FOOTNOTES:

FOOTNOTES:[78]Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.[79]Freeman, Med. Rec., March 28, 1896.[80]Busey and Kober, Rept. Health Off. of Dist. of Col., Washington, D. C., 1895, p. 299. These authors present in this report an elaborate article on morbific and infectious milk, giving a very complete bibliography of 180 numbers. They append to Hart's list (which is published in full) additional outbreaks which have occurred since, together with full data as to extent of epidemic, circumstances governing the outbreak, as well as name of original reporter and reference.[81]Smith, Theo., Journ. of Expt. Med., 1898, 3:451.[82]Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ. of Penn. Med. Bull., Sept. 1901.[83]Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell, Journ. Amer. Med. Ass'n, April 16, 1898.[84]Stille, Brit. Med. Journ., Aug. 19, 1899.[85]This test is made by injecting into the animal a small quantity of tuberculin, which is a sterilized glycerin extract of cultures of the tubercle bacillus. In a tuberculous animal, even in the very earliest phases of the disease, tuberculin causes a temporary fever that lasts for a few hours. By taking the temperature a number of times before and after injection it is possible to readily recognize any febrile condition. A positive diagnosis is made where the temperature after inoculation is at least 2.0° F. above the average normal, and where the reaction fever is continued for a period of some hours.[86]Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses animales, 1895.[87]C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.[88]Ostertag, Milch Zeit., 22:672.[89]Obermüller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d. Kais. Ges. Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898, p. 217.[90]Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.[91]Th. Smith. Journ. of Expt. Med., 1899, 4:217.[92]Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.[93]Hesse, Zeit. f. Hyg., 1900, 34:346.[94]Practically all of the finest butter made in Denmark is made from cream that has been pasteurized at temperatures varying from 160°-185° F.[95]Gebhardt, Virch. Arch., 1890, 119:12.[96]Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang, Milch Zeit., 1893, p. 672.[97]Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.[98]Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg, Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892, 18:16.[99]Schneider, Münch, med. Wochenschr., 1893, No. 27; Fröhner, Zeit f. Fleisch u. Milchhygiene, 1891, p. 55.[100]Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.[101]Nocard, Bull. Gén., 1885, p. 54.[102]Deutsche Viertelsjahr. f. offentl. Gesundheitspflege, 1890, 20:444.[103]Zeit. f. Fleisch und Milch hygiene, 11:114.[104]E. Roth, Deutsche Vierteljahresschr. f. offentl. Gesundheitspfl., 1890, 22:238[105]S. W. North, London Practitioner, 1889, 43:393.[106]Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.[107]Dabney, Phila. Med. News, 1893, 63:630.[108]Welphy, London Lancet, 1894, 2:1085.[109]Brit. Med. Journ., 1894, 1:815.[110]Mass. Bd. Health Rept., 1894, p. 765.[111]Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med. Journ., 1894, 2:829.[112]Hankin, Brit. Med. Journ., 1894, 2:613.[113]Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it capable of living from 20-30 days in milk.[114]Schüder (Zeit. f. Hyg., 1902, 38:34) examined the statistics of 638 typhoid epidemics. He found 71 per cent. due to infected drinking water, 17 per cent. to infected milk, and 3.5 per cent. caused by other forms of food.[115]Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.[116]Simpson, London Practitioner, 1887, 39:144.[117]Swithinbank and Newman, Bacteriology of Milk, p. 341.[118]Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and 73.[119]Baginsky, Hyg. Rund., 1895, p. 176.[120]Gaffky, Deutsch. med. Wochen., 18:14.[121]Flügge. Zeit., f. Hyg., 17:272, 1894.[122]Duval and Bassett, 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.

[78]Hart, Trans. Int. Med. Cong., London, 1881, 4:491-544.

[79]Freeman, Med. Rec., March 28, 1896.

[80]Busey and Kober, Rept. Health Off. of Dist. of Col., Washington, D. C., 1895, p. 299. These authors present in this report an elaborate article on morbific and infectious milk, giving a very complete bibliography of 180 numbers. They append to Hart's list (which is published in full) additional outbreaks which have occurred since, together with full data as to extent of epidemic, circumstances governing the outbreak, as well as name of original reporter and reference.

[81]Smith, Theo., Journ. of Expt. Med., 1898, 3:451.

[82]Dinwiddie, Bull. 57, Ark. Expt. Stat., June, 1899; Ravenel, Univ. of Penn. Med. Bull., Sept. 1901.

