Pea and soy protein isolate fractal gels: The role of protein composition, structure and solubility on their gelation behaviour

The gelation behaviour of two different pea protein isolates and one soy protein isolate were investigated with a focus on the role of the protein properties. Protein solubility was the lowest in pH 3 citrate-phosphate buffer (<10% w/w), increased in pH 7.4 phosphate-buffered saline (12–21% w/w), and was the highest in pH 7.6 MilliQ water (∼20–40% w/w). Heat-induced gelation conditions for the protein sources were sensitive to both the soluble and the insoluble fractions as obtained during extraction. At low protein concentrations (≤5% w/v), the proteins started to lose their viscoelastic behaviour and exhibited predominantly viscous properties. Fitting of the fractional Kelvin-Voigt model to the frequency sweeps showed an increase in the fractal gel strength with increasing protein concentration. Secondary structures of the soluble species showed mostly unordered proteins, suggesting that the proteins were denatured during the commercial extraction process although gelation has to date been suggested to be highly dependent on the denaturation of soluble proteins. Synchrotron Radiation Circular Dichroism measurements of the insoluble proteins showed a significant amount of ordered protein structures. SEM imaging of the gels also suggested a new gelation pathway in which insoluble proteins act as dispersed fillers within a continuous matrix of soluble proteins. This research elucidated the role of different protein fractions, globulins and albumins, and their secondary structure in the formation of a gel network and how this affects their viscoelastic behaviour.