Enhanced okara bioconversion and protein recovery through Aspergillus luchuensis fermentation and ultrasound...

Okara, a nutrient-rich residue generated during soybean processing, has considerable potential as a fermentation substrate. However, its compact fibrous matrix restricts microbial access to nutrients and hinders subsequent protein recovery. In this study, Aspergillus luchuensis MW-1, a strain with favorable fermentation performance, was screened and combined with ultrasound pretreatment and ultrasound-assisted protein extraction to improve okara conversion and protein recovery. MW-1 showed strong bioconversion capacity, with mycelial biomass yield reaching 55.1 g/100 g okara at 120 h and the protein content of fermented okara increasing to 27.55 % at 96 h through fungal biomass accumulation. Ultrasound pretreatment at 450 W for 10 min (20 kHz, 5 s on/5 s off) effectively disrupted the dense structure of okara and enhanced nutrient release. The contents of free amino acids and soluble dietary fiber increased by 64.10 % and 46.61 %, respectively, and the protein-enrichment period was shortened from 120 h to 72 h. The optimal condition for ultrasound-assisted extraction (UAE) was 450 W for 15 min, under which protein recovery reached 67.32 %. UAE also altered the structural characteristics of the recovered protein, as shown by reduced particle size, increased absolute ζ-potential, enhanced surface hydrophobicity, increased β-sheet content, and decreased α-helix and random coil contents. These structural changes improved protein functionality, increasing the emulsifying activity index from 38.12 to 47.64 m²/g and the foaming ability from 16.25 % to 20.25 %. These results demonstrate that ultrasound processing can jointly accelerate okara bioconversion, improve protein recovery, and enhance selected functional properties, providing a practical route for producing high-protein food ingredients from okara.