Protein isolates from Cajanus cajan L. as surfactant for o:w emulsions: pH and ionic strength influence on protein structure and emulsion stability

Abstract

This work has focused on the study of the influence of pH (2.1, 3.9, 6.3, 8.3) and ionic strength (μ) (0.10 and 0.54) on the formation and stability of oil in water (o:w) emulsions using a protein isolate of Cajanus cajan (PPI8) as emulsifier. Also, the influence of protein concentration was evaluated. Interfacial tension was better at pH2.1 and pH8.3, and it was improved at pH6.3 with the increase in ionic strength. Most emulsions showed a monomodal droplet size distribution, except at pH8.3 (μ: 0.54) where a bimodal distribution was observed. The critical concentration range of proteins was between 1.5 and 2.0% (μ: 0.10) and 1.0–1.5% (μ: 0.54). At pH6.3, the surface average diameter was independent of ionic strength (0.5%, 1.0%, and 2.0%). At low ionic strength, stable emulsions (protein concentration ≥1.0%) were obtained (up to 7 days) at pH2.1, pH6.3, and pH8.3, despite the differences in flocculation index (FI) observed. FI of emulsions remained constant up to 7 days of storage, except at pH2.1 (1.0%) and pH8.3 (2.0%). The emulsion stability at high ionic strength was pH dependent, at pH3.9 was improved even at low protein concentration due to the increase in protein solubility and apparent viscosity of emulsion. Because of their techno-functional properties, PPI8 could lead to the development of innovative products suitable for those consumers that demands for alternative plant-based protein sources.