Thermal acid hydrolysis modulates the solubility of quinoa protein: The formation of different types of protein aggregates

The poor solubility of plant-based proteins is the main factor limiting their utilization. In this study, how thermal acid hydrolysis (0, 0.05, 0.1, 0.2 and 0.5 M HCl; 90 °C) changed the solubility of quinoa proteins was investigated by monitoring the changes of the degree of hydrolysis (DH), sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS-PAGE) profiles, particle size, zeta-potential, surface hydrophobicity, intrinsic fluorescence and transmission electron microscopy (TEM) morphology with different hydrolysis time. We found that 11 S globulins and 2 S albumins were the major components of differently-sized quinoa protein particles (i.e., small/soluble or large/precipitated particles), which were the main form of quinoa proteins existing in water solution. Compared to precipitated fractions, 7 S globulins were more abundant in the soluble fractions of quinoa proteins. The solubility of quinoa proteins was determined by the combined effect of protein-protein interactions and acid hydrolysis, which was highly related to acid concentration. At 0 M HCl, heat-induced aggregation caused the decreased solubility because of the decreased electrostatic repulsion and hydrophobic interactions among quinoa protein molecules. At 0.05 and 0.1 M HCl, the solubility was also dominated by heat-induced protein aggregation without changes over 0.5–8 h treatment. At 0.2 M HCl, the solubility and the DH increased with hydrolysis time due to the formation of long and fibrillar protein aggregates. At 0.5 M HCl, strong acid hydrolysis greatly improved the solubility of quinoa proteins, forming short and worm-like strands and subsequently assembling into larger aggregates. This study would promote the utilization of quinoa proteins as an alternative protein.