## 3 Results and Discussion
### Evaluation of C. curvatus Growth on Different Concentrations of VFAs  
_C. curvatus_ appeared to tolerate higher concentrations of VFAs as can be seen by the VFA consumption (Figure 4), and biomass and lipid accumulation (Figure 5). Figure 3: (**A**) Biomass (Y\({}_{\rm X/S}\)) and (**B**) lipid (Y\({}_{\rm L/S}\)) yield for _R. toruloides_ when grown on different concentrations of synthetic VFAs and glucose (GC). At an initial concentration of 10 g/L, all the three different VFAs were completely consumed (Figure 4A). Similarly with _R. toruloides_, acetic acid was the most favorable VFA with a growth rate close to that of glucose. Increasing the concentration to 20 g/L did not impact the ability of the yeast to completely consume all three VFAs, albeit at a slower rate. By further increasing the concentration to 40 g/L, acetic and propionic acid were completely consumed, despite at a slower rate. However, the growth of _C. curvatus_ on butyric acid slowed down significantly, although a final consumption of 94.7% was finally reached. When the initial concentration increased to 60 g/L, acetic acid was again completely consumed, but no growth was observed with propionic and butyric acid (data not shown). Finally, to identify the upper limits of acetic acid tolerance, an initial concentration of 80 g/L was used, where the consumption of acetic acid was only 22.6% (data not shown). Increased tolerance of _C. curvatus_ to higher concentrations of acetic acid has been previously reported [48,49]. However, the maximum concentration that the yeast could completely utilize was 30 g/L [49], while in our study, the strain tested was capable of fully consuming a double concentration of acetic acid (60 g/L). In a previous study where the tolerance of _C. curvatus_ was tested on propionic and butyric acid [49], it was found that the highest VFAs concentration before yeast growth starts to slow down at 15 g/L for both acids. In our study, we demonstrate an increased tolerance of _C. curvatus_ towards propionic and butyric acid up to 40 g/L initial concentration. Figure 4: Consumption of VFAs and glucose throughout cultivation in 10 g/L (**A**), 20 g/L (**B**), 40 g/L (**C**), and 60 g/L (**D**) initial concentration for _C. curvatus_. The lipid and biomass accumulation of _C. curatus_ when using VFAs as a carbon source was different than that of _R. toruloides_ (Figure 5). Propionic and butyric acid seemed to also be favorable for the growth and lipid accumulation of _C. curatus_ alongside acetic acid. At 10 g/L initial carbon concentration (Figure 5A), the highest DCW was obtained with glucose (7.5 g/L), followed by butyric acid (6.5 g/L). DCW with acetic and propionic acid was similar (4.8 g/L and 4.9 g/L, respectively). The highest lipid accumulation was achieved with butyric acid (2.3 g/L) together with the highest lipid content (27.3\(\%\)\(w\)/\(w\)). Acetic and propionic acid proved to be also good substrates for lipids production (1.5 g/L for C2 and 1.4 g/L for C3) with a high lipid content (25.2% \(w\)/\(w\) for C2 and 23.3% \(w\)/\(w\) for C3). With glucose as a carbon source, the lipids production was lower (1.1 g/L and 17.3% \(w\)/\(w\)) compared to VFAs. When the concentration increased to 20 g/L (Figure 5B), biomass accumulation was the highest with butyric acid (10.1 g/L), followed by glucose (9.4 g/L), and acetic and propionic acid, which resulted in similar DCW values (6.4 g/L for C2 and 6.3 g/L for C3). Even though the lipid accumulation on butyric acid was the highest (4 g/L), this was not the case with the lipid content, which was higher for propionic acid (46.2% \(w\)/\(w\)), followed by acetic acid (45% \(w\)/\(w\)) and butyric acid (39.5% \(w\)/\(w\)). It can be  
Figure 5: DCW (g/L), total lipids (g/L), lipid content (% \(w\)/\(w\)) and substrate consumption (% \(w\)/\(w\)) on the last day of cultivation of _C.