## 3 Results and Discussion
### Impact of pH  
The influence of pH (6.5, 7.0, 7.5, 8.0) on the fermentation performance was studied. The kinetics of glycerol utilization and the formation of products are depicted in Figure 4.  
Figure 3: Changes in concentrations of glucose, metabolites (lactic acid, acetic acid), and dry cell weight (DCW) of _C. freundii_ AD119 in a fed-batch culture vs. time at (**a**) 30 and (**b**) 36 \({}^{\circ}\)C.  
Figure 2: Production of 3-hydroxypropionaldehyde (3-HPA) with _C. freundii_ AD119 in fed-batch cultures with a fixed pH value of 7.0 and stirring rate of 80 rpm at various process temperatures. The maximal concentration of 1,3-PD of about 40 g/L was reached in 48 h in fermentations performed at a pH of 7.0-8.0, which was significantly higher in comparison to the process conducted at a pH of 6.5 (Table 2). \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Metabolites**}} & \multicolumn{4}{c}{**pH Value**} \\ \cline{2-5}  & **6.5** & **7.0** & **7.5** & **8.0** \\ \hline
1,3-PD (g/L) & 10.50 \(\pm\) 0.04 a  & 43.30 \(\pm\) 2.07 b  & 40.46 \(\pm\) 2.02 b  & 38.61 \(\pm\) 2.10 b  \\ Lactic acid (g/L) & 6.79 \(\pm\) 0.05 a  & 10.82 \(\pm\) 0.05 ab  & 12.87 \(\pm\) 0.27 b  & 11.58 \(\pm\) 1.43 b  \\ Acetic acid (g/L) & 4.45 \(\pm\) 0.02 a  & 9.65 \(\pm\) 0.04 b  & 8.62 \(\pm\) 0.45 b  & 9.05 \(\pm\) 0.07 b  \\ Succin acid (g/L) & 1.59 \(\pm\) 0.01 a  & 4.85 \(\pm\) 0.04 b  & 5.77 \(\pm\) 0.13 b  & 5.15 \(\pm\) 0.26 b  \\ Ethanol (g/L) & 0.84 \(\pm\) 0.01 a  & 1.86 \(\pm\) 0.09 b  & 1.35 \(\pm\) 0.27 b  & 2.82 \(\pm\) 0.02 c  \\ DCW & 1.26 \(\pm\) 0.01 a  & 3.03 \(\pm\) 0.14 b  & 2.34 \(\pm\) 0.02 c [FONOTE:b]Footnote b: maximum concentration of 3-HPA achieved; Means \(\pm\)SD with different superscript letters in the same row differ significantly (\(p<\) 0.05).[ENDFOOTNOTE] & 1.85 \(\pm\) 0.19 ac  \\
3-HPA (mmol/g DCW) & 22.00 \(\pm\) 2.55 a  & 4.12 \(\pm\) 0.34 b  & 5.00 \(\pm\) 0.23 b  & 4.39 \(\pm\) 0.31 b  \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Final concentrations of metabolites (1,3-propanediol (1,3-PD), lactic acid, acetic acid, succinic acid, ethanol) and dry cell weight (DCW) in fed-batch cultures of _C. freundii_ AD119 at various pH values. Figure 4: Time course of dry cell weight (DCW) and metabolite production (1,3-propanediol (1,3-PD), lactic acid, acetic acid, succinic acid, ethanol) with _C. freundii_ AD119 in a fed-batch culture with a fixed temperature of 30 \({}^{\circ}\)C and stirring rate of 80 rpm at pH (**a**) 6.5, (**b**) 7.0, (**c**) 7.5, and (**d**) 8.0. Batches were also performed at these pH values for 250 h which revealed that the pH of 7.0 was the optimum as it allowed obtaining the highest concentration of 1,3-PD (Figure 5). The concentration of 1,3-PD obtained when the culture was performed at pH 6.5 was over three-fold lower. Moreover, the concentrations of by-products and biomass obtained at this pH level were also decreased. A premature reduction of the 1,3-PD production rate was observed; thus, the concentration of 3-HPA was determined (Figure 6). The accumulation of 3-HIPA was observed at each pH value. Its concentration was below 5 mmol/g DCW in all the cultures except the one conducted at pH 6.5. In that case, the maximum 3-HPA concentration was reached after 9 h of fermentation and was above 20 mmol/g DCW. The coincidence of the peak in 3-HIPA concentration and process cessation leads one to the assumption that the accumulation of the intermediate was  
Figure 5: Effect of pH values on 1,3-propanediol (1,3-PD) production with _C. freundii_ AD119 in 250 h batches. Figure 6: Production of 3-hydroxypropionaldehyde (3-HPA) with _C. freundii_ AD119 in fed-batch cultures with a fixed temperature of 30 \({}^{\circ}\)C and stirring rate of 80 rpm at various pH values. the probable reason behind the premature end of fermentation. It has been proven by [PERSON] et al. [1,17] that low pH during fermentation results in 3-HPA accumulation and decreased 1,3-PD production.