Heterogeneity in maize starch granule internal architecture deduced from diffusion of fluorescent dextran probes

Heterogeneity in maize starch granules was investigated by studying the diffusion of fluorescent dextran probes (20, 70 and 150 kDa) inside granules using fluorescence recovery after photobleaching combined with confocal microscopy. Access of probes to the interior of granules was greatly enhanced by limited (2.4%) amylolysis. The diffusion of probes within granules was found to be either 'fast' with diffusion coefficients in the order of 10^-6 cm² s^-1 or 'slow' with diffusion coefficients in the order of 10^-7 cm² s^-1, independent of the size of dextran probes or prior treatment of the granules by α-amylase. Results were compared with observations of pores and channels in granules by electron microscopy and by confocal microscopy after labelling with 8-amino-1,3,6- pyrenetrisulfonic acid. It is proposed that there is an inherent heterogeneity of internal architecture in maize starch granules due to the presence or absence in individual granules of (a) pores leading to a central cavity, resulting in 'fast' diffusion of dextran probes and (b) accessibility of the starch polymer matrix to dextran probes, leading to 'slow' diffusion behaviour. The observed heterogeneity of maize starch granule porosity has implications for chemical modification reactions and the kinetics of digestion with amylases.