Outlines of Dairy Bacteriology Part 2

Not only should the greatest care be paid to the condition of the cans and milk-pails, but all dippers, strainers, and other utensils that come in contact with the milk must be kept equally clean. Cloth strainers, unless attended to, are objectionable, for the fine mesh of the cloth retains so much moisture that they become a veritable hot-bed of bacterial life, unless they are daily boiled or steamed.

The inability to thoroughly render vessels bacteria-free with the conveniences which are generally to be found on the farm has led in some cases to the custom of washing and sterilizing the milk cans at the factory.

~Germ content of milk utensils.~ Naturally the number of bacteria found in different milk utensils after they have received their regular cleaning will be subject to great fluctuations; but, nevertheless, such determinations are of value as giving a scientific foundation for practical methods of improvement. The following studies may serve to indicate the relative importance of the utensils as a factor in milk contamination.

Two cans were taken, one of which had been cleaned in the ordinary way, while the other was sterilized by steaming. Before milking, the udder was thoroughly cleaned and special precautions taken to avoid raising of dust; the fore milk was rejected. Milk drawn into these two cans showed the following germ content:

No. bacteria Hours before per cc. souring.

Steamed pail 165 28-1/2 Ordinary pail 4265 23

Harrison[6] has shown how great a variation is in the bacterial content in milk cans. The utensils were rinsed with 100 cc. of sterile water and numerical determinations of this rinsing water made. In poorly cleaned cans, the average germ content was 442,000; in cans washed in tepid water and then scalded--the best farm practice--54,000, and in cans carefully washed and then steamed for five minutes, 880.

Another method used by the writer is to wash the utensil with 100 cc.

sterile wash water, using a sterile swab to remove dirt. Then repeat the process twice or more with fresh rinsing waters, making plate cultures from each. The following data were obtained from three such determinations:

No. bacteria in different washings. Total No.

I. II. III. bacteria.

7,800,000 1,450,000 49,000 9,299,000 283,000 43,400 35,000 361,400 1,685,000 105,000 61,400 1,851,400

~Infection of milk in udder cavity.~ A frequently neglected but considerable factor of infection is that which is attributable to the bacteria which are present in the udder and which are removed in large numbers during the milking process. An examination of the fore milk, i.

e., the first few streams from each teat, and that which is subsequently withdrawn, generally reveals a very much larger number of organisms in the fore milk.[7] Not infrequently will this part of the milk when drawn under as careful conditions as possible, contain several score thousand organisms per cc. If successive bacterial determinations are made at different periods of the milking, as shown in the following experiment, a marked diminution is to be noted after the first portion of the milk is removed:

_Bacterial content at different periods of milking._

Fore 200th 2000th 4300th 6500th Strippings.

milk. cc. cc. cc. cc.

Expt. 1 6,500 1,700 475 220 75 5 Expt. 2 8,100 1,650 400 240 50 10

By some observers it has been claimed that it is possible to secure absolutely sterile milk in the strippings but this is rarely so. It is quite probable that such reported results are due to the fact that too small quant.i.ties of milk were used in the examinations and so erroneous conclusions were drawn. This marked diminution in numbers indicates that the larger proportion of the organisms found in the fore milk are present in the lower portion of the udder and milk ducts. When consideration is given to the structure of the udder, it is readily apparent that infection will be greater here than above.

[Ill.u.s.tration: FIG. 8. Sectional view of udder showing teat with milk duct connecting exterior with the milk cistern. Milk sinuses are mostly shown in cross section interspersed and below the secreting tissue (Moore and Ward).]

The udder is composed of secreting tissue (_gland cells_) held in place by fibrous connective tissue. Ramifying throughout this glandular structure are numerous channels (_milk sinuses_) that serve to convey the milk from the cells where it is produced into the _milk cistern_, a common receptacle just above the teats. This cavity is connected with the exterior through the milk duct in the teat, which is more or less tightly closed by the circular sphincter muscles, thus preventing the milk from flowing out. The mucous membranes of the milk duct and cistern are naturally moist. The habits of the animal render it impossible to prevent infection of the external opening at the end of the teat and there is no mechanical reason why bacteria cannot readily find their way along the moist lining membrane for some distance. If organisms are adapted to this kind of an environment, ideal conditions exist for their multiplication, as moisture, warmth and suitable food supply are present. The question arises how far up into this organ is penetration possible? Within late years numerous observations have been made on the presence of organisms in the upper portion of the udder in contact with the secreting tissue.[8]

These investigations prove that bacteria are distributed throughout the whole of the udder, although numerically they are much less abundant in the region of the milk-secreting tissue than in the lower portion.

