Valorization of Soybean Residue (Okara) Through Microwave-Assisted Foam Mat Drying: A Study on Drying Kinetics and Functional Properties

Abstract

Soy okara, a nutrient-rich byproduct of tofu production, has limited use in food and feed due to its high content of oligosaccharides and enzyme inhibitors. Its high moisture content causes quick spoilage, leading to frequent waste and environmental concerns. To overcome these challenges, drying can extend its shelf life and enhance its potential for higher-value applications. This study investigates microwave-assisted foam mat drying (MFD) of okara, aiming to optimize foaming conditions and enhance drying and product quality. The optimal conditions—48.06% okara pulp, 2.64% egg white powder (EWP), 2.91% maltodextrin (MD), and 3.52 min of whipping time (WT)—enhanced foam stability and expansion. Drying at 300, 450, and 600 W followed the Page model (R² > 0.989), with higher power accelerating moisture removal. Microwave drying reduced moisture content and bulk density, resulting in improved water solubility index and flowability with increasing wattage. Foam-dried samples exhibited brighter color, lower browning index, and a porous microstructure. Structural analysis through XRD and SEM confirmed increased porosity and reduced crystallinity. FTIR spectra highlighted changes in functional groups due to the foaming agents, while TGA revealed that the foamed samples were thermally less stable, with lower degradation temperatures (Tp) and higher weight loss (ΔW) compared to the control samples. Additionally, foam samples retained total phenolic content (TPC) and antioxidant activity (DPPH inhibition), further increasing the potential value of dried okara. MFD offers a potentially scalable solution for valorizing soy okara, reducing waste, and enhancing resource efficiency, with promising applications in functional foods and nutraceuticals.