Particle size-starch–protein digestibility relationships in cowpea (Vigna unguiculata) Original Research Article

The kinetics of starch and protein digestion in hammer- and cryo-milled cowpea (70–370 μm) were investigated. The pH during the protein digestion reduced with time, and both the starch and protein digestion exhibited monophasic digestograms, which were suitably (r2 > 0.97, p < 0.001) described by a modified first-order kinetic model. The in vitro protein digestibility of the cowpea (>80%) was independent of the milling conditions. The hammer-milled cowpea digested more, but the reciprocal of its rate of protein digestion was independent of the square of the particle size. The rate of protein digestion in the cryo-milled cowpea inversely depended (p < 0.05) on the square of the particle size, with 67 × 10−7 cm2 s−1 as the diffusion coefficient. For the starch digestion, diffusion coefficients (cm2 s−1) were 0.6 × 10−7 (hammer-milled) and 0.3 × 10−7 (cryo-milled). The protein digestion proceeded at a much faster (100×) rate than the starch digestion.