| Soybean is one of the most important plant protein sources.Soybean protein has attracted more and more attention due to its high supply,abundant resources and high nutritional.However,the poor functional properties have limited the application of soybean protein in food industry due to its high density structure.The main study of this paper was to study physical modification of protein structure,using near-infrared spectrometer,Fourier transform infrared spectroscopy,fluorescence spectrometer,Zeta potential analyzer,laser particle-size analyzer,scanning electron microscope,laser light scattering instrument and other advanced means.Effects of physical pretreatment,high pressure homogenization,highintensity ultrasonication and high hydrostatic pressure,on the structure and physicochemical properties of SPI have been investigated.The mechanism was investigated by analyzing the modified protein structure and the state of molecules aggregation,which benefited to confirm the relationship among the physical pretreatment,structural changes and physicochemical properties.The results could provide technical and theoretical guidance for the development and utilization of soybean protein products.The main results are indicated as follows:1.High pressure homogenization pretreatment had a significant effect on the structure and physicochemical properties of SPI.(1)High pressure homogenization pretreatment could destroy the secondary structure of soybean protein isolate.HPH treatment led to a significant shift of NIR and FTIR characteristic peaks of SPI.Compared with the untreated-SPI,the ?-sheet and random coil contents of SPI were reduced significantly by HPH treatments,while their ?-helix and ?-turn content was increased.HPH treatment caused the hydrophobic groups of SPI expose at their surfaces and tertiary structure changed.The maximum fluorescence wavelength shifed from 343 nm to 338 nm.HPH treatment could expand the peptide chain of SPI and increase the negative surface charge.The maximum zeta-potential value was 26.25 mV,which improved the solubility of SPI in water.(2)The effective diameter of SPI decreased with the increasing of homogenization pressure.The effective diameter of SPI obtained at 80 MPa was the smallest(161.80 nm),and decreased by 25.5%.The scanning electron microscopy image of SPI showed that the structure of SPI changed from a spherical structure to a lamellar structure with smooth texture.(3)The structure of the SPI solution without HPH treatment showed a hard sphere,while those subjected to HPH treatment at 10 MPa became Gaussian coil,polydisperse(good solvent),Gaussian coil monodisperse(?-solvent)was got at 20 MPa and 30 MPa and hollow spheres was formed at higher pressure.Weight-average molecular weight,z-average mean radius of gyration,hydrodynamic radius of SPI all increased.The second virial coefficient is positive,improving the protein solubility in water.The foaming ability and foaming stability of SPI samples treated at lower pressure were improved.(4)HPH treatment-protease hydrolysis of SPI could significantly improve the degree of hydrolysis and antioxidant activity of enzymatic hydrolysis products.Compared with the untreated SPI,the hydrolysis degree of SPI at 30 MPa HPH treatment was 23.65%(increased by 143%).The higher DPPH radical-scavenging activity(70.44%)of the SPI was obtained after 4 cycles HPH treatment,and the antioxidant activity was increased by 30%.2.High-intensity ultrasonication pretreatment had a significant effect on the structure and physicochemical properties of SPI.(1)The secondary structure of SPI was disrupted by high-intensity ultrasonication pretreatment.HIU treatment led to a significant shift in characteristic peaks of SPI determined by NIR and FTIR.Compared with the control,all samples showed an increase in the ?-helix,?-sheet and random coil proportion and a decrease in ?-turn after HIU treatment.HIU treatment destroyed the internal hydrophobic interactions of protein molecules,which resulted in an increase in the hydrophobic areas of the proteins exposed to the surface of the molecules.The maximum fluorescence wavelength shifed from 343 nm to 336 nm.HIU treatment could expand the peptide chain of SPI and increase the negative surface charge.The maximum zetapotential value was 22.75 mV,which was conducive to the dissolution of soybean protein isolate in water.