Outlines of Dairy Bacteriology Part 10

[122] Duval and Ba.s.sett, Studies from the Rockefeller Inst. for Med.

Research, 2:7, 1904.

[123] Zeit. f. physiol. Chemie, 10:146; 9 Intern. Hyg. Cong. (London), 1891, p. 118.

[124] Vaughan and Perkins, Arch. f. Hyg., 27:308.

[125] Newton and Wallace (Phila. Med. News, 1887, 50:570) report three outbreaks at Long Branch, N. J., two of which occurred in summer hotels.

CHAPTER VI.

BACTERIA AND MILK SUPPLIES WITH ESPECIAL REFERENCE TO METHODS OF PRESERVATION.

To the milk dealer or distributor, bacteria are more or less of a detriment. None of the organisms that find their way into milk, nor the by-products formed by their growth, improve the quality of milk supplies. It is therefore especially desirable from the milk-dealer's point of view that these changes should be held in abeyance as much as possible. Then too, the possibility that milk may serve as a medium for the dissemination of disease-breeding bacteria makes it advisable to protect this food supply from all possible infection from suspicious sources.

In considering, therefore, the relation of bacteria to general milk supplies, the _economic_ and the _hygienic_ standpoints must be taken into consideration. Ordinarily much more emphasis is laid upon the first requirement. If the supply presents no abnormal feature as to taste, odor and appearance, unfortunately but little attention is paid to the possibility of infection by disease germs. The methods of control which are applicable to general milk supplies are based on the following foundations: (1) the exclusion of all bacterial life, as far as practicable, at the time the milk is drawn, and the subsequent storage of the same at temperatures unfavorable for the growth of the organisms that do gain access; (2) the removal of the bacteria, wholly or in part, after they have once gained access.

Until within comparatively recent years, practically no attention was given to the character of milk supplies, except possibly as to the percentage of b.u.t.ter fat, and sometimes the milk solids which it contained. So long as the product could be placed in the hands of the consumer in such shape as not to be rejected by him as unfit for food, no further attention was likely to be given to its character. At present, however, much more emphasis is being given to the quality of milk, especially as to its germ content; and the milk dealer is beginning to recognize the necessity of a greater degree of control.

This control must not merely concern the handling of the product after it reaches him, but should go back to the milk producer on the farm.

Here especially, it is necessary to inculcate those methods of cleanliness which will prevent in large measure the wholesale infection that ordinarily occurs.

The two watch words which are of the utmost importance to the milk dealer are _cleanliness_ and _cold_. If the milk is properly drawn from the animal in a clean manner and is immediately and thoroughly chilled, the dealer has little to fear as to his product. Whenever serious difficulties do arise, attributable to bacterial changes, it is because negligence has been permitted in one or both directions. The influence of cleanliness in diminis.h.i.+ng the bacterial life in milk and that of low temperatures in repressing the growth of those forms which inevitably gain access has been fully dealt with in preceding chapters. It is of course not practicable to take all of these precautions to which reference has been made in the securing of large supplies of market milk for city use, but great improvement over existing conditions could be secured if the public would demand a better supervision of this important food article. Boards of health in our larger cities are awakening to the importance of this question and are becoming increasingly active in the matter of better regulations and the enforcement of the same.

New York City Board of Health has taken an advanced position in requiring that all milk sold in the city shall be chilled down to 45 F.

immediately after milking and shall be transported to the city in refrigerator cars.

Reference has already been made to the application of the acid test (page 52) in the inspection of city milk supplies, and it is the opinion of the writer that the curd test (see page 76) could also be used with advantage in determining the sanitary character of milk. This test reveals the presence of bacteria usually a.s.sociated with dirt and permits of the recognition of milks that have been carelessly handled.

From personal knowledge of examinations made of the milk supplies in a number of Wisconsin cities it appears that this test could be utilized with evident advantage.

