DESICCATION, EVAPORATION, AND DRYING OF FOODS. 377 



quickly for they are readily attacked by microorganisms. These are 

 usually dried at a low temperature and in a partial vacuum. Other foods 

 are dried without recourse to evaporation by the use of the hydraulic 

 press or by centrifugal action, the latter in the manufacture of cane sugar. 

 The water available for the growth of microorganisms may be reduced 

 by the addition of some crystalline substance such as sugar or salt. The 

 usefulness of the latter depends largely upon their ability to create a 

 concentration of solutes too great for the growth of bacteria. At the 

 same time a considerable proportion of the water from that part of the 

 food into which the solutes will not penetrate, is abstracted by osmosis. 



Many food products do not require any additional drying, as they 

 naturally contain little moisture. Such are the grains and the products 

 manufactured from them, as flour. The drying in this instance has oc- 

 curred during the ripening process of the grain. When for any reason 

 this does not occur, the grain will mold. It has been found necessary in 

 many instances to kiln-dry corn. Grain, nuts, etc., are by their nature 

 adapted to keep under normal conditions for considerable periods. 

 Other foods require artificial drying. In these we have the intergrading 

 classes, which have been discussed above, those which contain a very 

 small percentage of water and those which have considerable water but 

 a high concentration of solutes. The absolute amount of water in the 

 food is by no means an index to the amount that is available for the 

 growth of microorganisms. Many foods are hygroscopic. Foods having 

 the same water content and percentage of solutes will behave very differ- 

 ently with reference to delivering up the water to any organism present. 



The effect of the concentration of solutes by drying is perhaps the 

 most important factor in the preservation of food. These substances 

 dissolved in the water may be actually antiseptic when concentrated, as 

 the acids of the juices of certain fruits. More often the sugars reach a 

 concentration so great as to prevent growth by plasmolyzing the cell con- 

 tents of the organisms. For every organism there is a maximum con- 

 centration reached sooner or later, beyond which growth is impossible. 



Dried foods may be divided into three groups, using the relative 

 abundance of carbohydrates, fats, and proteins as a basis for classification. 



Carbohydrate foods are usually preserved by drying. Many, such as 

 grains and nuts and the flours and meats prepared from them, do not 

 require artificial heating. They are, however, somewhat hygroscopic 

 and in damp climates enough moisture is taken up to allow the growth of 



