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ANTI-NUTRITIONAL CONSTITUENT OF COLOCASIA ESCULENTA ...

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A.l.4.3<br />

Hollenbeck et al., 1983; Carlson et al., 1983). Interestingly, subsequent investigation<br />

indicated that highly concentrated types of amylase inlubitors could potentially reduce<br />

such absorption in humans (Udani and Singh, 2007). Franco et al. (2000) noted that many<br />

studies have been undertaken to determine lhe value of a-amylase inhibitors as a means<br />

ofstrengthening a diverse range ofplants against insect and microbial pests.<br />

Lectins<br />

Lectins or phytohemagglutinins may be defined as large protein or glycoprotein<br />

molecules (Liener, 2000) and have been identified in more 1han 800 plant species,<br />

including legume seeds, pulses and tubers (Noman et al., 2007; Van Nevel et al., 1998;<br />

Ignacimulhu et al., 2000). Molecules which are found on lhe epilhelial cells of the<br />

intestinal walls and contain carbohydrate can be bound by lectins. The toxic potential of<br />

lectins is determined by lhe degree of lhis binding (Liener,2000). These proteins reflect<br />

lheir toxicity by inhibiting growlh in experimental animals (Liener, 1986) and by causing<br />

diarrhea, nausea, bloating and vomiting in humans (Liener, 1982). Because lhe presence<br />

of lectins irritates lhe intestinal wall., over-secretion of mucus from this membrane may<br />

affect its enzymatic and absorptive efficacy. When lectins are together wilh olher anti­<br />

nutrients, this harmful effect may be more pronounced (Francis et al., 2001). Lectins are<br />

classified into several groups based on lheir sugar-binding specificity. Francesca et al.<br />

(200I) observed that apparently most lectins have relative masses of between 100 000<br />

and 150 000 daltons and are composed oftetramers.<br />

Lectins have the ability to recognize carbohydrates which bind and agglutinate red blood<br />

cells and can therefore be used for blood typing (Pusztia, 1992). Autoclaving and aqueous<br />

heat treatment - 100°C for 10 minutes - are two melhods which eliminate lectins (Grant,<br />

1991). Thus, there would appear to be little cause for concern regarding toxicity if food is<br />

cookedproperly.<br />

16

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