The effects of different pH values, ranging from 4.0 to 7.0, on cell growth and -carotene production by
recombinant industrial wine yeast Saccharomyces cerevisiae T73-63 in a synthetic grape juice medium
was investigated. Based on the kinetic analysis of the batch fermentation process, a two-stage pH control
strategy was developed in which the pH was maintained at 7.0 for the first 24 h and then shifted to
5.0 after 24 h. Using this strategy, the highest -carotene production (50.39 mg/l) and the formation
rate (1.40 mg/l/h) were increased by 19.1% and 18.6%, respectively, compared to the maximum values
of constant pH fermentation. The oxidative stress during -carotene production was also determined
in terms of the catalase (CAT) and superoxide dismutase (SOD) activities. Oxidative stress appears to be
induced by the lowering of pH as indicated by the increase in activities of CAT and SOD due to pH shift from
pH 7.0 to pH 5.0. Pre-treating cells with ascorbic acid (an antioxidative agent) reversed the improvement
of -carotene production while addition of H2O2 enhanced it. Considering that induction of oxidative
stress is associated with increased -carotene production, it was concluded that the enhancement of
-carotene production by the low-pH strategy involved the induction of oxidative stress.