walnut protein was separated by acid precipitation and alkali dissolution. The final concentration of MDA prepared by adding 1,1,3,3- tetraethyl cyclopropane was , .1, 1,5,1 and 2 mmol L-1, respectively. It reacted with WPI for 24 hours at room temperature, and then it was removed by dialysis, and then it was freeze-dried in vacuum to prepare MDA oxidized walnut protein. The marked groups (sulfhydryl group, disulfide bond, amino group and carbonyl group), physical and chemical properties (solubility, turbidity, hydrophobicity, particle size and Zeta potential) and emulsifying function of the protein were determined.
MDA oxidation reduced the solubility of walnut protein from 68.74% to 11.88%, especially 5-2 mmol L-1 MDA had a significant effect on the solubility (P.5), but had little effect on the turbidity of protein solution (all values remained at about .32). In the modification of protein molecular groups, -1 mmol L-1 MDA has little effect on the contents of total sulfhydryl, disulfide bonds, free amino groups and carbonyl groups, but the concentration of MDA above 5 mmol L-1 has a significant effect on the decrease of total sulfhydryl and free amino groups and the increase of disulfide bonds and carbonyl groups (P.5). The results of polyacrylamide gel electrophoresis also showed that higher concentration of MDA had a significant effect on the increase of disulfide bond content, and promoted the formation of reducing disulfide bond and non-reducing valence bond between walnut protein molecules. Oxidation in MDA below .1 mmol L-1 did not affect the secondary structure of walnut protein, but above 1 mmol L-1 MDA, the contents of α-helix, β-sheet and β-turn were significantly reduced, and the content of random curl was increased. The change of protein fluorescence intensity with the increase of MDA concentration is the same as that of ordered structure in protein secondary structure, especially the concentration of MDA above 1 mmol L-1 can greatly reduce protein fluorescence intensity. Oxidation of -1 mmol L-1 MDA did not change the hydrophobicity of the protein, but the concentration of MDA above 1 mmol L-1 significantly reduced the hydrophobicity of the protein, and the maximum degree of hydrophobicity reduction was 1/1 of that of the control group. At the same time, the oxidation below 1 mmol L-1 MDA has no significant effect on the particle size and electric charge of walnut protein, but the concentration above 1 mmol L-1 MDA can significantly increase the particle size of walnut protein, and the maximum particle size reaches about 116nm at 1 mmol L-1 MDA, and the electric charge of walnut protein is significantly reduced, and it keeps decreasing with the increase of MDA concentration. In terms of emulsifying function of walnut, .1 mmol L-1 MDA oxidation can significantly reduce the emulsifying activity of protein, while 1 mmol L-1 MDA oxidation can significantly reduce the emulsifying stability; With the increase of MDA concentration to 2 mmol L-1, the emulsifying activity and emulsifying stability continued to decrease and both of them lost about 2/3 of their functionality.
in the oxidation system of lipid oxidation product MDA, the molecular structure of walnut protein (including its residue groups) was significantly modified by MDA, which promoted its cross-linking to form large aggregates and then changed its physicochemical properties, thus significantly reducing the emulsifying function of the protein.