## 3 Results
This cultivation variant allowed us to obtain approx. 2.15 \(\pm\) 0.04 g/L of GDL, which was an approx. 38% higher value in relation to bioreactor 1. The maximum concentration was recorded in 45 h of the reaction, and the cultivation was not carried out any longer. Figure 2: Growth curves illustrating of either OD\({}_{600}\) readings (blue line) and biomass yield (violet line) or viable cell counts (CFU/mL) (black line) and percentage of adhesion to decane (yellow line) for culture 1 (**a**), culture 2 (**b**), culture 3 (**c**), and culture 4 (**d**). Therefore, it can be assumed that it was possible to obtain a higher concentration of lactone while extending the time of the biotransformation reaction. During this cultivation, the highest concentration of the fragrance compound was obtained, unlike for bioreactor 1, during slow mixing (200 rpm) and at a much higher concentration of dissolved oxygen in the medium, approx. 76%. Both the oxygenation level of the medium and the analyzed yeast growth parameters (Figure 2b) indicated that the highest observed concentration of lactone in the medium was at the final stage of the stationary phase. Comparing the data from bioreactors 1 and 2, it can be seen that on the first day of the experiment, the growth of yeast cells in both cultures was similar--after 21 h in bioreactor 2, the biomass yield was 11.95 \(\pm\) 3.04 g CDW/L, while in bioreactor 1, it was 14.13 \(\pm\) 0.04 g CDW/L. The number of live cells was also comparable, at the level of approx. 8-8.2 log CFU/mL. The difference in cell growth became apparent on the second day of biotransformation. In bioreactor 2, cells died faster, so that after 43 h, the living cell population had decreased by almost four log orders--to 4.58 \(\pm\) 0.65 log CFU/mL. The hydrophobicity of yeast cells correlated with their growth phases. During the first 24 h of culture, the degree of cell adhesion to decane was over 60%, while at the end of the stationary/slow dying phase, it was only about 20.1% (42 h of culture). The third bioreactor culture (Figure 1c) was carried out by modifying the agitation speed (set at a constant level of 200 rpm) and the pH of the medium (initial pH = 7, not corrected during the cultivation) relative to the second bioreactor. The castor oil concentration was 50 g/L. Under these conditions, the maximum recorded concentration of GDL was 0.19 \(\pm\) 0.01 g/L (45 h of biotransformation). It was over 8 times lower than in the first culture and over 11 times lower than in the second culture. The concentration of dissolved oxygen in the medium was close to zero since the 12 th hour of cultivation. Following the yeast growth parameters (Figure 2c), it is clearly visible that between 15 h and 23 h, there was a relatively small increase in OD compared to the previous bioreactor cultures, from the value of 4.04 \(\pm\) 0.03 to 5.79 \(\pm\) 0.94. After 39 h of culture, the optical density was only 6.30 \(\pm\) 1.06. In this culture, the yield of yeast dry matter did not exceed 7.2 g CDW/L. The slight increase in biomass was accompanied by a small number of colonies. After 15 h of culture, the number of viable cells was 5.56 \(\pm\) 0.28 log CFU/mL and during the second day of culture, it increased statistically insignificantly to the value of 5.91 \(\pm\) 0.41 log CFU/mL. Between 15 h and 39 h, the hydrophobicity of _Yarrowia_ cells decreased from 49.7 \(\pm\) 5.3% to 24.4 \(\pm\) 4.1%. This may be related to the very low content of dissolved oxygen in the medium [39]. Figure 1d shows the data on the fourth bioreactor setting. This culture used a constant pH of 7 (throughout the cultivation period) and a constant agitation speed of 200 rpm, with a castor oil content of 75 g/L. As a result of the fourth biotransformation, 0.55 \(\pm\) 0.01 g/L of lactone was obtained in 46 h. Between 16 h and 25 h, the concentration of GDL increased from 0.09 \(\pm\) 0.01 g/L to 0.20 \(\pm\) 0.01 g/L, and the level of dissolved oxygen in the medium was 2-3%.