BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF
PRESERVATION.
To the milk dealer or distributor, bacteria are more or less of a
detriment. None of the organisms that find their way into milk, nor the
by-products formed by their growth, improve the quality of milk
supplies. It is therefore especially desirable from the milk-dealer's
point of view that these changes should be held in abeyance as much as
possible. Then too, the possibility that milk may serve as a medium for
the dissemination of disease-breeding bacteria makes it advisable to
protect this food supply from all possible infection from suspicious
sources.
In considering, therefore, the relation of bacteria to general milk
supplies, the _economic_ and the _hygienic_ standpoints must be taken
into consideration. Ordinarily much more emphasis is laid upon the first
requirement. If the supply presents no abnormal feature as to taste,
odor and appearance, unfortunately but little attention is paid to the
possibility of infection by disease germs. The methods of control which
are applicable to general milk supplies are based on the following
foundations: (1) the exclusion of all bacterial life, as far as
practicable, at the time the milk is drawn, and the subsequent storage
of the same at temperatures unfavorable for the growth of the organisms
that do gain access; (2) the removal of the bacteria, wholly or in part,
after they have once gained access.
Until within comparatively recent years, practically no attention was
given to the character of milk supplies, except possibly as to the
percentage of butter fat, and sometimes the milk solids which it
contained. So long as the product could be placed in the hands of the
consumer in such shape as not to be rejected by him as unfit for food,
no further attention was likely to be given to its character. At
present, however, much more emphasis is being given to the quality of
milk, especially as to its germ content; and the milk dealer is
beginning to recognize the necessity of a greater degree of control.
This control must not merely concern the handling of the product after
it reaches him, but should go back to the milk producer on the farm.
Here especially, it is necessary to inculcate those methods of
cleanliness which will prevent in large measure the wholesale infection
that ordinarily occurs.
The two watch words which are of the utmost importance to the milk
dealer are _cleanliness_ and _cold_. If the milk is properly drawn from
the animal in a clean manner and is immediately and thoroughly chilled,
the dealer has little to fear as to his product. Whenever serious
difficulties do arise, attributable to bacterial changes, it is because
negligence has been permitted in one or both directions. The influence
of cleanliness in diminishing the bacterial life in milk and that of low
temperatures in repressing the growth of those forms which inevitably
gain access has been fully dealt with in preceding chapters. It is of
course not practicable to take all of these precautions to which
reference has been made in the securing of large supplies of market milk
for city use, but great improvement over existing conditions could be
secured if the public would demand a better supervision of this
important food article. Boards of health in our larger cities are
awakening to the importance of this question and are becoming
increasingly active in the matter of better regulations and the
enforcement of the same.
New York City Board of Health has taken an advanced position in
requiring that all milk sold in the city shall be chilled down to 45 deg.
F. immediately after milking and shall be transported to the city in
refrigerator cars.
Reference has already been made to the application of the acid test
(page 52) in the inspection of city milk supplies, and it is the opinion
of the writer that the curd test (see page 76) could also be used with
advantage in determining the sanitary character of milk. This test
reveals the presence of bacteria usually associated with dirt and
permits of the recognition of milks that have been carelessly handled.
From personal knowledge of examinations made of the milk supplies in a
number of Wisconsin cities it appears that this test could be utilized
with evident advantage.
~"Sanitary" or "certified" milk supplies.~ In a number of the larger
cities, the attempt has been made to improve the quality of the milk
supplies by the installation of dairies in which is produced an
especially high grade of milk. Frequently the inspection of the dairy as
well as the examination of the milk at stated intervals is under the
control of milk commissions or medical societies and as it is customary
to distribute the certificate of the examining board with the product,
such milks are frequently known as "certified." In such dairies the
tuberculin test is used at regular intervals, and the herd inspected
frequently by competent veterinarians. The methods of control
inaugurated as to clean milking and subsequent handling are such as to
insure the diminution of the bacteria to the lowest possible point. The
bacterial limit set by the Pediatric Society of Philadelphia is 10,000
organisms per cc. Often it is possible to improve very materially on
this standard and not infrequently is the supply produced where it
contains only a few thousand organisms per cc. Where such a degree of
care is exercised, naturally a considerably higher price must be paid
for the product,[126] and it should be remembered that the development
of such a system is only possible in relatively large centers where the
dealer can cater to a selected high-class trade. Moreover, it should
also be borne in mind that such a method of control is only feasible in
dairies that are under individual control. The impossibility of
exercising adequate control with reference to the milking process and
the care which should be given the milk immediately thereafter, when the
same is produced on different farms under various auspices is evident.
PRESERVATION OF MILK SUPPLIES.
While much can be done to improve the quality of milk supplies by
excluding a large proportion of the bacteria which normally gain access
to the milk, and preventing the rapid growth of those that do find their
way therein, yet for general municipal purposes, any practical method of
preservation[127] that is applicable on a commercial scale must rest
largely upon the destruction of bacteria that are present in the milk.
The two possible methods by which bacteria can be destroyed after they
have once gained access is (1) by the use of chemical preservatives; (2)
by the aid of physical methods.
