MOLDS IN RELATION TO AIR 55 



Aeration. We have just seen that molding is stimulated 

 by stagnation of the air. This is not because molds do not 

 like air, but because air currents cause drying and deprive 

 them of the necessary moisture. Molds need oxygen, just 

 as we do, and hence require some air ; thus molding almost 

 always begins on the surface of any substance attacked. 



In this matter of aeration molds are quite different from 

 either bacteria or yeasts. Bacteria and yeasts, like fishes 

 and other aquatic animals, can live on the oxygen dissolved 

 in the water around them ; some bacteria can even grow 

 in the entire absence of free oxygen. Molds, on the other 

 hand, can never grow in its entire absence ; and although 

 they can probably use dissolved oxygen to some extent, 

 they apparently prefer to obtain it from the air. Some- 

 times one can actually drown a mold by keeping it beneath 

 the surface of a liquid in which it could grow well if it were 

 allowed to float on the surface. 



These facts show why molds do not cause the spoilage 

 of canned vegetables. A can of vegetables is ordinarily 

 sealed too tightly to allow the entrance of air; so some 

 kinds of bacteria may grow, but molds cannot. Further- 

 more, if air is able to get in, the bacteria can grow so well 

 in such material that they prevent mold growth. In canned 

 fruits or fruit juice molds cannot grow unless air gets in ; 

 but if the seal is not tight they can grow, because the 

 abundance of sugar and acid in such material prevents the 

 growth of bacteria which would keep down mold growth 

 if they could flourish. In jellies molds are ordinarily 

 the only microorganisms capable of growing, because of the 

 high acid content and large amount of sugar. Hence in 

 making jelly one does not have to be careful to keep out 



