CONTAMINATION OF MILK.
=Spoiling of milk.= Materials of animal origin are peculiarly prone to
undergo changes, rendering them unfit for use, and of these, milk is
exceedingly susceptible to such changes. This is due to the fact
that the composition of milk is especially adapted to bacterial
growth, and that the opportunity for entrance of such organisms is
likewise such as to permit of abundant contamination. The
consequence is that milk readily undergoes fermentative changes, due
to the development of one or another type of micro-organism.
=Milk, a suitable bacterial food.= While milk is designed by nature
for the nourishment of mammalian life, it is, curiously enough,
equally well adapted to the growth of these lowest forms of
vegetable life. The nutritive substances required by bacteria are
here sufficiently dilute to make possible rapid growth.
Milk also contains all the necessary chemical substances to make a
suitable bacterial food supply. Of the nitrogenous compounds,
albumen is in a readily assimilable form. Casein, the principal
nitrogenous constituent of milk, exists in an insoluble condition,
and cannot be directly utilized, until it is acted upon by digesting
enzymes. The fat in milk does not readily decompose, and while there
are a few bacteria capable of splitting this substance, the majority
of organisms are unable to utilize it. Milk sugar, on the other
hand, is an excellent food for most species.
[Illustration: Fig. 6.--Fat Globules and Bacteria.
Note the relative size of the fat globules of milk and the lactic
acid bacteria.]
=Sources of contamination.= Inasmuch as milk is especially exposed to
the inroads of bacterial growth, and because of the fact that much
of the contamination can easily be prevented, it is highly important
that the milk producer and dealer should be thoroughly cognizant of
the various sources of contamination. The different factors
concerned in contamination may be grouped as follows: the interior
of the udder; utensils, including all apparatus with which the milk
is brought in contact subsequent to withdrawal from the animal;
infection coming from the animal herself, from the milker, and the
surrounding air.
=Condition of milk when secreted.= Immediately after withdrawal from
the udder, milk always contains bacteria, yet in the secreting
cells of the udder of a healthy cow, germ life does not seem to be
present. Only when the gland is diseased are bacteria found in any
abundance. In the passage of the milk from the secreting cells to
the outside, it receives its first infection, so that when drawn
from the animal it generally contains a considerable number of
organisms.
A study of the structure of the udder shows the manner in which such
infection occurs.
=Structure of the udder.= The udder is composed of secreting tissue
(_gland cells_) that is supported by fibrous connective tissue. The
milk is elaborated in these cells and is discharged into microscopic
cavities, from whence it flows through the numerous channels (_milk
sinuses_) that ramify through the substance of the udder, until
finally it is conveyed into the _milk cistern_, a common receptacle
holding about one half pint that is located just above the teat.
This cavity is connected with the outside by a direct opening (_milk
duct_) through the teat. During the process of milking, the milk is
elaborated rapidly in the gland cells, and their contents upon
rupture of the milk cells, flow down into the cistern. The normal
contraction of the muscles at the lower opening of the outer duct
prevents the milk from passing out except when pressure is applied,
as in milking. The inner walls of the milk duct and cistern are
always more or less moist, and therefore afford a suitable place for
bacteria to develop, if infection once occurs, and conditions are
favorable for growth.
=Manner of invasion.= Two possible sources of invasion of the udder by
bacteria may exist. If bacteria are present in the circulating
blood, there is the possibility of organisms passing directly
through the tissues into the milk-secreting cells. The other
alternative is the possible direct contamination from the outside by
organisms passing up through the milk duct, and so spreading through
the open channels in the udder.
[Illustration: Fig. 7.--Sectional View of Udder.
Teat with milk duct connecting the exterior with the milk cistern.
Milk sinuses which conduct the milk from the secreting tissue to the
milk cistern. (After Moore & Ward.)]
=Number of bacteria in fore-milk.= If a bacteriological examination is
made of the milk drawn from each teat at different periods during
the milking process, it will be found that the fore-milk, _i.e._,
the first few streams, contains, as a rule, many more organisms per
cubic centimeter than that removed later. Not infrequently thousands
of organisms per cubic centimeter may be found in the first streams
while the middle milk, or strippings, will contain much smaller
numbers.
=Distribution and nature of bacteria in udder.= If the udder itself is
carefully examined as to its bacterial content, it appears that the
majority of organisms found is confined to the lower portion of this
organ, in the teat, milk-cistern and large milk-ducts; while
bacteria occur in contact with the secreting tissue, they are
relatively less abundant. This would seem to indicate that the more
probable mode of infection is through the open teat.
While there is no constant type of bacteria found in the fore-milk,
yet it is noteworthy that nearly all observers agree that the
organisms most commonly found are not usually the acid-producing, or
gas-generating type, so abundant on the skin or hairy coat of the
udder and which predominate in ordinary milks. Coccus forms,
belonging to both liquefying and non-liquefying types are most
generally present. Many of these produce acid slowly and in small
quantities.
