DESICCATION, EVAPORATION, AND DRYING FOODS 45 1 



until a part of the accumulation has been removed by diffusion or other- 

 wise. Since many of these actions are hydrolytic in nature, water for 

 both diffusion and hydrolysis must be present before the enzyme 

 can act. 



Bacteria are introduced in large numbers when food is handled and 

 probably constitute the most important factor in its destruction. If 

 moisture and temperature conditions are favorable, they bring about 

 undesirable changes. The amount of water present in foods may be 

 used as a basis for their classification into four groups: first, those in 

 which moisture is present in appreciable quantities in the interstices, 

 that is, those which seem wet. Under these conditions bacteria not 

 only multiply but spread rapidly through the medium by actual space 

 growth, by diffusion currents, and by their own motion. Second, 

 some foods may contain sufl&cient moisture for the abundant growth of 

 bacteria, but not free water for diffusion and distribution. In these 

 the spread of infection must be largely by direct growth of the organism 

 and will necessarily be slower than in the preceding. Third; the sub^ 

 stratum may be so dry that little or no growth of organisms may take 

 place, yet there is sufficient moisture so that they remain viable; 

 Foxurth, the food may be so dry that only those organisms that can 

 withstand relatively complete desiccation will survive. These 1 groups 

 cannot be differentiated entirely upon the basis of the percentage of 

 water present, for the character of the food itself and of the material 

 in solution are also important. 



Yeasts usually require sugars for their best development and are 

 therefore commonly present in foods containing this substance. They 

 are of importance therefore in fewer foods than bacteria. In nature, 

 they are frequently found upon fruitsj particularly those which contain 

 considerable quantities of sugar in the sap. They will be found also 

 upon the cut ends of twigs or grass culms where sugary sap has oozed 

 out. Colonies of considerable size may sometimes be seen upon corn 

 stubble during damp weather. They are commonly distributed by 

 flies and other insects which feed upon the sugary plant juices. They 

 are not motile, hence the spread of infection in any food must be by 

 direct growth. 



Molds, like bacteria, are ubiquitous and under proper conditions 

 will destroy almost any food. They grow readily in solutions and on 

 saturated substrata, but ordinarily, are prevented by the bacteria which 



