PRESERVATION OF MILK.
It has been shown in a previous chapter that milk becomes
contaminated with a multitude of bacteria not only on the farm where
it is produced, but during the various stages prior to its use. Many
of the bacteria which find their way into milk are readily able to
develop, and by their growth, render the milk unfit, or even harmful
for human food. With the most stringent precautions that can
reasonably be taken, it is impossible to avoid all contamination;
hence, all grades of milk will soon spoil, unless some means of
preservation is employed. Indeed, of all the foods classed as
perishable, milk is the one that most rapidly deteriorates. Produced
under ordinary conditions, it is unfit for ordinary use in a few
hours if kept at 70 deg. F.
There are three possible ways by which milk may be preserved: (1)
The removal of bacteria that have gained entrance to it; (2) The
prevention of growth of the contained bacteria; (3) The destruction
of the contained organisms. In practice at least two and sometimes
all of these methods are employed. The prevention of contamination,
a subject discussed in Chapter III is in reality one of the most
efficient means of preserving milk. In milk production, as
elsewhere, prevention is preferable to cure. Milk produced under
such conditions that its germ content is but a few thousand per
cubic centimeter will keep much longer than that handled in the
ordinary manner.
It might naturally be supposed that any method by which dirt is
removed from milk would improve the keeping quality of milk, due to
the reduction of bacteria, yet while the straining of the milk at
the time of milking removes dirt of various kinds, it does not
appreciably enhance the keeping quality, owing to the fact that the
bacteria adherent to the dirt particles are washed off in straining,
and pass through the pores of the strainer.
=Filtration of milk.= It is possible to remove all bacteria from water
and other fluids and thus render them sterile by passing through
filters of unglazed porcelain. This process can not be used with
milk for the fat globules are larger than the bacteria (see Fig. 6)
and any process that would remove the latter would also remove the
former. The term "filtration" is applied to a process used in some
European cities for the removal of the insoluble dirt that has been
introduced into the milk. Suitable containers are filled with layers
of coarse sand at the bottom and with finer sand at the top. The
milk is introduced at the bottom and is forced upward through the
sand. Such a filtering process is a very efficient means of removing
the dirt; but unless the filters are kept scrupulously clean, the
bacteria are likely to grow in the filtering material, so that the
number of organisms in the milk may actually be increased by the
filtering process. It is necessary to remove the sand daily and
thoroughly wash and sterilize the same. The extra care required in
keeping these sand filters in sanitary condition has been the great
objection to their employment in this country. Filters of other
material such as cellulose have been employed but with no marked
success.
=Clarifying milk.= A much more efficient and less troublesome means of
removing the insoluble foreign particles from milk is to pass it
through a cream separator, allowing the cream and skim milk to mix
in the same container. The slime that collects on the wall of the
separator bowl is made up of dirt, casein, bacteria, and the
cellular debris from the interior of the udder. The bacteria are
heavier than the milk serum, and would, therefore, be deposited on
the wall of the bowl were it not for other factors that in a measure
prevent this. The movement of the fat toward the center of the bowl
carries into the cream a considerable proportion of the bacteria in
the milk. The slime will always contain many more bacteria than the
milk, but the per cent of bacteria thus removed is relatively low,
due to the small amount of slime obtained from the milk, so that the
actual effect of clarification on the keeping quality of milk is
insignificant. The complete removal of all insoluble and therefore
visible dirt is, however, regarded of sufficient value to warrant
the use.
Machines designed especially for the clarification of milk are now
widely used. They differ from the cream separator in that the milk
is introduced at the outside of the bowl and hence there is no
separation of the fat from the serum. It is claimed that the removal
of the dirt, cells from the interior of the udder and bacteria is as
efficiently done as when the separator is used. The advantages
claimed for the machine are that it has no effect on the subsequent
gravity creaming of the milk and that less power is demanded than
for the separator.
