Effects of curdlan on wheat starch retrogradation and freeze-thaw stability in the extrusion system

Frozen pasta products have been ever increasingly favored in recent years, because of their convenience. Wheat starch is one of the most important components of buns, noodles, and cookies in the staple and leisure foods. The performance and quality of starch-based foods can depend seriously on the processing of wheat starches. Furthermore, starch-based foods easily lose water to be hard in the storage of retrogradation, indicating reduced elasticity. Therefore, it is very necessary to enhance the structure and properties of wheat starch. Starch can also be often modified by physical, chemical, biological and composite methods. Among them, physical modification has been widely used to modulate the properties of starch using hydrophilic colloids, due to their environmental friendliness, excellent performance and lower energy. Fortunately, the curdlan can be expected to compound with the starch for better functional properties, due to the excellent resistance to freeze-thaw stability and dehydration, water-holding, extreme stability, and emulsifying properties. In addition to being a prebiotic, curdlan can also improve the gastrointestinal health. The extrusion can be used as a biochemical reactor to promote the combination of wheat starch and curdlan, particularly for the high yield, easy operation, high degree of integration, and high temperature. The reason is that the high pressure, high shear and hydration can be generated to significantly change the structure and physicochemical properties of starch during operation. The complexes of curdlan and wheat starch can then be prepared to change the physicochemical properties of starch. The retrogradation and syneresis of wheat starch during freezing have been two of the most important limiting steps in the development of pasta products. This study aims to explore an effective way to inhibit the retrogradation and syneresis of wheat starch gels during freeze-thaw cycles. The composites with different concentrations of curdlan-wheat starch were investigated in the extrusion system. The effects of curdlan addition on the structural properties and freeze-thaw stability of wheat starch were analyzed using differential scanning calorimetry, X-ray diffractometry, and Fourier infrared spectrometry. In addition, the binding mechanism of curdlan-wheat starch composites in the extrusion field was revealed by combining the chemical reagents, Texture Analyzer and Scanning Electron Microscope. The results showed that the retrogradation of curdlan-wheat starch composites decreased and then increased with the increasing concentration of curdlan, while the syneresis rate showed a trend of decreasing and then increasing. The freeze-thaw stability of wheat starch was significantly improved to markedly inhibit the retrogradation with the addition of 0.6% curdlan in the extrusion system. The pasting temperature of wheat starch increased with the addition of 0.6% curdlan, compared with extruded starch. There was a decrease in the relative crystallinity, short-range ordering, and iodine binding of wheat starch, where the recrystallization of amylose was suppressed. Texture Analyzer and Scanning Electron Microscope revealed that the curdlan-wheat starch composites were dominated by the hydrogen bonding and weak electrostatic interaction forces in the extruded system. Furthermore, the curdlan-wheat starch composites presented a more intact and dense structure, indicating better freeze-thaw stability. The retrogradation was also effectively inhibited. The finding can provide a strong reference to produce wheat starch-based foods with resistance retrogradation and low syneresis using an extrusion process.