FERMENTATIONS IN MILK AND THEIR TREATMENT.
Under the conditions in which milk is drawn, it is practically
impossible to secure the same without bacterial contamination. The
result of the introduction of these organisms often changes its
character materially as most bacteria cause the production of more or
less pronounced fermentative processes. Under normal conditions, milk
sours, i. e., develops lactic acid, but at times this more common
fermentation may be replaced by other changes which are marked by the
production of some other more or less undesirable flavor, odor or change
in appearance.
In referring to these changes, it is usually customary to designate them
after the most prominent by-product formed, but it must be kept in mind
that generally some other decomposition products are usually produced.
Whether the organisms producing this or that series of changes prevail
or not depends upon the initial seeding, and the conditions under which
the milk is kept. Ordinarily, the lactic acid organisms grow so
luxuriantly in the milk that they overpower all competitors and so
determine the nature of the fermentation; but occasionally the milk
becomes infected with other types of bacteria in relatively large
numbers and the conditions may be especially suitable to the development
of these forms, thereby modifying the course of the normal changes that
occur.
The kinds of bacteria that find it possible to develop in milk may be
included under two heads:
1. Those which cause no appreciable change in the milk, either in taste,
odor or appearance. While these are frequently designated as the inert
bacteria, it must not be supposed that they have absolutely no effect on
milk. It is probably true in most cases that slight changes of a
chemical nature are produced, but the nature of the changes do not
permit of ready recognition.
2. This class embraces all those organisms which, as a result of their
growth, are capable of producing evident changes. These transformations
may be such as to affect the taste, as in the sour milk or in the bitter
fermentations, or the odor, as in some of the fetid changes, or the
appearance of the milk, as in the slimy and color changes later
described.
~Souring of milk.~ Ordinarily if milk is allowed to stand for several days
at ordinary temperatures it turns sour. This is due to the formation of
lactic acid, which is produced by the decomposition of the milk-sugar.
While this change is well nigh universal, it does not occur without a
pre-existing cause, and that is the presence of certain living bacterial
forms. These organisms develop in milk with great rapidity, and the
decomposition changes that are noted in souring are due to the
by-products of their development.
The milk-sugar undergoes fermentation, the chief product being lactic
acid, although various other by-products, as other organic acids
(acetic, formic and succinic), different alcohols and gaseous products,
as CO_{2}, H, N and methane (CH_{4}) are produced in small amounts.
In this fermentation, the acidity begins to be evident to the taste when
it reaches about 0.3 per cent., calculated as lactic acid. As the
formation of acid goes on, the casein is precipitated and incipient
curdling or lobbering of the milk occurs. This begins to be apparent
when the acidity is about 0.4 per cent., but the curd becomes more solid
with increasing acidity. The rapidity of curdling is also dependent upon
the temperature of the milk. Thus milk which at ordinary temperatures
might remain fluid often curdles when heated. The growth of the bacteria
is continued until about 0.8 to 1.0 per cent. acid is formed, although
the maximum amount fluctuates considerably with different lactic acid
species. Further formation then ceases even though all of the milk-sugar
is not used up, because of the inability of the lactic bacteria to
continue their growth in such acid solutions.
As this acidity is really in the milk serum, cream never develops so
much acid as milk, because a larger proportion of its volume is made up
of butter-fat globules. This fact must be considered in the ripening of
cream in butter-making where the per cent. of fat is subject to wide
fluctuations.
The formation of lactic acid is a characteristic that is possessed by a
large number of bacteria, micrococci as well as bacilli being numerously
represented. Still the preponderance of evidence is in favor of the view
that a few types are responsible for most of these changes. The most
common type found in spontaneously soured milk changes the milk-sugar
into lactic acid without the production of any gas. This type has been
described by various workers on European as well as American milks, and
is designated by Conn as the _Bact. lactis acidi_ type.[51] It is
subject to considerable variation under different conditions.
