ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

samples in this study were also higher than the values ofbetween 443 and 842 1lg/100 g
reported by Onayemi andNgiwe (1987).
However, levels of oxalate obtained from the unprocessed Amadumbe samples
investigated were lower than those recorded for three different local varieties of taro
grown under irrigation at Kaohsiung, Taiwan. The latter oxalate levels ranged between
234 and 411 mg/IOO g ofdry matter (Huang et al., 2007). Levels were also lower than the
oxalate contents of some tropical foods investigated by Holloway et al. (1989), who
reported on the oxalate content ofthe tubers of four different cultivars of taro grown in
Fiji. In their study, the total oxalates ranged from 65 mg/IOO g fresh weight (FW) for
Colocasia esculenta to 319 mg/lOO g for giant swamp taro. The oxalate content for the
investigated samples was also lower than that reported for paddy- and upland-cultivated
taro cultivars (Huang et al, 2007).
Cooking treatments were found to be effective in reducing the oxalate content of the
tubers under investigation. The highest losses of oxalate (54 per cent) occurred when
boiling the white Amadumbe sample. Boiling may cause considerable cell rupture and
facilitate the leakage of soluble oxalate into cooking water (Albihn and Savage, 2001).
When the samples were roasted and fried, there was an increase in oxalate content. This
apparent increase could be related to the relative increase in dry matter as Amadumbe is
roasted. Oxalate is broken down at 200 0 C, but this temperature is rarely reached when
roasting or frying. Similar cooking studies on oca (Oxalis tuberose) showed that boiling
considerably reduced the oxalate concentration in the whole tuber, while baking
increased the concentration ofsoluble oxalates in the cooked tissue (Albihn and Savage,
2001). Sangketkit et at. (2001), Savage (2002) and Quinteros et at. (2003) all observed a
similar trend regarding the decrease in soluble oxalate after boiling and the increase in
oxalate with baking.
Hui (1992) reported that ingestion of 5 g or more of oxalic acid could be fatal to humans
while Munro and Bassir (1969) determined the limit of oxalate toxicity in man to be 2-5
gllOO g ofthe sample. Care should be taken in high-quantity consumption ofAmadumbe,
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