From the standpoint of the consumer, all processes by which dirt is
removed from milk are objectionable, since they make the milk
appear cleaner and better than it really is, the harm having been
done when the dirt with the adherent bacteria found its way into the
milk. The removal of the foreign matter that has been introduced
into the milk will have but little effect in reducing the number of
bacteria, since a large part of the organisms will have been washed
off the insoluble material. All of these processes improve the
appearance of the milk but have little or no influence in increasing
its keeping quality or its healthfulness.
=Preservation by cold.= The only legitimate way of preventing the
growth of bacteria in milk is by holding it at temperatures at which
the ordinary forms of bacteria cannot thrive. Bacterial growth is
greatly checked at temperatures approximating 50 deg. F., or below,
although certain types multiply at the freezing point or slightly
above. If food products are actually congealed, no germ growth
occurs, and they may be kept quite indefinitely, but this process
cannot be successfully applied to milk, as the fat and casein are
physically changed, so that a normal emulsion can not again be made
when the frozen milk is melted. The fat separates in visible masses
as though the milk had been partially churned. On account of this
fact milk must be stored at temperatures above the freezing point.
In Denmark efforts have been made to preserve milk, that is to be
shipped long distances, by freezing a portion of the milk, and
placing a block of the frozen milk in each can after cooling the
main mass of milk nearly to the freezing point. Even this method has
not proven practical, and at present reliance is placed on thorough
chilling of the milk. At 32 deg. F., the lactic bacteria cannot grow,
but other types, such as certain of the putrefactive forms grow
slowly; the milk may, therefore, have no objectionable odor or taste
and yet be swarming with bacteria. In cities the practice is
followed of placing cream in cold-storage during the cooler periods
of summer in preparation for an increased demand, during hot weather
or on holidays. It seems probable that poisoning from ice cream may,
at times, be due to the use of such cream.
=Preservation by the use of antiseptics.= Many chemical substances
prevent the growth of bacteria when added to food supplies; such
substances thus used are called _preservatives_. In the past some of
these have been used in milk to a great extent, but at present, on
account of stringent pure food laws, they are employed only to a
slight extent. There is a great temptation for the small milk dealer
in the city to employ them to preserve the excess of milk from day
to day, as through the use of a few cents worth of some preparation,
many dollars worth of milk may be kept from spoiling until it can be
sold to the unsuspecting consumer.
Formalin has been most widely used in milk because it is a most
efficient preservative; it is cheap and cannot be detected by the
consumer, although it injures the digestibility of the casein. One
ounce will keep one thousand pounds of milk sweet for twenty-four to
forty-eight hours. Borax, boric acid, and salicylic acid have also
been used, but these substances must be employed in much larger
quantities than formalin. Bicarbonate of soda has sometimes been
used although it is not a true preservative. Its effect is based
upon the neutralization of the acid produced by bacterial growth.
The treated milk does not taste sour so quickly, and the curdling of
the milk is also delayed.
Many proprietary compounds for milk preservation have been placed on
the market in the past, but the use of all of these is illegal in
most states. The federal law also prohibits their use in all dairy
products that pass into interstate commerce.
Within recent years a method for the preservation of milk was
introduced by a Danish engineer, Budde, which consists of adding to
milk a very small amount of peroxid of hydrogen which is a very
efficient antiseptic. The peroxid is decomposed by some substance in
the milk; the products of decomposition being water and free oxygen.
The peroxid together with the application of heat at a comparatively
low temperature (122 deg. F.) is sufficient to destroy the larger part
of the bacteria in the milk. Practical difficulties are encountered
in the commercial application, so that it is probable the process
will never be a commercial success.
For the preservation of composite samples of milk for analytical
purposes, such as the Babcock test, strong disinfectants, as
corrosive sublimate, are employed. This material is very poisonous,
and leaves the milk unchanged in appearance. Some coloring matter is
therefore usually mixed with the sublimate in making the
preservative tablets, so as to render their use more conspicuous.
Corrosive sublimate not only stops all bacterial growth, but quickly
destroys the life of the cells. Bichromate of potash is generally
employed in the preservation of composite samples for the Hart
casein test.
