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Production of a thermostable pullulanase by DO-stat fed-batch fermentation of recombinant Escherichia coli BL 21 was investigated in a 5 L of fermentor. The effect of three oxygen control strategies, glucose feedback, shifting fermentor pressure, and adding oxygen-enriched air, on cell growth and pullulanase expression were examined. The oxygen-transfer capacity was found to be enhanced with increasing fermentor pressure and oxygen ratio in oxygen-enriched air, but the cell growth and pullulanase production were restrained under high fermentor pressure. The highest cell density and pullulanase activity reached 55.1 g/L and 412 U/mL, respectively, in the case by adding oxygen-enriched air, which was suggested as an effective approach to enhance both cell growth and pullulanase production. PRACTICAL APPLICATIONS: This thermostable pullulanase displayed optimal activity at 90°C and pH 5.4, which could be applied for one-step saccharification of starch biomass. The optimization of the DO-stat fed-batch fermentation in high cell density level would provide a research basis for its industrialization.
Chi et al. (Mon,) studied this question.