TITRATION AND ADJUSTMENT OP CULTURE MEDIA 221 



as stated above, even after fourteen hours autoclaving. We 

 are continuing this work. 



In the usual preparation of media from meat the total amount 

 of heating in the autoclave varies from one half hour, at about 

 15 pounds pressure, for sterilization of ordinary broth, to two 

 and one-half hours in the preparation and sterilization of agar. 



It developed in the tests made on the corrected meat infusions 

 that the change in acidity in the first half hour in the auto- 

 clave (the usual time for the steriHzation of finished media) 

 varied from nothing to 0.3 per cent at the room temperature 

 figures, while in boiling temperature figures the change in acidity 

 was 0.3 to 0.4 per cent (see chart 6). 



The nature of the acid products which are formed as a result 

 of the hydrolytic decomposition on boiling with sodium hydroxid 

 may be different from those produced by hydrolysis alone when 

 boiling unadjusted media. Therefore, while the reaction may 

 be adjusted in each to the same point of acidity, the behavior 

 of the media toward the various organisms may not be the same. 



Similar tests to those above were made on Liebig's beef ex- 

 tract dissolved in tap water, filtered and titrated in a similar 

 fashion. These tssts showed no change in the reaction of the 

 uncorrected series even after eight hours heating. Although 

 a beef extract solution is undoubtedly quite stable," as com- 

 pared with meat infusions, a sample corrected^^ ^o neutral, after 

 two hours in the autoclave, rose 0.2 per cent in acidity. This 

 was corrected to neutral once more and did not change again 

 although heated four hours longer. 



While the above change is almost negligible, the addition of 

 peptone to this as to other media, raises the change in acidity 

 further. This must be taken into consideration in the prepara- 

 tion of media with beef extract when delicate end points are 

 desired, 



" This stability is due probably to very prolonged heating in the preparation 

 of the beef extract itself. 



^2 Correction based on room temperature titration. 



