## 4 Discussion
Research on _Yarrowia_ yeast has shown that this microorganism has genes coding for enzymes specializing in the degradation of hydrophobic substrates ([PERSON] et al., 2017). _Yarrowia lipolytica_ cells contain six acyl-CoA oxidases (Aox1 to Aox6, encoded by the POX1 to POX6 genes). POX gene products show different substrate carbon chain degradation abilities. Aox2 oxidase is specific for long-chain substrates, while Aox3 shows activity towards short-chain compounds, leading the oxidation process of the 10-carbon chain and thus contributing to the degradation of the GDL produced ([PERSON] et al., 2017). This was confirmed in the studies by [PERSON] et al. (2017) comparing the production of GDL by the wild W29 strain and the MLYP40-2p mutant strain of the yeast _Yarrowia lipolytica_. According to [PERSON] and [PERSON] (2018) the most probable route for GDL degradation is the opening of the lactone ring through gamma-lactonase activity, followed by the activation of the CoA esters and \(\beta\)-oxidation. The lactone oxidation pathway is also possible, producing a dicarboxylic acid after delactonization. According to [PERSON] et al. (2018), an intense decrease in lactone concentration may also occur after the depletion of ricinoleic acid in the medium. Regarding the efficient hydrolysis of castor oil and biotransformation of ricinoleic acid, the contact of the lipid droplet surfaces with the cells of microorganisms is also important. Although the present research did not show a statistically significant difference between the efficiency of GDL synthesis on substrates with added surfactants and the control ones, the literature from this field includes some reports confirming that surfactants play a key role in the degradation of hydrophobic substrates, and thus indirectly influence the results of GDL synthesis [13]. It is believed, among other factors, that they can accelerate the rate of transport of substrates into the interior of cells and contribute to a greater degree of development of the interface surface. [PERSON] et al. [13] drew these conclusions on the basis of research carried out on the synthesis of GDL, in which both ionic and nonionic surfactants were added to the medium. When using saponin, Triton X-100, Tween 80, CTAB (hexadecyltrimethylammonium bromide), and SDS (sodium dodecyl sulfate) in the biotransformation of methyl ricinoleate to GDL, it was found that the highest concentration of aroma compound (0.234 g/L) was possible in the presence of Tween 80. This is consistent with our test results, in which the highest concentration of GDL in the tested media was also recorded in the presence of this surfactant. In the authors' research, the efficiency was approximately 2.5-3 times lower when using Triton X-100, the presence of which was associated with the shorter viability of yeast cells. The authors also suggested that the ionic surfactants used (SDS or CTAB) had a toxic effect on microorganisms (especially due to interactions with cell membranes), which may explain the complete lack of GDL in the samples extracted from these media. The presented research showed that mechanical emulsification of the substrate was extremely important from the point of view of the level of GDL synthesis, which is not mentioned in the extensive literature on the biosynthesis of GDL by microorganism cells. Creating finely dispersed droplets contributes heavily to a medium's stability. The dispersed lipid phase ensures better contact of the substrate with the enzyme, higher lipolytic activity of cells, higher level of castor oil hydrolysis, and thus higher biotransformation efficiency. Thanks to emulsification, the conversion yield of GDL per biomass formed and specific rate of GDL production were more than twice as high as in nonemulsified substrates. Another factor that determines the level of GDL biosynthesis is the size of the inoculum introduced into the reaction.