184 AGRICULTURAL BACTERIOLOGY 



perature of boiling water above 212 F., the temperature 

 of steam when confined increases rapidly above that point. 

 Under a steam pressure of fifteen pounds to the square 

 inch, a temperature of 248 F. is attained, which is suffi- 

 cient to destroy all forms of bacterial life, even the most 

 resistant spores. 



In the treatment of many foods, the heating process can 

 not be continued for too long a period without injury to 

 the physical properties of the product, as, for instance, 

 with milk heating at excessive temperatures causes the 

 casein to curdle. In the treatment of peas, if heated at too 

 high a temperature or for a prolonged period, some of the 

 peas crack open, allowing the contents to escape. This 

 results in a turbid liquid which is unattractive and gives 

 the impression of an abnormal fermentation. To prevent 

 this the cans should be exposed for a longer time at a lower 

 temperature. In the earlier days of the canning industry 

 much loss was occasioned from the development of abnor- 

 mal fermentations, due to the growth of gas-producing bac- 

 teria, the spores of which were not destroyed by insufficient 

 sterilization. Even under the anaerobic conditions that 

 prevailed in the closed container, luxuriant germ growth 

 could occur. 



With milk, peas, and vegetables, the reaction of the liquid 

 is sufficiently neutral to permit of the ready germination 

 of any spores that may have escaped destruction in the 

 heating process. Since gas is generally produced as a re- 

 sult of such fermentation, spoiled food products preserved 

 in tin containers can usually be detected by the bulging 

 of the ends of the can. This pressure may develop to the 

 point where the cans actually explode. In the treatment 

 of meat products, there is no practical danger of over- 

 heating, so the losses that occur are relatively small. 



In household preservation of vegetables, sterility can be 



