Citrus fruits are characteristic in containing high levels of ascorbic acid as well as relatively high levels of certain flavonoids. In their peel, citrus fruits also contain the unique glucaric and galactaric acid conjugates of hydroxycinnamic acids, mainly as feruloyl and p-coumaroyl conjugates at levels of 170-250mgkg-1 in oranges and 3-10 times less in lemons and grapefruits.39,40 It appears that the antioxidant potencies of these particular conjugates have not been systematically tested.
Ascorbic acid is considered one of the major nutrients in citrus fruits, owing to its activity as vitamin C, and it seems plausible that the presence of ascorbic acid may influence the antioxidant potency of citrus products. The ascorbic acid levels in various processed citrus juice products manufactured in Florida (orange juices, grape juices) range from ~300 to 450mgl-1.41
Flavonoids in the edible part of citrus fruits are dominated by hesperidin, which is a compound exhibiting only limited antioxidant and antiradical potency in various assay test systems.23 Hesperidin concentrations in citrus are in the range 5400-5500mgkg-1 dry weight based on analyses of 66 different citrus species.42 When the ABTS^+ radical trapping efficency of orange juice was evaluated in the TEAC assay, the antioxidant activity of orange juice was mainly ascribed to the presence of hesperidin, naringin and narirutin.17 In contrast, neither orange juice, tangerine juice, grapefruit juice nor hesperidin exerted antioxidant activity on human LDL after ex vivo spiking in plasma,43 and although extracts of grapefruit inhibited ascorbate/iron induced in vitro lipid oxidation of human liver micro-somes to the same degree as peach extracts, but less efficiently than plum extracts, they only exhibited very weak antioxidant activity when the same liver microsomes were oxidised by NADPH or when the oxidising substrate was phosphatidylcholine.36
Citrus essential oils, which contain a large number of volatile components, notably high levels of limonone, exert radical scavenging effects against DPPH, where the essential oil of the Korean lemon variety Ichang lemon, Tahiti lime and Eureka lemon were found to be especially strong radical scavengers on DPPH in vitro.44 Individual volatile components of citrus, notably terpinolene, geraniol and gamma-terpinene also exhibited pronounced radical scavenging activities on DPPH.44 However, no clear relationship between specific essential oil constituents or essential oil composition and antiradical scavenging efficiency has been established. Extracts from citrus peel and seeds contain glycosylated fla-vanones and polymethoxylated flavones, especially of naringin, neohesperidin, hesperidin and narirutin, as well as hydroxycinnamates, with the flavanone content in the peels being higher than in the seeds.45,46 In a model system using citronellal as the oxidising substrate, seed extracts of various citrus fruits exhibited greater antioxidant activity than the corresponding extracts of peels, but no clear relationship could be established between antioxidant activity and phenolic composition of these peel and seed extracts.45 Thus, citrus products contain a range of very different types of antioxidant compounds, which are furthermore distributed differently in the separate fruit fractions.
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