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.