Relationship between structure and properties of extruded enzymatic soybean protein and emulsifying activity evaluation of hydrolysate emulsion

Soybean protein, one type of high-quality plant protein has been widely used in the food processing field, because of its excellent water-holding capacity, and gelation functional properties. However, the soybean protein is still lacking in emulsion, due to the dense network structure and limited molecular flexibility. In this study, an appropriate modification was adopted to expand the structure of soybean protein for better emulsification properties. An optimal enzymatic digestion was exposed to the internal functional groups of soybean protein for better functional properties. Specifically, Alcalase cut off the peptide bonds in the peptide chain, and then the soybean protein was broken down into many short peptides, finally exposing the hydrophobic amino acids embedded within protein molecules. As a result, the surface hydrophobicity of soybean protein increased to improve the structural characteristics. The extrusion treatment also promoted the unfolding of protein structure, thus exposing the internal hydrophobic groups. There was also some variation in the structure of soybean protein. Therefore, a systematic investigation was implemented to explore the relationship and mechanism between the emulsification and structure of soybean protein enzymatic digestion products that are treated by extrusion with enzymatic digestion. The effect of Alcalase treatment time on the protein structure (flexibility, surface hydrophobicity, and secondary structure) and emulsion interfacial properties (rheology, emulsifying activity, and emulsifying stability) of extruded enzymatic digestion products was studied under the conditions of different time (0, 10, 20, 30, 40, and 50 min) of cold pressed soybean meal powder pretreated by extrusion. The improvement performance of soybean protein digestion emulsion on the stability of egg liquid was explored by mixing the soybean protein digestion emulsion with egg liquid, according to different mass ratios. A new idea was provided to apply the experimental and theoretical research into the practical factory. The results showed that the flexibility and surface hydrophobicity of soybean protein enzymatic digestion products after extrusion showed a trend of first increasing and then decreasing with the extension of enzymatic digestion time. Furthermore, the flexibility of extruded soybean protein enzymatic digestion products reached the maximum of 0.40 after 30 mins of enzymatic digestion. The surface hydrophobicity of extruded soybean protein enzymatic digestion products reached the maximum of 44.96 after 30 mins of enzymatic digestion. The secondary structure of extruded soybean protein enzymatic digestion products was shifted from the ordered to a disordered state. Meanwhile, the content of β-turn and random coil increased, whereas, the spiral content of β-sheet and α-helix decreased. The storage and loss modulus first increased and then decreased in the soybean protein digestion emulsion at the interface with the increase of the degree of enzymatic digestion. The content of interface peptides also showed a similar trend of first increasing and then decreasing with the extension of enzymatic digestion time. The emulsifying activity and stability of extruded soybean protein digestion emulsion also reached the optimal values of 28.45 m²/g and 61.05 min, respectively, after 30 min of digestion emulsion. The stability of extruded soybean protein digestion emulsion was verified by mixing the emulsion with egg liquid at a mass ratio of 3:3 and then analyzed by multiple light scattering. The small thermal instability index (thermal instability index, TSI) was achieved in the emulsion system, indicating better stability. Therefore, the extruded soybean protein digestion emulsion can be expected to effectively improve the stability of egg liquid. The utilization rate of plant-based proteins can also be improved, such as egg allergies. This finding can provide a theoretical basis for the application of soybean protein hydrolysates in the field of emulsified foods (such as salad dressing).