Curiously enough if milk which has been drawn with special care is
examined immediately after milking, the lactic organisms are not usually
found. They are incapable of development in the udder itself, as shown
by injections into the milk cistern. They abound, however, on hay, in
dust, in the barn air, on the hairy coat of the animal, and from these
sources easily gain access to the milk. In this medium they find an
exceptionally favorable environment and soon begin a very rapid growth,
so that by the time milk is consumed, either in the form of milk or milk
products, they make up numerically the larger portion of the bacteria
present.
Another widely disseminated, although numerically less prevalent, type
is _B. lactis aerogenes_. This type forms gas in milk so that the soured
milk is torn by the presence of gas bubbles. It also grows more
luxuriantly in contact with the air.
Other types occur more or less sporadically, some of which are capable
of liquefying the casein of milk while at the same time they also
develop lactic acid. Conn and Aikman refer to the fact that over one
hundred species capable of producing variable quantities of lactic acid
are already known. It is fair to presume, however, that a careful
comparative study of these would show that simply racial differences
exist in many cases, and therefore, that they are not distinct species.
As a group these bacteria are characterized by their inability to
liquefy gelatin or develop spores. On account of this latter
characteristic they are easily destroyed when milk is pasteurized. They
live under aerobic or anaerobic conditions, many of them being able to
grow in either environment, although, according to McDonnell,[52] they
are more virulent when air is not excluded.
While growth of these lactic forms may go on in milk throughout a
relatively wide range in temperature, appreciable quantities of acid are
not produced except very slowly at temperatures below 50 deg. F.[53]
From the standpoint of frequency the most common abnormal changes that
occur in milk are those in which gases of varying character are
developed in connection with acids, from the milk sugar. Other volatile
products imparting bad flavors usually accompany gas production. These
fermentations are of most serious import in the cheese industry, as they
are especially prone to develop in the manufacture of milk into certain
types of cheese. Not often is their development so rapid that they
appear in the milk while it is yet in the hands of the milk producer,
but almost invariably the introduction of the causal organisms takes
place while the milk is on the farm. Numerous varieties of bacteria
possess this property of producing gas (H and CO_{2} are most common
although N and methane (CH_{4}) are sometimes produced). The more common
forms are those represented by _B. lactis aerogenes_ and the common
fecal type, _B. coli commune_. The ordinary habitat of this type is dirt
and intestinal filth. Hence careless methods of milk handling invite
this type of abnormal change in milk.
It is a wide-spread belief that thunder storms cause milk to sour
prematurely, but this idea has no scientific foundation. Experiments[54]
with the electric spark, ozone and loud detonations show no effect on
acid development, but the atmospheric conditions usually incident to a
thunder storm are such as permit of a more rapid growth of organisms.
There is no reason to believe but that the phenomenon of souring is
wholly related to the development of bacteria. Sterile milks are never
affected by the action of electric storms.
~"Gassy" milks.~ Where these gas bacteria abound, the amount of lactic
acid is generally reduced, due to the splitting up of some of the sugar
into gaseous products. This type of germ life does not seem to be able
to develop well in the presence of the typical lactic acid non
gas-forming bacteria.
[Illustration: FIG. 16. Cheese made from "gassy" milk.]
~"Sweet curdling" and digesting fermentations.~ Not infrequently milk,
instead of undergoing spontaneous souring, curdles in a weakly acid or
neutral condition, in which state it is said to have undergone "sweet
curdling." The coagulation of the milk is caused by the action of enzyms
of a rennet type that are formed by the growth of various species of
bacteria. Later the whey separates more or less perfectly from the curd,
producing a "wheyed off" condition. Generally the coagulum in these
cases is soft and somewhat slimy. The curd usually diminishes in bulk,
due to the gradual digestion or peptonization of the casein by
proteid-dissolving enzyms (tryptic type) that are also produced by the
bacteria causing the change.
A large number of bacteria possess the property of affecting milk in
this way. So far as known they are able to liquefy gelatin (also a
peptonizing process) and form spores. The Tyrothrix type of bacteria (so
named by Duclaux on account of the supposed relation to cheese ripening)
belongs to this class. The hay and potato forms are also digesters.
Organisms of this type are generally associated with filth and manure,
and find their way into the milk from the accumulations on the coat of
the animal.
