## 3 Results and Discussion
### Single-Cell Oil (SCO)  
#### 3.4.1 Fatty Acid Composition of the SCO  
The fatty acid profile of the two yeasts is depicted in Figure 8 and is mainly comprised of palmitic acid (C16:0) (9.3-31.4%), stearic acid (C18:0) (3.0-22.0%), oleic acid (C18:1) (4.8-50.65%), and linoleic acid (C18:2) (4.3-23.4%). This composition is in accordance with previous studies that showed an abundance of C18 and C16 fatty acids [3; 46; 48; 49; 52; 55; 56; 57; 58; 59; 60]. Although the fatty acid profiles had similar major fatty acids alongside the tested conditions, an impact of the carbon source on the fatty acids was observed. For _R. toruloides_, a small amount of myristic acid (C14:0) was observed (up to 1.8%) when glucose and acetic acid are used as substrates. Additionally, the fatty acids for _R. toruloides_ showed a higher abundance of palmitic acid (C16:0) compared to _C. curvatus_, which contained higher amounts of stearic acid (C18:0) instead. Increased production of margaric acid (C17:0) (up to 25%) and heptadecenoic acid (C17:1) (up to 23.9%) was noted when propionic acid was used as a substrate, which was higher for _C. curvatus_ than for _R. toruloides_. The oleic (C18:1) and linoleic (C18:2) acid content was similar between the two yeasts.  
It is worth noting that when acetic and butyric acid were used as carbon sources for the growth of _C. curvatus_, there was an increased formation of the long-chain fatty acid lignoercic acid (C24:0) (1.9-4.6%) compared to glucose (1.3-2.9%) and to the lignoercic acid produced by _R. toruloides_ (0.4-1.2%). Small production of lignoercic acid was also reported in another study using acetic acid to test the growth of _C. curvatus_[60] and _R. toruloides_[46]. For _C. curvatus_, a small production of pentacosanoic acid (C25:0) (up to 6.5%) was also noted in some cases. The shift from a C/N ratio of 50 to 100 for glucose and acetic acid did not result in changes in the fatty acid profile for either yeast. The same observation was previously reported by [PERSON] et al. [58].  
Figure 8: _Cont._When waste-derived VFAs were used, a small production of margaric acid (C17:0) (2.5%) and heptadecenoic acid (C17:1) (1.0%) was noticed in _C. curvatus_, which is due to the presence of propionic acid, together with a higher amount of lignoeceric acid (4.6%) and pentacosanoic acid (6.5%). _R. toruloides_ had a similar fatty acid profile to _C. curvatus_, with a production of margaric acid (2.7%) and heptadecenoic acid (2.9%). In addition, there was a higher production of lignoeceric acid (4.3%) as well.  
Table 3 shows the levels of saturated fatty acids (SFAs), monounsaturated fatty acids (MUFAs), and polyunsaturated fatty acids (PUFAs) for both yeasts and all the tested conditions. In all cases, when propionic acid was used as a carbon source, the produced lipids had a higher percentage of SFAs (50.9-65.9%) and the lowest MUFA content (16.0-16.6% for _RT_ and 20-44.8% for CC). When BSG VFAs were used, the lipids produced by _C. curvatus_ had higher amounts of SFAs (45.1%), while the lipids produced by _R. toruloides_ had higher amounts of MUFAs (57.2%).  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & & _C. curvatus_ & & & \\ \hline
**Concentration (g/L)** & **Substrate** & **SFA (\%)** & **MUFA (\%)** & **PUFA (\%)** & **OCFAs/Total Lipids (\%)** & **OCFAs (g/L)** \\ \hline  & GC & 37.8 & 42.7 & 19.6 & 0.0 & 0.00 \\  & C2 & 42.1 & 43.0 & 14.9 & 0.4 & 0.01 \\  & C3 & 54.1 & 38.4 & 7.5 & 58.1 & 0.80 \\  & C4 & 42.5 & 45.9 & 11.6 & 4.8 & 0.11 \\ \hline  & GC & 42.1 & 45.3 & 12.6 & 0.0 & 0.00 \\  & C2 & 43.2 & 44.1 & 12.7 & 0.5 & 0.01 \\
20 & C3 & 50.9 & 44.8 & 4.4 & 67.7 & 1.75 \\  & C4 & 43.8 & 45 & 11.1 & 2.7 & 0.11 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Percentage (%) of saturated FAs (SFAs), monounsaturated FAs (MUFAs), polyunsaturated FAs (PUFAs), and odd-chain FAs (OCFAs) of the total FAs produced.  
Figure 8: Fatty acid (FA) profile of the lipids produced with (**A**) _C. curvatus_, and (**B**) _R. toruloides_.