GELATIN 35 



Although temperatures above 100 C. are much more 

 destructive to the solidifying property than that of 100 C., 

 it is possible to sterilize a medium containing 12% to 15% 

 of gelatin in the autoclav (7 to 8 Ibs. pressure) at 112 to 

 113 C. for twenty minutes or at 15 Ibs. pressure (120 C. 

 for five minutes) without impairing its usefulness as a solid 

 culture medium. 



This use of steam under pressure (dry steam) is almost 

 necessary in the case of a gelatin medium to effect sterili- 

 zation, since gelatin, from its source, method of preparation, 

 and later liabilities to contamination, is almost certain to 

 contain or bear upon its surface a large number of very 

 resistant spores. Heating at 100 C. for thirty minutes 

 on three or even four or five consecutive days is not always 

 efficient, as these spores do not always germinate within 

 twenty-four hours after heating and, referring to the data 

 above, it is readily seen that the lowering of the lique- 

 faction point is not to be considered as negligible in the 

 process of intermittent sterilization. 



Gelatin possesses another property which renders it 

 valuable for bacteriological work: i.e., in gelatin plate 

 cultures no water of condensation ordinarily collects on the 

 cover of the Petri dish (as with agar) later to drop on the 

 surface of the gelatin and thus obliterate forms of colonies 

 and cause isolated colonies to become contaminated with 

 neighboring ones. The storing of this medium either 

 in test tubes or in plates, sterile or inoculated, is thus 

 rendered much more simple than with agar. 



REFERENCE 



VAN DERHEIDE, C.C.: Gelatinose Losungen und Verflussigungspunkt 

 der Nahrgelatine, Arch. f. Hyg., Bd. 30, 1897, pp. 82-115. 



