180 AGBICULTUEAL AND INDUSTEIAL BACTEBIOLOGY 



determines osmotic pressure, and this is an important factor 

 in determining the ability of microorganisms to grow. Most 

 microorganisms cannot develop in concentrated sugar solu- 

 tions. Because of their relatively high sugar content, many 

 fruits are in consequence readily preserved by drying, and 

 may contain a much higher percentage of water than could 

 other carbohydrate foods, such as flour, without deteriora- 

 tion. 



The amount of moisture that must be removed may also 

 in part be determined by the concentration of acids present. 

 Essential oils are also important in some instances, although 

 the drying process may occasionally remove some of these. 

 The malic acid of the apple is a factor of no inconsiderable 

 importance in preserving dried apples. 



The amount of water available for the growth of micro- 

 organisms is not always the amount which is revealed upon 

 comparison of the moist and dry weight of the food 

 material. Many foods hold their water tenaciously, not 

 yielding it readily to microorganisms. Then, too, in some 

 instances the water is not equally distributed throughout 

 the food material. Butter, for example, retains water in the 

 form of small droplets. Butter, therefore, containing 15 

 per cent of moisture may afford much better moisture con- 

 ditions for microorganisms' growth and yield up moisture 

 more readily than some other food containing 30 per cent. 

 When the butter fat is completely freed from water it does 

 not change readily. During the process of drying, further- 

 more, external layers of the food may be changed so as to 

 render the penetration of microorganisms difficult. For 

 example, meats which are dried or partially dried soon come 

 to have the external layers saturated with fats and oils, con- 

 stituting a waterproof exterior through which microorgan- 

 isms cannot readily penetrate. 



The kinds of organisms present or the nature of the 

 enzymes may also determine the amount of moisture which 



