Effects of amylose content on the physicochemical properties of sodium carboxymethyl rice starches

Nine native rice starches, prepared from different strains of rice with the amylose content between 14.67 and 29.09 percent, were cmployed in the preparation of sodium carboxymethyl rice starches (SCMRSs). The reaction was carried out at 50 deg C for 20 minutes, using monochloroacetic acid as a reagent under alkaline condition and 1-propanol as a solvent. The degree of substitution was determined and the physicochemical properties, including water solubility, pH and viscosity of 1 percent w/v solution, X-ray diffraction analyses and film-forming property were investigated. The degree of substitution of the prepared SCMRSs ranged from 0.2459 to 0.4034. All SCMRSs were freely soluble in water, the pH of 1 percent w/v solutions were between 8.6 and 9.9 and the viscosities ranged from 32.7 to 66.0 mPa-s. The degree of substitution showed a significant positive correlation with the amylose content, while the pH and the viscosity did not. The X-ray diffraction of all SCMRSs showed the loss of crystallinity which was possibly due to the pregelatinization of starch molecule by water and heat. At 3 percent w/v concentration, most SCMRS pastes formed clear films with varying film characteristics, e.g., from flaky and brittle to soft and gluey or elastic, depending on the amylose content of the native starches. The results from this study suggested that amylose content affected the physicochemical properties of not only the native starches but also the corresponding SCMRSs.