Effect of starch properties on parboiling process and some physicochemical properties of parboiled rice

The optimum soaking period for two varieties (IR42 and IR62) differing in gelatinization temperature ranged from 7-9 hours at 60 deg C. The water content in the soaked rough rices ranged from 41.2 to 43.4% dry basis which was sufficient for parboiling. At high temp. (70 deg C), water uptake was faster in IR42 (low gelatinization temp.) than in IR62 (intermediate gelatinization temp.). Steaming the soaked rough rice at 0 kg/cm2 (100 deg C) for 30 min and 10 min at 1.0 kg/cm2 (120 deg C) brought about complete starch gelatinization based on endosperm degradation in 1.0% KOH. After soaking of 12 rough rice samples differing in amylose content and gelatinization temp. at 60 deg C for 8 hours, low gelatinization temp. varieties had higher water content (43.1 to 51.4%) than intermediate gelatinization temp. varieties (41.4-42.8%) except for Pajam. Pajam had high water content (47.8%). Water content of soaked rough rices was also highest for waxy and low amylose rices. The samples steamed at 0 kg/cm2 for 30 min and 1.0 kg/cm2 or 1.5 kg/cm2 for 10 min had almost similar total milling yields to those of raw rice except in IR65 (waxy). IR65 had low milling yield even at 0 kg/cm2 for 30 min steaming. Head rice yield in the parboiled samples was higher than that of the raw rice. IR65, IR5 and BR4 had lower head rice yields at 1.5 kg/cm2 parboiling pressure compared to 0 kg/cm2 and 1.0 kg/cm2. The total and head rice yields in all test varieties were similar at 0 kg/cm2 or 1.0 kg/cm2 parboiling pressure. The light yellow color was associated with the samples at 0 kg/cm2 pressure whereas 1.0 kg/cm2 parboiling pressure produced grayish color. Amylograph properties such as peak viscosity, final viscosity at 95 deg C, viscosity cooled to 50 deg C and consistency decreased with an increase in severity of parboiling except for IR24 and IR64 at 0 kg/cm2 pressure. IR24 and IR64 steamed at 0 kg/cm2 pressure had higher final viscosity at 95 deg C than raw