Artifacts Produced on the Surface of Starch Granules in the Scanning Electron Microscopy

Abstract

The surface of starch granules, which was coated rather thickly with carbon and gold and then kept in high humidity, showed some cracks in the scanning electron micrographs (Fig . 1). A weak electron beam caused the cracks on the surface of the coated starch granules (Fig . 2), but the greater the intensity of the electron beam (over 15 kV), the more severe the cracks on the exposed surface (Fig. 3). To obtain a high magnification, it is necessary to use a high electron beam intensity, for example 20-25 kV, which causally resulted in the appearance of the protuberances (Fig. 4) or indentations (Fig. 5), giving a pockmarked pattern on the surface of starch granules, especially when the granules were coated thinly. The cracks can be reproduced intentionally by keeping the coated granules in high humidity (over 90% RH). When the coated starch granules were kept in high humidity for a long time (2-3 days), the whole surface of the granules became rugose, including the cracks, during scanning in the microscope (Figs. 6 and 7). The observed cracking patterns were not uniform, namely, there were regular patterns like brick-works (Fig. 1), or fitful! patterns like the superficial appearance of a volley-ball (Fig . 8), and in other cases, intricate lines crossed over the surface. The cracks were observed with several kinds of starches (potato, sweet potato, yam, tapioca, corn, barley, broad bean, gingko, etc.), and were more evident with large starch granules having smooth surfaces.