As shown in Fig. 2, during batch cultivation cells were grown at a high specific growth rate of approximately 0.7 h−1. At 6.5 h, when glucose was exhausted, the highest acetate level was reached (1.74 g l−1) and afterward the acetate was consumed by the cells. The PSA production was consistent with the biomass formation pattern and the final PSA concentration was 0.47 g l−1. After 4.9 h the reactor could not provide sufficient oxygen to the cells, which led to the decrease of dissolved oxygen concentration to zero. From this time onwards, OTR (oxygen transfer rate) and CPR (carbon production rate) (Fig. 3) were also exhibiting another growth pattern, indicating that anaerobic respiration as well as aerobic oxidation occurred.
The oxygen transfer limitation, the high amount of acetate production and the low PSA productivity indicated that the batch cultivation was suboptimal. Therefore, fed-batch cultivations were exploited. Considering that the specific growth rate of 0.7 h−1 used in the batch cultivation was higher than the critical specific growth rate for acetate formation, a lower specific growth rate (0.25 h−1) was tested in the following fed-batch cultivation.