The influence of the ion content of whey on the pH‐activity profile of the β‐galactosidase from Aspergillus oryzae

The pH‐dependence of the reaction kinetics of lactase (β‐galactosidase) from Aspergillus oryzae in different reaction media is presented in terms of a two‐parameter model. The lactase from A. oryzae seems to have replaced the A. niger lactase on the market owing to a better activity/price ratio and may be utilised for lactose hydrolysis in acid as well as in neutral milk products. Its pH optimum is around pH 4.5. However, in the neutral pH‐range its activity depended strongly on the salt content of the substrate solution. For example, its activity in whey (pH 6.5) fell to only 30% of its expected activity in a pure lactose solution at the same pH. The whey effect was the same for both soluble and immobilised lactase. The two parameter kinetic model, which included a term for competitive product inhibition gave excellent agreement with experimental data, and may thus be useful for the prediction of reactor performance with this enzyme.