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

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-ß-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.

Soluble and insoluble phenolic compounds and antioxidant activity of immature calamondin affected by solvents and heat treatment.

Hot water extract of immature calamondin peel contains high total phenolic content, which shows significant correlation to DPPH scavenging potency. By heat treatment, the extraction yields of naringin, tangeretin, ferulic acid, p-coumaric acid and gallic acid increased, but the amount of 3',5'-di-C-ß-glucopyranosylphloretin (DGPP) decreased drastically. The major soluble phenolic compounds in the nonpolar extract are nobiletin and tangeretin, while DGPP and hesperidin are in the hot water extract. For insoluble phenolic compounds, ferulic acid, p-coumaric acid and sinapic acid are mainly in ester linkage form. After heat treatment, gallic acid and p-coumaric acid are the major increased soluble and insoluble phenolic acids, respectively. This indicates that high temperature heating (150°C) probably produces two major effects: (1) degradation of flavonoids, such as DGPP and hesperidin; (2) destruction of the cell wall structure, leading to an increase in soluble nobiletin, tangeretin and gallic acid, as well as insoluble ferulic and p-coumaric acids.

Flavonoid compositions and antioxidant activity of calamondin extracts prepared using different solvents.

Calamondin has been demonstrated to exhibit antioxidant function and tyrosinase inhibitory activity, which might be attributed to its flavonoid compounds. To improve their application, the flavonoid compositions and antioxidant activity of calamondin extracts, prepared by different solvents, were investigated. The results showed that total phenolic and flavonoid contents of extracts from peel of calamondin were higher than that from pulp, except the flavonoid content in hot water extract. The flavonoids found in extracts of calamondin were 3',5'-di-C-ß-glucopyranosylphloretin (DGPP), naringin, hesperidin, nobiletin, tangeretin, and diosmin. DGPP exhibited the highest quantity, while naringin and hesperidin were the other two major flavonoids. The content of DGPP in hot water extract of peel was higher than in extracts of organic solvents, however, the contents of nobiletin and tangeretin were found only in extracts of organic solvents. The highest levels of total flavonoids and DGPP were obtained in hot water extract from peel at 90°C. The extracts of hot water and ethyl acetate showed higher 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging potency than that of ethanol and methanol. A positive relationship existed between total phenolic contents and DPPH scavenging potency (p < 0.01), while total flavonoid compositions also showed correlation (p < 0.05). Thus, DGPP, naringin, and hesperidin might contribute to antioxidant activity. Collectively, the hot water extract of calamondin peel might have potential for health food and cosmetic applications due to its good antioxidant activity and high level of DGPP.