Structural and physicochemical properties of soy protein isolate films at different hard-to-soft particle ratios

The influences of hard-to-soft particle ratios on the microstructure and properties of soy protein isolate (SPI)-based films were examined to elucidate how particle morphology affects film performances. Increasing the hard particle content led to the progressive rise in the surface roughness of the SPI films. The tensile strength (TS) and elongation at break (EAB) of SPI films prepared with soft particles were 5.86 MPa and 104.57%, respectively. The highest TS was achieved at a hard-to-soft particle ratio of 3:1, whereas EAB consistently decreased as hard particle content increased. Multiple light scattering analysis demonstrated that as the hard particle content increased, the transmitted light intensity at the bottom of the solution became lower than that in the middle and upper regions. At a hard-to-soft particle ratio of 3:1, the SPI films exhibited higher thermal stability (53.83 °C) and greater FTIR peak intensity for the amide A band compared to other films. These findings indicated that a hard-to-soft particle ratio of 3:1 enhanced the SPI film stability and film strength, suggesting that adjusting hard-to-soft particle ratios is an effective strategy for improving SPI film properties. • SPI film’s structure could be regulated by hard-to-soft particle ratios. • Compact SPI films could be formed from 3:1 hard-to-soft particle ratio. • Interaction of hard and soft particles could improve SPI film’s tensile strength.