Bromelain hydrolysis and CaCl₂ coordination promote the fibrillation of quinoa protein at pH 7

Quinoa protein has high nutritional value, containing all the essential amino acids, but its weak gelling property limits its processing and application. The fibrillar aggregates of plant proteins formed at pH 2 can improve the gelling property, but the fibrils dissociate as the pH values shift. Here, we found that green enzymatic hydrolysis at pH 7 could substitute acid hydrolysis to generate thermal-induced fibrillar aggregates from quinoa protein isolate (QPI). When the enzyme to substrate ratio (E/S) increased to 0.075%, the electrostatic repulsion increased, but the hydrophobic interaction did not vary significantly, and the formation of fibrillar aggregates was favored by the synergistic effect of these two molecular driving forces. Whereas a continuous increase in the E/S resulted in fewer fibrillar aggregates, which was due to the decreased electrostatic repulsion and enhanced hydrophobic interaction. Furthermore, when the E/S was 0.075%, the parallel β-sheet content increased and the length of fibrillar aggregates extended from 175 nm to 327 nm as the concentration of CaCl₂ increased from 0 to 120 mmol/L. The coordination between Ca²⁺ and carboxyl groups could be bridging mode. However, the increase of CaCl₂ concentration to 160 and 200 mmol/L caused fibrillar aggregates to cluster and become shorter due to the electrostatic shielding effects, leading to the decreased gelling properties. Here, we presented an innovative method to prepare QPI fibrillar aggregates at neutral condition, and it showed limited bromelain hydrolysis and appropriate CaCl₂ addition could improve the gelation of QPI by regulating its fibrillation.