## 3 Results
### Bioreactor Scale Experiments  
Four batch cultures of _Y. lipolytica_ KKP 379 strain in a laboratory bioreactor have been conducted to evaluate the effect of the previously selected factors. In each culture, changes in the pH of the culture medium and the rate of oxygen consumption have been monitored (Figure 2). Generally, in all variants of the culture medium, the yeast completed the logarithmic growth phase after nearly 16 h, thus entering the stationary phase associated with the production of metabolites such as acids, which was reflected in a significant decrease in pH, but most importantly the cells began to accumulate storage lipids. For M1 medium, in addition to the modification related to KH2 PO4 (3.0 g/dm3) and (NH4)2 SO4 (3.0 g/dm3) doses, the level of Na2 HPO4 supplementation (1.1 g/dm3) was also adjusted due to the buffering properties of the medium. The molar ratios C/N and C/P were 71.8:1 and 114.9:1, respectively. A significant increase in the oxygen demand of the cells has been observed from the 20 th hour of culture onwards, resulting in a decrease in the dissolved oxygen content of the medium (Figure 2A). During the entire cultivation process, the biomass yield value was marginal. Therefore, the high oxygen demand could be related to the increased lipid synthesis process due to the beginning of the stationary growth phase rather than to the oxygen demand caused by the intensive biomass growth, which did not occur. After 63 h of cultivation in mineral medium M1 with phosphorus and nitrogen source limitation, the efficiency of intracellular lipid accumulation by wild-type _Y. lipolytica_ cells was 47.44%, which is considered an astonishing result. The yeast cells also efficiently utilized the waste carbon source, leaving only 9.11 g/dm3 unused. Despite the lipid yield result, considering the increase in biomass, the amount of SCO obtained per unit of substrate was very low (Table 3). Figure 2: Changes in relative oxygen consumption and pH of the strain _Y. lipolytica_ KKP 379 grown in (**A**) M1, (**B**) M2, (**C**) M3 and (**D**) M4 medium with 5% rapeseed waste post-frying oil as a carbon source. The residual oil content of the substrate was a reflection of the biomass yield in the M2 substrate (C/N = 26.7:1, C/P = 114.9:1). A significant increase in biomass yield after 2 days of culture combined with a decrease in the residual carbon source in the medium was noticeable (Figure 2B). After 88 h of culture, the cells had used almost all of the waste post-frying oil, reducing its initial content in the culture medium from 50 to 1.56 g/dm\({}^{3}\), achieving at the same time the highest biomass yield (11.10 g\({}_{\rm d.m. }/\)dm\({}^{3}\)) among the other yeast culture variants (Table 3). In addition, there was a gradual decrease in oxygen demand at the end of the culture (71 h). Cells no longer multiplied intensively; moreover, lipids were not efficiently accumulated, gaining a final yield of 20.9%. Oxygen turned out to be a factor worth observing also in the case of the next batch culture in M3 medium. The addition of (NH\({}_{4}\))\({}_{2}\)SO\({}_{4}\) was further increased to 10 g/dm\({}^{3}\) (C/N = 21.5:1). Similarly, the level of the addition of phosphorus sources was slightly increased, KH\({}_{2}\)PO\({}_{4}\) 3.5 g/dm\({}^{3}\) and Na\({}_{2}\)HPO\({}_{4}\) 2.5 g/dm\({}^{3}\) (C/P = 81.6:1). In culture medium, the dissolved oxygen content remained constant from the 15 th hour until the end of the cultivation process. The amplitude range of the agitation speed appeared to be sufficient to maintain the oxygenation level of the culture medium at a level close to 30%. The constantly low level of oxygen consumption by the cells indicated low oxygen demand, which could be associated with moderate growth of yeast biomass (Figure 2C). Although, the biomass yield was only 4.56 g\({}_{\rm d.m.}/\)dm\({}^{3}\). The final waste post-frying oil content in the medium was relatively low at 12.44 g/dm\({}^{3}\).