Phenolic antioxidants

A large variety of phenolic phytochemicals that have several beneficial functions on human health are present in plants and especially in fruits. The phenolic phytochemicals are generally present in their glycosidic and nonglycosidic forms. The glycosides are mainly confined to hydrophilic regions in the cells, such as in vacuoles and apoplasts, probably because of their higher water solubility (197,198). Glycosylation of the hydroxyl groups on the phenolic ring of a phenolic phytochemical renders the molecule more water soluble and less reactive toward free radicals (6). Glucose is the most commonly involved sugar in glycoside formation, although phenolic glycosides of galactose, rhamnose, xylose, and arabinose; and disaccharides such as rutinose, have also been reported in plants (6). Polymeric phenolics such as tannins exist primarily as condensed tannins or proanthocyanidins and are formed biosynthetically by the condensation of single catechins and flavonols. They are present either as soluble tannins or bound to the cell wall. Hydrolysable tannins are esters of a sugar with either gallic acid (gallotannins) or ellagic acid (ellagitannins). Tannins, even though they have higher antioxidant properties than individual simple phenolics, are usually not bioavailable, and are to some extent antinutritive in their function because of their ability to bind and precipitate biological macromol-ecules such as proteins and carbohydrates (109). The total phenolic phytochemical content in plant foods also varies greatly. Their presence in plant foods is largely influenced by genetic factors and environmental conditions. Other factors, such as cultivar, variety, maturity, processing, and storage, also influence the content of plant phenolics (199-201). The effects of processing and storage on the changes and content of polyphenols in cranberry (202), plum (203), and grape juice (204) have been evaluated.

As a consequence of evidence that consumption of fruits and vegetables has been linked to decreased manifestation of chronic diseases, there has been a constant increase in the demand for diets rich in phenolic phytochemicals. Vast variation in the amounts of phenolic antioxidants available via diet (205) coupled with reduced bioavailability and functionality has led to an urgent need to develop innovative strategies to enrich diets with phenolics and specifically phenolic antioxidants with consistent phytochemical profile for enhanced health functionality.

Among many strategies, two are important to enrich phenolic antioxidants. The first is genetic modification of cultivars to produce plants that will yield fruits and vegetables with higher phenolic concentration. Currently, in terms of genetic improvement, breeding strategy coupled with micropropagation using tissue culture is being developed (206-208). These strategies, along with genetic modification, could be directed toward phytochemical enrichment and quality improvement. However, this method presents important issues, such as regulation of key metabolites by multiple genes and biochemical pathways, acceptance of genetically modified foods, and relative time and economic considerations that are involved (209). Another exciting strategy that can be used is the bioprocessing of botanicals using solid-state bioprocessing and synergies to generate phytochemical profiles with enhanced health functionality. This strategy can be used for juice and pulp as well as pomace that remains after the juice is extracted from the fruits. Fermentation of fruit juices, such as grape juice to wine, has already been shown to improve nutritive and health promoting activities (210-212). Solid-state bioprocessing done on the pulps using food grade fungi can result in enrichment of the pulps with phenolic antioxidants and functionally important phenolic phytochemicals, and also improve phytochemical profile consistency.

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