Limited Alcalase hydrolysis improves the thermally-induced gelation of quinoa protein isolate (QPI) dispersions

Gelation is critical in many food applications of plant proteins. Herein, limited hydrolysis by Alcalase was used to promote thermally induced gelation of quinoa protein isolates (QPI). Mechanical properties of various QPI gels were characterised by small and large oscillatory shear deformation rheology while the microstructural features were observed by confocal laser scanning microscopy (CLSM). Both the gel strength and microstructure are strongly related to the hydrolysis time. The maximum gel strength (∼100 Pa) was achieved after Alcalase hydrolysis for 1 min, which was ∼20 folds higher than that of untreated QPI. Extended hydrolysis up to 5 min progressively decreased the gel strength. A string-like interconnected protein network was formed after proteolysis. The change of gel strength with hydrolysis time correlated well to the Gʹ(20°C)/Gʹ(90°C) value and results of intrinsic fluorescence and surface hydrophobicity. The Gʹ(20°C)/Gʹ(90°C) value is sensitive to hydrogen bonds formation while the intrinsic fluorescence and surface hydrophobicity are associated with protein unfolding and exposure of hydrophobic groups. Therefore, both hydrogen bonding and hydrophobic interactions are critical in improving the gel strength of QPI hydrolysates. Finally, FTIR analysis revealed that protein secondary structures are affected by the proteolysis and formation of inter-molecular hydrogen bonds between polypeptides. This study provides an efficient strategy for improving thermally induced gelation of QPI and enables a deep understanding of QPI gelation mechanism induced by Alcalase hydrolysis.