Gelatinization and rheological properties of rice starch/xanthan mixtures: Effects of molecular weight of xanthan and different salts

Effects of molecular weight (Mw) of xanthan (XG) and salts (0.1 M NaCl or CaCl2) on the pasting, thermal, and rheological properties of rice starch (RS) were studied. A series of five XG samples, having various Mw, was prepared by homogenization of native XG solutions in the presence or absence of salts. The presence of salts greatly reduced the intrinsic viscosities, [η], of all XG solutions. Rapid visco-analysis (RVA) data showed that XG addition increased the peak, breakdown, final, and setback viscosities of RS, either in the presence or absence of salts, whereas the pasting temperatures were unaffected. Differential scanning calorimetry (DSC) demonstrated that the gelatinization temperatures of RS were unaffected by XG addition but slightly increased by CaCl2 addition, whereas the gelatinization enthalpies (ΔH) were significantly decreased by additions of XG and salts. Dynamic shear data revealed weak gel-like behaviour in all paste samples in which their rigidity was decreased by XG addition. Flow tests showed that all pastes exhibited time-dependent shear-thinning (thixotropic) with yield stress behaviour in which the hysteresis loop areas were significantly decreased by XG addition, whereas the other rheological parameters varied differently among the samples, with and without added salt. In general, the effects of XG addition on the pasting and rheological properties of RS were more pronounced with increasing Mw of XG and these effects depended on salts added.