[83]Ravenel, Journ. of Comp. Med. & Vet. Arch., Dec. 1897; Hartzell, Journ. Amer. Med. Ass'n, April 16, 1898.

[84]Stille, Brit. Med. Journ., Aug. 19, 1899.

[85]This test is made by injecting into the animal a small quantity of tuberculin, which is a sterilized glycerin extract of cultures of the tubercle bacillus. In a tuberculous animal, even in the very earliest phases of the disease, tuberculin causes a temporary fever that lasts for a few hours. By taking the temperature a number of times before and after injection it is possible to readily recognize any febrile condition. A positive diagnosis is made where the temperature after inoculation is at least 2.0° F. above the average normal, and where the reaction fever is continued for a period of some hours.

[86]Martin, Brit. Med. Journ. 1895, 1:937; Nocard, Les Tuberculoses animales, 1895.

[87]C. O. Jensen, Milch Kunde und Milch hygiene, p. 69.

[88]Ostertag, Milch Zeit., 22:672.

[89]Obermüller, Hyg. Rund., 1897, p. 712; Petri, Arb. a. d. Kais. Ges. Amte, 1898, 14: 1; Hormann und Morgenroth, Hyg. Rund., 1898, p. 217.

[90]Rabinowitsch, Zeit. f. Hyg., 1897, 26: 90.

[91]Th. Smith. Journ. of Expt. Med., 1899, 4:217.

[92]Russell and Hastings, 18 Rept. Wis. Expt. Stat., 1901.

[93]Hesse, Zeit. f. Hyg., 1900, 34:346.

[94]Practically all of the finest butter made in Denmark is made from cream that has been pasteurized at temperatures varying from 160°-185° F.

[95]Gebhardt, Virch. Arch., 1890, 119:12.

[96]Scheurlen, Arb. a. d. k. Ges. Amte, 1891, 7:269; Bang, Milch Zeit., 1893, p. 672.

[97]Moore, Year Book of U. S. Dept. Agr., 1895, p. 432.

[98]Weigel and Noack, Jahres. d. Ges. Med., 1890, p. 642; Weissenberg, Allg. med. Cent. Zeit., 1890, p. 1; Baum, Arch. f. Thierheilkunde, 1892, 18:16.

[99]Schneider, Münch, med. Wochenschr., 1893, No. 27; Fröhner, Zeit f. Fleisch u. Milchhygiene, 1891, p. 55.

[100]Feser, Deutsche Zeit. f. Thiermed., 1880, 6:166.

[101]Nocard, Bull. Gén., 1885, p. 54.

[102]Deutsche Viertelsjahr. f. offentl. Gesundheitspflege, 1890, 20:444.

[103]Zeit. f. Fleisch und Milch hygiene, 11:114.

[104]E. Roth, Deutsche Vierteljahresschr. f. offentl. Gesundheitspfl., 1890, 22:238

[105]S. W. North, London Practitioner, 1889, 43:393.

[106]Sedgwick and Chapin, Boston Med. & Surg. Journ., 1893, 129:485.

[107]Dabney, Phila. Med. News, 1893, 63:630.

[108]Welphy, London Lancet, 1894, 2:1085.

[109]Brit. Med. Journ., 1894, 1:815.

[110]Mass. Bd. Health Rept., 1894, p. 765.

[111]Turner, London Practitioner, 1892, 49:141; Munro, Brit. Med. Journ., 1894, 2:829.

[112]Hankin, Brit. Med. Journ., 1894, 2:613.

[113]Heim (Arb. a. d. Kais. Gesundheitsamte, 1889, 5:303) finds it capable of living from 20-30 days in milk.

[114]Schüder (Zeit. f. Hyg., 1902, 38:34) examined the statistics of 638 typhoid epidemics. He found 71 per cent. due to infected drinking water, 17 per cent. to infected milk, and 3.5 per cent. caused by other forms of food.

[115]Kitasato. Arb. a. d. Kais. Gesundheitsamte, 1:470.

[116]Simpson, London Practitioner, 1887, 39:144.

[117]Swithinbank and Newman, Bacteriology of Milk, p. 341.

[118]Schottelius and Ellerhorst. Milch Zeit., 1897, pp. 40 and 73.

[119]Baginsky, Hyg. Rund., 1895, p. 176.

[120]Gaffky, Deutsch. med. Wochen., 18:14.

[121]Flügge. Zeit., f. Hyg., 17:272, 1894.

