Gelatin 231 



GELATIN. 



The culture-medium known as gelatin is bouillon to which 

 10 per cent, of gelatin is added. It has the decided ad- 

 vantage over bouillon that it is not only an excellent food 

 for bacteria, and, like the bouillon, transparent, but also is 

 solid at the room temperature. Nor is this all: it is a 

 transparent solid that can be made liquid or solid at will. 

 Leffmann and La Wall have examined commercial gelatins 

 and found that many of them contain sulfur dioxid in quanti- 

 ties as great as 835 parts per million. As the varying quan- 

 tity of this impurity may modify the growth of the culture, 

 pure gelatin should be demanded, and all gelatin should be 

 washed for some hours in cold running water after being 

 weighed and before being added to the bouillon. It is pre- 

 pared as follows: 



To 1000 c.c. of meat-infusion or to 1000 c.c. of water 

 containing 2 grams of beef-extract in solution, 10 grams 

 of peptone, 5 grams of salt, and 100 grams of gelatin ("Gold 

 label" is the best commercial article) are added, and heated 

 until the ingredients are dissolved. The solution reacts 

 strongly acid and must be corrected by titration, as already 

 described. It must then be returned to the fire and boiled 

 for about an hour. As gelatin is apt to burn when boiled 

 over the direct flame, double boilers have been suggested, 

 but unless the outer kettle is filled with brine or saturated 

 calcium chlorid solution, they are very slow, and when 

 proper care is exercised there is really no great danger of 

 the gelatin burning. It must be stirred occasionally, and 

 the flame should be so distributed by wire gauze or by placing 

 a sheet of asbestos between it and the kettle as not to act 

 upon a single point. At the end of the hour the albumins 

 of the meat-infusion will be coagulated and the gelatin 

 thoroughly dissolved. Giinther has shown that the gelatin 

 congeals better if allowed to dissolve slowly in warm water 

 before boiling. As much water as has been lost by vaporiza- 

 tion during the process of boiling should be replaced. It 

 is well to cool the liquid to about 60 C., add the water mixed 

 with the white of an egg to clear the liquid, boil again for 

 half an hour, and filter. 



If the filter paper be of good quality, properly folded 



