Effects of sampling methods on starch granule size measurement of potato tubers under a light microscope

Light-microscopic measurement of starch granule size is the preferred approach in most laboratories because it is simple, rapid and visual and because it can study both size and shape. However, potato juice consists of starch granules with very different sizes and precipitation/movement speeds, which causes differences when sampling the juice and taking the microscopic images. The previously described method is to scrape and transfer some juice from potatoes using a razor blade directly to a slide with some water for microscopic observation. In this study we used a tape-hole chamber on the microscopic slide 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 efficient 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.