Root and tuberous vegetables

Potato tubers (Solanum tuberosum), sweet potatoes (Ipomoea batatas), carrots (Daucus carota) and red beets (Beta vulgaris L.) all contain antioxidant substances, but they are very different types of chemicals. Potatoes contain ascorbic acid and are characterised by high levels of conjugated hydroxycinnamates, present at 500-1200mgkg-1 dry weight, and chlorogenic acid dominates.82 The phenolics are concentrated in the potato skins; red skinned cultivars harbour up to 7gkg-1 of p-coumaryl-anthocyanin conjugates in the peels and around only 25% of this level in the flesh82 and pelargonidin-3-rutinoside-5-glucoside appears to be the dominant anthocyanin compound in red-fleshed potatoes.83 Concentrated aqueous extracts of red and brown potato skins, respectively, contained up to 12.5g kg-1 of hydroxycinnamates, and chlorogenic acid accounted for 60-65 wt% of these, followed by caffeic acid (22-24wt%).84 Ferulic acid and protocatechuic acid are also among the major phenolic acids in potato peels.84

Homogenised potatoes and sweet potatoes only exhibited medium ORAC compared with, for example, kale, garlic, spinach and onions.79 Ethanolic extracts of whole potatoes have been demonstrated to reduce oxidising DPPH radicals and to inhibit linoleic acid oxidation in suspension.85 More concentrated extracts of potato peels efficiently retarded carotene bleaching coupled to linoleic acid oxidation,84 and slowed the oxidation of soybean oil (active oxygen method).86 By 1964 hot water extracts of potato peels had been demonstrated to exert weak antioxidant activity in retarding development of thiobarbituric acid reactive substances when added to beef slices and in slowing the bleaching time of a carotene-lard solution adsorbed onto filter paper.87 A large portion of the antioxidant activity of these extracts was ascribed to the presence of quercetin derivatives, caffeic acid and chlorogenic acid.87,88 This assumption has been corroborated and refined in several later investigations, which attribute most of antiradical scavenging effects and antioxidant activities exerted by potatoes and potato extracts to the presence of chlorogenic, protocatechuic and caffeic acid.85,86 Anthocyanins extracted from the flesh of coloured potatoes also delay oxidation when tested in an aqueous assay using linoleic acid.89

Methanolic extracts of sweet potatoes also exhibit antioxidant activity to retard linoleate oxidation. The phenolics in a methanolic sweet potato extract were identified mainly as caffeoylquinic acids, notably chlorogenic acid, and various 'iso' chlorogenic acids, but the antioxidant activity of this sweet potato extract was not directly related to the phenolic profile, being ascribed as a result of a synergistic action of both phenolic compounds and amino acids.90 Peonidin glucoside, an anthocyanin purified from purple sweet potatoes, also exhibited antioxidant activity on linoleate oxidation.73 Recently, a proteinaceous trypsin inhibitor isolated from sweet potatoes was demonstrated to be able to exert antiradical scavenging activity against the DPPH radical and to capture hydroxyl radicals as measured by electron paramagnetic resonance after addition of picomole levels of the inhibitor; the DPPH scavenging efficiency was about one-third that of glutathione.91 Whether this radical scavenging efficiency may have any relation or quantitative relevance to true antioxidant effectiveness in food or biological systems remains to be investigated.

Carrots are very rich in alpha- and beta-carotenes that range in content from 4000-8700 mg per 100g (alpha) and 7000-16000 mg per 100g (beta) in different orange carrot varieties.92-95 The major phenolic compound in carrots is chloro-genic acid, but dicaffeoylquinic acids, and several other hydroxycinnamic-quinic acid conjugates are also present; in total, the level of conjugated hydroxycinna-mates is about 1.6mgkg-1 and ascorbic acid contents are 30-50 mg kg-1 fresh carrot weight (Table 3.3).92 Carrot blends and extracts only exert very weak antioxidant activities compared to other vegetables,54,80 but extracts of carrot peel and leaves have been shown to inhibit formation of diene hydroperoxides in pure methyl linoleate at 40°C, although the inhibitory activities were <50% of those of potato peel extracts at the same level of addition.54 At the time of writing, no clear relationship between the antioxidant activity of carrots and their contents of carotenoids, ascorbic acid or hydroxycinnamates has been recognised. Methanolic extracts of peels of sugar beet and red beetroot contain the same total level of phenolics (about 4.2 mgg-1 dry weight of starting material) and exhibited strong antioxidant activities in pure methyl linoleate at 40°C, almost blocking oxidation when 500 ppm dry weight base were added.54 Betacyanins, the major colour compounds in red beets, were shown to exert potential anti-oxidant activities in various model systems, including isolated turkey muscle microsomes, human LDL and solubilised linoleate.96 These compounds contain a phenolic and a cyclic amine group, where the structure of the latter resembles that of ethoxyquin, a strong antioxidant permitted for use in feeds in the USA (but not in Europe).

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