Application of ultrasound to enhance functional properties of soy okara obtained after protein extraction of soy press cake

[EMBARGOED UNTIL 8/1/2024] Soy okara is an insoluble byproduct from the production of soymilk, tofu, and soy protein extraction. Recently, there has been increasing interest in alternative processing of soy protein with the use of soy press cake as starting material instead of the traditional solvent-extracted soybean meal. While the utilization of okara has been studied, the literature is still limited, especially those that focus on okara from soy protein processing of soy press cake. This environmentally-friendly approach could become practical with an increasing demand of plant-based food and plant-based protein as well as environmental awareness. In this study, soy press cake was utilized as the initial material for protein extraction, and the resulting okara was subjected to freeze-drying. Okara dispersions were employed to create oil-in-water emulsions with different volume fractions ([phi]) of 0.2, 0.4, and 0.6. The effects of sonication during protein extraction (with sonication: SOK; without sonication: UOK) and during the preparation of the okara dispersion (0, 10, and 20 minutes) on the physical and emulsification properties were comprehensively examined. This investigation resulted in the production of okara with varying yields, compositions, secondary structures, and functional properties. The application of sonication during protein extraction significantly decreased the yield, protein content, and fat content of the okara (e.g., SOK<UOK, P < 0.05). Okara obtained without sonication before protein extraction (UOK) exhibited smaller mean diameters (D[4, 3]). No significant effect of sonication on charge property or protein structure was observed (P > 0.05). Sonication during dispersion preparation could significantly enhance the physical and functional properties of both freeze dried UOK and SOK powders. Sonication reduced the size of the okara particles and modified the secondary structure of the okara protein, resulting in a notably higher content of [beta]-sheet and lower [alpha]-helix. The okara, which was predominantly composed of polysaccharides and protein, exhibited strong adsorption activity at the oil-water interface as well as excellent emulsification properties at different [phi]. Types of okara (e.g., UOK vs SOK), and sonication during dispersion preparation influenced droplet/particle sizes, rheological properties and stability. In conclusion, value-added okara from soy press cake can be prepared to have enhanced functional properties for food applications.