=Destruction of bacteria in milk.= Actual destruction of the life of
bacterial cells by heat is one of the most important ways for
preserving milk. Heat easily destroys the vegetating, growing
bacteria, while the spores, of which there are always a number in
milk, are very resistant. If, however, the growing organisms are
destroyed, the milk will keep much longer than if it had not been so
treated.
The process of pasteurization was first used by the French
bacteriologist, Pasteur, for the treatment of the wines of his
native district which were likely to undergo undesirable types of
fermentations due to bacteria. From the wine industry it was applied
in the brewing industry, and was later found to be of the greatest
service in the dairy industry. The process of pasteurization may be
briefly defined, as the heating of milk to temperatures, varying
from 140 deg. F. and upward for a longer or shorter time, and
subsequently cooling to a low temperature, so as to prevent the
germination of the spores that are not destroyed by the heating.
=Effect of heat on milk.= When milk is heated it undergoes more or
less profound changes, depending on the temperature and time of
heating. Some of these changes are of practical importance, since
they are more or less evident, and objectionable to the consumer.
In raw milk the fat globules are largely found in larger or smaller
aggregates, rather than uniformly distributed throughout the serum.
The surface of a mass of fat globules is smaller in proportion to
the volume of the mass than is the case with single globules, hence
globule clusters encounter less resistance in their passage through
the serum, either as they rise to the surface in gravity creaming,
or in the separator bowl. If these clusters are broken up, so
that the globules are uniformly distributed, the milk will cream
much less rapidly and completely. In the process known as
"homogenization" of milk, the individual fat globules are broken
into such small globules, that they cannot overcome the viscosity
of the serum, and they remain distributed throughout the milk. In
such cases, no cream rises, and even the cream separator is unable
to remove the fat from such milk.
In selling bottled milk, it is highly desirable that the cream line
should show distinctly. In normal milk, this line forms in a few
hours, but where milk is heated to a high temperature, and agitated
at the same time, the clusters of fat globules are broken apart and
the creaming power injured. This physical change is dependent not
only on the temperature, but also on the time of exposure. A
momentary exposure at 160 deg. F., or for 20 minutes at 145 deg. F., is
about the maximum limit which can be applied to milk without
material injury to the creaming property.
[Illustration: Fig. 25.--Fat Globules in Raw Milk.
In raw milk the fat globules are in masses of varying sizes. These
rise to the surface quickly in gravity creaming.]
The body or consistency of pasteurized cream may be restored by
allowing the cream to stand for several days at low temperatures,
or by the addition of a small amount of sucrate of lime. This
substance, known to the dairy trade as "viscogen," is made by adding
to a thick solution of cane sugar, some freshly slaked lime. The
sugar solution permits of the dissolving of a much larger amount of
the lime than is possible in water. When the liquid is allowed to
settle, the clear solution is then decanted off and is used at the
rate of about one part to 100 to 150 parts of cream. The fat
globules are, by its action, brought into aggregates and the body of
the cream thus restored. Viscogen contains nothing that is at all
harmful, but milk and cream to which it is added must be sold under
some distinctive name as "visco-cream," since the laws of
practically all states do not allow the addition of any substance
whatever to milk or cream.
[Illustration: Fig. 26.--Fat Globules in Heated Milk.
When milk is heated the masses of globules are broken up and fat
globules are uniformly distributed throughout the milk.]
[Illustration: Fig. 27.--Creaming of Milk.
The cylinder on the left contains raw milk; that in the center, milk
heated to 140 deg. F. for twenty minutes; on the right, milk heated to
160 deg. F. for twenty minutes. The dark line indicates the depth of the
cream after twenty-four hours. The breaking up of the fat globule
clusters delays greatly the rising of the cream.]
Heated milk has a taste unlike that of raw milk; to one not
accustomed to it the taste is objectionable. This change is due to
some extent to the expulsion of the carbon dioxide from the milk.