Conn[55] has separated the rennet enzym from bacterial cultures in a
relatively pure condition, while Fermi[56] has isolated the digestive
ferment from several species.
Duclaux[57] has given to this digesting enzym the name _casease_ or
cheese ferment. These isolated ferments when added to fresh milk possess
the power of causing the characteristic curdling and subsequent
digestion quite independent of cell development. The quantity of ferment
produced by different species differs materially in some cases. In these
digestive fermentations, the chemical transformations are profound, the
complex proteid molecule being broken down into albumoses, peptones,
amido-acids (tyrosin and leucin) and ammonia as well as fatty acids.
Not infrequently these fermentations gain the ascendency over the normal
souring change, but under ordinary conditions they are held in abeyance,
although this type of bacteria is always present to some extent in milk.
When the lactic acid bacteria are destroyed, as in boiled, sterilized
or pasteurized milk, these rennet-producing, digesting species develop.
~Butyric acid fermentations.~ The formation of butyric acid in milk which
may be recognized by the "rancid butter" odor is not infrequently seen
in old, sour milk, and for a long time was thought to be a continuation
of the lactic fermentation, but it is now believed that these organisms
find more favorable conditions for growth, not so much on account of the
lactic acid formed as in the absence of dissolved oxygen in the milk
which is consumed by the sour-milk organisms.
Most of the butyric class of bacteria are spore-bearing, and hence they
are frequently present in boiled or sterilized milk. The by-products
formed in this series of changes are quite numerous. In most cases,
butyric acid is prominent, but in addition to this, other organic acids,
as lactic, succinic, and acetic, are produced, likewise different
alcohols. Concerning the chemical origin of butyric acid there is yet
some doubt. Duclaux[58] affirms that the fat, sugar and casein are all
decomposed by various forms. In some cases, the reaction of the milk is
alkaline, with other species it may be neutral or acid. This type of
fermentation has not received the study it deserves.
In milk these organisms are not of great importance, as this
fermentation does not readily gain the ascendency over the lactic
bacteria.
~Ropy or slimy milk.~ The viscosity of milk is often markedly increased
over that which it normally possesses. The intensity of this abnormal
condition may vary much; in some cases the milk becoming viscous or
slimy; in others stringing out into long threads, several feet in
length, as in Fig. 17. Two sets of conditions are responsible for these
ropy or slimy milks. The most common is where the milk is clotted or
stringy when drawn, as in some forms of garget. This is generally due to
the presence of viscid pus, and is often accompanied by a bloody
discharge, such a condition representing an inflamed state of the udder.
Ropiness of this character is not usually communicable from one lot of
milk to another.
[Illustration: FIG. 17. Ropy milk.]
The communicable form of ropy milk only appears after the milk has been
drawn from the udder for a day or so, and is caused by the development
of various species of bacteria which find their way into the milk after
it is drawn. These defects are liable to occur at any season of the
year. Their presence in a dairy is a source of much trouble, as the
unsightly appearance of the milk precludes its use as food, although
there is no evidence that these ropy fermentations are dangerous to
health.
There are undoubtedly a number of different species of bacteria that are
capable of producing these viscid changes,[59] but it is quite probable
that they are not of equal importance in infecting milk under natural
conditions.
In the majority of cases studied in this country,[60] the causal
organism seems to be _B. lactis viscosus_, a form first found by Adametz
in surface waters.[61] This organism possesses the property of
developing at low temperatures (45 deg.-50 deg. F.), and consequently it
is often able in winter to supplant the lactic-acid forms. Ward has found
this germ repeatedly in water tanks where milk cans are cooled; and
under these conditions it is easy to see how infection of the milk might
occur. Marshall[62] reports an outbreak which he traced to an external
infection of the udder; in another case, the slime-forming organism was
abundant in the barn dust. A defect of this character is often
perpetuated in a dairy for some time, and may therefore become
exceedingly troublesome. In one instance in the writer's experience, a
milk dealer lost over $150 a month for several months from ropy cream.
Failure to properly sterilize cans, and particularly strainer cloths, is
frequently responsible for a continuance of trouble of this sort.
