The objective was to test ultrasound treatments on spinach leaves during extraction, and conventional extraction was used as a control. The effects of different combinations of the ultrasonic water bath factors tested on phenolic compound yields included frequency (37 and 80 kHz), exposure time (5, 10, 15, 20, 25 and 30 min), temperature (30, 40, and 50 C), and ultrasonic power (30%, 50%, and 70%). The best conditions for extraction yields were ultrasonic frequency of 37 kHz, extraction time of 30 min, reaction temperature of 40 C, and ultrasonic power of 50%. The mean yield (mg/100 g), total phenol (mg gallic acid/g DW), flavonoids (mg/g DW), % DPPH free-radical scavenging activity, and % ferric reducing antioxidant power were all high (64.88 ± 21.84, 33.96 ± 11.30, 27.37 ± 11.85, 64.18 ± 16.69 and 70.25 ± 9.68). Treatments were significantly different. The interaction among the ultrasonic parameters was significant. Temperature and power had significant effects on all other dependent variables.
Altemimi, Ammar, Choudhry, Ruplal, Watson, Dennis G. and Lightfoot, David A. "Effects of ultrasonic treatments on the polyphenol and antioxidant content of spinach extracts." Ultrasonics Sonochemistry 24 (Jan 2015): 247–255. doi:http://dx.doi.org/10.1016/j.ultsonch.2014.10.023.