STERILISATION : pasteurisaTioIsi ;;; 



chief of which are based upon the destructive action of high 

 temperatures, or upon the poisonous nature of certain chemical 

 compounds. 



{a) By maintaining a low temperature, food-stuffs may be 

 kept for an indefinite time without change. In a frozen state 

 meat and milk are transported from one part of the world 

 to another without damage, and it is well known that fruit, jam, 

 milk, bread, and flesh 'keep' best in cool, dry situations. In 

 such cases the low temperature merely checks the development 

 of the bacteria present for a time, and does not destroy them ; a 

 sterile condition, therefore, is not produced by processes of this 

 kind, and when the temperature is raised the organisms begin 

 their work with unabated vigour. 



(b) The keeping qualities of substances are greatly increased 

 by roasting, baking, boiling, or steaming, for most bacteria 

 are killed by these processes. The most certain and complete 

 sterility is obtained by the application of heat. A temperature 

 of 150° C. maintained for an hour is sufficient to destroy all forms 

 of bacteria, their spores included. Even the boiling of liquids, 

 such as water, milk, and broth, at 100° C. kills all bacteria con- 

 tained in them if continued long enough, but certain spores resist 

 this temperature for several hours. Usually it is only needful to 

 keep most common liquids at 100° C. for fifteen or twenty 

 minutes to destroy all bacteria in a vegetative condition and 

 a great many of their spores also. 



By repeated heating on three or four successive days, during 

 which intervals the spores are allowed to germinate, it is possible 

 to completely sterilise a liquid without going above a temperature 

 of 100° C. 



' Pasteurisation ' or heating for twenty minutes at 70° C, 

 followed by rapid cooling, is a process sufficient to kill almost 

 all objectionable bacteria occurring in milk and other liquids, 

 without materially altering the taste or composition of the latter. 

 It does not, however, destroy the spores of these organisms, and 



