Cereal alpha-amylase assay reagent cannot be used at pH 3-4.  However, you can perform a similar assay using blocked p-nitophenol maltoheptaoside (BPNPG7). Basically dissolve the BPNPG7 vial contents in 10 mL with water.   Incubate 0.2 mL of your enzyme (buffered) with 0.2 mL of BPNPG7 for 10 min at a set temperature.  Terminate the reaction by heating in a boiling water bath for 2 min. Then add 0.2 mL of alpha-glucosidase (E-TSAGL; at 10 U/mL in 0.2 M tris buffer, pH 7.0) and incubate for 10 min.  Add 3 mL stopping reagent (Trizma base pH 8.5) and read colour at 400 nm.

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