42 GROWTH OF BACTERIA 



carried out in open tanks at a temperature of 100 C, 

 the small opening in the centre of the lid being closed 

 with molten solder after the heating had been sufficiently 

 prolonged to destroy all living germs and their spores. 



In order to ensure more certain sterility and reduce 

 the time necessary for the process, the tins are now 

 generally heated in large closed autoclaves under pres- 

 sure to 1 1 5 C. or more. 



Common salt (NaCl) is one of the most useful sub- 

 stances for the preservation of food. The addition of a 

 small amount assists the keeping quality of butter and 

 cheese ; larger quantities are employed in the " curing " 

 of bacon, and a strong brine is capable of preventing the 

 bacterial decomposition of beef, pork, fish, and other 

 kinds of meat. 



Boric acid (H 3 BO 3 ) and borax (Na 2 B 4 O 7 + ioH 2 O) 

 have also an inhibiting effect on the development of 

 bacteria, and have been extensively employed for the 

 temporary preservation of butter, cream, and other dairy 

 products. It is, however, doubtful if these compounds 

 are altogether harmless ; if used in sufficient amount to 

 check bacteria, they are liable to produce derangements 

 of the digestive system when the " preserved " food is 

 consumed by human beings. 



