Antioxidants in fruits, berries and vegetables
3.2 Antioxidants from fruits and berries: overview
Fruits and berries are good sources of antioxidants, including carotenoids, ascor-bic acid, tocopherols, flavonoids and phenolic acids. It has been known for a long time that the phenolics, as well as some of the other antioxidant components, are closely associated with the sensory attributes of fresh and processed fruits, berries and other plant foods. Especially, the contribution to colour by carotenoids (yellow to orange and red) and anthocyanins (red to purple and blue) is well known. Also the specific involvement of some of the phenolic substances in flavour development and taste sensation is amply documented.1Phenolic com-pounds, including those having potent antioxidant activity, are also substrates for undesirable, oxidative browning reactions occurring during bruising of fruits, when fruits are cut or during their processing.
The possible beneficial biological functions of the traditional antioxidant vitamins, i.e. ascorbic acid, a-tocopherol and to a certain extent beta-carotene (provitamin A) have been intensively studied for at least 50 years and continue to receive high research attention. More recently, the antioxidant functions of flavonoids and other phenolic compounds have received increased attention. The biological roles of these plant phenolics that exert antioxidant activity are yet to be completely clarified, but evidence pointing to the possibility that phenolic phytochemicals also exert various protective effects in humans is accumulating.
Because of the possible benefits of phenolic phytochemicals to human health, data on their quantitative occurrence and composition in various fruits and berries are gradually emerging in the literature. Therefore it is now known that flavonoids and other phenolic compounds are particularly abundant in fruits and berries. However, as it is generally recognised in relation to compilation of food compositional data, there are large variations in the levels of the constituents reported, depending on the species investigated, harvest time, fruit maturity stage, geographical origin etc.
Differences in the methods employed for extraction and analyses also strongly affect the results and there are thus some inconsistencies in the available data, or very broad ranges for the levels of certain constituents in various fruits.
Some studies have evaluated the phenolic contents in fruits at more than one ripening stage. In the case of plums as well as with red grapes intended for wine making, a marked increase in the content of phenolics of potential antioxidant potency was seen in the fully ripe stage in comparison with the less ripe stage.2,3 In contrast, no clear differences were observed in other fruits, e.g. peaches and
nectarines,3so it appears that there is no general rule correlating phenolic content and antioxidant potency with the fruit ripening stage.
Antioxidant composition (anthocyanins, flavanols and proanthocyanidins, flavonols, hydroxycinnamates, carotenoids, vitamin C and vitamin E) of selected, commonly consumed fruits and berries is presented in Table 3.1. Large amounts of anthocyanins (up to 8100 mg kg-1) are found in the strongly coloured fruits and berries including bilberries (wild clone of blueberries), blackcurrants, cherries, cranberries, red grapes and raspberries. The amount of flavanols is generally below 150 mg kg-1 with larger amounts found in blackcurrants, cranberries, red wine grapes, peaches, plums and red raspberries. Apart from a few exceptions such as cranberries and red grapes, fruits and berries are generally also low in flavonols and high in phenolic acids such as hydroxycinnamates. Large amounts of hydroxycinnamates are present in cherries (300–1930 mg kg-1), plums (121–
896 mg kg-1) and peaches (81–750 mg kg-1). High molecular weight phenolics, tannins, are also found in fruits and berries with large amounts of ellagitannins in red raspberries (2200 mg kg-1) and cloudberries (1800–2600 mg kg-1) and moderate amounts in strawberries (90–200 mg kg-1).4 The vitamin C content of fresh fruits and berries is generally high while that of provitamin A caro-tenoids and vitamin E is low. Blackcurrants (1200–1500 mg kg-1), cloudberries (1000 mg kg-1), strawberries (550–1000 mg kg-1) and orange (510 mg kg-1) are very rich in vitamin C. One exceptional berry is sea buckthorn berry with extremely large amounts of vitamin C (2000 mg kg-1) as well as high amounts of beta-carotene (15 mg kg-1) and vitamin E (32 mg kg-1).
Food processing of fruits and berries into juices and jams, and drying of fruits generally result in lower amounts of antioxidant compounds. For example, losses of anthocyanins in juices and purées of strawberries, strawberry and blackcurrant syrups, cranberry juice, raspberry juice and wine have been reported5–9as well as phenolic degradation during processing of apple juice.10On the other hand, the manufacturing process had no effect on the qualitative anthocyanin profile of commercial jams made from strawberries, blackberries, raspberries, blueberries, blackcurrants and cherries.11
In domestic berry processing practices, a quercetin loss of 15% was observed in strawberry jam, 85% loss in blackcurrant juice, 40% loss in bilberry soup and 85% loss in lingonberry juice in their making procedures.12Flavanols are effec-tively extracted into apple cider, blackcurrant juice and red wine, the amounts being higher than those of the raw materials.13–17An increase in ellagic acid in raspberry jams was reported to occur, most likely owing to release of ellagic acid from ellagitannins with the thermal treatment,18although according to Häkkinen et al.19 ellagic acid content in strawberry jam was 80% that of unprocessed strawberries.
