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1922 
 
 UNITED STATES DEPARTMENT T)F AGRICULTURE 
 
 BULLETIN No. 342 
 
 Washington, D. C. 
 
 Issued January 8, 1916; revised October 10, 1922 
 
 THE PRESENT STATUS OF THE PASTEURIZATION 
 
 OF MILK. 
 
 By S. HENRY AYERS, 
 Bacteriologist, Dairy Division, Bureau of Ani, 
 
 CONTENTS. 
 
 Meaning of the term pasteurization- 
 Value of pasteurization 
 
 Electrical and ultra-violet-ray treat- 
 ment of milk 
 
 Extent of pasteurization in the 
 
 United States 
 
 Methods of pasteurization 
 
 Advantages of low-temperature pas- 
 teurization 
 
 Temperatures and methods most suit- 
 able for pasteurization 
 
 Supervision of the process 
 
 B 
 
 7 
 
 10 
 
 11 
 33 
 
 Handling milk after pasteurization- 
 
 Cost of pasteurizing milk 
 
 Bacteria which survive pasteuriza- 
 tion 
 
 The colon test for efficiency of pas- 
 teurization 
 
 Past and present theories of pasteur- 
 ization 
 
 Pasteurization and vitamins 
 
 The necessity for pasteurization 
 
 References to literature- 
 
 15 
 16 
 
 16 
 
 20 
 
 20 
 22 
 24 
 25 
 
 MEANING OF THE TERM PASTEURIZATION. 
 
 The term "pasteurization" originated from the experiments of 
 Louis Pasteur, in France. From 1860 to 1864, in experiments on the 
 u diseases " of wine, he found that heating for a few moments at 
 temperatures of from 122 to 140 F. was sufficient to prevent abnor- 
 mal fermentations and souring in wine. A little later he found 
 that by a similar heating beer could be preserved from souring. 
 The application of the process gave rise to the term " pasteurization." 
 As applied under commercial conditions, pasteurization is the process 
 of heating for a short or a long period, as the different processes 
 demand, at temperatures usually between 140 and 185 F. As ap- 
 plied to milk for direct consumption, pasteurization should mean 
 a process of heating to 145 F. and holding at that temperature for 
 30 minutes. The process is followed by rapid cooling. 
 
 elieved that the term pasteurization should be applied only 
 ocess of heating at 145 F. for a period of 30 minutes. 
 
 .48 22 1 
 
 4933 
 
DEPARTMENT OF AGRICULTURE. 
 
 PASTEURIZATION. 
 
 From a sanitary standpoint, the value of pasteurization is of 
 greatest importance when market milk is under consideration. The 
 pasteurization of milk, when the process is properly performed, 
 affords protection from pathogenic organisms. Such disease-pro- 
 ducing bacteria as Bacillus tuberculosis , B. diphtherias, B. typhi, 
 and other organisms of the typhoid-paratyphoid group, and the 
 dysentery bacillus, when heated at 140 F. for 20 minutes or more 
 are destroyed, or at least lose their ability to produce disease. 
 
 Occasionally results are reported, such as those of Twiss (30), 1 
 which again open the question as to the destruction of certain patho- 
 genic organisms by pasteurization. Using test organisms of the 
 typhoid-paratyphoid group, she obtained results which indicated 
 that there was not a complete destruction of these organisms when 
 heated in milk at 140 F. and even at 149 F. for 30 minutes. Krum- 
 wiede and Noble (24) , however, using some of the same test organ- 
 isms of the typhoid-paratyphoid group as used by Twiss, found that 
 they did not survive heating for 10 minutes at 140 F. They further 
 pointed out that the apparent heat resistance of the strains used by 
 Twiss was due to the method of determining their thermal death 
 point. 
 
 According to Mohler (25), pasteurization offers protection against 
 foot-and-mouth disease. He makes the following statement : 
 
 Milk which has been pasteurized for the elimination of tubercle and typhoid 
 bacilli will not prove capable of transmitting the disease (foot-and-mouth) to 
 persons or animals fed with it. 
 
 In view of the outbreak of foot-and-mouth disease in this country 
 a few years ago this statement is of importance. 
 
 The abortuslike bacteria in the udders of healthy cows which were 
 demonstrated by Evans (15) may also be considered in a discussion 
 of pasteurization. Although their sanitary significance has not been 
 definitely established, it is interesting to observe that it was found 
 by Evans (16) that both the pathogenic and lipolytic varieties could 
 be destroyed by heating to 125 F. for 30 minutes or to 145 F. for 
 30 seconds. 
 
 Within recent years several epidemics of septic sore throat have 
 been traced to milk. In some of these epidemics it was found 
 possible, by pasteurization, to destroy streptococci which were iso- 
 lated from throats of infected people and which were believed to 
 be the infective agents. Pasteurization, properly performed, seems 
 to protect against epidemics of this kind, but until the organism 
 
 'See References to literature. 
 
STATUS OF PASTEURIZATION 01 ^XIJxA. 3 
 
 which causes the disease is definitely knpwij.it is imp-^ssjljje. i<; say 
 that it affords absolute protection. 
 
 Since it is quite generally believed that the streptococci are the 
 causative agents of septic sore throat, the ability of certain of this 
 group of organisms to stand temperatures above that of pasteuriza- 
 tion naturally presents a grave situation. If pathogenic streptococci 
 are able to survive the usual process of pasteurization, the value of 
 the process, from a sanitary standpoint, is materially lowered. 
 
 Experience with the use of properly pasteurized milk and the de- 
 termination of the thermal death point of pathogenic streptococci 
 by various investigators indicate very clearly, however, that the 
 thermal death point of these organisms is relatively low and that 
 they are readily destroyed by proper pasteurization. Thus Ham- 
 burger (IT), who studied the epidemic of septic sore throat in 
 Baltimore in 1912, traced this epidemic to a certain milk supply. 
 Advice was given to boil all milk, and the dairy connected with 
 the epidemic raised the temperature of its flash pasteurization to 
 160 F. ; then it changed to the holder process by which the milk 
 was heated to 145 F. and held for a period of 30 minutes. The 
 cases of sore throat that followed were neither so severe nor so 
 numerous and did not follow the milk supply, but appeared to 
 have been transmitted from individual to individual. Hamburger 
 (18) also found that a streptococcus isolated from a patient having 
 a case of sore throat was killed by heating in milk at 145 F. for 
 30 minutes. 
 
 Again, Capps and Miller (12) who studied the Chicago epi- 
 demic of septic sore throat, traced it to a dairy where the milk was 
 pasteurized by the flash process at 160 F. On certain dates they 
 found that there was a pronounced failure to pasteurize and follow- 
 ing these dates there were outbreaks of septic sore throat. These 
 authors believed that the final responsibility for the epidemic rested 
 on the inadequate and unreliable pasteurization. They state that 
 the absolute protection of the children of the Michael Reese Hos- 
 pital from infection by efficient pasteurization demonstrates this 
 point. Bray (11), who studied an epidemic of tonsillitis of tuber- 
 culous patients, traced the epidemic to a milk supply of one farm 
 where a carrier presumably infected the milk. Forty cases of 
 tonsillitis resulted among 400 people. As soon as the epidemic 
 broke out the milk was pasteurized, and from that time only 1 
 case appeared. 
 
 From the results achieved from the proper pasteurization of milk 
 it seems evident that the thermal death point of pathogenic strep- 
 tococci, which cause septic sore throat, is relatively low. This belief 
 
BULLETIN/ 3a2j:U,. : S. DEPARTMENT OF AGRICULTURE. 
 
 is bo^^oi^by.-'tliEesiXite-O'f the studies of Davis (13), who found 
 that streptococci isolated from cases of sore throat were readily killed 
 by heating at 140 F. for 30 minutes. He also found that none of 24 
 strains of pathogenic hemolytic streptococci of human origin resisted 
 heating at 140 F. for 30 minutes. He makes the following statement : 
 
 I know of no evidence that strains of streptococci pathogenic to man can resist 
 the usual temperature of pasteurization, 145 F., for 30 minutes. 
 
 Further evidence that pathogenic streptococci are destroyed by 
 proper pasteurization was presented by the results obtained by Ayers, 
 Johnson, and Davis (7), who found that 27 strains of these organisms 
 were always destroyed by heating at 140 F. for 30 minutes. 
 
 Epidemics of scarlet fever have been traced to milk supplies, and 
 in such cases pasteurization has been resorted to, with apparently 
 satisfactory results, as a means of safeguarding the public health. 
 
 Pasteurization is of value from a commercial standpoint so far as 
 it increases the keeping quality of the milk and assists in preventing 
 financial losses by souring. As practiced at the present time, commer- 
 cial pasteurization, with reasonable care, destroys about 99 per cent 
 of the bacteria (this percentage varies, depending upon the propor- 
 tion of heat-resistant bacteria in the milk), and while it does not 
 prevent the ultimate souring of milk, it does delay the process. At 
 the present time pasteurization is the best process for the destruction 
 of bacteria in milk on a commercial scale. 
 
 ELECTRICAL AND ULTRA-VIOLET-RAY TREATMENT OF MILK. 
 
 Many attempts have been made to destroy bacteria in milk by 
 means of electricity, but no process has been devised which has been 
 commercially applied to any great extent. 
 
 Alternating currents have been most extensively worked with, be- 
 cause direct currents were found to produce undesirable chemical 
 changes in milk. While the proper application of suitable alternating 
 currents has resulted in bacteria reductions similar to those produced 
 by pasteurization, it appears to be an open question as to whether 
 the action of the electric current is due to the heat generated or to 
 the direct action of electricity on the bacterial cells. 
 
 Thornton (28), who studied this question in England, came to the 
 conclusion that the destruction of bacteria must be regarded as due 
 largely to thermal changes rather than electrical, but thought his 
 results indicated some electrical action on the molecular structure of 
 the bacteria. Beattie (8, 9), also working in England on the same 
 problem, came to the conclusion that heat was not the principal fac- 
 tor in the destruction of bacteria by electricity, but found that to 
 obtain satisfactory results the temperature should not be below 145 
 
STATUS OF PASTEURIZATION OF MILK. 5 
 
 F. In the United States an electric process has been investigated by 
 Anderson and Finkelstein (1). Their conclusion as to the cause of 
 the destruction of the bacteria is as follows : 
 
 The destruction of bacteria in the " " process is apparently due to 
 
 the heat produced by the electric current rather than to the electric current 
 
 itself. The " " process furnishes a method for producing a very 
 
 sudden high temperature for a brief period of time. 
 
