| Walnut meal is a by-product of oil extraction and rich in protein.Walnut protein contains high contents of essential amino acids,and long-term consumption of walnut protein is beneficial to human health.In order to utilize walnut meal more effectively,in recent years,many attempts have been conducted to study the extraction process,composition and functional properties of walnut protein.However,different processing methods for walnut kernels and oil extraction often lead to significant differences in structure and functional properties of the protein.Therefore,in order to promote the high-value utilization of walnut meal and to improve the utilization rate of walnut protein resources,this study was to explore the structural properties of walnut protein in the heated and supercritical defatted walnut meals,and the physicochemical properties and antioxidant activity of the protein were also compared.The main conclusions are as follows:1.Structural analysis of walnut protein suggested that the microstructure of natural walnut protein was relatively loose,showing a flaky and compact platy structure after heating and supercritical defatting,respectively.X-ray diffraction results showed that walnut protein with different treatment methods had a high proportion of ?-sheet structure.In the process of roasting and supercritical defatting,the content of ?-sheet and a-helix structure was reduced,whereas the content of ?-turn and random coil structure was increased,indicating that the secondary structure of walnut protein changed from order to disorder state.Especially,more ?-sheet structures were destroyed by the supercritical extraction process.Heating and supercritical defatting treatment also reduced the thermal stability of walnut protein.2.The isoelectric point of walnut protein is 4.5,and the solubility curve of walnut protein presented a U-shape at pH 2.0?12.0.The solubility of walnut protein decreased with the increasing NaCl concentration(0?1.0 mol/L).At pH 2.0,NaCl has a greater effect on the solubility of walnut protein than that at pH 10.0.Heat treatment reduced the solubility of walnut protein under different pH and NaCl conditions.3.Emulsifying properties of walnut protein were analyzed.The emulsifying capacity and emulsifying stability of walnut protein with different treatment methods initially increased and then decreased with the NaCl concentration increasing from 0 to 0.6 mol/L.The emulsifying capacity of the protein was the greatest when the NaCl concentration was 0.3 mol/L,and the emulsifying stability was found to be the best at 0.1 mol/L of NaCl.In addition,heat treatment reduced the emulsifying capacity of walnut protein,but improved the emulsifying stability.The emulsifying stability of the walnut protein was also improved by supercritical defatting treatment.4.The foaming properties and foam stability of the protein were the lowest at pH 4.5(the pI of the protein),and the foaming properties were the highest at pH 12,with a value of 37.11%,47.56%,8.96%and 14.87%,respectively.The walnut protein treated by soxhlet extraction but without heating had the best water holding capacity,which was 1.74 g/g.Heating and supercritical defatting treatment lead to denaturation of walnut protein and thus reduced the water holding capacity.Supercritical defatting treatment significantly improved the fat holding capacity of the protein.In addition,heating and supercritical defatting treatment increased the viscosity of the walnut protein solution.5.Heat treatment enhanced the ability of walnut protein to scavenge free radicals.The measurement of peroxide value suggested that,heat treatment delayed the oxidation of walnut powder in storage.The walnut powder after heating and pulverizing still maintained good color,flavor and tissue state during storage,indicating that the walnut powder after heating had good oxidation stability. |
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