~Chemical preservatives.~ Numerous attempts have been made to find some
chemical substance that could be added to milk which would preserve it
without interfering with its nutritive properties, but as a general rule
a substance that is toxic enough to destroy or inhibit the growth of
bacterial life exerts a prejudicial effect on the tissues of the body.
The use of chemicals, such as carbolic acid, mercury salts and mineral
acids, that are able to entirely destroy all life, is of course
excluded, except when milk is preserved for analytical purposes; but a
number of milder substances are more or less extensively employed,
although the statutes of practically all states forbid their use.
The substances so used may be grouped in two classes:
1. Those that unite chemically with certain by-products of bacterial
growth to form inert substances. Thus bicarbonate of soda neutralizes
the acid in souring milk, although it does not destroy the lactic acid
bacteria.
2. Those that act directly upon the bacteria in milk, restraining or
inhibiting their development. The substances most frequently utilized
are salicylic acid, formaldehyde and boracic acid. These are nearly
always sold to the milk handler, under some proprietary name, at prices
greatly in excess of what the crude chemicals could be bought for in the
open market. Formaldehyde has been widely advertised of late, but its
use is fraught with the greatest danger, for it practically renders
insoluble all albuminous matter and its toxic effect is greatly
increased in larger doses.
These substances are generally used by milk handlers who know nothing of
their poisonous action, and although it may be possible for adults to
withstand their use in dilute form, without serious results, yet their
addition to general milk supplies that may be used by children is
little short of criminal. The sale of these preparations for use in milk
finds its only outlet with those dairymen who are anxious to escape the
exactions that must be met by all who attempt to handle milk in the best
possible manner. Farrington has suggested a simple means for the
detection of preservalin (boracic acid).[128] When this substance is
added to fresh milk, it increases the acidity of milk without affecting
its taste. As normal milk tastes sour when it contains about 0.3 per
cent lactic acid, a milk that tests as much or more than this without
tasting sour has been probably treated with this antiseptic agent.
~Physical methods of preservation.~ Methods based upon the application of
physical forces are less likely to injure the nutritive value of milk,
and are consequently more effective, if of any value whatever. A number
of methods have been tried more or less thoroughly in an experimental
way that have not yet been reduced to a practical basis, as electricity,
use of a vacuum, and increased pressure.[129] Condensation has long been
used with great success, but in this process the nature of the milk is
materially changed. The keeping quality in condensed milk often depends
upon the action of another principle, viz., the inhibition of bacterial
growth by reason of the concentration of the medium. This condition is
reached either by adding sugar and so increasing the soluble solids, or
by driving off the water by evaporation, preferably in a vacuum pan.
Temperature changes are, however, of the most value in preserving milk,
for by a variation in temperature all bacterial growth can be brought to
a standstill, and under proper conditions thoroughly destroyed.
~Use of low temperatures.~ The effect of chilling or rapid cooling on the
keeping quality of milk is well known. When the temperature of milk is
lowered to the neighborhood of 45 deg. F., the development of bacterial
life is so slow as to materially increase the period that milk remains
sweet. Within recent years, attempts have been made to preserve milk so
that it could be shipped long distances by freezing the product, which
in the form of milk-ice could be held for an indefinite period without
change.[130] A modification of this process known as Casse's system has
been in use more or less extensively in Copenhagen and in several places
in Germany. This consists of adding a small block of milk-ice (frozen
milk) to large cans of milk (one part to about fifty of milk) which may
or may not be pasteurized.[131] This reduces the temperature so that the
milk remains sweet considerably longer. Such a process might permit of
the shipment of milk for long distances with safety but as a matter of
fact, the system has not met with especial favor.
[Illustration: FIG. 22. Microscopic appearance of normal milk showing
the fat-globules aggregated in clusters.]
~Use of high temperatures.~ Heat has long been used as a preserving agent.
Milk has been scalded or cooked to keep it from time immemorial. Heat
may be used at different temperatures, and when so applied exerts a
varying effect, depending upon temperature employed. All methods of
preservation by heat rest, however, upon the application of the heat
under the following conditions:
1. A temperature above the maximum growing-point (105 deg.-115 deg. F.)
and below the thermal death-point (130 deg.-140 deg. F.) will prevent
further growth, and consequently fermentative action.
2. A temperature above the thermal death-point destroys bacteria, and
thereby stops all changes. This temperature varies, however, with the
condition of the bacteria, and for spores is much higher than for
vegetative forms.
Attempts have been made to employ the first principle in shipping milk
by rail, viz., prolonged heating above growing temperature, but when
milk is so heated, its physical appearance is changed.[132] The methods
of heating most satisfactorily used are known as sterilization and
pasteurization, in which a degree of temperature is used approximating
the boiling and scalding points respectively.
[Illustration: FIG. 23. Microscopic appearance of milk heated above 140
deg. F., showing the homogeneous distribution of fat-globules. The
physical change noted in comparison with Fig. 22 causes the diminished
consistency of pasteurized cream.]
~Effect of heat on milk.~ When milk is subjected to the action of heat, a
number of changes in its physical and chemical properties are to be
noted.
_1. Diminished "body."_ When milk, but more especially cream, is heated
to 140 deg. F. or above, it becomes thinner in consistency or "body," a
condition which is due to a change in the grouping of the fat globules.
