Aspergillus foetidus as a potent producer for β-galactosidase utilizing lemon peels and coffee waste powder: production optimization, purification, kinetic and thermodynamic characterization

Background: The main obstacle facing the utilization of microbial enzymes in industrial applications is the high cost of production substrates. As a result of the mentioned different wastes (coffee powder waste, dates nawah powder, molokhia stems, pea peels, lemon peels, and corn cobs) were investigated as low-cost nutritional substrates for the production of microbial β-galactosidase in this study. The purification of the enzyme and its kinetic and thermodynamics were investigated.

Results: β-galactosidase was effectively produced by Aspergillus foetidus utilizing lemon peels and coffee powder waste by solid-state fermentation technique. The production yield was improved through Plackett-Burman Design declaring the significant effect of lemon peels and coffee waste powder, and beef extract quantities on A. foetidus β-galactosidase production. Followed by Central Composite Design investigating each factor with five levels resulting in 37363.1 U.ml^{- 1} production. The enzyme was fully purified by gel filtration technique through Sephadex G-150 giving one band with a molecular weight 40 KDa on SDS-PAGE gel. The maximal β-galactosidase activity was obtained at 50 °C with 0.4% ONPG. Cu²⁺, Fe²⁺, and Hg²⁺ showed severe inhibitory effect on pure enzyme activity. Energy required for enzyme activation (E_a) and denaturation (E_d) were determined to be 17.40, and 43.86 KJ.mol^{- 1}, respectively. Parameters reflecting β-galactosidase thermal stability at 40, 45, and 50 °C as T_1/2 and D-values values were determined to be 283.92, 209.43, and 168.56 min, and 943.34, 695.84, and 560.06 min, respectively.