3.2 Fed-batch fermentation
Different concentrations of medium components have been previously tested, and the optimal condition was used in this study for the fed-batch process (Liberman et al., 2011). Figure 2 shows that the highest OD was between 14-15, thus 1.5 fold higher than in the batch process. However, dry cell weight was not statistically different, 4.43 ± 0.17 g/L, perhaps because the medium was more concentrated for Soytone in the batch process (20 g/L) than in fed-batch (5 g/L), which could have affected the physiology of the microorganism. Since the OD of the culture is a measurement of light scattering, which can vary according to size, density, opacity and complexity of cells, changes in the medium composition could alter the relation between OD and dry cell weight.
Compared to the batch process, which had no glucose at the end of cultivation, there was some glucose remaining (6.9 ± 1.5 g/L) at the beginning of the stationary phase (5 h of cultivation) in the fed-batch process. Since a higher OD was reached, we hypothesize that the bacteria stopped growing because of high concentrations of inhibitor products, of which lactate would be a reasonable possibility, as observed by Callewaert and De Vuyst (2000), during fed-batch cultivation of another Gram-positive bacteria, Lactobacillus amylovorus . Lactate production in the fed-batch process was 28.4 g/L, which was 1.2 to 1.4 fold higher than in the batch process. The same increase in lactate production was observed when Ding and Tan (2006) compared batch and fed-batch processes for cultivation of Lactobacillus casei. On the other hand, acetate production was 1.4 to 2.3 fold lower in fed-batch than in batch process.