Effect of Whey Protein Concentrate on Solubility and Emulsion Stability of Pine Kernel Protein

To study effects of protein-protein interactions on solubility, structure and emulsion properties of pine kernel protein (PKP), PKP and whey protein concentrate (WPC) were used as research objects, and PKP-WPC complex protein was prepared by pH-cycle method. The structure and surface properties of complex protein were analyzed by SDS-PAGE, intrinsic fluorescence spectra, ultraviolet-visible spectra, circular dichroism, fluorescent probe and ζ- potential, and then emulsions with oil phase volume fractions of 3%, 10% and 50% were prepared using PKP-WPC complex protein as material, respectively. Finally, emulsion properties were tested. The results showed that when mass ratio of WPC addition to PKP was 1.0∶1.0 and pH of system went through a pH-cycle from 7.0 to 12.0 and back to 7.0, water solubility of PKP improved from 48.53% to 92.43%. SDS-PAGE results showed that PKP-WPC complex protein completely retained subunits of PKP and WPC. Intrinsic fluorescence spectra, ultraviolet-visible spectra and circular dichroism showed that electrostatic interaction, hydrophobic interaction and hydrogen bond were major forces driving the interaction between PKP and WPC, and interactions of PKP and WPC endowed complex protein with high structural toughness and resistance to acid-induced conformational folding. The addition of WPC changed secondary structure of PKP, the amount of α-helix, β-turn and random coil structures increased, while relative content of β-sheet structures decreased. PKP-WPC complex protein had higher surface charges (-34.74 mV) to resist protein aggregation. Compared with emulsions prepared from PKP, emulsions prepared from PKP-WPC showed reduced mean particle diameter and creaming index, increased absolute value of ζ-potential, and significantly improved stability. Emulsions properties varied considerably depending on oil phase volume fraction. The composite emulsion with 3% oil phase volume fraction had small and uniformly distributed droplets, and its stability was better than composite emulsion with 10% and 50% oil phase volume fraction. The study improved PKP solubility by adding WPC through pH-cycle method, and obtained PKP emulsions with better stability. The study could provide a theoretical basis for the development of new protein products, broaden application of pine kernel protein in processed foods, and promote development of PKP-WPC double protein emulsion research.