The bacteria coming from the interior of the udder are of small
practical significance since they do not grow rapidly at the
temperatures at which milk is stored. If the milk is protected from
contamination from other sources, the bacteria from the udder will
ultimately cause it to spoil, but under ordinary conditions other
forms are present in such greater numbers, and grow so much more
rapidly in milk, that the udder forms have small opportunity to
exert any effect.
It is interesting to note that the bacteria found in the udder are
similar to those that seem to be most abundant in such glandular
tissues as the liver and spleen. This fact increases the probability
that these comparatively inert coccus forms of the udder may
originate directly from the blood stream. The organisms that
normally are found in the udder exert no harmful effects on the
gland. It might be thought that due to the presence of abundant food
and a favorable temperature that growth would be abundant, but such
is not the case. At times the udder may be invaded by forms that are
not held in check by the natural factors and an inflammation of the
udder is likely to result.
=Germicidal property of milk.= It has been claimed that freshly drawn
milk, like other body fluids, possesses germicidal properties,
_i.e._, the power of destroying bacteria with which it may be
brought in contact. If milk is carefully examined bacteriologically,
hour by hour, after it is withdrawn from the udder, it will
generally be found that there is at first not only no increase in
number of organisms during a longer or shorter period when it is
kept at temperatures varying from 40 deg. to 70 deg. F., but that an
actual reduction not infrequently takes place. When cultures of bacteria,
such as _B. prodigiosus_, a red organism, lactic acid organisms,
and even the yellow, liquefying coccus, so commonly found in the
fore-milk, are artificially introduced into the udder, it has been
found that no growth occurs and that in the course of a few days the
introduced organisms actually disappear. Whether this failure to
colonize can be regarded as evidence of a germicidal property or not
is questionable. In fact, this question is a matter of but little
practical importance in the handling of milk since, under the best
of conditions, the keeping quality of the milk is not materially
enhanced. It may be of importance in inhibiting growth in the udder.
=Rejection of fore-milk.= The fact that the fore-milk contains per
cubic centimeter so much more germ life than the remainder of the
milk has led some to advocate its rejection when a sanitary milk
supply is under consideration. While from a purely quantitative
point of view, this custom may be considered advantageous, in
practice, however, it is hardly worth while since it is not at all
certain that the rejection will have any effect on the keeping
quality or healthfulness of milk. This is especially true if the
ends of the teats are thoroughly cleaned before milking. It is true
that the fore-milk is relatively deficient in fat so that the loss
of butter fat occasioned by the rejection of the first few streams
is comparatively slight.
=Contamination from utensils.= One of the most important phases of
contamination is that which comes from the utensils used to hold the
milk from the time it is drawn until it is utilized. Not only is
this important because it is a leading factor in the infection of
milk, but because much improvement can be secured with but little
trouble, and it is especially necessary that the dairy student
should be made familiar with the various conditions that obtain.
Pails and cans used to hold milk may be apparently clean to the eye,
and yet contribute materially to the germ content of the milk placed
in them. Not only does much depend upon their condition, but it is
equally important to take into consideration their manner of
construction. Dairy utensils should be simple in construction,
rather than complex. They should be made so that they can be readily
and easily cleaned, or otherwise the cleaning process is apt to be
neglected.
Of first importance are those utensils that are used to collect the
milk and in which it is handled while on the farm. The warm milk is
first received in pails, and unless these are scrupulously cleaned,
an important initial contamination then occurs. As ordinarily
washed, the process falls far short of ridding the utensils of the
bacterial life that is adherent to the inner surface of the pail.
Then, too, all angles or crevices afford an excellent hiding place
for bacteria, and it is very important to see that all seams are
well soldered. Round corners and angles flushed with solder greatly
facilitate thorough cleaning of utensils. Tin utensils are
recognized as most satisfactory.
Shipping cans are likely to serve as greater infecting agents than
pails for they are subject to more wear and tear and are harder to
clean. As long as the surface is bright and smooth, it may be easily
cleaned, but large utensils, such as cans, are likely to become
dented and rusty in spots on the inner side. The storage of milk in
such utensils results in its rapid deterioration. The action of
rennet has been found to be greatly retarded where milk comes in
contact with a rusty iron surface. It is also probable that some of
the abnormal flavors in butter are due to the action of acid cream
on iron or copper surfaces from which the tin has been worn. It is
equally important that attention be paid to the care of strainers,
coolers, and the small utensils. Cloth strainers are more or less of
a hotbed for bacterial growth, for unless they are boiled, and then
dried quickly and thoroughly, germ growth will continue apace in
them, as long as they contain any moisture.
=Milking machines and farm separators.= The introduction of these
special types of dairy machinery in the handling of milk on the farm
has materially complicated the question of the care of milk. Both of
these types of apparatus are much more complicated than the usual
milk utensil; consequently, the danger of imperfect cleaning is
thereby increased. This is still further accentuated by the fact
that cleansing of utensils on the farm can never be done so well as
at the factory or milk depot where steam is available. The milking
machine may be easily kept in a comparatively germ-free condition,
but unless this is done, it contributes its quota of germ life to
the milk.
