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ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

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(Njintang et al., 2006), whereas tannin content was similar to the values for potato and<br />

sweet potato (Noman et aI., 2007). The level of phenolic compounds in plant sources<br />

depends on factors such as cultivation techniques, cultivar, growing conditions and<br />

ripening process, among others (Naczk and Shahidi, 2006). A reduction in these<br />

compounds during processing may have been due to heat degradation of the tannin<br />

molecules or to the formation of water-soluble complexes (Uzogara et al., 1990; Bakr<br />

and Gawish, 1991). The reduction during boiling was the highest (38 per cent and 24 per<br />

cent ofthe white and purple tubers respectively) and may be attributed to leaching out of<br />

the phenol into the cooking medium under the influence of the concentration gradient<br />

(Uzogara et aI., 1990; Vijayakumani et aI., 1997). The flavonoid content was also lower<br />

than that ofpotato (Chu et al., 2000) and much lower than that ofsweet potato (Huang et<br />

aI., 2006). Flavonoid content decreased considerably during processing. The tubers are<br />

partially hydrolyzed during boiling and these hydrolytic changes influence both their<br />

distrIbution between the lipidic and aqueous phases and their reaction with lipidic free<br />

radicals (Price et al., 1998).<br />

One very important factor reducing the consumption of cocoyaru is the presence of<br />

oxalates which impart an acrid taste or cause irritation when foods containing them are<br />

eaten (Holloway et al., 1989). Oxalates can have a harmful effect on human nutrition and<br />

health, especially by reducing calcium absorption and aiding the formation of kidney<br />

stones (Noonan and Savage, 1999). The levels of oxalates in the locally grown<br />

Amadumbe are therefore also important in the assessment ofnutritional status. The tissue<br />

in this study was sampled after the removal of the Amadumbe skin as the skin is never<br />

cooked or eaten and is known to contain high levels ofoxalates. Holloway et al., (1989)<br />

reported that the presence ofoxalate becomes less concentrated from the skin towards the<br />

centre of the laro corm. The total oxalate content of the corms in this study showed<br />

variation in plants from different localities. Oxalate values for the crude extract of the<br />

local Colocasia species were found to be much higher than the values ranging between<br />

430 and 1560 Ilgl100 g obtained by using anion-exchaoge high performance liquid<br />

chromatography (Huang and Taoadjadja, 1992). The oxalate values obtained for the<br />

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