A pH control strategy for increased β-carotene production during batch fermentation by recombinant industrial wine yeast

The effects of different pH values, ranging from 4.0 to 7.0, on cell growth and β-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 β-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 β-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 β-carotene production while addition of H₂O₂ enhanced it. Considering that induction of oxidative stress is associated with increased β-carotene production, it was concluded that the enhancement of β-carotene production by the low-pH strategy involved the induction of oxidative stress.