KELATIONSHIP OF MICROORGANISMS 181 



must be removed in order to make food keep. In general 

 bacteria seem to require somewhat more available moisture 

 than do the molds. Yeasts and molds grow fairly well in 

 solutions that are somewhat acid, the yeasts requiring also 

 sugar for their development. 



The methods used in the process of desiccation also 

 influence the keeping qualities. Certain fruits, for example, 

 are generally sulphured, that is, exposed to the fumes of 

 S0 2 or sulphurous acid in order to bleach them. This 

 effectually destroys many microorganisms which are pres- 

 ent. Other foods may be exposed to the direct rays of the 

 sun during the process of drying and the microorganisms 

 present destroyed. 



Many methods are used for the drying of foods. Drying 

 is expedited by the use of high temperatures, by the use of 

 air which is relatively dry, by increasing the rapidity of air 

 circulation by blowing or fanning, and by the use of a par- 

 tial vacuum. 



Dried foods may be divided into groups, using as a basis 

 of classification the proportion of carbohydrates, fats, or 

 proteins, present. 



The carbohydrate foods such as grains, and the flours and 

 meals produced from them usually are sufficiently dried 

 during the process of ripening and manufacture so that 

 they will keep indefinitely. Fats and oils usually keep well 

 providing they are completely freed from water. Proteins 

 are usually somewhat more difficult to dry and to preserve. 



Many starchy products produced by the baker and the 

 manufacturer, such as macaroni, vermicelli, yeast cake, 

 crackers, biscuits, etc., are dried for preservation. Methods 

 have also been developed for the rapid desiccation of vege- 

 tables and fruits. 



Many dried fruits may contain as much as 30 per cent of 

 moisture and yet not spoil. Syrups, molasses, sorghum, 

 jams, jellies, preserves, etc., are readily preserved because 