Ward's conclusions are "that milk when secreted by the glands of a healthy udder is sterile. It may however, immediately become contaminated by the bacteria which are normally present in the smaller milk ducts of the udder."

~Nature of bacteria in fore milk.~ Generally speaking the number of different species found in the fore milk is not large, and of those which do appear many occur at only occasional intervals. Moore[9] in the examination of 9 udders found 20 different forms, and of these only 3 species predominated, all of which proved to be micrococci. Streptococci have also been quite frequently reported. Freudenreich[10] found the most common types to be cocci, belonging to both the liquefying and non-liquefying cla.s.s.

Peptonizing[11] and spore-bearing[12] species have also been reported as well as gas-producing[13] forms allied to the colon bacillus. Such findings are, however, due in all probability to accidental invasion.

Most investigators report the absence of the distinctively lactic-acid group of organisms.[14]

~Origin of bacteria in udder.~ There is no question but that many of the types of bacteria found in the udder gain access from the outside. Those belonging to the spore-bearing, digesting and intestinal types have such a favorable opportunity for introduction from outside and are so unlikely to have come directly from the body of the animal, that the external source of infection is much more probable. Whether this explanation answers the origin of the cocci that are so generally found in the upper portion of the udder is questionable. The statement is ordinarily made that the inner tissues of healthy organs are bacteria-free, but the studies of Ford[15] seem to indicate that 70 per cent. of such organs, removed under aseptic conditions from guinea pigs, rabbits, dogs and cats contained living organisms. Others have reported similar results in which cocci have been found[16] very similar to those occurring in the udder. These findings increase the probability that the origin of this type is from the blood. The persistence of certain species in the udder for months as noted by Ward indicates possibility of growth of some forms at least. Stocking[17] has shown where cows are not milked clean that the germ content of succeeding milkings is greatly increased.

~Artificial introduction of bacteria into udder.~ If bacteria are capable of actually developing in the udder proper, it ought to be possible to easily demonstrate this by the artificial introduction of cultures. In a number of cases[18] such experiments have been made with various saprophytic forms, such as _B. prodigiosus_, lactic acid bacilli and others. In no case has it appeared evident that actual growth has occurred, although the introduced organism has been demonstrated in diminishing numbers for 5-6 days. Even the common lactic acid germ and a yellow liquefying coccus isolated from the fore milk failed to persist for more than a few days when thus artificially introduced. This failure to colonize is indeed curious and needs explanation. Is it due to unsuitable environmental conditions or attributable to the germicidal influence of the milk?

Various body fluids are known to possess the property of destroying bacteria and it is claimed by Fokker[19] that this same property was found in freshly drawn milk. This peculiarity has also been investigated by Freudenreich,[20] and Hunziker[21] who find a similar property.

No material increase in germ content takes place in milk for several hours when chilled to 40-70 F.; on the other hand an actual, but usually not a marked decrease is observed for about 6 hours. This phenomenon varies with the milk of different cows. Nothing is known as to the cause of this apparent germicidal action. The question is yet by no means satisfactorily settled, although the facts on which the hypothesis is based are not in controversy. If such a peculiarity belongs to milk, it is not at all improbable that it may serve to keep down the germ content in the udder. Freudenreich[22] found that udders which were not examined for some time after death showed abundant growth, which fact he attributed to the loss of this germicidal property.

The infection of the whole milk can be materially reduced by rejecting the fore milk, but it is questionable whether such rejection is worth while, except in the case of "sanitary" dairies where milk is produced with as low a germ content as possible. The intrinsic loss in b.u.t.ter fat in the fore milk is inconsiderable as the first few streams contain only about one-fifth the normal fat content.

