Effects of Different Soybean and Coconut Oil Additions on the Physicochemical and Sensory Properties of Soy Protein–Wheat Protein Mixture Subjected to High-Moisture Extrusion

A protein mixture was prepared using a blend of soybean protein isolate, soybean protein concentrate, and wheat protein through high-moisture extrusion. This study investigated the effects of soybean oil/coconut oil additions (2%, 5%, and 8%) on the physiochemical properties of a soy protein–wheat protein mixture subjected to high-moisture extrusion. The protein extrudates underwent assessment for textural properties, fiber degree, sensory evaluation, microstructure, protein solubility, and protein secondary structure. The findings indicated that plant oils significantly reduced the hardness, springiness, and chewiness of the extrudates, and 5% plant oil significantly increased the fiber degree of the extrudates. In addition, the highest fiber degree and sensory evaluation score were achieved with 5% coconut oil. Observation of the macro- and microstructure indicated that the presence of unsaturated fatty acids in soybean oil did not benefit the improvement of the fibrous structure of protein extrudates during high-moisture extrusion processing. SDS-PAGE and FTIR results revealed that coconut oil, rich in saturated fatty acids, caused the clustering of medium- and low-molecular-weight subunits in texturized protein. Additionally, coconut oil elevated the ratio of 11S protein subunits containing sulfur-based amino acids and facilitated a shift from β-turn to β-sheet. The inclusion of plant oils increased the development of hydrogen and disulfide bonds, resulting in a denser, fibrous structure. DSC demonstrated that plant oils reduced the thermal stability of the texturized proteins but enhanced the order of protein structure.