CHAPTER XVI. 

 THE PRESERVATION OF FOOD PRODUCTS. 



THE SPOILING OF FOOD. 



Practically all the materials raised on the farm as food for 

 man or animals may serve also as food for microorganisms. If 

 any of the various fungi attack foods, their presence is shortly 

 made manifest by signs of decomposition. The food, we say, 

 begins to spoil. Any one of our food products that contains sufficient 

 water is sure to be attacked sooner or later by some of the various 

 types of microorganisms, especially by the three classes we have 

 recognized. 



By Bacteria. These are particularly adapted for feeding upon 

 proteid matter, but they are not fond of sugar. Hence we find 

 them especially concerned in the spoiling of proteid foods. Meats, 

 milk, eggs, wheat, and other cereals, all contain unusually large 

 quantities of proteids .and are liable to putrefaction, whenever 

 they are moist enough; and their putrefaction will practically 

 always be found to be due to bacteria. In all attempts to preserve 

 these substances it is to be remembered that we are dealing with 

 bacteria, some of which are liable to form spores and for that reason 

 are difficult to destroy. Pure sugar solutions, on the other hand, 

 will not undergo a bacterial fermentation, although impure sugars 

 may do so. 



By Yeasts. Although sugars are not attacked by bacteria, 

 they are the favorite food of yeasts, which destroy them by setting 

 up an alcoholic fermentation. If the sugar is in considerable 

 abundance, it serves as a partial protection against bacterial gro wth, 

 although it favors yeast activities. From this it follows that fruit 

 juices in particular are subject to yeast fermentation, but are not 

 specially liable to bacterial action. All such substances as fruit 



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