The insipid taste of boiled water is, in part, due to its freedom
from carbon dioxide. The production of this cooked flavor is
dependent upon the time and temperature of exposure. It has been
claimed that heated milk is less digestible than raw, and a
considerable amount of experimental work has been done, both on
animals and children, in order to determine the relative
digestibility of heated and raw milk. The results obtained have been
contradictory. It is claimed that heated milk causes such diseases
as rickets, scurvy and marasmus in children. It is probably true
that milk heated to the boiling point is less fitted as food for the
young child than raw milk, but, on the other hand, it has not been
proven that properly pasteurized milk is an unsuitable food for
children. The best evidence has been accumulated in recent years, in
many of the large cities of this country and of Europe, where
pasteurized milk has been used with the greatest success in the
feeding of children of all ages.
The heated milk does not curdle readily when rennet is added due to
the precipitation of the lime salts by heat. The curdling power can
be restored by the addition of soluble lime salts or of acids.
=Purpose of pasteurization.= There are two reasons for the
pasteurization of milk: (1) To improve the keeping quality; (2) To
destroy any pathogenic bacteria it may contain. The first may be
called the economic reason; the second, the hygienic reason fur
pasteurization. In the selection of a proper pasteurizing
temperature, two factors must be taken into account: First, the
effect of heat on milk, and second, the temperature necessary to
destroy those forms of bacteria that are of the greatest importance,
as far as the keeping properties are concerned, and the pathogenic
bacteria that might possibly be present in the milk. The lactic
acid bacteria are non-spore-bearing and are not resistant to heat.
Most of them are destroyed when the milk is heated to 140 deg. F. for
fifteen minutes or to 160 deg. F. for a moment. To insure proper keeping
quality, somewhat higher temperatures must be employed, such as 145 deg.
to 150 deg. F. for fifteen to twenty minutes.
Milk pasteurized at these temperatures will, as a rule, undergo an
acid fermentation in much the same manner as will raw milk. The rate
with which the acid develops is of course much slower than in the
raw milk, due to the destruction of 95 to 99 per cent of the
acid-forming bacteria. If the milk has been pasteurized at higher
temperatures, the acid fermentation may not appear. The spores of
the spore-bearing organisms will be left; these may germinate and
cause their characteristic change in milk, which, as previously
noted, is usually a sweet-curdling or a digesting fermentation.
Since the changes they produce in the milk are not evident at first,
it might be used as food even though it was so far advanced in
decomposition as to be undesirable or even harmful as food. Indeed
one of the objections urged against pasteurization is that it
destroys the natural safe guard, the acid-forming bacteria. Many
people are so accustomed to use this as the indication of spoiled
milk that they will use milk long after it should be used if it does
not show an acid fermentation.
The butyric acid organisms are spore forming and may at times
produce their characteristic fermentation in pasteurized milk. The
milk shows gas formation and develops an objectionable odor.
The pathogenic bacteria most likely to be present in the milk are
the typhoid and the tubercle organisms. The typhoid bacillus is no
more resistant to heat than the ordinary acid-forming bacteria, and
all milk that has been heated, so as to impart to it satisfactory
keeping properties, will certainly be free from typhoid bacilli. It
has sometimes been asserted that the tubercle bacillus is very
resistant to heat; some claiming that it is necessary to heat milk
to 200 deg. F. in order to destroy it. Other experimenters have asserted
that lower temperatures would suffice, but the temperatures were
still above those at which the milk is physically and chemically
changed by the heating process. More recent work has shown that not
all sources of error were avoided in the earlier attempts to
determine the thermal death point of the tubercle bacillus, as, for
example, it has been shown by the authors that the "scalded film"
that forms on the surface of milk when heated in an open vessel will
protect the bacteria imbedded in it. It has also been shown by the
authors that a temperature of 140 deg. F., for twenty minutes or 160
deg. F. for one minute will destroy the tubercle bacilli in milk, in
case the heating is done with sufficient thoroughness to insure all
particles of the milk being heated to the same temperature for these
periods of time.
