Dilatational rheological and nuclear magnetic resonance characterization of oil-water interface: Impact of pH on interaction of soy protein isolated and soy hull polysaccharides

The effects of pH on the interaction of soy hull polysaccharides (SHP) and soy protein isolated (SPI) at the oil-water interface were investigated by analyzing the particle size, ζ-potential, dilatational rheological properties of the interface adsorbed layer and nuclear magnetic resonance (NMR). The results showed that particle size, penetration rate (Kp), reorganization rate (Kr) and dilatational viscoelasticity of SPI/SHP increased, but diffusion rate (Kdiff) and interfacial pressure (π) decreased with the increasing of pH value from 2 to 4, suggesting that SHP might adsorb on the surface of SPI through the electrostatic attraction. As pH increasing from 5 to 9, the mean volume diameter (d4,3) of SPI/SHP emulsions decreased significantly. Besides, the SPI/SHP emulsions at pH 8–9 had higher ζ-potential and smaller d4,3. The π values of SPI/SHP samples increased at pH 5–8 then declined at pH 9, while the Kdiff increased and Kp decreased with increasing pH from 6 to 9. Moreover, the shorter spin-spin relaxation time and increasing line-width relatively were observed for emulsions at pH 8 and 9, suggesting that more restriction of mobility of oil droplets and increased interaction between biopolymers and oil molecules. All the results showed that pH had significant effects on the interaction between proteins and polysaccharides from 2 to 9. These findings could support the application of SHP in the food industry and provide a theoretical basis for the lipid-soluble nutrient delivery system.