Potato tubers, contain a lot of starch .. Starch is 70-80% of all the dry substances of the tuber. There is starch in the cells in the form of layered starch grains ranging in size from 1 to 100 microns, but more often 20-40 microns.
The starch content can be determined by cutting the potatoes and rubbing them with fingers.
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From @v0inter