rennet extract does not curdle the casein in a normal manner.
This condition can be restored, in part at least, by the addition of
soluble lime salts, such as calcium chlorid; but in our experience,
desirable results were not obtained where heated milks to which this
calcium solution had been added were made into cheddar cheese.
Considerable experience has been gained in the use of heated milks in
the manufacture of certain types of foreign cheese. Klein[182] finds
that Brick cheese can be successfully made even where the milk is heated
as high as 185 deg. F. An increased weight is secured by the addition of
the coagulated albumin and also increased moisture.
~Bacteria in rennet.~ In the use of natural rennets, such as are
frequently employed in the making of Swiss cheese, considerable numbers
of bacteria are added to the milk. Although these rennets are preserved
in salt, alcohol or boric acid, they are never free from bacteria.
Adametz[183] found ten different species and from 640,000 to 900,000
bacteria per cc. in natural rennets. Freudenreich has shown that rennet
extract solutions can be used in Swiss cheese-making quite as well as
natural rennets; but to secure the best results, a small quantity of
pure lactic ferment must be added to simulate the conditions that
prevail when natural rennets are soaked in whey, which, it must be
remembered, is a fluid rich in bacterial life.
Where rennet extract or tablets are used, as is generally the case in
cheddar making, the number of bacteria added is so infinitesimal as to
be negligible.
~Development of acid.~ In the manufacture of cheddar cheese, the
development of acid exerts an important influence on the character of
the product. This is brought about by holding the curds at temperatures
favorable to the growth of the bacteria in the same. Under these
conditions the lactic-acid organisms, which usually predominate, develop
very rapidly, producing thereby considerable quantities of acid which
change materially the texture of the curds. The lactic acid acts upon
the casein in solutions containing salt, causing it to dissolve to some
extent, thus forming the initial compounds of digestion.[184] This
solution of the casein is expressed physically by the "stringing" of the
curds on a hot iron. This causes the curds to mat, producing a close,
solid body, free from mechanical holes. Still further, the development
of this acid is necessary for the digestive activity of the pepsin in
the rennet extract.
In some varieties of cheese, as the Swiss, acid is not developed and the
character of the cheese is much different from that of cheddar. In all
such varieties, a great deal more trouble is experienced from the
production of "gassy" curds, because the development of the
gas-producing bacteria is held in check by the rapid growth of the
lactic acid-producing species.
~Bacteria in green cheese.~ The conditions under which cheese is made
permit of the development of bacteria throughout the entire process. The
cooking or heating of curds to expel the excessive moisture is never so
high as to be fatal to germ life; on the contrary, the acidity of the
curd and whey is continually increased by the development of bacteria in
the same.
The body of green cheese fresh from the press is, to a considerable
extent, dependent upon the acid produced in the curds. If the curds are
put to press in a relatively sweet condition the texture is open and
porous. The curd particles do not mat closely together and "mechanical
holes," rough and irregular in outline, occur. Very often, at relatively
high temperatures, such cheese begin to "huff," soon after being taken
from the press, a condition due to the development of gas, produced by
gas-generating bacteria acting on the sugar in the curd. This gas finds
its way readily into these ragged holes, greatly distending them, as in
Fig. 30.
[Illustration: FIG. 30. _L_, a sweet curd cheese direct from the press.
"Mechanical" holes due to lack of acid development; _P_, same cheese
four days later, mechanical holes distended by development of gas.]
~Physical changes in ripening cheese.~ When a green cheese is taken from
the press, the curd is tough, firm, but elastic. It has no value as a
food product for immediate use, because it lacks a desirable flavor and
is not readily digestible. It is nothing but precipitated casein and
fat. In a short time, a deep-seated change occurs. Physically this
change is demonstrated in the modification that the curd undergoes.
Gradually it breaks down and becomes plastic, the elastic, tough curd
being changed into a softened mass. This change in texture of the cheese
is also accompanied by a marked change in flavor. The green cheese has
no distinctively cheese flavor, but in course of time, with the gradual
change of texture, the peculiar flavor incident to ripe cheese is
developed.
The characteristic texture and flavor are susceptible of considerable
modification that is induced not only by variation in methods of
manufacture, but by the conditions under which the cheese are cured. The
amount of moisture incorporated with the curd materially affects the
physical appearance of the cheese, and the rate of change in the same.
The ripening temperature, likewise the moisture content of the
surrounding air, also exerts a marked influence on the physical
properties of the cheese. To some extent the action of these forces is
purely physical, as in the gradual loss by drying, but in other respects
they are associated with chemical transformations.
~Chemical changes in ripening cheese.~ Coincident with the physical
breaking down of the curd comes a change in the chemical nature of the
casein. The hitherto insoluble casein is gradually transformed into
soluble nitrogenous substances (_caseone_ of Duclaux, or _caseogluten_
of Weigmann). This chemical phenomenon is a breaking-down process that
is analogous to the peptonization of proteids, although in addition to
the peptones and albumoses characteristic of peptic digestion,
amido-acids and ammonia are to be found. The quantity of these lower
products increases with the age of the cheese.
The chemical reaction of cheese is normally acid to phenolphthalein,
although there is generally no free acid, as shown by Congo red, the
lactic acid being converted into salts as fast as formed. In very old
cheese, undergoing putrefactive changes, especially on the outside, an
alkaline reaction may be present, due to the formation of free ammonia.
The changes that occur in a ripening cheese are for the most part
confined to the proteids. According to most investigators the fat
remains practically unchanged, although the researches of Weigmann and
Backe[185] show that fatty acids are formed from the fat. In the green
cheese considerable milk-sugar is present, but, as a result of the
fermentation that occurs, this is rapidly converted into acid products.
~Bacterial flora of cheese.~ It might naturally be expected that the green
cheese, fresh from the press, would contain practically the same kind of
bacteria that are in the milk, but a study of cheese shows a peculiar
change in the character of the flora. In the first place, fresh cottage
cheese, made by the coagulation of the casein through the action of
acid, has a more diversified flora than cheese made with rennet, for the
reason, as given by Lafar,[186] that the fermentative process is farther
advanced.
When different varieties of cheese are made from milk in the same
locality, the germ content of even the ripened product has a marked
similarity, as is illustrated by Adametz's work[187] on Emmenthaler or
Swiss hard cheese, and Schweitzer Hauskaese, a soft variety. Of the nine
species of bacilli and cocci found in mature Emmenthaler, eight of them
were also present in ripened Hauskaese.
Different investigators have studied the bacterial flora of various
kinds of cheese, but as yet little comparative systematic work has been
done. Freudenreich[188] has determined the character and number of
bacteria in Emmenthaler cheese, and Russell[189] the same for cheddar
cheese. The same general law has also been noted in Canadian[190] and
English[191] cheese. At first a marked decrease in numbers is usually
noted, lasting for a day or two. This is followed by an enormous
increase, caused by the rapid growth of the lactic-acid type. The
development may reach scores of millions and often over a hundred
million organisms per gram. Synchronous with this increase, the
peptonizing and gas-producing bacteria gradually disappear. This rapid
development, which lasts only for a few weeks, is followed by a general
decline.
