4S2 MICROBIOLOGY OF SPECIAL INDUSTBIES 



find the optimum condition for their growth in such conditions. For 

 example, it is commonly observed that wet silage rots when exposed 

 to air supports a 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 wiU the bacterial colony. In certain foods, therefore, as 

 meals and flours, molds are more destructive than are the bacteria. 

 Usually they will multiply with less moisture. 



Inhibition of Growth of Microorganisms in Dried Food 



In a few cases, the development of microorganisms is prevented by 

 the absence of sufficient moisture in the medium to support growth. 

 This is not nearly so common as might appear at first thought. It 

 occurs in some foods as olive oil, starches, meals, cane sugar, etc., that 

 have little or no free water. Frequently the drjring results in a con- 

 centration of the solutes, beyond the point to which microorganisms 

 can adapt themselves to the osmotic pressure. When it is remembered 

 that a 50 per cent solution of cane sugar is capable of exerting a pressure 

 of about 226 kg. (500 pounds) per square inch, it will be reaUzed 

 that considerable readjustment is necessary in the cell of a yeast plant 

 that can grow in such a medium. Dr3dng also sometimes changes the 

 former relationship of cells and tissue constituents so that protective 

 layers may be formed. For example, in curing pork, the fat which is 

 structurally isolated in distinct cells for the most part becomes diffused 

 throughout the outer layers of the tissues and forms a water-free and 

 water-proof exterior. Foods are sometimes subjected during the 

 process of drying to sufficient heat to destroy the microorganisms con- 

 tained. At other times they are exposed to the germicidal action of 

 the direct rays of the sun or to the fumes of some disinfectant or 

 bleaching agent as sulphur dioxid or smoke. 



Methods of Drying 



The reduction of the water in foods below the minimum required 

 for the growth of microorganisms is accomplished in a variety of ways. 

 Most commonly heat is employed, either the sun's ray or some artificial 



