Dear Maryam,
Is this the product?
https://uk.vwr.com/store/product/2340985/starch-from-corn-sigma-aldrich
If so, the average molecular size is in the description.
However, if this is not it, and you absolutely can't find this information, remember that Starch is a polysaccharide that might have variable molecular weight. In the case of soluble starch (which I guess its your case), the most accurate way would be to run a high-performance size exclusion chromatography (HPSEC), and use a set of standards so you can estimate the average molecular weight from your sample.
As a polysaccharide, many variables, including extraction and pretreatments, are able to change its molecular size (e.g hydrolysis), and therefore there is no such categorical affirmation for all types of starch regarding its molecular size.
Best regards,
For the determination of Viscosity Average M.Wt. one may use capillary viscometer(Ostwald type).
I suggest using the potato starch which is from Sigma-Aldrich and well used in many formulations.
The challenge with MW determination of starch is that you need to disrupt the granule structure of starch to liberate the starch molecules without breaking glycosidic bonds within the latter. [Breaking 1 single glycosidic bond per starch molecule results in a reduction of MW by 50 %!]. To this aim many solvents have been tested with results that may differ by a factor 100.
Until now the best procedure is to dissolve the granules in 90 % aqueous DMSO, precipitate the starch with ethanol, and redissolve it in boiling water (preferably with N2 flush). MW is measured by SEC-MALLS (size exclusion chromatography coupled with multi-angle laser light-scattering detection). In this way maize amylopectin has MW 5.10^8 g/mol. Because maize starch contains ca 25 % amylose with MW ca 10^6 g/mol, MW of maize starch may be approximated as 3.75.10^8 g/mol. See the attachment.
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