 It seems evident from a review of the literature that in the use of 
 electricity, as it has been applied, sufficient heat is generated by elec- 
 tricity, or a combination of steam and electricity, to raise the milk 
 to the pasteurizing temperature. Since the temperatures reached 
 are in themselves destructive to most nonspore-forming bacteria, 
 the problem of determining whether the effect of electricity is due 
 to heat or direct electric action is a difficult one. 
 
 The use of ultra-violet rays for the destruction of bacteria in milk 
 has not proved to be of value as a commercial process. Experiments 
 with these rays carried on by Ayers and Johnston (5) showed that 
 while the rays cause great destruction of bacteria in milk, when ex- 
 posed under suitable conditions, the process in its present state of 
 development can not replace that of pasteurization on a commercial 
 scale. It is difficult to obtain the proper exposure of milk to the 
 rays on a scale sufficient to permit of practical operation and im- 
 practicable to secure suitable bacteria reductions without seriously 
 injuring the flavor of the milk. 
 
 EXTENT OF PASTEURIZATION IN THE UNITED STATES. 
 
 Pasteurization when first practiced by milk dealers in this country 
 was carried on more or less secretly, and, except as a means of pre- 
 serving the milk, was regarded by them as a process of no value. As 
 the practice became more general the subject of pasteurization was 
 studied, and its value as a means of destroying disease-producing 
 bacteria was recognized. In consequence of the recognition of the 
 merits of the process there has been during the last 20 years a rapid 
 increase in the quantity of milk pasteurized. Jordan (23) , in a paper 
 published in 1913. stated that 10 years previously only about 5 per 
 cent of the milk supply of Xew York City was pasteurized ; figures 
 from other sources show that about 40 per cent in 1912, 88 per cent 
 in 1914, and 98 per cent in 1921 was pasteurized. In Boston very 
 little milk was pasteurized in 1902, but in 1915 80 per cent, while at 
 present about 90 per cent is so treated. In many of the smaller cities 
 there have been corresponding increases in the quantity of milk 
 pasteurized during the last few years. 
 
 The general tendency in this country to-day is toward the pasteuri- 
 zation of all milk for direct consumption, with the exception of 
 
BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 certified or equivalent grades of milk from tuberculin-tested herds. 
 Some idea of the increase in the extent of pasteurization in the United 
 States from 1915 to 1921 may be gained by a study of Table 1. The 
 figures were obtained from a questionnaire sent to health officers. In 
 1915 the figures were based on 344 replies and 379 in 1921. 
 
 TABLE 1. Extent of pasteurization of milk in cities m the United States of 
 more than 10,000 population in 1915 and 1921. 
 
 Population of 
 
 Number of 
 cities 
 answering. 
 
 50 per cent 
 or more 
 pasteurized. 
 
 10 to 50 per cent 
 pasteurized. 
 
 to 10 per cent 
 pasteurized. 
 
 None 
 pasteurized. 
 
 cities. 
 
 
 
 
 
 
 
 1915 
 
 1921 
 
 1915 
 
 1921 
 
 1915 
 
 1921 
 
 1915 
 
 1921 
 
 1915 
 
 1921 
 
 
 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 Perct. 
 
 More than 500,000. . 
 100,001 to 500,000. . . 
 
 9 
 40 
 
 12 
 42 
 
 77.8 
 30.0 
 
 100.0 
 90.5 
 
 22.2 
 50.0 
 
 
 9.5 
 
 
 15.0 
 
 
 
 
 
 5.0 
 
 
 
 
 75,001 to 100,000... . 
 50,001 to 75,000 
 
 19 
 30 
 
 15 
 34 
 
 26.3 
 12.3 
 
 73.3 
 
 67.7 
 
 42.1 
 50.0 
 
 26.7 
 14.7 
 
 21.0 
 20.0 
 
 
 2.9 
 
 10.6 
 16.7 
 
 
 14.7 
 
 25,001 to 50,000 
 
 78 
 
 62 
 
 16.7 
 
 59.7 
 
 39.7 
 
 25.8 
 
 15.4 
 
 3.2 
 
 28.2 
 
 11.3 
 
 10,001 to 25,000 
 
 168 
 
 126 
 
 6.0 
 
 33.3 
 
 23.8 
 
 18.3 
 
 10.7 
 
 9.5 
 
 59.5 
 
 38.9 
 
 It will be noted that since 1915 there has been a great increase in 
 the percentage of cities in which more than 50 per cent of the milk 
 is pasteurized. There has been during the same period a marked 
 decrease in the percentage of cities having no pasteurized milk. 
 
 Table 1 does not contain any data from cities of less than 10,000 
 population, but replies from 88 such cities showed the following 
 figures : In 22 cities 50 per cent or more of the milk was pasteurized, 
 in 12 others from 11 to 50 per cent, and in two cities 10 per cent 
 or less was so treated. Fifty-two of the 88 cities reported no pas- 
 teurized milk. It seems evident, therefore, that the process of pas- 
 teurization is being used extensively in this country even in the 
 small cities. 
 
 A study of the available figures on the extent of pasteurization 
 revealed a few more facts which may be of interest. In 1915 milk 
 was pasteurized in about 62 per cent of the cities with a population 
 above 10,000, and in 1921 in about 80 per cent of such cities. The 
 increase in pasteurization in small cities, 10,000 to 25,000, is shown 
 by the fact that in 1915 about 40 per cent of these cities reported 
 pasteurized milk compared with approximately 61 per cent in 1921. 
 
 Considering these figures as a whole the increasing trend of pas- 
 teurization is plain. 
 
 A good idea of the present extent of pasteurization may be 
 obtained from Table 2. It will be observed that there is an increas- 
 ing tendency, which follows their increasing population, for cities 
 to have pasteurized milk and also to pasteurize a higher percentage 
 of the supply. 
 
STATUS OF PASTEURIZATION OF MILK. 
 
 TABLE 2. Proportion of cittes having pasteurized milk and average per cent of 
 their milk supply ichich VMS pasteurized in 1921. 
 
 
 Number of 
 
 Number of 
 
 Per cent 
 
 Average 
 
 
 cities 
 
 cities 
 
 of cities 
 
 percent 
 
 Population of cities. 
 
 reporting 
 pasteur- 
 ized milk. 
 
 with no 
 pasteur- 
 ized milk. 
 
 with 
 pasteur- 
 ized milk. 
 
 of milk 
 pasteur- 
 ized. 
 
 More than 500 000 
 
 12 
 
 
 
 100 
 
 95 
 
 100 001 to 500 000 
 
 42 
 
 
 
 100 
 
 72 
 
 75 001 to 100 000 
 
 15 
 
 
 
 100 
 
 68 
 
 50'001 to 75 000 ... 
 
 29 
 
 5 
 
 85 
 
 65 
 
 25 001 to 50 000 
 
 55 
 
 7 
 
 89 
 
 58 
 
 10 000 to 25,000 . 
 
 77 
 
 49 
 
 61 
 
 51 
 
 Less than 10,000 
 
 36 
 
 52 
 
 41 
 
 53 
 
 Total 
 
 266 
 
 113 
 
 
 
 
 
 
 
 
 For those who are particularly interested in the quantity of milk 
 pasteurized in various cities, Table 3 has been prepared. In it is a 
 list of 266 cities that reported pasteurized milk and the approxi- 
 mate quantity of milk pasteurized. The cities are listed in order of 
 their population given in the 1920 census and include all cities re- 
 porting pasteurization up to the time the table was prepared. It is 
 particularly interesting to note the extent to which milk is 
 pasteurized even in the small cities. 
 
 METHODS OF PASTEURIZATION. 
 
 At present three processes of pasteurization are practiced in this 
 country. The first is known as the flash, or continuous process; the 
 second, the holder, or holding process; and the third is known as 
 pasteurization in the bottle. 
 
 TABLE 3. Approximate quantity of milk pasteurized in various cities as shown 
 by returns from questionnaire sent in 1921. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Per cent 
 of milk 
 pas- 
 teurized. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Per cent 
 of milk 
 pas- 
 teurized. 
 
 New York N Y 
 
 5 620 048 
 
 98 
 
 Columbus, Ohio 
 
 237,031 
 
 75 
 
 Chicago 111 
 
 2 701 705 
 
 98 
 
 Louisville Kv 
 
 234 891 
 
 85 
 
 Philadelphia Pa 
 
 1. 823' 799 
 
 98 
 
 St. Paul, Minn 
 
 234, 595 
 
 60 
 
 Detroit Mich 
 
 993 678 
 
 98 
 
 Akron 6hio 
 
 208,435 
 
 98 
 
 Cleveland Ohio 
 
 796 841 
 
 98 
 
 Omaha Nebr 
 
 191 601 
 
 30 
 
 St Louis 'MO 
 
 772*892 
 
 92 
 
 Worcester, Mass 
 
 179, 754 
 
 65 
 
 Boston Mass 
 
 748 060 
 
 90 
 
 Syracuse N. Y 
 
 171,717 
 
 66 
 
 Baltimore Md 
 
 73'i' v">fi 
 
 98 
 
 Richmond V a 
 
 171 667 
 
 97 
 
 Pittsburgh Pa 
 
 . r >xv .343 
 
 95 
 
 New Haven, Conn 
 
 162, 537 
 
 55 
 
 Los Angeles Calif 
 
 576 673 
 
 86 
 
 Memphis Tenn 
 
 162, 351 
 
 50 
 
 Buffalo N. Y. 
 
 506,775 
 
 100 
 
 Dallas, Tex 
 
 158, 976 
 
 70 
 
 Sn Franris^o fialif 
 
 506 676 
 
 85 
 
 Dayton, Ohio 
 
 152, 559 
 
 95 
 
 Milwaukee Wis 
 
 457 147 
 
 98 
 
 Houston Tex 
 
 138,296 
 
 50 
 
 Washington D.C . 
 
