The hydrolysis of lactose in skimmed milk and beta-galactosidase inactivation studies were carried out in three different devices bioreactor, sonicator and homogeniser to evaluate the performance of such reactors that have different operational systems. The experiments were carried out using beta-galactosidase produced from Kluyveromyces marxianus lactis. At the optimum process conditions obtained from the experiments performed in bioreactor, sonicator and homogeniser, 89%, 90% and 54% of lactose were hydrolysed and the enzyme lost its activity by 14%, 13% and 24%, respectively, at the end of the processing time of 30 min. The commercial milk lactose content (1 g/L lactose) was reached at 60 min for bioreactor and sonicator. After evaluation of the data, it was found that the kinetics of hydrolysis and enzyme inactivation could be represented by a first-order kinetic model and a single-step non-first-order enzyme inactivation kinetic model, respectively, for all process conditions applied. The activation energy for hydrolysis reaction and the enzymatic inactivation energy values were also calculated.