APPLICATIONS AND SOURCES OF ULTRAVIOLET 83 



.shading" similar to the coefficient of absorption of lifiuid.s which defines 

 the "effective depth of agitation" for granular materials, analogous to 

 the effective depth of penetration of water. Experience indicates that the 

 effective depth of agitation is of the order of ten times the diameter of 

 granulated products. 



The theory of treating granulated and powdered solids is practically 

 the same as for highly absorptive liquids, with effective depths of agita- 

 tion of the order of the effective depth of penetration into sugar syrups. 

 Although the bacteria on the surface of dry sugar crystals, for example, 

 may be about ten times as susceptible to ultraviolet killing as are those 

 in water, this is offset by the presence of the individual sugar crystals 

 on which the bacteria ride at the surface of the layer for only about a 

 tenth of the time. When the crystals are in the irradiated layer, bac- 

 teria on them are irradiated only about a fifth of the time on the exposed 

 sides of the moving crystals. The result is that granular particles can 

 be disinfected with only about one-fifth the efficiency of water disinfec- 

 tion and that the method is not practical on powdery materials. The 

 disinfection of the surfaces of granular materials is well illustrated by the 

 commercial method used on canner's sugar. 



SUGAR 



Thermoduric bacteria survive the vacuum evaporator temperatures of 

 sugar-syrup concentration and, rejected by the sugar crystals during 

 formation, remain in the final film of dilute syrup left on the crystal sur- 

 faces. Ordinarily harmless, they may cause serious spoilage in canned 

 foods and beverages. 



Such sugar, preferably in coarse crystals in a layer of about 3>-^ in. at 

 rest, is continually vibrated, stirred, or cascaded on a conveyor under 

 closely spaced germicidal lamps (Fig. 2-10) providing of the order of 

 23,000 ultraviolet mw/scj ft of conveyor surface. The length and speed 

 of the conveyor may be such as to provide a total exposure time of 15-5 

 sec for an exposure of the order of 500 mw-min/sq ft. 



GRAINS AND SMOOTH-SKIN FRUIT 



An ultraviolet method has been reported by Ewest and Leicher (1939) 

 to be effective in reducing the superficial mold contamination of hard 

 grains such as that which develops after storage in the tropics. A similar 

 but simpler method is reported by Matelsky (1950) as effective on 

 smooth-skinned fruit such as cherries. 



ULTRAVIOLET-INDUCED MUTANTS FOR NEW FUNGI 



The use of ultraviolet to produce mutants of fungi in a search for new 

 or better commercial characteristics deserves mention because of its 