In normal milk, the butter fat for the most part is massed in
microscopic clots as (Fig. 22). When exposed to 140 deg. F. or above for
ten minutes these fat-globule clots break down, and the globules become
homogeneously distributed (Fig. 23). A _momentary_ exposure to heat as
high as 158 deg.-160 deg. may be made without serious effect on the cream
lime; but above this the cream rises so poorly and slowly that it gives
the impression of thinner milk.
_2. Cooked Taste._ If milk is heated for some minutes to 160 deg. F., it
acquires a cooked taste that becomes more pronounced as the temperature
is further raised. Milk so heated develops on its surface a pellicle or
"skin." The cause of this change in taste is not well known. Usually it
has been explained as being produced by changes in the nitrogenous
elements in the milk, particularly in the albumen. Thoerner[133] has
pointed out the coincidence that exists between the appearance of a
cooked taste and the loss of certain gases that are expelled by heating.
He finds that the milk heated in closed vessels from which the gas
cannot escape has a much less pronounced cooked flavor than if heated in
an open vessel. The so-called "skin" on the surface of heated milk is
not formed when the milk is heated in a tightly-closed receptacle. By
some[134] it is asserted that this layer is composed of albumen, but
there is evidence to show that it is modified casein due to the rapid
evaporation of the milk serum at the surface of the milk.
_3. Digestibility._ Considerable difference of opinion has existed in
the minds of medical men as to the relative digestibility of raw and
heated milks. A considerable amount of experimental work has been done
by making artificial digestion experiments with enzyms, also digestion
experiments with animals, and in a few cases with children. The results
obtained by different investigators are quite contradictory, although
the preponderance of evidence seems to be in favor of the view that
heating does impair the digestibility of milk, especially if the
temperature attains the sterilizing point.[135] It has been observed
that there is a noteworthy increase in amount of rickets,[136] scurvy
and marasmus in children where highly-heated milks are employed. These
objections do not obtain with reference to milk heated to moderate
temperatures, as in pasteurization, although even this lower temperature
lessens slightly its digestibility. The successful use of pasteurized
milks in children's hospitals is evidence of its usefulness.
_4. Fermentative changes._ The normal souring change in milk is due to
the predominance of the lactic acid bacteria, but as these organisms as
a class do not possess spores, they are readily killed when heated above
the thermal death-point of the developing cell. The destruction of the
lactic forms leaves the spore-bearing types possessors of the field, and
consequently the fermentative changes in heated milk are not those that
usually occur, but are characterized by the curdling of the milk from
the action of rennet enzyms.
_5. Action of rennet._ Heating milk causes the soluble lime salts to be
precipitated, and as the curdling of milk by rennet (in cheese-making)
is dependent upon the presence of these salts, their absence in heated
milks greatly retards the action of rennet. This renders it difficult to
utilize heated milks in cheese-making unless the soluble lime salts are
restored, which can be done by adding solutions of calcium chlorid.
~Sterilization.~ As ordinarily used in dairying, sterilization means the
application of heat at temperatures approximating, if not exceeding,
212 deg. F. It does not necessarily imply that milk so treated is sterile,
i. e., germ-free; for, on account of the resistance of spores, it is
practically impossible to destroy entirely _all_ these hardy forms. If
milk is heated at temperatures above the boiling point, as is done where
steam pressure is utilized, it can be rendered practically germ-free.
Such methods are employed where it is designed to keep milk sweet for a
long period of time. The treatment of milk by sterilization has not met
with any general favor in this country, although it has been more widely
introduced abroad. In most cases the process is carried out after the
milk is bottled; and considerable ingenuity has been exercised in the
construction of devices which will permit of the closure of the bottles
after the sterilizing process has been completed. Milks heated to so
high a temperature have a more or less pronounced boiled or cooked
taste, a condition that does not meet with general favor in this
country. The apparatus suitable for this purpose must, of necessity, be
so constructed as to withstand steam pressure, and consequently is
considerably more expensive than that required for the simpler
pasteurizing process.
~Pasteurization.~ In this method the degree of heat used ranges from 140
deg. to 185 deg. F. and the application is made for only a limited length
of time. The process was first extensively used by Pasteur (from whom it
derives its name) in combating various maladies of beer and wine. Its
importance as a means of increasing the keeping quality of milk was not
generally recognized until a few years ago; but the method is now
growing rapidly in favor as a means of preserving milk for commercial
purposes. The method does not destroy all germ-life in milk; it affects
only those organisms that are in a growing, vegetative condition; but if
the milk is quickly cooled, it enhances the keeping quality very
materially. It is unfortunate that this same term is used in connection
with the heating of cream as a preparatory step to the use of pure
cultures in cream-ripening in butter-making. The objects to be
accomplished vary materially and the details of the two processes are
also quite different.
While pasteurizing can be performed on a small scale by the individual,
the process can also be adapted to the commercial treatment of large
quantities of milk. The apparatus necessary for this purpose is not
nearly so expensive as that used in sterilizing, a factor of importance
when other advantages are considered. In this country pasteurization has
made considerable headway, not only in supplying a milk that is designed
to serve as children's food, but even for general purposes.
~Requirements essential in pasteurization.~ While considerable latitude
with reference to pasteurizing limits is permitted, yet there are
certain conditions which should be observed, and these, in a sense, fix
the limits that should be employed. These may be designated as (1) the
_physical_, and (2) the _biological_ requirements.
