Drying effect on flavonoid composition and antioxidant activity of immature kumquat

A seven flavonoids in hot water extract of immature kumquat (Citrus japonica var. margarita) were identified and quantified (mg/100 g fresh fruit): 3′,5′-di-C-β-glucopyranosylphloretin (DGPP, 285.9 ± 2.9 mg/100 g), acacetin 8-C-neohesperidoside (margaritene, 136.2 ± 2.6 mg/100 g), acacetin 6-C-neohesperidoside (isomargaritene, 119.1 ± 1.8 mg/100 g), fortunellin (acacetin 7-O-neohesperidoside, 28.5 ± 0.7 mg/100 g), apigenin 8-C-neohesperidoside (16.9 ± 0.1 mg/100 g), poncirin (isosakuranetin 7-O-neohesperidoside, 5.1 ± 0.1 mg/100 g), and rhoifolin (apigenin 7-O-neohesperidoside, 2.0 ± 0.1 mg/100 g). When immature kumquat was dried at 110 and 130 °C for 0.5 h, the antioxidant activity, total phenolic content and identified flavonoids increased. The UV absorbance of browning products of immature kumquat dried at 130 °C for 1.5 h increased dramatically, while the identified flavonoids decreased. Therefore, it was concluded that drying below 130 °C for 1.0 h, could release phenolic compounds, which resulted in the increasing antioxidant activity. Drying at 130 °C for 1.5 h, it might be due to the effect of formed browning products.