The slimy substance formed in milk comes from various constituents of
the milk, and the chemical character of the slime produced also varies
with different germs. In some cases the slimy material is merely the
swollen outer cell membrane of the bacteria themselves as in the case of
_B. lactis viscosus_; in others it is due to the decomposition of the
proteids, but often the chief decomposition product appears to come from
a viscous fermentation of the milk-sugar.
An interesting case of a fermentation of this class being utilized in
dairying is seen in the use of "lange wei" (long or stringy whey) which
is employed as a starter in Holland to control the gassy fermentations
in Edam cheese. This slimy change is due to the growth of
_Streptococcus Hollandicus_.[63]
~Alcoholic fermentations.~ Although glucose or cane-sugar solutions are
extremely prone to undergo alcoholic fermentation, milk sugar does not
readily undergo this change. Where such changes are produced it is due
to yeasts. Several outbreaks attributable to such a cause have been
reported.[64] Russell and Hastings[65] have found these milk-sugar
splitting yeasts particularly abundant in regions where Swiss cheese is
made, a condition made possible by the use of whey-soaked rennets in
making such cheese.
Kephir and Koumiss are liquors much used in the Orient which are made
from milk that has undergone alcoholic fermentation. Koumiss was
originally made from mare's milk but is now often made from cows' milk
by adding cane sugar and yeast. In addition to the CO_{2} developed,
alcohol, lactic acid, and casein-dissolving ferments are formed. Kephir
is made by adding to milk Kephir grains, which are a mass of yeast and
bacterial cells. The yeasts produce alcohol and CO_{2} while the
bacteria change the casein of milk, rendering it more digestible. These
beverages are frequently recommended to persons who seem to be unable to
digest raw milk readily. The exact nature of the changes produced are
not yet well understood.[66]
~Bitter milk.~ The presence of bitter substances in milk may be ascribed
to a variety of causes. A number of plants, such as lupines, ragweed and
chicory, possess the property of affecting milk when the same are
consumed by animals. At certain stages in lactation, a bitter salty
taste is occasionally to be noted that is peculiar to individual
animals.
A considerable number of cases of bitter milk have, however, been traced
to bacterial origin. For a number of years the bitter fermentation of
milk was thought to be associated with the butyric fermentation, but
Weigmann[67] showed that the two conditions were not dependent upon each
other. He found that the organism which produced the bitter taste acted
upon the casein.
Conn[68] observed a coccus form in bitter cream that was able to impart
a bitter flavor to milk. Sometimes a bitter condition does not develop
in the milk, but may appear later in the milk products, as in the case
of a micrococcus which Freudenreich[69] found in cheese.
Harrison[70] has traced a common bitter condition in Canadian milk to a
milk-sugar splitting yeast, _Torula amara_ which not only grows rapidly
in milk but produces an undesirable bitterness in cheddar cheese.
Cream ripened at low temperatures not infrequently develops a bitter
flavor, showing that the optimum temperature for this type of
fermentation is below the typical lactic acid change.
Milk that has been heated often develops a bitter condition. The
explanation of this is that the bacteria producing the bitter substances
usually possess endospores, and that while the boiling or sterilizing of
milk easily kills the lactic acid germs, these forms on account of their
greater resisting powers are not destroyed by the heat.
~Soapy milk:~ A soapy flavor in milk was traced by Weigmann and Zirn[71]
to a specific bacillus, _B. lactis saponacei_, that they found gained
access to the milk in one case from the bedding and in another instance
from hay. A similar outbreak has been reported in this country,[72] due
to a germ acting on the casein and albumen.
~Red milk.~ The most common trouble of this nature in milk is due to
presence of blood, which is most frequently caused by some wound in the
udder. The ingestion of certain plants as sedges and scouring rushes is
also said to cause a bloody condition; madders impart a reddish tinge
due to coloring matter absorbed. Defects of this class can be readily
distinguished from those due to germ growth because they are apparent at
time of milking. Where blood is actually present, the corpuscles settle
out in a short time if left undisturbed.
