Preservation of Foods. 231 



as it is placed in the silo, the cells are revived and normal 

 silage is formed, when without the addition of water the 

 silage would spoil. 



In the preparation of sauerkraut the fresh tissue is 

 treated in very much the same way as is that placed in a 

 silo, except that salt is added and the cabbage is packed 

 until the interspaces are filled with liquid instead of gases 

 as in silage. In this weak brine, acid-producing bacteria 

 grow, apparently forming from the sugar and starches 

 of the cabbage, lactic and acetic acids, that prevent the 

 growth of the putrefactive bacteria. Sauerkraut is usu- 

 ally stored in open vessels. On the surface of the acid 

 liquid, molds grow which gradually neutralize the acid. 

 At last a point is reached where the putrefactive bacteria 

 can begin to grow. The sauerkraut then changes into an 

 offensive mass of decomposing tissue. 



Preservation by canning. In many of the processes 

 of food preservation, the product is less appetizing than 

 the fresh material. In order to avoid this as far as pos- 

 sible, the process of preservation by canning has been 

 introduced. In the canning of most materials the bac- 

 teria are absolutely destroyed by heating where the ex- 

 posure is made either at the boiling point of water for a 

 considerable period or for a shorter period of exposure at 

 higher temperatures. In the canning of corn, peas, 

 meats, fish, etc., the latter process is employed. The- 

 canned foods are sterile and will keep for an indefinite 

 period. In the canning of very acid vegetables, as to- 

 matoes, it is not necessary to render them perfectly free 

 from living bacteria, as the acid prevents germ growth. 

 If the heating is sufficient to kill the mold spores, yeasts, 

 and bacteria that do not form spores, the tomatoes will 

 keep. Some bacterial spores will be left but they can not 



