Protein nutritional quality of cowpea and navy bean residue fractions

Abstract

Cereal-legume protein complementation has long been recommended as a suitable strategy for augmenting the protein quality of cereal and legume based foods.However, the use of the insoluble legume residue, following protein extraction for cereal-legume protein complementation has not been widely studied. In fact, legume residue is considered a waste by-product. The protein quality of cowpea residue-wheat and navy bean residue-wheat diets was determined using in-vivo and in-vitro protein digestibility assays with an AIN-93G diet as control. The diets were fed to laboratory rats over 4 weeks. The in-vitro digestibility of the diets was assessed using the pH drop and pH stat enzymatic methods. The proximate composition, limiting amino acid profile and phytohemagglutinin activity were also determined. All six diets had lower levels of the sulphur amino acid requirements for rats as expected but had higher than the FAO/WHO recommended levels for pre-school children. The cowpea residue diets had higher levels of limiting amino acids than the navy bean residue diets. Phytohemagglutinin activity was only detectable in the raw cowpea and navy bean samples. All cowpea residue diets, the 30% and 70% navy bean residue diets and the control diet supported growth while the 100% navy bean residue diet resulted in weight loss. The in-vitro digestibility ranged from 77.82% - 84.54% and 66.51% - 79.59% for the cowpea residue and the navy bean residue diets, respectively. These ranges were lower than the control (98.1%) but correlated highly to those obtained using the in-vivo true protein digestibility method; 73.7% - 87.5% and 62.6% - 78.2%, respectively. These findings suggest that the cowpea residue diets had higher protein quality overall than the navy bean residue diets. In addition, it suggests that the 30:70 ratio of cowpea residue to wheat diet had the highest protein quality of all the 6 experimental diets.Legume residues after protein extraction could be recommended for human food if complemented with a cereal, particularly as it meets the amino acid pattern for pre-school children. Finally, in-vitro assays can also be reliably used to assess the protein quality of foods.