~"Sanitary" or "certified" milk supplies.~ In a number of the larger cities, the attempt has been made to improve the quality of the milk supplies by the installation of dairies in which is produced an especially high grade of milk. Frequently the inspection of the dairy as well as the examination of the milk at stated intervals is under the control of milk commissions or medical societies and as it is customary to distribute the certificate of the examining board with the product, such milks are frequently known as "certified." In such dairies the tuberculin test is used at regular intervals, and the herd inspected frequently by competent veterinarians. The methods of control inaugurated as to clean milking and subsequent handling are such as to insure the diminution of the bacteria to the lowest possible point. The bacterial limit set by the Pediatric Society of Philadelphia is 10,000 organisms per cc. Often it is possible to improve very materially on this standard and not infrequently is the supply produced where it contains only a few thousand organisms per cc. Where such a degree of care is exercised, naturally a considerably higher price must be paid for the product,[126] and it should be remembered that the development of such a system is only possible in relatively large centers where the dealer can cater to a selected high-cla.s.s trade. Moreover, it should also be borne in mind that such a method of control is only feasible in dairies that are under individual control. The impossibility of exercising adequate control with reference to the milking process and the care which should be given the milk immediately thereafter, when the same is produced on different farms under various auspices is evident.

PRESERVATION OF MILK SUPPLIES.

While much can be done to improve the quality of milk supplies by excluding a large proportion of the bacteria which normally gain access to the milk, and preventing the rapid growth of those that do find their way therein, yet for general munic.i.p.al purposes, any practical method of preservation[127] that is applicable on a commercial scale must rest largely upon the destruction of bacteria that are present in the milk.

The two possible methods by which bacteria can be destroyed after they have once gained access is (1) by the use of chemical preservatives; (2) by the aid of physical methods.

~Chemical preservatives.~ Numerous attempts have been made to find some chemical substance that could be added to milk which would preserve it without interfering with its nutritive properties, but as a general rule a substance that is toxic enough to destroy or inhibit the growth of bacterial life exerts a prejudicial effect on the tissues of the body.

The use of chemicals, such as carbolic acid, mercury salts and mineral acids, that are able to entirely destroy all life, is of course excluded, except when milk is preserved for a.n.a.lytical purposes; but a number of milder substances are more or less extensively employed, although the statutes of practically all states forbid their use.

The substances so used may be grouped in two cla.s.ses:

1. Those that unite chemically with certain by-products of bacterial growth to form inert substances. Thus bicarbonate of soda neutralizes the acid in souring milk, although it does not destroy the lactic acid bacteria.

2. Those that act directly upon the bacteria in milk, restraining or inhibiting their development. The substances most frequently utilized are salicylic acid, formaldehyde and boracic acid. These are nearly always sold to the milk handler, under some proprietary name, at prices greatly in excess of what the crude chemicals could be bought for in the open market. Formaldehyde has been widely advertised of late, but its use is fraught with the greatest danger, for it practically renders insoluble all alb.u.minous matter and its toxic effect is greatly increased in larger doses.

These substances are generally used by milk handlers who know nothing of their poisonous action, and although it may be possible for adults to withstand their use in dilute form, without serious results, yet their addition to general milk supplies that may be used by children is little short of criminal. The sale of these preparations for use in milk finds its only outlet with those dairymen who are anxious to escape the exactions that must be met by all who attempt to handle milk in the best possible manner. Farrington has suggested a simple means for the detection of preservalin (boracic acid).[128] When this substance is added to fresh milk, it increases the acidity of milk without affecting its taste. As normal milk tastes sour when it contains about 0.3 per cent lactic acid, a milk that tests as much or more than this without tasting sour has been probably treated with this antiseptic agent.

~Physical methods of preservation.~ Methods based upon the application of physical forces are less likely to injure the nutritive value of milk, and are consequently more effective, if of any value whatever. A number of methods have been tried more or less thoroughly in an experimental way that have not yet been reduced to a practical basis, as electricity, use of a vacuum, and increased pressure.[129] Condensation has long been used with great success, but in this process the nature of the milk is materially changed. The keeping quality in condensed milk often depends upon the action of another principle, viz., the inhibition of bacterial growth by reason of the concentration of the medium. This condition is reached either by adding sugar and so increasing the soluble solids, or by driving off the water by evaporation, preferably in a vacuum pan.

Temperature changes are, however, of the most value in preserving milk, for by a variation in temperature all bacterial growth can be brought to a standstill, and under proper conditions thoroughly destroyed.

~Use of low temperatures.~ The effect of chilling or rapid cooling on the keeping quality of milk is well known. When the temperature of milk is lowered to the neighborhood of 45 F., the development of bacterial life is so slow as to materially increase the period that milk remains sweet.

