Chemical and Nutritional Properties of Dry and Wet Milling Products of Red Cowpeas

Abstract

AbstractRed cowpea flour, and starch and protein fractions from dry and wet milling processes were evaluated for their chemical and nutritional quality. Red cowpea starch contained 24.2% amylose, slightly higher than mung bean starch with 21.2% amylose. Sugars in red cowpea flour from dry processing contained 3% stachyose, 2.1% raffinose, 1.9% sucrose, 0.5% glucose and 1.1% fructose, significantly higher than those in the flour from wet processing. Most of these sugars were collected in the protein fraction after air classification of dry-milled flour. Fatty acid profiles of dry- and wet-dehulled flour, protein fraction, and protein isolate were similar. However, there were indications of changes in the lipids of wet-dehulled flour. Amino acid profiles of the cowpea flours and proteins produced with both techniques were also similar to one another having sulfur-containing amino acids as a limiting factor. SDS-PAGE showed prominent molecular weights of red cowpea proteins to be between 45,000 and 65,000 daltons. Heat treatment in the wet extraction of cowpea protein appeared to cause dissociation, aggregation and denaturation of the protein, resulting in proteins of very low solubility and low biological quality. The pattern of solubility vs pH of red cowpea proteins from both wet- and dry-dehulled flours showed the isoelectric point at pH 4.4. Tannin content in red cowpea flours ranged from 3 to 4.5mg/g sample, where the trypsin inhibitor content was between 3.8 and 15.6 TUI/mg samples. Trypsin inhibitor activity in the protein isolate was about 75% lower than that of the flour. PER of cowpea flours from both dry and wet dehulling was about 1.6–1.7, compared with 1.4 for whole seeds and 1.2 for protein isolate. Pressure cooking at 120°C for 20min increased the PER of whole seeds to 2.1. These quality parameters indicated that for red cowpeas, dry processing generally produced products superior than those obtained from wet processing.