Abstract

Sweet potato starch was subjected to cross-linking to achieve high content of resistant starch (RS) under heat-moisture treatment and the physicochemical properties of cross-linked sweet potato starches were investigated. The cross-linking agent was sodium trimetaphosphate and sodium tripolyphosphate (STMP/STPP). A maximum RS content (72.45%) was obtained after 10% STMP/STPP (99/1% w/w) at 120°C and pH 11.5 with a 20% moisture level. Through the analysis of ³¹P (phosphorus) nuclear magnetic resonance (³¹P NMR) spectroscopy, the RS-72.45% contained distarch monophosphate (DSMP) and monostarch monophosphate (MSMP) was not detected. Fourier-transform infrared spectroscopy (FTIR) presented the cross-linking reaction, confirmed by the comparison of native curves. After cross-linking, in the starches with high moisture treatment (≥25%) a loss of birefringence and surface erosion were observed. X-ray diffraction patterns showed that the crystal type was unchanged but with slight alteration in the relative crystallinity of cross-linked starches. Differential scanning calorimetry (DSC) showed gelatinization temperature increased as the moisture content increased, while gelatinization enthalpy apparently decreased.

Key words: Sweet potato starch, Heat-moisture treatment, ³¹P Nuclear magnetic resonance spectroscopy, optimization, X-ray difraction (XRD), scanning electron microscopy (SEM), polarized light microscopy.