This research studied the effects of alcalase hydrolysis on the structure, physicochemical properties and aggregation behavior of quinoa protein isolate (QPI), and explored its effects on the gelling properties of QPI.

QPI was extracted by alkaline extraction and acid precipitation method at 4 ℃. Alcalase with different enzyme -substrate ratios was added to the QPI solution to hydrolyze the protein. Thereafter, the changes of the composition, particle size, Zeta potential, solubility, and surface hydrophobicity (S₀-ANS) of QPI were analyzed, and the correlation between these changes and the Th T fluorescence intensity and morphology of protein thermal aggregates were discussed. Furthermore, the concentration of QPI dispersion was elevated to form heat-induced QPI gels. The correlation among alcalase hydrolysis, aggregation behavior and gelling properties of QPI was discussed through the analysis of the microstructure, texture and protein secondary structure of QPI gels. Meanwhile, the reasons for the changes in the gelling properties of QPI induced by alcalase hydrolysis were revealed.

With the increase of enzyme substrate ratio (E/S), the protein particle size of QPI dispersion (2%, w/v) gradually decreased, and its electronegativity increased first and then decreased. Meanwhile, the surface hydrophobicity of QPI gradually increased as the E/S increased. At the same time, the Th T fluorescence intensity of QPI aggregates showed an upward trend as E/S rose from 0 to 0.08%, and then declined when the E/S further increased to 0.14%. Through TEM observation, short fibrils (145-306 nm) and long fibrils (217-406 nm) were formed when E/S was 0.05% and 0.08%, respectively. However, with the further increase of E/S ratio, the length of fibrils became shorter, and more amorphous aggregates appeared. In addition, it was found that the aggregation behavior of QPI had a significant effect on its gelling properties (P<0.05). QPI gels showed enhanced hardness, higher storage modulus and a denser network structure, when fibrillar aggregation was dominant. Compared with short fibrils, long fibrils exerted a more significant effect to improve the gelling properties. Furthermore, alcalase showed a significant effect on the protein secondary structure of QPI gels, and the contents of β-sheet and random coil rose first and then decreased with the increase of E/S.

The limited alcalase hydrolysis promoted QPI to form ordered fibrillar aggregates, and further improved its gelling properties. When the E/S was 0.08%, QPI formed the longest fibrils, and QPI gels exhibited the highest hardness as well as the most favorable viscoelastic properties. Meanwhile, the protein secondary structure of QPI gels was ordered, and their microstructure was dense. However, the higher degree of hydrolysis was unfavorable to form fibrillar aggregates and failed to improve the gelling properties. Therefore, limited alcalase hydrolysis could significantly improve the weak gelling properties of QPI through fibrillation.