## 4 Discussion
Therefore, yeast cells could absorb free fatty acids without the need for a high production of lipases, as confirmed by [PERSON] et al. [23], who observed a lower hydrolytic activity of _Y. lipolytica_ CECT 1240 (ATCC 18942) in a medium with oleic acid than in a medium containing olive oil. It can be deduced that there exists a level of free fatty acids that activates lipase synthesis, but exceeding the concentration gives a signal to inhibit enzyme production. Olive oil and corn oil were recognized as the best inducers of lipase synthesis in _Y. lipolytica_ 681 cultures [25]. Likewise, [PERSON] and [PERSON] [24] evaluated the effect of plant oils (olive, extra virgin olive, canola, corn, sunflower, and soybean oils) on lipase production by three different strains of _Y. lipolytica_. The maximum hydrolytic activity was detected for _Y. lipolytica_ NBRC 1658 in the canola oil medium, and _Y. lipolytica_ IFO 1195 in the soybean oil medium, while the carbon source did not affect the synthesis of lipases by the local wild-type strain. In turn, for _Y. lipolytica_ CECT 1240, a higher lipase activity was achieved in a culture with sunflower oil rather than olive oil [23]. [PERSON] and [PERSON] [25] were not able to find simple relationships between hydrolytic activity and the composition of the lipid substrate present in the medium (plant oil or fish oil) when growing 22 different _Y. lipolytica_ yeast strains. [PERSON] et al. [26] do not confirm the hypothesis that oleic acid is the only or the main stimulator of the synthesis of lipolytic enzymes by the yeast _Y. lipolytica_. The authors prepared cultures in which olive oil, rapeseed oil, and sunflower oil were used as lipid substrates. In the medium containing rapeseed oil, the activity concerning the olive oil was 5% higher, and the highest hydrolytic activity was achieved in the medium containing sunflower oil. The results by [PERSON] and [PERSON] support the hypothesis that a high oleic acid content in triacylglycered molecules is not the only or main factor that stimulates the high hydrolytic activity of _Y. lipolytica_ yeast, and that there are other factors determining enzyme synthesis, which the authors of the present study did not investigate. Comparing the results of different authors may prove difficult or even impossible, due to the lack of a standardized method for determining lipase activity, their wide variety, different culture conditions, and the different strains used in other authors' studies. Nevertheless, the results obtained so far show that the extracellular hydrolytic activity of the _Y. lipolytica_ species is a result of at least several factors, and it is most likely a mistake to correlate it only with the oleic acid content of the plant oil used as a carbon source in the culture medium. In addition to the factors mentioned above that make it difficult to compare results between different research centers, another important piece of this puzzle is the appropriate selection of the substrate for the determination of hydrolytic activity, which was also undertaken in the current work. Similarly, [PERSON] et al. [27] also used two esters of \(p\)-nitrophenyl, namely laurate and butyrate esters, for the comparison of the activity of esterases synthesized by _Anoxyobacillus gonensis_ A4. The hydrolytic activity of the esterases for these two substrates was 48 U/L and 347 U/L, respectively. Based on these results, it can be seen that the esterases hydrolyze both of these substrates; however, the activity against \(p\)-nitrophenyl butyrate was about seven times higher compared to that against \(p\)-nitrophenyl laurate. The determination of the substrate specificity of esterases towards short-chain fatty acids allowed for the development of a method for the determination of these enzymes, using \(p\)-nitrophenyl butyrate and fatty acid esters with a carbon chain length of more than 10 carbon atoms that have been reserved for lipases [27].