in the β-galactosidase purification processes. Additionally, the influence levels of the pH (6.5, 7.7, or 7.5), membrane molecular weight cut-off (30, 50, 60, or 100 kDa), operating pressure (1.5, 2.0, or 2.5 kgf/cm2), and ionic strength of the ultrafiltration using NaCL or KCl (0....