The pasteurization of milk can be done in such a manner as to impart
to it good keeping qualities and to insure its freedom from
pathogenic bacteria, and yet not impair its physical and chemical
properties, but much of the so-called pasteurized milk placed on the
market is not treated in accordance with proper hygienic methods.
[Illustration: Fig. 28.--The Pott's Discontinuous Pasteurizer.
The milk is placed in the inner compartment. For heating and
cooling, hot or cold water is passed between the jackets.]
=Methods of pasteurization.= In order to destroy the bacteria in milk,
it is necessary that the milk be heated for a varying time dependent
upon the temperature employed. A lower temperature for a
considerable period may exert the same effect on the bacteria as a
higher temperature for a shorter time. In practice, two types of
pasteurizing machines are employed, depending on the temperature at
which the milk is to be treated. The discontinuous machines or
intermittently operated pasteurizers are those in which the milk is
heated for any desired time at any temperature. Such machines
consist of jacketed containers the inner receptacle being filled
with milk, while the outer space between the walls is filled with
circulating hot water or steam. The milk is kept agitated by the
rotation of the machine. After it is heated, it is cooled in the
same container by replacing the hot water first with cold water,
then ice water. The disadvantage of this process is that the
capacity of the machine is limited which precludes its use in places
where large quantities of milk or cream are handled; for the
pasteurization of limited quantities, it is very successful, as
every particle of milk or cream is under the direct control of
the operator and may be thoroughly and efficiently treated.
As pasteurization was introduced for the treatment of market milk,
and for the preparation of cream for butter, machines have been
devised which permit large quantities, as thousands of pounds, to be
handled per hour. It is evident under these conditions that the milk
must be heated for only a short time, and hence a higher temperature
must be employed. These machines are called "continuous flow"
pasteurizers since the milk passes through them in a constant
stream. The period of exposure is very short, in some only a few
seconds; hence, they are sometimes called "flash" pasteurizers.
[Illustration: Fig. 29.--A Continuous Pasteurizer.
The milk is exposed but a short time since it flows through the
heater in a constant stream.]
All machines of this type possess the obvious disadvantage that it
is impossible to heat all of the milk for a uniform period. The milk
in contact with the walls of the machine flows much more slowly than
in the middle of the stream, just as the current near the bank is
less rapid than in mid-stream. In none of the machines yet devised
have the designers been able to overcome this disadvantage. In a
test of one of the most widely used pasteurizers of this type, it
was found that some of the milk passed through the machine in 15
seconds, while the larger part of it was held for about 30 seconds,
and some as long as forty-five to sixty seconds. If the temperature
employed had been such as to destroy the bacteria in that part of
the milk heated for the minimum time, hygienic safety would be
assured, but in order to avoid injuring the physical properties of
the milk, the tendency is to use as low a temperature as possible,
so that the milk heated for the minimum time may often contain
organisms that have passed through the machine uninjured.
Many devices have been proposed for the heating and cooling of the
milk. In many of the pasteurizers, the milk flows in a thin stream
over a metal surface, on the opposite side of which is the heating
agent, usually steam; while in others, the milk is allowed to flow
through a vat in which revolve a series of discs into which steam is
passed. The discs are of considerable size; thus, making a large
heating surface; the milk is thus heated quickly, and is constantly
stirred by the rotation of the heating discs. In other types the
milk passes into the bottom of a chamber in which a dasher revolves
at a rapid rate. This catches the milk, throwing it in a thin film
onto the wall of the chamber, which is heated with steam on the
opposite side. From such machines, of which the Fjord, the Jensen,
and the Reid machines are types, the milk may be forced to a
considerable height. These are widely used in this country for the
pasteurization of milk and cream for butter making.
Milk that has been heated must be cooled at once by the use of cold
water and ice. In order to economize in the use of both steam and
cooling agents, the so-called regenerative machines were devised.
The essential feature of these machines lies in the fact that the
cold milk inlet and the hot milk outlet are on opposite sides of a
single partition; thus the inflowing cold milk is partially heated
by means of the already treated hot milk which it is desired to
cool.
