## 3 Results and Discussion
### Single-Cell Oil (SCO)
#### 3.4.3 SCO from Oleaginous Yeasts towards the Production of Odd-Chain Fatty Acids (OCFAs)  
When propionic acid was provided as a carbon source for _C. curvatus_, margaric acid (C17:0) accounted for up to 25% of the total fatty acids and heptadecenoic acid up to 23.9% of the total fatty acids. At the same time, there was a huge decrease in the amount of C18 fatty acids and an increased formation of odd-chain fatty acids such as pentadecanoic acid (C15:0) (up to 4.1%) and nonadecanoic acid (C19:0) (up to 9.8%). For _R. toruloides_, the same behavior was observed with propionic acid, where margaric acid up to 19.7% was formed and heptadecenoic acid up to 11.1%. However, no production of nonadecanoic acid was noted. When butyric acid was used, both yeasts synthesized a very small amount of margaric acid of up to 2%. The use of propionic acid as a substrate has already been associated with the biosynthesis of odd-chain fatty acids [3; 50; 61]. However, the main OCFAs that have been reported are C15:0, C17:0, and C17:1, while in our study, the formation of C19:0 and C25:0 were also noted by _C. curvatus_. It has also been shown that the combination of propionic and butyric acid in the absence of acetic acid for _C. curvatus_ increases the OCFAs [49]. Additionally, it has been indicated that the even-numbered fatty acids, acetic and butyric acid, are inducing the biosynthesis of even-numbered fatty acids since they are converted to acetyl-CoA, in contrast to the propionic acid which is converted to propionyl-CoA [37; 55]. This is also verified by the findings of this study.  
Table 3 summarizes the percentage of OCFAs in the total lipids, alongside their concentration. It can be thus assumed that the formation of OCFAs should be combined with a satisfactory total lipid concentration to yield high production. The use of propionic acid alone does not yield high lipid concentrations, especially for _R. toruloides_; therefore, the OCFAs concentration is low. As it has been previously suggested, the combination of different VFAs in different ratios will benefit not only the lipids production but also the formation of OCFAs. This combination will differently impact the two yeasts since they show different responses toward individual VFAs. By the observations of this study, it can be assumed that _R. toruloides_ will result in higher lipids production by an increase in the acetic acid concentration, while _C. curvatus_ by an increase in the butyric acid concentration, which is in line with the reports of [PERSON] et al. [49]. It can be also concluded that the production of OCFAs is more pronounced for _C. curvatus_ than _R. toruloides_, indicating the suitability of this yeast to use VFAs towards microbial oils that can be applied not only in the traditional biofuel industry but also for simultaneous production of high-quality fatty acids with a wide range of applications. Further studies are needed in order to optimize the production of microbial lipids towards the formation of OCFAs, combining different VFAs in varying ratios with different cultivation methods. Metabolic studies can further improve the naturally produced amounts of OCFAs by oleaginous yeasts. This approach was employed using _Yarrowia lipolytica_ and propionic acid as a carbon source. The maximum concentration of OCFAs that was produced by the engineered strain reached 0.75 g/L [37], however, in our study, the maximum amount of naturally occurring OCFAs reached 2.17 g/L by _C. curvatus_ using 40 g/L of propionic acid. With the waste-derived VFAs as substrate, _C. curvatus_ produced 0.15 g/L OCFAs, while _R. toruloides_ 0.10 g/L.