Particle size-starch-protein digestibility relationships in cowpea (Vigna unguiculata)

The kinetics of starch and protein digestion in hammer- and cryo-milled cowpea (70-370 mu m) were investigated. The pH during the protein digestion reduced with time, and both the starch and protein digestion exhibited monophasic digestograms, which were suitably (r(2) > 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 x 10(-7) cm(2) s(-1) as the diffusion coefficient. For the starch digestion, diffusion coefficients (cm(2) s(-1)) were 0.6 x 10(-7) (hammer-milled) and 0.3 x 10(-7) (cryo-milled). The protein digestion proceeded at a much faster (100x) rate than the starch digestion. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.