In order to avoid the disadvantages of the continuous machines,
viz., lack of control, an apparatus has recently been devised which
can handle large quantities of milk, heating the same to any
temperature for any desired time. In such a machine the milk is
first heated in a continuous heater, and is then passed into large
tanks in which it is allowed to remain for the desired time, and
from which it flows over the coolers. Such an apparatus is called a
"holding" machine, and is probably the most feasible type of
pasteurizer now on the market, when all factors are considered. In
some of the continuous machines, an attempt is made to accomplish
the same result, by building the machine so that the milk requires
fifteen to twenty minutes for passage through the machine, but in
all such cases the same disadvantage of variation in rate of flow,
as in other continuous flow type of machines obtains.
=Tests of pasteurizing machines.= It is possible for the operator to
test the rate of flow in a machine, so as to determine whether all
of the milk is heated for a uniform time. This is done most easily
in the following manner: The machine is first filled with water,
heating the same to the desired temperature, and regulating the rate
of flow as it would be if milk was used. The flow of water is then
turned off, and a stream of milk containing a known per cent of fat
admitted to the machine. The time elapsing between the admission of
milk to the machine, and that at which the first sign of turbidity
is noted at the outlet, will be the minimum period necessary for any
portion of the milk to flow through the machine. At frequent
intervals thereafter, samples of the outflowing liquid may be
collected, noting the time at which each sample is taken. The
percentage of fat in the various samples is determined by the
Babcock test; at the moment when all of the water has been removed,
the sample taken will show the same fat content as the milk used.
The samples taken previous to this will show a lower fat test,
dependent upon the relative amount of water and milk. In this
manner, the minimum, the maximum, and the average period of exposure
of milk in the machine tested, can be determined with exactness.
The accompanying table gives results that were obtained in the
testing of one of the continuous types of machines. The machine in
question required about three hundred pounds of milk to fill it and
was supposed to handle 1,000 pounds per hour. Thus theoretically it
should require twenty minutes for any portion of the milk to pass
through the machine. As will be seen from the data, some of the milk
passed through within seven minutes after the water was shut off and
the milk turned on. The figures also show that not all of the water
had been replaced by the milk in even 45 minutes. In actual practice
like results will be obtained, and a portion of the milk will be
heated to the temperature employed but a short time. In this, the
vegetating bacteria will not be wholly destroyed.
=========================================================
Trial | |Per cent of fat in milk coming from
| | machine at following times
|Per cent|---------------------------------------
| of fat| MINUTES
| in milk|---------------------------------------
| | 7 | 11 | 15 | 19 | 23 | 27 | 36 | 47
--------+--------+----+----+----+----+----+----+----+----
No. I | 4.0 | 0.2| 0.8| 1.6| 2.0| 2.4| 2.6| |
No. II | 3.8 | 0.2| 0.6| 1.5| 1.8| 2.2| 2.6| 3.0| 3.4
No. III | 3.5 | 0.7| 1.9| 2.4| 2.8| 2.8| 3.0| 3.4| 3.4
=========================================================
=Pasteurization of small quantities of milk.= It is often desirable to
treat a small quantity of milk for home use, in which case the
commercial types of pasteurizers are out of the question. This
treatment can be done in a number of ways, consideration always
being paid to the manner of heating which should be done under such
conditions, as have been shown to be necessary for efficient
pasteurization. Milk may be heated in tall, narrow cans which are
placed in hot water. In the household, milk may be treated by
placing the filled bottle in a pail having a false bottom so the
bottle shall not be broken when the pail is placed on the stove. The
pail should be filled with water so that its level is about the same
as that of the milk. The water is then heated to the desired
temperature, maintained for the requisite period of time, and is
then cooled as rapidly as possible. During the heating, the mouth of
the bottle should be covered, either with an inverted glass tumbler,
or the paper cap may be left in place, simply punching a small hole
through it so as to permit of the insertion of a thermometer.
[Illustration: Fig. 30.--A Pasteurizer for Use in the Home.
