276 EXPERIMENT STATION RECORD. 



higher, both the raw and the pasteurized milk soured in 24 hours with solid 

 curds and a little gas. These experiments indicated that pasteurization does 

 not prevent milk from souring, but merely delays the process. 



A few experiments were made in using the " holder " process for pasteurizing. 

 The milk was heated in the same " flash " machine used in the preceding experi- 

 ments, from which it flowed into a steamed-covered can where it was held at 

 2° lower than when pasteurized. Twenty minutes" holding was found to be 

 almost as eflicient as the 40 minutes' holding. Another experiment was carried 

 out under the same conditions, except that the milk was pasteurized in a jack- 

 eted can and lield for varying lengths of time. Two samples heated to 60° 

 and held for 40 minutes and 2 samples heated to 65.6° all curdled with an acid 

 curd in 48 hours, when held at room temperature. On the other hand, one of 

 the 2 samples heated to 71.1° curdled with a rennet curd after 24 hours at room 

 temperature, while the other curdled after 48 hours with an acidity of 0.41 per 

 cent and was partly digested. 



" These experiments show that a high temperature for a considerable period 

 is required to prevent souring. It is evident, too, that peptonizing bacteria 

 are of no more importance than in raw milk, for the milk must be held 4 or 5 

 days before their numbers in pasteurized milk are as great as those in raw 

 milk. The results also indicate that milk always sours when pasteurized under 

 conditions that would prevail in commercial work, although the souring is 

 delayed." 



An examination of the pasteurized milk of a small city showed that at the 

 time the consumer would receive it the average group proportion was : " Pep- 

 tonizers, 9.92 per cent ; lactic-acid bacteria, 56.70 per cent ; alkali and inert 

 forms, 33.97 per cent. After holding 24 hours at room temperature the pro- 

 portion of acid bacteria rapidly increased to 99.03 per cent, while the alkali 

 or inert forms decreased to 0.19 per cent and the peptonizers to 1.14 per cent. 

 The average acidity was 0.64 per cent, indicating a sour milk." 



Studies of commercially pasteurized milk obtained from the regular milk 

 supply of Boston and New York also showed that commercially pasteurized 

 milk always sours because of the development of lactic-acid bacteria, which, 

 on account of their high thermal death point survive pasteurization, and per- 

 haps in some cases because of subsequent infection with acid-forming bacteria 

 during cooling and bottling. 



The results in this paper hold only for commercially pasteurized milk heated 

 from 60° to 65.6° in the "holder" process, or up to 71.1° in the flash process. 

 It is pointed out that it is not reasonable to expect the use of high temperatures 

 in commercial pasteurization. At the above temperatures milk, when lieated 

 under commercial conditions, may be expected eventually to sour, and the 

 bacterial content will be comparable to the average group curves for pasteurized 

 milk as shown in this paper. These temperatures would be sufficiently high to 

 afford protection against pathogenic bacteria and yet would probably leave in 

 the milk the maximum proportion of lactic-acid bacteria and the group pro- 

 portions would be very similar to those of all grades of market milk. If higher 

 temperatures are used a degree of heat may be reached which will result in 

 the destruction of all but spores, which, when allowed to develop, will un- 

 doubtedly produce peptonization of the milk. 



" The relative proportion of the groups of peptonizing, lactic acid, and alkali 

 or inert bacteria is approximately the same in efficiently pasteurized milk as it 

 is in clean raw milk. In both cases the alkali or inei-t forms constitute the 

 largest group, the lactic-acid bacteria next, while the peptonizers are in the 

 minority. When both of these milks — the efficiently pasteurized and clean raw 