~Infection of milk after withdrawal from animal.~ The germ content of the milk, when it is being drawn from the animal is immediately increased upon contact with the atmosphere. These organisms are derived from the surrounding air and the utensils in which the milk is received and stored. The number of organisms which find their way into the milk depends largely upon the character of the surroundings. Bacteria are so intimately a.s.sociated with dirt, dust and filth of all kinds that wherever the latter are found, the former are sure to be present in abundance.

The most important factors in the infection of the milk after withdrawal are the pollution which is directly traceable to the animal herself and the condition of the milk utensils. Fortunately both of these sources of contamination are capable of being greatly minimized by more careful methods of handling.

~Infection directly from the cow.~ It is a popular belief that the organisms found in milk are derived from the feed and water which the cow consumes, the same pa.s.sing directly from the intestinal tract to the milk by the way of the blood circulation. Such a view has no foundation in fact as bacteria absorbed into the circulation are practically all destroyed in the tissues by the action of the body fluids and cells.[23]

While organisms cannot pa.s.s readily from the intestine to the udder, yet this must not be interpreted as indicating that no attention should be given to the bacterial character of the material consumed. The water supply given should be pure and wholesome 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 rations with which animals are now fed leads to the production of softer fecal discharges 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 circ.u.mstances will permit.

[Ill.u.s.tration: FIG. 9. Showing the bacterial contamination arising from hair. These hairs were allowed to fall on a sterile gelatin surface. The adherent bacteria developed readily in this medium, and the number of bacteria thus introduced into the milk from these hairs can be estimated by the number of developing colonies.]

The animal herself 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 collected with care 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 ma.s.ses 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.

The amount of actual impurities found in milk is often considerable and when it is remembered that about one-half of fresh manure dissolves in milk,[24] and thus does not appear as sediment, the presence of this undissolved residue bespeaks filthy conditions as to milking. From actual tests made, it is computed that the city of Berlin, Germany consumes about 300 pounds of such dirt and filth daily. Renk has laid down the following rule with reference to this insoluble dirt: If a sample of milk shows any evidence of impurity settling on a transparent bottom within two hours, it should be regarded as too dirty for use.

While the number of organisms here introduced is at all times large, the character of the species is of even greater import. Derived primarily from dirt and fecal matter, it is no wonder that such forms are able to produce very undesirable fermentative changes.

~Influence of milker.~ The condition of the milker is not to be ignored in determining all possible factors of infection, for when clothed in the dust-laden garments that have been worn all day, a favorable opportunity for direct contamination is possible. The filthy practice of wetting the hands with milk just before milking is to be condemned. The milker's hands should be washed immediately before milking in clean water and dried. A pinch of vaseline on hands is sometimes used to obtain a firmer grasp and prevents the ready dislodgment of scales.[25] It must also be borne in mind that the milker may spread disease through the milk. In typhoid fever and diphtheria, the germs often remain in the system for weeks and thus make infection possible. Stocking[26] has shown that the individual milker exerts a potent influence on the total germ content of milk, even where the procedure is quite the same. In sanitary dairies milkers are usually clad in white duck suits.

~Milking by machinery.~ Several mechanical devices have been invented for milking, some of which have been tested bacteriologically as to their efficiency. Harrison[27] has examined the "Thistle" machine but found a much higher germ content than with hand-drawn milk. The recent introduction of the Burrel-Lawrence-Kennedy machine has led to numerous tests in which very satisfactory results have been obtained. If the rubber parts of the milker are thoroughly cleaned and kept in lime water solution, they remain nearly sterile. When milk is properly handled, the germ content may be greatly reduced.

~Reduction in dirt and adherent bacteria.~ No factor of contamination is so susceptible of improvement as that which relates to the reduction in filth and dirt which gains access during and immediately subsequent to the milking. The care which is taken to keep the coat of the animal clean by grooming lessens very much the grosser portion of such contamination, but with a dry, hairy coat, fine scales and dust particles must of necessity be dislodged.[28] Ordinarily the patron thinks all evidence of such dirt is removed if the milk is strained, but this process only lessens the difficulty; it does not overcome it.

Various methods are in use, the effectiveness of which is subject to considerable variation. Some of these look to the elimination of the bacteria after they are once introduced into the milk; others to the prevention of infection in the first place.

