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BACKGROUND Today, ultrasound is increasingly utilized in enzyme modification. Strongly dependent on the specific operational conditions, the modification effect brought by ultrasound can be activation and inactivation of enzymes. This work aims to study the ultrasound mechanisms under different conditions, to investigate the respective roles of free radical effect and mechanical effect in pectinase activation and inactivation, and to reveal the influence of pectinase concentration on the ultrasound-modification effect. RESULTS When ultrasound was introduced to a liquid system, generation of free radicals was positively correlated with ultrasound intensity and treatment duration, but negatively correlated with temperature. Thiourea with a concentration of 4 mM was selected as a free radical scavenger to effectively shield ultrasound free radicals. The highest enzyme activity of pectinase solutions at 0.1, 1.0, and 10.0 mg mL-1 was obtained at the same ultrasound intensity of 4.50 W mL-1 and time of 15 min, where the enzyme activity was increased by 68.24%, 20.98% and 18.83%, respectively. Furthermore, the addition of thiourea enhanced the enzyme activity at each tested ultrasound intensity and time, especially those exceeding the best conditions; it also eliminated the redshift phenomenon that was previously presented in the fluorescence spectra of pectinase samples. CONCLUSION Pectinase concentrations did not change the optimum ultrasound conditions for enzyme modification, but pectinase with a low concentration was more vulnerable to ultrasound treatment. During modification, ultrasound mechanical effects dominated in pectinase activation, while free radical effects dominated in the inactivation process. This article is protected by copyright. All rights reserved.