The farm separator is more widely used than the milking machine and
in actual practice the grossest carelessness prevails in the matter
of its care. Frequently it is not taken apart and thoroughly
cleansed, but is rinsed out by passing water through the machine. It
is impossible by such a treatment to remove the slime that collects
on the wall of the bowl; the machine remains moist and bacterial
growth can go on. Such a machine represents a most important source
of contamination of milk and cream and it is probable that the
widespread introduction of the hand separator has contributed more
to lower the quality of cream delivered at the factory than any
other single factor.
=Contamination from factory by-products.= The custom of returning
factory by-products in the same set of cans that is used to bring
fresh milk is a prominent cause of bad milk. Whey and skim milk are
rich in bacterial life, and not infrequently are so handled as to
become a foul, fermenting mass. If the cans used to transport this
material are not scrupulously cleaned on the farm, transfer of
harmful bacteria to the milk is made possible. In this way the
carelessness of a single patron may be the means of seeding the
whole factory supply. This custom is not only liable to produce a
poor quality of milk, but it is more or less of a menace to all the
patrons of a factory, inasmuch as the opportunity always obtains
that disease-producing organisms may thus be introduced into the
supply. Not infrequently is tuberculosis thus spread through the
medium of factory by-products.
[Illustration: Fig. 8.--Whey Disposal.
Whey barrels at a Wisconsin Swiss cheese factory. Each patron's
share is placed in a barrel which is so situated that it is
impossible to empty it completely; thus it is not cleaned during the
season.]
The manufacture of Swiss cheese presents a striking example of the
disregard which factory operators show toward the employment of
bacteriological principles. In these factories, the custom is widely
practiced of apportioning the patrons' allotment of whey into
individual barrels which are supposed to be emptied each day. As
these barrels are, however, rarely ever cleaned from the beginning
to the end of the season, they become very foul, and the whey placed
in them from day to day highly polluted. It is this material which
is taken back to the farms in the same set of cans that is used for
the fresh milk. When one recalls that the very best type of milk is
essential for the making of a prime quality of Swiss cheese, and
that to secure such, the maker insists that the patron bring the
product to the factory twice daily, the before mentioned practice
appears somewhat inconsistent.
=Treatment of factory by-products.= To overcome the danger of
infecting milk from factory by-products with either undesirable
fermentative organisms, or disease-producing bacteria, the most
feasible process is to destroy these organisms by the application of
heat. In Denmark, some portions of Germany, and in some of the
states in this country, laws exist which require the heating of all
skim milk before it is returned to the farm. This is done by the
direct use of exhaust steam, or running the product through heaters.
The treatment of whey in cheese factory practice is especially
important since the warm whey must be stored for a number of hours
before it is returned to the farms. Even under the best of
conditions the whey is certain to be in an advanced state of
fermentation when placed in the milk cans, and it only needs the
infection of the whey tank with harmful bacteria to cause great loss
on account of the injury of the product by these bacteria. Among
Canadian factories the custom of heating the whey as it passes from
the cheese vat to whey tank has been introduced, and where ever
adopted has been retained, because, it has resulted in such an
improvement of the cheese that the gain was much greater than the
cost, which is estimated at not over fifty cents per ton of cheese.
The whey is heated not to exceed 155 deg. F.; the hot whey serves to
scald the whey tank and as the mass of whey is usually quite large,
it does not cool to a point where bacterial growth can take place
for a number of hours. The whey is thus quite sweet when returned to
the farm and has greater feeding value. The heating also prevents
the creaming of the whey in the tank and thus avoids the soiling of
the cans with grease which is most difficult to remove.
Where compulsory legislation is in force it is generally required
that these by-products be heated to a temperature of at least 176 deg.
F. This is done so as to destroy effectually the organisms of
tuberculosis, and especially to permit of the utilization of the
so-called Storch test,[1] which enables a person to determine
quickly whether milk or whey has been heated or not.
[1] Storch (40 Rept. Expt. Stat., Copenhagen, 1898) has devised
a test whereby it can be determined whether this treatment has
been carried out or not; milk contains a soluble enzyme known as
peroxidase which has the property of decomposing hydrogen
peroxid. If milk is heated to 176 deg. F., (80 deg. C.) or above, this
enzyme is destroyed, so that the above reaction no longer takes
place. If potassium iodide and starch are added to unheated milk
and the same treated with hydrogen peroxid, the decomposition of
the latter agent releases oxygen which acts on the potassium
salt, which in turn gives off free iodine that turns the starch
blue.
=Cleaning utensils.= Various processes are applied to dairy utensils
to cleanse them. In removing visible dirt and foreign matter, much
of the bacterial life is mechanically eliminated, but most of the
cleaning processes fail to destroy the germ life in these utensils.
