Interaction of oxalic acid with divalent metals—calcium, magnesium and zinc—during the cooking of cowpea (Vigna unguiculata)

Two varieties of cowpea (Vigna unguiculata)—‘Ife brown’ and ‘Kano white’—were examined for their oxalate contents. At least 80% of the oxalate, 35% of the calcium and 64% of the magnesium were found in the soluble form. Zinc was, however, not detected in the soluble form. The ‘Ife brown’ variety showed higher levels of the oxalate and minerals. Heating the cowpeas in water showed increases in both the insoluble oxalate and the soluble mineral levels, the changes being noticeable after only 20 min of heating at 140 °C. Cooking the seeds in salt solutions led to increases in the levels of insoluble oxalate, implying in-seed interaction between the oxalate and the metal ions.