Comparison of quinoa proteins modified by thermal acid hydrolysis, enzymatic hydrolysis, and ultrasonication as Pickering emulsifiers

Abstract

The poor functionality of seed storage proteins is the major factor limiting their practical utilization. Although thermal acid hydrolysis, enzymatic treatment, and ultrasonication enhance quinoa protein functionality, their effects are rarely compared under identical conditions. Herein, quinoa protein isolate (QPI) was extracted from air-classified quinoa flours without the defatting step, as this step greatly reduces extraction efficiency, and subsequently modified via thermal acid hydrolysis, moderate tryptic hydrolysis, and ultrasonication. The solubility, morphology, surface and emulsifying properties of QPI, acid-hydrolyzed QPI (AQPI), enzyme-hydrolyzed QPI (EQPI) and ultrasonication-modified QPI (UQPI) were evaluated to elucidate how these modifications influenced QPI performance as a Pickering emulsifier under identical conditions. The results showed that the solubility and wettability of all modified QPIs were improved. Both QPI and modified QPIs were able to form oil-in-water emulsions with an average droplet size of ∼13 μm. The interfacial protein concentration increased linearly with emulsifier concentration from 0.5 to 2 % w/w, with AQPI and EQPI exhibiting higher adsorption capacity than QPI and UQPI, which was ascribed to a contact angle closer to 90° (i.e., a balanced hydrophilic-hydrophobic interface). Over 14 days of storage, all emulsions stabilized by QPI and modified QPIs remained resistant to flocculation and coalescence due to high protein adsorption, with EQPI providing the smallest droplet size increase (12.5 to 15.3 μm). Increasing the concentration of QPI and modified QPIs improved the creaming stability of all emulsions. This study demonstrates that these modification methods, particularly moderate tryptic hydrolysis, effectively tailored quinoa proteins for Pickering emulsions.