Bioactive Compounds as Important Antioxidants

Many of the bioactive compounds are strong antioxidants, i.e. they slow or prevent the oxidation of other chemicals. Oxidation reactions can involve the production of free radicals, which can form dangerous chain reactions. Bioactive compounds from agro-residues vary widely in chemical structure and function and are grouped accordingly (Kris-Etherton et al. 2002). Important bioactive compounds from agroresidues include phytochemicals, viz. phenolic compounds, carotenoid and tocopherols. Table 11.2 shows the important bioactive compounds present in agro-residues and their mode of action.

Phytochemicals

They are pronounced as “fight-o-chemicals”, i.e. they fight to protect health. Phytochemicals naturally occur in fruit and vegetables that work together with vitamins, minerals and fibre to promote health benefits in many ways. Phytochemicals that are present in the diet and have been associated with health benefits include

Table 11.2 Important bioactive compounds present in agro-processing wastes and their mode of action

Fig. 11.1 The general breakdown of plant based phenols

glucosinolates, sulphur-containing compounds of the alliaceae, terpenoids (carotenoids, monoterpenes) and various groups of polyphenols (anthocyanins, flavones, flavan-3-ols, isoflavones, stilbenoids, ellagic acid, etc.). The bioactivity of these phytochemicals has been, to some extent, associated to their antioxidant properties, i.e. capacity to scavenge free radicals which are involved in the onset of many of chronic degenerative diseases (LDL oxidation in atheroma plaque development, DNA oxidation and cancer, oxidation and ageing, inflammation, etc.).

11.5.1.1 Phenolic Compounds

Polyphenols are secondary plant metabolites that are derived through pentose phosphate, shikimate and phenylpropanoid pathways. Phenolic compounds are found predominantly in the by-products than in the edible portions as they tend to accumulate in the dermal tissues of plant body because of their potential role in the protection against UV rays, as attractants in fruit dispersal and also as defence chemicals against pathogens. Phenolics may act as phytoalexins (Popa et al. 2008), antifeedants, attractants for pollinators, contributors to plant pigmentation, antioxidants and protective agents against UV light (Naczk and Shahidi 2006). Polyphenolic compounds are divided into several classes based on their structural diversity (Fig. 11.1). Of these, flavonoids, phenolic acids and tannins (hydrolyzable and condensed) are regarded as the main dietary phenolic compounds (D'archivio et al. 2007).

The antioxidant activity of a phenolic compound is directly proportional to its chemical structure. Following relationships between chemical structure and antioxidant activity of phenolic compounds have been encountered:

1. The antioxidant potential of phenolics depends on the number and arrangement of the hydroxyl groups (Sang et al. 2002). For phenolic compounds, the more OH groups there are in the ring, the larger is the TEAC (trolox equivalent antioxidant capacity) value (Villano et al. 2005).

2. The contribution of the 3-OH group in flavonoids is very significant (Heijnen et al. 2001). Blocking the 3-hydroxyl group in the B ring (i.e. rutin) decreases the antioxidant activity (Villano et al. 2005).

3. The presence of an ortho-dihydroxy substitution in the B ring confers higher stability to the radical structure and participates in electron delocalization and plays an important role in the antioxidant activity (Yao et al. 2004).

Phenolics have been considered powerful antioxidants in vitro (Frankel et al. 1995) and have been proved to be more potent antioxidants than vitamins E, C and carotenoids (Rice-Evans et al. 1997). The inverse relationship between fruit, vegetable intake and risk of cardiovascular and neurodegenerative diseases, cancer, diabetes and osteoporosis has partially been ascribed to phenolics (Scalbert et al. 2005a, b). Their antioxidant activity lies in their ability to donate a hydrogen or electron and their ability to delocalize the unpaired electron within the aromatic structure. They can also protect biological molecules against oxidation.

