Preparation and in vitro evaluation of calcium-induced soy protein isolate nanoparticles and their formation mechanism study

Soy protein isolate (SPI) nanoparticles (28–179 nm) were prepared by employing a cold gelation method with a slight modification. The obtained nanoparticles exhibited uniform size distribution and spherical shape with a unique honeycomb-like core structure. Nanoparticle characteristics including size, surface charge and hydrophobicity could be adjustable by changing calcium concentration and environmental pH. Generally, higher calcium concentration and lower pH led to formation of nanoparticles with larger size, lower surface charge and hydrophobicity. Both protein conformation and nanoparticle dissociation studies indicated that calcium likely shielded negative charges on the SPI polypeptide chains, and functioned as a salt-bridge to permit polypeptide chains to approach one another. In this process, calcium favoured the development of β-sheet structures to form SPI aggregates stabilised by hydrogen bonding. These aggregates were then associated to build SPI nano-networks through hydrophobic interactions. In vitro study indicated that the SPI nanoparticles were non-toxic and mainly located in the cytoplasm when uptaken into Caco-2 cells.