Compositional, functional and nutritional characterization of ultrafiltration processed cowpea (Vigna unguiculata) and navy bean (Phaseolus vulgaris) protein fractions

Ultrafiltration technology is increasingly being used to simultaneously purify, concentrate and fractionate macromolecules without the application of heat or the use of either extreme chemical or physical conditions. This is particularly important in isolating proteins since it results in very little modification of structure and functionality. Ultrafiltration is now a unit operation in the dairy industry. In the vegetable protein industry, most of the research has been limited to soy protein. There is thug tremendous potential to evaluate the effects of ultrafiltration processing on the physico-chemical properties of proteins from under-utilized leguminous species.;The optimum conditions for aqueous extraction of protein from milled cowpea and navy bean seeds was determined using particle sizes 0.79mm--6.35mm, pH values 2--12 for 60 minutes each at 25 or 50°C. The legume flours were extracted three times and the extracts combined and pumped through a plate and frame ultrafiltration system. The flow rate (L/hr), flux (L/m 2/hr), protein content, recovery, molecular weight characterization and functional properties of freeze dried fractions were compared to a commercial soy protein isolate (SPI). The protein quality of the residue remaining after aqueous alkali extraction was determined. Legume and wheat flour diet blends were made up to 10% protein and contained 30%, 70% and 100% by weight of the cowpea and navy bean residue supplemented with whole-wheat flour to 100%. Arrowroot starch was added as the major carbohydrate source. A 2% albumin diet was used to determine metabolic nitrogen, and a modified AIN-93G diet used as a control.;Alkaline pH was more effective than acidic pH in extracting protein. Particle size had a highly significant inverse relationship and temperature had no effect. Results indicate that the optimum protein extraction conditions were particle size of 1.59mm, at pH 10 and 25°C. Permeation flux ranged from 0.18--4.03 L/m2/hr and 3.12--24.80 L/m 2/hr for cowpea and navy bean, respectively. Protein content of the fractions ranged from 18--53%. About 84% of the protein in the extract was recovered in the cowpea protein isolate (CPI), and 80% in navy bean isolate (NBI). Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) indicated similar discrete band patterns for all protein fractions. The physico-chemical properties of cowpea and navy bean proteins were significantly affected by ultrafiltration (UF) processing, and there were significant differences between the experimental fractions and soy protein isolate (SPI). The protein quality of all experimental diets was significantly lower than that of the modified AIN-93G diet except the 30% cowpea diet. Diets with cowpea or navy bean as the primary source of protein were considered poor quality protein sources. The extraction and ultrafiltration system employed, provided discrete protein fractions that can be used as ingredients in the food industry; the 30%CP wheat flour diet blend could be recommended for use as food for pre-school children, ages 2--5 yr.