Kinetics of starch digestion in sorghum as affected by particle size

The mechanisms of starch digestion in cryomilled and hammer-milled sorghum grain were investigated by an in vitro procedure using 10 particle sizes (≅120–560 μm). The samples exhibited essentially monophasic digestograms, and three digestion models (Duggleby, first-order kinetic and Peleg models) were tested for describing the digestograms. Irrespective of the particle size, starch digestion followed first-order kinetics, and the modified first-order exponential model and Peleg model adequately (mean relative deviation modulus < 10%) described the digestograms. With or without an intercept, there was a significant linear relationship (r2 > 0.61; p < 0.01) between the reciprocal of the digestion rate constant and the square of the particle size consistent with digestion proceeding by a diffusion-controlled mechanism. The reciprocal of the slope of the relationship 0.4–0.9 × 10−7 cm2 s−1 provides an estimate of the diffusion coefficient of α-amylase within the milled grains. Particle size affected digestion kinetics, water absorption index (WAI), pasting properties, and water solubility index (WSI) of the samples, as did hammer-milling. However, differential scanning calorimetry showed no significant effects of the hammer-milling on starch gelatinisation properties. Although particle size was the primary determinant of digestion properties, small but detectable changes to the grain particle structure due to the hammer-milling affected digestion kinetics, presumably because of additional frictional heat and mechanical effects. Suggestions were made on how to modulate starch digestion, for example to achieve efficient energy delivery to animals from sorghum-based feed.