Within recent years, attempts have been made to preserve milk so that it could be s.h.i.+pped long distances by freezing the product, which in the form of milk-ice could be held for an indefinite period without change.[130] A modification of this process known as Ca.s.se's system has been in use more or less extensively in Copenhagen and in several places in Germany. This consists of adding a small block of milk-ice (frozen milk) to large cans of milk (one part to about fifty of milk) which may or may not be pasteurized.[131] This reduces the temperature so that the milk remains sweet considerably longer. Such a process might permit of the s.h.i.+pment of milk for long distances with safety but as a matter of fact, the system has not met with especial favor.

[Ill.u.s.tration: FIG. 22. Microscopic appearance of normal milk showing the fat-globules aggregated in cl.u.s.ters.]

~Use of high temperatures.~ Heat has long been used as a preserving agent.

Milk has been scalded or cooked to keep it from time immemorial. Heat may be used at different temperatures, and when so applied exerts a varying effect, depending upon temperature employed. All methods of preservation by heat rest, however, upon the application of the heat under the following conditions:

1. A temperature above the maximum growing-point (105-115 F.) and below the thermal death-point (130-140 F.) will prevent further growth, and consequently fermentative action.

2. A temperature above the thermal death-point destroys bacteria, and thereby stops all changes. This temperature varies, however, with the condition of the bacteria, and for spores is much higher than for vegetative forms.

Attempts have been made to employ the first principle in s.h.i.+pping milk by rail, viz., prolonged heating above growing temperature, but when milk is so heated, its physical appearance is changed.[132] The methods of heating most satisfactorily used are known as sterilization and pasteurization, in which a degree of temperature is used approximating the boiling and scalding points respectively.

[Ill.u.s.tration: FIG. 23. Microscopic appearance of milk heated above 140 F., showing the h.o.m.ogeneous distribution of fat-globules. The physical change noted in comparison with Fig. 22 causes the diminished consistency of pasteurized cream.]

~Effect of heat on milk.~ When milk is subjected to the action of heat, a number of changes in its physical and chemical properties are to be noted.

_1. Diminished "body."_ When milk, but more especially cream, is heated to 140 F. or above, it becomes thinner in consistency or "body," a condition which is due to a change in the grouping of the fat globules.

In normal milk, the b.u.t.ter fat for the most part is ma.s.sed in microscopic clots as (Fig. 22). When exposed to 140 F. or above for ten minutes these fat-globule clots break down, and the globules become h.o.m.ogeneously distributed (Fig. 23). A _momentary_ exposure to heat as high as 158-160 may be made without serious effect on the cream lime; but above this the cream rises so poorly and slowly that it gives the impression of thinner milk.

_2. Cooked Taste._ If milk is heated for some minutes to 160 F., it acquires a cooked taste that becomes more p.r.o.nounced as the temperature is further raised. Milk so heated develops on its surface a pellicle or "skin." The cause of this change in taste is not well known. Usually it has been explained as being produced by changes in the nitrogenous elements in the milk, particularly in the alb.u.men. Th.o.e.rner[133] has pointed out the coincidence that exists between the appearance of a cooked taste and the loss of certain gases that are expelled by heating.

He finds that the milk heated in closed vessels from which the gas cannot escape has a much less p.r.o.nounced cooked flavor than if heated in an open vessel. The so-called "skin" on the surface of heated milk is not formed when the milk is heated in a tightly-closed receptacle. By some[134] it is a.s.serted that this layer is composed of alb.u.men, but there is evidence to show that it is modified casein due to the rapid evaporation of the milk serum at the surface of the milk.

_3. Digestibility._ Considerable difference of opinion has existed in the minds of medical men as to the relative digestibility of raw and heated milks. A considerable amount of experimental work has been done by making artificial digestion experiments with enzyms, also digestion experiments with animals, and in a few cases with children. The results obtained by different investigators are quite contradictory, although the preponderance of evidence seems to be in favor of the view that heating does impair the digestibility of milk, especially if the temperature attains the sterilizing point.[135] It has been observed that there is a noteworthy increase in amount of rickets,[136] scurvy and marasmus in children where highly-heated milks are employed. These objections do not obtain with reference to milk heated to moderate temperatures, as in pasteurization, although even this lower temperature lessens slightly its digestibility. The successful use of pasteurized milks in children's hospitals is evidence of its usefulness.

