## 3 Results and Discussion
### Interactions between MUAE Factors of Response Surface  
The three-dimensional response surfaces and contour plots for quercetin extraction demonstrate the effects and interactions between the extraction factors (Figure 2A-D). The surface showed an interaction between ultrasound temperature and ultrasound time. It indicated that the extraction yield of quercetin decreased when both ultrasound temperature and time were increased simultaneously (Figure 2A). It is speculated that increasing the ultrasonication time may lead to continuous product release, causing the extraction solvent to become saturated with the extracted products [34]. In addition, an increase in the temperature can accelerate the process of solvent evaporation. It can reduce the contact surface between the solvent and the solid, thereby decreasing the extraction yield of quercetin. Figure 2B examines the interactions between the two parameters of ethanol concentration and ultrasound time. Considering the ethanol concentration at an ultrasound time of 5 min, increasing the ethanol concentration increased the yield of quercetin extraction. However, in general, the simultaneous decrease in both parameters indicated an increase in the maximum value of extraction yield. The possible reason is reducing evaporation by decreasing temperature and adding water as a co-solvent, which may increase the swelling degree of the onion skins in the solvent medium and enhance the contact surface between the solvent and the solid [35]. Hence, it can raise the yield. The response surface in Figure 2C shows an effective interaction between microwave time and ultrasound temperature. Nevertheless, microwave time alone did not significantly affect the quercetin extraction yield. At an ultrasound temperature of 60 \({}^{\circ}\)C, decreasing the microwave time led to a reduction in extraction yield. This is likely due to insufficient microwave energy being applied to break down the cell walls of the onion skin, limiting the release of quercetin into the solvent. However, at an ultrasound temperature of 80 \({}^{\circ}\)C, reducing the microwave time improved the extraction yield. At higher temperatures, prolonged microwave exposure may lead to thermal degradation of bioactive compounds like quercetin, reducing the yield. The reduction in contact between the solvent and the solid sample, along with an increase in the concentration of solid load due to solvent evaporation, can also contribute to decreased yield. Additionally, excessive microwave energy at higher temperatures may destroy or alter the structure of bioactive compounds, further lowering the extraction yield [36]. The slope of the response surface in the 3-D graph (Figure 2D) analyzes the effects of ethanol concentration and ultrasound temperature on quercetin extraction yield. In the temperature range of 60-80 \({}^{\circ}\)C, a decrease in ethanol concentration (50-70% \(\
u/\
u\), water) at higher temperatures, such as 80 \({}^{\circ}\)C, led to a reduction in extraction yield. However, at lower temperatures (closer to 60 \({}^{\circ}\)C), increasing the ethanol concentration improved the extraction yield. This effect occurs because, at higher temperatures, the structure of bioactive compounds may degrade, while at lower temperatures, increasing the ethanol concentration enhances the solvent's ability to dissolve and interact with these compounds, thus improving the extraction yield. In general, reducing the ultrasound temperature while increasing ethanol concentration improves the extraction yield. At higher temperatures, the structure of bioactive compounds and flavonoids may be decomposed, leading to a lower extraction yield [37]. Figure 2: **(A–D)**. Response surface and contour plots for the MUAE of quercetin from red onion skin waste with respect to ultrasound temperature and ultrasound time (**A**); ethanol concentration and ultrasound time (**B**); microwave time and ultrasound temperature (**C**); and ethanol concentration and ultrasound temperature (**D**).