Influence of Pretreatment and Particle Size Modification on Proximate Composition and Physicofunctional Properties of Soymilk By-Product (Okara)—A Proof-of-Concept Study

Abstract

The study investigated the effects of pretreatment and particle size on the proximate composition and physicochemical properties of soymilk residue (okara). A 3 × 3 factorial design involving three processes (fermentation, steaming and no treatment) and three sieve sizes (125, 154 and 180 μm) were used for this study. Proximate composition and physicofunctional properties including colour, bulk density, water and oil absorption capacity and swelling index were determined using standard methods. The proximate composition of the flours was determined using standard protocols. Results indicate that processing method and sieve size had no significant effect on moisture (6.94%–7.25%), ash (4.02%–4.24%) and energy value (471.58–481.97), whereas protein, fat and total carbohydrate (27.27%–34.63%) were significantly affected. Furthermore, particle size (125 μm) increased protein content in untreated samples (36.00%). In contrast, fermented samples (125 and 154 μm) decreased protein (29.28%–29.28%) while increasing crude fat (24.68%–25.41%). The colour parameters increased with sieve size, with untreated samples observing higher L^∗ (80.22–81.46) and a^∗ (3.83–4.27), indicating greater colour intensity and visual appeal. The combined effect of processing operations and varying particle sizes significantly affected the oil absorption capacity (93.5%–128.8%) and tapped bulk density. However, the water absorption (223.2%–260.8%), swelling index (2.458–2.583) and loose bulk density (0.33–0.43 g/mL) were not significantly affected. Steaming appeared to be the most suitable pretreatment for industrial applications, owing to improved colour and functional properties (WAC and OAC). The coarse fraction (180 μm) may be suitable for enhancing baked goods and meat analogues, while the fine untreated fraction (125 μm) may be ideal for high-protein fortification in complementary foods. Overall, these findings suggest the potential of okara as a high-value industrial ingredient.