Relationship between granule size and in vitro digestibility of maize and potato starches

Maize and potato starch granules were separated by a sedimentation method into a range of fractions based on their size. The surface weighted mean diameter[D(3, 2)] of separated granules was in good agreement with predictions from Stokes' law of sedimentation. In vitro digestion of fractionated starches by alpha-amylase was well fitted by first-order kinetics, with the digestion rate coefficient (K) showing an inverse square relation with granule size, consistent with either a diffusion-controlled or surface-controlled mechanism. Apparent diffusion coefficients of alpha-amylase obtained by fitting the size dependence were 7.40 (maize starch) and 1.35 (potato starch) x 10(-10) cm(2) s(-1) respectively. A correlation between K and specific granule surface area was also obtained for both starches, consistent with a role for surface area in controlling amylase digestion rates. Differences in K values are consistent with electron microscopy of partially digested granules, suggesting that an external surface-controlled mechanism may be operating for potato starch, and that the effective surface area of maize starch is greater than predicted from granule diameter due to surface pores and channels. (C) 2010 Elsevier Ltd. All rights reserved.