Individual and combined ultrasonic–enzymatic treatments for modifying soybean residue (okara) fiber: impacts on structure, physicochemical, and functional properties

This study investigated the effects of ultrasonic, enzymatic, and combined ultrasonic-enzymatic treatments on the structural, physicochemical, and functional properties of soybean residue (okara) dietary fiber (DF). Under the optimal combined ultrasonic-enzymatic treatment conditions, the soluble DF (SDF) yield and water-holding capacity (WHC) of the modified okara DF increased to 8.00 ± 1.04 g/100 g and 14.88 ± 0.29 g/g respectively, representing enhancements of 193.04% and 40.11% compared with unmodified okara DF. Structural analysis showed reduced particle size, a more uniform size distribution, and disrupted microstructures characterized by wrinkles, voids, and fragmentation. Physicochemical properties were also improved (p < 0.05), with WHC, oil-holding capacity (OHC), and swelling capacity (SC) increasing by 40.11%, 82.81%, and 40.19%, respectively. Moreover, the combined treatment exhibited stronger functional effects than individual methods, yielding 4.82- and 10.07-fold increases in cholesterol and bile salt adsorption capacities, and 1.35- and 7.78- fold enhancements in DPPH and hydroxyl radical scavenging activities. These enhancements are attributed to structural modifications, including reduced particle size, increased specific surface area, exposure of functional groups, and decreased crystallinity, which collectively improve the functional properties of okara DF. Overall, this study highlights the synergistic advantages of ultrasonic-enzymatic treatment and provides theoretical and technical support for the high-value utilization of okara DF.