Comprehensive investigation and comparison of surface microstructure of fractionated potato starches

Potato starch (PS) granules were separated into four fractions based on their volume-weighted mean diameters: very small (VS), small (S), medium (M) and large (L) granules. The particle size distribution and morphology of the starch fractions were investigated by laser particle size analysis, confocal laser scanning microscopy (CLSM), and scanning electron microscopy (SEM). The starch granules were spherical in the PS-VS and PS-S fractions, ellipsoid in the PS-M fraction, and elongated in the PS-L fraction. The nanoscale surface features of fractionated potato starch granules were further investigated using atomic force microscopy (AFM) and low-temperature nitrogen adsorption. AFM images showed that different fractionated granules had different nanoscale `blocklet' structures on their surfaces, indicating differences in the molecular self-assembly of amylopectin side chains at the surfaces. Image analysis showed that PS-VS and PS-S fractions had more homogeneous surfaces than PS-M and PS-L fractions. Nitrogen adsorption analysis showed that the PS-VS and PS-S fractions had a narrow range of pore diameters, indicating a more homogeneous morphology of the smaller granules. Conversely, PS-M and PS-L fractions had broad range of pore diameters and a heterogeneous surface structure. The surface texture and nanoscale structures of the starch granules were significantly correlated to the pasting properties of starch, providing a new perspective on the structure-function relationship of starch granules.