Kinetic modelling of the thermal and pH inactivation of a thermostable β-galactosidase from Thermus sp. strain T2

β-Galactosidase from Thermus sp. strain T2 is a promising thermostable enzyme for the hydrolysis of lactose at temperatures from 60 to 70 °C. In this work, thermal and pH stability of such enzyme were investigated in a lacteous buffer containing 50 g L−1 lactose. Temperatures ranged from 60 to 110 °C while pH values cover the pH range for the potential application of the enzyme to hydrolyse lactose in dairy industry: from 3 to 4 of the acid whey to 6.5 of milk, as well as the basic pH used for immobilisation-stabilisation of the enzyme. Thermal inactivation kinetics followed two trends depending on the temperature range: from 60 to 90 °C an increase in the activity as the temperature rises was followed by its final decrease to a slightly active species; however, at higher temperatures follows a simple exponential trend with no residual activity. A kinetic model involving two consecutive first-order reactions with residual activity was chosen for the first temperature range, and a simple first-order reaction model was taken for the higher temperature range. With pH, again, two different tendencies were observed: a biphasic behaviour in the acidic range with final stabilisation of the activity depending on the pH value and a two-stage behaviour in the neutral-basic range, with a fast deactivation followed by a certain reactivation. As in the case of lower temperatures, a two first-order reaction kinetic model with residual activity fits deactivation data. The known presence of aggregates in initial enzyme preparations suggests inactivation routes where an initial disruption of aggregates leads to trimers, which are finally broken or reorganization of the aggregates.