In rinsing, washing, or even scalding, the water is not applied at a
sufficiently high temperature to destroy effectively the bacteria.
These processes are primarily used for the removal of dirt and other
matter. To facilitate such removal, washing powders of various kinds
are frequently employed; some of these possess considerable
disinfecting action. All utensils after cleansing should be
thoroughly rinsed in clean, hot water. Even where no further
treatment is given, a careful cleaning may so reduce the germ
content on the inner surface of utensil as to render contamination
therefrom relatively unimportant. Most of the contamination in a
well cleaned utensil comes from the cracks and angles, which permit
of the collection of the dirt. If these are properly attended to,
thorough cleaning and rinsing alone will accomplish much.
To exert an actual germ-destroying effect on the bacterial content
of the utensil, resort must be had to boiling or steaming. To treat
utensils so as to render them wholly germ-free would be impractical
under ordinary commercial conditions, as it would consume too much
time, although with proper apparatus, this process is not
impossible, but it is well within the limits of practicability in
factory treatment to apply steam for a short period of time. Where
cans, pails and such utensils, are steamed for a minute or so after
being thoroughly cleaned, the germ content is greatly reduced. In a
series of tests by Harrison, the germ content of a set of cans
cleaned in an ordinary way was 442,000 bacteria per cubic centimeter
in 100 cubic centimeters of wash water; in a set washed in tepid
water and then scalded--the best farm practice--it was 54,000 per
cubic centimeter, while in cans carefully washed and then steamed
for 5 minutes, it was reduced to 880 per cubic centimeter. It would
not be worth while to institute measures that would accomplish the
destruction of this small residual content.
The use of steam, therefore, is of great service in eliminating
bacterial life in all utensils. In apparatus of at all complicated
design, it is absolutely necessary. Of course, ordinarily, steam can
be applied only at the factory, as the farm does not usually afford
facilities for its easy generation. This fact has led in some cases
to the adoption of the method of cleaning and sterilizing the cans
at the factory rather than to await their arrival at the farm. This
custom is most frequently followed in milk supply plants.
It is also very important in cleaning dairy utensils to see that
they are rapidly and thoroughly dried after being washed and
steamed. As pointed out above, the short period of steaming that can
be followed in practice does not kill all the bacteria. If moisture
is retained, conditions permit of the growth of the undestroyed
organisms. Tests made on glass milk bottles showed that considerable
growth occurred in the condensation water even after quite thorough
sterilization. Some of the devices used for the sterilization of
such utensils as milk cans are so arranged that, after steam has
been introduced, hot air is passed into the can until it is
thoroughly dried. Other utensils such as cloth strainers become
sources of contamination unless the articles are thoroughly and
quickly dried after cleaning.
In a general way, it may be said that whenever a utensil is so
constructed and in such a condition that every portion of its
surface can be reached by a cloth or a brush, it can be kept in a
sanitary condition. But whenever any portion cannot be thus reached,
whether it is an angle or a seam in a pail or can, the interior of
the separator bowl, or in the pipes used for conducting milk,
contamination is certain to result from such places, unless extreme
care is taken to destroy the bacteria therein by steaming.
=Contamination from the animal.= In the process of milking, the
bacterial content of the milk is materially increased. In part this
comes from the utensils into which the milk is drawn, but the animal
herself, the milker, as well as the surrounding air, also contribute
to a varying extent. Of these factors, the one fraught by far with
the most consequence, is the influence of the animal herself. It is
a popular belief that the organisms found in milk are derived from
the feed and water which the animal consumes, but under normal
conditions, the bacteria consumed in food pass through the
intestinal canal and do not appear in the circulation. It must not
be assumed, however, that the character of feed and water supply is
of no moment. Stock should be given pure and wholesome water and no
decomposed or spoiled food should be used.
The infection traceable directly to the cow is modified materially
by the conditions under which the animal is kept and the character
of the feed consumed. The nature of the fecal matter is in part
dependent upon the character of the food. The more nitrogenous the
ration fed, the softer are the fecal discharges, producing a
condition which is more likely to soil the coat of the animal unless
care is taken. The same is true with animals kept on pasture in
comparison with those fed dry fodder.
Stall-fed animals, however, are more likely to have their flanks
fouled, unless special attention is paid to the removal of the
manure. All dairy stalls should be provided with a manure drop which
should be cleaned as frequently as circumstances will permit.
[Illustration: Fig. 9.--Bacteria on Hairs.
Each colony on the hair represents one or more bacteria that were
adherent to the hair when it was placed on the surface of the solid
culture-medium.]
The animal contributes materially to the quota of germ life finding
its way into the milk through the dislodgment of dust and filth
particles adhering to its hairy coat. The nature of this coat is
such as to favor the retention of these particles. Unless care is
taken, the flanks and udder become polluted with fecal matter, which
upon drying is displaced with every movement of the animal. Every
hair or dirt particle so dislodged and finding its way into the
milk-pail adds its quota of organisms to the liquid. This can be
readily demonstrated by placing cow's hairs on the moist surface
of gelatin culture plates. Almost invariably bacteria will be found
in considerable numbers adhering to such hairs, as is indicated in
Fig. 9.