~Physical requirements.~ _1. Avoidance of scalded or cooked taste._ The
English and American people are so averse to a scalded or cooked flavor
in milk that it is practically impossible for a highly heated product to
be sold in competition with ordinary raw milk. In pasteurization then,
care must be taken not to exceed the temperature at which a permanently
cooked flavor is developed. As previously observed, this point varies
with the period of exposure. A momentary exposure to a temperature of
about 170 deg. F. may be made without any material alteration, but if
the heat is maintained for a few minutes (ten minutes or over), a
temperature of 158 deg. to 160 deg. F. is about the maximum that can be
employed with safety.
_2. Normal creaming of the milk._ It is especially desirable that a
sharp and definite cream line be evident on the milk soon after
pasteurization. If this fails to appear, the natural inference of the
consumer is that the milk is skimmed. If the milk be heated to a
temperature sufficiently high to cause the fat-globule clusters to
disintegrate (see Figs. 22 and 23), the globules do not rise to the
surface as readily as before and the cream line remains indistinct.
Where the exposure is made for a considerable period of time (10 minutes
or more), the maximum temperature which can be used without producing
this change is about 140 deg. F.; if the exposure is made for a very
brief time, a minute or less, the milk may be heated to 158 deg.-160 F.
deg. without injuring the creaming property.
_3. No diminution in cream "body."_ Coincident with this change which
takes place in the creaming of the milk is the change in body or
consistency which is noted where cream is pasteurized at too high a
temperature. For the same reason as given under (2) cream heated above
these temperatures is reduced in apparent thickness and appears to
contain less butter-fat. Of course the pasteurizing process does not
change the fat content, but its "body" is apparently so affected. Thus a
25 per cent. cream may seem to be no thicker or heavier than an 18 per
cent. raw cream. This real reduction in consistency naturally affects
the readiness with which the cream can be whipped.
~Biological requirements.~ _1. Enhanced keeping quality._ In commercial
practice the essential biological requirement is expressed in the
enhanced keeping quality of the pasteurized milk. This expresses in a
practical way the reduction in germ life accomplished by the
pasteurizing process. The improvement in keeping quality depends upon
the temperature and time of exposure, but fully as much also on the way
in which the pasteurized product is handled after heating. The lowest
temperature which can be used with success to kill the active,
vegetative bacteria is about 140 deg. F., at which point it requires
about ten minutes exposure. If this period is curtailed the temperature
must be raised accordingly. An exposure to a temperature of 175 deg. F.
for a minute has approximately the same effect as the lower degree of
heat for the longer time.
The following bacteriological studies as to the effect which a variation
in temperature exerts on bacterial life in milk are of importance as
indicating the foundation for the selection of the proper limits. In the
following table the exposures were made for a uniform period (20
minutes):
_The bacterial content of milk heated at different temperatures._
Number of bacteria per cc. in milk.
45 deg. C. 50 deg. C. 55 deg. C. 60 deg. C. 65 deg. C. 70 deg. C.
Unheated 113 deg. F. 122 deg. F. 131 deg. F. 140 deg. F. 149 deg. F. 158 deg. F.
Series I. 2,895,000 ---- 1,260,000 798,000 32,000 5,770 3,900
Series II. 750,000 665,000 262,400 201,000 950 700 705
Series III. 1,350,000 1,100,000 260,000 215,000 575 610 650
Series IV. 1,750,000 ---- 87,360 ---- 4,000 3,500 3,600
It appears from these results that the most marked decrease in
temperature occurs at 140 deg. F. (60 deg. C.). It should also be observed
that an increase in heat above this temperature did not materially
diminish the number of organisms present, indicating that those forms
remaining were in a spore or resistant condition. It was noted, however,
that the developing colonies grew more slowly in the plates made from the
highly heated milk, showing that their vitality was injured to a greater
extent even though not killed.
_2. Destruction of disease bacteria._ While milk should be pasteurized
so as to destroy all active, multiplying bacteria, it is particularly
important to destroy any organisms of a disease nature that might find
their way into the same. Fortunately most of the bacteria capable of
thriving in milk before or after it is drawn from the animal are not
able to form spores and hence succumb to proper pasteurization. Such is
the case with the diphtheria, cholera and typhoid organisms.
The organism that is invested with most interest in this connection is
the tubercle bacillus. On account of its more or less frequent
occurrence in milk and its reputed high powers of resistance, it may
well be taken as a standard in pasteurizing.
~Thermal death limits of tubercle bacillus.~ Concerning the exact
temperature at which this germ is destroyed there is considerable
difference of opinion. Part of this arises from the inherent difficulty
in determining exactly when the organism is killed (due to its failure
to grow readily on artificial media), and part from the lack of uniform
conditions of exposure. The standards that previously have been most
generally accepted are those of De Man,[137] who found that thirty
minutes exposure at 149 deg. F., fifteen minutes at 155 deg. F., or ten
minutes at 167 deg. F., sufficed to destroy this germ.
