Isolation and characterization of starches from two cowpea (Vigna unguiculata) cultivars

This study was conducted to evaluate and compare the gross chemical compositions and functional properties of starches derived from two cultivars of cowpea. White cowpea starch (WCS) and brown cowpea starch (BCS) were isolated from their respective seeds. The starch yield (40.00%) with its residual protein content (0.09%) from white cowpea seeds was higher than that obtained from brown cowpea seeds. The apparent amylose (AAM) content of WCS was lower than in BCS. Moisture, fats and ash contents for the WCS and BCS were 11.54 and 10.18%, 0.05 and 0.07%, 0.03 and 0.05%, respectively, and their pH was 6.98 and 6.93, respectively. Bulk density of WCS was lower than in BCS; in contrast the percentage dispersibility of WCS was higher than in BCS. The shapes of the starch granules obtained from SEM were round to elliptical with many granules occurring in clusters. WCS granules were clustered while BCS granules were singled. Fissures were also observed on the surfaces of BCS granules. When heated from 55 to 95oC at 10oC intervals, the swelling power (SP) and water solubility index (WSI) of both starches were evaluated. SP of WCS and BCS increased with increased temperature, but were more pronounced in BCS. The WSI of WCS increased progressively with increased temperature until a decrease occurred after 85oC, for BCS the WSI values were inconsistent with increase in temperature. Pasting parameters (PP) were evaluated using RVA. Significant differences were observed in the PP of the two cowpea starches, especially in PV, TV, FV and SV. The results revealed that cultivar difference has an effect on pasting properties of cowpea starch.