Questions and answers to edibility problem of the Canavalia ensiformis seeds – A review

Abstract

The seed of Canavalia ensiformis (jackbean), a highly productive large-seeded tropical legume, contains about 300 g crude protein and 600 g carbohydrates kg −1 dry matter. It, however, contains toxic and antinutritional factors which limit its use as human food or animal feed. The trypsin inhibitors in the seed have, however, been reported to be easily inactivated by moist heat. The toxic alkaline non-protein amino acid, canavanine (present at about 50 g kg −1 seed dry matter), a naturally occuring analog of l-arginine, has also been reported to induce reduced feed intake in non-ruminants but at the equivalent of about 300 g kg −1 dietary level of the raw seed, or in the condition of dietary arginine deficiency. The concentrations of the specialized secondary plant biomolecules – cyanogenic glycosides, saponins, alkaloids and terpenoids – have also been shown to drop below detectable levels following 1 h of cooking. Urease and canatoxin which were isolated from the seed are highly toxic if injected into experimental animals but exert no toxic effects if orally administered, and thus cannot be classified as antinutritional factors. Concanavalin A (Con A), the most studied of plant lectins, appears to be the most important toxic and antinutritional factor in the seed, being highly resistant to heat treatments and to proteolytic digestion in the gut. It requires 3 h of cooking at 96°C or 45 min of pressure cooking; 48 h of soaking the seed in water prior to cooking for 2 h; 72 h of soaking in water prior to ordinary cooking for 1 h; pressure-cooking for 15 min, to completely eliminate it from the seed. Complete inactivation of Con A in the seed can, however, be easily achieved if the seed is broken into pieces and cooked for 1 h or pressure-cooked for 15 min.