To detect starch in agar plates I saw a paper that floods the plate with Lugol's iodine reagent 0.01M so the plate gets dark blue if there is starch. I tried it but I didn't get it, maybe the concentration should be different, maybe it needs an exact time for staining agar plates without being degraded by sunlight...Has anyone ever tried it? Or any idea of why don't my plates get stained? Thank you
Kindly check https://www.sas.upenn.edu/LabManuals/biol275/Table_of_Contents_files/21-DiagnosticTests.pdf
Also check https://microbiologyinfo.com/starch-hydrolysis-test/
Several factors may affect the starch-iodine interaction, which may result in lower colour intensity and/or a shift in wavelength for maximum absorbance.
1. Amylose-lipid complexes (e.g. cereal starches or presence of fatty acids, monoglycerides or certain phospholipids) in sample diminish the amount of amylose-iodine complexation resulting in lower colour intensity (S. Bhatnagar, Cereal Chem. 71 (1994) 582-587)
2. Solvent pH or composition: Staining is optimal at neutral or (slightly) acidic pH. Presence of alkali, DMSO or alcohols in sample diminish colour intensity (A.J. Barrett, J. Macromol. Sci. A 35 (1988) 711-722.
3. Some proteins may interfere with the starch-iodine reaction (R.L. Searcy, Clin. Chim. Acta 12 (1965) 631-638).
4. Molecular degradation of starch (e.g. presence of active amylases in sample) will generally lead to a diminished starch-iodine interaction and a shift to lower maximum wavelength (W. Banks, C.T. Greenwood, Starch and its components, Edinburgh Univesity Press, 1975).
5. Decrease of amylose-amylopectin ratio of starch (e.g. in waxy starches) results in a lowering of iodine binding capacity and a shift to lower maximum wavelength = colour shift from blue to pink red (W. Banks, see above).
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