Journal of Cereal Science 50 (2009) 198–204
Effect of particle size on kinetics of starch digestion in milled barley
and sorghum grains by porcine alpha-amylase
Ghaid J.S. Al-Rabadi, Robert G. Gilbert, Michael J. Gidley*
Centre for Nutrition and Food Sciences, School of Land Crop & Food Sciences, The University of Queensland, St Lucia, Brisbane, Qld 4072, Australia
The influence of milled grain particle size on the kinetics of enzymatic starch digestion was examined. Two types of cereals (barley and sorghum) were ground, and the resulting grounds separated by size using sieving, with sizes ranging from w0.1 to w3 mm. In vitro enzymatic digestion was performed, using pancreatic alpha-amylase, amyloglucosidase and protease, to determine fractional-digestion rates over 24 h. The resulting glucose production rate data were well fitted by simple first-order kinetics. For each sieve screen size, the digestion rate of barley was always higher than that of sorghum. The rate coefficients for digestion showed a decrease with increasing size, and could be well fitted by an inverse square relationship. This is consistent with the supposition that starch digestion in these systems is controlled by diffusion of enzyme through the grain fragment. Apparent diffusion coefficients of alphaamylase obtained by fitting the size dependence were 0.76 (sorghum) and 1.7 (barley) x 10-7 cm2 s-1, 9 (sorghum) and 4 (barley) times slower than predicted for a molecule of the size of alpha-amylase in water.