[122]Duval and Bassett, 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.

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

Until within comparatively recent years, practically no attention was given to the character of milk supplies, except possibly as to the percentage of butter fat, and sometimes the milk solids which it contained. So long as the product could be placed in the hands of the consumer in such shape as not to be rejected by him as unfit for food, no further attention was likely to be given to its character. At present, however, much more emphasis is being given to the quality of milk, especially as to its germ content; and the milk dealer is beginning to recognize the necessity of a greater degree of control. This control must not merely concern the handling of the product after it reaches him, but should go back to the milk producer on the farm. Here especially, it is necessary to inculcate those methods of cleanliness which will prevent in large measure the wholesale infection that ordinarily occurs.

The two watch words which are of the utmost importance to the milk dealer arecleanlinessandcold. If the milk is properly drawn from the animal in a clean manner and is immediately and thoroughly chilled, the dealer has little to fear as to his product. Whenever serious difficulties do arise, attributable to bacterial changes, it is because negligence has been permitted in one or both directions. The influence of cleanliness in diminishing the bacterial life in milk and that of low temperatures in repressing the growth of those forms which inevitably gain access has been fully dealt with in preceding chapters. It is of course not practicable to take all of these precautions to which reference has been made in the securing of large supplies of market milk for city use, but great improvement over existing conditions could be secured if the public would demand a better supervision of this important food article. Boards of health inour 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 associated with dirt and permits of the recognition of milks that have been carelessly handled. From personal knowledge of examinations made of the milk supplies in a number of Wisconsin cities it appears that this test could be utilized with evident advantage.

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

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

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

Chemical preservatives.Numerous attempts have been made to find some chemical substance that could be added to milk which would preserve it without interfering with its nutritive properties, but as a general rule a substance that is toxic enough to destroy or inhibit the growth of bacterial life exerts a prejudicial effect on the tissues of the body. The use of chemicals, such as carbolic acid, mercury salts and mineral acids, that are able to entirely destroy all life, is of course excluded, except when milk is preserved for analytical purposes; but a number of milder substances are more or less extensively employed, although the statutes of practically all states forbid their use.

The substances so used may be grouped in two classes:

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

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

These substances are generally used by milk handlers who know nothing of their poisonous action, and although it may be possible for adults to withstand their use in dilute form, without serious results, yet their addition togeneral 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 shipped long distances by freezing the product, which in the form of milk-ice could be held for an indefinite period without change.[130]A modification of this process known as Casse's system has been in use more or less extensively in Copenhagen and in several places in Germany. This consists of adding a small block of milk-ice (frozen milk) to large cans of milk (one part to about fifty of milk) which may or may not be pasteurized.[131]This reduces the temperature so that the milk remains sweet considerably longer. Such a process might permit of the shipment of milk for long distances with safety but as a matter of fact, the system has not met with especial favor.

Fig. 22. Microscopic appearance of normal milk showing the fat-globules aggregated in clusters.Fig. 22. Microscopic appearance of normal milk showing the fat-globules aggregated in clusters.

Use of high temperatures.Heat has long been used as a preserving agent. Milk has been scalded or cooked to keep it from time immemorial. Heat may be used at differenttemperatures, 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 shipping milk by rail, viz., prolonged heating above growing temperature, but when milk is so heated, its physical appearance is changed.[132]The methods of heating most satisfactorily used are known as sterilization and pasteurization, in which a degree of temperature is used approximating the boiling and scalding points respectively.

Fig. 23.Fig. 23.

Microscopic appearance of milk heated above 140° F., showing the homogeneous distribution of fat-globules. The physical change noted in comparison with Fig. 22 causes the diminished consistency of pasteurized cream.]

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

1. Diminished "body."When milk, but more especially cream, is heated to 140° F. or above, it becomes thinner in consistency or "body," a condition which is due to a change in the grouping of the fat globules. In normal milk, the butter fat for the most part is massed in microscopic clots as (Fig. 22). When exposed to 140° F. or above for ten minutes these fat-globule clots break down, and the globules become homogeneously distributed (Fig. 23). Amomentaryexposure 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 pronounced as the temperature is further raised. Milk so heated develops on its surface a pellicle or "skin." The cause of this change in taste is not well known. Usually it has been explained as being produced by changes in the nitrogenous elements in the milk, particularly in the albumen. Thoerner[133]has pointed out the coincidence that exists between the appearance of a cooked taste and the loss of certain gases that are expelled by heating. He finds that the milk heated in closed vessels from which the gas cannot escape has a much less pronounced cooked flavor than if heated in an open vessel. The so-called "skin" on the surface of heated milk is not formed when the milk is heated in a tightly-closed receptacle. By some[134]it is asserted that this layer is composed of albumen, but there isevidence to show that it is modified casein due to the rapid evaporation of the milk serum at the surface of the milk.

