Status of polymorphism, physicochemical properties and in vitro digestibility of dual retrogradation-annealing modified rice starches.

In this research, for the first time, dual retrogradation-annealing modification was applied to three rice starches with 22.7%, 9.8% and 0.3% apparent amylose content (AAC). Amylose partially inhibited the effect of gelatinization on amylopectin crystallinity. Polymorphism in retrograded starches was feeble and unstable. Annealing principally caused amylopectin crystal growth by lamellar rearrangement and simultaneous degeneration of weaker lamellae. Reappearance of physical growth rings occurred. Altered crystalline arrangement had direct effect on cooked paste viscosity, gel texture, swelling power, suspension turbidity, and freeze-thaw stability of the samples. A new prominent X-ray diffraction peak (2θ = 29.1) confirmed development of novel polymorphic form other than characteristic A-, B- and V-types. Crystallite melting enthalpy (ΔH) was higher in dual modified starches (9.9 J/g, 12.9 J/g, 19.0 J/g) than native samples (ΔH = 7.4 J/g, 9.1 J/g, 12.1 J/g). Starch-lipid complexes formed the major portion of the crystalline phase within the modified starch matrices. Amylopectin played the major role in crystalline reformation and formed prominent amylopectin-lipid complexes. Simultaneous conversion of slowly digestible starch to resistant starch occurred. Enhanced thermostability, enzyme resistance and functional properties may be suitably targeted for food and non-food applications of the dual modified rice starches.