_4. Fermentative changes._ The normal souring change in milk is due to the predominance of the lactic acid bacteria, but as these organisms as a cla.s.s do not possess spores, they are readily killed when heated above the thermal death-point of the developing cell. The destruction of the lactic forms leaves the spore-bearing types possessors of the field, and consequently the fermentative changes in heated milk are not those that usually occur, but are characterized by the curdling of the milk from the action of rennet enzyms.

_5. Action of rennet._ Heating milk causes the soluble lime salts to be precipitated, and as the curdling of milk by rennet (in cheese-making) is dependent upon the presence of these salts, their absence in heated milks greatly r.e.t.a.r.ds the action of rennet. This renders it difficult to utilize heated milks in cheese-making unless the soluble lime salts are restored, which can be done by adding solutions of calcium chlorid.

~Sterilization.~ As ordinarily used in dairying, sterilization means the application of heat at temperatures approximating, if not exceeding, 212 F. It does not necessarily imply that milk so treated is sterile, i. e., germ-free; for, on account of the resistance of spores, it is practically impossible to destroy entirely _all_ these hardy forms. If milk is heated at temperatures above the boiling point, as is done where steam pressure is utilized, it can be rendered practically germ-free.

Such methods are employed where it is designed to keep milk sweet for a long period of time. The treatment of milk by sterilization has not met with any general favor in this country, although it has been more widely introduced abroad. In most cases the process is carried out after the milk is bottled; and considerable ingenuity has been exercised in the construction of devices which will permit of the closure of the bottles after the sterilizing process has been completed. Milks heated to so high a temperature have a more or less p.r.o.nounced boiled or cooked taste, a condition that does not meet with general favor in this country. The apparatus suitable for this purpose must, of necessity, be so constructed as to withstand steam pressure, and consequently is considerably more expensive than that required for the simpler pasteurizing process.

~Pasteurization.~ In this method the degree of heat used ranges from 140 to 185 F. and the application is made for only a limited length of time. The process was first extensively used by Pasteur (from whom it derives its name) in combating various maladies of beer and wine. Its importance as a means of increasing the keeping quality of milk was not generally recognized until a few years ago; but the method is now growing rapidly in favor as a means of preserving milk for commercial purposes. The method does not destroy all germ-life in milk; it affects only those organisms that are in a growing, vegetative condition; but if the milk is quickly cooled, it enhances the keeping quality very materially. It is unfortunate that this same term is used in connection with the heating of cream as a preparatory step to the use of pure cultures in cream-ripening in b.u.t.ter-making. The objects to be accomplished vary materially and the details of the two processes are also quite different.

While pasteurizing can be performed on a small scale by the individual, the process can also be adapted to the commercial treatment of large quant.i.ties of milk. The apparatus necessary for this purpose is not nearly so expensive as that used in sterilizing, a factor of importance when other advantages are considered. In this country pasteurization has made considerable headway, not only in supplying a milk that is designed to serve as children's food, but even for general purposes.

~Requirements essential in pasteurization.~ While considerable lat.i.tude with reference to pasteurizing limits is permitted, yet there are certain conditions which should be observed, and these, in a sense, fix the limits that should be employed. These may be designated as (1) the _physical_, and (2) the _biological_ requirements.

~Physical requirements.~ _1. Avoidance of scalded or cooked taste._ The English and American people are so averse to a scalded or cooked flavor in milk that it is practically impossible for a highly heated product to be sold in compet.i.tion with ordinary raw milk. In pasteurization then, care must be taken not to exceed the temperature at which a permanently cooked flavor is developed. As previously observed, this point varies with the period of exposure. A momentary exposure to a temperature of about 170 F. may be made without any material alteration, but if the heat is maintained for a few minutes (ten minutes or over), a temperature of 158 to 160 F. is about the maximum that can be employed with safety.

_2. Normal creaming of the milk._ It is especially desirable that a sharp and definite cream line be evident on the milk soon after pasteurization. If this fails to appear, the natural inference of the consumer is that the milk is skimmed. If the milk be heated to a temperature sufficiently high to cause the fat-globule cl.u.s.ters to disintegrate (see Figs. 22 and 23), the globules do not rise to the surface as readily as before and the cream line remains indistinct.

Where the exposure is made for a considerable period of time (10 minutes or more), the maximum temperature which can be used without producing this change is about 140 F.; if the exposure is made for a very brief time, a minute or less, the milk may be heated to 158-160 F. without injuring the creaming property.