Abstract: In this study, orthogonal array design was employed to optimize the conditions for ultrasonic-assisted extraction of flavonoids from apple peel. Along with this, the stability of flavonoids extracted from apple peel was evaluated. The optimal extraction conditions were extraction temperature of 60 ℃, ethanol concentration of 60%, extraction duration of 20 min, and material-to-liquid ratio of 1:25. Under the optimal extraction conditions, the extraction rate of flavonoids was 27.56 mg/g. Meanwhile, the extracted flavonoids were stable at pH 6.0 and unstable at a temperature over 40 ℃. Light illumination could result in a decrease of stability. In addition, H2O2, Na2SO3, NaNO2, Pb2+, Cu2+, and Fe3+ could obviously reduce the absorbance of flavonoids; on the other hand, Mg2+ and Ca2+ could result in an increase in the absorbance of apple peel flavonoids. Vitamin C, Zn2+, Al3+, and K+ did not reveal obvious effect on the absorbance of apple peel flavonoids. Therefore, ultrasonic-assisted extraction reveals high efficiency and reproducibility with RSD of 0.55% in extracting apple peel flavonoids. Flavonoids from apple peel have weak tolerance to high temperature as well as strong acid and alkali. Although VC, Zn2+, Al3+, and K+ exhibit less effect on the stability of apple peel flavonoids, H2O2, Na2SO3, NaNO2, Mg2+, Ca2+, Pb2+, Cu2+ and Fe3+ have obvious impact on the stability of apple peel flavonoids. 

Key words: apple peel, flavonoids, ultrasonic-assisted extraction, stability