Minimal salt medium was used to facilitate the subsequent purification of the peptidase compared with a cultivation in milk. The minimal salt medium (Liao, 1989) had the following composition: 2.0 g L−1 KH2PO4, 7.0 g L−1 K2HPO4, 0.2 g L−1 MgSO4·7H2O, 2.0 g L−1 (NH4)2SO4 and 8.0 g L−1 glycerol (86%, v/v). The pH value of the medium was 7.1 ± 0.1 (not adjusted). An amount of 60 mL peptidase inducer per L sterile minimal salt medium was added, as described by McKellar and Cholette (1984).