 437 571 
 
 91 
 
 Hartford, Conn 
 
 138,036 
 
 70 
 
 Newark" N 'j 
 
 414 524 
 
 80 
 
 Grand Rapids, Mich... 
 
 137,634 
 
 90 
 
 Cincinnati Ohio 
 
 401 247 
 
 98 
 
 Paterson N J 
 
 135, 875 
 
 80 
 
 Minneapolis, Minn 
 
 380 582 
 
 94 
 
 Youngstown. Ohio 
 
 132,358 
 
 92 
 
 Kansas Citv, Mo 
 
 324 410 
 
 50 
 
 Springfield Mass 
 
 129,614 
 
 95 
 
 Seattle Wash 
 
 315 312 
 
 85 
 
 New Bedford Mass 
 
 121.217 
 
 40 
 
 Rochester, N. Y 
 
 295.750 
 
 65 
 
 Fall River, Mass 
 
 120,485 
 
 55 
 
 Portland, Oreg 
 
 2o> 2SS 
 
 55 
 
 Trenton N. J 
 
 119,289 
 
 60 
 
 Denver Colo 
 
 256 491 
 
 80 
 
 Nashville Tenn 
 
 118,342 
 
 40 
 
 Providence. R. I.. 
 
 237. 595 
 
 60 
 
 Salt Lake Citv. Utah . . . 
 
 118, 110 
 
 88 
 
8 
 
 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 TABLE 3. Approximate quantity of mills pasteurized in various cities as shown 
 by returns from questionnaire sent in 1921 Continued. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Per cent 
 of milk 
 pas- 
 teurized. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Per cent 
 of milk 
 pas- 
 teurized. 
 
 Norfolk Va. . 
 
 115,777 
 113,344 
 112,759 
 111, 958 
 110, 168 
 107,784 
 101, 177 
 100, 176 
 99,148 
 98, 917 
 95,283 
 93,372 
 93,091 
 91,599 
 91,558 
 91,295 
 88,723 
 87,091 
 85,264 
 83,327 
 78,384 
 77,939 
 77,560 
 73,502 
 70,983 
 69,272 
 67, 957 
 67,327 
 66,800 
 65,651 
 65,142 
 61,903 
 60,840 
 60,331 
 60,203 
 59.316 
 59,183 
 58,593 
 57, 895 
 57,327 
 56,378 
 56, 208 
 56,036 
 55,593 
 54,948 
 53,844 
 53,150 
 52,548 
 51,252 
 50,842 
 50,670 
 50,022 
 48,395 
 48,374 
 47,876 
 45,566 
 45,393 
 44, 955 
 44, 938 
 44,616 
 44, 255 
 43,464 
 43,050 
 42, 726 
 42, 515 
 41,707 
 41,611 
 41,534 
 41,534 
 41,326 
 41,013 
 40, 472 
 40, 296 
 40, 120 
 40,079 
 39,675 
 39,671 
 
 range, Eas 
 
 50 
 60 
 34 
 91 
 66 
 96 
 65 
 98 
 75 
 46 
 87 
 70 
 85 
 80 
 70 
 75 
 36 
 90 
 80 
 75 
 75 
 46 
 33 
 75 
 90 
 84 
 100 
 50 
 75 
 92 
 50 
 41 
 80 
 97 
 75 
 20 
 94 
 85 
 80 
 70 
 100 
 75 
 82 
 80 
 70 
 25 
 50 
 20 
 8 
 95 
 95 
 40 
 50 
 30 
 63 
 75 
 33 
 75 
 45 
 40 
 15 
 50 
 55 
 100 
 60 
 85 
 33 
 35 
 14 
 50 
 13 
 75 
 90 
 98 
 20 
 100 
 60 
 
 t Orange a 
 
 San Jose Calif 
 
 39,642 
 39, 141 
 37, 748 
 37, 524 
 37,295 
 37,234 
 36, 230 
 36, 198 
 36,192 
 35,596 
 35,000 
 34, 764 
 34, 273 
 33,813 
 33,162 
 32,804 
 31,017 
 30,255 
 30, 105 
 30, 070 
 29,053 
 28,806 
 28, 379 
 28,064 
 27, 824 
 27,700 
 27,644 
 26, 341 
 26,224 
 25,202 
 24, 057 
 23, 594 
 23, 127 
 22,897 
 22,779 
 22,561 
 22,486 
 21,961 
 21, 719 
 21, 626 
 21, 393 
 21,284 
 20,906 
 20, 879 
 20, 474 
 20,292 
 20,065 
 19,861 
 19, 653 
 19, 516 
 19,441 
 19, 336 
 19, 143 
 18,661 
 18,539 
 18,060 
 17, 734 
 17, 677 
 17,033 
 17,004 
 16, 971 
 16,843 
 15, 820 
 15, 873 
 15,868 
 15,831 
 15, 731 
 15, 482 
 15, 462 
 15, 195 
 15, 157 
 15,100 
 15,044 
 14,648 
 14, 375 
 14,323 
 
 3 village, e 
 
 70 
 50 
 88 
 15 
 99 
 99.5 
 90 
 75 
 15 
 50 
 95 
 10 
 50 
 73 
 25 
 50 
 75 
 90 
 85 
 40 
 25 
 90 
 40 
 20 
 80 
 75 
 75 
 10 
 80 
 50 
 30 
 98 
 33 
 100 
 30 
 45 
 90 
 50 
 15 
 50 
 7 
 80 
 65 
 100 
 85 
 50 
 50 
 8 
 100 
 75 
 20 
 15 
 70 
 25 
 80 
 8 
 90 
 100 
 50 
 25 
 20 
 99 
 12 
 100 
 10 
 55 
 10 
 80 
 20 
 95 
 40 
 16 
 50 
 84 
 50 
 10 
 
 ind South 
 
 Albany, N. Y 
 
 Dubuque, Iowa 
 
 
 Brookline Mass 
 
 "The Oranges" i N. J 
 
 Columbia, S. C. . 
 
 Wilmington, Del 
 
 Lorain Onio 
 
 Reading, Pa' 
 
 Evanston 111 
 
 Kansas City, Kans 
 
 
 Yonkers, N. Y 
 
 
 Lynn, M'ass 
 
 
 Dnliith. Minn 
 
 
 Elizabeth, N. J 
 
 Newport News, v a. 
 
 Erie, Pa 
 
 PougnKeepsie, IN . i 
 
 Somerville, Mass 
 
 
 Flint, Mich 
 
 
 Jacksonville, Fla 
 
 
 Oklahoma, Okla 
 
 Ogden,Utah 
 
 Canton, Ohio 
 
 Green Bay, Wis 
 Newport, R.I 
 
 Honolulu, Hawaii 
 Manchester, N. H 
 St. Joseph, Mo 
 El Paso, Tex 
 Allentown, Pa 
 South Bend, Ind . . . 
 Portland, Me 
 
 Colorado Springs, Colo 
 Lynchburg, Va 
 Phoenix, Ariz 
 Alameda, Calif 
 Nashua,N.H 
 Hagerstown, Md 
 Mansfield, Ohio 
 
 Plninfipld N J 
 
 Charleston, S. C 
 Johnstown, Pa 
 Binghamton, N. Y 
 Rockford, 111 
 Little Rock, Ark 
 Saginaw, Mich 
 Springfield, Ohio 
 Altoona, Pa 
 
 Everett, Wash 
 Rome,N.Y 
 Kearny, N. J 
 Sioux Falls, S. Dak 
 Burlington, Iowa 
 Middletown, Ohio 
 Greenville, S. C 
 
 Holyoke, Mass 
 
 Sandusky , onio 
 Burlington, Vt 
 
 Springfield 111 
 
 Beverly, Mass.' 
 
 Racine, Wis 
 
 La FayettMnd 
 
 Chattanooga, Tenn 
 
 Fargo, N. Dak 
 Durham, N . C 
 
 Gary Ind 
 
 Logansport, Ind 
 
 Wheeling W Va 
 
 Boise, Idaho 
 
 Berkeley/Calif 
 
 Beloit Wis 
 
 Long Beach, Calif 
 
 Eau Claire, Wis 
 Braddock, Pa 
 
 Haverhill Mass 
 
 Elyria, Ohio 
 
 
 Tucson, Ariz 
 
 
 Mason City, Iowa 
 
 Tampa Fla 
 
 Greensboro, N. C 
 
 Roanoke Va 
 
 Chicago Heights, 111 
 
 Niagara Falls N Y 
 
 Ann Arbor, Mich 
 
 Topeka Kans 
 
 Santa Barbara. Calif 
 
 Winston-Salem N C 
 
 Dunkirk, N. Y 
 
 Jackson Mich 
 
 Winona, Minn 
 
 
 Wausau, Wis 
 
 
 Yakima Wash 
 
 Elmira N Y' 
 
 Alexandria, Va 
 
 Cicero 111 
 
 Anniston, Ala 
 
 New Castle, Pa 
 
 Hackensack, N. J 
 
 Fresno Calif 
 
 Framingham, Mass 
 
 Galveston Tex 
 
 Ithaca N Y 
 
 Montgomery Ala 
 
 Gardner, Mass 
 
 PuebFo Colo 
 
 Richmond Calif 
 
 Mt. Vernon,N. Y 
 
 Corning, N . Y 
 
 Salem, Mass 
 
 Champaign, 111 
 
 Perth Am boy, N . j 
 
 Peekskill, N. Y 
 
 Butte, Mont' 
 
 ChilMcothe, Ohio 
 
 Lexington Ky 
 
 Marshalltown, Iowa 
 
 Pittsfield, Mass 
 
 North Tonawanda, N. Y 
 
 Lima, Ohio 
 
 Fitchburg, Mass. 
 
 Mishawaka Ind 
 
 Kenosha, Wis 
 
 Albuquerque N Mex 
 
 Stockton, Calif 
 
 Billings Mont 
 
 Everett, Mass 
 
 
 Wichita Falls, Tex 
 
 Geneva N Y 
 
 Hamilton. Ohio 
 
 Tiffin Ohio 
 
 Superior, Wis 
 
 Bridgeton, N. J 
 
 1( 'The Oranges" include O 
 Orange township. 
 
 nd West Orange, South Orang 
 
STATUS OF PASTEURIZATION OF MILK. 
 
