In plate tests, you should see a cleaning zone where microbial cultures grow.  The Red Starch is depolymerised by alpha-amylase and will diffuse throughout the plate giving a zone of lighter colour.  A concentration of 0.1% is correct.  You may try even lower.  Ensure that the pH of the Agar is in line with enzyme requirements, i.e. pH 4.5-5.5 for fungal and 6.0-7.0 for bacterial alpha-amylases.

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