More recently it has been demonstrated,[138] and these results
confirmed,[139] that if tuberculous milk is heated in closed receptacles
where the surface pellicle does not form, the vitality of this disease
germ is destroyed at 140 deg. F. in 10-15 minutes, while an exposure at
160 deg. F. requires only about one minute.[140] If the conditions of
heating are such that the surface of the milk is exposed to the air, the
resistance of bacteria is greatly increased. When heated in open vessels
Smith found that the tubercle organism was not killed in some cases where
the exposure was made for at least an hour. Russell and Hastings[141]
have shown an instance where the thermal death-point of a micrococcus
isolated from pasteurized milk was increased 12.5 deg. F., by heating it
under conditions that permitted of the formation of the scalded layer.
It is therefore apparent that apparatus used for pasteurization should
be constructed so as to avoid this defect.
~Methods of treatment.~ Two different systems of pasteurization have grown
up in the treatment of milk. One of these has been developed from the
hygienic or sanitary aspect of the problem and is used more particularly
in the treatment of cream and relatively small milk supplies. The other
system has been developed primarily from the commercial point of view
where a large amount of milk must be treated in the minimum time. In the
first method the milk is heated for a longer period of time, about
fifteen minutes at a relatively low temperature from 140 deg.-155 deg.
F.; in the other, the milk is exposed to the source of heat only while
it is passing rapidly through the apparatus. Naturally, the exposure
under such conditions must be made at a considerably higher temperature,
usually in the neighborhood of 160 deg. F.
The types of apparatus used in these respective processes naturally
varies. Where the heating is prolonged, the apparatus employed is built
on the principle of a _tank_ or _reservoir_ in which a given volume of
milk may be held at any given temperature for any given period of time.
When the heat is applied for a much shorter period of time, the milk is
passed in a continuous stream through the machine. Naturally the
capacity of a continuous-flow apparatus is much greater than a machine
that operates on the intermittent principle; hence, for large supplies,
as in city distribution, this system has a great advantage. The question
as to relative efficiency is however one which should be given most
careful consideration.
~Pasteurizing apparatus.~ The problems to be solved in the pasteurization
of milk and cream designed for direct consumption are so materially
different from where the process is used in butter-making that the type
of machinery for each purpose is quite different. The equipment
necessary for the first purpose may be divided into two general classes:
1. Apparatus of limited capacity designed for family use.
2. Apparatus of sufficient capacity to pasteurize on a commercial scale.
~Domestic pasteurizers.~ In pasteurizing milk for individual use, it is
not desirable to treat at one time more than will be consumed in one
day; hence an apparatus holding a few bottles will suffice. In this case
the treatment can best be performed in the bottle itself, thereby
lessening the danger of infection. Several different types of
pasteurizers are on the market; but special apparatus is by no means
necessary for the purpose. The process can be efficiently performed by
any one with the addition of an ordinary dairy thermometer to the common
utensils found in the kitchen. Fig. 24 indicates a simple contrivance
that can be readily arranged for this purpose.
The following suggestions indicate the different steps of the process:
1. Use only fresh milk.
2. Place milk in clean bottles or fruit cans, filling to a uniform
level, closing bottles tightly with a cork or cover. If pint and quart
cans are used at the same time, an inverted bowl will equalize the
level. Set these in a flat-bottomed tin pail and fill with warm water to
same level as milk. An inverted pie tin punched with holes will serve as
a stand on which to place the bottles during the heating process.
3. Heat water in pail until the temperature of same reaches 155 deg. to
160 deg. F.; then remove from source of direct heat, cover with a cloth
or tin cover, and allow the whole to stand for half an hour. In the
preparation of milk for children, it is not advisable to use the
low-temperature treatment (140 deg. F.) that is recommended for
commercial city delivery.
[Illustration: FIG. 24. A home-made pasteurizer.]
4. Remove bottles of milk and cool them as rapidly as possible without
danger to bottles and store in a refrigerator.
~Commercial pasteurizers.~ The two methods of pasteurization practiced
commercially for the preservation of milk and cream have been developed
because of the two types of machinery now in use. Apparatus constructed
on the reservoir or tank principle permits of the retention of the milk
for any desired period of time. Therefore, a lower temperature can be
employed in the treatment. In those machines where the milk flows
through the heater in a more or less continuous stream, the period of
exposure is necessarily curtailed, thereby necessitating a higher
temperature.
~Reservoir pasteurizers.~ The simplest type of apparatus suitable for
pasteurizing on this principle is where the milk is placed in shotgun
cans and immersed in water heated by steam. Ordinary tanks surrounded
with water spaces can also be used successfully. The Boyd cream ripening
vat has also been tried. In this the milk is heated by a swinging coil
immersed in the vat through which hot water circulates.
In 1894 the writer[142] constructed a tank pasteurizer which consisted
of a long, narrow vat surrounded by a steam-heated water chamber. Both
the milk and the water chambers were provided with mechanical agitators
having a to-and-fro movement.
[Illustration: FIG. 25. Pott's pasteurizer.]
Another machine which has been quite generally introduced is the Potts'
rotating pasteurizer. This apparatus has a central milk chamber that is
surrounded with an outer shell containing hot water. The whole machine
revolves on a horizontal axis, and the cream or milk is thus thoroughly
agitated during the heating process.
