Effects of extrusion temperature on the functional and structural properties of soybean protein isolate and proanthocyanidin complex

The interaction between macromolecules and small molecules has gradually attracted widespread interest in recent years, especially for the interaction between proteins and polyphenols in food components. Phenolic compounds are the parts of the secondary metabolites, mainly found in plant species with great structural diversity. They can exist as glycosides or aglycones, mainly composed of polymeric or monomer structures. Protein is widely found in animals and plants. In the course of food processing, the protein and polyphenols can inevitably coexist in the same system and then interact with each other, which often changes the structure of protein levels. There are some effects of the interaction between polyphenols and proteins on the functional properties of proteins, mainly including the hydration properties (water retention, viscosity, and solubility) and surface properties (foaming and emulsification). The beneficial interactions can improve the nutritional and functional properties of foods. However, the interaction between proteins and polyphenols is susceptible to a variety of factors, including internal factors (self factors, such as the structural characteristics and relative concentrations of proteins and polyphenols), and external factors (environmental factors, mainly including the solvent composition and solution parameters, such as temperature, ionic strength, and pH). The extrusion technique is a physical modification to integrate multiple unit operations, such as material mixing, homogenization, cooking, and molding. In this study, an attempt was made to clarify the effect of proanthocyanidins on the physicochemical properties of soy protein as a representative plant protein, considering the important role of proteins and phenols in health, nutrition, and food quality. Specifically, an investigation was implemented on the effects of different extrusion temperatures (40, 60, 80, 100, and 120℃) on the functional and structural properties of Soybean Protein Isolation (SPI) and Grape Seed Procyanidins Extractive (GSPE) complex. The indicators were selected as emulsifying activity index, emulsion stability index, zeta potential, and particle size. Fluorescence and infrared spectroscopy were used to analyze the functional properties and structure of SPI in the composite system. The results show that there were significantly improved solubility, emulsifying activity, emulsifying stability, absolute value of Zeta potential, and water holding capacity of SPI-GSPE composite after extrusion treatment, compared with the extruded SPI. But, there was an observable decrease in the surface hydrophobicity and oil-holding capacity of the SPI-GSPE composite. Specifically, the solubility, oil retention, and emulsification activity of SPI-GSPE composite first increased, then decreased, and finally reached the maximum at 80℃, with the increase of extrusion temperature. By contrast, the surface hydrophobicity of the SPI-GSPE composite first decreased, then increased, and finally reached the minimum at 80℃. The absolute value of Zeta potential, emulsification stability, and water retention all decreased with the increase in temperature. The particle-size analysis showed that the SPI and GSPE formed a more compact complex after extrusion. The fluorescence spectrum showed that the composite and extrusion with the GSPE were used to extend the skeleton of SPI polypeptide chain for the protein structure, indicating a significant quenching effect on the SPI fluorescence. Ir spectra showed that the non-covalent cross-linking of GSPE to SPI varied greatly in the secondary structure of proteins. The composition and extrusion with the GSPE increased the α-helix, irregular crimp, and β-corner contents of the SPI, but decreased the β -fold contents. This finding can provide important theoretical guidance for the utilization of SPI and GSPE resources. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.