ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

A.1.4.7
when the levels are high enough (Bradbury and Nixon, 1998). These crystals play a role
to the formation ofkidney stones (Noonan and Savage, 1999).
When oxalic acid is consumed, it irritates the lining ofthe gut and can prove fatal in large
doses. Hui (1992) stated that intake of5g or more ofoxalic acid could be fatal to humans
while Munro and Bassir (1969) estimated the threshold ofoxalate toxicity in man to be 2­
5g1100g of the sample. Oxalic acid is a common and wide-spread component of most
plant families. While the levels ofthis acid in these plants are generally low, it is the high
concentrations in the leaves and conus ofplants consumed daily that are ofconcern.
Oxalate is an anti-nutrient which under normal conditions is confined to separate
compartments. However, when it is processed and/or digested, it comes into contact with
the nutrients in the gastrointestinal tract (Kaushaiya et al., 1988). When released, oxalic
acid binds with nutrients, rendering them inaccessible to the body. Iffood with excessive
amounts ofoxalic acid is consumed regularly, nutritional deficiencies are likely to occur,
as well as severe irritation to the lining ofthe gut.
Most taro cultivars have an astringent taste and can cause swelling of lips, mouth and
throat if eaten unprocessed. This causticity is caused by closely-packed, needle-like,
calcium oxalate crystals, which can penetrate soft skin (Bradbury and Nixon, 1998).
Thereafter, an irritant, probably a protease, present on the sheathlike bundle of needles
(raphides) can cause discomfon in the tissue (Bradbury and Nixon, 1998; Paul et al.,
1999). Both the tubers and the leaves can give this reaction (FAO, 1992) but this effect is
reduced by cooking (Bradbury and Nixon, 1998).
Phytate
Phytic acid, which is hexaphosphate of myo-inositol, is very common in the plant
kingdom and is found mainly in mature seeds such as legumes, fruits, vegetables and
cereal grains (Chan et aI., 2007). Phytic acid is the primary storage compound of
phosphorus in plants, accounting for up to 80 per cent of the total phosphorus (Raboy,
2001; Steiner et aI., 2007). Josefsen et aI., (2007) reponed that the negatively charged
23