The granules Rob scraped from the dress are likely potato starch granules.
Starch is a biopolymer composed of multiple glucose units linked to each other. Starch is mainly found in plants, the most common types include:
Additionally, the types of starch differ due to the size of the granules.
Based on this, it can be concluded the starch Rob removed from the dress was likely potato because the size of the granules was 75um and this matches only the size of potatoes starch granules.
Note: This question is incomplete because the context is missing; here is the missing part:
The granules, though which were smooth and oval-shaped with a diameter of about 75 um (micrometers), came from the stain itself. He added a drop of a weak, yellowish iodine solution to the slide. The granules turned dark blue. Under Treatment Plan he wrote "Neckline stain: use amylase cleaning solution" - an enzymatic solution specific for removing starch"
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