High-resolution Imaging of Starch Granule Structure using Atomic Force Microscopy

Atomic force microscopy (AFM) was carried out for the precise structural analysis of starch granules. AFM is a useful technique to obtain — under atmospheric conditions — high-resolution images which are as good as those obtained by conventional scanning and transmission electron microscopy. The samples for observation by AFM do not require complicated preparations such as the drying and metal coating as in the case of electron microscopy. The AFM technique is suitable for the precise structural observation of bio-materials in which damage caused by drying and heating must be avoided and the effect of the thickness of the metal coating must be considered. Several preparation methods for exposing the bare inner surface of starch granules were studied in the beginning. AFM structural analyses on the nanometer scale were carried out on starch granules from five different botanical sources which were prepared by a physical method. It was directly demonstrated that fine particles of approximately 30 nm in diameter existed inside each granule and occasionally formed a straight chain structure. These fine particles may correspond to the individual blocklet or single cluster which has been proposed in the starch structural model previously discussed.