## 4 Discussion  
Based on the obtained results, it cannot be denied that the investigated factor, which was plant oil type, did not affect the hydrolytic activity and specific activity of the four different _Y. lipolytica_ strains, although there was no significant effect of the carbon source used in the culture media. However, some dependencies and directions for further work on stimulating lipase synthesis in plant oil media were indicated. Within strains of _Y. lipolytica_ yeasts, a large variability in physiological abilities is observed, and much research is devoted to the search for special features among strains of this species. Furthermore, metabolite biosynthesis is often dependent not only on specific yeast strains, but also on culture conditions, mainly carbon and nitrogen sources, oxygenation, temperature, or pH [18; 19; 20; 21]. This strain-dependent behavior was described by [PERSON] et al. [9], who conducted a study in which 20 strains of _Y. lipolytica_ of various origins (Po river, dairy products, or refrigerated food) were compared in terms of their lipolytic and proteolytic profiles, as well as their possible solutions for the valorization of untreated cheese whey. Very often, as in the current study, statistical methods are used to assess or select many factors, optimize the biosynthesis of a selected metabolite, or screen strains; e.g., in the optimization of lipase production, using fractional factorial design to evaluate inoculum concentration, temperature, and agitation [20]; or in the optimization of pH, culture time, and glucose concentration using Response Surface Methodology for high-yield citric acid production [21]. In the case of the biosynthesis of extracellular lipases, a lot of research in the scientific literature has been devoted to the use of appropriate carbon sources in the form of lipids, especially those with a high content of oleic acid. In the present study, the strain _Y. lipolytica_ KKP 379 produced a \"true\" lipase, showing high activity towards oleic acid ester in the model reaction of the hydrolysis of _p_-nitrophenyl esters. The activity of extracellular lipases was higher than for laurate esters, but lower than for butyric acid ester. In this study, hypotheses which assumed that there exists a minimum threshold concentration of free fatty acids present in plant oil (used in culture medium as a sole carbon source), which is essential for initiating the expression of extracellular lipases, were acknowledged. On the other hand, no connection between the production of lipases, the total composition of fatty acids in triacylglycerlecrols, and their concentration in the _sn_-2 positions of triacylglycerlecrols in plant oils was found. Plant oils are certainly good lipase secretion stimulators, but their connection with enzyme activity is not straightforward, and is influenced by other factors. Figure 8: Hydrolytic activity for crude extracellular lipases of _Y. lipolytica_ KKP 379 immobilized on Lewati support, determined by the hydrolysis of _p_-nitrophenyl laureate under optimal reaction conditions, at different times. The assumption that oleic acid serves as an inducer of _LIP2_ gene expression has been explored by [PERSON] et al. [22]. Most authors consider olive oil as an excellent carbon source for stimulating the synthesis of lipases in oleaginous yeast, including _Y. lipolytica_. The stimulatory action of olive oil on microbial lipase synthesis is attributed to the high content of oleic acid in glycerol esters, which is an inducer of the _LIP2_ promotor [11]. However, it should be noted that extracellular lipases expressed by different _Y. lipolytica_ strains are not equally activated by the presence of olive oil and other plant oils used in the production of microbial lipases [23]. Some plant oils have been shown to stimulate lipase activity of the yeast to a greater extent than olive oil [11; 24]. Fatty acids may be largely free and unbound in the triacylglycerlycerols of plant oils.