![]() Condensed tannins are called proanthocyanidins due to the fact that when heated in an acidic alcohol solution, they can be degraded into anthocyanidins via an acid-catalyzed oxidation process ( Naumann et al., 2017). Regarding the condensed tannins, they are either oligomers or polymers of flavan-3-ol bonded via the interflavan carbon bond. Several oligomeric compounds with molecular weight (MW) ranging from 200 to 5000 Da are formed from various intermolecular oxidation reactions ( Khanbabaee and van Ree, 2001). The formation of oxidative linkage between the components of these structures’ accounts for the great differences in their structures. The hydrolysable tannins contain a central glucose core in an esterified form with gallic acid. Tannins are commonly subdivided into 2 groups: Hydrolysable and condensed tannins. Ferulic acid is another common phenolic acid found in cereals mostly esterified with hemicelluloses ( Dai and Mumper, 2010). Caffeic acid is mostly found in vegetables and fruits which are mostly esterified with quinic acid. There are 2 classes of phenolic, they are acids-benzoic acid derivatives (gallic acid) and cinnamic acid derivatives such as coumaric and ferulic acid. The commonest flavonoids are quercetin, catechin, naringenin, cyanidin-glycoside, and daidzein ( D’Archivio et al., 2007 Dai and Mumper, 2010). ![]() The differences in the structure of each subgroup are partly attributed to the pattern and degree of hydroxylation, prenylation, glycosylation, or methoxylation. This grouping operates on the oxidation state of central C ring in the flavonoid structure. There are six subgroups of flavonoid, these are flavones, flavanones, flavonols, flavanols, anthocyanins, and isoflavones. Structurally, flavonoids composed of a flavan nucleus with 15 carbon atoms arranged in 3 rings such as C6–C3–C6 labelled A, B, and C. Among the plants, phenolics are lignans, tannins, phenolics acids, stilbenes, and flavonoids.įlavonoids are the major polyphenols in human diets. Phenolics are known to account for the differences in the flavour and colour of different wine brands. Phenolics can also added to the colour of many fruits and vegetables. Likewise, phenolic compounds are responsible for the bitterness of fruits due to their interaction with salivary glycoprotein. Phenolics are found mainly in fruits, legumes, vegetables, tea, wine, coffee, and accounts for the organoleptic characteristics of plant food. Their presence in all plant organs makes them a vital ingredient of the human diet ( Balasundram et al., 2006 Shah et al., 2018). The compounds participate in plant defence against ultra violet (UV), pathogens, and other predators. They range from the simple phenolic such as phenolic acids to the complex compounds like tannins. They are the most common secondary plant metabolites with over 8000 known structures. Compounds with more or single aromatic rings coupled to a single or more hydroxyl groups are commonly called phenolic.
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