_1. Straining the milk._ Most of the visible, solid particles of filth, such as hairs, dirt particles, etc., can be removed by simple straining, the time-honored process of purification. As ordinarily carried out, this process often contributes to instead of diminishing the germ life in milk. The strainer cloths unless washed and thoroughly sterilized by boiling harbor mult.i.tudes of organisms from day to day and may thus actually add to the organisms present. Various methods have been suggested for this simple process, but the most practical and efficient strainer is that made of fine wire gauze to which is added 3-4 layers of cheese cloth, the whole to set over the storage milk can.

_2. Filtration._ In Europe especially, the system of cleaning milk by filtration through sand, gravel and other substances has been quite extensively used. These filters are built in sections and the milk pa.s.ses from below upward. The filtering substance is washed in hot water immediately after use and then steamed and finally baked. While it is possible to remove the solid impurities in this way, the germ content cannot be greatly reduced.[29] Cellulose filters have also been suggested[30] as an improvement over the sand filters. Methods of filtration of this character have not been used under commercial conditions here in this country.

_3. Clarification in separator._ Within recent years the custom has grown of clarifying milk or removing the visible dirt by pa.s.sing the milk through a centrifugal separator the cream and skim milk being remixed after separation. This process naturally removes the solid impurities as dirt, hairs, epithelial scales and cells, also some of the casein, making what is known as centrifuge slime. This conglomerate ma.s.s is incomparably rich in germ life and the natural inference would be that the bacterial content of the milk would be greatly reduced by this procedure. Eckles and Barnes[31] noted a reduction of 37 to 56 per cent.

of the bacteria but others have failed to observe such reductions.[32]

This condition is explained by the more thorough breaking up of the bacterial ma.s.ses in the process, thus apparently not reducing them in numbers.

It is somewhat surprising that in spite of the elimination of much organic matter and bacteria, such clarified milk sours as rapidly as the untreated product.[33]

The mechanical shock of separation ruptures the cl.u.s.ters of fat globules and so delays creaming and also lessens the consistency of cream derived from such milk. This practical disadvantage together with the increased expense of the operation and the failure to materially enhance the keeping quality of the product outweigh the advantage which might come from removal of solid impurities which can be largely accomplished on the farm by efficient straining.

_4. Washing the udder._ If a surface is moist, bacteria adherent to it cannot be dislodged by ordinary movements. Thus the air over snow-covered mountains or oceans is relatively devoid of germ life. The method of moistening the udder is applied with success to the hairy coat of the animal thus subserving the double purpose of cleaning the animal and preventing in large measure the continual dislodgment of dust particles. After these parts have been well carded to remove loose hairs and dirt particles, the skin should be thoroughly moistened with clean water and then wiped. It has been urged that this procedure lessens the yield of milk but Eckles[34] concludes from experiments that when the animal becomes accustomed to this treatment, no noticeable change in amount of milk or b.u.t.ter-fat is produced.

The effectiveness of this method in reducing the actual amount of dirt and filth introduced into the milk as well as the great diminution in germ life is shown by the instructive experiments of Fraser[35] who found that the actual amount of dirt dislodged from udders of apparently clean animals during the milking process was three and one-half times as much as when the cow's udders were washed. From udders visibly polluted one ounce of such filth was removed in 275 pounds of milk, while from cows whose udders had been washed, the same amount of dirt was distributed through 24,030 pounds.

Fraser observed as a result of 420 examinations that the average germ content of 4-inch culture dishes under clean but unwashed udders was 578, while under washed animals it was reduced to 192. From numerous tests made in the writer's laboratory, it is evident that the germ content of the milk in the pail is increased from 20,000-40,000 bacteria _per minute_ during the milking period. By far the larger part of this pollution can be easily prevented by cleaning and dampening the udder.

_5. Diminishing exposed surface of pail._ The entrance of organisms into the milk can be greatly reduced by lessening the area of the milk pail directly exposed to the dust shower. A number of so-called sanitary or hygienic milk pails have been devised for this purpose. In one case the pail is smaller at the top than bottom, but in most of them the common form is kept and the exposed area is lessened by means of a cover, the milk being received through a narrower opening. In some cases, strainers are also interposed so as to remove more effectually the coa.r.s.e particles. It is necessary to have these covers and strainers constructed in such a way so they can be easily removed and cleaned.