Research On Extrusion To Physic-chemical Properties Of Purple Sweet Potato Powder

In our study, purple sweet potato powder (PWPP) obtained from Chang Le was extruded with twin-screw extruder. We evaluated the impact of parameters (screw speed, feed speed, barrel temperature and material moisture) on the gelatinization degree of samples as well as the radial expansion rate and the density. We also studied the changes in the physic-chemical characteristics and the structure of PWPP. Based on these results, and combining with quality requirements (GB/T 30645-2014) of traditional cloud chip cake, we explored the techniques and the formula of chip cake production with the addition of puffed PWPP. The main results are as follows:We analyzed the technological parameters associated with PWPP during extrusion. We studied the effects of screw speed, feeding speed, barrel temperature, and moisture content on the degree of gelatinization, the radial expansion rate, and density. The optimal parameters of each single factor were determined as:screw speed 25Hz, feed rate 10Hz, barrel temperature 165℃, and moisture content 14%.Based on RSM experimental design, we studied the radial expansion rate and the degree of gelatinization. With Design Expert 8.0.6 a mathematical model was established to determine the optimal value for each parameter as well as the effects of interaction among parameters on radial expansion rate and degree of gelatinization. Our results suggested a significant effect from the interaction of screw speed with feed rate, screw speed with material moisture content, and barrel temperature with moisture content on the radial expansion rate; the interaction of screw speed with feed rate, screw speed with barrel temperature, screw speed with material moisture content, and feed rate with barrel temperature had significant impacts on the degree of gelatinization. The optimal parameters were determined as:screw speed 24Hz, feed rate 13Hz, barrel temperature 164℃, material moisture content 13%. Under these conditions, the radial expansion rate and the degree of gelatinization were 3.71% and 91.4%, repectively.The content of water, starch, lipid, protein and anthocyanin in PWPP significantly declined after extrusion with reducing sugar increased. The capacity in water and oil absorption decreased in PWPP, while water solubility and the degree of gelatinization greatly increased. The absorption peak of the DSC gelatinization endotherm curves was smaller than that of PWPP prior to extrusion, meaning the extrusion process results in full gelatinization. With extrusion, the shape of starch particle transformed from the elliptical structure into a mutually adhesive, irregular blocky one. After extrusion, a new absorption peak appeared at 1019.32 cm-1, suggesting a new substance emerged in PWPP after extrusion. The PWPP paste was both pseudoplastic fluid before and after extrusion, with viscosity significantly decreased and stability of suspensions increased after extrusion, which led to convenient blending. With extrusion, the three-dimensional network structure of gel becomes more fragile and the state of deformation recovered faster at a high speed shear.We explored the techniques and the formula of chip cake production with addition of PWPP. The optimal formula for purple sweet potato chip cake was determined as:glutinous rice flour 100g at baseline, the best accessories for each added were puffed powder 15%, sugar 60%, water 20%,sesame oil 1%, honey 5%. With this formula the chip cake demonstrated good sensory and texture qualities. With fuzzy mathematics, the results with scientific and objective investigations provided a theoretical foundation and guidance for future industrial production.