~Continuous-flow pasteurizers.~ The demand for greater capacity than can
be secured in the reservoir machines has led to the perfection of
several kinds of apparatus where the milk is heated momentarily as it
flows through the apparatus. Most of these were primarily introduced for
the treatment of cream for butter-making purposes, but they are
frequently employed for the treatment of milk on a large scale in city
milk trade. Many of them are of European origin although of late years
several have been devised in this country.
The general principle of construction is much the same in most of them.
The milk is spread out in a thin sheet, and is treated by passing it
over a surface, heated either with steam directly or preferably with hot
water.
Where steam is used directly, it is impossible to prevent the "scalding
on" of the milk proteids to the heated surface.
In some of these machines (Thiel, Kuehne, Lawrence, De Laval, and
Hochmuth), a ribbed surface is employed over which the milk flows, while
the opposite surface is heated with hot water or steam. Monrad, Lefeldt
and Lentsch employ a centrifugal apparatus in which a thin layer of milk
is heated in a revolving drum.
In some types of apparatus, as in the Miller machine, an American
pasteurizer, the milk is forced in a thin sheet between two heated
surfaces, thereby facilitating the heating process. In the Farrington
machine heated discs rotate in a reservoir through which the milk flows
in a continuous stream.
One of the most economical types of apparatus is the regenerator type (a
German machine), in which the milk passes over the heating surface in a
thin stream and then is carried back over the incoming cold milk so
that the heated liquid is partially cooled by the inflowing fresh milk.
In machines of this class it requires very much less steam to heat up
the milk than in those in which the cold milk is heated wholly by the
hot water.
A number of machines have been constructed on the principle of a
reservoir which is fed by a constantly flowing stream. In some kinds of
apparatus of this type no attempt is made to prevent the mixing of the
recently introduced milk with that which has been partially heated. The
pattern for this reservoir type is Fjord's heater, in which the milk is
stirred by a stirrer. This apparatus was originally designed as a heater
for milk before separation, but it has since been materially modified so
that it is better adapted to the purposes of pasteurization. Reid was
the first to introduce this type of machine into America.
~Objections to continuous flow pasteurizers.~ In all continuous flow
pasteurizers certain defects are more or less evident. While they
fulfill the important requirement of large capacity, an absolute
essential where large volumes of milk are being handled, it does not of
necessity follow that they conform to all the hygienic and physical
requirements that should be kept in mind. The greatest difficulty is the
shortened period of exposure. The period which the milk is actually
heated is often not more than a minute or so. Another serious defect is
the inability to heat _all_ of the milk for a uniform period of time. At
best, the milk is exposed for an extremely short time, but even then
portions pass through the machine much more quickly than do the
remainder. Those portions in contact with the walls of the apparatus are
retarded by friction and are materially delayed in their passage, while
the particles in the center of the stream, however thin, flow through
in the least possible time.
The following simple method enables the factory operator to test the
period of exposure in the machine: Start the machine full of water, and
after the same has become heated to the proper temperature, change the
inflow to full-cream milk, continuing at the same rate. Note the exact
time of change and also when first evidence of milkiness begins to
appear at outflow. If samples are taken from first appearance of milky
condition and thereafter at different intervals for several minutes, it
is possible, by determining the amount of butter-fat in the same, to
calculate with exactness how long it takes for the milk to entirely
replace the water.
Tests made by the writer[143] on the Miller pasteurizer showed, when fed
at the rate of 1,700 pounds per hour, the minimum period of exposure to
be 15 seconds, and the maximum about 60-70 seconds, while about
two-thirds of the milk passed the machine in 40-50 seconds. This
manifest variation in the rate of flow of the milk through the machine
is undoubtedly the reason why the results of this type of treatment are
subject to so much variation. Naturally, even a fatal temperature to
bacterial life can be reduced to a point where actual destruction of
even vegetating cells does not occur.
~Bacterial efficiency of reservoir pasteurizers.~ The bacterial content of
pasteurized milk and cream will depend somewhat on the number of
organisms originally present in the same. Naturally, if mixed milk
brought to a creamery is pasteurized, the number of organisms remaining
after treatment would be greater than if the raw material was fresh and
produced on a single farm.
An examination of milk and cream pasteurized on a commercial scale in
the Russell vat at the Wisconsin Dairy school showed that over 99.8 per
cent of the bacterial life in raw milk or cream was destroyed by the
heat employed, i. e., 155 deg. F. for twenty minutes duration.[144] In
nearly one-half of the samples of milk, the germ content in the
pasteurized sample fell below 1,000 bacteria per cc., and the average of
twenty-five samples contained 6,140 bacteria per cc. In cream the germ
content was higher, averaging about 25,000 bacteria per cc. This milk
was taken from the general creamery supply, which was high in organisms,
containing on an average 3,675,000 bacteria per cc. De Schweinitz[145]
has reported the germ content of a supply furnished in Washington which
was treated at 158 deg. to 160 deg. F. for fifteen minutes. This supply
came from a single source. Figures reported were from 48-hour-old agar
plates. Undoubtedly these would have been higher if a longer period of
incubation had been maintained. The average of 82 samples, taken for the
period of one year, showed 325 bacteria per cc.
[Illustration: FIG. 26. Effect of pasteurizing on germ content of milk.