In the ripening of cheese a question arises as to whether the process
goes on throughout the entire mass of cheese, or whether it is more
active at or near the surface. In the case of many of the soft cheese,
such as Brie and limburger, bacterial and mold development is
exceedingly active on the exterior, and the enzyms secreted by these
organisms diffuse toward the interior. That such a condition occurs in
the hard type of cheese made with rennet is extremely improbable. Most
observers agree that in this type of cheese the ripening progresses
throughout the entire mass, although Adametz opposes this view and
considers that in Emmenthaler cheese the development of the specific
aroma-producing organism occurs in the superficial layers. Jensen has
shown, however, that the greatest amount of soluble nitrogenous products
are to be found in the innermost part of the cheese, a condition that is
not reconcilable with the view that the most active ripening is on the
exterior.[192]
The course of development of bacteria in cheddar cheese is materially
influenced by the ripening temperature. In cheese ripened at relatively
low temperatures (50 deg.-55 deg. F.),[193] a high germ content is
maintained for a much longer period of time than at higher temperatures.
Under these conditions the lactic-acid type continues in the ascendancy
as usual. In cheese cured at high temperatures (80 deg.-86 deg. F.) the
number of organisms is greatly diminished, and they fail to persist in
appreciable numbers for as long a time as in cheese cured at temperatures
more frequently employed.
~Influence of temperature on curing.~ Temperature exerts a most potent
influence on the quality of the cheese, as determined not only by the
rate of ripening but the nature of the process itself. Much of the poor
quality of cheese is attributable to the effect of improper curing
conditions. Probably in the initial stage of this industry cheese were
allowed to ripen without any sort of control, with the inevitable result
that during the summer months the temperature generally fluctuated so
much as to impair seriously the quality. The effect of high temperatures
(70 deg. F. and above) is to produce a rapid curing, and, therefore, a
short lived cheese; also 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 very apt to undergo abnormal fermentations, more especially
those of a gassy character.
[Illustration: FIG. 31. Influence of curing temperature on texture of
cheese. Upper row ripened eight months at 60 deg. F.; lower row at 40
deg. F.]
Where cheese is ripened at low temperatures, ranging from 50 deg. F. down
to nearly the freezing temperatures, it is found that the quality is
greatly improved.[194] Such cheese are thoroughly broken down from a
physical point of view even though they may not show such a high per
cent of soluble nitrogenous products. They have an excellent texture,
generally solid and firm, free from all tendency to openness; and,
moreover, their flavor is clean and entirely devoid of the sharp,
undesirable tang that so frequently appears in old cheese. The keeping
quality of such cheese is much superior to the ordinary product. The
introduction of this new system of cheese-curing promises much from a
practical point of view, and undoubtedly a more complete study of the
subject from a scientific point of view will aid materially in
unraveling some of the problems as to flavor production.
~Theories of cheese curing.~ Within the last few years considerable study
has been given the subject of cheese curing or ripening, in order to
explain how this physical and chemical transformation is brought about.
Much of the misconception that has arisen relative to the cause of
cheese ripening comes from a confusion of terms. In the ordinary use of
the word, ripening or curing of cheese is intended to signify the sum
total of all the changes that result in converting the green product as
it comes from the press into the edible substance that is known as cured
cheese. As previously shown, the most marked chemical transformation
that occurs is that which has to do with the peptonization or breaking
down of the casein. It is true that under ordinary conditions this
decomposition process is also accompanied with the formation of certain
flavor-producing substances, more or less aromatic in character; but it
by no means follows that these two processes are necessarily due to the
same cause. The majority of investigators have failed to consider these
two questions of casein decomposition and flavor as independent, or at
least as not necessarily related. They are undoubtedly closely bound
together, but it will be shown later that the problems are quite
different and possibly susceptible of more thorough understanding when
considered separately.
In the earlier theories of cheese ripening it was thought to be purely a
chemical change, but, with the growth of bacteriological science,
evidence was forthcoming that seemed to indicate that the activity of
organisms entered into the problem. Schaffer[195] showed that if milk
was boiled and made into cheese, the casein failed to break down.
Adametz[196] added to green cheese various disinfectants, as creolin and
thymol, and found that this practically stopped the curing process. From
these experiments he drew the conclusion that bacteria must be the cause
of the change, because these organisms were killed; but when it is
considered that such treatment would also destroy the activity of enzyms
as well as vital ferments, it is evident that these experiments were
quite indecisive.
A determination of the nature of the by-products found in maturing
cheese indicates that the general character of the ripening change is a
peptonization or digestion of the casein.
Until recently the most widely accepted views relating to the cause of
this change have been those which ascribed the transformation to the
activity of micro-organisms, although concerning the nature of these
organisms there has been no unanimity of opinion. The overwhelming
development of bacteria in all cheeses naturally gave support to this
view; and such experiments as detailed above strengthened the idea that
the casein transformation could not occur where these ferment organisms
were destroyed.
The very nature of the changes produced in the casein signified that to
take part in this process any organism must possess the property of
dissolving the proteid molecule, casein, and forming therefrom
by-products that are most generally found in other digestive or
peptonizing changes of this class.
~Digestive bacterial theory.~ The first theory propounded was that of
Duclaux,[197] who in 1887 advanced the idea that this change was due to
that type of bacteria which is able to liquefy gelatin, peptonize milk,
and cause a hydrolytic change in proteids. To this widely-spread group
that he found in cheese, he gave the generic name _Tyrothrix_ (cheese
hairs). According to him, these organisms do not function directly as
ripening agents, but they secrete an enzym or unorganized ferment to
which he applies the name _casease_. This ferment acts upon the casein
of milk, converting it into a soluble product known as _caseone_. These
organisms are found in normal milk, and if they function as casein
transformers, one would naturally expect them to be present, at least
frequently, if not predominating in the ripening cheese; but such is not
the case. In typical cheddar or Swiss cheese, they rapidly disappear (p.
168), although in the moister, softer varieties, they persist for
considerable periods of time. According to Freudenreich, even where
these organisms are added in large numbers to the curd, they soon
perish, an observation that is not regarded as correct by the later
adherents to the digestive bacterial theory, as Adametz and Winkler.
Duclaux's experiments were made with liquid media for isolation
purposes, and his work, therefore, cannot be regarded as satisfactory as
that carried out with more modern technical methods. Recently this
theory has been revived by Adametz,[198] who claims to have found in
Emmenthaler cheese a digesting species, one of the Tyrothrix type, which
is capable of peptonizing the casein and at the same time producing the
characteristic flavor of this class of cheese. This organism, called by
him _Bacillus nobilis_, the Edelpilz of Emmenthaler cheese, has been
subjected to comparative experiments, and in the cheese made with pure
cultures of this germ better results are claimed to have been secured.
Sufficient experiments have not as yet been reported by other
investigators to warrant the acceptance of the claims made relative to
the effect of this organism.