 TABLE 3. Approximate quantity of in ilk pasteurized in various cities as shown 
 &*/ returns from questionnaire scut in 1921 Continued. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Percent 
 of milk 
 pas- 
 teurized. 
 
 City. 
 
 Popula- 
 tion, 1920 
 census. 
 
 Per cent 
 of milk 
 pas- 
 teurized. 
 
 Hiizh Point, X. C 
 
 14,302 
 13, f*04 
 13, 722 
 13, 521 
 13,351 
 13,294 
 13,212 
 13, 181 
 13,104 
 12,718 
 12.548 
 12, 456 
 12, 451 
 12,400 
 12, 227 
 11,996 
 li;920 
 11,634 
 11,585 
 11,475 
 11,253 
 10,968 
 10,909 
 10,790 
 10, 476 
 10. 42.-, 
 10, 305 
 10, 179 
 10, 169 
 10, 145 
 10, 118 
 9,901 
 9,497 
 9,474 
 
 20 
 
 50 
 
 75 
 5 
 10 
 65 
 35 
 50 
 30 
 25 
 4 
 10 
 95 
 95 
 75 
 7 
 80 
 66 
 5 
 45 
 17 
 50 
 25 
 15 
 100 
 75 
 20 
 60 
 100 
 65 
 25 
 95 
 44 
 
 Ashland, Ohio .. 
 
 9,249 
 9,130 
 9,076 
 
 8,849 
 8.743 
 8,664 
 
 8,478 
 
 7,' 435 
 7,348 
 
 7,348 
 7,311 
 7,272 
 7,132 
 7,096 
 7,062 
 7,016 
 6,964 
 6,757 
 6,729 
 6,011 
 5,939 
 5,917 
 5,900 
 5,8.82 
 5 776 
 
 70 
 50 
 GO 
 50 
 50 
 30 
 40 
 23 
 75 
 25 
 80 
 75 
 75 
 56 
 25 
 25 
 25 
 30 
 85 
 50 
 10 
 50 
 75 
 25 
 55 
 50 
 75 
 100 
 100 
 10 
 50 
 45 
 97 
 
 1-villf 1 Pa 
 
 Chipnewa Falls, Wis 
 
 Rochester Minn 
 
 Bedford Ind 
 
 Branch, X. J 
 
 Warerville, Me 
 
 Wellsville, Ohio.. 
 
 Washington, Ind 
 
 Cortland X Y 
 
 Hanover, Pa 
 
 Eureka, Calif 
 
 Mitchell, S. Dak 
 
 Saratoga X. Y 
 
 Manhattan. Kans 
 
 Cambridge, Ohio 
 
 DeKalb.Iil... 
 
 Marquette, Mich 
 
 Salein, X . J 
 
 Morristown N J 
 
 Marblehead, Mass 
 
 La wreiice, Kans 
 
 Seymour, Ind 
 
 
 Xiles Mich 
 
 Vsburv Park X J 
 
 Painesville, Ohio 
 Princeton. Ind 
 
 Benton Harbor, Mich 
 
 Tuscaloosa. Ala 
 
 Bemidji, Minn 
 
 Independence, Kans 
 
 Roswell N. Mex 
 
 Martins Ferrv, Ohio 
 
 Rumford Falls. Me 
 
 Frankfort, Ind 
 
 Xewark, N. Y ' 
 
 Fairfield, Coun 
 
 Napa, Calif 
 
 Bristol. Va 
 
 Arkansas Citv. Kans . 
 
 Florence, S. C 
 
 Medina, N. Y 
 
 Plat ts burg X Y 
 
 
 Ehvood, Ind 
 
 Princeton, X J 
 
 Minot, X. Dak 
 
 Palo AJto Calif 
 
 Bucvrus, Ohio 
 
 Fredericksburg, Va 
 
 Salein. Ohio 
 
 
 Chic-kasha, Okla 
 
 Delphos Ohio 
 
 5, 745 
 5,679 
 4,236 
 3,924 
 2,440 
 1,500 
 
 Ilion, X Y 
 
 St \iarys Ohio 
 
 Whiting, Ind 
 
 Oberlin Ohio 
 
 Austin, Minn 
 
 
 Connersvilie, Ind 
 
 Rutherford, X. J 
 
 Pullman, Wash 
 
 Davis, Calif, 
 
 Webster drove, Mo 
 
 
 
 The flash process consists in heating rapidly to the pasteurizing 
 temperature, then cooling quickly. In this process the milk is heated 
 from 30 seconds to 1 minute only, usually at a temperature of 160 
 F. or above. In view of the previously mentioned requirements for 
 pasteurized milk this process should not be considered suitable for 
 proper pasteurization. Several cities now prohibit the use of the 
 flash process for the pasteurization of milk. 
 
 In the holder process the milk is heated to temperatures of from 
 140 to 150 F. and held for approximately 30 minutes, after which 
 it is rapidly cooled. Sometimes the milk, instead of being held at a 
 certain temperature in one tank for 30 minutes, is merely retarded 
 in its passage through several tanks or other retarding device so that 
 the length of time required for the milk to pass through is about 30 
 minutes. In such cases, however, there is not always assurance that 
 all the milk is held for the desired time. The holder process has 
 almost entirely replaced the flash process, and is the one most used 
 in this country. 
 
 Pasteurization in bottles is the latest development of the process 
 to be used on a practical scale. This process, as first practiced, con- 
 sisted in putting the raw milk into bottles with water-tight seal caps, 
 107148 22 2 
 
10 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 then immersing them in hot water until heated to 145 F. and hold- 
 ing them at that temperature for from 20 to 30 minutes. The cool- 
 ing was accomplished by gradually lowering the temperature of the 
 water until that of the milk reached 50 F. 
 
 The advantage of this process is in the fact that the milk after 
 heating is not exposed until it reaches the consumer, thereby elimi- 
 nating any danger of reinfection with disease-producing organisms 
 through handling. For this process to be successful, however, it is 
 necessary that the temperature of the milk in bottles be measured at 
 the bottom of the bottle, and that the holding period of 30 minutes 
 begin when the temperature at the bottom has reached 145 F. This 
 is essential, because the milk in the top heats faster than that in 
 the bottom of the bottle. 
 
 The matter of seals is also important. They should be absolutely 
 water-tight, as the bottles are submerged in water, and during cool- 
 ing a defective cap might allow infection by polluted cooling water. 
 The disadvantage of this process is in the increased cost of pasteuri- 
 zation, caused by the cost of the seal caps. It is claimed, however, 
 that the saving in milk losses by pasteurization in bottles makes up 
 for the added expense of caps. It is now possible to pasteurize milk 
 in this manner without using water-tight caps. This is accomplished 
 by the aid of devices which fit over the tops and necks of the bottles, 
 thereby protecting the ordinary paper caps from the water which is 
 sprayed on the bottles for the purpose of heating or cooling. This 
 method of protecting the tops permits the use of the ordinary caps 
 and seems to remove the possible danger of polluted water infecting 
 the milk. 
 
 ADVANTAGES OF LOW-TEMPERATURE PASTEURIZATION. 
 
 In general, the trend of pasteurization has been toward the holder 
 process, and with this tendency the use of lower temperatures has 
 become more common. As a general rule, when the holder process 
 is used milk is heated to about 145 F. for from 20 to 30 minutes and 
 to at least 160 F. for 1 minute when the flash process is used. From 
 bacteriological, chemical, and economical standpoints it is highly 
 desirable that milk be pasteurized at the lower temperature. 
 
 From a bacteriological standpoint, pasteurization at 145 F. for 
 30 minutes gives assurance, so far as we know, of a complete de- 
 struction of nonspore-forming disease-producing bacteria and at 
 the same time leaves in the pasteurized milk the maximum percent- 
 age of the bacteria that cause milk to sour (lactic-acid bacteria) 
 and only a small percentage of those that cause it to decompose 
 (peptonizers). When higher temperatures are used, while the total 
 number of all kinds of bacteria is reduced, the percentage of lactic- 
 
STATUS OF PASTEURIZATION OF MILK. 11 
 
 acid bacteria becomes less and less and the peptonizing group in- 
 creases until at 180 F., or above, the lactic-acid bacteria are prac- 
 tically destroyed and most of the bacteria left belong to the pep- 
 tonizing group. The heat-resistant lactic-acid bacteria which sur- 
 vive pasteurization at 145 F. for 30 minutes play* an important 
 role in the souring of commercially pasteurized milk. 
 
 From a chemical standpoint, the advantage of the lower tempera- 
 ture is in the fact that milk pasteurized at 145 F. for 30 minutes 
 does not undergo any appreciable change which should affect its 
 nutritive value or digestibility. According to Rupp (26), the solu- 
 ble phosphates of lime and magnesia do not become insoluble and 
 the albumin does not coagulate. At 150 F. about 5 per cent of 
 the albumin is rendered insoluble, and the amount increases with 
 higher temperatures to 160 F., when about 30 per cent of the al- 
 bumin is coagulated. The heating period in Rupp's experiments 
 was 30 minutes. 
 
 From an economic standpoint the advantages of pasteurization 
 at low temperatures is in the saving in the cost of heating and cool- 
 ing the milk. Bowen (10) has shown that the flash process of pas- 
 teurization requires approximately 17 per cent more heat than the 
 holder process. There is, of course, a correspondingly wider range 
 through -which the milk must be cooled, which also adds to the 
 cost of pasteurization. This is owing to the fact that in the holder 
 process milk may be heated to 145 F. and held for 30 minutes, 
 while to obtain the same bacteriological reduction with the flash 
 process, with one-minute heating, the milk would have to be heated 
 to 165 F., and even then the complete destruction of disease-pro- 
 ducing bacteria might be questionable. 
 
 TEMPERATURES AND METHODS MOST SUITABLE FOR PASTEUR- 
 IZATION. 
 
