## 3 Results
### Effect of the Culture Mode on the Sterol Content and the Elemental Composition of the Biomass  
The content of sterols in microbial oils extracted from biomass cultured in molasses media and in medium with post-frying rapeseed oil has been also described. The content of cholesterol, dehydroregosterol, ergosterol, campesterol, stigmasterol and \(\beta\)-sistosterol determined in analyzed oils samples has been shown in Table 3. The lowest total sterol content 3.25 mg/goil was noted for microbial lipids from O1-fb culture, in which yeast biomass was fed with rapeseed oil after frying. In SCO derived from medium with post-frying oil, \(\beta\)-sitosterol proved to be the dominant sterol at 44.29%, followed by campesterol (25.22%), ergosterol (13.86%) and dehydroregosterol (12.67%). Stigmasterol accounted for 2.96% of the total sterols content and cholesterol 1.02%.  
In SCO sample from M1-b culture of _Yarrowia_ yeast in molasses medium, the total sterol content was 8.34 mg/goil, while in M2-fb culture sterol content amounted to 68.40 mg/goil. After 62 h in both variants of molasses culture M1-b and M2-fb, the sterol with the highest proportion was ergosterol, with a content of 57.95% and 87.94% respectively. In the culture M2-fb with molasses hydrolysate, the ergosterol content was 60.16 mg per gram of lipids extracted from biomass. In the M1-b medium without feeding mode, the ergosterol content at the end of the culture was 5.00 mg/g of oil. The another sterol with a significant percentage was \(\beta\)-sitosterol (18.25%) and campesterol (10.24%). Dehydroregosterol, cholesterol and stigmasterol were determined in smaller amounts (Table 3).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Culture Variant** & **Time** & **C16:0** & **C18:0** & **C18:1** & **C18:2** & **C18:3** & **C20:0** & **C22:0** \\ \hline M1-b & 62 h & 11.67 & 5.51 & 52.66 & 22.49 & 7.68 & n.d. & n.d. \\ \hline M2-fb & 62 h & 13.69 & 4.65 & 44.43 & 30.75 & 6.48 & n.d. & n.d. \\ \hline \multirow{6}{*}{O1-fb} & 40 h & 3.01 & 9.25 & 60.36 & 18.23 & 4.20 & 4.09 & 1.06 \\ \cline{2-8}  & 48 h & 4.09 & 11.08 & 58.43 & 18.88 & 2.94 & 3.75 & 0.92 \\ \cline{1-1} \cline{2-8}  & 62 h & 4.21 & 10.86 & 61.81 & 16.41 & 1.76 & 4.15 & 1.05 \\ \cline{1-1} \cline{2-8}  & 72 h & 5.20 & 10.13 & 63.17 & 15.26 & 2.23 & 3.61 & 0.68 \\ \cline{1-1} \cline{2-8}  & 90 h & 4.09 & 8.49 & 66.94 & 14.41 & 0.72 & 4.93 & 0.51 \\ \hline \end{tabular}
\end{table}
Table 2: The fatty acids profile (%) of microbial lipids extracted from biomass cultured in M1-b, M2-fb and O1-fb media.  
The elemental composition of _Y. lipolytica_ yeast biomass cultured in waste media was dominated by carbon, nitrogen, potassium, sodium, phosphorus and sulfur (Table 4). Yeast biomass fed with a lipid carbon source in the form of post-frying rapeseed oil was characterized by a higher content of carbon (601.12 g/kg), iron (0.59 g/kg) and aluminum (55.47 mg/kg) compared to cultures in molasses media. In contrast, biomass from M1-b and M2-fb cultures was richer in minerals such as nitrogen, sodium, sulfur, potassium, manganese, copper, zinc, cobalt and strontium. All biomass samples contained similar amounts of phosphorus (9.84-11.96 g/kg), magnesium (0.65-0.84 g/kg), lead (0.90-1.13 mg/kg) and barium (0.54-0.77 mg/kg). Cells fed with molasses hydrolysate obtained from M2-fb medium were richer than biomass from M1-b batch culture in potassium (47.54 g/kg), phosphorus (11.96 g/kg), calcium (2.13 g/kg), manganese (197.35 mg/kg), zinc (209.80 mg/kg) and strontium (6.45 mg/kg). On the other hand, biomass samples from the M1-b culture compared to the M2-fb culture contained more carbon (447.87 g/kg), nitrogen (65.56 g/kg), sulfur (13.61 g/kg), iron (0.46 g/kg), aluminum (19.25 mg/kg), chromium (10.71 mg/kg), nickel (15.08 mg/kg) and zirconium (10.71 mg/kg).