Effect of Extrusion Treatment on Retrogradation and Rheological Properties of Rice Starch/Glutelin Composite System

The retrogradation of rice starch is the major factor that restricts its application. Recent studies have suggested that extrusion combined with glutelin has an obvious inhibitory effect on the retrogradation of rice starch. This study was executed in order to clarify the influence of extrusion parameters including screw speed, extrusion temperature, raw material moisture content and mass ratio of rice starch to glutelin on the retrogradation properties of rice starch-glutelin mixed system. Furthermore, how extrusion treatment could delay the regeneration of the mixed system was analyzed using a rapid viscosity analyzer (RVA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, low-field nuclear magnetic resonance (LF-NMR), scanning electron microscopy (SEM), laser scanning confocal microscopy (LSCM) and a rheometer. The results showed that the most pronounced retarding effect on the retrogradation of the mixed system was observed when the screw speed was 300 r/min, the extrusion temperature was 90 ℃, the moisture content of the raw material was 34%, and the starch/protein ratio (material ratio) was 91:9. Extrusion treatment significantly decreased the peak viscosity, disintegration value and retrogradation value of the composite system from (3005.00 ± 25.00), (1193.00 ± 6.00), and (164.33 ± 3.51) cP to (429.00 ± 5.00), (213.00 ± 3.00) and (27.05 ± 0.14) cP, respectively (P < 0.05), and reduced the relative crystallinity from (20.18 ± 0.13)% to (2.17 ± 0.43)%. Moreover, the endothermic peak of the mixture disappeared, and the ratio between the peak intensities at 1047 and 1022 cm^-1 in the infrared spectra was decreased significantly to 0.43 ± 0.08 (P < 0.05). After extrusion treatment, the microstructure became more uniform and compact, the water retention property was enhanced, the shear stress was decreased, the hysteresis loop area was increased, and the storage modulus (G’) and the loss modulus (G”) were decreased, indicating good resistance to retrogradation. The results of this study provide a theoretical reference for the application of extrusion treatment in delaying the retrogradation of rice starch–glutelin mixed system.