Effects of Sampling Methods on Starch Granule Size Measurement of Potato Tubers under a Light Microscope

Measurement of starch granules by lightmicroscope is the preferred approach in most laboratories because it is simple, rapid and visual and because both size and shape can be investigated. However, juice from potato tubers consists of starch granules of very different sizes and precipitation/movement speeds which can cause artefacts when sampling the juice and recording microscopic images. In the previously described method, a razor blade was used to scrape and transfer juice from potato tubers directly to a drop of water on a slide for microscopic observation. In this study we used chambers made from tape on microscopic slides to reduce the cover-slip-induced shifting of small and medium granules. We improved the starch measurement reproducibility by testing various juice sampling methods. The reproducibility between repeated experiments using 10 cultivars was increased from a correlation coefficient r = 0.815 in the razor-blade-scraping method to r = 0.923 in a squeezing-juice method. The largest starch granule detected was 151 μm in length. Sampling methods (using a razor-blade or a garlic press) strongly influenced the granule length values measured from the same potato tuber. The results indicated that (1) The squeezing- juice approach is more reproducible, and (2) The average length of starch granules is one of the most reproducible scores but varies according to juice-sampling methods.