~Lactic-acid bacterial theory.~ It has already been shown that the
lactic-acid bacteria seems to find in the green cheese the optimum
conditions of development; that they increase enormously in numbers for
a short period, and then finally decline. This marked development,
coincident with the breaking down of the casein, has led to the view
which has been so ably expounded by Freudenreich[199] that this type of
bacterial action is concerned in the ripening of cheese. This group of
bacteria is, under ordinary conditions, unable to liquefy gelatin, or
digest milk, or, in fact, to exert, under ordinary conditions, any
proteolytic or peptonizing properties. This has been the stumbling-block
to the acceptance of this hypothesis, as an explanation of the breaking
down of the casein. Freudenreich has recently carried on experiments
which he believes solve the problem. By growing cultures of these
organisms in milk, to which sterile, freshly precipitated chalk had been
added, he was able to prolong the development of bacteria for a
considerable period of time, and as a result finds that an appreciable
part of the casein is digested; but this action is so slow compared with
what normally occurs in a cheese, that exception may well be taken to
this type of experiment alone. Weigmann[200] inclines to the view that
the lactic-acid bacteria are not the true cause of the peptonizing
process, but that their development prepares the soil, as it were, for
those forms that are more directly concerned in the peptonizing process.
This they do by developing an acid substratum that renders possible the
more luxuriant growth of the aroma-producing species. According to
Gorini,[201] certain of the Tyrothrix forms function at high
temperatures as lactic acid producing bacteria, while at lower
temperatures they act as peptonizers. On this basis he seeks to
reconcile the discrepancies that appear in the experiments of other
investigators.
~Digestive milk enzym theory.~ In 1897 Babcock and the writer[202] showed
that milk underwent digestive changes spontaneously when bacterial
activity was suspended by the addition of such anaesthetics as ether,
chloroform and benzol. The chemical nature of the by-products produced
by this auto-digestion of milk resembles quite closely those found in
ripened cheese, except that ammonia is not produced as is the case in
old cheese. The cause of the decomposition of the casein, they found to
be due to the action of a milk enzym which is inherent to the milk
itself. This digestive ferment may be separated from fresh milk by
concentrating centrifuge slime extracts by the usual physiological
reagents. This ferment, called by them _galactase_, on account of its
origin in milk, is a proteolytic enzym of the tryptic type. Its activity
is destroyed by strong chemicals such as formaldehyde, corrosive
sublimate, also when heated to 175 deg. F. or above. When such extracts
are added to boiled milk, the digestive process is started anew, and the
by-products produced are very similar to those noted in a normal cheese.
Jensen[203] has also shown that the addition of pancreatic extracts to
cheese accelerated the formation of soluble nitrogenous products.
The action of galactase in milk and cheese has been confirmed by
Freudenreich[204] and Jensen,[205] as well as by American investigators,
and this enzym is now generally accepted as one of the factors concerned
in the decomposition of the casein. Freudenreich believes it is able to
change casein into albumose and peptones, but that the lactic-acid
bacteria are chiefly responsible for the further decomposition of the
nitrogen to amid form.
Failure before to recognize the presence of galactase in milk is
attributable to the fact that all attempts to secure sterile milk had
been made by heating the same, in which case galactase was necessarily
destroyed. A brief exposure at 176 deg. F. is sufficient to destroy its
activity, and even an exposure at lower temperatures weakens its action
considerably, especially if the reaction of the medium is acid. This
undoubtedly explains the contradictory results obtained in the ripening
of cheese from pasteurized milk, such cheese occasionally breaking down
in an abnormal manner.
The results mentioned on page 172, in which cheese failed to ripen when
treated with disinfectants,--experiments which were supposed at that
time to be the foundation of the bacterial theory of casein
digestion--are now explicable on an entirely different basis. In these
cases the casein was not peptonized, because these strong disinfectants
destroyed the activity of the enzyms as well as the bacteria.
Another important factor in the breaking down of the casein is the
_pepsin_ in the rennet extract. The digestive influence of this agent
was first demonstrated for cheddar cheese by Babcock, Russell and
Vivian,[206] and simultaneously, although independently, by Jensen[207]
in Emmenthaler cheese. In this digestive action, only albumoses and
higher peptones are produced. The activity of pepsin does not become
manifest until there is about 0.3 per cent. acid which is approximately
the amount developed in the cheddar process. These two factors
undoubtedly account for by far the larger proportion of the changes in
the casein; and yet, the formation of ammonia in well ripened cheese is
not accounted for by these factors. This by-product is the main end
product of proteid digestion by the liquefying bacteria but their
apparent infrequency in cheese makes it difficult to understand how they
can function prominently in the change, unless the small quantity of
digestive enzyms excreted by them in their growth in milk is capable of
continuing its action until a cumulative effect is obtained. Although
much light has been thrown on this question by the researches of the
last few years, the matter is far from being satisfactorily settled at
the present time and the subject needs much more critical work. If
liquefying bacteria abound in the milk, doubtless they exert some
action, but the role of bacteria is doubtless much greater in the
production of flavor than in the decomposition of the curd.
~Conditions determining quality.~ In determining the quality of cheese,
several factors are to be taken into consideration. First and foremost
is the flavor, which determines more than anything else the value of the
product. This should be mild and pleasant, although with age the
intensity of the same generally increases but at no time should it have
any bitter, sour, or otherwise undesirable taste or aroma. Texture
registers more accurately the physical nature of the ripening. The
cheese should not be curdy and harsh, but should yield quite readily to
pressure under the thumb, becoming on manipulation waxy and plastic
instead of crumbly or mealy. Body refers to the openness or closeness of
the curd particles, a close, compact mass being most desirable. The
color of cheese should be even, not wavy, streaked or bleached.
For a cheese to possess all of these characteristics in an optimum
degree is to be perfect in every respect--a condition that is rarely
reached.
So many factors influence this condition that the problem of making a
perfect cheese becomes exceedingly difficult. Not only must the quality
of the milk--the raw material to be used in the manufacture--be
perfectly satisfactory, but the factory management while the curds are
in the vat demands great skill and careful attention; and finally, the
long period of curing in which variation in temperature or moisture
conditions may seriously affect the quality,--all of these stages, more
or less critical, must be successfully gone through, before the product
reaches its highest state of development.
It is of course true that many phases of this complex series of
processes have no direct relation to bacteria, yet it frequently happens
that the result attained is influenced at some preceding stage by the
action of bacteria in one way or another. Thus the influence of the
acidity developed in the curds is felt throughout the whole life of the
cheese, an over-development of lactic-acid bacteria producing a sour
condition that leaves its impress not only on flavor but texture. An
insufficient development of acid fails to soften the curd-particles so
as to permit of close matting, the consequence being that the body of
the cheese remains loose and open, a condition favorable to the
development of gas-generating organisms.
~Production of flavor.~ The importance of flavor as determining the
quality of cheese makes it imperative that the nature of the substances
that confer on cheese its peculiar aromatic qualities and taste be
thoroughly understood. It is to be regretted that the results obtained
so far are not more satisfactory, for improvement in technique is hardly
to be expected until the reason for the process is thoroughly
understood.
