PRESERVATION OF FOOD PRODUCTS FROM I5ACTERIA. 309 



and his compost heap with equal readiness, and a large part 

 of a farmer's labor is directed toward the application of 

 means to prevent the growth of bacteria where they are not 

 wanted. There is no sharp line separating the process de- 

 vised for preventing bacterial growth and those mentioned in 

 the last chapter producing special fermentations. The ensiling 

 of corn is a means of preventing its decay, and at the same time 

 a means of furnishing it with special flavors which increase the 

 value of the product. In the preparation of hay the chief aim 

 is to prevent subsequent decay and decomposition by bacteria, 

 but, as we shall see, even in this process fermentations play a 

 part. But although the two purposes run together in some 

 food products, they are, in general, quite distinct. 



PREVENTING THE ACCESS OF BACTERIA. 



If it were possible to protect various food products from the 

 presence of bacteria they might be preserved almost indefi- 

 nitely. But this is not possible except by hermetical seal- 

 ing. Bacteria are so abundant in all water and air that they 

 are sure to get into any mass of food which is simply exposed 

 to the air. 



Certain natural foods, however, do have a certain amount 

 of protection against bacteria. Fruits in general are excellent 

 food for bacteria and undergo ready decay. The decay of 

 fruit has been carefully studied by bacteriologists, and has 

 proved to be due to a considerable number of species of bac- 

 teria. No single species appears to be particularly concerned 

 but a considerable variety may contribute to the phenomenon. 

 (Fig. 12, page 80.) These species of decomposing organ- 

 isms are so abundant in air and water that it is simply impos- 

 sible to keep them away from fruit and, consequently, an 

 indefinite preservation of fruit is impossible. But for a while 

 the attack of bacteria is delayed by the presence of the fruit 

 skin. A tough, smooth skin, like that of the apple, is not 