Black square represents bacteria of raw milk; small white square, those
remaining after pasteurization.]
~Bacterial efficiency of continuous-flow pasteurizers.~ A quantitative
determination of the bacteria found in milk and cream when treated in
machinery of this class almost always shows a degree of variation in
results that is not to be noted in the discontinuous apparatus.
[Illustration: FIG. 27. Reid's Continuous Pasteurizer.]
Harding and Rogers[146] have tested the efficiency of one of the Danish
type of continuous pasteurizers. These experiments were made at 158 deg.,
176 deg. and 185 deg. F. They found the efficiency of the machine not
wholly satisfactory at the lower temperatures. At 158 deg. F. the average
of fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum
range from 62,790 to 120. Twenty-five examinations at 176 deg. F. showed
an average of only 117, with a range from 300 to 20. The results at 185
deg. F. showed practically the same results as noted at 176 deg. F.
Considerable trouble was experienced with the "scalding on" of the milk
to the walls of the machine when milk of high acidity was used.
Jensen[147] details the results of 139 tests in 1899, made by the
Copenhagen Health Commission. In 66 samples from one hundred thousand to
one million organisms per cc. were found, and in 22 cases from one to
five millions. Nineteen tests showed less than 10,000 per cc.
In a series of tests conducted by the writer[148] on a Miller
pasteurizer in commercial operation, an average of 21 tests showed
12,350 bacteria remaining in the milk when the milk was pasteurized from
156 deg.-164 deg. F. The raw milk in these tests ran from 115,000 to
about one million organisms per cc.
A recently devised machine of this type (Pasteur) has been tested by
Lehmann, who found that it was necessary to heat the milk as high as
176 deg. to 185 deg. F., in order to secure satisfactory results on the
bacterial content of the cream.
The writer tested Reid's pasteurizer at 155 deg. to 165 deg. F. with the
following results: in some cases as many as 40 per cent. of the bacteria
survived, which number in some cases exceeded 2,000,000 bacteria per
cc.
~Pasteurizing details.~ While the pasteurizing process is exceedingly
simple, yet, in order to secure the best results, certain conditions
must be rigidly observed in the treatment before and after the heating
process.
It is important to select the best possible milk for pasteurizing, for
if the milk has not been milked under clean conditions, it is likely to
be rich in the spore-bearing bacteria. Old milk, or milk that has not
been kept at a low temperature, is much richer in germ-life than
perfectly fresh or thoroughly chilled milk.
The true standard for selecting milk for pasteurization should be to
determine the actual number of bacterial _spores_ that are able to
resist the heating process, but this method is impracticable under
commercial conditions.
The following method, while only approximate in its results, will be
found helpful: Assuming that the age or treatment of the milk bears a
certain relation to the presence of spores, and that the acid increases
in a general way with an increase in age or temperature, the amount of
acid present may be taken as an approximate index of the suitability of
the milk for pasteurizing purposes. Biological tests were carried out in
the author's laboratory[149] on milks having a high and low acid
content, and it was shown that the milk with the least acid was, as a
rule, the freest from spore-bearing bacteria.
This acid determination can be made at the weigh-can by employing the
Farrington alkaline tablet which is used in cream-ripening. Where milk
is pasteurized under general creamery conditions, none should be used
containing more than 0.2 per cent acidity. If only perfectly fresh milk
is used, the amount of acid will generally be about 0.15 per cent with
phenolphthalein as indicator.
[Illustration: FIG. 28. Diagram showing temperature changes in
pasteurizing, and the relation of same to bacterial growth.
Shaded zone represents limits of bacterial growth, 50 deg.-109 deg. F.
(10 deg.-43 deg. C.), the intensity of shading indicating rapidity of
development. The solid black line shows temperature of milk during the
process. The necessity for rapid cooling is evident as the milk falls
in temperature to that of growing zone.]
Emphasis has already been laid on the selection of a proper limit of
pasteurizing (p. 114). It should be kept constantly in mind that the
thermal death-point of any organism depends not alone on the temperature
used, but on the period of exposure. With the lower limits given, 140
deg. F., it is necessary to expose the milk for not less than fifteen
minutes. If a higher heat is employed (and the cooked flavor
disregarded) the period of exposure may be curtailed.
~Chilling the milk.~ It is very essential in pasteurizing that the heated
milk be immediately chilled in order to prevent the germination of the
resistant spores, for if germination once occurs, growth can go on at
relatively low temperatures.
The following experiments by Marshall[150] are of interest as showing
the influence of refrigeration on germination of spores:
Cultures of organisms that had been isolated from pasteurized milk were
inoculated into bouillon. One set was left to grow at room temperature,
another was pasteurized and allowed to stand at same temperature, while
another heated set was kept in a refrigerator. The unheated cultures at
room temperature showed evidence of growth in thirty trials in an
average of 26 hours; 29 heated cultures at room temperature all
developed in an average of 50 hours, while the heated cultures kept in
refrigerator showed no growth in 45 days with but four exceptions.
Practically all of the rapid-process machines are provided with
especially constructed cooling devices. In some of them, as in the
Miller and Farrington, the cooling is effected by passing the milk
through two separate coolers that are constructed in the same general
way as the heater. With the first cooler, cold running water is
employed, the temperature often being lowered in this way to 58 deg. or
60 deg. F. Further lessening of the temperature is secured by an
additional ice water or brine cooler which brings the temperature down
to 40 deg.-50 deg. F.
