An SEC−MALLS Study of Molecular Features of Water‐soluble Amylopectin and Amylose of Tef [Eragrostis tef (Zucc.) Trotter] Starches

Abstract

AbstractThe molecular features of five tef starches along with those of commercial normal maize starch were investigated by size‐exclusion chromatography with multi‐angle laser light scattering‐differential refractive index detection (SEC/MALLS‐DRI) after solubilization in water by cooking in a household pressure cooker. The weight‐average molar mass ($\displaystyle \bar M_w $) and weight‐average root‐mean square radius of gyration (<Rg>w) of the amylopectin (AP) of tef starches ranged from 10.1×107 g/mol (156 nm) to 16.5×107 g/mol (205 nm) with a mean of 13.9×107 g/mol (186 nm). The AP $\displaystyle \bar M_w $ of the tef starches was considerably smaller than that of maize starch ($\displaystyle \bar M_w $ = 19.6×107 g/mol, <Rg>w = 207 nm). These considerably smaller AP molecules in tef starches were most probably responsible for the low paste viscosity of tef starches as compared to maize starch. In most tef starches, the polydispersity index (PI) of the AP was broader than that of the AP of maize starch. The intermediate fraction (IN) $\displaystyle \bar M_w$ 1.0−1.6, mean = 1.1) of most tef starches were similar to those of the IN of maize starch. The amylose (AM) $\displaystyle \bar M_w $ (range 1.5×106−3.0×106 g/mol, mean = 2.2×106 g/mol) and size (range 176−214 nm, mean = 191 nm) of most tef starches was also apparently similar to that of the maize starch ($\displaystyle \bar M_w $ = 2.3×106 g/mol, <Rg>w = 193 nm), but the polymer distribution was narrower. The AM−iodine complex of the tef starches had a λmax range of 611−679 nm and the absorption shifted toward longer wavelengths by 8−14 nm as compared to the maize starch AM−iodine complex. The blue value (absorption at λmax) for 1 mg/mL of tef AM had a range of 2.3−2.8 (mean = 2.5), whereas for the maize starch, the mean was 2.2. The branched nature of tef starches was also investigated by debranching with isoamylase and determination of chain lengths (DPn) of the branches by size exclusion chromatography with refractive index detector (SEC‐RI). The AP in tef starches had a polymodal distribution with a periodicity similar to that of cereal starches. The branches had DPn values of A = 11, B1 = 16, B2 = 46 (range 46−47), B3 = 70 (range 69−72) and B4 = 118 (range 113−123). The outer (A + B1) chains were shorter than those of maize starch AP with abundance (74%, w/w) only slightly less than that of the maize starch (75%, w/w). The slow rate of retrogradation, the slightly lower percent crystallinity, the lower gelatinization temperatures and the lower gelatinization enthalpy observed for tef starches (as compared to maize starch) are probably related to the shorter outer (A + B1) chain lengths of their amylopectin molecules, and may be the foundation of the comparably good keeping quality of tef injera, the main staple in the Ethiopian diet.