Kinetic analysis of the fermentation process controlled at individual
pH conditions demonstrated that cell growth and sugar
consumption were facilitated at a higher pH, while -carotene production
was optimal at a lower pH. Hence, maintaining one single
pH throughout the fermentation process may not be optimal for
maximal carotenoid production. As presented in Fig. 1, the cell
growth entered mid-exponential phase after 24 h when controlled
pH was at 7.0 (Fig. 1A), while -carotene concentration (the specific
-carotene content) increased more quickly at a pH of 5.0 after
a 24 h cultivation compared to that of other pHs (Fig. 1C). Therefore,
a two-stage pH control strategy was proposed. In this strategy,
pH was controlled at 7.0 in the first 24 h of fermentation to facilitate
the cell growth and then switched to 5.0 after 24 h to enhance
the -carotene production (Fig. 2). Using this strategy, the biomass (DCW) continuously increased and reached 11.40 g/l at 36 h. The
highest specific -carotene content (4.62 mg/g DCW) was achieved
at 32 h, which was 19.4% higher than that of constant 7.0 pH condition;
after which, the value decreased slightly (4.42 mg/g DCW at 36 h).