Feb. IS, 1915 



Colon Bacilli and Pasteurization 



407 



THE PRESENCE OF COLON BACILLI AS A TEST OF THE EFFICIENCY 



OF PASTEURIZATION 



The growth of colon bacilli which survive pasteurization is a matter 

 of considerable importance, particularly when the presence of B. coli 

 in pasteurized milk is considered as an index of the efficiency of the 

 process. We have therefore studied the effect of pasteurization at 62.8° 

 C. (145° F.) on two cultures of colon bacilli which were known to be able 

 to sur\ave heating at that temperature. Flasks of sterile skim milk 

 were inoculated with several cubic centimeters of an i8-hour-old broth 

 culture of a colon bacillus. The number of bacteria in the milk was 

 determined before heating and again after the pasteurized milk had been 

 allowed to stand for 24, 48, and 72 hours at room temperature. The 

 bacteria at the end of the 24-hour period were determined by placing as 

 high as 3 cubic centimeters in large petri plates. Table IV shows the 

 results of an experiment with the two colon cultures, GV and HO. Three 

 flasks of milk were inoculated from each culture with the same amount 

 of broth, but a bacterial count was made on only one of the three flasks 

 before heating. 



Table IV. — Growth of Bacillus coli in milk liealedfor jo minutes at 62.8° C. {145° F.) 

 and held at room temperature 



It may be seen from the table that, although the cultures survived the 

 pasteurization at 62.8° C. (145° F.), there was a very great cell destruc- 

 tion, as the bacterial count after the flasks had stood for 24 hours at 

 room temperature was very low. However, in 48 hours' time there was 

 a very large bacterial increase and even more after 72 hours. 



A similar experiment was repeated with the same cultures, except that 

 the milk after heating was held in a refrigerator at 8 C. (46.4° F.). 

 The results in Table V show again the great destruction of bacterial 

 cells which takes place during the heating process. A few bacteria 

 survived, but very little increase took place at the low temperature. 

 75012°— 15 5 



