34 GENERAL MICROBIOLOGY 



If superdried at about 130 C., or superheated when in 

 the gelatinous state either for a short time at a temperature 

 above 100 C., or for a long time at 100 C., as in inter- 

 mittent sterilization, the gelatin is so modified that its 

 redissolving or resolidifying power respectively is lost. In 

 superdrying, the loss of the redissolving property is laid to 

 the too close contact of the constituent particles, a change 

 in the physical state; in the superheated gelatin, the loss of 

 the resolidifying power is probably due to the disintegra- 

 tion of the gelatin molecule, a more purely chemical 

 phenomenon. This loss of the gelatinizing property is also 

 caused by the enzymic activities of many microorganisms 

 and is also a disintegration process. 



Gelatin possesses a liquefaction point which, however, 

 varies considerably under different conditions. Ordinarily, 

 media containing 12% to 15% gelatin will liquefy or melt 

 at a temperature in the vicinity of 24 to 26 C., solidify- 

 ing again at 8 to 10 C. to a clear, transparent jelly. As a 

 consequence, gelatin media may be employed only for 

 organisms which do not require a higher temperature than 

 22 to 24 C. for development. Overheating in the process 

 of preparation or sterilization will cause a considerable 

 lowering of the liquefaction point, perhaps ultimately so 

 low that the medium will be liquid at room temperature 

 (20 to 21 C.) It will readily be seen how the latter 

 gelatin medium could not handily be used for the isola- 

 tion of organisms. A few data will assist in fixing this in 

 mind. 



The solidifying property of gelatin varies in inverse 

 proportion with the time of heating during the process of 

 sterilization; its liquefying point is lowered on an average of 

 2 C. for each hour of heating at 100 C. This makes clear 

 why such care must be taken in the preparation of a gelatin 

 medium, in the fractional sterilization of this medium in 

 streaming steam, and why immediate cooling is necessary 

 aftei; each fractionation in the process of its preparation. 