A milk bottle with a tumbler for a cover. The cover prevents the
formation of the "scalded layer" on the milk during the heating and
also protects the mouth of the bottle from dust.]
=Efficiency of pasteurizing.= It is easy to destroy over 99 per cent
of the bacteria present by the use of any of the modern types of
machines. The number remaining after treatment will be largely
dependent, other things being equal, upon the number of bacteria
before pasteurization. The pasteurizing process is not one by which
poor milk can be changed into good milk, nor is it legitimate to use
the process in place of cleanliness, as is sometimes done. There is
a legitimate field for the process in the handling of market milk,
as well as in the creamery; but it should be used to improve the
keeping quality, and to insure the freedom of the milk from
pathogenic bacteria, when other protective measures have been
carried as far as possible under the prevailing conditions.
=Details of process.= If the process is to be successful, due
attention must be given to certain details. In the treatment of
market milk, care should be taken to use only that in which the
acidity has not materially increased. A fair standard is about 0.2
per cent. High acid milk usually means old milk or dirty milk,
either of which is very likely to contain many more spore-bearing
bacteria than clean, fresh milk. The greater the number of spores,
the more rapidly will the pasteurized milk spoil. If it is possible
to exercise any selection of milk prior to pasteurization, the rapid
test for determination of acidity will prove of great advantage.
Care should be taken to prevent fluctuations in the temperature to
which the milk is heated. With varying steam pressure and variations
in the rate of flow of milk, these fluctuations may be very
considerable. Regulators are now made that will control the
temperature within narrow limits.
In all pasteurized milk as it flows from the machine, there will
remain some living bacteria. The spores will not be destroyed by any
pasteurizing process, and under commercial conditions, vegetating
bacteria are also present. If the milk is not quickly chilled after
heating, these forms will grow, and their development is
particularly hastened by the destruction of the lactic bacteria, the
acid of which would otherwise hold them in check. The result is
that, unless immediately chilled, pasteurized milk spoils almost as
rapidly as though it had not been heated at all. Efficient and rapid
cooling are, therefore, as essential a portion of the process as the
heating itself.
Care should also be taken to protect the milk from contamination
after treatment. Every utensil with which it comes in contact should
be sterilized. The bottles should be thoroughly washed and
sterilized and subsequently protected from dust until used.
=Sterilization of milk.= It is possible to render milk sterile by the
use of temperatures above the boiling point of water, where it is
heated in a closed vessel, in which steam under pressure is
generated. Such milk is often found in the European markets. In our
own country, the only milk of this kind is the so-called "evaporated
milk." In this process sweet fresh milk is evaporated in vacuum pans
to about one-third of the original volume. This is then placed in
tin cans, which are treated, as in the canning of such vegetables as
peas and corn, by heating the milk to 230 deg. or 240 deg. F. for a few
minutes. In this process, the bacteria (spores as well as vegetating
forms) are completely killed, and the milk acquires a brownish tint,
due to the caramelization of the sugar. The appearance of the
product is very similar to cream, and previous to the passage of
the pure food law, it was sold as evaporated cream.
Condensed milk is not wholly free from bacteria, but is sufficiently
thick, by reason of its treatment so that the contained bacteria
cannot grow. They remain dormant in the milk, but as soon as it is
diluted to a normal consistency, growth takes place, and the milk
rapidly spoils. Condensed milk is prepared by adding cane sugar to
fresh sweet milk, then evaporating the mixture to one-third the
original volume, forming a semi-solid product. Syrups owe their
keeping qualities to the same factor, as condensed milk, _i.e._, the
high consistency.
Milk is also preserved by wholly evaporating the water, thus leaving
a dry powder, which on being mixed with water again will have much
the same properties as the original milk. Various methods have been
devised for the preparation of these milk powders, all of which have
been patented by the inventors. If the powder is to be kept for long
periods, skim milk must be used, since the fat slowly undergoes
changes which cause it to have a rancid odor. These dry preparations
are largely used by bakers in place of fresh milk.