On the Drying of Rough Rice

Abstract

The laboratory test was conducted to provide the fundamental information about the most efficient method of rough rice drying. 1) Hulls have a function to prevent kernels from rapid dehydration, acordingly, during rough rice drying, dehydration in hulls is easy and rapid but that in kernels is hard and slow. When dehydration in kernels is enforced by immoderate drying, cracks are formed in kernels as the time elapses. It is, therefore, thought that the alternate repetition of rapid drying of hulls by relatively high temperature for a short time and transfusing of moisture from wet kernels to dried hulls during tempering is the most efficient method of rough rice drying. As shown in fig. 1, after drying by temperature of 65°C for 15 minutes, moisture content in hulls dropped from 17.7% (wet basis) to 11.0%, whereas, moisture content in kernels dropped only from 21.7% to 20.9%, and during tempering moisture in kernels removed into dried hulls and moisture content in them became 20.0% and 14.0% respectively. (The moisture content of 14% in hulls is fairly lower than the moisture conten of hulls equilibrated under natural condition (*), on account of hysteresis.) Thus, the kernels were dried from an initial moisture content of 21.7% to a final moisture content of 14.3% during 6 passes in a total drying time of 63 minutes. 2) The amount of moisture removed in each drying has influences to efficiency of drying and to crack formation in kernels, and the critical amount of dehydration in each drying that does not form crack in kernels is varied by the initial moisture content in rice. In the moist kernel, above about 25% in moisture content (dry basis), crack scarcely occures by drying on account of softness of kernel tissue. Consequently, as shown in fig. 2, the critical amount of safety-dehydration is raised from 1.0% to 1.5% (dry basis) according as the initial moisture content rises from 25% to 27%. When the inital moisture content is less than 25%, the lower the initial moisture content becoms, the more easily crack is formed, therefore, the critical amount of safety-dehydration drops from 0.9% to 0.6%. The same relation between the critical amount of dehydration and the initial moisture content is observed in unhulled rice. But, the curve of the critical amount of dehydration in unhulled rice has more steep inclination than that in hulled rice, because the amount of dehydration in unhulled rice is a total of amounts of dehydration in hull and kernel, and the amounts of dehydration in hull becomes larger as moisture content in unhulled rice rises higher. When drying is not so rapid as this experiment, the critical amounts of dehydration are more or less higher than those in these figures. 3) The time of tempering required to equilibrate the moisture between hulls and kernels is effected by the amount of dehydration and also by ambient temperature. In this experiment, it is about 2 hours under the condition that the amount of dehydration is in the range of the critical dehydration in fig. 2 and ambient temperature is about 40°C as shown in fig. 4. (fig. 5.) The equilibration of moisture among the portions in kernel is also accomplished in 2 hours. 4) This drying method has no harmful influence on rice in regard to germination and cooking characteristics. (table 1) Whereas, when ambient temperature rises 66°C by drying temperature of 76°C, germination is slightly injured and the total solids content in residual liquid becomes appearently less. 5) This principle on rough rice drying will be applicable not only to articficial drying but also to sunn drying in tropics and will be avairable to preventing the occurence of "sun checking".