Abstract
Abstract The effect of vacuum from 100 to 700 mm of Hg (gauge) in accelerating the tempering process of parboiled paddy in double-pass drying was studied. The required parboiled paddy was obtained by a combination soaking treatment of paddy at 80°C for 0·75 h in the first-stage and 70°C for 3·5 h in the second-stage followed by a steaming treatment at 98·07 kPa for 600 s. In all the parboiled paddy double-pass drying experiments, the first-pass drying was carried out with hot air at 90°C up to the critical moisture content, followed by tempering treatments and the second-pass drying at 75°C in a fluidized bed dryer. For accelerated tempering a vessel with provisions for vacuum control, measurement of relative humidity, and quick loading and unloading was developed. Completion of tempering process was monitored using a tempering index based on relative humidity of the overhead space of the tempering grains in the vessel. The critical moisture content of the parboiled paddy in double-pass drying, based on the minimum total in-dryer time was obtained as 20·48% d.b. A progressive reduction of tempering time was observed with the increase in the level of the vacuum applied. Compared to the control sample for a 95% completion of tempering, the reduction in total drying time achieved for all vacuum tempering levels above 500 mm of Hg (gauge) was above 50%. For the 100% completion of tempering, the maximum time reduction of 72% compared with control was obtained at 700 mm of Hg (gauge).
Published Version
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