11.5.1.2 Carotenoids

Carotenoids are a diverse group of over >600 different compounds that contribute to the yellow to red colours found in many foods. Carotenoids are polyenes consisting of 3–13 conjugated double bonds and up to six carbon ring structures at one or both ends of the molecule. Carotenoids containing oxygen are known as xanthophylls (e.g. lutein and zeaxanthin) while those without oxygen are known as carotenes (e.g. lycopene and β-carotene). The carotenoids have several potential health benefits. Lutein and zeaxanthin which are found in high concentrations in the human eye have been postulated to be beneficial to age-related macular degeneration and cataracts (Stringham and Hammond 2005). Although carotenoids are generally thought to be beneficial for health, clinical trials have found that large doses of β-carotene increase the risk of lung cancer (Bendich 2004). In general, carotenoids are not strong antioxidants when added to food but relatively unstable in food systems because they are susceptible to light, oxygen and auto-oxidation (Xianquan et al. 2005).

11.5.1.3 Ascorbates

Ascorbic acid (vitamin C) is a commonly used antioxidant in many food systems for maintaining organoleptic quality. In its natural forms, ascorbic acid (e.g. L-ascorbic acid and D-isoascorbic acid) functions both as a reducing agent and as an oxygen scavenger. In addition, the metal-sequestering activity of ascorbic acid, which forms metal-ascorbate complexes that are less reactive with oxygen than with metal ions alone, provides antioxidant activity (Martell 1982). The oxygen-scavenging activity of ascorbic acid is effective in trapping both singlet oxygen and a superoxide anion, thus producing ascorbate free radicals (Zhang and Fung 1994). Ascorbate radicals so produced also react with peroxyl radicals to produce hydroperoxides (nonradical species) and the oxidized form of ascorbic acid, namely, dehydroascorbic acid. The conversion of native ascorbic acid to a salt form increases its stability and versatility in different food systems at the expense of biological activity. In muscle systems, ascorbic acid has been used to delay the formation of metmyoglobin in fresh meat products; it has also been used to prevent enzymatic browning in fresh fruits and vegetables. However, L-ascorbic acid has been shown to be ineffective as an antioxidant in poultry meat (King et al. 1995). The addition of ascorbates and sodium citrate to milk also provides protection against the loss of the lipid-soluble vitamins A and D. In clinical studies, ascorbic acid has been shown to prolong the survival of patients that have terminal cancers (Cameron and Pauling 1978). Vitamin C is one of the most popular and least toxic antioxidant of foods and has been widely used as a dietary supplement to prevent oxidative stress-mediated diseases (Gardner et al. 2002).

11.5.1.4 Tocopherols

Several lines of evidence indicate that α-tocopherol is the most effective phenolic antioxidant for reducing lipid peroxidation. The hydrophobic character of α-tocopherol enables it to be a strong antioxidant in lipid systems, where it functions as a radical scavenger and terminates the propagation of radical chain reactions by reacting with peroxyl radicals and generating unreactive phenoxyl radicals and hydroperoxide products. Dietary supplementation with vitamin E increases the plasma tocopherol concentration and the potential for associated antioxidative protection. Vitamin E is transported in plasma lipoproteins, where it can exert a positive role against peroxidative damage. In the dynamic scheme of antioxidation reactions, one mole of α-tocopherol reacts with two lipid-peroxyl radicals, yielding 14 lipid hydroperoxides and seven oxidized tocopherol molecules. Numerous studies support the contention that vitamin E is involved in antioxidant defence and chronic disease. For example, vitamin E has been shown to delay the oxidation of lowdensity lipoproteins (LDL), implicated to be an early step in the development of atherosclerosis. Other clinical studies have reported that α-tocopherol, although important, is not the sole factor that determines the resistance of LDL to oxidative stress (Maiorino et al. 1995). The antioxidant activity of α-tocopherol has also been shown to reduce the cytotoxic activity of lipid peroxides in tumour cells (Maiorino et al. 1995). Higher concentrations of vitamin E can also reduce transition metals and produce oxy radicals from redox reactions (Iwatsuki et al. 1995).

 
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