520 MICROBIOLOGY OF FOODS 



A 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 rays or from some 

 artificial 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 is sufficient for drying. For other types 

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

 relative humidity. Some foods cannot be dried at high temperatures 

 because of their instability. They are usually dried at a low tempera- 

 ture and in a partial vacuum. Other foods are dried without recourse 

 to evaporation by the use of hydraulic presses or by centrifugation, the 

 latter in the manufacture of cane sugar. The water 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 method 

 depends largely upon the creation of a concentration of solutes too 

 great for the growth of the 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 extracted by osmosis. 



Many food products do not require any special 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 and immediately following 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. 

 Grains, nuts, etc., are by their nature adapted to keep under normal 

 conditions for considerable periods, although there is usually present in 

 nuts sufficient moisture to allow of the slow action of lipolytic enzymes 

 and consequent development of rancidity. Other foods require artifi- 

 cial drying. In these we have the intergrading classes which we have 

 discussed above; those which contain a small percent of water and those 

 which contain considerable water but a high concentration of solutes. 

 The absolute amount of water in a food is by no means the 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 solute^ will behave very differently with reference to 

 delivering up the water to an organism present. 



The effect of the concentration of solutes by drying is perhaps 

 the most important factor in the preservation of foods. These sub- 