There are a number of chromogenic or color-producing bacteria that are
able to grow in milk, but their action is so slow that generally they
are not of much consequence. Moreover their development is usually
confined to the surface of the milk as it stands in a vessel. The most
important is the well-known _B. prodigiosus_. Another form found at
times in milk possessing low acidity[73] is _B. lactis erythrogenes_.
This species only develops the red color in the dark. In the light, it
forms a yellow pigment. Various other organisms have been reported at
different times.[74]
~Blue milk.~ Blue milk has been known for many years, its communicable
nature being established as long ago as 1838. It appears on the surface
of milk first as isolated particles of bluish or grey color, which
later become confluent, the blue color increasing in intensity as the
acidity increases. The causal organism, _B. cyanogenes_, is very
resistant toward drying,[75] thus accounting for its persistence. In
Mecklenberg an outbreak of this sort once continued for several years.
It has frequently been observed in Europe in the past, but is not now so
often reported. Occasional outbreaks have been reported in this country.
~Other kinds of colored milk.~ Two or three chromogenic forms producing
still other colors have occasionally been found in milk. Adametz[76]
discovered in a sample of cooked milk a peculiar form (_Bacillus
synxanthus_) that produced a citron-yellow appearance which precipitated
and finally rendered soluble the casein. Adametz, Conn, and List have
described other species that confer tints of yellow on milk. Some of
these are bright lemon, others orange, and some amber in color.
Still other color-producing bacteria, such as those that produce violet
or green changes in the milk, have been observed. In fact, almost any of
the chromogenic bacteria are able to produce their color changes in milk
as it is such an excellent food medium. Under ordinary conditions, these
do not gain access to milk in sufficient numbers so that they modify the
appearance of it except in occasional instances.
~Treatment of abnormal fermentations.~ If the taint is recognized as of
bacterial origin (see p. 57) and is found in the mixed milk of the herd,
it is necessary to ascertain, first, whether it is a general trouble, or
restricted to one or more animals. This can sometimes be done by
separating the milk of the different cows and noting whether any
abnormal condition develops in the respective samples.
~Fermentation tests.~ The most satisfactory way to detect the presence of
the taints more often present is to make a fermentation test of one kind
or another. These tests are most frequently used at the factory, to
enable the maker to detect the presence of milk that is likely to prove
unfit for use, especially in cheese making. They are based upon the
principle that if milk is held at a moderately high temperature, the
bacteria will develop rapidly. A number of different methods have been
devised for this purpose. In Walther's lacto-fermentator samples of milk
are simply allowed to stand in bottles or glass jars until they sour.
They are examined at intervals of several hours. If the curdled milk is
homogeneous and has a pure acid smell, the milk is regarded as all
right. If it floats in a turbid serum, is full of gas or ragged holes,
it is abnormal. As generally carried out, no attempt is made to have
these vessels sterile. Gerber's test is a similar test that has been
extensively employed in Switzerland. Sometimes a few drops of rennet are
added to the milk so as to curdle the same, and thus permit of the more
ready detection of the gas that is evolved.
~Wisconsin curd test.~ The method of testing milk described below was
devised at the Wisconsin Experiment Station in 1895 by Babcock, Russell
and Decker.[77] It was used first in connection with experimental work
on the influence of gas-generating bacteria in cheese making, but its
applicability to the detection of all taints in milk produced by
bacteria makes it a valuable test for abnormal fermentations in general.
In the curd test a small pat of curd is made in a glass jar from each
sample of milk. These tests may be made in any receptacle that has been
cleaned in boiling water, and to keep the temperature more nearly
uniform these jars should be immersed in warm water, as in a wash tub or
some other receptacle. When the milk is about 95 deg. F., about ten drops
of rennet extract are added to each sample and mixed thoroughly with the
milk. The jars should then remain undisturbed until the milk is
completely curdled; then the curd is cut into small pieces with a case
knife and stirred to expel the whey. The whey should be poured off at
frequent intervals until the curd mats. If the sample be kept at blood
heat (98 deg. F.) for six to eight hours, it will be ready to examine.
[Illustration: FIG. 18. Improved bottles for making curd test. _A_, test
bottle complete; _B_, bottle showing construction of cover; _S_, sieve
to hold back the curd when bottle is inverted; _C_, outer cover with _(D
H)_ drain holes to permit of removal of whey.]
