Antioxidants from vegetables overview

The antioxidants present in commonly consumed vegetables include ascorbic acid, tocopherols, carotenoids and phenolic compounds such as flavonols and phenolic acids (Table 3.3). In comparison to fruits and berries, vegetables generally contain much lower amounts of antioxidant compounds. A large amount of vitamin C is found in sweet red pepper (1850mgkg-1) and significant amounts in Brussels sprouts (up to 900mgkg-1) and broccoli (750-830 mg kg-1), while

Table 3.3 Antioxidant compounds in selected vegetables and their products, mgkg fresh weight

Vegetable

Flavonols

Hydroxy-

Carotenoids

Vitamin C

Vitamin E

(quercetin)

cinnamates

(beta-carotene)

Broccoli

15-6571-72-134

62-14872

4-2770,94

750-83068,121

7121

- boiled

640117

7117

Brussels sprouts

0-671

4.394

900117

4117

Carrots

11-77070,94

60117

4117

- boiled

101117

42117

4117

Onions

3 40-42071,105

0.194

75117

0.4117

- blanched

210-290105

- fried

220-370105

0.2117

57117

8117

Pea

1.4-1.6105

3.694

200117

2117

- boiled

0.8-1.0105

3.6117

- fried

1.3-2.0105

Potatoes

14076

0.194

100117

1117

- boiled

100117

1117

Spinach

tr71

8-24070

600117

12117

Tomatoes

2-1471

0.2-62394*

140117

7117

- juice

1371

140117

7117

- ketchup

9994 *

80117

723117

Sweet red pepper

1.2-3370

1850117

* Lycopene.

the amounts of vitamin E are generally below 10mgkg-1 in vegetables. According to Hussein et al.68 although there was significant loss in vitamin C during storage of broccoli and green peppers, in most cases there was no difference in loss of vitamin C or beta-carotene between the processed and unprocessed vegetables, and the packaging systems. After storage, artichokes stored at different temperatures showed a decrease of about 40% in the vitamin C content which was most likely to have been associated with the ability of the polyphenol oxidase to catalyse the oxidation of ascorbic acid.69 Carotenoids contribute to antioxidant activity, with beta-carotene (1-644mgkg-1) and lutein (up to 203 mgkg-1 in spinach70) present in all vegetables and lycopene dominating in tomatoes (0.2-623mgkg-1) and tomato products (Table 3.3). As a result of food processing involving heat treatment carotenoids undergo isomerisation70 which may decrease their antioxidant activity. On the other hand thermal processing is reported to increase carotenoid concentration, perhaps owing to greater extractability, enzymatic degradation and unaccounted losses of moisture and soluble solids.20

In fresh vegetables only glycosylated flavonols and other flavonoids are present but aglycones may be found as a result of food processing.71 Quercetin levels in vegetables are generally below 10 mgkg-1, except for onions (340-347mgkg-1), kale (110-120mgkg-1) and broccoli (30-166mgkg-1), while kaempherol has only been detected in kale (210-470 mg kg-1), endive (15-90mgkg-1), broccoli (60mgkg-1) and leek (10-60mgkg-1).71,72 The content of other flavonoids in vegetables is very low with some exceptions such as flavanones in celery leaves (apigenin, 750mgkg-1)71 or anthocyanins in purple sweet potatoes.73 In general, flavonol levels in processed foods are lower than in fresh products.74 Crozier et al.75 studied the effect of cooking on the quercetin content of onions and tomatoes. With both vegetables, boiling reduced the quercetin content by 80%, microwave cooking by 65% and frying by 30%. All vegetables contain phenolic acids such as hydroxycinnamates where either caffeic acid, ferulic acid, sinapic acid or coumaric acid has been conjugated with quinic acid and/or esterified with for example sugars.76,77 According to Clifford76 commercial varieties of American potato may contain up to 1400 mgkg-1 dry weight caffeoylquinic acids. In broccoli several hydroxycinnamic acid esters have been isolated in amounts of 62-148 mgkg-1.78

Vegetable extracts such as root and tuberous crops (carrots, potatoes, sweet potatoes, red beets etc.), cruciferous vegetables (cabbage, Brussels sprouts, broccoli etc.), green leafy vegetables (lettuce, spinach etc.), onions, tomatoes and other vegetables have been screened for antioxidant activity using different oxidation systems and methods to measure antioxidant activity. Cao et al.79 reported that the antioxidant score of vegetables measured by ORAC assay decreased in the following order: kale > garlic > spinach > Brussels sprouts > alfalfa sprouts

> broccoli flowers > beets > red bell pepper > onion > corn > eggplant > cauliflower > potato > sweet potato > cabbage > leaf lettuce > string bean > carrot > yellow squash > iceberg lettuce > celery > cucumber.

Results on spiking plasma with vegetable extracts showed that beans, garlic, onions, asparagus, beet, potato and broccoli ranked highest in inhibiting the oxidation of the LDL and VLDL fractions.80 On oxidation of pure methyl linoleate at 40°C, the antioxidant activity was the following: pea, legume > cucumber, leaf

> pea > onion > carrot.54 Compared to the poor activity (10-37% inhibition) of these vegetables in inhibiting lipid oxidation, the peel extracts of beetroot, sugar beet and potato showed remarkable antioxidant activity ranging from 86 to 99% inhibition. By measuring the ORAC, Gazzani et al.81 reported that when prepared at 2°C, most vegetable juices showed initial pro-oxidant activity. This pro-oxidant activity was very high for eggplant, tomato and yellow bell pepper. In general the antioxidant activity increased after heat treatment suggesting that the pro-oxidant activity is due to peroxidases which are inactivated at high temperature during food processing.

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