As for other antioxidant compounds, peeling and juicing result in substantial losses of provitamin A carotenoids, often surpassing those associated with heat treatment.20Moreover, the stability of carotenoids differs in different foods even when the same processing conditions are used. Ascorbic acid of fruit juices such as orange, peach, grapefruit, pineapple, apple and mango juice is readily oxidised
Fruit and vegetable processing Fruit or berry Anthocyanins Flavanols and Flavonols Hydroxycinnamates Carotenoids Vitamin C Vitamin E
proanthocyanidins (b-carotene)
Apple 4–54 0–15116 17–704,74 263–3084 0.494 40117 2117
– juice 0–298118 2.5119 0.1–16210,17,120 0.2117 300117 0117
Bilberry 3450–46354 13–294 41–1954,12 170–3474 0.594 150117 19117
– soup 612 0.0194 20117 5117
Blueberry 3970–484029 63–7029 115–13929 226–31529
Blackcurrant 130–81004,62 205–3744 133–1574,12 104–1674 194 1200117 23117
– juice 2417 3612 0.1117 380–42112,117 11117
Cherry, sweet, red 31–450022–31 20–6328–31 10–2329 100–190028–31 1.294 70121 1121
Cloudberry 7–154 2–64 34–904 90–1284 1.494 1000117 31117
Cranberry 460–17204,6,122,123 2854 139–334122–124 1914 0.294 120121 10121
– juice 18–512124
Grapes, table, red 72.5–765125 1–160118 13–25125 5–19125 0.3117 50117 7117
– wine, red 0.6–38513,15 0–500118 10–5513,15 4–1313,15 tr121 0121 0121
Grapes, table, white 0125 0125 10–13.574,125 5.5125 0.3117 50117 7117
– wine, white 015 0–10615 1–3415 tr121 0121 0121
Orange 0–570 136–163126 0–570,74 510117 4117
– juice 0.194 300–45041 2117
Peach 0–17.827 24.5–7003,27 0–11.927 54–14827 0.994 80121 10121
– canned 0127 tr127 tr127 11–29127 1.094 20117 20117
Plum 19–7631,37 140–6003 5.7–2737 500–90023,37 4.3121 54121 8.6121
– dried (prune) 037 037 4237 180037 1.494 0117 18121
Raspberry, red 200–22004,29 4–4804,29 6–394,29,128 3–3512,29 0.194 296–38012,117 11117
Sea buckthorn 15121 2000121 32121
berry
Strawberry 202–7904,29 9–1844,29 7–1744,29,128 14–694,29 0.194 420–600117,1286117
Strawberry jam 4–2211 11.412 0.04117 80–23612,117 1.0117
and lost during staying of the juices with losses ranging from 29 to 41% when stored at room temperature for four months.21 Kalt et al.22 found a marked difference in the stability of ascorbate in green leafy vegetables when compared with fruits. For example, in spinach more than 90% of the ascorbate was lost within three days after harvest when stored at ambient temperature while losses in ascorbate during storage of blueberries, raspberries and strawberries were minimal.22
Antioxidant activity of fruits and berries and their products reported in many studies varies widely and this is partly due to the use of different oxidation systems and methods to analyse antioxidant compounds. For antioxidant testing, either extracts or juices of fruits and berries have been used resulting in differ-ent antioxidant compositions owing to choice of extraction solvdiffer-ents (e.g. either water-soluble or lipid-soluble compounds extracted by one method) or use of fil-tration (e.g. possible losses of antioxidant compounds). The literature has much focused on the antioxidant effects of flavonoids and phenolic acids although ascorbic acid, carotenoids and tocopherols also contribute to the antioxidant activity of fruits and berries. Many of the flavonoids and phenolic acids exert comparable or better radical scavenging activity than vitamin C and E in radical scavenging activity assays.23
It is beyond the scope of the present treatise to discuss the problem that arises because the absolute and relative efficiencies of many natural antioxidants vary depending on the test method employed, and especially that the free-radical trapping methods (DPPH, ORAC, TEAC and TRAP assays) may not always mimic the complex multifunctional antioxidant mechanisms of natural anti-oxidants. It is important to note, however, that relevant antioxidant mechanisms of natural antioxidants and polyphenols such as metal chelation, inhibition of oxidative enzymes etc are overlooked in many of the currently employed rapid free-radical antioxidant test assays. In addition, the possible influences of factors such as antioxidant solubility, partitioning, ionic charge, complexing/interaction with other compounds, type of initiation, pH of the system and so on are not considered in simple radical scavenging tests (discussed in detail by Frankel and Meyer).24Glycosides of polyphenols have often been found to be less active as antioxidants compared to the corresponding aglycones in radical scavenging tests.21 However, this may be an artefact as with more realistic substrates, for example in LDL and phospholecithin liposomes in vitro assays, the glycoside/aglycone issue appears to be more complex. Thus, when evaluated at the same micromolar addition level on copper-catalysed LDL in vitro, rutin exerted better antioxidant potency than quercetin. Likewise, chlorogenic (5¢-caffeoylquinic) acid was better than caffeic acid on human LDL oxidation in vitro, when oxidation was induced with AAPH,25 while no significant differ-ences between antioxidant potency of these two compounds could be established when the LDL oxidation was induced with copper ions.26Some of the differences in results obtained in different in vitro antioxidant tests with anti-oxidant compounds present in fruits and berries are summarised and exemplified in Table 3.2.