In Vitro Digestibility, Structural and Functional Properties of Millettia speciosa Champ. Seed Protein

As an underutilized industrial byproduct generated during bioactive compound extraction from <i>Millettia speciosa</i> Champ. seeds, the residual protein fraction represents a promising sustainable resource for valorization. <i>Millettia speciosa</i> Champ. seed protein (MP) was extracted, and its fundamental physicochemical and functional properties were evaluated for potential applications in the food industry. Structural characterization revealed that MP had a molecular weight distribution with major components at 14.0 kDa and 116.0 kDa, with respective denaturation temperatures of 79.75 °C and 91.77 °C. The main structure of MP included different proportions of intramolecular α-helices and random coils in different pH microenvironments, based on circular dichroism spectroscopy. The MP displayed similar solubility profiles to the soy protein isolate (SP), but with lower solubility at slightly acidic pH, low solubility at pH 5.0, and comparable solubility above pH 8.0. Functional assessments showed that MP possessed emulsifying, foaming, water-binding, and fat-absorption capacities comparable to those of SPI, although the in vitro digestibility was relatively lower. These findings indicate that MP may serve as a safe and nutritious functional ingredient for health-oriented food products.