Dirt particles are even richer in germ life. Not only is there the
dislodgment of hairs, epithelial scales, and masses of dirt and
filth, but during the milking process, as at all other times, every
motion of the animal is accompanied by a shower of _invisible_
particles, more or less teeming with bacterial life. All of this
material contains organisms that are more or less undesirable in
milk. Bacteria concerned in gassy fermentations and those capable of
producing obnoxious taints are particularly common, so that this
type of pollution is especially undesirable in milk.
=Amount of dirt in milk.= When one remembers that the larger part of
fresh manure is of such a nature that it does not appear as
sediment, the presence of evident filth in milk must bespeak
careless methods of handling.
The sediment or dirt test is used quite extensively to ascertain the
amount of dirt milk may contain. By means of a cotton filter, the
insoluble residue is removed and is made evident upon a layer of
absorbent cotton. Milk that would show with difficulty any evidence
of dirt upon ordinary examination reveals such defects very readily
in this test.
=Exclusion of dirt.= It is better to keep bacteria out of milk, so far
as practicable, rather than to attempt to remove them after they
have once gained entrance. As is usual, prevention of trouble is
much more easily accomplished than removing the difficulty after it
once occurs.
[Illustration: Fig. 10.--Dirt from Milk.
The dirt adherent to each of the filters was obtained from one pint
of milk. The milks tested were produced on different farms.]
Much reduction as to the amount of dirt that finds its way into milk
may be accomplished by improved stable environment. The fouling of
the udder and flanks comes from wading in dirty water, muddy yards,
and from improper type of stalls. Barnyards are often a disgrace
through the accumulation of manure and seepage. Cows wading in such
mire cannot but accumulate mud and filth to a material degree on the
teats and udder. Greater care as to drainage of the barnyard and the
paving of same with gravel, cinders, etc., will permit of its being
kept clean, and so prevent the fouling of animals. But more
important than the yard is the stall which the animal occupies in
the stable. The essential feature is to have a stall of such
construction as to keep the animal out of her own manure when she
lies down. To accomplish this, it is necessary to have a manure drop
behind the stall proper so that the feces and urine are kept out of
the bed of the stall as much as possible.
[Illustration: Fig. 11.--The Model Stall.
A stall of this type keeps the animals clean, and thus aids greatly
in producing good milk.]
Most of the stalls widely advertised in the farm press seek to
accomplish this in one way or another, usually by some arrangement
by which the cow is forced back when standing and drawn forward on
lying down. In Fig. 11 a type of stall is illustrated that
accomplishes this most successfully; the essential feature being a
2x3-inch wood strip nailed to the stall floor immediately in front
of the hind feet of the animal when in a standing position. When the
animal lies down, she crowds forward to avoid lying on this strip,
and thus is out of contact with the manure, except such as is
carried onto the bedding by the hind feet. By the use of this stall
it is possible to keep the animals free from all accumulations of
manure.
Effort should be made to prevent fouling of the animals rather than
in cleaning them after once soiled. It is very evident that where
the cattle come to the milker with muddy udders, they will not be so
cleaned before milking as to prevent a large amount of such dirt
from entering the milk. However, when all that can be done towards
keeping the cows clean has been accomplished, a small amount of
grooming will greatly reduce the contamination coming from them.
The kind of bedding used in the stalls may have a marked influence
on the contamination coming from the animal. If the straw is dusty,
partially rotten and moldy, the bacteria and molds adhere to the
coat of the animal and are thus introduced into the milk. In the
case of cattle on pasture, no visible evidences of dirt are usually
present but the hair is covered with the dust coming from the soil.
There is very good reason to believe that the quality of milk is
influenced by the type of pasture on which the cows graze, due to
the difference in the types of bacteria in the surface soil. The
milk from animals on low land is more likely to show undesirable
fermentations than that from those grazing on higher lands. This is
not due to the influence of the feed as is often supposed but rather
to the dirt from the coat of the animal.
=Washing the udder.= If a surface is moist, dust and the adherent
bacteria cannot be easily dislodged. The air over snow-covered
mountains or over oceans is relatively free from bacteria. The udder
and flanks of the animals can be carded to remove the loose hairs
and the evident dirt; the fine dust can now be removed by wiping
with a clean damp cloth just before the milking process. The actual
washing and wiping of the udder and flanks still further reduces the
contamination coming from the animal; experiments show a reduction
of fully three-fourths of total contamination. Clipping the udder
and flanks also aids in keeping the animal clean.
It is often asserted that the treatment of the animals in these ways
reduces the yield of milk. It is certain that such an effect will
persist for only a short time and there is reason to believe that
grooming increases the yield.