 In view of the advantages of the lower temperature for heating 
 it is believed that the temperature of pasteurization should be 
 145 F. and that the milk should be held at that temperature 
 for 30 minutes. It has been found that heating at 140 F. 
 for that length of time will destroy pathogenic bacteria, provided 
 all the milk is heated to that point and held the full length of time. 
 But it has been shown by Schorer and Rosenau (27) that it is diffi- 
 cult to do this under commercial conditions. These investigators 
 tested the destruction of pathogenic organisms by inoculating milk 
 with B. diphthei^ue, B. typhi, and B. tuberculosis and pasteurizing 
 it in 100-gallon lots under commercial conditions. They found that 
 
12 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 sometimes the organisms were not all destroyed, and in this connec- 
 tion state : 
 
 Nothing in our experiments throws any doubt upon the thermal death points 
 of the microorganisms tested. We are sure that if the milk reaches 140 F. 
 and is held there for 20 minutes it will kill tubercle, typhoid, and diphtheria 
 bacilli. Our experiments show that milk pasteurized at this temperature for the 
 specific time may not always, in practice, reach these minimum requirements. 
 It is therefore evident that a liberal factor of safety is necessary in the opera- 
 tion of this type of pasteurizer under commercial conditions. 
 
 They state further: 
 
 Perhaps the best temperature to meet practical conditions is 145 F. and the 
 milk should be held from 30 to 45 minutes. This should give sufficient leeway. 
 If the pasteurizer is set at 145 F. care will probably be taken that it does 
 not go above 148 F. on account of destroying the cream line, and it is not likely 
 that the mixed milk in the holding tank would drop below 140 F., which is 
 the minimum. 
 
 Other experiments are reported by Pease and Heulings (in the 
 Report of the Committee on Milk Supply of American Public Health 
 Association, 1920), in which the destruction of pathogenic organisms 
 was tested under commercial conditions of pasteurization. Some of 
 the pathogenic types were found living after heating to from 140 
 to 141 F. and holding for 15 minutes, but none were found alive after 
 30 minutes' holding. Here again is evidence of the narrow margin 
 of safety when milk is pasteurized at 140 F. for 30 minutes, and 
 the committee expressed the following opinion : 
 
 The committee feels that while enough has been done to indicate clearly that 
 a proper application of heat to a temperature of 140 F. for a minimum period 
 of 30 minutes will destroy substantially all the pathogenic bacteria in milk, 
 still they believe, as already expressed, that a margin of safety for biological 
 reasons calls for the use of higher temperatures of not lower than 145 F. 
 
 The United States Department of Agriculture, since 1910, (2) has 
 advised the use of a temperature of 145 F. for a period of 30 minutes 
 for the pasteurization of milk. Besides insuring an ample margin of 
 safety, a temperature of 145 F. causes a considerably greater destruc- 
 tion of bacteria in milk than 140 F. when held for the same period 
 of 30 minutes. 
 
 Extensive experiments (3) in the research laboratories of the 
 Dairy Division have shown that the thermal death point of a con- 
 siderable number of bacteria lies between 140 and 145 F.; there- 
 fore an increase of 5 above 140 F. produces a great increase in the 
 destruction of bacteria. 
 
 There is a marked tendency in commercial work to pasteurize at 
 or near the minimum temperature requirement necessary to destroy 
 pathogenic organisms, namely, 140 F. Such seems to be the case 
 because of the fear of injuring the cream line. In fact, the opinion 
 is often expressed by milk-plant operators that a temperature of 
 
STATUS OF PASTEURIZATION OF MILK. 13 
 
 145 F. can not be used because of the marked loss in cream line. 
 Harding (19) has studied the effect of temperature on the cream line 
 in a number of different plants throughout the country, and has come 
 to the following conclusion : 
 
 The data here presented show that the volume of cream on milk begins 
 measurably to decrease when the temperature of pasteurization rises from 142 
 F. to 144 F. As the temperature goes higher, the decrease in the volume of 
 cream becomes rapidly more pronounced ; at 145 F. it amounts to slightly more 
 than 10 per cent by volume; at 146 F. it amounts to 1C.6 per cent; and at 
 148 F., to approximately 40 per cent. 
 
 It may be said, however, that there are plants in this country, in- 
 cluding some of the largest, in which milk is successfully pasteurized 
 at 145 F., and this temperature is maintained for 30 minutes. It is 
 also interesting to note that at the 1921 meeting of the International 
 Dairy and Milk Inspectors' Association, Pease reported experiments 
 carried on by Heulings and him which showed that pasteurization at 
 145 F. for 30 minutes did not decrease the cream line when the milk 
 was properly heated and cooled. 
 
 The method of pasteurization, whether it is the holder or in-the- 
 bottle process, is not so important provided the process is such that 
 the milk is heated to 145 F. and that all of it is held for 30 minutes. 
 The great majority of plants pasteurize by the holder process, and it 
 is gratifying to observe that the flash process is but little used. 
 Replies to a questionnaire sent to numerous cities in this country 
 showed only 33 plants using the flash process in 18 cities out of the 
 266 which supplied information on this subject. Five cities reported 
 that the flash process was not allowed, while one permitted its use 
 but would not allow the milk to be labeled " Pasteurized." 2 
 
 SUPERVISION OF THE PROCESS. 
 
 Intelligent supervision of the pasteurizing process is absolutely 
 necessary and can not be provided unless there is a thorough knowl- 
 edge of the primary object of pasteurization and the bacteriological 
 principles involved. 
 
 The primary object is the destruction of any disease-producing 
 bacteria which may be in the milk and the handling of the pasteurized 
 milk in such manner that it can not be reinfected. When this object 
 is accomplished it is found that a large percentage of the bacteria 
 in the milk are destroyed and its keeping quality greatly unproved. 
 
 The primary object can be accomplished by heating all the milk to 
 145 F. and holding it for a period of 30 minutes. It is then only 
 necessary to cool the milk immediately over thoroughly cleaned and 
 
 2 For information on pasteurizing equipment the reader is referred to United States 
 Department of Agriculture Bulletin No. 890, Milk-Plant Equipment. 
 
 107148 22 3 
 
14 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 steamed coolers, to run into thoroughly cleaned and steamed, or other- 
 wise sterilized, bottles through a thoroughly cleaned and sterilized 
 bottle filler, then to cap the bottles with sterilized caps and place the 
 milk in low-temperature refrigerators. 
 
 This process sounds relatively simple, yet at every step problems are 
 encountered which may defeat the primary object. 
 
 First of all, it must be kept in mind that bacteria are too small to 
 be seen by the naked eye and that they are distributed in the air of 
 the milk plant, upon the equipment with which milk comes in contact, 
 and upon the hands of employees. Flies also carry millions of bac- 
 teria. When milk comes to the plant to be pasteurized the logical 
 thing to do is to see that it comes in contact only with apparatus 
 which has been thoroughly cleaned and thoroughly steamed. The 
 equipment may appear clean, but since bacteria can not be seen 
 with the naked eye, a clean (to the eye) tank or pipe may contain 
 many millions. Means must be taken to destroy as many of them as 
 possible. To do this steam is usually employed, for steam at 205 F. 
 or above for a period of 2 to 5 minutes will destroy disease-producing 
 bacteria and all but spores of the harmless types. Equipment so 
 treated may be called bacteriologically clean, but must be visibly 
 clean before application of the steam if satisfactory results are to be 
 expected. 
 
 When the equipment is in this condition, the milk can be pasteur- 
 ized. At this point the object is to heat all the milk to 145 F. and 
 hold it for 30 minutes. In intelligent supervision many problems 
 are encountered at this step in the process. They are well dis- 
 cussed in a paper entitled Pasteurization of Milk, which is a report 
 of the Committee on Milk Supply of the Sanitary Engineering Sec- 
 tion of the American Public Health Association, 1920. Briefly, the 
 principal points are : 
 
 1. Heat all the milk to 145 F. 
 
 2. Hold all the milk for 30 minutes. (Some continuous-flow systems do 
 not do this.) 
 
 3. Watch for leaking valves, also pipe lines which hold milk below the pas- 
 teurizing temperature. 
 
 4. Have accurate recording thermometers so arranged as to show the total 
 heating period. Recording thermometers should be frequently checked against 
 a standard thermometer of unquestionable accuracy. 
 
 5. Watch for foam on the milk. This may stay in the vats for hours at a 
 warm temperature suitable for bacterial development. 
 
 After proper heating and holding, the pathogenic organisms have 
 been destroyed and the total number of bacteria reduced to a mini- 
 mum point. The next problem is to cool and bottle the milk with- 
 out reinfection, particularly with disease-producing bacteria. 
 
STATUS OF PASTEURIZATION OF MILK. 15 
 
 To do this, bacteriologically clean coolers, bottle fillers, bottles, 
 and sterilized caps are necessary ; and what is of greatest importance 
 is to see that the pasteurized milk does not come in contact with 
 human hands, or with apparatus, including bottles and caps, touched 
 by the hands after being sterilized. The hands of milk handlers 
 constitute perhaps the most dangerous source of reinfection in the 
 plant, for they may convey pathogenic organisms. Through such 
 channel milk may be contaminated by carriers of many diseases. 
 
 In order to guard against such possibilities, all employees who 
 handle apparatus or milk in the plant or during delivery should 
 undergo frequent medical examination, and any diseased persons 
 or carriers should be prevented from working in positions in which 
 they have even indirect contact with milk, milk equipment, or deliv- 
 ery of the product. 
 
 It is perhaps unnecessary to say that flies are also a very serious 
 menace to the milk supply. They must be kept out of milk plants, 
 for it is impossible to tell when they may infect the milk. This in- 
 fection can occur directly by flies getting into the milk or indirectly 
 through contamination of equipment or containers. 
 
 At every step in the pasteurization of milk, one is compelled to 
 think of the process in terms of bacteria in order to supervise it 
 intelligently. 
 
 HANDLING MILK AFTER PASTEURIZATION. 
 
 Pasteurization of milk destroys about 99 per cent of the bacteria ; 
 consequently the milk is not sterile. On account of this fact, pas- 
 teurized milk is still a perishable product, and must be handled 
 with the same care as raw milk. This is a point for both the con- 
 sumer and the milkman to remember. 
 