More convenient types of this test than the improvised apparatus just
alluded to have been devised by different dairy manufacturers.
Generally, they consist of a special bottle having a full-sized top,
thus permitting the easy removal of the curd. The one shown in Fig. 18
is provided with a sieve of such construction that the bottles will
drain thoroughly if inclined in an inverted position.
~Interpretation of results of test.~ The curd from a good milk has a firm,
solid texture, and should contain at most only a few small pin holes. It
may have some large, irregular, "mechanical" holes where the curd
particles have failed to cement, as is seen in Fig. 19. If gas-producing
bacteria are very prevalent in the milk, the conditions under which the
test is made cause such a rapid growth of the same that the evidence of
the abnormal fermentation may be readily seen in the spongy texture of
the curd (Fig. 20). If the undesirable organisms are not very abundant
and the conditions not especially suited to their growth, the "pin
holes" will be less frequent.
[Illustration: FIG. 19. Curd from a good milk. The large irregular holes
are mechanical.]
Sometimes the curds show no evidence of gas, but their abnormal
condition can be recognized by the "mushy" texture and the presence of
"off" flavors that are rendered more apparent by keeping them in closed
bottles. This condition is abnormal and is apt to produce quite as
serious results as if gas was formed.
~Overcoming taints by use of starters.~ Another method of combatting
abnormal fermentations that is often fruitful, is that which rests upon
the inability of one kind of bacteria to grow in the same medium in
competition with certain other species.
Some of the undesirable taints in factories can be controlled in large
part by the introduction of starters made from certain organisms that
are able to obtain the ascendency over the taint-producing germ. Such a
method is commonly followed when a lactic ferment, either a commercial
pure culture, or a home-made starter, is added to milk to overcome the
effect of gas-generating bacteria.
[Illustration: FIG. 20. Curd from a badly tainted milk. Large ragged
holes are mechanical; numerous small holes due to gas. This curd was a
"floater."]
A similar illustration is seen in the case of the "lange wei" (slimy
whey), that is used in the manufacture of Edam cheese to control the
character of the fermentation of the milk.
This same method is sometimes applied in dealing with certain abnormal
fermentations that are apt to occur on the farm. It is particularly
useful with those tainted milks known as "sweet curdling." The ferment
organisms concerned in this change are unable to develop in the
presence of lactic acid bacteria, so the addition of a clean sour milk
as a starter restores the normal conditions by giving the ordinary milk
bacteria the ascendency.
~Chemical disinfection.~ In exceptional instances it may be necessary to
employ chemical disinfectants to restore the normal conditions. Of
course with such diseases as tuberculosis, very stringent measures are
required, as they are such a direct menace to human life, but with these
abnormal or taint-producing fermentations, care and cleanliness, well
directed, will usually overcome the trouble.
If it becomes necessary to employ chemical substances as disinfecting
agents, their use should always be preceded by a thorough cleansing with
hot water so that the germicide may come in direct contact with the
surface to be disinfected.
It must be borne in mind that many chemicals act as deodorants, _i.e._,
destroy the offensive odor, without destroying the cause of the trouble.
_Sulfur_ is often recommended as a disinfecting agent, but its use
should be carefully controlled, otherwise the vapors have but little
germicidal power. The common practice of burning a small quantity in a
room or any closed space for a few moments has little or no effect upon
germ life. The effect of sulfur vapor (SO_{2}) alone upon germ life is
relatively slight, but if this gas is produced in the presence of
moisture, sulfurous acid (H_{2}SO_{3}) is formed, which is much more
efficient. To use this agent effectively, it must be burned in large
quantities in a moist atmosphere (three lbs. to every 1,000 cubic feet
of space), for at least twelve hours. After this operation, the space
should be thoroughly aired.
_Formalin_, a watery solution of a gas known as formaldehyde, is a new
disinfectant that recent experience has demonstrated to be very useful.
It may be used as a gas where rooms are to be disinfected, or applied as
a liquid where desired. It is much more powerful in its action than
sulfur, and it has a great advantage over mercury and other strong
disinfectants, as it is not so poisonous to man as it is to the lower
forms of life.