(2)The effective diameter of SPI decreased with the increasing of power and time.The effective diameter of SPI obtained at 800W-10 min HIU treatment was the smallest(141.23 nm),and decreased by 35%.The scanning electron microscopy image of SPI showed that the structure of SPI changed from a spherical structure to a lamellar structure with more smooth texture.(3)The structure of the SPI solution without HIU treatment showed a hard sphere,while those subjected to HIU treatment at low power HIU treatment became hollow spheres,Gaussian coil,polydisperse(good solvent)and Gaussian coil monodisperse(?-solvent)was formed after medium and high power ultrasound treatment.Weight-average molecular weight,z-average mean radius of gyration,hydrodynamic radius of SPI all increased.The second virial coefficient is positive,improving the protein solubility in water.The foaming ability and foaming stability of SPI samples after ultrasonic treatment were significantly improved.(4)The degree of hydrolysis and antioxidant activity of soybean protein isolate were significantly increased by ultrasonic treatment.The degree of hydrolysis increased with the prolongation of ultrasonic treatment time.Compared with the untreated-SPI,the degree of hydrolysis of SPI treated with HIU treatment at 1000 W for 25 min was the highest(22.33%),and increased by 134.4%.The higher DPPH radical-scavenging activity(85.90%)of the SPI hydrolysates was obtained HIU treatment at 1800 W for 30 min,and the antioxidant activity was increased by 58.62%.3.High hydrostatic pressure treatment had a significant effect on the structure and physicochemical properties of SPI.(1)The secondary structure of SPI was changed by high hydrostatic pressure pretreatment.HHP treatment led to a significant shift in characteristic peaks of SPI measured by NIR and FTIR.Compared with the control,SPI showed a decrease in the ?-sheet and random coil proportion and an increase ?-helix and ?-turn after 100-400 MPa treatment.While the pressure increased to 500 MPa,the ?-helix,?-sheet and random coil contents of SPI were reduced,their ?-turn content was increased.The hydrophobic portions which buried inside the protein molecules were exposed after HHP treatment.The tertiary structure of SPI was damaged.The maximum fluorescence wavelength shifed from 343 nm to 339 nm.HHP treatment could expand the peptide chain of SPI and increase the positive surface charge.The maximum zeta-potential value was 13.65 mV.(2)The effective diameter of SPI decreased with the increasing of pressure and time.The effective diameter of SPI obtained at 300MPa-30 min HHP treatment was the smallest(192.6 nm),and decreased by 11.4%.The scanning electron microscopy image of SPI showed that the structure of SPI changed from a spherical structure to a irregular polymerized flat spherical structure with rough surface.(3)The structure of SPI was changed from a hard sphere to hollow spheres after HHP treatment.Weight-average molecular weight,z-average mean radius of gyration,hydrodynamic radius of SPI all increased.The second virial coefficient of SPI after medium pressure treatment is negative,which was not conducive to the dissolution of the protein in water.The foaming ability of SPI samples after HHP treatment were improved.(4)The degree of hydrolysis and antioxidant activity of SPI were significantly increased by HHP treatment.The degree of hydrolysis decreased with the increasing pressure.Compared with the untreated-SPI,the degree of hydrolysis of SPI treated 500 MPa for 30 min was highest(27.13%),increasing by 184.4%.The higher DPPH radical-scavenging activity(70.14%)was obtained at 100 MPa for 15 min,and the antioxidant activity increased by 29.54%.4.The secondary and tertiary structures of soybean protein isolate was changed by different physical pretreatments.The protein peptide chain expanded,and the active sites of the enzyme were exposed more,which accelerated the enzymatic hydrolysis rate of soybean protein isolate and significantly increased the antioxidant activity of the enzymatic hydrolysis products.The antioxidant activity of enzymatic hydrolysis products obtained by HIU treatment was higher than that of the other two pretreatments,and the foaming ability and foaming stability of SPI samples were significantly improved.Therefore,HIU pretreatment could be used as an auxiliary means for the production of soybean protein products. |
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