 Milk after pasteurization should be cooled to about 40 F. and 
 kept at that temperature until delivery. During warm weather it 
 should be iced on the delivery wagons. From a sanitary standpoint 
 all milk, whether raw or pasteurized, should be delivered as soon as 
 possible, in order that the consumer may get it in the best condition. 
 In the best pasteurized milk, when held at about 40 F., there is only 
 a slight bacterial increase during the first 24 hours. In many cases 
 the pasteurization and delivery may be so arranged that the con- 
 sumer gets the milk before much, if any, change has taken place in 
 the bacterial content. For the benefit of the consumer the word 
 " Pasteurized " should be printed on the cap, as it is only right for 
 him to know whether he is using raw or pasteurized milk. Some people 
 object to pasteurized milk, especially for infant feeding, while others 
 desire it. It has been the experience of numerous milk dealers that 
 the labeling of their product has greatly increased their trade. 
 
16 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 COST OF PASTEURIZING MILK. 
 
 The present cost of pasteurization has been estimated by Bo wen 
 from the cost given in his earlier paper (10) on the assumption that 
 the average price of coal has increased 2.04 times and that milk- 
 plant labor and equipment have increased 50 per cent over the prices 
 of 1913, the year in which his paper was written. He obtained the 
 information from a series of tests in five establishments which were 
 considered to represent the average city milk plant. The pasteuriz- 
 ing equipment in each consisted of a heater, a holding tank, a 
 regenerator, and a cooler. The cost of operation was based on the 
 pasteurizing cycle, starting with the initial temperature of the raw 
 milk and raising it to the pasteurizing temperature, then cooling to 
 the initial temperature of the raw milk. He based the costs on daily 
 interest at 6 per cent per annum on capital invested in pasteurizing 
 equipment, and depreciation and repairs per day at 25 per cent 
 per annum; interest per day at 6 per cent per annum on capital 
 invested in mechanical equipment for pasteurizing, such as engines, 
 boilers, etc., and depreciation and repairs per day at 10 per cent per 
 annum. Other costs figured were labor, coal now estimated at $8.16 
 a ton, cooling water now estimated at $0.75 per 1,000 cubic feet, and 
 refrigeration now estimated at $2 a ton. With these new estimates 
 substituted for the old figures, Bowen calculates that the average 
 cost of pasteurizing 1 gallon of milk is approximately $0.0049, or a 
 little less than one-half cent. 
 
 BACTERIA WHICH SURVIVE PASTEURIZATION. 
 
 It has been stated that about 99 per cent of the bacteria in milk 
 are destroyed by pasteurization; consequently about 1 per cent of 
 the bacteria remain alive, and the kinds left depend entirely on the 
 temperature to which the milk is heated and the number of heat- 
 resistant bacteria in the milk. From studies of the bacteria which 
 survive pasteurization, it is possible to show graphically the hypo- 
 thetical relations of the bacterial groups in raw milk and in milk 
 pasteurized by the holder process at various temperatures under 
 laboratory conditions. 
 
 The bacterial flora of the various kinds of milk is represented in 
 Figure 1 by columns of equal length divided into sections, which, in 
 a general way, show the relative proportion of the bacterial groups. 
 
 From the figure it may be seen that raw milk contains four prin- 
 cipal groups of bacteria the acid, inert, alkali, and peptonizing. 
 The acid group is divided again into two the acid-coagulating, 
 which coagulates milk within 14 days, and the acid group, which 
 merely produces acid and does not coagulate it in less time than that. 
 In raw milk the inert group is the largest. 
 
STATUS OF PASTEURIZATION OF MILK. 
 
 17 
 
 In milk pasteurized at 145 F. the great increase in the proportion 
 of the acid-coagulating and acid groups is plainly shown. The per 
 cent of the alkali and peptonizing groups is reduced. At 160 F. 
 the total-acid group is still the largest, but the acid-coagulating 
 group is made up of bacteria which coagulate very slowly. At this 
 
 '62.ec. 
 
 77C. 82.2C. 
 
 93.3 
 
 FIG. 1. The hypothetical relation of the bacterial group to raw and pasteurized milk. 
 
 temperature the alkali group is greatly reduced, and the peptonizing 
 reduced to the minimum. At 170 F. the total-acid group remains 
 about the same, but the organisms produce acid and coagulate the 
 milk very slowly. The alkali group is practically destroyed although 
 
18 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 occasionally a sample may show a fairly high per cent. The most 
 important change is in the peptonizing group. At this temperature 
 the ratio of this group to the total number of bacteria begins to 
 increase. The increase when milk is pasteurized at 180 F. is even 
 more striking. At this temperature more than 75 per cent of the 
 bacteria which survive are peptonizers. No organisms of the acid- 
 coagulating group are found, and only a small per cent of the acid 
 group. Occasionally a few of the alkali group may be found. At 
 190 F. and 200 F. the bacterial groups which survive are about the 
 same in their relative sizes as at 180 F. 
 
 It is very evident that when the bacterial flora of pasteurized milk 
 is under Discussion the temperature of the process is of fundamental 
 importance. From Figure 1 the bacterial groups left in milk pas- 
 teurized at different temperatures may be seen at a glance. It must 
 be remembered, however, that the relations of the bacterial groups 
 represent only average conditions and that the bacterial flora of every 
 sample of milk must not be expected to conform exactly* to these 
 averages. Variations in methods and conditions in the production of 
 milk may considerably influence the bacterial group relations of an 
 individual sample. 
 
 The results in Figure 1 may perhaps be better explained in popular 
 terms. When milk is pasteurized at 145 F. for 30 minutes, most 
 of the bacteria . (lactic-acid) left alive in it are of the kind which 
 cause it to sour, and there are present only a few bacteria (pep- 
 tonizing) which cause it to decompose. As the milk stands, the acid 
 formers grow and cause the milk to sour instead of decompose. 
 When milk is pasteurized at 180 F. for 30 minutes, however, the bac- 
 teria (lactic-acid) which cause the souring of milk are practically all 
 destroyed, and those which are alive (peptonizing) continue to grow 
 and cause the milk to decompose. 
 
 Not only do certain types of lactic- acid bacteria survive pas- 
 teurization but some also grow at the pasteurizing temperature. 
 Sometimes upon long-continued heating at 140 to 145 F. for 
 several hours, milk sours in the holding tanks due to the growth 
 of these organisms. The ordinary period of holding does not provide 
 sufficient time for their development, so this type of souring is not 
 encountered in milk plants except when there is an interruption 
 in the pasteurizing process due to some abnormal condition. 
 
 SURVIVAL OF STREPTOCOCCI. 
 
 Since the general groups of bacteria which survive pasteuriza- 
 tion have been discussed, let us now consider a more specific, group. 
 It has been the custom of some authorities to consider the presence 
 of streptococci in pasteurized milk an indication of an ineffective 
 process. As already pointed out, pathogenic streptococci are readily 
 
STATUS OF PASTEURIZATION OF MILK. 19 
 
 destroyed by proper pasteurization. In a study of the subject (4), 
 however, it was found that certain strains of streptococci are able 
 to survive pasteurizing temperatures. 
 
 The thermal death points of 139 cultures of streptococci isolated 
 from cow feces. from the udder and mouth, and from milk and cream, 
 showed a wide variation when the milk was heated for 30 minutes 
 under conditions similar to pasteurization. At 140 F., the lowest 
 pasteurizing temperature, 89 cultures, or 64.03 per cent, survived; 
 at 145 F., the usual temperature for pasteurizing, 46, or 33.07 per 
 cent, survived; and at 160 F., 3 cultures, or 2.16 per cent, survived; 
 all these were destroyed at 165 F. The streptococci from the udder, 
 on the whole, were less resistant and those from milk and cream 
 more resistant to heat than those from the mouths and feces of 
 the cows. 
 
 Two classes of streptococci seem to survive pasteurization: (1) 
 Streptococci which have a low majority thermal death point (the 
 temperature at which a majority of the bacteria are killed), but 
 among which a few cells are able to survive the pasteurizing tem- 
 perature. This ability of a few bacteria may be due to certain 
 resistant characteristics peculiar to them or it may be caused by 
 some protective influence in the milk. (2) Streptococci which have 
 a high majority thermal death point, and which, when such is the 
 case, survive because this point is above the temperature of pas- 
 teurization. This ability to resist destruction by heating is a per- 
 manent characteristic of certain strains of streptococci. 
 
 These streptococci which have a high thermal death point above 
 the pasteurizing temperature undoubtedly play an important part 
 in the occasional high counts found in pasteurized milk. Such 
 counts are sometimes observed when the count of the raw milk runs 
 the same as usual. As the proportion of these heat-resistant types 
 vary in milk their numbers may at times reach such figures that 
 their survival of the pasteurizing process gives an abnormally high- 
 count product. The presence and variation of their numbers in milk 
 therefore is a matter which must be given consideration in connec- 
 tion with bacteria standards for pasteurized milk. 
 
 It is evident that certain varieties of streptococci are able to survive 
 pasteurization, while others are probably always destroyed. Numer- 
 ous investigators have studied the thermal death point of streptococci 
 isolated from patients having septic sore throat and have found that 
 the organism was destroyed by pasteurization at 145 for 20 minutes. 
 These results, together with the protection which proper pasteuriza- 
 tion seems to afford against epidemics of that disease caused by milk 
 supplies, indicate that the varieties of streptococci associated with or 
 responsible for the disease are among the varieties which have a low 
 thermal death point. 
 
20 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 THE COLON TEST FOR EFFICIENCY OF PASTEURIZATION. 
 
 In a study (6) of the ability of colon bacilli to survive pasteuriza- 
 tion it was found that certain strains could survive pasteurization at 
 145 F. for 30 minutes. On examining 174 cultures of colon bacilli it 
 was found that at 140 F., the lowest pasteurizing temperature, 95 
 cultures survived; at 145 F., the usual temperature for pasteuriza- 
 tion, 12 survived. In each case the heating period was 30 minutes. 
 Considerable variation was observed in the thermal death point of 
 the colon bacilli which survived at 145 F. When the cultures which 
 withstood the first heating were again heated it was found that many 
 did not survive, and in each subsequent heating different results 
 were obtained. Colon bacilli have a low majority thermal death 
 point but on account of the resistance of a few cells, they may survive 
 the pasteurizing process. 
 
