The amyloglucosidase (AMG) present in ground malt is most probably of fungal origin, i.e. A. niger.  The flour should be extracted with 100 mM sodium acetate buffer, pH 4.5 at 1 gram flour per 10 mL of buffer.  You should note that the PNP-beta-maltoside substrate is also susceptible to hydrolysis by alpha-glucosidase, so it would be difficult to know which of these enzymes you are measuring.

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