DESICCATION, EVAPORATION AND DRYING OF FOODS 517 



out many color changes may occur which interfere with the appearance 

 of the food product. For example, it is a common observation that the 

 cut surface of an apple turns dark upon exposure to the air. This is 

 due to the presence of an oxidizing enzyme. If a product resulting 

 from the drying of apples is not to be too dark colored, either unusual 

 care must be used in the preparation or some method of inhibiting the 

 action of the enzyme or bleaching the product must be used. The 

 ripening process of fruits is due to the transformation of cell contents and 

 constituents by the enzymes contained. When this process continues 

 too long over-ripening and spoilage occur. In other words, substances 

 which are ill-flavored may develop in foods w r hich are not dried rapidly 

 enough or are dried insufficiently. 



Bacteria are present in large numbers upon the surfaces of many 

 raw foods. More are added during the process of handling. They 

 probably constitute the most important single item bringing about 

 destruction or deterioration, and whenever moisture and temperature 

 conditions are favorable they rapidly bring about undesirable changes. 

 Carbohydrates present in the foods are hydrolyzed and fermented, fats 

 are frequently hydrolyzed, and proteins broken down into simpler 

 compounds. In general they require somewhat more moisture for 

 their development than the yeasts and the molds. 



Food materials may be divided into two principal groups: those in 

 which the desirable food constituent is in solution in water; and those ' 

 in which the principal food constituent is more or less insoluble. There 

 are many foods which combine both characteristics. Raisins, for 

 example, contain a considerable amount of sugar in solution in the 

 water present. Those foods which are relatively insoluble may again be 

 divided into four groups, using the amount of water as a basis for 

 classification. First, those in which moisture is present in appreci- 

 able quantities in the interstices, that is, those which seem wet. These 

 usually furnish optimum conditions for the growth of the bacteria 

 which multiply rapidly and spread through the medium by actual 

 space growth, by convection currents, and by their own power of motion. 

 Second, some foods may contain moisture sufficient for the abundant 

 growth of bacteria but not free water which will allow of rapid dis- 

 tribution. In these the spread of the microorganisms must be largely 

 by direct growth and will necessarily be slower than in the preceding. 

 Third, the substance may be so dry that little or no growth of the 