The view that is most generally accepted is that this most important
phase of cheese curing is dependent upon bacterial activity, but the
organisms that are concerned in this process have not as yet been
satisfactorily determined. In a number of cases, different species of
bacteria have been separated from milk and cheese that have the power of
producing aromatic compounds that resemble, in some cases, the peculiar
flavors and odors that characterize some of the foreign kinds of cheese;
but an introduction of these into curd has not resulted in the
production of the peculiar variety, even though the methods of
manufacture and curing were closely followed. The similarity in germ
content in different varieties of cheese made in the same locality has
perhaps a bearing on this question of flavor as related to bacteria. Of
the nine different species of bacteria found in Emmenthaler cheese by
Adametz, eight of them were also present in ripened Hauskaese. If
specific flavors are solely the result of specific bacterial action, it
might naturally be expected that the character of the flora would
differ.
Some suggestive experiments were made by Babcock and Russell on the
question of flavor as related to bacterial growth, by changing the
nature of the environment in cheese by washing the curds on the racks
with warm water. In this way the sugar and most of the ash were removed.
Under such conditions the character of the bacterial flora was
materially modified. While the liquefying type of bacteria was very
sparse in normal cheddar, they developed luxuriantly in the washed
cheese. The flavor at the same time was markedly affected. The control
cheddar was of good quality, while that made from the washed curds was
decidedly off, and in the course of ripening became vile. It may be
these two results are simply coincidences, but other data[208] bear out
the view that the flavor was to some extent related to the nature of the
bacteria developing in the cheese. This was strengthened materially by
adding different sugars to washed curds, in which case it was found that
the flavor was much improved, while the more normal lactic-acid type of
bacteria again became predominant.
~Ripening of moldy cheese.~ In a number of foreign cheeses, the peculiar
flavor obtained is in part due to the action of various fungi which grow
in the cheese, and there produce certain by-products that flavor the
cheese. Among the most important of these are the Roquefort cheese of
France, Stilton of England, and Gorgonzola of Italy.
Roquefort cheese is made from goat's or cow's milk, and in order to
introduce the desired mold, which is the ordinary bread-mold,
_Penicillium glaucum_, carefully-prepared moldy bread-crumbs are added
to the curd.
At ordinary temperatures this organism develops too rapidly, so that the
cheese to ripen properly must be kept at a low temperature. The town of
Roquefort is situated in a limestone country, in a region full of
caves, and it is in these natural caves that most of the ripening is
done. These caverns are always very moist and have a temperature ranging
from 35 deg. to 44 deg. F., so that the growth of the fungus is retarded
considerably. The spread of the mold throughout the ripening mass is
also assisted in a mechanical way. The partially-matured cheese are run
through a machine that pricks them full of small holes. These slender
canals allow the mold organism to penetrate the whole mass more
thoroughly, the moldy straw matting upon which the ripening cheese are
placed helping to furnish an abundant seeding of the desired germ.
When new factories are constructed it is of advantage to introduce this
necessary germ in quantities, and the practice is sometimes followed of
rubbing the walls and cellars of the new location with material taken
from the old established factory. In this custom, developed in purely an
empirical manner, is to be seen a striking illustration of a
bacteriological process crudely carried out.
In the Stilton cheese, one of the highly prized moldy cheeses of
England, the desired mold fungus is introduced into the green cheese by
exchanging plugs taken with a cheese trier from a ripe Stilton.
~Ripening of soft cheese.~ The type of ripening which takes place in the
soft cheeses is materially different from that which occurs in the hard
type. The peptonizing action does not go on uniformly throughout the
cheese, but is hastened by the development of molds and bacteria on the
outside that exert a solvent action on the casein. For this reason, soft
cheeses are usually made up in small sizes, so that this action may be
hastened. The organisms that take part in this process are those that
are able to form enzyms (similar in their action to trypsin, galactase,
etc.), and these soluble ferments gradually diffuse from the outside
through the cheese.
Most of these peptonizing bacteria are hindered in their growth by the
presence of lactic acid, so that in many cases the appearance of the
digesting organisms on the surface is delayed until the acidity of the
mass is reduced to the proper point by the development of other
organisms, principally molds, which prefer an acid substratum for their
growth.
In Brie cheese a blue coating of mold develops on the surface. In the
course of a few weeks, a white felting appears which later changes to
red. This slimy coat below the mold layer is made up of diverse species
of bacteria and fungi that are able to grow after the acid is reduced by
the blue mold. The organisms in the red slimy coat act upon the casein,
producing an alkaline reaction that is unfavorable to the growth of the
blue mold. Two sets of organisms are, therefore essential in the
ripening process, one preparing the soil for the ferment that later
produces the requisite ripening changes. As ordinarily carried on, the
process is an empirical one, and if the red coat does not develop as
expected, the maker resorts to all kinds of devices to bring out the
desired ferment. The appearance of the right form is dependent, however,
upon the proper reaction of the cheese, and if this is not suitable, the
wished-for growth will not appear.
INFLUENCE OF BACTERIA IN ABNORMAL CHEESE PROCESSES.
The reason why cheese is more subject to abnormal fermentation than
butter is because its high nitrogen content favors the continued
development of bacteria for some time after it is made. It must be
borne in mind, in considering the more important of these changes, that
not all defective conditions in cheese are attributable to the influence
of living organisms. Troubles frequently arise from errors in
manufacturing details, as too prolonged cooking of curds, too high
heating, or the development of insufficient or too much acid. Then
again, the production of undesirable flavors or impairment in texture
may arise from imperfect curing conditions.
Our knowledge regarding the exact nature of these indefinite faults is
as yet too inadequate to enable many of these undesirable conditions to
be traced to their proper source; but in many cases the taints observed
in a factory are due to the abnormal development of certain bacteria,
capable of evolving unpleasant or even putrid odors. Most of them are
seeded in the milk before it comes to the factory and are due to
careless manipulation of the milk while it is still on the farm. Others
gain access to the milk in the factory, owing to unclean conditions of
one sort or another. Sometimes the cheese-maker is able to overcome
these taints by vigorous treatment, but often they pass on into the
cheese, only to detract from the market value of the product. Most
frequently these "off" flavors appear in cheese that are cured at too
high temperatures, say above 65 deg. F.
~"Gassy" fermentations in cheese.~ One of the worst and at the same time
most common troubles in cheese-making is where the cheese undergoes a
fermentation marked by the evolution of gas. The presence of gas is
recognized by the appearance either of spherical or lens-shaped holes of
various sizes in the green cheese; often they appear in the curd before
it is put to press. Usually in this condition the curds look as if they
had been punctured with a pin, and are known as "pin holey" curds. Where
the gas holes are larger, they are known as "Swiss holes" from their
resemblance to the normal holes in the Swiss product. If the development
of gas is abundant, these holes are restricted in size. Often the
formation of gas may be so intense as to cause the curds to float on the
surface of the whey before they are removed. Such curds are known as
"floaters" or "bloaters."
If "gassy" curds are put to press, the abnormal fermentation may
continue. The further production of gas causes the green cheese to
"huff" or swell, until it may be considerably distorted as in Fig. 33.
In such cases the texture of the cheese is greatly injured, and the
flavor is generally impaired.
[Illustration: FIG. 33. Cheese made from gassy milk.]
Such abnormal changes may occur at any season of the year, but the
trouble is most common in summer, especially in the latter part.
This defect is less likely to occur in cheese that is well cheddared
than in sweet curd cheese. When acidity is produced, these gassy
fermentations are checked, and in good cheddar the body is so close and
firm as not readily to permit of gaseous changes.
