Coacervation in pea protein solutions: The effect of pH, salt, and fractionation processing steps

Abstract

Legumes such as pea and soy have the ability to form coacervates in solution, which depends on the pH and salt concentration. We studied the phenomenon of coacervation for pea protein solutions in which the proteins were exposed to different extents of fractionation (i.e., neutral extraction, neutral extraction followed by freeze drying, alkaline extraction and alkaline extraction followed by isoelectric precipitation). The pH of the dispersed fractions was varied between 5.5 and 6.5 and the salt concentration between 0 and 500 mM NaCl. Confocal microscopy and particle size measurements were used to confirm the presence of spherical shaped protein-rich domains; a signature of the concentrated coacervate phase. It was found that the mildest processed fraction – obtained after protein extraction from the flour and subsequent removal of solids by centrifugation – formed coacervates as well as aggregates (i.e., non-spherical domains), between pH 6.0 and 6.5. At pH 6.25 only coacervates were observed. When 50 mM NaCl was added at pH 6.25, the coacervate average diameter increased from approximately 1 to 5 μm. When the salt concentration was increased to ≥200 mM NaCl, no coacervates were observed anymore. The coacervates formed at pH 6.25 with 0 and 50 mM NaCl were examined further. It turned out that the coacervates contained pea globulins, with legumin being most abundant. Pea albumins were not found in the coacervates. The internal protein content of the coacervates formed at pH 6.25 was around 45 wt %. When the mildest processed fraction was freeze dried, coacervates could still be formed at pH 6.25 and low salt concentrations (≤50 mM NaCl). After alkaline extraction, dispersions with both aggregates and coacervates were observed at pH 6.25. Isoelectric precipitated pea protein isolate did not show coacervates at any of the conditions tested. Our research shows that mildly fractionated pea protein was most suitable to form coacervates.