Continuous 2-keto-L-gulonic acid fermentation from L-sorbose by Ketogulonigenium vulgare DSM 4025

A single-stage continuous fermentation process for the production of 2-keto-L-gulonic acid (2KGA) from L-sorbose using Ketogulonigenium vulgare DSM 4025 was developed. The chemostat culture with the dilution rate that was calculated based on the relationship between the 2KGA production rate and the 2KGA concentration was feasible for production with high concentration of 2KGA. In this system, 112.2 g/L of 2KGA on the average was continuously produced from 114 g/L of L-sorbose. A steady state of the fermentation was maintained for the duration of more than 110 h. The dilution rate was kept in the range of 0.035 and 0.043 h(-1), and the 2KGA productivity was 3.90 to 4.80 g/L/h. The average molar conversion yield of 2KGA from L-sorbose was 91.3%. Under the optimal conditions, L-sorbose concentration was kept at 0 g/L. Meanwhile, the dissolved oxygen level was changing in response to the dilution rate and 2KGA concentration. In the dissolved oxygen (DO) range of 16% to 58%, it was revealed that the relationship between DO and D possessed high degree of positive correlation under the L-sorbose limiting condition (complete consumption of L-sorbose). Increasing D closer to the critical value for washing out point of the continuous fermentation, DO value tended to be gradually increased up to 58%. In conclusion, an efficient and reproducible continuous fermentation process for 2KGA production by K. vulgare DSM 4025 could be developed using a medium containing baker's yeast without using a second helper microorganism.