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Thermal stability of lactase (β-galactosidase) enzyme has been studied by a variety of physico-chemical methods. β-galactosidase is the main active ingredient of medications for lactose intolerance. It is typically produced industrially by the Aspergillus oryzae filamentous fungus. Lactase was used as a model to help understand thermal stability of enzyme-type biopharmaceuticals. Enzyme activity (hydrolyzation of lactose) of β-galactosidase was determined after storing the solid enzyme substance at various temperatures. For a better understanding of the relationship between structure and activity changes we determined the mass and size of the molecules with gel electrophoresis and dynamic light scattering and detected aggregation processes. A bottom-up proteomic procedure was used to determine the primary amino acid sequence and to investigate changes in the N-glycosylation pattern of the protein. NMR and CD spectroscopic methods were used to observe changes in higher order structures and to reveal relationships between structural and functional changes.