[Illustration: Fig. 12.--Sanitary Milk Pails.
The small opening is very efficient in keeping the dirt out of
milk.]
=Sanitary milk pails.= The entrance of organisms into the milk can be
greatly reduced by lessening the area of the milk pail exposed to
the dust shower. To accomplish this purpose a number of so-called
sanitary or hygienic milk pails have been devised. In some cases,
these are the regular type of pail provided with a cover having a
small opening through which the milk is received. In other cases, a
strainer is interposed so as to remove more effectually the coarse
particles. While pails of this type are successful in the removal of
a large part of the dirt, and consequently reduce materially the
bacterial content of the milk, yet they must be of simple
construction, so that they can be kept in a clean condition in order
to adapt them for general practical use. The use of such a utensil
increases materially the keeping quality of the milk.
[Illustration: Fig. 13.--Sanitary Milk Pails.
The Stadtmueller pail and the Truman pail, two of the most practical
of the small-topped pails.]
Stocking has shown that under ordinary barn conditions, the use of
small-topped pails reduced the number of bacteria 95 per cent; with
dirty cows the reduction in bacteria amounted to 97 per cent. A
six-inch opening presents only one-fourth as large an exposure as a
twelve inch, so that the reduction in bacterial content is greater
than the lessening in the size of the openings of the pails. The
ordinary pail receives dust not only from the udder, but also from
the flank which is usually a more important source of contamination
than the udder itself, while the small-topped pail receives only
that from the udder.
[Illustration: Fig. 14.--use of Sanitary Milk Pails.
The open pail is fully exposed to the falling dust while the hooded
pail excludes much of the dust and dirt coming from the animal.]
=Milking machines.= Where the milk is removed from the udder by
machine methods, instead of by hand, it is possible to eliminate
nearly all external contamination from the animal and her
surroundings. The only opportunity for infection is then through the
leakage of air around the teat cups. Care should be taken to see
that the teats are in a clean condition before applying the suction
cups. The main problem in the use of a milking machine is to keep
the apparatus in an aseptic condition. Immersion of the teat cups
and the rubber connections in lime water, brine solution, or other
mild antiseptics, prevents bacterial development. Hastings has found
that milk having a germ content of less than 10,000 bacteria per
cubic centimeter may be produced by the use of a properly handled
milking machine.
=Contamination from the milker.= While the milker is a small factor in
comparison with the animal in the matter of contamination, yet he
can not be neglected, as it is within his power to affect profoundly
the quality of the milk. His personal habits as to cleanliness and
his appreciation of the precautions necessary in the production of
clean milk have much to do with the contamination of the milk. The
milking should be done with dry hands, although a little vaseline
may be used with effect. The hands should be washed before milking
as milk is certain to come in contact with them to some extent. The
milking should be done with the whole hand rather than stripping
between the thumb and finger; the clothing should be covered with
clean overalls and jumper, or at least a clean apron should be worn
during the milking. If these are of white material, more frequent
laundering is likely to result.
=Contamination from air.= It is difficult to disassociate the
contamination arising from the condition of the air from that
derived directly from the animal. Barn operations of various kinds
result in the production of dust, particularly where dry forage,
such as hay or straw, is handled. Where manure is given an
opportunity to dry, dust is readily produced, and such material is
particularly replete with bacterial life. Some kinds of dust, such
as that originating from ground grains, or shavings that may be used
for bedding, contain a small amount of bacterial life in comparison
with the dust from hay, or other dry fodder. In a dried condition,
the slightest movement is apt to dislodge these fine particles, and
they float in the air for considerable periods of time. If milk is
drawn and exposed to the air of the barn during the feeding
operations, it is subject to the dust shower that is present. Where
the storage can is allowed to stand in the stable during the
milking, even though it is covered with a strainer, this
accumulation of microscopic particles is added to the milk, as they
readily pass the meshes of the finest strainer.
[Illustration: Fig. 15.--contamination From the Air.
This culture plate, three inches in diameter, was exposed for 30
seconds in the barn during feeding of dry fodder. A 12-inch pail
exposes over 18 times the surface of this plate.]
=Removal of dirt after introduction.= The more primitive method of
improving the quality of milk, so far as its dirt content is
concerned, is to attempt to remove the grosser particles of
contamination after entrance. In the case of straining, the method
is usually applied at the time of milking, but in the case of
filtering and clarifying, it is carried out at the milk station, in
an effort to improve the appearance of milk and overcome the
influence of careless methods of the producer. By the use of
strainers, either metallic or cloth, it is possible to remove
particles of hair, undissolved dirt and manure, but it must be
remembered that these grosser _visible_ particles of pollution are
not really the cause of the troubles which may ensue in improperly
handled milk. The bacteria which are adherent to these foreign
particles are in large measure washed off in the process of
straining, and pass through the meshes of the finest strainer. The
main service, therefore, of straining is to improve the appearance
of the milk, and it has no effect on the quality in any way.
