The activities of SOD and CAT can indicate the level of oxidative
stress in cells and are often used to evaluate the oxidative stress
caused by ROS [38]. The specific activities of both enzymes under
pH-shifting and constant pH (7.0) conditions were examined. As
shown in Fig. 3, the specific activities of SOD and CAT were elevated
by shifting pH to 5.0 after a 24 h cultivation. The specific activities of
CAT at 30 h, 36 h and 40 h were 38.1%, 43.6% and 47.2% higher than
those of the single pH at 7.0, respectively