The study on starch granules by using darkfield and polarized light microscopy

Abstract

A deep understanding of starch granule properties is important for its food and non-food industrial applications. On the other hand, the advance of optical microscopy has been ongoing. In this study, optical microscopies including transmission light microscopy, darkfield microscopy (DFM), differential interference contrast microscopy and polarized light microscopy (PLM) are compared for studying starch granules. It was found that detailed morphology and size of native starch granules could be precisely obtained by DFM, while crystalline is reflected by the Maltese cross under PLM. Optical images showed that native starch granules from rice and maize are polygonal, while starch granules of sweet potato are oval-shaped. Starch granules from sweet potato have the largest size of 15.2 ± 3.8 μm. After gelatinization, starches aggregated and chunks were formed under DFM, accompanied by the vanishing of crystalline (Maltese cross) under PLM. More importantly, the technique can visualize gelatinization progress of single rice starch granules in situ with a time scale of 100 ms at 1 mol/L NaOH. Due to gelatinization, micelle structures of granules with a decreasing size were determined by DFM, while the branched structure with an increased size was observed through PLM. The gelatinization at different subregions of starch granules was heterogeneous; structural changes at specific subregions are relatively faster. The combination of PLM and TLM may become an important technique for studying starch properties, especially obtaining detailed dynamic changes.