The article concern modelling of starch liquefaction in the presence of alpha-amylase from Bacillus globigii. The considered models are based on Michaelis-Menten equation and take into account the possibility of action of two enzyme isoforms, negative cooperation with substrate or more/less susceptible fractions of the substrate. The enzyme inactivation and influence of initial starch concentration on substrate conversion degree in equilibrium state is also considered.