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

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

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

Sterilization.As ordinarily used in dairying, sterilization means the application of heat at temperatures approximating, if not exceeding, 212° 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 entirelyallthese hardy forms. If milk is heated at temperatures above the boiling point, as is done where steam pressure is utilized, it can be rendered practically germ-free. Such methods are employed where it is designed to keep milk sweet for a long period of time. The treatment of milk by sterilization has not met with any general favor in this country, although it has been more widely introduced abroad. In most cases the process is carried out after the milk is bottled; and considerable ingenuity has been exercised in the construction of devices which will permit of the closure of the bottles after the sterilizing process has been completed. Milks heated to so high a temperature have a more or less pronounced boiled or cooked taste, a condition that does not meet with general favor in this country. The apparatus suitable for this purpose must, of necessity, be so constructed as to withstand steam pressure, and consequently is considerablymore 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 butter-making. The objects to be accomplished vary materially and the details of the two processes are also quite different.

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

Requirements essential in pasteurization.While considerable latitude with reference to pasteurizing limits ispermitted, 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) thephysical, and (2) thebiologicalrequirements.

Physical requirements.1. Avoidance of scalded or cooked taste.The English and American people are so averse to a scalded or cooked flavor in milk that it is practically impossible for a highly heated product to be sold in competition with ordinary raw milk. In pasteurization then, care must be taken not to exceed the temperature at which a permanently cooked flavor is developed. As previously observed, this point varies with the period of exposure. A momentary exposure to a temperature of about 170° 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 clusters to disintegrate (see Figs. 22 and 23), the globules do not rise to the surface as readily as before and the cream line remains indistinct. Where the exposure is made for a considerable period of time (10 minutes or more), the maximum temperature which can be used without producing this change is about 140° 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.

3. No diminution in cream "body."Coincident with this change which takes place in the creaming of the milk is the change in body or consistency which is noted where cream is pasteurized at too high a temperature. For the same reason as given under (2) cream heated above these temperatures is reduced in apparent thickness and appears to contain less butter-fat. Of course the pasteurizing process does not change the fat content, but its "body" is apparently so affected. Thus a 25 per cent. cream may seem to be no thicker or heavier than an 18 per cent. raw cream. This real reduction in consistency naturally affects the readiness with which the cream can be whipped.

Biological requirements.1. Enhanced keeping quality.In commercial practice the essential biological requirement is expressed in the enhanced keeping quality of the pasteurized milk. This expresses in a practical way the reduction in germ life accomplished by the pasteurizing process. The improvement in keeping quality depends upon the temperature and time of exposure, but fully as much also on the way in which the pasteurized product is handled after heating. The lowest temperature which can be used with success to kill the active, vegetative bacteria is about 140° F., at which point it requires about ten minutes exposure. If this period is curtailed the temperature must be raised accordingly. An exposure to a temperature of 175° F. for a minute has approximately the same effect as the lower degree of heat for the longer time.

The following bacteriological studies as to the effect which a variation in temperature exerts on bacterial life in milk are of importance as indicating the foundation for the selection of the proper limits. In the followingtable the exposures were made for a uniform period (20 minutes):

45° C.50° C.55° C.60° C.65° C.70° C.Unheated113° F.122° F.131° F.140° F.149° F.158° F.Series I.2,895,000——1,260,000798,00032,0005,7703,900Series II.750,000665,000262,400201,000950700705Series III.1,350,0001,100,000260,000215,000575610650Series IV.1,750,000——87,360——4,0003,5003,600

It appears from these results that the most marked decrease in temperature occurs at 140° F. (60° C.). It should also be observed that an increase in heat above this temperature did not materially diminish the number of organisms present, indicating that those forms remaining were in a spore or resistant condition. It was noted, however, that the developing colonies grew more slowly in the plates made from the highly heated milk, showing that their vitality was injured to a greater extent even though not killed.

