Transforming soy proteins into nanoporous aerogels using supercritical carbon dioxide (SC-CO2) drying

Abstract: Soybean is a widely cultivated legume crop for its high protein (35-40%) and oil (~20%) contents. While soybean oil is widely used in food, the protein-rich defatted meal mostly goes to animal feed (98%), despite being a complete protein source with numerous health benefits. This study investigated the formation and characterization of aerogels from soy protein isolate (SPI) at different pHs (6-10) using supercritical carbon dioxide (SC-CO₂) drying. The formed SPI hydrogels and aerogels were characterized for their rheological properties, textural properties, morphology, crystallinity, chemical structure, thermal stability, and solubility. The gels formed at pH values of 7.0-10.0 were able to form strong gels, where all the gels showed a shear-thinning behavior. The hardness of the gels increased with the increase in pH from 7.0 to 10.0. The resulting SPI aerogels upon SC-CO₂ drying showed outstanding properties, such as surface areas of 222-278 m²/g, pore sizes of 8-11 nm, and pore volumes of 1.88-3.13 cm³/g. Their densities were ~0.21 g/cm³ with high porosities of ~83%. Further, SEM images of aerogels showed three-dimensional open porous structures, where the structures were more heterogeneous at higher pH values, leading to a higher surface area. XRD and FTIR data provided information about the crystallinity and changes in the chemical structure of proteins, respectively. The main thermal degradation peak for aerogels was ~295 °C, which was slightly lower than that for SPI powder at 303 °C. The water solubility of SPI powder increased with the formation of aerogels from 17-19% to 30-36%. Overall, this study highlights the potential for generating SPI aerogels with high surface areas for their use in food and pharmaceutical industries.