14 EXPERIMENTAL DAIRY BACTERIOLOGY 



Preparation of gelatin. Prepare gelatin as follows : In- 

 fuse meat, strain, weigh cooker, and bring weight of infusion 

 to 1000 grams, as in the preparation of bouillon. Add 1 per 

 cent of peptone and 10 per cent of gelatin. Warm on water 

 bath until peptone and gelatin are dissolved, not allowing 

 the temperature to rise above 50 C. Titrate and bring re- 

 action to -}- 1 per cent. Eestore weight, taking into account 

 the weight of the gelatin added. Cook thirty minutes over 

 boiling water. Boil two minutes over free flame, stirring 

 constantly. 



Allow liquid to stand a few moments to settle. Filter 

 through cotton (see p. 7). Test reaction, adjust if necessary. 

 Eecord final reaction. Tube and sterilize. The filtering of 

 gelatin is more difficult than in the case of broth. No trouble 

 will be experienced if the filtering is not attempted until the 

 coagulum is firm and solid and the liquid gelatin appears 

 perfectly clear. If the proteids are not firmly coagulated, 

 gelatin is very difficult to filter, as the fine sediment clogs 

 the pores of the filter. If the cooking is continued until the 

 albumen is completely coagulated and the funnel is arranged, 

 as shown in Fig. 3, no trouble should be experienced. 



Prolonged heating injures the solidifying properties of 

 gelatin. If the sterilizing is done by the discontinuous 

 method, the gelatin should be placed in cold water after 

 each heating. If the autoclave is employed, the exposure 

 should not exceed five minutes at 120 C. (15 pounds pres- 

 sure). After removal from the autoclave the gelatin should 

 be cooled at once by placing in cold water. It is preferable 

 to use but 7-10 pounds pressure and continue the exposure 

 for ten or twelve minutes. 



Exercise. Each student will prepare 1 liter of gelatin. 



