β-Galactosidase is an enzyme that catalyzes the hydrolysis of lactose. It has potential importance due to various applications in the food and dairy industries, involving lactose-reduced ingredients. The properties of two b-galactosidase enzymes, crude and purified, from different sources, Kluyveromyces marxianus CCT 7082 and Kluyveromyces marxianus ATCC 16045, were analyzed. The pH and temperature optima, deactivation energy, thermal stability and kinetic and thermodynamic parameters were determined, as well as the ability to hydrolyze lactose and produce galactooligosaccharides. Purification process improved the properties of the enzymes, and the results showed that purified enzymes from both strains had a higher optimum temperature, and lower values of Km, thus showing greater affinity for o-nitrophenyl-b-D-galactopiranoside than the crude enzymes. 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