=Production of clean milk.= The problem of clean milk is important,
whatever may be the use to which milk may be put. It is important in
the manufacture of butter, but owing to the fact that the fat is not
readily acted upon by bacteria, it is not so sensitive to bacterial
conditions, as when the milk is made into cheese. In this product,
the bacterial condition of the milk is a matter of prime importance.
In milk destined for direct consumption, the exclusion of the
bacteria becomes yet more important. While it is impossible to
exclude bacteria so completely that milk will not undergo
fermentative changes, yet for domestic consumption it is preferable
to have milk with as low bacterial content as can readily be
secured. The highest type of market milk, that known as sanitary, or
certified, is produced under such extreme conditions of care as to
contain the minimum germ content. To accomplish these results
requires such stringent control as to increase greatly the cost of
the product. Pure, clean milk can be produced at a very slight
increase in cost over the regular expense of milk production, if the
right kind of attention is given to certain details of a practical
character. Improvement in our milk supplies must largely come from
this source, for any improvement to be permanent must be made to
pay, and it requires considerable education to secure the
co-operation of consumers and their willingness to pay for any
material increase in the quality of the product.
In the foregoing factors concerned in the contamination of milk, it
is of course impossible to measure accurately the influence of the
different sources of infection, as these are continually subject to
variation in every case. As a rule, the most important factors are
those pertaining to the utensils and the condition of the animal
herself. If these two factors are brought under reasonable control,
the major portion of contamination that ordinarily obtains is done
away with. The application of the remedial or preventive measures
heretofore mentioned will greatly reduce the germ content of the
milk.
=Cooling of milk on farm.= Bacterial growth is directly related to
temperature conditions, and with summer temperatures, such
development goes on apace, unless it is checked by early cooling.
The larger portion of bacteria that find their way into milk,
especially those that are previously in contact with the air, are in
a dormant condition, and are therefore not stimulated into immediate
growth, unless reasonably high temperatures prevail. In milk, which
comes from the animal at blood heat, this growth is greatly
stimulated. To counteract this effect, milk should be chilled as
soon after milking as possible. If the temperature is immediately
lowered to 50 deg. F., or lower, actual cell development is greatly
retarded, and the rate of souring, and other fermentative changes
thereby diminished. In this country ice is liberally used in
accomplishing this result. In Europe, the use of ice is much less
common. The employment of such artificial means of refrigeration
makes possible the shipment of milk for long distances by rail. New
York city now receives milk that is produced in Canada and
northeastern Ohio.
[Illustration: Fig. 16.--Effect of Cooling Milk.]
=Aeration of milk=. The custom has been extensively recommended of
subjecting milk to the influence of air in the belief that such
exposure permits of the interchange of gases that would improve the
quality. In practice, this process, known as aeration, is carried on
in different ways. In some cases, air is forced into the milk; in
others, the milk is allowed to distribute itself in a thin sheet
over a broad surface, falling in drops or tiny streams through the
air. Whenever this process is carried on at a temperature lower
than that of the milk, it results in more or less rapid cooling.
In earlier times, aeration was generally recommended and practiced,
especially in connection with the cheese industry, but carefully
controlled experiments fail to show that the process exerts any
material influence on the rate of germ development. If it is carried
out in an atmosphere more or less charged with bacteria, as in the
barn or stable, it is more than likely to add to the bacterial
content of the milk. While to some extent odors may be eliminated by
the process, the custom is not followed so generally now as it used
to be some years ago.
=Absorption of taints.= A tainted condition in milk may result from
the development of bacteria, acting upon various constituents of the
milk, and transforming these in such a way, as to produce
by-products that impair the flavor or appearance of the liquid; or
it may be produced by the milk being brought in contact with any
odoriferous or aromatic substance, under conditions that permit of
the direct absorption of such odors.
This latter class of taints is entirely independent of bacterial
action, and is largely attributable to the physical property which
milk possesses of absorbing volatile odors. This direct absorption
may occur before the milk is withdrawn from the animal, or
afterwards if exposed to strong odors.
It is not uncommon for the milk of animals advanced in lactation to
have a more or less strongly marked odor and taste; sometimes it is
apt to be bitter, at other times salty to the taste. It is a defect
that is peculiar to individual animals, and is liable to recur at
approximately the same period in lactation. The peculiar "cowy" or
"animal odor" of fresh milk is an inherent peculiarity that is due
to the direct absorption of volatile elements from the animal
herself.