In Swiss cheese, which is essentially a sweet curd cheese, these
fermentations are very troublesome. Where large holes are formed in
abundance (blaehen), the trouble reaches its maximum. If the gas holes
are very numerous and therefore small it is called a "nissler."
Sometimes the normal "eyes" are even wanting when it is said to be
"blind" or a "glaesler."
[Illustration: FIG. 34. Block Swiss cheese showing "gassy"
fermentation.]
One method of procedure which is likely to cause trouble in Swiss
factories is often produced by the use of sour, fermented whey in which
to soak the natural rennets. Freudenreich and Steinegger[209] have shown
that a much more uniform quality of cheese can be made with rennet
extract if it is prepared with a starter made from a pure lactic
ferment.
The cause of the difficulty has long been charged to various sources,
such as a lack of aeration, improper feeding, retention of animal gases,
etc., but in all these cases it was nothing more than a surmise. Very
often the milk does not betray any visible symptom of fermentation when
received, and the trouble is not to be recognized until the process of
cheese-making is well advanced.
Studies from a biological standpoint have, however, thrown much light on
this troublesome problem; and it is now known that the formation of gas,
either in the curd or after it has been put to press, is due entirely to
the breaking down of certain elements, such as the sugar of milk, due to
the influence of various living germs. This trouble is, then, a type
fermentation, and is, therefore, much more widely distributed than it
would be if it was caused by a single specific organism. These
gas-producing organisms are to be found, sparingly at least, in almost
all milks, but are normally held in check by the ordinary lactic
species. Among them are a large number of the bacteria, although yeasts
and allied germs are often present and are likewise able to set up
fermentative changes of this sort. In these cases the milk-sugar is
decomposed in such a way as to give off CO_{2} and H, and in some cases,
alcohol. Russell and Hastings[210] found a lactose-splitting yeast in a
severe outbreak of gassy cheese in a Swiss factory. In this case the gas
did not develop until the cheese were a few weeks old. In severe cases
the cheese actually cracked to pieces.
According to Guillebeau, a close relation exists between those germs
that are able to produce an infectious inflammation (mastitis) in the
udder of the cow and some forms capable of gas evolution.
If pure cultures of these gas-producing bacteria are added to perfectly
sweet milk, it is possible to artificially produce the conditions in
cheese that so frequently appear in practice.
~Treatment of "pin-holey" curds.~ When this type of fermentation appears
during the manufacture of the cheese, the maker can control it in part
within certain limits. These methods of treatment are, as a rule, purely
mechanical, as when the curds are piled and turned, and subsequently
ground in a curd mill. After the gas has been forced out, the curds are
then put to press and the whole mats into a compact mass.
Another method of treatment based upon bacteriological principles is the
addition of a starter to induce the formation of acid. Where acid is
developed as a result of the growth of the lactic-acid bacteria, the
gas-producing species do not readily thrive. Another reason why acid
aids in repressing the development of gas is that the curd particles are
partially softened or digested by the action of the acid. This causes
them to mat together more closely, and there is not left in the cheese
the irregular mechanical openings in which the developing gas may find
lodgment.
Another method that is also useful with these curds is to employ salt.
This represses gaseous fermentations, and the use of more salt than
usual in making the cheese will very often restrain the production of
gas. Tendency to form gas in Edam cheese is controlled by the addition
of a starter prepared from slimy whey (lange wei) which is caused by the
development of an acid-forming organism.
Some have recommended the custom of washing the curds to remove the whey
and the gas-producing bacteria contained therein. Care must be taken not
to carry this too far, for the removal of the sugar permits
taint-producing organisms to thrive.[211]
The temperature at which the cheese is cured also materially affects the
development of gas. At high curing temperatures, gas-producing organisms
develop rapidly; therefore more trouble is experienced in summer than at
other seasons.
If milks which are prone to undergo "gassy" development are excluded
from the general supply, it would be possible to eliminate the source of
the entire trouble. To aid in the early recognition of such milks that
are not apparently affected when brought to the factory, fermentation or
curd tests (p. 76) are of great value. The use of this test in the hands
of the factory operator often enables him to detect the exact source of
the trouble, which may frequently be confined to the milk delivered by a
single patron.
~"Fruity" or "sweet" flavor.~ Not infrequently the product of a factory
may acquire during the process of ripening what is known as a "sweet" or
"fruity" flavor. This flavor resembles the odor of fermented fruit or
the bouquet of certain kinds of wine. It has been noted in widely
different sections of the country and its presence bears no relation to
the other qualities of the cheese. The cause of this trouble has
recently been traced[212] to the presence of various kinds of yeasts.
Ordinarily yeasts are rarely present in good cheese, but in cheese
affected with this trouble they abound. The addition of starters made
from yeast cultures resulted in the production of the undesirable
condition.
~Mottled cheese.~ The color of cheese is sometimes cut to that extent that
the cheese presents a wavy or mottled appearance. This condition is apt
to appear if the ripening temperature is somewhat high, or larger
quantities of rennet used than usual. The cause of the defect is
obscure, but it has been demonstrated that the same is communicable if a
starter is made by grating some of this mottled cheese into milk. The
bacteriology of the trouble has not yet been worked out, but the defect
is undoubtedly due to an organism that is able to grow in the ripening
cheese. It has been claimed that the use of a pure lactic ferment as a
starter enables one to overcome this defect.
~Bitter cheese.~ Bitter flavors are sometimes developed in cheese
especially where the ripening process is carried on at a low temperature
in the presence of an excess of moisture for a considerable length of
time.
Guillebeau[213] isolated several forms from Emmenthaler cheese which he
connected with udder inflammation that were able to produce a bitter
substance in cheese.
Von Freudenreich[214] has described a new form _Micrococcus casei amari_
(micrococcus of bitter cheese) that was found in a sample of bitter
cheese. This germ is closely related to Conn's micrococcus of bitter
milk. It develops lactic acid rapidly, coagulating the milk and
producing an intensely bitter taste in the course of one to three days.
When milk infected with this organism is made into cheese, there is
formed in a few days a decomposition product that imparts a marked
bitter flavor to the cheese.
Harrison[215] has recently found a yeast that grows in the milk and also
in the cheese which produces an undesirable bitter change.
It is peculiar that some of the organisms that are able to produce
bitter products in milk do not retain this property when the milk is
worked up into cheese.
~Putrid or rotten cheese.~ Sometimes cheese undergoes a putrefactive
decomposition in which the texture is profoundly modified and various
foul smelling gases are evolved. These often begin on the exterior as
small circumscribed spots that slowly extend into the cheese, changing
the casein into a soft slimy mass. Then, again, the interior of the
cheese undergoes this slimy decomposition. The soft varieties are more
prone toward this fermentation than the hard, although the firm cheeses
are by no means exempt from the trouble. The "Verlaufen" or "running" of
limburger cheese is a fermentation allied to this. It is where the
inside of the cheese breaks down into a soft semi-fluid mass. In severe
cases, the rind may even be ruptured, in which case the whole interior
of the cheese flows out as a thick slimy mass, having sometimes a putrid
odor. The conditions favoring this putrid decomposition are usually
associated with an excess of moisture, and an abnormally low ripening
temperature.
