Effect of Dry- and Wet-Milling Processes on Chemical, Physicochemical Properties and Starch Molecular Structures of Rice Starches

Abstract

Rice starches from low amylose (Pathum Thani 1), medium amylose (RD 7) and high amylose (Leuang 11) varieties prepared from dry- and wet-milling processes were investigated. Dry-milled rice starches contained significantly (p<0.05) higher amounts of damaged starch than wet-milled rice starches in all rice varieties. The scanning electron micrographs (SEM) of starch granules confirmed that damaged starches occurred from the dry-milling process more than from the wet-milling process. The enthalpy required to gelatinize dry-milled rice starches (8.11-9.53 J/g) from all rice varieties was significantly (p<0.05) lower than those of wet-milled rice starches (9.24-12.48 J/g). The peak viscosity (257.75-353.80 RVU) and final viscosity (248.64-391.00 RVU) of dry-milled rice starches measured by Rapid Visco Analyser (RVA) were significantly (p<0.05) lower than those of wet-milled rice starches. Rice starch molecular weight distributions obtained by size-exclusion chromatography with RI and multiangle laser light scattering detection (SEC-RI-MALLS) showed that dry-milled rice starch samples contained lower amounts of the high molecular weight portion of starch (amylopectin) than wet-milled samples. The molecular weight of both amylopectin from dry-milled rice starches (0.95-1.7 x 10⁷ Da) was lower than that of wet-milled rice starches (1.11-1.90 x 10⁷ Da), suggesting that the dry-milling process caused a higher degree of starch fragmentation than the wet-milling process.