Broccoli (Brassica olearacea L. cv Italica L.), Brussels sprouts (B. olearacea L. Gemmifera), red cabbage (B. olearacea L. cv Rubra), white cabbage (B. olearacea L. cv Alba) and cauliflower (B. olearacea L. cv Botrytis) have been reported to show significant antioxidant properties against lipid peroxidation.97 Phenolic compounds such as flavonols and hydroxycinnamic acids in the cruciferous vegetables may be responsible for the antioxidant activity rather than the main bioactive compounds in cruficers, namely glucosinolates.98,99 According to Plumb et al.78 purified glucosinolates exhibited only weak antioxidant properties and thus are unlikely to account for the antioxidant effects of extracts from cruciferous vegetables. Compared to other vegetables and cauliflower, kale (B. olearacea L. cv Acephala), Brussels sprouts and broccoli were found to exert higher antioxi-dant activity.70,80,97,100 White cabbage was reported to show more than 80% inhibition of coupled oxidation of beta-carotene and linoleic acid81 and it was also an active hydroxyl radical scavenger.98 However, while measuring lipid peroxidation in microsomes containing specific cytochrome P450s cabbage, cauliflower and Brussels sprouts were found to be pro-oxidants.72 Food processing involving heat treatment seems to have different effects on various cruciferous vegetables depending on the choice of the antioxidant activity measurement. Boiled (15 min) Brussels sprouts were found to promote peroxidation of human liver microsomes and of phospholipid liposomes,98 while boiled (5 min) broccoli exhibited 96% inhibition of oxidation of beta-carotene linoleic acid emulsion100 and boiling for up to 30min improved the antioxidant activity of white cabbage.81
Onion (Allium cepa) has been studied for antioxidant activity both in lipid oxidation models54'81'87,88'97,10^102 and in radical scavenging assays.79,80 Both yellow and red onion were shown to be poor antioxidants towards oxidation of methyl linoleate,54 moderately active towards coupled oxidation of beta-carotene and linoleic acid102 and highly active towards oxidation of lower density lipopro-teins.81 Onion had also a poor antioxidant score in the ORAC activity test while garlic (Allium sativum L.) expressed a score four times higher.79 Yin and Chen103 reported that the presence of garlic bulb, garlic greens, Chinese leek, scallion, onion bulb and shallot bulb significantly delayed lipid oxidation of phosphatidylcholine liposomes. While a thiosulphinate, allicin, is responsible for the antioxidant activity of garlic bulb,104 compounds other than allicin are involved in determininig the antioxidant effect of other Allium members.
Makris and Rossiter101 assessed the impact of domestic processing, including chopping, maceration and boiling on onion bulbs. While quercetin 3,4'-diglucoside and quercetin-4'-monoglucoside were virtually unaffected by chopping, boiling for 60 minutes caused overall flavonol losses of 20.6% in the onions. In contrast, Ewald et al.105 reported that the greatest loss of quercetin and kaempherol in onion took place during the peeling, trimming and chopping before blanching. Further processing by cooking, frying and warm-holding of blanched onion had only a small effect on flavonoid content. Chopping did not considerably influence the antioxidant capacity of onion bulbs, but boiling did provoke notable changes measured by the coupled oxidation system of beta-carotene and linoleic acid.101 Boiling of juiced onion for 10min resulted in pro-oxidant activity that was reversed into antioxidant activity with prolonged heat treatment.81 On the other hand, incubation of pulped onion at 37°C resulted in improved antioxidant activity partly caused by the enzymatic (endogenous glycosidases and gly-cosyltransferases) conversion of quercetin diglucosides into the monoglucoside and aglycone forms.102 After six hours of incubation, 75% of the total quercetin existed in the aglycone form.106 It was suggested that the increment of anti-oxidant activity through enzymes naturally present in vegetables could be used to replace food antioxidants.
The antioxidant activity of green leafy vegetables such as spinach (Spinacia olearacea L.) has been reported to be low against inhibition of oxidation of LDL80 and moderate towards oxidation of linoleic acid.97 On the other hand spinach expressed a very high ORAC activity while that of leaf lettuce and iceberg lettuce was poor.79 According to Beom et al.,106 blending spinach with other vegetables resulted in increased antioxidant activity in iron-catalysed model systems. Differently processed, that is, minced or enzymatically juiced spinach samples, were found to inhibit formation of lipid hydroperoxides but to act as pro-oxidants in cooked meat.107 The antioxidant activity of spinach decreased during storage after modified atmosphere packaging (MAP) which could be due to decrease in the ascorbic acid content.108 The authors also reported a 50% loss of total flavonoids and 60% loss of vitamin C in the cooking water while boiling spinach. However, the vitamin C content of the cooked tissue was higher than in spinach stored in MAP.
Tomato (Lycopersicon esculentum) was reported to exert antioxidant activity in some studies86,88 while in other experiments it showed no antioxidant activity109 or acted as pro-oxidant.85 In beef homogenates, tomato significantly inhibited lipid peroxidation.81 The antioxidant effect of tomato is most likely to result from synergism between several phytochemical compounds and it is not due to lycopene content alone as pure lycopene and several other carotenoids act as pro-oxidants in the lipid environment.100,110,111 In a study by Wenli et al.,n2 lycopene concentrate extracted from tomato paste containing 50% lycopene and 50% other lipid-soluble substances (probably including tocopherols) was shown to scavenge oxygen radicals effectively and to inhibit lipid peroxidation. Lycopene in tomatoes seems to be more stable compared to other carotenoids to changes during peeling and juicing of vegetables.113 Among commercial juices tested, tomato juice has a higher oxygen radical absorbance capacity than orange juice and apple juice.52 According to Anese et al.,114 antioxidant activity of tomato juice decreased after an initial 2-5 hours of heating but was restored after prolonged heating. Gazzani et al.81 report that while boiled vegetable juices were generally found to exert antioxidant activity, tomato juice was pro-oxidant. These contradictory findings may be explained by differences in the amounts of the antioxidant compounds in the tomato juices because Gazzani et al.81 used a filtration method resulting in loss of most of the juice coloration. Apart from lycopene, another interesting antioxidant compound, naringenin chalcone, is present in tomato skin (64mgkg-1) and may be present in juice, paste and ketchup.55 In tomato processing to ketchup, naringenin chalcone is transformed to naringenin.
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