## 3 Results  
### Batch and Fed-Batch Yarrowia lipolytica Cultures  
Three experiments were carried out in media containing by-products of the agro-food industry in a laboratory bioreactor under the conditions outlined in the methodology section (Section 2.2). There was evaluated the effect of the culture mode on the growth of yeast strain _Y. lipolytica_ KKP 379 and its ability to accumulate lipids. Microbial utilization of hydrophilic and hydrophobic low-cost substrates from the culture media (molasses and rapeseed oil after fish frying) was also investigated. In M1-b and M2-fb media, molasses hydrolysate was the only source of carbon and the main source of minerals. In the case of M1-b batch culture (Figure 1a), the lag phase lasted about 10 h. High oxygen demand of yeast cells was observed throughout the culture period. When the cells entered the logarithmic growth phase, the sugar content in the medium had already been at a low level of 8.29 g/L. After 16 h of culture in molasses medium, the biomass yield was only 2.9 g\({}_{\text{DCW}}\)/L, and after 24 h its value slightly increased to 3.0 g\({}_{\text{DCW}}\)/L. The low content of the carbon source in the cultivation medium made biomass growth constricted. With a very low sugar content of 3.81 g/L, the biomass yield after 40 h of culture was only 3.5 g\({}_{\text{DCW}}\)/L. In the final stage of culture, the yeast consumed almost the entire pool of available carbon source in the medium, reducing its content from 29 g/L to 2.7 g/L. The pH of the medium remained relatively constant at 5.5 until about 40 h, after which time it began to decrease slightly to 2.89 after 62 h.  
In M2-fb yeast culture (Figure 1b), the medium was supplemented with molasses hydrolysate after 16, 24, 40 and 48 h. It is worth noting that feeding the culture at the beginning of the logarithmic phase of growth promoted biomass growth. Admittedly, the adaptation phase was longer than in M1-b medium and the biomass yield after 16 h of culture was several times lower than at the same point in M1-b culture. After 48 h the biomass yield was 10.84 gDCW/L compared to 5.10 gDCW/L in the no-feeding mode. Finally, supplying hydrolysate to the culture medium as a source of essential components for the cells resulted in a biomass yield of 11.85 gDCW/L. Taking into account that wild yeast _Y. lipolytica_ is unable to break down sucrose, which is the main sugar of molasses, the cheap substrate was treated with acid to hydrolyze the disaccharide. Thus, the hydrolysate of molasses required neutralization before being added to the culture. Therefore, supplementing the medium with ingredients necessary for growth caused periodic fluctuations in the pH. As a result of the 62-h yeast culture, the content of simple sugars in the M2-fb medium decreased to 3.28 g/L. The initial content of hydrolyzed molasses was 50 g/L, which corresponded to 29 g/L of simple sugars. Taking into account the sum of molasses portions with which the M2-fb medium was supplemented during the culture, the yeast utilized a total of 141.72 g/L of simple sugars from the waste medium. The subsequent O1-fb culture was conducted in mineral medium with 5% post-frying rapeseed oil as a hydrophobic source of carbon for _Yarrowia_ cells (Figure 1c). In contrast to cultures conducted in molasses media, the culture was carried out for 90 h. Twice a day, 250 mL of medium was withdrawn from the bioreactor, and then the culture was supplemented with the defined amount of oily waste, as well as 250 mL of sterile concentrated YNB medium. The adaptation phase lasted nearly 24 h. After 16 h of culture, the level of waste oil in the culture medium decreased to 31.7 g/L, while the biomass yield was 0.6 gDCW/L. In an effort to provide the cells with the needed energy source, an addition of 20 g/L of post-frying rapeseed oil was used. Nonetheless, over the next 8 h, the yeast  
Figure 1: Changes in the oxygenation of the medium, pH, biomass yield of _Y.