Figure 4: the O2uptake rate (OUR) of the four laboratory scale fed batch fermentations with increasing molar ratios of F:G and decreasing glucose concentration (see Table 10). The 4 fermentations had F:G ratios increasing from 0 (LF1, black), 3 (LF2, red), 5 (LF3, blue) to 7 (LF4, green).
The base titrant used in the protocol is ammonia, which simultaneously serves as N-source for biomass formation. Despite the low pH of the carbon feed solutions that contain high formic acid concentrations, the titrant dosing to the fermentations is limited (between 100 and 125 g 25 wt% NH3), and varied little between the 4 fermentations. The ammonia level in the broth was between 0.4 and 0.9 g/l for all 4 fermentations at all timepoints.
The key results of fermentations LF1-4 are summarized in Table 2. The results agree with the chemostat findings which showed that co-feeding formic acid with glucose increases the biomass yield on glucose up to a molar ratio of about 5:1, and that further increasing the ratio gives no benefit in terms of yield. Again, these results correspond well with the predicted output of the fermentations shown in Error! Reference source not found. D with one exception: the highest yield for the formic acid: glucose ratio of about 7:1 that is predicted by the model is not observed in practice. The cause of this discrepancy is that the model assumes a fixed, positive ATP yield for each molecule of formic acid consumed, whereas the chemostat results indicated a decoupling of ATP formation from formic acid dissimilation past a F:G ratio of 5:1.
Table 2: Average biomass yield on glucose (Yxs) obtained in the laboratory scale and pilot scale fermentations