~Rusty spot.~ This name is applied to the development of small
yellowish-red or orange spots that are formed sometimes throughout the
whole mass of cheddar cheese. A close inspection shows the colored
points to be located along the edges of the curd particles. According to
Harding,[216] this trouble is most common in spring and fall. The cause
of the difficulty has been traced by Connell[217] to the development of
a chromogenic bacterium, _Bacillus rudensis_. The organism can be most
readily isolated on a potato surface rather than with the usual
isolating media, agar or gelatin.
~Other pigment changes.~ Occasionally, with the hard type of cheese, but
more frequently with the softer foreign varieties, various abnormal
conditions arise that are marked by the production of different pigments
in or on the cheese. More frequently these are merely superficial and
affect only the outer layers of the cheese. Generally they are
attributable to the development of certain chromogenic organisms
(bacteria, molds and yeasts), although occasionally due to other causes,
as in the case of a blue discoloration sometimes noted in foreign cheese
made in copper kettles.[218]
De Vries[219] has described a blue condition that is found in Edam
cheese. It appears first as a small blue spot on the inside, increasing
rapidly in size until the whole mass is affected. This defect he was
able to show was produced by a pigment-forming organism, _B.
cyaneo-fuscus_. By the use of slimy whey (lange wei) this abnormal
change was controlled.
~Moldy cheese.~ With many varieties of cheese, especially some of the
foreign types, the presence of mold on the exterior is not regarded as
detrimental; in fact a limited development is much desired. In hard
rennet cheese as cheddar or Swiss, the market demands a product free
from mold, although it should be said that this condition is imposed by
the desire to secure a good-looking cheese rather than any injury in
flavor that the mold causes. Mold spores are so widely distributed that,
if proper temperature and moisture conditions prevail, these spores will
always develop. At temperatures in the neighborhood of 40 deg. F. and
below, mold growth is exceedingly slow, and often fructification does
not occur, the only evidence of the mold being the white, felt-like
covering that is made up of the vegetating filaments. The use of
paraffin has been suggested as a means of overcoming this growth, the
cheese being dipped at an early stage into melted paraffin. Recent
experiments have shown that "off" flavors sometimes develop where cheese
are paraffined directly from the press. If paraffin is too hard, it has
a tendency to crack and separate from the rind, thus allowing molds to
develop beneath the paraffin coat, where the conditions are ideal as to
moisture, for evaporation is excluded and the air consequently
saturated. The use of formalin (2% solution) has been suggested as a
wash for the outside of the cheese. This substance or sulfur is also
applied in a gaseous form. Double bandaging is also resorted to as a
means of making the cheese more presentable through the removal of the
outer bandage.
The nature of these molds has not been thoroughly studied as yet. The
ordinary blue-green bread mold, _Penicillium glaucum_, is most
frequently found, but there are numerous other forms that appear,
especially at low temperatures.
~Poisonous cheese.~ Cases of acute poisoning arising from the ingestion of
cheese are reported from time to time. Vaughan has succeeded in showing
that this condition is due to the formation of a highly poisonous
alkaloid which he has isolated, and which he calls _tyrotoxicon_.[220]
This poisonous ptomaine has also been demonstrated in milk and other
milk products, and is undoubtedly due to the development of various
putrefactive bacteria that find their way into the milk. It seems quite
probable that the development of these toxic organisms can also go on
in the cheese after it is taken from the press.
~Prevention or cheese defects.~ The defective conditions previously
referred to can rarely be overcome in cheese so as to improve the
affected product, for they only become manifest in most cases during the
later stages of the curing process. The only remedy against future loss
is to recognize the conditions that are apt to prevail during the
occurrence of an outbreak and see that the cheese are handled in such a
way as to prevent a recurrence of the difficulty.
Many abnormal and undesirable results are incident to the manufacture of
the product, such as "sour" or "mealy" cheese, conditions due to the
development of too much acid in the milk or too high a "cook." These are
under the direct control of the maker and for them he alone is
responsible. The development of taints due to the growth of unwelcome
bacteria that have gained access to the milk while it is yet on the farm
are generally beyond the control of the cheese maker, unless they are so
pronounced as to appear during the handling of the curds. If this does
occur he is sometimes able, through the intervention of a starter or by
varying some detail in making, to handle the milk in such a way as to
minimize the trouble, but rarely is he able to eliminate it entirely.
One of the most strenuous duties which the maker must perform at all
times is to point out to his patrons the absolute necessity of their
handling the milk in such a way as to prevent the introduction of
organisms of a baleful type.
FOOTNOTES:
[178] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 112; Campbell, Trans.
High. & Agr. Soc. Scotland, 5 ser., 1898, 10:181.
[179] Winkler, Milch Zeit. (Hildesheim), Nov. 24, 1900.
[180] Campbell, No. Brit., Agric., May 12, 1897.
[181] Weigmann, Milch Zeit., No. 50, 1889.
[182] Klein, Milch Zeit. (Hildesheim), No. 17, 1900.
[183] Adametz, Landw. Jahr., 18:256.
[184] Van Slyke and Hart, Bull. 214, N. Y. Expt. Stat., July 1902.
[185] Milch Zeit., 1898, No. 49.
[186] Lafar, Technical Mycology, p. 216.
[187] Adametz, Landw. Jahr., 18:228.
[188] Freudenreich, Landw. Jahr. d. Schweiz, 4:17; 5:16.
[189] Russell, 13 Rept. Wis. Expt. Stat., 1896, p. 95.
[190] Harrison and Connell, Rev. gen. du Lait, Nos. 4, 5, 6, 7 and 8,
1903-04.
[191] Lloyd, Bath and West of Eng. Soc. Rept., 1892, 2:180.
[192] Freudenreich, Landw. Jahr. d. Schweiz, 1900; Adametz, Oest. Molk.
Zeit., 1899, No. 7.
[193] Russell, 14 Wis. Expt. Stat., 1897, p. 203. Harrison and Connell,
Rev. gen. du Lait Nos. 4, etc., 1903-04.
[194] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901. Dean,
Harrison and Harcourt, Bull. 121, Ont. Agr'l. Coll., June 1902.
[195] Schaffer, Milch Zeit., 1889, p. 146.
[196] Adametz, Landw. Jahr., 18:261.
[197] Duclaux, Le Lait, p. 213.
[198] Adametz, Oest. Molk. Zeit., 1900, Nos. 16-18.
[199] Freudenreich, Landw. Jahr. d. Schweiz, 1897, p. 85.
[200] Weigmann, Cent. f. Bakt., II Abt., 1898, 4:593; also 1899, 5:630.
[201] Gorini, Abs. in Expt. Stat. Rec., 11:388.
[202] Babcock and Russell, 14 Rept. Wis. Expt. Stat., 1897, p. 161.
[203] Jensen, Cent. f. Bakt., II Abt., 3:752.
[204] Freudenreich, Cent. f. Bakt., II Abt., 1900, 6:332.
[205] Jensen, Ibid., 1900, 6:734.
[206] 17 Rept. Wis. Expt. Stat., 1900, p. 102.
