PH—Postharvest Technology: Development of an Accelerated Tempering Process for drying Parboiled Paddy

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).