 The colon test as an index of the efficiency of the process of 
 pasteurization is complicated by the ability of certain strains to 
 survive a temperature of 145 F. for 30 minutes and to develop 
 rapidly when the pasteurized milk is held under certain tempera- 
 ture conditions met during storage and delivery. Consequently 
 the presence of a few colon bacilli in pasteurized milk under ordi- 
 nary market conditions does not necessarily indicate that the milk 
 was not properly heated. The presence of a large number of colon 
 bacilli immediately after the heating process indicates that the milk 
 has not been heated to 145 F. for 30 minutes and the test properly 
 applied should be valuable in control work. Fermentation tubes 
 can be used for making the test, but when gas formation is noted 
 the presence of colon bacilli should be demonstrated by further tests. 
 Often anaerobic spore formers are encountered which survive pas- 
 teurization and give the typical fermentation tube test. 
 
 PAST AND PRESENT THEORIES OF PASTEURIZATION. 
 
 pksteiiHzatlo^i at present^ looked upon with favor by medical 
 men, sanitarians, dairymen, and consumers, but the art has not been 
 developed without opposition, and its value is not universally ac- 
 cepted. Most of the objections to pasteurized milk have been based 
 on theory or on experiments in which the milk was pasteurized at 
 high temperatures. In view of our modern theories they are of no 
 great importance. 
 
 One of the greatest objections to pasteurized milk has been that 
 the heating destroyed the lactic-acid bacteria and that putrefactive 
 organisms were left, which, when relieved from the restraining action 
 of the acid-forming bacteria, would develop, forming toxins and 
 putrefactive products. It was believed that the milk, because it 
 was not sour, would be consumed in that condition. This objection 
 
STATUS OF PASTEURIZATION OF MILK. 21 
 
 was based on experiments in which milk was heated to temperatures 
 near the boiling point and can not be applied to milk pasteurized at 
 low temperatures. From the results of many years' work in the 
 Dairy Division on commercial pasteurized milk, it has been found 
 that such milk sours, as raw milk does, but that the souring is 
 delayed when compared with the souring of the same grade of raw 
 milk. Pasteurized milk sours in a manner similar to that of a high 
 grade of raw milk, and there is no more reason to fear the over- 
 growth of putrefactive organisms than there is in any high-grade 
 milk. Pasteurization for 30 minutes at temperatures of about 145 
 F., as is generally practiced in this country, does not destroy all 
 the lactic-acid organisms, and those which survive play an important 
 role in the souring of commercially pasteurized milk. 
 
 Another objection to pasteurized milk has been that bacteria grow 
 faster in it than in raw milk. In spite of several experiments which 
 seem to prove this point, it has never been thoroughly established. 
 It has been found that the rate of bacterial increase is approximately 
 the same when the comparison is made between raw milk and pasteur- 
 ized milk having about the same bacterial content. 
 
 It is often stated that pasteurization, even if it does destroy 
 bacteria, does not destroy poisonous products of their growth. This 
 can hardly be considered a real objection, for if they are present in 
 raw milk they must be consumed with it, and if pasteurization does 
 not destroy them the pasteurized milk would be no worse than raw 
 milk. 
 
 The question as to whether pasteurization destroys beneficial 
 enzyms is still an open one. In the light of our present knowledge 
 of the enzyms in milk and the part they play in the digestive process 
 it is quite impossible to settle the question of their importance. It 
 is evident, however, that the low temperatures now in use in pasteuri- 
 zation have little effect on the commonly recognized enzyms. 
 
 The opponents of pasteurization have raised an objection on the 
 ground of its direct influence on the milk producer. It has been 
 asserted that pasteurization would cause lax methods of production 
 on the farm, for the reason that farmers would know that the milk 
 was to be pasteurized and, therefore, they could be careless in its pro- 
 duction. There seems to be some basis for this objection, but in any 
 city where there is any inspection of the raw-milk supply the same 
 inspection can and should be continued even though the milk is to 
 be pasteurized. 
 
 From a chemical standpoint ^serious objections have been raised 
 against pasteurized milk, because the heating produces changes which 
 render the milk less digestible, particularly in the case of infants. 
 As has already been stated, however, Rupp (26) has found that milk 
 
22 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 pasteurized at 145 F. for 30 minutes does not undergo any appreci- 
 able chemical change. He found that soluble phosphates do not 
 become insoluble, that the albumin does not coagulate, and that when 
 higher temperatures are used chemical changes do occur. He also 
 developed the fact that 5 per cent of the albumin is rendered insoluble 
 in milk heated for 30 minutes at 150 F., while at 160 F. 30.78 per 
 cent of the albumin is coagulated. Further evidences that low-tem- 
 perature pasteurization does not injure the digestibility and nutritive 
 value of milk are shown by the results of feeding experiments with 
 babies. According to Weld (31), a number of babies that were fed 
 raw milk and pasteurized milk showed only a slight difference in 
 the average net daily gain in weight during the feeding period. The 
 slight difference was in favor of pasteurized milk. Hess (21), how- 
 ever, has found that milk pasteurized for 30 minutes at 145 F. may 
 cause, in infants, a mild form of scurvy, which yields readily to so 
 simple a remedy as orange juice. 
 
 High-temperature pasteurization of earlier days must not be con- 
 fused with low -temperature pasteurization of the present day. Many 
 of the objections which have been raised to pasteurization have been 
 founded on the observation of milk heated to high temperatures. 
 The fallacy of the objections to pasteurization have been shown, 
 however, through scientific research in the last few years, and as a 
 result the value of the process has been firmly established. 
 
 PASTEURIZATION AND VITAMINS. 
 
 The discovery of vitamins within recent years has shown how im- 
 possible it is to estimate nutritive requirements solely in terms of 
 digestible protein, carbohydrate, fat, and inorganic salts. But little 
 is known of the real chemical nature of vitamins, except that they are 
 necessary for normal growth and health. Three vitamins are now 
 recognized, known as vitamin A (soluble in fat) and vitamins B 
 and C (soluble in water). Most authorities now agree that fat- 
 soluble A and water-soluble B are essential for growth, and water- 
 soluble C, the antiscorbutic vitamin, may also play a part in this 
 relation. 
 
 Because of the limited character of the infant's diet the vitamin 
 content of its food is more important than that of the adult's, as the 
 latter has a great variety of foods. Fortunately, milk has been 
 found to be a food containing the three vitamins and the effect of 
 pasteurization on the vitamin content is of importance. 
 
 Fat-soluble A and water-soluble fe have been found to be quite 
 resistant to heat, and it is agreed that pasteurization has little or 
 no effect upon them. The antiscorbutic vitamin C, however, is quite 
 sensitive to heat above 122 F. While the destruction of this vita- 
 
STATUS OF PASTEURIZATION OF MILK. 23 
 
 min depends upon the temperature, length, and condition of heat- 
 ing, as well as the reaction of the material in which it exists, there 
 seems to be little doubt that pasteurization of milk, under usual com- 
 mercial conditions, at 145 F. for 30 minutes, weakens the antiscor- 
 butic property of the milk. 
 
 Hess and Fish (20), in 1914, in studying scurvy in children found 
 that some cases of scurvy developed when milk was used which had 
 been pasteurized at 145 for 30 minutes. 
 
 After further studies on this subject Hess (22) made the following 
 statement : 
 
 Although pasteurized milk is to be recommended on account of the security 
 which it affords against infection, we should realize that it is an incomplete 
 food. Unless antiscorbutics, such as orange juice, the juice of an orange peel, 
 or potato water is added, infants will develop scurvy on this diet. This form 
 of scurvy takes some months to develop and may be termed subacute. It must 
 be considered not only the most common form of this disorder, but one which 
 passes most often unrecognized. In order to guard against it, infants fed ex- 
 clusively on a diet of pasteurized milk should be given antiscorbutics far earlier 
 than is at present the custom, even as early as the first month in life. 
 
 In the course of the development of infantile scurvy, growth both in weight 
 and in length is markedly affected. Under these conditions weight ceases to 
 increase, and a stationary plane is maintained for weeks or for months. There 
 is quick response, however, on the administration of orange juice or its equiva- 
 lent ; indeed sapergrowth is thereupon frequently manifested. 
 
 PASTEURIZED MILK FOR INFANTS. 
 
 A rational view must be taken of the use of pasteurized milk. 
 Shall the protection against infection, which is made available by 
 the proper pasteurization of milk, be discarded because of its defi- 
 cient antiscorbutic property, or shall its protection be accepted and 
 the deficiency in vitamin C be made up by feeding orange juice or 
 other antiscorbutics ? 
 
 Perhaps the feeding of infants calls for even further thought than 
 is generally given. As Eddy (14) in his recent book points out, there 
 are two points to be kept in mind in infant nutrition. The first is 
 that the vitamin content of cow's or human milk is dependent pri- 
 marily on the food eaten by the producer of the milk. In other 
 words, milk is merely a mobilization of vitamins eaten, and if the 
 diet is to yield a milk rich in vitamins the food eaten must also be 
 rich. He further points out the fact that cereals are poor in vitamins 
 and green grasses rich in them, and that this brings up the question 
 of winter feeding if the milk supply is used for infants, and he sug- 
 gests that the variability in vitamins A and B in milk may at times 
 make it necessary to supplement the diet. 
 
 The second point brought out by Eddy expresses what appears to 
 be the most reasonable attitude toward the use of pasteurized milk 
 
24 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 for infant feeding according to our present knowledge of vitamins, 
 and it is therefore quoted : 
 
 The second point in regard to milk lies in the effect of pasteurization. This 
 measure is now well-nigh universal and in America at least has played a tre- 
 mendous part in the reduction of infant mortality, especially in the summer 
 months. At present, however, we know that this treatment while removing 
 dangerous germs may also eliminate the antiscorbutic factor. The sensible 
 attitude then is to recognize this fact and if a clean whole milk is not available 
 retain the pasteurization and meet the vitamin deficiency by other agents. 
 Such agents are orange juice and tomato juice, and experience has already 
 shown that these juices can be well tolerated by infants much earlier than 
 used to be thought possible. 
 