[207] Jensen, Landw. Jahr. d. Schweiz, 1900.
[208] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
[209] Cent. f. Bakt. 1899, p. 14.
[210] Bull. 128, Wis. Expt. Stat., Sept. 1905.
[211] Babcock and Russell, 18 Rept. Wis. Expt. Stat., 1901.
[212] Harding, Rogers and Smith, Bull. 183, N. Y. (Geneva) Expt. Stat.,
Dec., 1900.
[213] Guillebeau, Landw. Jahr., 1890, p. 27.
[214] Freudenreich, Fueehl. Landw. Ztg., 43:361.
[215] Harrison, Bull. 123 Ont. Agr'l. Coll., May, 1902.
[216] Bull. 183, N. Y. (Geneva) Expt. Stat., Dec. 1900.
[217] Connell, Bull. Canadian Dept. of Agr., 1897.
[218] Schmoeger, Milch Zeit., 1883, p. 483.
[219] De Vries, Milch Zeit., 1888, pp. 861, 885.
[220] Zeit. f. physiol. Chemie, 10:146.
INDEX.
Acid, effect of, on churning, 137;
in butter-making, 138.
Acid test, 52.
Aeration of milk, 59.
Aerobic bacteria, 7.
Alcoholic fermentation in milk, 72.
Anaerobic bacteria, 7.
Animal, influence of, on milk infection, 34.
Animal odor, 56.
Anthrax, 94.
Antiseptics, 9, 88.
Aroma, of butter, 140.
Bacillus: definition of, 2.
_acidi lactici_, 64;
_cyaneo-fuscus_, 188;
_cyanogenus_, 74;
_foetidus lactis_, 157;
_lactis aerogenes_, 65;
_lactis erythrogenes_, 74;
_lactis saponacei_, 67;
_lactis viscosus_, 71;
_nobilis_, 162, 174;
_prodigiosus_, 74;
_rudensis_, 188;
_synxanthus_, 75;
_tuberculosis_, 84.
Bacteria:
on hairs, 35;
kinds in milk, 63;
in barn air, 42;
in milk pails, 27;
in butter, 154;
classification of, 4;
in cheese, 160;
culture of, 17;
in cream, 128;
discovery of, 1;
external conditions affecting, 8;
form of, 2;
in butter, 142;
in butter-making, 127;
in centrifuge slime, 39;
In fore milk, 28;
in rennet, 163;
In separator slime, 39;
manure, 37;
number of, in milk, 50.
Distribution of:
milk of American cities, 50;
European cities, 50;
in relation to cheese, 168.
Of disease:
anthrax, 94;
cholera, 98;
diphtheria, 99;
lockjaw, 94;
toxic, 100;
tuberculosis, 84;
typhoid fever, 98.
Methods of study of:
culture, 15;
culture media, 13;
isolation, 14.
Bitter butter, 158;
cheese, 189;
milk, 72.
Bloody milk, 74.
Blue cheese, 191;
milk, 74.
Bovine tuberculosis, 84.
Brie cheese, 182.
Butter:
bacteria in, 154;
bitter, 158;
"cowy," 157;
fishy, 159;
lardy, 157;
moldy, 158;
mottled, 156;
oily, 158;
putrid, 156;
rancid, 155;
tallowy, 157;
turnip flavor in, 157.
Making:
aroma, 140;
flavor in, 140;
pure culture, 143;
in ripening of cream, 136.
Butyric acid fermentation, 69.
By-products of factory, methods of preserving, 25.
Casease, 68.
Caseone, 68.
Centrifugal force, cleaning milk by, 38.
Cheese:
bacterial flora of, 168;
bitter, 189;
blue, 187;
Brie, 182;
Edam, 72, 162;
Emmenthaler, 185;
flavor of, 179;
gassy fermentations in, 183;
Gorgonzola, 180;
molds on, 191;
mottled, 189;
"nissler," 185;
poisonous, 192;
putrid, 190;
ripening of moldy, 180;
ripening of soft, 181;
Roquefort, 180;
rusty spot in, 188;
Stilton, 180;
Swiss, 185.
Making and curing:
chemical changes in curing, 166;
influence of temperature on curing, 169;
influence of rennet, 177;
physical changes in curing, 165;
prevention of defects, 193;
starters in, 161;
temperature in relation to bacterial influence, 169.
Theories of curing:
digestive, 173;
galactase, 175, 177;
lactic acid, 174.
Chemical changes in cheese-ripening, 166.
Chemical disinfectants in milk:
bleaching powder, 81;
corrosive sublimate, 81;
formalin, 80;
sulfur, 80;
whitewash, 81;
vitriol, 81.
Chemical preservatives, 80.
Children, milk for, 45.
Cholera in milk, 98.
Classification by separator, 38.
Coccus, definition of, 2.
Cold, influence on bacteria, 8, 48.
Contamination of milk through disease germs, 95, 191.
Covered milk pails, 41.
Cream, bacterial changes in, 135;
mechanical causes for bacteria in, 135;
pasteurized, 113;
restoration of consistency of pasteurized, 132.
Ripening of, 136;
advantage of pure cultures in, 144;
by natural starters, 142;
characteristics of pure cultures in, 145;
objections to pure cultures in, 146;
principles of pure cultures in, 143;
propagation of pure cultures, 151;
purity of commercial starters, 150;
home-made starters in, 146.
Creaming methods, 134.
Curd test, 76.
Dairy utensils a source of contamination, 21.
Diarrhoeal diseases, 100.
Digesting bacteria, 67.
Digestibility of heated milk, 111.
Diphtheria, 99.
Dirt in milk, 34.
Dirt, exclusion of, 36.
Disease germs in milk, 95;
effect of heat on, 91;
origin of, 83.
Disinfectants, 9:
carbolic acid, 81;
chloride of lime, 81;
corrosive sublimate, 81;
formalin, 80;
sulfur, 80;
vitriol salts, 81;
whitewash, 79.
Disinfectants in milk:
alkaline salts, 106;
boracic acid, 106;
formalin, 106;
preservaline, 107;
salicylic acid, 106.
Domestic pasteurizing apparatus, 119.
Drugs, taints in milk due to, 56.
Drying, effect of, 8.
Edam cheese, 72, 162.
Emmenthaler cheese, 185.
Endospores, 3.
Enzyms, 10.
Factory by-products, 22;
treatment of, 25.
Farrington alkaline tablet, 52.
Fecal bacteria, effect of, on butter, 35.
Fermentation:
In cheese: gassy, 183.
In milk:
alcoholic, 72;
bitter, 72;
blue, 74;
butyric, 69;
digesting, 67;
gassy, 66;
kephir, 72;
koumiss, 72;
lactic acid, 63;
lange-wei, 72;
red, 74;
ropy, 69;
slimy, 69;
soapy, 73;
souring, 63;
sweet curdling, 67;
treatment of, 75.
Tests, 76;
Gerber's, 76;
Walther's, 76;
Wisconsin curd, 76.
Filtration of milk, 38.
Fishy butter, 159.
Flavor:
of butter, 140;
of cheese, 179.
Foot and mouth disease, 93.
Fore milk, 28.
Formaldehyde, 80.
