APPENDIX A 121 



The criterion employed for the Pasteurizing temperature is the 

 temperature necessary to kill such pathogenic organisms as the 

 typhoid and tuberculosis bacteria, and also the majority, if not all, 

 of the lactic-acid bacteria. If the time allowed for heating be short, 

 then the temperature must be higher, and vice versa, within certain 

 limits. 



The principle of Pasteurization is somewhat similar to that of 

 fractional sterilization ; that is, non-spore-forming bacteria are killed 

 by the temperatures employed. The process is to be regarded as one 

 which retards germ development rather than as one which prevents it. 



Pasteurization is also extensively used in the production of beer. 

 The beer, in bottles or jugs, is submerged for half an hour in water 

 at a temperature of 60-65 C. 



STERILIZATION BY MEANS OF FILTRATION 



Liquids which cannot be treated with chemicals or with heat 

 may be sterilized by nitration. In the preparation of enzymes and 

 toxins this method of treatment is extensively employed. 



The filters used are cylindrical vessels, closed at one end like a 

 test tube, which are called bougies. The Chamberland filters are 

 made of porous porcelain, hard-burned and unglazed. The Berkefeld 

 filters are made of kieselguhr, a fine, diatomaceous earth. The 

 Chamberland has the finer pores ; the Berkefeld filters are quicker ; 

 with either one, however, it is necessary to use aspiration or pressure 

 to hasten the filtration. 



The simplest way to use such a filter, when a small quantity of 

 liquid is to be filtered, is to place the liquid inside the filter and force 

 it out by means of compressed air. Wrap the nipple end of the 

 bougie with a firm band of absorbent cotton about 7~10 cm. wide. 

 Invert the bougie in a glass cylinder 5-10 cm. longer than the bougie, 

 so that only the nipple end projects from the mouth of the cylinder. 

 The cotton wrapping should fit rather firmly in the mouth of the 

 cylinder, and the nipple of the bougie should be wired to prevent its 

 slipping down into the cylinder. Wrap this apparatus in manila 

 paper and sterilize it in the dry oven for two hours at 140 C. 



After the apparatus is cool, insert a small funnel in the nipple of 

 the bougie and pour in the liquid to be filtered, taking care not to 

 wet the cotton surrounding the upper end of the bougie. Connect 

 the bougie nipple with an air pump by means of heavy-walled 



