Effect Of Extrusion On Composition And Protein Structure Of Cold-pressed Soybean Meal

Extrusion technology as a multi-step,multi-function and heat and mechanical combination of processing methods,it can improve the quality of materials by changing the state of the organization,has been widely used in food processing.The structure of soybean protein can be changed by extrusion treatment to improve its functional properties.However,the mechanism of protein structure change is not clear because the extrusion process involves a variety of energy input and output.The interaction mechanism between soybean protein,lipid and polysaccharide during extrusion is still unclear.The functional properties of soybean protein could not be effectively regulated and stably improved.Therefore,this study divided the extruder into five sections according to the temperature of different sections and screw structure of the extruder.The interaction between soybean protein,fatty acids and polysaccharides in soybean meal was explored and analyzed with the impact of extrusion process on each component of cold-pressed soybean meal as the entry point.Then the effects of different extrusion temperatures,material moisture and screw speed on the structure of soybean protein in cold-pressed soybean meal were determined.To reveal the structural changes of soybean protein in cold-pressed soybean meal and the molecular mechanism of its formation during extrusion.The main research conclusions are as follows:(1)The cold-pressed soybean meal contains rich oil and fat except 52.48%protein,and the three sections of the material are in molten state,which is the key section of the protein structure change in the extrusion process.Under the action of non-covalent bond forces such as disulfide bond,hydrogen bond and hydrophobic interaction,new SPI aggregates are formed.When the cold-pressed soybean meal enters the third stage,the material state rapidly changes to the molten state,and the small molecular structure formed after the destruction of coldpressed soybean meal begins to polymerize into new aggregates.When the material enters the fourth stage,the denaturation peak is 93.18℃.It shows that when the material enters the fourth section,the internal SPI is basically completely denatured.The oil content at the fifth section of the extruder reached the highest value of 15.74%,indicating that the extrusion process promoted the binding between protein and oil,the relative contents of palmic acid and linoleic acid increased,and the fatty acid content at the fifth section increased from 12.01%and 52.88%to 13.17%and 55.22%,respectively.(2)When the extrusion temperature is 150℃,the aggregates formed by the proteins are relatively uniform.At 160℃,the relative contents of α-helix and β-corner structure reached the highest values,which were 25.20%and 17.98%,respectively.The Zeta potential reached the highest value of-14.47 mV.The system was relatively uniform at 160℃ and 170℃.Moreover with the increase of extrusion temperature,the average particle size of SPI at 150℃increased to a maximum of 153.24 d.nm,temperature over 160℃,7S component stripe again appear,this may be due to the phenomena of excessive extrusion temperature condition destroyed has formed SPI aggregate,the total sulphur content and disulfide bond content declined slightly,temperature is too high,is not conducive to the exposure of the internal thiol groups and the formation of disulfide bond.When the screw speed is 130 r/min,the maximum value of NSI is 16.37%.The total sulfhydryl group and disulfide bond content reached the highest values of 23.35 μmol/g and 10.54μmol/g,respectively,with 11S component as the most obvious band.The particle size and volume distribution are relatively uniform,indicating that the extrusion conditions are conducive to the formation of stable SPI aggregates.Material moisture content was 20%,the total sulphur content and disulfide bond content reached the lowest,20.34 μmol/g and 10.52 μmol/g,is not conducive to the exposure of the internal thiol groups and the formation of disulfide bond,under the condition of 20%and 22%,material moisture,low content of the formation of disulfide bond makes the SPI after extrusion for the stability of the high solubility of the structure of the protein aggregates.(3)The soybean protein in cold pressed soybean meal denaturated and aggregated in the barrel of the extruder.The solubility slightly increases to 42.41%at the first stage.With the push of the screw,the solubility decreases sharply.When SPI enters the third stage of the screw of the extruder,the solubility reaches 15.65%,and then slowly decreases.After the soybean protein passes through the third stage of the extruder,the soybean protein begins to reaggregate and cross-link the molecular chains under the action of high temperature and high shear force,and finally forms protein aggregates with larger molecular weight.The main forces involved in this process are disulfide bond,hydrophobic interaction and electrostatic interaction.The flexibility of soybean protein increased gradually to 0.418 after extrusion,which was higher than that of unextruded soybean protein.