Formalin, 80.
Fruity flavor in cheese, 188.
Galactase in cheese, 175.
Gassy fermentations:
in cheese, 183;
in milk, 67;
in Swiss cheese, 167.
Glaesler, 185.
Gorgonzola cheese, 180.
Growth of bacteria, essential conditions for, 4;
in milk, 46.
Hair, bacteria on, 35.
Heat, influence on bacterial growth, 8.
Heated milk:
characteristics of, 109;
action toward rennet, 112;
body, 110;
digestibility, 111;
fermentative changes, 111;
flavor, 110;
hydrogen peroxid test in, 23;
Storch's test, 23.
Hygienic milk, bacteria in, 45.
Infection of milk:
animal, 34;
dairy utensils, 21;
fore milk, 28;
milker, 36.
Isolation of bacteria, methods of, 14.
Kephir, 72.
Koumiss, 72.
Lactic acid:
fermentation in milk, 63;
theory in cheese-curing, 174.
Lange-wei, 72.
Lardy butter, 157.
Light, action on bacteria, 9.
Manure, bacteria in, 33.
Methods:
of isolation, 14;
culture, 15.
_Micrococcus casei amari_, 189.
Microscope, use of, 17.
Milk:
a bacterial food medium, 19;
bacteria in, 48.
Disease organisms in:
anthrax, 94;
cholera, 98;
diphtheria, 99;
foot and mouth disease, 93;
poisonous, 101;
ptomaines, 101;
scarlet fever, 99;
tuberculosis, 84;
typhoid fever, 98.
Contamination, 20:
from air, 42;
from animal odors, 55;
dirt, 34;
distinction between bacterial and non-bacterial, 57;
fore milk, 28;
infection in factory, 59;
milker, 36;
relative importance of various kinds, 43;
utensils, 21.
Milk fermentations:
alcoholic, 72;
bitter, 72;
bloody, 74;
blue, 74;
butyric acid, 69;
gassy, 66, 167;
kephir, 72;
koumiss, 72;
lactic acid, 63;
red, 72;
ropy, 69;
slimy, 69;
soapy, 74;
souring, 63;
sweet curdling, 67;
tests for, 76;
treatment of, 75;
yellow, 75.
Milk, heated:
action towards rennet, 112;
digestibility, 111;
flavor of, 110;
fermentative changes in, 111;
hydrogen peroxid test, 110.
Milking machines, influence of, on germ content, 37.
Milk preservation:
chemical agents in, 106;
condensation, 107;
freezing, 108;
heat, 108;
pasteurization, 113;
sterilization, 112.
Milk-sugar as bacterial food, 19.
Mold, in butter, 158;
in cheese, 191.
Mottled cheese, 189.
"Nissler" cheese, 185.
Odors, direct absorption of, in milk, 55.
_Oidium lactis_, 159.
Oily butter, 158.
Pasteurization of milk;
acid test in, 128;
bacteriological study of, 124, 126, 149;
for butter, 147;
for cheese, 162;
for direct use, 113;
of skim milk, 25;
details of, 128;
temperature and time limit in, 118.
Pasteurizing apparatus:
continuous flow, 122;
coolers, 131;
Danish, 123;
domestic, 119;
Farrington, 122;
intermittant flow, 121;
Miller, 122;
Potts, 121;
regenerator, 122;
Reid, 126;
Russell, 121;
testing rate of flow, 124.
_Penicillium glaucum_, 159, 180, 190.
Pepsin, 10.
Physical changes in cheese-ripening, 165.
Poisonous bacteria:
in cheese, 192;
in milk, 100, 101.
Preservaline, 167.
Preservation of milk:
by exclusion, 103;
chemical agents, 106;
condensing, 107;
filtration, 38;
freezing, 108;
pasteurization, 112;
physical agents, 107;
sterilization, 112.
Ptomaine poisoning, 101.
Pure cultures, 15.
Pure culture starters:
advantages of, 144;
characteristics of, 145;
home-made cultures compared with, 146;
propagation of, 151.
Putrid cheese, 190;
butter, 156.
Rancidity in butter, 155.
Red milk, 74.
Rennet:
action in heated milk, 112;
bacteria in, 163;
influence of, on cheese-ripening, 177.
Restoration of consistency in pasteurized cream, 132.
Ripening of cheese:
moldy cheese, 180;
soft cheese, 181.
Of cream, 136;
artificial starters, 143;
natural starters, 142;
principles of pure culture starters in, 143.
Ropy milk, 69.
Roquefort cheese, 180.
Rusty spot in cheese, 190.
Rusty cans: effect of, on acidity, 53.
Sanitary milk, 45, 104.
Sanitary pails, 41.
Scarlet fever in milk, 99.
Separator slime:
bacteria in, 39;
tubercle bacillus in, 93.
Scalded layer, resistance of bacteria in, 91.
Skim-milk, a distributor of disease, 96.
Slimy milk, 69.
Soapy milk, 74.
Soft cheese, ripening of, 186.
Sources of contamination in milk:
barn air, 42;
dairy utensils, 21;
dirt from animals, 34;
factory cans, 25;
fore-milk, 28;
milker, 36.
Souring of milk, 63.
Spirillum, definition of, 2.
Spores, 3.
Starters:
in cheese-making, 161;
in butter-making, 142;
propagation of, 151;
pure cultures in cream-ripening, 143.
Sterilization of milk, 112.
_Streptococcus Hollandicus_, 72, 162.
Stilton cheese, 181.
Storch's test, 23.
Sulfur as a disinfectant, 81.
Sweet curdling milk, 68.
Sweet flavor in cheese, 188.
Swiss cheese, 177;
gassy fermentations in, 24, 185.
Taints, absorption of, 55.
Taints, bacterial vs. physical, 58.
Taints in milk, absorption of, 55.
Taints, use of starters in overcoming, 79.
Taints in butter:
putrid, 156;
rancidity, 155;
turnip flavor, 157.
Tallowy butter, 157.
Temperature:
effect on bacterial development, 6, 48;
effect of low, 108;
effect of high, 108;
and time limit in milk pasteurization, 113.
Tests for milk:
fermentation, 76;
Storch's, 23;
acid, 52.
Theories in cheese-curing:
digestive, 171;
galactase, 175, 177;
lactic acid, 174.
Trypsin, 10.
Tubercle bacillus:
in milk, 88;
in separator slime, 93;
thermal death limits, 117.
Tuberculin test, 86.
Tuberculosis, bovine, 84.
Turnip flavor in butter, 157.
Typhoid fever, 98.
Tyrogen, 162.
Tyrotoxicon, 101, 190.
Udder:
artificial introduction of bacteria into, 32;
milk germ-free in, 19;
infection of, 28;
washing, 89;
tuberculosis in, 87.
Viscogen, 132.
Water: as a source of infection, 61.
Whey, pollution of vats, 23;
method of preserving, 25;
treatment of, in vats, 25.
Whitewash, 81.
Wisconsin curd test, 76.
Yeasts:
alcoholic ferments in milk, 73;
fruity flavor in cheese, 186;
gassy due to yeasts, 186;
in bitter cheese, 189;
in canned butter, 159;
kephir, 72.