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 α-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 α-amylase obtained by fitting the size dependence were 7.40 (maize starch) and 1.35 (potato starch) × 10 ^-10 cm² 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.