2. Destruction of disease bacteria.While milk should be pasteurized so as to destroy all active, multiplying bacteria, it is particularly important to destroy any organisms of a disease nature that might find their way into the same. Fortunately most of the bacteria capable of thriving in milk before or after it is drawn from the animal are not able to form spores and hence succumb to proper pasteurization. Such is the case with the diphtheria, cholera and typhoid organisms.

The organism that is invested with most interest in this connection is the tubercle bacillus. On account of its more or less frequent occurrence in milk and its reputed highpowers of resistance, it may well be taken as a standard in pasteurizing.

Thermal death limits of tubercle bacillus.Concerning the exact temperature at which this germ is destroyed there is considerable difference of opinion. Part of this arises from the inherent difficulty in determining exactly when the organism is killed (due to its failure to grow readily on artificial media), and part from the lack of uniform conditions of exposure. The standards that previously have been most generally accepted are those of De Man,[137]who found that thirty minutes exposure at 149° F., fifteen minutes at 155° F., or ten minutes at 167° F., sufficed to destroy this germ.

More recently it has been demonstrated,[138]and these results confirmed,[139]that if tuberculous milk is heated in closed receptacles where the surface pellicle does not form, the vitality of this disease germ is destroyed at 140° F. in 10-15 minutes, while an exposure at 160° F. requires only about one minute.[140]If the conditions of heating are such that the surface of the milk is exposed to the air, the resistance of bacteria is greatly increased. When heated in open vessels Smith found that the tubercle organism was not killed in some cases where the exposure was made for at least an hour. Russell and Hastings[141]have shown an instance where the thermal death-point of a micrococcus isolated from pasteurized milk was increased 12.5° F., by heating it under conditions that permitted of the formation of the scalded layer. It is therefore apparent that apparatus used forpasteurization should be constructed so as to avoid this defect.

Methods of treatment.Two different systems of pasteurization have grown up in the treatment of milk. One of these has been developed from the hygienic or sanitary aspect of the problem and is used more particularly in the treatment of cream and relatively small milk supplies. The other system has been developed primarily from the commercial point of view where a large amount of milk must be treated in the minimum time. In the first method the milk is heated for a longer period of time, about fifteen minutes at a relatively low temperature from 140°-155° F.; in the other, the milk is exposed to the source of heat only while it is passing rapidly through the apparatus. Naturally, the exposure under such conditions must be made at a considerably higher temperature, usually in the neighborhood of 160° F.

The types of apparatus used in these respective processes naturally varies. Where the heating is prolonged, the apparatus employed is built on the principle of atankorreservoirin which a given volume of milk may be held at any given temperature for any given period of time.

When the heat is applied for a much shorter period of time, the milk is passed in a continuous stream through the machine. Naturally the capacity of a continuous-flow apparatus is much greater than a machine that operates on the intermittent principle; hence, for large supplies, as in city distribution, this system has a great advantage. The question as to relative efficiency is however one which should be given most careful consideration.

Pasteurizing apparatus.The problems to be solved in the pasteurization of milk and cream designed for direct consumption are so materially different from where the process is used in butter-making that the type of machinery for each purpose is quite different. The equipment necessary for the first purpose may be divided into two general classes:

1. Apparatus of limited capacity designed for family use.

2. Apparatus of sufficient capacity to pasteurize on a commercial scale.

Domestic pasteurizers.In pasteurizing milk for individual use, it is not desirable to treat at one time more than will be consumed in one day; hence an apparatus holding a few bottles will suffice. In this case the treatment can best be performed in the bottle itself, thereby lessening the danger of infection. Several different types of pasteurizers are on the market; but special apparatus is by no means necessary for the purpose. The process can be efficiently performed by any one with the addition of an ordinary dairy thermometer to the common utensils found in the kitchen. Fig. 24 indicates a simple contrivance that can be readily arranged for this purpose.

The following suggestions indicate the different steps of the process:

1. Use only fresh milk.

2. Place milk in clean bottles or fruit cans, filling to a uniform level, closing bottles tightly with a cork or cover. If pint and quart cans are used at the same time, an inverted bowl will equalize the level. Set these in a flat-bottomed tin pail and fill with warm water to same level as milk. An inverted pie tin punched with holes will serve as a stand on which to place the bottles during the heating process.

3. Heat water in pail until the temperature of same reaches 155° to 160° F.; then remove from source of direct heat, cover with a cloth or tin cover, and allow the whole to stand for half an hour. In the preparation of milk for children, it is not advisable to use the low-temperature treatment (140° F.) that is recommended for commercial city delivery.

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