376 MICROBIOLOGY OF SPECIAL INDUSTRIES. 



Unlike bacteria, the molds extend through and over food when there is 

 no visible water film. The spores are much better adapted to air dis- 

 persal than are bacterial cells, and the hyphae penetrate more rapidly 

 than will the bacterial colony. In certain foods, therefore, as meals and 

 flours, molds are more destructive than are the bacteria. Usually they 

 will multiply with less moisture. 



INHIBITION OF GROWTH OF MICROORGANISMS IN DRIED FOOD. 



In a few cases, the development of microorganisms is prevented by the 

 absence of sufficient moisture in the medium to support growth. This 

 is not nearly so common as might appear at first thought. It occurs in 

 some foods as olive oil, starches, meals, cane sugar, etc., that have little 

 or no free water. Frequently the drying results in a concentration of 

 the solutes, beyond the point to which microorganisms can adapt them- 

 selves to the osmotic pressure. When it is remembered that a 50 per cent 

 solution of cane sugar is capable of exerting a pressure of nearly 226.796 

 kg. (500 pounds) per square inch, it will be realized that considerable 

 readjustment is necessary in the cell of a yeast plant that can grow in 

 such a medium. Drying also sometimes changes the former relationship 

 of cells and tissue constituents so that protective layers may be formed. 

 For example, in curing pork, the fat which is structurally isolated in 

 distinct cells for the most part becomes diffused throughout the outer 

 layers of the tissues and forms a water-free and water-proof exterior. 

 Foods are sometimes subjected during the process of drying to sufficient 

 heat to destroy the microorganisms contained. At other times they are 

 exposed to the germicidal action of the direct rays of the sun or to the 

 fumes of some disinfectant or bleaching agent as sulphur dioxid or smoke. 



METHODS OF DRYING. 



The 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 ray or 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, ex- 

 posure 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 foods must be dried 



