ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

A.l.4.4 Total polyphenols
Polyphenols are a large and varied class ofmetabolites widely spread throughout the plant
kingdom: they are a complex but important group ofnaturally occurring compounds (Ryan
et aI, 1999). PhenoIics are secondary metabolites: that is, they are not directly involved in
any metabolic process (FAO, 1995). Plants produce them during natura1 development
(Harllorne, 1982) and as a result of stress conditions such as wounding, infection and
ultraviolet radiation (Beckman, 2000).
These compounds, derived from phenylalanine and tyrosine, are an extremely diversified
group ofphytochemicals (Shahidi and Naczk, 2004). This class ofplant metabolites contains
more than 8000 known compounds, ranging form simple phenols - such as phenol itself ­
through to materials ofcomplex and variable composition, such as tannins (Harllorne, 1993;
Bravo, 1998). One thing that all phenolic compounds have in common is that their molecular
structme includes an aromatic hydrocarbon group to which a hydroxyl fimctional group (­
OH) is attached.
Plant foods mostly contain phenolic acids in the bound form. The principal phenolic acids
which may be found in plants are alternative derivatives of hydroxybenzoic and
hydroxycinnamic acids, the most common of which are hydroxycinnamic acids (Mattila
and Hellstrom, 2007). Examples of the latter are caffeic, p-coumaric and ferolic acids,
which are often found in foods as simple esters with quinic acid or glucose. These
derivatives differ in the patterns ofthe hydroxylations and methoxylations oftheir aromatic
rings (Shahidi and Naczk, 1995; Hermann, 1989). The methoxylations of their pungent
rings and the patterns of the hydroxylations differentiate these derivatives (Shahidi and
Naczk, 1995; Hermann, 1989).
Phenolics may add to the smen, taste (bitterness, sharpness), colour and oxidative
stability of food (Maga, 1978; Robbins, 2003). The dissociability of their -OH group
renders phenols acidic. They are also easily oxidized and form polymers (dark
aggregates). Phenolic compounds discolour and darken because of enzyme activity, as
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