Synthesis and properties of carboxymethyl kudzu root starch

Carboxymethyl kudzu root starch was synthesized for the first time under different reaction conditions. Optimal degree of substitution (DS) of 0.92 and reaction efficiency (RE) of 66.1% were obtained at 40 °C, 3 h in isopropanol–water reaction mixture ratio 1:0.08. The ratio of sodium hydroxide and sodium monochloroacetate moles to anhydroglucose unit moles for the optimal DS and RE were 2.02 and 1.39. The influence of sodium hydroxide concentration, sodium monochloroacetate concentration, and water content, type of organic solvent, reaction time, and temperature were evaluated for degree of substitution (DS) and reaction efficiency (RE). Wide angle X-ray diffractometry revealed that starch crystallinity was reduced after carboxymethylation. Both thermogravimetry (TG) and derivative thermogravimetry (DTG) results show that thermal stability improved after carboxymethylation. IR spectrometry showed new bands at ν = 1597, 1415, and 1323 cm−1 when starch underwent carboxymethylation. The broad-band 13C NMR spectra of the ultrasonically degraded carboxymethyl starch showed a peak at δ = 177.85 ppm which was assigned to carbonyl carbon.