Enhanced physicochemical and functional properties of pea (Pisum sativum) protein by pH-shifting and ultrasonication combined process

In recent years, pea protein as a novel food ingredient has drawn increasing attention due to its high nutritional value, hypoallergenic, and low price. As an amphiphilic molecule, protein is known as a natural and bio-safe emulsifier. However, similar to other legume proteins, the low water solubility and poor functional properties of pea protein limit its applications in the food industry. This study was undertaken to investigate the effects of pH-shifting in combination with ultrasonication on the structural and physicochemical properties of pea protein isolate (PPI). PPI dispersions (30 mg/ml each) were treated with ultrasonication, pH-shifting, and pH-shifting in combination with ultrasound and compared to control (no treatment). Water solubility, particle size, solution turbidity, surface hydrophobicity, free sulfhydryl group content, and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of the soluble pea protein obtained by the above treatments were determined. The PPI samples (10 mg/ml) treated with pH-shifting at pH 12 in combination with ultrasound (pH12+U5), which had highest solubility, were used to prepare nanoemulsions (0.25% oil) and nanocomplexes loaded with vitamin D3 (VD3). Storage stability, photooxidation protective ability, and morphological structure of the PPI-stabilized nano-systems were examined. The pH12+U5 treatment increased the solubility of PPI from 8.17% (Control) to 60.83%, and reduced the volume-weighted mean diameters D [4, 3] of the soluble protein aggregates from 206.9 (Control) to 45.2 nm. The surface hydrophobicity of the pH12+U5-treated PPI was significantly higher than that of the native protein, while its free sulfhydryl group content was slightly decreased. Structural rearrangement of the treated PPI was observed in the SDS-PAGE, showing that the alkaline pH-shifting and ultrasonic treatment can disrupt covalent and non-covalent bonds. Even though there was no significant improvement in the antioxidant activity of the pH12+U5-processed protein compared to the native PPI, it exhibited good radical scavenging ability. After exposure to UV-light (312 nm, 15 W) for 180 minutes, the VD3 retained in the PPI-based nanoemulsion and nanocomplex was 74.22% and 65.37%, respectively, in contrast to 8.71% in the Control, demonstrating a good photooxidation protection ability of the nano-structures. Besides, the D [4, 3] of the droplets in the nanoemulsion and nanocomplex stabilized by the pH12+U5-treated PPI were 113.93 and 88.90 nm, respectively, and both nano-systems exhibited good stability during storage for 30 days. In summary, the combination of pH-shifting and ultrasonication effectively improved the structural and physicochemical properties of pea protein isolate. The pea protein isolate processed with this new method would be a promising carrier to deliver and protect lipophilic bioactive components in food products, which could lead to foods with improved flavor, nutritional value, and shelf life.