FOOD REQUIREMENTS OF MOLDS 53 



From this it is easy to understand why certain kinds of 

 food mold while certain others spoil from the growth of 

 bacteria. If a food contains much sugar, like jelly, or much 

 acid, like lemons, bacteria cannot grow in it ; while molds, 

 if neither the sugar content nor the acid is too high, are 

 not inhibited. If, on the other hand, a moist food is low in 

 sugar and acid, ordinary decay bacteria are likely to grow 

 so rapidly in it that the molds cannot get started. 



This is not the whole story, however. Yeasts, as well as 

 molds, like much sugar and tolerate considerable acid. We 

 must look further into the pecuUarities of these organisms 

 before we can learn why some foods spoil from the action 

 of molds and others from that of yeasts. 



Moisture. A factor of primary importance in the 

 growth of all microorganisms is water. A vigorous growth 

 of molds requires an abundance of moisture, and in dry 

 material they will not grow at all. There is, however, a 

 very distinct difference in this respect between molds and 

 either bacteria or yeasts. Bacteria and yeasts ordinarily 

 prefer liquids to solids, and grow only in solids that have a 

 high percentage of moisture. If, therefore, the moisture con- 

 tent of any material is quite low, like that of flour stored 

 in a damp place, molds can grow but neither yeasts nor 

 bacteria. This particular illustration is interesting: flour 

 is not acid and contains practically no sugar, but molds will 

 grow in it because its low moisture content prevents the 

 growth of bacteria or yeasts. 



It must be emphasized, however, that some moisture is 

 necessary even for molds. Flour that is stored in a dry 

 place can be kept indefinitely without molding. In fighting 

 molds, therefore, one line of attack is to exclude all moisture. 



