Effect of dual hydroxypropyl-carboxymethyl modification on the physicochemical properties of mung bean starch

Abstract

Modified mung bean starches were prepared by dual carboxymethylation and hydroxypropylation reactions. The reactions were used to react with native mung bean starch step by step to yield carboxymethyl mung bean starch (CMMS), hydroxypropyl-carboxymethyl mung bean starch (HPCMMS) and carboxymethyl-hydroxypropyl mung bean starch (CMHPMS). The modified starches were investigated for physicochemical properties. The degree of substitution (DS) of modified starches was 0.25, 0.27 and 0.31 for CMMS, HPCMMS and CMHPMS, respectively. The molar substitution (MS) was 0.05 and trace for HPCMMS and CMHPMS, respectively. Scanning electron microscope and X-ray diffraction showed that the reactions did not alter starch granules and crystal structure. IR experiments confirmed the substitution of carboxymethyl groups in modified starches. All of modified starches were water-soluble, 1% w/v of starch solutions yielded viscosity of 507.13, 301.63 and 255.61 at a shear rate 100 s -1 for CMMS, HPCMMS and CMHPMS, respectively. The 3% w/v solutions yielded 1075.17, 823.80 and 514.88 at the same shear rate. The water uptake investigations of modified starches showed that CMMS took up water less than the others. CMMS, however, showed the highest viscosity. Based on the high viscosity and good water uptake ability, CMMS and HPCMMS are considered hydrophilic polymers which can potentially be used as a controlledrelease agent for pharmaceutical preparations.