 It seems, therefore, that the only serious effect of pasteurization 
 on the vitamins is on the antiscorbutic vitamin C, and it is evident 
 that the feeding of orange or tomato juice, or other antiscorbutic, 
 readily makes up for the deficiency of this vitamin in pasteurized 
 milk. 
 
 THE NECESSITY FOR PASTEURIZATION. 
 
 The need for safeguarding the milk supply is amply proved by 
 the numerous epidemics traced to milk. Trask (29) reported 179 
 epidemics of typhoid fever from 1881 to 1907, of which 107 were in 
 the United States, 51 epidemics of scarlet fever, including 25 in this 
 country, during the same period, and 23 epidemics of diphtheria 
 from 1879 to, 1907, including 15 in the United 'States. These were 
 all traced to milk. He also listed 7 epidemics of sore throat, most 
 of which occurred in England. Since 1907 several epidemics of 
 septic sore throat have been traced to milk. Among these may be 
 mentioned the epidemics at Boston, Chicago, and Baltimore, and 
 others which have occurred in smaller cities. 
 
 The problem of pasteurization is not based simply on the question 
 of which is preferable, raw or pasteurized milk, but rather upon the 
 most economical and practical way of producing a safe milk supply. 
 
 In connection with the possibility of transmission of disease 
 through the agency of milk, certain fundamental facts must be 
 recognized. 
 
 1. That such possibilities exist as demonstrated by epidemics of the past. 
 
 2. That certain diseases transmitted to man, such as tuberculosis, may come 
 from diseased animals. The danger from this source can be prevented by the 
 elimination of tuberculous cattle from producing herds on the basis of the 
 tuberculin test. 
 
 3. That the freeing of the herds from tuberculosis offers no protection against 
 other diseases, as typhoid fever, diphtheria, and septic sore throat, because the 
 pathogenic organisms causing these diseases may come from infected water sup- 
 plies or probably in most cases from human carriers of disease. 
 
 The term "carriers" is used to designate persons who carry the 
 disease-producing bacteria. In the case of diphtheria, carriers har- 
 
STATUS OF PASTEURIZATION OF MILK. 25 
 
 bor the diphtheria organisms and discharge them from the nose or 
 throat. Typhoid carriers discharge typhoid bacilli in their feces or 
 urine. Diphtheria carriers may become so after having an acute 
 attack of the disease or from other carriers. Typhoid carriers are 
 particularly important, because from 2 to 4 per cent of the persons 
 who have had typhoid fever continue, as evidence shows, to discharge 
 the typhoid bacilli in their feces or urine or both and become chronic 
 carriers. 
 
 Persons suffering from sore throat are a menace to the milk 
 supply, and probably the organisms responsible for septic sore throat 
 are sometimes carried in the throat of apparently normal individuals. 
 
 It is manifestly impossible to have a medical examination of all per- 
 sons engaged in producing and handling milk. Yet such examinations 
 at frequent intervals would be necessary, together with tuberculin 
 testing and the assurance of unpolluted water supplies on every farm, 
 in order to safeguard the milk supply of the Nation to the same 
 extent that is now possible by proper pasteurization. The apprecia- 
 tion of the need for pasteurization is distinctly shown by the marked 
 increase in pasteurization in the United States. 
 
 REFERENCES TO LITERATURE. 
 
 ANDERSON, A. K., and FINKELSTEIN, K. 
 
 (1) 1919. A study of the electro-pure process of treating milk. In Jour. 
 
 Dairy Science, v. 2, no. 5, p. 374-406. 
 AYEBS, S. H., and JOHNSON, W. T., jr. 
 
 (2) 1910. The bacteriology of commercially pasteurized and raw market 
 
 milk. U. S. Dept. Agr., Bur. Anim. Indus. Bui. no. 126. 
 
 (3) 1913. A study of the bacteria which survive pasteurization. U. S. 
 
 Dept. Agr., Bur. Anim. Indus. Bui. no. 161. 
 
 (4) 1914. Ability of streptococci to survive pasteurization. In U. S. 
 
 Dept. Agr., Jour. Agr. Research, v. 2, no. 4, p. 321-330. 
 
 (5) 1914. The destruction of bacteria in milk by ultra-violet rays. In 
 
 Centbl. Bakt. [etc.], Abt. 2, Bd. 40, No. 1/8, p. 109-131. 
 
 (6) 1915. Ability of colon bacilli to survive pasteurization. In U. S. 
 
 Dept. Agr., Jour. Agr. Research, v. 3, no. 5, p. 401-410. 
 and DAVIS, B. J. 
 
 (7) 1918. The thermal death point and limiting hydrogen ion concentra- 
 
 tion of pathogenic streptococci. In Jour. Infec. Diseases, v. 23, no. 
 3, p. 290-300. 
 BEATTIE, J. M. 
 
 (8) 1916. The electrical treatment of milk for infant feeding. In Jour. 
 
 State Med., London, v. 24, no. 4, p. 97-113. 
 
 (9) 1920. On the destruction of bacteria in milk by electricity. Gt. Brit. 
 
 Med. Research Committee, Spec., Rpt. no. 49. 
 BOWEN, J. T. 
 
 (10) 1914. The cost of pasteurizing milk and cream. U. S. Dept. Agr., 
 
 Bui. no. 85. 
 
26 BULLETIN 342, U. S. DEPARTMENT OF AGRICULTURE. 
 
 BEAT, H. A. 
 
 (11) .1915. A milk borne epidemic of tonsillitis in tuberculous patients. 
 
 In Jour. Amer. Med. Assoc., v. 64, no. 14, p. 1127-1130. 
 CAPPS, J. A., and MILLER, J. L. 
 
 (12) 1912. The Chicago epidemic of streptococcus sore throat and its re- 
 
 lation to the milk supply. In Jour. Amer. Med. Assoc., v. 58, no. 24, 
 p. 1848-1852. 
 DAVIS, D. J. 
 
 (13) 1918. The relation of streptococci to bovine mastitis and septic sore 
 
 throat. In Amer. Jour. Pub. Health, v. 8, no. 1, p. 40-46. 
 EDDY, W. H. 
 
 (14) 1921. The vitamine manual. Williams & Wilkins- Co., Baltimore, 
 
 Md. 
 EVANS, ALICE C. 
 
 (15) 1916. The bacteria of milk freshly drawn from normal udders. In 
 
 Jour. Infect. Diseases, v. 18, no. 5, p. 437-476. 
 
 (16) 1917. The large number of Bact. abortus var. lipolyticus which may 
 
 be found in milk. In Jour. Bact., v. 2, no. 2, p. 185-186. 
 HAMBURGER, L. P. 
 
 (17) 1912. An epidemic of septic sore throat in Baltimore and its relation 
 
 to a milk-supply, a preliminary report. In Jour. Amer. Med. Assoc., 
 v. 58, no. 15, p. 1109-1111. 
 
 (18) 1913. The Baltimore epidemic of streptococcus or septic sore throat 
 
 and its relation to a milk supply. In Bui. Johns Hopkins Hosp., 
 v. 24, no. 263, p. 1-11. 
 HARDING, H. A. 
 
 (19) 1921. Effect of temperature of pasteurization on the creaming ability 
 
 of milk. 'Univ. of Illinois Agr. Exp. Station Bui. 237. 
 HESS, A. F., and FISH, M. 
 
 (20) 1914. Infantile scurvy: The blood, the blood-vessels, and the diet. 
 
 In Amer. Jour. Diseases Children, v. 8, no. 6, p. 385-405. 
 HESS, A. F. 
 
 (21) 1915. Infantile scurvy. II. A new aspect of symptomatology, path- 
 
 ology, and diet. In Jour. Amer. Med. Assoc., v. 65, no. 12, p. 1003- 
 1006. 
 
 (22) 1916. Infantile scurvy. III. Its influence on growth (length and 
 
 weight). In Amer. Jour. Diseases Children, v. 12, no. 2, p. 152-165. 
 JORDAN, E. O. 
 
 ( 23 ) 1913. The municipal regulation of milk supply. In Jour. Amer. Med. 
 
 Assoc., v. 61, no. 26, p. 2286-2291. 
 KRTJMWIEDE, C., and NOBLE, W. C. 
 
 (24) 1921. On the claim that some typhoid-paratyphoid strains survive 
 
 the milk pasteurization. In Jour. Infect. Diseases, v. 29, no. 3, p. 
 310-312. 
 MOHLER, J. R. 
 
 (25) 1909. Conditions and diseases of the cow injuriously affecting the 
 
 milk. In U. S. Hyg. Lab. Bui. no. 56, p. 501-526. 
 RUPP, PHILIP. 
 
 (26) 1913. Chemical changes produced in cows' milk by pasteurization. 
 
 U. S. Dept. Agr., Bur. Anim. Indus. Bui. no. 166. 
 SCHORER, E. H., and ROSENAU, M. J. 
 
 (27) 1912. Tests of the efficiency of pasteurization of milk under prac- 
 
 tical conditions. In Jour. Med. Research, v, 26, no. 1, p. 127-158. 
 
STATUS OF PASTEURIZATION OF MILK. 27 
 
 THORNTON. W. M. 
 
 1912. The electrical conductivity of bacteria and the rate of steriliza- 
 tion of bacteria by electric currents. In Proc. Roy. Soc., London, 
 Ser. B. Biological Sciences, v. 85, no. B 580, p. 331-344. 
 TBASK, J. W. 
 
 1909. Milk as a cause of epidemics of typhoid fever, scarlet fever, 
 
 and diphtheria. In U. S. Hyg. Lab. Bui. no. 56, p. 25-149. 
 Twiss, EDITH M. 
 
 1920. The effect of pasteurizing temperatures on the paratyphoid 
 
 group. In Jour. Infect. Diseases, v. 26, no. 2, p. 165-170. 
 WELD, I. C. 
 
 1912. George M. Oyster, jr. Baby milk philanthropy, report for the 
 first 18 months beginning Apr. 24, 1911, and ending Oct. 24, 1912. 
 
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