Effect of pH and ionic strength on heat-induced pea protein isolate aggregation and gel formation

Abstract

Heat-induced aggregation and gelation of salt extracted pea protein isolates (PPI) was studied as a function of NaCl concentration (0.0–0.4 M) and pH (3.5–8.5). It was hypothesized that an isolate extracted with NaCl, and subsequently dialyzed would show different composition and techno-functional properties depending on its ionic environment. Protein solubility of PPI was affected by NaCl concentrations and pH, with the lowest solubility measured at pH 4.5, regardless of NaCl concentrations. At pH 3.5, solubility was high at low ionic strengths and decreased with increasing salt. At pH between 4.5 and 7, protein solubility increased in solutions at higher NaCl concentrations. At alkaline pH, where proteins are highly charged, salt concentrations did not affect solubility. Heating induced extensive protein aggregation in the presence of NaCl. However, in the case of heated samples in deionized water at pH 3.5 and 8.5, limited aggregation was noticed. These results were confirmed using atomic force microscopy on water redispersed samples. Analysis of viscoelastic properties at the least gelation concentration showed that pH and ionic strength affected not only the stiffness but also the linear viscoelastic regime. This work clearly demonstrated that solubility and thermal stability of PPI are affected by charge properties and how the structure and properties of pea protein aggregates may be modulated through careful control of pH and ionic environment, ultimately affecting the bulk properties of pea protein heat-induced gels. • Pea protein isolate extracted with salt and then dialyzed has high solubility and heat stability at low ionic strength. • Salt increased protein solubility at pH 4.5–8.5 but decreased it at pH 3.5. • Salt affects pea protein aggregation differently before and during heating. • Salt enhanced protein aggregation during heating. • Salt addition greatly reduced the least gelation concentration of pea protein gels and results in soft and brittle gels.