Comparative study of the emulsifying properties of blue lupin, white lupin, and soybean protein isolates

This study investigated the emulsifying properties of isoelectric pH-precipitated blue lupin (BLPI), white lupin (WLPI), and soybean (SPI) protein isolates at various protein concentrations and pH values (3, 5, 7, and 9). The solubility, secondary, and tertiary structure of the protein isolates at different pH were determined. The emulsion characteristics evaluated included the droplet size, microstructure characterization, coalescence index (CI), and flocculation index (FI). The solubility of the protein isolates was influenced by pH, with the lowest solubility recorded at pH 4-5. At pH 5, all the protein isolates had more R-sheet structure than alpha-helix, while at pH 9, the isolates contained the helical conformation more than R-sheet. In general, mean oil droplet size (d(3,2)) was bigger at pH 3 and 5 (similar to 5-16 mu m) when compared to pH 7 and 9 (<6 mu m). The presence of small spherical and uniform oil droplet emulsions (<9 mu m) stabilized by WLPI, BLPI, and SPI at pH 7 and 9 was confirmed using confocal laser scanning microscopy. Oil droplet coalescence was not the most prominent mechanism of emulsion instability for both lupin isolates and SPI, but rather bridging flocculation, with the FI highest at pH 5 (50-100%).