In the case of SOD at the
corresponding time points, the values were 34.3%, 49.0% and 57.8%,
respectively. This indicates that oxidative stress was induced by
low-pH and yeast is under oxidative stress during -carotene formation.
In the second experiment, the antioxidative agent ascorbic
acid (10.0 mM) was added at 22 h at a constant fermentation pH
of 7.0 to pre-treat the cells for 2 h. The pH was then shifted to 5.0.
Ascorbic acid is a good antioxidant that protects yeast from oxidative
damage (from scavenging ROS), and its presence can alleviate
the antioxidative role of -carotene in oxidative stress and reduce
carotenoid production [39,40]. As expected, the pre-treatment with
ascorbic acid reversed the improvement of -carotene production
by the low-pH stress strategy because there were no significant
differences in specific -carotene content between the pH-shifting
and constant pH (7.0) condition (Fig. 4). This suggests that ascorbic
acid is a substitute for -carotene to protect the cells from oxidative
stress and thus counteract the stimulation of -carotene formation
by low-pH stress.