THE MILK CONTRACTOR 287 



face. The importance of the fact was perceived at once for heating to 

 155F. for 20 min. affects the albumin, enzymes and cream line and is 

 nearly high enough to impart a cooked flavor to milk whereas at the lower 

 temperature trouble with these constituents and properties is avoided. 

 Other observers, notably Hesse, Russell and Hastings, and Rosenau, con- 

 firmed Smith's findings. The thermal death points of the common dis- 

 ease germs have been determined by several bacteriologists. In this 

 country the findings of Rosenau are accepted. They are published in 

 Hygienic Laboratory Bulletin 42 and show that: 



"The tubercle bacillus loses its virulence and infective power for guinea-pigs 

 when heated in milk at 140F. for 20 min., or at 149F. for a much shorter time; 

 in other words, it may be considered dead. 



"The typhoid bacillus when heated for 2 min. at 140F. is killed; the great 

 majority of these organisms are killed by the time the temperature reaches 138F. 

 and few survive to 140F. 



"The diphtheria bacillus succumbs at comparatively low temperature; it 

 often fails to grow after heating to 131F.; occasionally some survive until the 

 milk reaches 140F. 



"The cholera vibrio is usually destroyed at 131F. and only once survived 

 to 140F. 



' 'Both the Shiga and Flexner types of dysentery bacillus were experimented 

 with. The bacillus is somewhat more resistant to heat than the typhoid bacillus 

 but the great majority of the dysentery germs are killed by the time the milk 

 reaches 140F. They sometimes withstand heating to 140F. for 5 min.; heating 

 to this temperature for 10 min. always kills them. 



"The evidence with regard to M. melitensis is meager. The organism is not 

 destroyed by heating to 131F. for a short time; the great majority of these micro- 

 cocci are killed at 136.4F. and all are killed at 140F. 



"A temperature of 140F. for 20 min. is sufficient to destroy the virus of 

 scarlet fever, streptococci and other pathogenic organisms. 



"Milk heated to 140F. and maintained at that temperature for 20 min. may 

 be considered safe so far as conveying infection with any of these organisms is 

 concerned." 



These laboratory findings were tested in 1912 on a commercial scale 

 by Schorer and Rosenau who had placed at their disposal for the purpose, 

 a pasteurizing plant that was to be dismantled. It was equipped to 

 handle 8,000 Ib. of milk an hour by the holder process. The principal 

 parts of the system were the receiving vat, the milk pump, the heater 

 and the holder. Four runs were made with milk heavily seeded with 

 cultures of diphtheria, typhoid fever, bovine and human tuberculosis. 

 The results obtained in these experiments are briefly set forth in Table 83. 



From these experiments it was concluded that by heating milk to 

 140F. and holding it at that temperature for 20 min. the germs of diph- 

 theria, typhoid fever and tuberculosis are surely killed but that probably 

 practical conditions are best met by pasteurizing at 145F. for 30 to 45 



