DESICCATION, EVAPORATION, AND DRYING EOODS 453 



source. In localities where the humidity of the air is low, as in many 

 of the irrigated fruit districts of the western United States, exposure 

 to the rays of the sun results in rapid drying. With other types of 

 foods and in more humid regions, artificial heat is used to reduce this 

 relative humidity. Some foods cannot be dried at high temperatures 

 because of their instability. In most cases such fogds must be dried 

 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 

 occurred during the ripening process of the grain. When for any 

 reason this does not occur, the grain wiU 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 con- 

 siderable 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 differently 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 



