Abstract:

The space of ultrasonic bath was divided according to height and position into high and low ultrasonic irradiation
surface each with five sections such as P1, P2, P3, P4 and P5. The effects of ultrasonic-assisted extraction parameters
including solvent type and locally ultrasonic positions on the stability and antioxidant capacity of caffeic acid under different
ultrasonic irradiation surfaces (at 6 cm and 14 cm high from the bottom) were comparatively analyzed in the present study.
The results showed that caffeic acid was markedly degraded in low irradiation surface (P < 0.05). However, ultrasonic
energy had no significant impact on the stability of caffeic acid in high irradiation surface. The type of solvent was one of
the key factors in determining the stability of caffeic acid and 80% ethanol was the optimal solvent. In the meantime, the
ultrasonic energy distribution was uneven and dynamic in the local positions, and the content of caffeic acid was decreased
significantly in P4 or P5 of ultrasonic bath (P < 0.05). Moreover, the antioxidant activities of caffeic acid as determined by
DPPH, FRAP and ABTS methods exhibited a big difference. Simultaneously, the position of P5 greatly affected antioxidant
activity of caffeic acid evaluated by DPPH and FRAP methods, and of its degraded products had much higher antioxidant
activity. Additionally, antioxidant capacity of caffeic acid as determined by ABTS method was positively correlated with
its content, but DPPH free radical scavenging capacity had a negative correlation. These findings indicated that appropriate
ultrasonic energy could change greatly the stability of caffeic acid and simultaneously enhance its antioxidant capacity in
low irradiation surface.

Key words: ultrasonic irradiation surface, locally ultrasonic position, caffeic acid, antioxidant capacity