Effects of extraction methods on the composition, structure, and gelling mechanism of pea proteins

Abstract

A a systematic understanding of pea protein composition, conformation and functionalities as impacted by extraction methods is limited. Moreover, previous works focused on investigating the effects of gelling conditions on gel properties and showed that pea protein had significantly lower gelling capacity and weaker gel texture than soy proteins. However, gelling mechanism of pea protein has not been fully understood yet. Six protein isolation methods were selected, including air-classification (AC), alkaline extraction followed by isoelectric precipitation (AI) or ultrafiltration (AU), salt solution extraction followed by dialysis (SD) or ultrafiltration (SU) and micellar precipitation (MP). The pea protein compositions were analyzed by size exclusive-HPLC. The structures of proteins were revealed by Fourier-transform infrared spectrum, intrinsic fluorescence, and surface hydrophobicity. This study found that recovery methods determine the protein composition. Those obtained by ultrafiltration and dialysis contains albumins, whereas precipitation methods specifically retained globulins. Pea proteins prepared by alkaline solution had higher 11S/7S ratio and surface hydrophobicity than those by salt solution. The protein isolates by different methods showed large variation in gelling properties. Pea protein extracted by MP, AU or SU formed gels with a compressive strength of 60-80 kPa, comparable to soy protein gels. AU, SU, MP experienced high degree of unfolding upon heating, resulting in the exposure of interaction regions. The appropriate level of 7S allowed unfolded proteins to aggregate in a more organized manner through intra-floc links. These together led to homogeneous percolating-like microstructure with greater strength. Instead, the aggregates triggered by extraction through SD method prevented protein unfolding, leading to coarse particulate structure, and weak gels. This study showed the impact of extraction methods on protein composition and structure in relation to their gelling properties. The generated knowledge will help industry target the improved pea protein gels for specific applications.