## 4 Conclusions  
This research was built upon a long-standing history of previous studies demonstrating this model yeast species' safety and nutritional potential as a source of nutrients. This study assessed the suitability of oleaginous yeast of the _Y. lipolytica_ species for the production of microbial oil and determined the specific qualitative characteristics of the biomass and oil, such as the protein content of the biomass, the unsaturated fatty acid content of the yeast lipids, and the oxidative stability of the oil. The effect of the culture water used and the duration of the culture on the yield of the obtained microbial oil was evaluated. Obtaining oleaginous microbial oil is gaining popularity, and more and more research is being undertaken to introduce alternative oil sources with similar properties to vegetable oils, such as rapeseed or palm oil, in food technology. Post-frying waste oil from the food industry was an easily assimilable carbon source for the yeast _Y. lipolytica_. Untreated tap water used for the preparation of the culture medium did not significantly affect the yeast biomass yield obtained, the microbial oil biosynthesis efficiency or its quality compared to the results obtained for biomass multiplied in a distilled water-based medium, except for a significant effect on the oxidative stability of the oil. The oleaginous yeast biomass of _Y. lipolytica_ and the microbial oil were characterized by high nutritional value. The yeast biomass contained high amounts of lipids (39.43-50.53%), proteins (24.16-27.03%), and unsaturated fatty acids, essential dietary components. The lipids of oleaginous yeast contained predominantly unsaturated fatty acids (92.56-93.39%), mainly oleic acid (62.73-66.44%) and linoleic acid (19.40-21.40%). The time of batch culture of oleaginous yeast of the species _Y. lipolytica_ influenced the yield and quality of the microbial oil (stock lipids) obtained from it. The yeast biomass from the 20 h culture was characterized by the lowest oxidative stability of the oil (more than twice as low as the 65 h culture). Using untreated tap water to prepare the culture medium containing post-frying oil as a carbon source did not affect the yield or nutritional value of the biomass. The results from both distilled and tap water-based cultures were similar to each other. In terms of culture time, the oxidative stability of the oil was shorter after a shorter culture time, and a shorter culture time was associated with a shorter maximum oxidation time. In addition, the microbiological analysis of the tap water and the microbial oil obtained confirmed their safety for use, demonstrating that culturing _Y. lipolytica_ in waste frying oil and untreated tap water can yield safe and nutritionally valuable biomass. This further highlights the novelty and potential of this approach for future food applications. The issue could be pursued further, as it is an interesting alternative for the future use of oleaginous microorganisms in oil production and its introduction into human nutrition. Conceptualization, A.U.F. ; investigation, A.U.F., J.K., M.G., A.P., I.P. and D.N. ; resources, A.U.F., M.G. and D.N. ; data curation, A.U.F., J.K. and I.P. ; writing--original draft preparation, A.U.F. and J.K.; writing--review and editing, A.U.F. ; visualization, A.U.F. and J.K.; supervision, A.U.F. ; project administration, A.U.F. ; funding acquisition, A.U.F. All authors have read and agreed to the published version of the manuscript. **Funding:** This study was financially supported by sources of the Polish Ministry of Science and Higher Education grant no. SKN/SP/601167/2024 titled \"Kapsulkowany olej mikrobiologiczny jako zroddo cennych skladnikow bioa4 kywnych/Encapsulated microbial oil as a source of valuable bioactive components\". **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable.