Sheeting has been widely used to form the gluten network structure for the high product quality of the wheat-based food products. This study aimed to systematically investigate the effects of sheeting parameters on the molecular structural characteristics, rheological properties, moisture distribution, and gelatinization properties of high-fiber dough and the specific volume, height-diameter ratio, hardness, and microstructure of high-fiber steamed buns. The key sheeting parameters were selected as the different sheeting cycles (0, 5, 10, 15, 20, and 25 passes with a gradient of 5 passes) and three folding-feeding modes, including Two-fold Folding with 90° Spinning and Backward feeding (FHR), Trisection Folding with 90° Spinning and Backward feeding (FTR), and Four-fold Folding with 90° Spinning and Backward feeding (FHTR). A series of advanced analytical techniques was employed, including Fourier transform infrared spectroscopy, X-ray diffraction, rapid viscosity analysis, texture profile analysis, dynamic rheology measurement, low-field nuclear magnetic resonance (LF-NMR) with magnetic resonance imaging (MRI), and scanning electron microscopy (SEM), to characterize and evaluate the samples. The results showed that moderate sheeting effectively optimized the performance of high-fiber dough. There were the uniform distribution of moisture, extensibility, anti-gelatinization, the ordered protein secondary structures, and starch crystallinity of high-fiber dough. The optimal FTR was achieved in the 15 sheeting cycles among all samples. Correspondingly, the high-fiber steamed buns exhibited the best quality with a specific volume of 2.75 mL/g, a height-diameter ratio of 0.72, and a hardness reduced to 850 g. Meanwhile, the high-fiber dough formed a dense and stable gluten network, with a more uniform moisture distribution and high rheological properties. In contrast, excessive sheeting led to adverse effects: ordered protein structures were degraded into random coils and β-turns, damage to starch granule structures and gluten network continuity, the extensibility, and gas-holding capacity of high-fiber dough. Ultimately, the significant deterioration also caused the quality of high-fiber steamed buns, such as the reduced specific volume, height-diameter ratio, high hardness, and uneven microstructure. Three folding-feeding modes showed that: The FHR offered the higher processing efficiency, due to the fewer folding layers but insufficient stability in structure and quality; The FTR with moderate folding layers shared the uniform distribution of mechanical forces during sheeting, suitable for the full extension and cross-linking of gluten proteins, thus indicating the best stability in both high-fiber dough properties and high-fiber steamed buns quality; The FHTR with excessive folding layers led to uneven mechanical force distribution and easy lateral dislocation between layers, resulting in limited effect on the formation and reinforcement of the gluten protein network and structural stability. Appropriate sheeting cycles and folding-feeding modes were the key technical factors to enhance the quality of high-fiber steamed buns. Especially, excessive sheeting was avoided during industrial production. There was the regulatory mechanism of sheeting on the physicochemical and structural properties of high-fiber dough and the quality of high-fiber steamed buns. The finding can provide practical guidance for the industrial production of high-quality high-fiber steamed buns in food processing.

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.

In order to investigate the synergistic effect of extrusion and polyphenol treatment on the physicochemical and structural characteristics of corn starch (CS), four polyphenols (ferulic acid, gallic acid, quercetin and curcumin) were selected in this study. The solubility, swelling power, pasting properties, thermal characteristics, antioxidant capacity, in vitro digestibility and crystal structure of the extruded CS with polyphenols were evaluated. The results showed that extrusion combined with polyphenol treatment resulted in an increase in the solubility, swelling power and pasting temperature of CS, and a decrease in the gelatinization enthalpy, peak viscosity and setback value, as well as a significant improvement in the scavenging activity against 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) radical cation and anti-digestive properties. The binding of polyphenols to CS was dominated by hydrogen bonding during the extrusion process, leading to a decrease in the short-range orderliness of CS and eventually the formation of a special V-shaped crystal structure. After the combined treatment, the structure of CS became denser as observed by scanning electron microscope (SEM). This study provides theoretical guidance for the application of polyphenols in extruded starch-based functional foods.