  This study aimed to optimize the extraction conditions in order to maximize
the recovery yields of quercetin and total flavonoids from red onion skin
wastes using sequential microwave-ultrasound-assisted extraction. Five
effective factors on quercetin extraction yield were investigated using
response surface methodology. The method was successfully performed under
optimal conditions of 60 s microwave irradiation followed by 15 min sonication
at 70 {\deg}C, 70% ethanol with solvent to solid ratio of 30 mL/g. Based on the
optimization results, ultrasound temperature was found to be a highly
significant and influential factor for the recovery. The maximum recovery
yields of quercetin and total flavonoids from red onion skin were estimated to
be 10.32 and 12.52%, respectively. The predicted values for quercetin (10.05%)
and total flavonoids (12.72%) were very close to the experimental results. The
recovery yields obtained from different extraction methods were as follows:
ultrasound-microwave-assisted extraction (7.66% quercetin and 10.18% total
flavonoids), ultrasound-assisted extraction (5.36% quercetin and 8.34% total
flavonoids), and microwave-assisted extraction (5.03% quercetin and 7.91 %
total flavonoids). The validity of the projected model was examined by the
obtained experimental data; in which, the validated model was suitable for the
recovery of the valuable products from onion skin wastes for further scale-up
in the food processes.
