Antioxidants in fruits, berries and vegetables
3.3 Stone fruits
a tendency for contents to be higher in white flesh peach cultivars than in yellow flesh cultivars.3,27Plums contain higher levels of epicatechin than catechin, with the total levels of these flavanol diastereoisomers being 5–50 mg kg-1fresh weight of whole plums.28 More recent values for the total amount of flavanols, which include procyanidins, in the plum flesh alone are in the range 140–600 mg kg-1 fresh weight (Table 3.1).3The higher values obtained in the more recent investi-gations3may be a result both of a more comprehensive extraction technique as well as use of better analytical methodology.
Cherries, both sweet and sour, appear to be richer in anthocyanins as well as in hydroxycinnamic acid derivatives than peaches, nectarines and plums. Sweet cher-ries contain up to 3500–4500 mg kg-1 of anthocyanins, 100–1900 mg kg-1 hydroxycinnamates as caffeic acid and p-coumaric acid derivatives, where especially 3¢-p-coumaryl quinic acid can account for 50–75% and neochlorogenic acid for 15–60% of the hydroxycinnamates, depending on the variety.28–30Sour cherries have been shown to harbour higher levels of flavan-3-ols than sweet cherries, mainly epicatechin and catechin: estimates of total contents are in the range 70–170 mg kg-1 for sour cherries in contrast to 20–60 mg kg-1 in sweet cherries (Table 3.1).28–31Individually these compounds exhibit strong antiradical activity, for example in the DPPH in vitro assay, when evaluated at different micromolar concentrations.32The flavanols are also effective inhibitors of human LDL oxidation in vitro.33Catechin especially is one of the individual phenolic constituents that are present at high levels in red wines and have high antioxidant activity in inhibiting oxidation of human LDL in vitro.15A number of flavanone, flavone and flavonol compounds as well as a chlorogenic acid methyl ester and some novel cinnamoyl derivatives – namely the cyclopenta-2,3 and -2,5-diols of caffeic acid – have been purified from sour cherries, varieties ‘Balaton’ and
‘Montmorency’.34In an antioxidant assay using phosphatidyl choline liposomes as the oxidising substrate, these novel sour cherry compounds exhibited antioxi-dant activities that were comparable to the activities of TBHQ, BHT and caffeic acid.34Knowledge on their quantitative occurrence in cherries is scarce, however.
The extracts of two different varieties of sweet cherries were superior to various berry extracts (blueberries, raspberries, blackberries and strawberries) in inhibiting lipid oxidation in an in vitro phosphatidyl lecitine model system; in contrast, the relative antioxidant activities of the same cherry extracts on human LDL oxidation in vitro were lower than that of blackberries and raspberries, but higher than that of blueberries and strawberries when evaluated at the same micromolar concentration of 10mM total phenols.29The antioxidant activities of phenolic extracts of berries against lecithin liposomes were significantly posi-tively correlated to the content of hydroxycinnamates, but the amount of flavanols correlated to the antioxidant potency of extracts of berries in neither the in vitro LDL oxidation systems nor in the lecithin liposome assay.29 Extracts of sweet cherries were found to be the best among a large number of other fruits in inhibit-ing oxidation in vitro of a pool of LDL+ VLDL; sweet cherries had an IC50(i.e.
the amount required to inhibit oxidation by 50%) of only 0.10mM total phenols, while red grapes ranked second with IC50= 0.27 mM.35Nectarines, peaches and
plums were less potent and were ranked as numbers 14, 15 and 10, respectively, on this antioxidant potency scale.35
At a concentration of 10mM total phenols as gallic acid equivalents, extracts of whole clingstone peach cultivars were demonstrated to inhibit human LDL oxidation in vitro by 44–84% depending on the cultivar.27Also extracts of the flesh and skins of the peaches exhibited antioxidant activity against LDL oxida-tion in vitro. Extracts of peach peels contained more total phenols ranging from 910–1920 mg kg-1 as gallic acid equivalents than the extracts from flesh, where phenols levels were in the range 430–770 mg kg-1: Chang et al.27found a statis-tically significant linear correlation between relative antioxidant activity and con-centration of total phenols of peach extracts of 0.76. Thus, the relative antioxidant activity of peel extracts was better than the extracts of whole peach and peach flesh extracts, even though the percentage inhibition at 10mM was in a similar range for all types of peach extracts. The results signified that the antioxidant activity was widely distributed among the extracted peach phenolics. In peaches, the anthocyanins are mainly confined to the peel tissue.3,27When the antioxidant activity of clingstone peach extracts was evaluated against LDL oxidation in vitro, however, a significant correlation between antioxidant activity and anthocyanins could not be established. In contrast, a strong correlation, r = 0.96, was found between the percentage relative inhibitory activity and redness of whole peach extracts when colour was measured on the Hunter scale.27
Plums contain high levels of hydroxycinnamic acids (Table 3.1), notably neochlorogenic and chlorogenic acids, with neochlorogenic acid as the dominant compound with content levels in the range 500–770 mg kg-1fresh weight.28 Indi-vidually, these compounds exert potent antioxidant activity on human LDL oxi-dation in vitro and have been shown to inhibit totally the LDL oxioxi-dation in vitro at addition levels of 10mM in total phenols.26Plum extracts tested in vitro were better inhibitors of lipid oxidation in human liver microsomes and phosphatidyl choline than peach, apple, grapefruit and pear extracts.36Analyses of methano-lic extracts of freshly harvested, unprocessed prune plums, cultivar La petite d’Agen, showed the mean concentration of phenolics to be about 1100 mg kg-1 fresh weight, where neochlorogenic acid constituted 73 wt% of the phenols (807 mg kg-1) and chlorogenic acid was 13 wt% (144 mg kg-1); only low amounts of 3¢-coumarylquinic acid (10 mg kg-1) were detected.37The level of anthocyanins in these plums were 76 mg kg-1, while there was 54 mg kg-1 catechin and 27 mg kg-1 of other flavonols, mainly rutin.37In a study where five Californian plum cultivars were analysed for their phenolic content, high levels of anthocy-anins, about 1600 mg kg-1fresh weight, were found in the skin of the blue plum cultivar ‘Angeleno’. The ‘Angeleno’ anthocyanins were dominated by cyanidin 3-glucoside (about 1040 mg kg-1) and cyanidin-3-rutinoside (560 mg kg-1). Other red and blue plum varieties also contained mainly these two anthocyanin gluco-sides in their skin, but at lower levels, in the range 130–700 mg kg-1. In all the red and blue plum varieties evaluated, only low levels of anthocyanins were detected in the flesh part.3 In pitted prunes, anthocyanins and catechin were absent, and hydroxycinnamates – dominated by neochlorogenic acid – made up
98% by weight of the phenolic material, where the mean concentrations of phenols were 1840 mg kg-1.37Extracts of prunes as well as of prune juice were shown to inhibit the copper catalysed oxidation of lipids in human LDL signifi-cantly at 5–10mM test levels with the prune extract exerting higher antioxidant activity than the prune juice.37ORAC measurements evaluated on a per 100 gram weight basis ranked the ‘antioxidant power’ of dried plums, that is prunes, the highest among a range of other fruits. Thus, by this measurement, the dried plums exerted an antioxidant score of 5770, while fresh plums scored 949;38however, part of the increase could be due to the greater dry matter content in dried plums compared to fresh plums.