Pilot scale fed-batch cultivations
After the optimal F:G ratio that had been determined in the chemostat experiments had been confirmed in the laboratory scale fed-batch experiments, the final step of the investigation was to test whether the obtained positive results of formic acid co-feeding on biomass yield were robust to scaling up to pilot scale. To this end, two out of the four variations of the laboratory scale fed-batch protocol were scaled up to pilot scale, having F:G ratios of 0 (experiment PF1, scale up of LF1), and 5 (PF2, scale up of LF3). Again, the dynamic fermentation process model proved valuable in qualitatively predicting the time profiles of key fermentation parameters during the seed and main fermentation stage before executing the experiments (see Supplementary Materials 1).
The 300 l pilot scale fed batch process demonstrated that also at this scale Y. lipolytica readily consumes the mixed glucose/ formic acid feed up to a molar ratio of 1:5. Throughout the fermentations, supernatant samples were taken and analysed for residual formic acid, and the concentrations were always low (<0.05 g/l) or below the detection limit. Figure 5 shows the O2 uptake rate profiles of the 2 pilot scale fermentations with the batch phases ending between 10-12h, followed by the carbon-limited fed batch phases. The oscillation of the OUR of PF2 between 12-16h was caused by a technical deviation when starting up the feed. The actual OUR profiles are in good agreement with the profiles of Error! Reference source not found. B that were simulated to design the experiment.