Many kinds of feed consumed by the animal produce a more or less
pronounced taint or flavor in the milk. With some plants, such as
garlic, leeks, turnips, and cabbage, the odor is so pronounced as to
render the milk quite unfit for use. In some states along the
Atlantic seaboard, wild plants of this character in woodland
pastures may be so abundant as to make it impossible to pasture
milch animals. The difficulty in such cases is due to absorption of
the volatile principles into the circulation of the animal, and if
such feed is consumed shortly before milking, the characteristic
odors appear in the milk. If consumed immediately after the milk is
withdrawn from the animal, sufficient time may elapse so that the
peculiar odors are dissipated before the milk is again secreted. The
same principle applies in a lesser degree to the use of certain
green fodders that are more suitable for feed, such as rape, green
rye, or even silage. Silage produces a distinct, but not unpleasant
odor in milk, but newly pastured rye often confers so strong an odor
as to render the milk unusable.
Where certain drugs are employed in the treatment of animals, such
as belladonna, castor oil, sulfur, or turpentine, the peculiar odors
may reappear in the milk. Such mineral poisons as arsenic have been
known to persist for a period of three weeks before elimination.
On account of the elimination of many drugs, unchanged, from the
animal in the milk, the milk of any animal that is receiving
medicine should not be used for human food. When such milk is mixed
with that of a number of other animals and when it is used by
adults, no harm is likely to result, but when the dilution is not
great and the milk is used for young children it may affect them
through its content of the drug. The feed may not only affect the
quality of milk but its value as food. One of the most prominent of
American dairymen, who has for many years produced milk especially
for children's use, has said that he could feed his cows so as to
make ill every child receiving the milk.
=Absorption of odors after milking.= If milk is brought in contact
with strong odors after being drawn from the animal, it will absorb
them readily, as in the barn, where frequently it is exposed to the
odor of manure and other fermenting organic matter.
It has long been a popular belief that milk evolves odors and cannot
absorb them so long as it is warmer than the surrounding air, but
from experiments of one of us (R), it has been definitely shown that
the direct absorption of odors takes place much more rapidly when
the milk is warm than when cold, although under either condition, it
absorbs volatile substances quite rapidly.
The custom of straining the milk in the barn has long been
deprecated as inconsistent with proper dairy practice, and in the
light of the above experiments, an additional reason is evident why
this should not be done.
Even after milk is thoroughly cooled, it may absorb odors, as is
noted where the same is stored in a refrigerator with certain
fruits, meats, fish, etc.
=Distinguishing bacterial from other taints.= In perfectly fresh milk
it is relatively easy to distinguish between taints caused by the
growth of bacteria and those attributable to direct absorption. If
the taint is evident at time of milking, it is in all probability
due to character of feed consumed, or possibly to medicines. If,
however, the intensity of the taint grows more pronounced as the
milk becomes older, then it is probably due to living organisms
which require a certain period of incubation before their
by-products are most evident.
Moreover, if the difficulty is of bacterial origin, it can be
frequently produced in another lot of milk (heated or sterilized is
preferable) by inoculating the same with some of the original milk.
Not all abnormal fermentations are able, though, to compete with the
lactic acid bacteria, and hence outbreaks of this sort soon die out
by the re-establishment of more normal conditions.
=Factory contamination.= As the time element is of importance in the
production of troubles due to bacteria, it follows that infection of
milk on the farm is fraught with more consequence than factory
contamination, as the organisms introduced would have a longer
period of development. Nevertheless, the conditions in the factory
are by no means to be ignored, as they not infrequently permit the
milk to become seeded with highly undesirable types. A much more
rigid control can be exercised in the factory, where steam is at
hand as an aid in the destruction of organisms. In the cleaning of
pumps and pipes, steam is absolutely necessary to keep such
apparatus in a sanitary condition.
The water supply of the factory is a matter of prime importance, as
water is used so extensively in all factory operations. When taken
from a shallow well, especially if surface drainage from the
factory is possible, the water may be contaminated to such an extent
as to introduce undesirable bacteria in such numbers that the normal
course of fermentation may be changed. The quality of the water,
aside from flavor, can best be determined by making a curd test (p.
99) which is done by adding some of the water to boiled milk, and
incubating the same. If "gassy" fermentations occur, it signifies an
abnormal condition. In deep wells, pumped as thoroughly as is
generally the case with factory wells, the germ content should be
very low, ranging from a few score to a few hundred bacteria per
cubic centimeter at most. The danger from ice is much less, for the
reason that good daily practice does not sanction using ice directly
in contact with milk or cream. Then, too, water is largely purified
in the process of freezing, although if secured from a polluted
source, reliance should not be placed in this method of
purification, for even freezing does not destroy all vegetating
bacteria.
The ordinary house fly is an important source of contamination in
creameries, cheese factories and city milk plants. They are of
importance not only in increasing the number of fermentative
bacteria in milk but they may serve to contaminate it with
disease-producing organisms. The windows of all places where milk is
handled, whether on the farm or elsewhere should be screened.
It should be kept in mind in the handling of milk and other dairy
products that human food is being prepared and that cleanliness is
desirable from every point of view, and that the methods of
handling and production should compare with those used in the
preparation of foods which like milk cannot be cleaned when once
polluted. Desirability, keeping quality, healthfulness and the value
of every product made from milk depends upon the extent and amount
of contamination.