Abstract: The effects of ultrasonic-assisted extraction parameters (ultrasonic treatment time, extraction temperature and ultrasonic power) on total phenolic content and antioxidant capacity of methanol extracts from Citrus unshiu peels under different ultrasonic frequencies (20, 60 kHz and 100 kHz) were comparatively analyzed in the present study. The results showed that increasing ultrasonic treatment time, extraction time and ultrasonic power resulted in a notable increase in total phenolic content and antioxidant capacity. However, the optimal extraction conditions for achieving the maximum total phenolic content and antioxidant capacity were different. The extract obtained by 60 kHz ultrasonic-assisted extraction presented higher total phenolic content and antioxidant capacity when compared to those obtained at 20 kHz and 100 kHz. Citrus unshiu peel extracts exhibited an excellent correlation between total phenolic content and antioxidant capacity. The correlation coefficients for the extracts obtained at ultrasonic frequencies of 20, 60 kHz and 100 kHz were 0.8228, 0.8354 and 0.8435 at 15 ℃, 0.7706, 0.8735 and 0.8895 at 30 ℃, 0.8626, 0.9248 and 0.8622 at 40 ℃, respectively. The correlation coefficients for the extracts obtained at ultrasonic powers of 3.2, 8, 30 W and 56 W were 0.9143, 0.9958 and 0.7971, respectively. These findings indicate that ultrasonic treatment at low frequencies can effectively enhance the antioxidant capacity of Citrus unshiu peel extracts.

Key words: Citrus unshiu peel, extraction, total phenolic content, antioxidant capacity, ultrasonic frequency