DESICCATION, EVAPORATION, AND DRYING OF FOODS. 375 



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 sufficient '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 neces- 

 sarily be slower than in the preceding. Third, the substratum may be 

 so dry that little or no growth of organisms may take place, yet there is 

 sufficient moisture so that they remain viable. Fourth, the food may be 

 so dry that only those organisms that can withstand relatively complete 

 desiccation will survive. These 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 fruits, 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 find the 

 optimum condition for their growth in such conditions. For example, 

 it is commonly observed that wet silage rots when exposed to air sup- 

 ports a luxuriant growth of bacteria, while drier silage becomes moldy. 