In the economical use of ice the ice itself should be applied as closely
as possibly to the milk to be cooled, for the larger part of the
chilling value of ice comes from the melting of the same. To convert a
pound of ice at 32 deg. F. into a pound of water at the same temperature,
if we disregard radiation, would require as much heat as would suffice to
raise 142 pounds of water one degree F., or one pound of water 142 deg. F.
The absorptive capacity of milk for heat (specific heat) is not quite
the same as it is with water, being .847 for milk in comparison with 1.0
for water.[151] Hot milk would therefore require somewhat less ice to
cool it than would be required by any equal volume of water at the same
temperature.
~Bottling the product.~ If the milk has been properly pasteurized, it
should, of course, be dispensed in sterilized bottles. Glass bottles
with plain pulp caps are best, and these should be thoroughly sterilized
in steam before using. The bottling can best be done in a commercial
bottling machine. Care must be taken to thoroughly clean this apparatus
after use each day. Rubber valves in these machines suffer deterioration
rapidly.
[Illustration: FIG. 29. Relative consistency of pasteurized cream before
(A) and after (B) treatment with viscogen as shown by rate of flow down
inclined glass plate.]
~Restoration of "body" of pasteurized cream.~ The action of heat causes
the tiny groupings of fat globules in normal milk (Fig. 22) to break up,
and with this change, which occurs in the neighborhood of 140 deg. F.,
where the milk is heated for about 15 minutes and at about 160-165 deg.
F. where rapidly heated in a continuous stream, the consistency of the
liquid is diminished, notwithstanding the fact that the fat-content
remains unchanged. Babcock and the writer[152] devised the following
"cure" for this apparent defect. If a strong solution of cane sugar is
added to freshly slacked lime and the mixture allowed to stand, a clear
fluid can be decanted off. The addition of this alkaline liquid, which
is called "viscogen," to pasteurized cream in proportions of about one
part of sugar-lime solution to 100 to 150 of cream, restores the
consistency of the cream, as it causes the fat globules to cluster
together in small groups.
The relative viscosity of creams can easily be determined by the
following method (Fig. 29):
Take a perfectly clean piece of glass (plate or picture glass is
preferable, as it is less liable to be wavy). Drop on one edge two or
three drops of cream at intervals of an inch or so. Then incline piece
of glass at such an angle as to cause the cream to flow down surface of
glass. The cream, having the heavier body or viscosity, will move more
slowly. If several samples of each cream are taken, then the aggregate
lengths of the different cream paths may be taken, thereby eliminating
slight differences due to condition of glass.
FOOTNOTES:
[126] From 10 to 16 cents per quart is usually paid for such milks.
[127] Much improvement in quality could be made by more careful control
of milk during shipment, especially as to refrigeration; also as to the
care taken on the farms. The use of the ordinary milking machine (see
page 37), would go far to reduce the germ content of milk.
[128] Farrington, Journ. Amer. Chem. Soc., Sept., 1896.
[129] Hite, Bull. 58, West Va. Expt. Stat., 1899.
[130] Milch Zeit., 1895, No. 9.
[131] Ibid., 1897, No. 33.
[132] Bernstein, Milch Zeit., 1894, pp. 184, 200.
[133] Thoerner, Chem. Zeit., 18:845.
[134] Snyder, Chemistry of Dairying, p. 59.
[135] Doane and Price (Bull. 77, Md. Expt. Stat., Aug. 1901) give quite
a full resume of the work on this subject in connection with rather
extensive experiments made by them on feeding animals with raw,
pasteurized and sterilized milks.
[136] Rickets is a disease in which the bones lack sufficient mineral
matter to give them proper firmness. Marasmus is a condition in which
the ingested food seems to fail to nourish the body and gradual wasting
away occurs.
[137] De Man, Arch. f. Hyg., 1893, 18:133.
[138] Th. Smith, Journ. of Expt. Med., 1899, 4:217.
[139] Russell and Hastings, 17 Rept. Wis. Expt. Stat., 1900, p. 147.
[140] Russell and Hastings, 21 Rept. Ibid., 1904.
[141] Russell and Hastings, 18 Rept. Ibid., 1901.
[142] Russell, Bull. 44, Wis. Expt. Stat.
[143] Russell, 22 Wis. Expt. Stat. Rept., 1905, p. 232.
[144] Russell, 12 Wis. Expt. Stat. Rept., 1895, p. 160.
[145] De Schweinitz, Nat. Med. Rev., 1899, No. 11.
[146] Harding and Rogers. Bull. 182, N. Y. (Geneva) Expt. Stat., Dec.,
1899.
[147] Jensen, Milchkunde und Milch Hygiene, p. 132.
[148] 22 Wis. Expt. Stat. Rept., 1905, p. 236.
[149] Shockley, Thesis, Univ. of Wis., 1896.
[150] Marshall, Mich. Expt. Stat., Bull. 147, p. 47.
[151] Fleischmann, Landw. Versuchts Stat., 17:251.
[152] Babcock and Russell, Bull. 54, Wis. Expt. Stat., Aug. 1896.