EVAPORATION AND DRYING OF FOODS 519 



few cases the development of microorganisms is absolutely inhibited 

 by the absence of sufficient moisture in the food to support growth. 

 Many foods which appear to be dry nevertheless contain an appreciable 

 amount of moisture. The amount necessary to bring about appreci- 

 able changes or detrimental changes is sometimes not very great. For 

 example, the amount of moisture present in raw sugar appears to con- 

 stitute a real loss to the manufacturer. Some foods, such as olive oil, 

 starches, meals, cane sugar, etc., have little or no free water. Others 

 contain an appreciable amount of water and yet do not deteriorate, 

 usually because the drying has resulted in a concentration of the solutes 

 beyond the point to which the 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 225 

 kilograms per square inch, it will be realized that considerable capa- 

 city for readjustment is necessary in the cell of any yeast or mold that 

 can grow in such a medium. 



In the process of drying, the former relationships of tissue cells 

 and tissue constituents may be so changed that protective layers are 

 formed. For example, in curing pork, the fat which was structurally 

 isolated in distinct cells for the most part becomes diffused through 

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

 exterior. The keeping quality of the dried food is sometimes in part 

 dependent on the destruction of microorganisms by heat during the 

 process of drying. In other cases they are exposed to the germicidal 

 action of the direct rays of the sun, or to the action of some disinfectant 

 or bleaching agent, such as sulphur dioxide or smoke. 



METHODS OF DRYING 



The rapidity with which foods may be dried depends upon the 

 amount of water present in the food, the texture and size of the particles, 

 the temperature, the relative humidity of the atmosphere, and the 

 rapidity of the current of air which carries away the moisture. The 

 rapidity with which foods must be dried in order to remain palatable 

 depends in part upon how subject they are to the attacks of molds, 

 yeasts, or bacteria, in part upon the rapidity of ghanges brought about 

 by autolytic enzymes, and in part upon the changes in flavor and tex- 

 ture which may be brought about by the application of too high 

 a temperature. 



