Antioxidant activity and effective compounds of immature calamondin peel

The antioxidant activity and the flavonoids of mature and immature calamondin (Citrus mitis Blanco) peel were investigated. The hot water extract of immature calamondin peel exhibited the highest oxygen radical absorbance capacity (ORAC), reducing power, and superoxide scavenging effect. 3′,5′-Di-C-β-glucopyranosylphloretin, naringin, hesperidin, nobiletin, and tangeretin are the five major flavonoids found in hot water extract with the levels of 6888 ± 522, 2333 ± 157, 1350 ± 94, 165 ± 13, and 8 ± 4 mg/100 g dry basis, respectively. The contents of nobiletin and tangeretin increased after ripening. The hot water extract of immature calamondin peel was fractionated using a semi-preparative HPLC. Fraction VI showed the highest ORAC value (28.02 ± 2.73 mmol Trolox equivalents (TE)/g fraction) and two compounds, naringin and hesperidin, were identified as the major active components attributed to the antioxidant activity. Fraction V contained 3′,5′-di-C-β-glucopyranosylphloretin, which revealed low ORAC value with 7.43 mmol TE/g fraction. However, it might also contribute to antioxidant activity in immature calamondin peel due to its greatest quantity.