EXPERIMENT STATION BULLETINS. 547 



LITERATURE REVIEW. 



A non-spore-forming bacterium (Bact. Tjulgaricum) was studied by 

 Leichmaun (12) who states that to kill it in milk an exposure of 70° C. 

 for two hours was required. The belief of Russell (16) is tliat to a 

 large extent the lactic acid bacteria are destroyed by the pasteurization 

 process. Marshall (13) observed that a large number of the samples of 

 milk pasteurized at 68° C. for twenty minutes loppered with the pro- 

 duction of acid, although no true lactics Avere found on plating. Among 

 those isolated were four non-spore forming bacteria, three of which are 

 able to survive 80° C. for twenty minutes in bouillon and the fourth 

 withstood TO'' C. for the same length of time. A number of streptococci- 

 and members of the colon group whose death point is high were isolated 

 by Harrison (10). Gage and Stoughton (8) isolated a strain of B. coli 

 whose thermal death point in bouillon was 80° 0. They attempted to 

 increase its resistance to heat by subculturing from the surviving few, 

 but no increase in resistance was obtained. Dornie and Daire (7) 

 state that Bacillus surgeri (an organism of the Bact. dulgaricum group) 

 isolated from pasteurized whey resists a temperature of 85° C. for five 

 minutes. Ayers and Johnson (1, p. 58) say that a temperature of 62.8° 

 C. held for thirty minutes wovild be sufficiently high to afford protection 

 against pathogenic bacteria and yet would leave in the milk the maxi- 

 mum proportion of lactic-acid bacteria and the group proportions would 

 be very similar to those of all grades of market milk. In their conclu- 

 sions they assume that the souring of pasteurized milk is due to the de- 

 velopment of lactic acid bacteria that, on account of their high thermal 

 death point in milk, survive pasteurization. White and Avery (22, p. 

 171) in their study of seventeen diflferent strains of Bact. hulgaricum 

 observed that a minimum exposure in whey of fifteen minutes at 60° 

 C. was necessary to kill all the strains. Pease (14) states that mem- 

 bers of the colon group are more difficult to kill by pasteurization than 

 is Bact. tuberculosis. In commercial pasteurized milk of Wahington, 

 Kinj^oun (11) found high counts of colon bacilli and of streptococci. 

 He attributed their presence to dirty milk inefficiently pasteurized. 

 Data given by Ayers and Johnson (2) show that 1.2 percent to 4.5 

 percent of the lactic acid bacteria in raw milk survive pasteurization at 

 62.8° C. for thirty minutes. They give the thermal death point of a 

 lactic acid organism isolated from pasteurized milk as 79.4° C. for 

 thirty minutes in bouillon. Some acid-forming bacteria were found that 

 have a thermal death point in bouillon ranging between 82° C. and 

 93.3° C. The determination of the thermal death point of one hundred 

 and thirty-nine strains of streptococci isolated from various sources re- 

 vealed to Ayers and Johnston (3) that 33.07 percent of the strains sur- 

 vived, in milk, a temperature of 145° F. for thirty minutes, that 2.58 

 percent survived at 160° F. and that all succumb at 165° F. They ob- 

 served that the typical streptococci (those forming long chains) were 

 less resistant to heat — seventeen from the eighteen strains were killed 

 at 145° F.— than the atypical of wliich, at 145° F, 38.46 percent of the 

 strains survived. Also that the strains from the udder are more re- 



