Effect of milling and particle size on functionality and physicochemical properties of cowpea flour

Cowpeas (Vigna unguiculata) were milled through 0.5-, 1.0-, and 2.0-mm screens, and the flour was subsequently separated into different particle-size ranges. Such procedures caused only minimal changes in moisture. fat, protein, ash, and total carbohydrate. The amount of extractable starch, however. varied from 34.5 to 52%. The effects of both mill screen and sieve mesh size were significant (P < 0.05). Differences in milling and separation procedures resulted in significant variations in water absorption (0.41-2.81 g of water/g of flour), solids lost (0.34-1.17 g/g of flour), and protein solubility (21.2-37.4%) (P < 0.05). Finely milled flours (91% moisture) had lower initial gelatinization temperatures (70-73 degrees C), as measured by differential scanning calorimetry (DSC) (P < 0.01). Gelatinization peaks in high-moisture flour were similar to that of pure starch. At lower moisture, a second peak was observed indicative of protein. Light-scattering analysis showed that different conditions produced a bimodal particle-size distribution when samples were suspended in water. The small size had relatively constant diameters (19-21 mu m) and was associated with starch granules. The latter had a large size distribution and varying peak size and was associated with aggregated flour particles. These results indicate that changes in processing produces cowpea flours with differing chemical and physical properties.