Background and objectives:The effects of ultrasound pretreatment with different levels of power output (200, 400, and 600 W) on hydrolysis degree of Quinoa protein isolate (QPI) and antioxidant activity of QPI hydrolyzate were studied.The secondary structure, surface hydrophobicity, intrinsic fluorescence, and scanning electron microscopy (SEM) of QPI treated by ultrasound were measured.The untreated QPI and high-temperature treatment (90°C for 30 min) were used as control. Findings:The results showed that ultrasound pretreatment at 400 W significantly improved the hydrolysis degree of QPI and antioxidant activity of QPI hydrolyzate. The changes in secondary structure, surface hydrophobicity, and intrinsic fluorescence indicated the unfolding of QPI after ultrasound. The SEM results showed that ultrasound treatment resulted in the deformation of QPI. Conclusions:The improvement in degree of hydrolysis might be related to the changes in structure induced by ultrasound treatment, which supported more groups for the process of enzymatic hydrolysis. Significance and novelty: The power output selection of ultrasound pretreatment is essential for improvement of accessibility of QPI to the alcalase and preparation of antioxidant peptides. K E Y W O R D Santioxidant activity, Quinoa protein isolate, structure, ultrasound pretreatment