## 3 Results  
Impact of Concentration of Castor Oil, pH of the Medium, and Agitation Speed on Synthesis of \(\gamma\)-Decalactone in the Batch Culture Carried Out in the bioreactor  
Four bioreactor batch cultures were carried out according to the Taguchi design method (Table 1) aimed at optimizing three parameters of the biotransformation process in terms of GDL synthesis. In each culture, apart from lactone concentration (Figure 1), yeast growth parameters were also monitored, namely optical density, biomass yield, and the number of yeast colonies, as well as their hydrophobicity, by performing the MATH test (Figure 2). These parameters were selected as significant based on literature data and our previous research on GDL synthesis [13, 37]. In the first batch culture (Figure 1a), the substrate (castor oil) concentration was 75 g/L, a variable agitation rate ranging from 200 to 500 rpm was used, and the substrate pH initially at 7 was not corrected throughout the process cycle. The use of such parameters made it possible to obtain a maximum of 1.56 \(\pm\) 0.04 g/L of GDL in 37 h of biotransformation. On the first day of the biotransformation, the concentration of lactone increased exponentially, reaching the level of 1.26 \(\pm\) 0.13 g/L already in the 20 th hour of the reaction. After reaching the maximum (37 h), the concentration decreased, reaching the value of 1.01 \(\pm\) 0.11 g/L in the 64 th hour of cultivation. Literature data confirm a decrease in GDL concentration in biotransformation reactions conducted by wild-type yeast strain _Y. lipolytica_[13, 23, 38]. Therefore, in order to fully capture the kinetics of \(\gamma\)-decalactone biosynthesis by _Yarrovia_ yeast, cultures in bioreactor 1 were performed for 64 h. Observing the maximum concentration of the aroma compound at about 37 h of cultivation, the time of biotransformation reactions in subsequent bioreactors was reduced to two days. When analyzing the changes in GDL concentration in relation to the process parameters, it can be noticed that only after lowering the pH value below 5 in the medium, the concentration of GDL exceeding 1 g/L was recorded. The maximum concentration of lactone occurred during the period of intensive mixing (500 rpm), with the dissolved oxygen content in the medium at the level of approx. 10%. The parameters of yeast growth in bioreactor 1 (Figure 2a) indicated that on the first day of the experiment, the yeast cells were in the log phase. The OD increased dynamically, reaching the value of 14.06 \(\pm\) 1.79 in 20 h. During the second day of biotransformation, the cells went into the stationary phase, and since the 40 th hour of cultivation, the OD value remained at approx. 20. The biomass yield correlated with the OD values. Within 38 h of culture, a more than threefold increase in cellular biomass was observed, from the level of 7.22 \(\pm\) 1.87 g CDW/L in the 14 th hour of culture to 24.43 \(\pm\) 1.73 g CDW/L in 38 h. The number of cells in the initial period of the exponential phase, expressed as log CFU/mL, was less than 6 (16 h). After 48 h of culture, the number of colonies increased 100 times. Figure 1: Changes in dissolved oxygen, agitation speed, and concentration of \(\gamma\)-decalactone in the batch cultures of \(\gamma\). _liquid/fat_ yeast carried out in accordance with L4 orthogonal array. Batch culture parameters: (a) bioreactor culture 1—glutation speed of 200-500 rpm, unregulated pH, castor oil concentration of 75 g/L3 (b) bioreactor culture 2—agitation speed of 200–500 rpm, regulated pH, castor oil concentration of 50 g/L3 (c) bioreactor culture 3—agitation speed of 200 rpm, regulated pH, castor oil concentration of 50 g/L4 (d) bioreactor culture 4—agitation speed of 200 rpm, regulated pH, castor oil concentration of 75 g/L. In the second bioreactor culture (Figure 1b), the acidity of the medium and the concentration of the introduced substrate were modified. The pH was kept constant at 7 throughout the biotransformation process, and the concentration of castor oil was reduced from 75 g/L to 50 g/L.