Rice starch isolation by neutral protease and high-intensity ultrasound

Abstract

Abstract The efficacy of a neutral protease and combinations of neutral protease and high-intensity ultrasound for isolation of rice starch was investigated. Rice flour slurry (33%) was treated with a neutral protease at 0.01, 0.03, or 0.05% (on flour basis) or combinations of 0.03% neutral protease and high-intensity ultrasound at 25, 50, or 75% amplitude for 15, 30, or 60 min concurrently or sequentially. The starch yield using neutral protease alone ranged from 62.5 to 71.8% with 0.53–0.88% residual protein content and 0.99–1.81% damaged starch content. The starch yield greatly improved to 79.8–86.7% with 0.50–0.96% residual protein content and 0.98–1.87% damaged starch content when neutral protease treatment was combined with high-intensity ultrasound. Increasing sonication time at 50% amplitude during protease digestion improved the starch yield from 83.9 to 86.1%, but did not improve those treated at 75% amplitude. The preferred combination was neutral protease digestion for 2 h followed by sonication at 75 or 50% amplitude for 15 or 30 min. The starch structure analyzed by a high performance size-exclusion chromatography and scanning electron microscopy revealed no damage to the molecular structure or to the starch granule surface, however, the peak viscosities of rice starch increased after sonication. The combination of neutral protease and high-intensity ultrasound was an effective technique for isolation of rice starch without generating high salt wastes.