Phenolic content, antioxidant activity and effective compounds of kumquat extracted by different solvents

• Positive correlation existed between antioxidant activity and phenolic content.
• Most flavonoids in hot water extract of kumquat were C-glycoside conjugated.
• 3′,5′-Di-C-β-glucopyranosylphloretin is the major antioxidant flavonoid in kumquat.

The total phenolic and flavonoid content of extracts from peel of kumquat were higher than those from pulp, and those extracted from immature kumquat were higher than those from mature kumquat. The highest levels of phenolic and flavonoid content were obtained in hot water extracts. The flavonoids of kumquat extracted from hot water were mainly soluble conjugated compounds, including C-glycosides, such as 3′,5′-di-C-β-glucopyranosylphloretin (DGPP), acacetin 8-C-neohesperidoside (margaritene), acacetin 6-C-neohesperidoside (isomargaritene), apigenin 8-C-neohesperidoside, and O-glycosides, such as acacetin 7-O-neohesperidoside (fortunellin), isosakuranetin 7-O-neohesperidoside (poncirin) and apigenin 7-O-neohesperidoside (rhoifolin). A positive relationship existed between total phenolic content and DPPH scavenging potency (p < 0.001). Total flavonoid content showed a similar correlation (p < 0.001) to DPPH scavenging potency. The effective flavonoids contributing to antioxidant activity were DGPP and apigenin 8-C-neohesperidoside, which could be extracted in high amounts, by hot water at 90 °C, from immature kumquat peel.