Chemical composition, amino acid digestibility, and true metabolizable energy of cowpeas as affected by roasting and extrusion processing treatments using the cecectomized rooster assay

Nutritional differences due to variety, source, and effects of heat processing on cowpeas (Vigna unguiculata) were assessed. The precision-fed cecectomized rooster assay was used to determine amino acid digestibility and TME of cowpeas. Mozambican cowpea (nhemba) treatments included either raw or roasted, whereas those (black-eyed beans) sourced from the United States were raw, roasted, or extruded. Crude protein content on a DM basis was similar for raw nhemba (296 g/kg) in comparison to raw black-eyed beans (299 g/kg). Raw nhemba had slightly higher amino acid content (except tryptophan) than raw black-eyed beans. The level of trypsin inhibitors of nhemba was 3 times higher than black-eyed beans (6,700 vs 2,200 TIU/g). Heat treatment generally had no effect on amino acid concentration except for lysine, which was reduced in both beans by heat. Additionally, all heat treatments reduced the concentration of trypsin inhibitors to below 2,000 TIU/g. Amino acid digestibility for the nhemba cowpeas was lower than for the black-eyed cowpeas except for tryptophan. Roasting had no effect on amino acid digestibility in black-eyed beans, but increased digestibility of several amino acids in nhemba cowpeas, and the average increase was 3.4 percentage units. The TME_n of raw nhemba cowpeas was significantly increased (2,806 vs 3,347 kcal/kg DM) by roasting and the TME_n of raw black-eyed cowpeas was significantly increased by extrusion (3,164 vs 3,535 kcal/kilogram DM).