518 MICROBIOLOGY OF FOODS 



organisms may take place yet there is sufficient moisture so that they 

 remain viable for long periods of time. Fourth, the food may be so 

 dry that only those organisms which withstand relatively complete 

 desiccation will survive. These groups cannot be differentiated from 

 each other wholly on the basis of the percentage of water present. The 

 manner in which the water is held, and the substances which may be 

 in solution in the water are also very important. 



Yeasts are present upon the surfaces of many fruits. They usually 

 require sugars for their best development, and are therefore commonly 

 present in foods containing this substance. Yeasts will 'also be found 

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

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

 stubble during damp weather. These yeasts are commonly distributed 

 by flies and other insects which feed upon the plant juices. The yeasts 

 are not motile, hence their spread in any food must be as a result of 

 direct growth or of convection currents. 



Molds, like the bacteria, are ubiquitous and under proper conditions 

 will destroy most types of food. They grow readily on solutions and 

 on saturated substrata, but frequently are overgrown by bacteria under 

 these conditions. For example, wet silage rots when exposed to air 

 and supports luxuriant growth of bacteria, while drier silage becomes 

 moldy. Unlike bacteria the molds extend through and over food when 

 ' there is no visible water film. The spores are much better adapted to 

 air dispersal than are bacterial cells, and the hyphae penetrate more 

 rapidly than will the bacterial colonies. In certain foods, therefore, 

 such as meals and flours, molds are more destructive than are bacteria. 

 Usually they will multiply with somewhat less moisture. 



FACTORS WHICH INHIBIT GROWTH OF MICROORGANISMS IN DESICCATED 



FOODS 



The factors which appear to be of greatest importance in inhibiting 

 the growth of microorganisms in dried foods are: the relatively complete 

 absence of free water, concentration of solutes, formation of water- 

 free protective layers, and the action of heats, sunlight, sulphur dioxide, 

 smoke, or other disinfectants or bleaching agents. 



The amount of water remaining in a desiccated food is probably the 

 most important single factor in determining its keeping qualities. In a 



