Heterologous Expression and Enzymatic Characterization of a Stable β-Galactosidase from Aspergillus niger

β-Galactosidase is an important enzyme for lactose hydrolysis because it catalyzes the conversion of lactose into glucose and galactose. In this study, Aspergillus niger C18, which showed β-galactosidase-producing ability during preliminary screening, was selected as the gene source. A β-galactosidase gene from this strain was cloned into the pET28a vector and heterologously expressed in Escherichia coli. Solid-state fermentation conditions were optimized to produce the native enzyme as a reference for comparison. The enzymatic properties of the recombinant enzyme were then systematically characterized and compared with those of the native enzyme. The recombinant β-galactosidase exhibited favorable thermal and pH stability. After incubation for 2 h at its optimal pH and optimal temperature, the recombinant enzyme retained 88.9% and 94.1% of its initial activity, respectively; specifically, 88.9% corresponded to pH stability and 94.1% corresponded to thermal stability. These results indicate favorable stability of the recombinant enzyme under the tested conditions. Thin-layer chromatography and high-performance liquid chromatography analyses confirmed that the recombinant enzyme efficiently hydrolyzed lactose in a model lactose solution, achieving more than 99.0% lactose degradation after 12 h of reaction. These findings suggest that β-galactosidase derived from A. niger C18 is a promising candidate for lactose hydrolysis.