BACTERIA IN INDUSTRIES 71 



their presence everywhere makes it impossible to preserve meat 

 and fruit for more than a few days without applying special 

 methods. This fact, coupled with the necessity for conserving 

 a supply for the months when such foods are scarce, has given 

 rise to one of the most important industries. Canning of meats 

 and fruit is simply preserving them from the attack of micro- 

 organisms ; heating kills all the bacteria present, and hermetically 

 sealing prevents others from gaining access. 



The process of canning was practised as early as 1804. Monsieur 

 Appert of Paris found that food in sealed vessels would keep in- 

 definitely if, after being sealed, the containers were kept for one 

 hour in boiling water. His method is still employed except that 

 after an interval of a day a second heating is now given to destroy 

 forms that might have been in a spore stage on the first day. 

 Canning is really a practical application of fractional sterilization. 



Drying is one of the oldest and simplest methods of preserving 

 food from bacterial attack. Exposure to sun and air deprives the 

 substances of their moisture and consequently renders them 

 unsuitable for bacterial growth. Smoke-dried meats in addition 

 to losing their moisture are impregnated with antiseptic substances 

 such as creosote, which are present in varying amounts in wood 

 smoke. It has been found, however, that smoking cannot be 

 depended upon to destroy disease-producing organisms in con- 

 taminated meats. 



The addition of chemical substances that prevent the growth 

 of bacteria has long been practiced ; vinegar is a familiar example. 

 Meat placed in brine and fruit in a thick sugar sirup are preserved, 

 because the density of the solution being greater than that of the 

 bacterial cell, water is drawn from the microorganism rather than 

 supplied to it. Strongly bactericidal substances are sometimes 

 used, such as borax, salicylic acid, and formaldehyde. They may 

 accomplish the purpose for which they were added, but except in 

 very small amounts they are injurious to the consumer. 



The cold-storage method of food preservation has as its prin- 

 ciple the inhibition of bacterial growth by a low temperature. 

 Bacterial multiplication ceases at a few degrees above freezing 



