One unit is the amount of enzyme required to release one
micromole of glucose from starch per minute, under standard assay
conditions (starch substrate) (10 mg/mL starch; pH 4.5; 40˚C).
Or
One unit is the amount of enzyme required to release one micromole of p-nitrophenol from p-nitrophenol beta-maltoside (in the presence of excess beta-glucosidase) (10 mM pNP betamaltoside; pH 4.5; 40˚C).