Effect of cooking on the profile of phenolics, tannins, phytate, amino acid, fatty acid and mineral nutrients of whole-grain and decorticated vegetable cowpea (Vigna unguiculata L. Walp)

An experiment was conducted to determine the profile of phenolics, tannins, phytate, amino acid, fatty acid and mineral nutrients in whole-grain and decorticated vegetable cowpea before and after cooking. Two cowpea varieties: C-152-White, having big grain and S-1552-White, having small grain with black eye were used for the study. Decortication of cowpea grain resulted in significant reduction (P <= 0.05) of total phenolics, total tannins and phytate contents of the two cowpea varieties. Comparison showed that cooking resulted in 19, 28, 28 and 37% losses in phenolics in whole-grain C-152, decorticated C-152, whole-grain S-1552 and decorticated S-1552, respectively. After cooking, whole-grain and decorticated C-152 retained 52.1 and 57.7% tannins while S-1552 retained 59.4 and 49.5%, respectively. In the whole grain, C-152 lost 72.2% phytate after cooking while S-1552 lost 70.3%. For the decorticated samples, C-152 lost 72.4% phytate after cooking while S-1552 lost 71.2%. For the two cowpea varieties, decortication resulted in significant (P <= 0.05) losses of Ca, Fe, Mn, Mg, Zn, K and Cu while there was no significant effect on protein and amino acid profile. Cooking did not cause any significant change in the mineral nutrient composition of either the whole-grain or decorticated cowpea. For the two cowpea varieties, bulk of the fat consisted of palmitic acid (23.2-27.3%) and stearic acid (53.5-59.8%) in the ratio of 1:2, approximately. Upon cooking, losses of crude protein were recorded in the whole-grain and decorticated samples of the two cowpea varieties.