_Bleaching powder or chloride of lime_ is often recommended where a
chemical can be advantageously used. This substance is a good
disinfectant as well as a deodorant, and if applied as a wash, in the
proportion of four to six ounces of the powder to one gallon of water,
it will destroy most forms of life. In many cases this agent is
inapplicable on account of its odor.
_Corrosive sublimate_ (HgCl_{2}) for most purposes is a good
disinfectant, but it is such an intense poison that its use is dangerous
in places that are at all accessible to stock.
For the disinfection of walls in stables and barns, common thin _white
wash_ Ca(OH)_{2} is admirably adapted if made from freshly-burned quick
lime. It possesses strong germicidal powers, increases the amount of
light in the barn, is a good absorbent of odors, and is exceedingly
cheap.
Carbolic acid, creosote, and such products, while excellent
disinfectants, cannot well be used on account of their odor, especially
in factories.
For gutters, drains, and waste pipes in factories, _vitriol salts_
(sulfates of copper, iron and zinc) are sometimes used. These are
deodorants as well as disinfectants, and are not so objectionable to use
on account of their odor.
These suggestions as to the use of chemicals, however, only apply to
extreme cases and should not be brought into requisition until a
thorough application of hot water, soap, a little soda, and the
scrubbing brush have failed to do their work.
FOOTNOTES:
[51] Guenther and Thierfelder, Arch. f. Hyg., 25:164, 1895; Leichmann,
Cent. f. Bakt., 2:281, 1896; Esten, 9 Rept. Storrs Expt. Stat., p. 44,
1896; Dinwiddie, Bull. 45, Ark. Expt. Stat., May, 1897; Kozai, Zeit. f.
Hyg., 38:386, 1901; Weigmann, Hyg. Milk Congress, Hamburg, 1903, p. 375.
[52] McDonnell, Inaug. Diss., Kiel. 1899, p. 39.
[53] Kayser, Cent. f. Bakt. II. Abt. 1:436.
[54] Treadwell, Science, 1894, 17:178.
[55] Conn, 5 Rept. Storrs Expt. Stat., 1892, p. 396.
[56] Fermi, Arch. f. Hyg., 1892, 14:1.
[57] Duclaux, Le Lait, p. 121.
[58] Duclaux, Principes de Laiterie, p. 67.
[59] Guillebeau (Milch Zeit., 1892, p. 808) has studied over a dozen
different forms that possess this property.
[60] Ward, Bull. 165, Cornell Expt. Stat., Mch., 1899; also Bull. 195,
Ibid., Nov., 1901.
[61] Adametz, Landw. Jahr., 1891, p. 185.
[62] Marshall, Mich. Expt. Stat., Bull. 140.
[63] Milch Zeit., 1899, p. 982.
[64] Duclaux, Principes de Laiterie, p. 60. Heinze and Cohn, Zeit. f.
Hyg., 46: 286, 1904.
[65] Bull. 128, Wis. Expt. Stat., Sept. 1905.
[66] Freudenreich, Landw. Jahr. d. Schweiz, 1896, 10; 1.
[67] Weigmann, Milch Zeit., 1890, p. 881.
[68] Conn, 3 Rept. Storrs Expt. Stat., 1890, p. 158.
[69] Freudenreich, Fuehl. Landw. Ztg. 43: 361.
[70] Harrison, Bull. 120 Ont. Agr'l. Coll., May, 1902.
[71] Milch Zeit. 22:569.
[72] Marshall, Bull. 146, Mich. Expt. Stat., p. 16.
[73] Grotenfelt, Milch Zeit., 1889, p. 263.
[74] Menge, Cent. f. Bakt., 6:596; Keferstein, Cent. f. Bakt., 21:177.
[75] Heim, Arb. a. d. Kais. Gesundheitsamte, 5:578.
[76] Adametz, Milch Zeit., 1890, p. 225.
[77] 12 Rept. Wis. Expt. Stat., 1895, p. 148; also Bull. 67, Ibid.,
June, 1898.