CHAPTER XIII 

 THE CONTAMINATION OF FOODS 



The decomposition of organic matter is due to the action 

 of m irniiirirHM isms that utilize the various compounds as 

 food, and leave, as a result of their life processes, more sim- 

 ple substances, or by-products. Since most of these chants 

 all'ect the quality of foods that are used by man, or even the 

 domestic animals, it is desirable to protect food supplies in 

 general, so far as practicable, from the action of such 

 microorganisms. Especially in the temperate zone is this 

 question of food preservation of great importance, for the 

 season during which plant growth takes place is short, and 

 vegetable matter must be stored for use during the colder 

 period of the year. Under the complex conditions in which 

 we now live, the question of protection of food during the 

 process of distribution is likewise of great importance. 



While the action of most microorganisms in food sup- 

 plies does not enhance the nutritive properties of foods, 

 certain types are used to advantage in the preparation of 

 some foods, as in the fermentation industries, in which the 

 raw materials are transformed by the action of living or- 

 ganisms. Some by-products are used as food, or they may 

 be of service in the preparation of food, as is the case with 

 carbon-dioxide, formed by the action of yeast on sugar, 

 which serves as a leaven to Braise" or lighten the dough in 

 bread-making. 



Milk. In the following pages the discussion is limited 

 chiefly to milk and dairy products, as virtually all phases 

 of the relation of microorganisms to foods are well illus.- 



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