Soy protein nano-aggregates with improved functional properties prepared by sequential pH treatment and ultrasonication

The functional properties of soluble nano-sized soy protein aggregates produced by pH treatment followed by ultrasonication (US) were investigated. Commercial soy protein isolate (SPI) was subjected to pH treatment under acidic (pH 2-4) or alkaline (pH 9-12) conditions followed by US before neutralizing to pH 7. The pH treatment at pH 12 followed by sonication for 5 min (pH12 + US5) was the most effective in reducing the sizes and turbidity of soluble protein aggregates, enhancing protein solubility and surface hydrophobicity, and modifying protein subunits. The pH12 + US5 treated SPI increased protein solubility from 1.49% for the control to 82.73%, producing soluble protein aggregates with average size of 22 nm. SPI nanoemulsions prepared using pH12 + US5 treated protein and canola oil had particle sizes in the range of 70-117 nm. Nanoemulsions and nanocomplexes (no oil) prepared with the SPI nano-aggregates provided good protection of vitamin D₃ against UV exposure (180 min), with retention of 73.5 and 70.7%, respectively, compared to 5.2% in the control. This new treatment may offer an effective method to modify the functional properties of commercial SPI for use as building blocks for preparing nanoparticles and nano-structures to protect and deliver bioactive compounds.