Xia Xuanze, Li Yan, Qian Haifeng, Zhang Hui, and Wang Li from the School of Food Science of Jiangnan University used pea protein as raw materials to study the effect of three modification methods: extrusion treatment, phosphorylation treatment, and extrusion co-phosphorylation treatment on pea protein. The effects of disulfide bond content, secondary structure content, subunit composition, and surface hydrophobicity, solubility, water and oil retention, emulsifying activity and emulsifying stability. The results showed that the structure and functional properties of pea protein changed under different modification conditions. Compared with extrusion treatment and phosphorylation treatment, extrusion synergistic phosphorylation treatment had the most obvious effect on the physical and chemical properties of pea protein. Compared with unmodified pea protein, the content of disulfide bonds in pea protein treated with extrusion synergistic phosphorylation increased by 26.25%; the content of β-sheets in the secondary structure increased by 15.06%, and the content of β-turns decreased by 9.85%; The content of glycinin A and glycinin B in the base composition decreased; the surface hydrophobicity increased by 101.95%. The solubility, oil retention, emulsifying activity and emulsifying stability of pea protein after extrusion synergistic phosphorylation treatment were significantly higher than those of unmodified pea protein (P<0.05). The functional improvement effect is not as good as that of extrusion co-phosphorylation treatment, especially the solubility of pea protein is reduced after extrusion treatment.
Bowl bean is an important leguminous crop with an annual global production of about 13.5 million tons and is currently cultivated in more than 90 countries [1]. The protein in pea is considered as an emerging vegetable protein resource in the food industry due to its high nutritional value, low cost, hypoallergenicity and non-GMO status [2]. The amino acid composition of pea protein is relatively balanced. The protein content of lysine is higher than that of grain protein, but the content of sulfur-containing amino acids is relatively low [3]. Pea protein is easily denatured by heat during processing, resulting in poor solubility, and natural pea protein has poor emulsifying and foaming properties [4], which limits its application in the food industry.
At present, modification methods such as physical, chemical, enzymatic or synergistic treatment are common technical means to solve the problem of limited application of proteins. The mechanism of action is to improve the functional properties of pea protein by changing the internal structure. Peng et al. [5] found that when pea protein was heat-treated at 95 °C for 30 min, its subunits would be connected to aggregates through disulfide bonds, which would increase the surface hydrophobicity and reduce the O/W interfacial tension, thereby improving its emulsification activity and emulsification. stability. Liu et al[6] studied the effect of phosphorylation modification on the physicochemical properties of pea protein and found that the content of α-helix and β-sheet of phosphorylated pea protein increased, the content of β-turn and random coil decreased, and the solubility increased by 171.21%. Oil retention increased by 73.31%, emulsifying activity increased by 63.07%, emulsifying stability increased by 69.08%, and foaming increased by 114.28%. Zhou et al. [7] found that the hydrolysis of pea protein was more sufficient after papain enzymatic co-extrusion treatment, and the solubility and DPPH free radical scavenging ability were improved. Current studies have found that the synergistic use of several modification methods has a better effect on improving the functional properties of pea protein, and can reduce the defects of single method modification. Studies have shown that extrusion treatment, as a physical method, can expand, aggregate and rearrange proteins through heat, pressure and shear force [8]; phosphorylation treatment, as a chemical modification, can introduce negatively charged phosphoric acid group, which enhances the electrostatic repulsion between proteins and changes the spatial structure of proteins. Extrusion treatment and phosphorylation treatment alone have significant effects on protein structure. However, there are few reports on the use of extrusion and phosphorylation synergistic treatment to modify pea protein.
In this experiment, three modification methods of extrusion treatment, phosphorylation treatment and extrusion-co-phosphorylation treatment were used to modify pea protein. Fourier transform infrared spectroscopy, sodium dodecyl sulfate-polyacrylamide gel electrophoresis were used to modify pea protein (SDS-PAGE), protein free sulfhydryl and disulfide bond content determination and other analytical methods to study the effect of modification treatment on protein structure, in order to improve the functional properties of pea protein and expand its application range.
Extrusion treatment, phosphorylation treatment and extrusion co-phosphorylation treatment all effectively changed the structural and functional properties of pea protein. Comprehensive comparison, extrusion co-phosphorylation treatment has more obvious effects on the physical and chemical properties of pea protein. The protein structure analysis of the three modified pea proteins by infrared spectroscopy and SDS-PAGE showed that the secondary structure of pea protein was changed, the content of β-sheet was increased, and the content of β-turn was decreased; the content of free sulfhydryl group was decreased, and the content of disulfide bond was increased; The subunit composition was changed, and the content of legumelin A and legumelin B was decreased; the surface hydrophobicity was increased. The analysis results of the functional properties of the three modified pea proteins showed that phosphorylation treatment and extrusion synergistic phosphorylation treatment could improve the solubility of pea protein, while extrusion treatment reduced its solubility. The protein oil retention, emulsifying activity and emulsifying stability were improved, but in these functional properties, the improvement effect of single extrusion treatment and phosphorylation treatment was not as good as that of extrusion synergistic phosphorylation treatment. This study shows that extrusion synergistic phosphorylation treatment can improve the processing characteristics of pea protein and increase the added value of pea protein products, thereby expanding the application scope of pea protein in the food industry.