_Applied sciences_  
_Article_  
**Concept of Batch and Fed-Batch Cultures of _Yarrowia lipolytica_ as a Valuable Source of Sterols with Simultaneous Valorization of Molasses and Post-Frying Rapeseed Oil**  
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Y. lipolytica_ make the species able to utilize both simple and complex wastes as sources of carbon, nitrogen, phosphorus and other essential components [8; 9].  
The main byproduct of sugar mills is the brown leachate, obtained when sucrose crystals are separated in the final stage of sugar production, called molasses. Generally speaking, the production of 1 ton of raw sugar involves the production of 0.38 tons of molasses. The thick and viscous syrup consists of up to 20% (\(w/w\)) water and 80% (\(w/w\)) dry matter. Molasses contains large amounts of sucrose and reducing sugars (20-60% \(w/w\)), in smaller amounts of inorganic salts, nitrogenous compounds, vitamins and colour-forming substances. Because of the composition, molasses is used as a low-cost substrate necessary for yeast cell growth [10; 11].  
Regarding the ability of oleaginous _Y. lipolytica_ cells to grow and accumulate significant amounts of microbial lipids in media with hydrophobic carbon sources, there was established, that waste cooking oils (WCOs) may be considered as an efficient source of carbon stimulating lipase activity and promoting the Single Cell Oils (SCOs) accumulation process [9; 12; 13]. In the food industry, microbial oils have gained popularity as additives in infant formula and animal feed. SCOs are primarily a source of triacylglycerols, sterols and sterol esters similar to those from plant sources. Monounsaturated fatty acids stand out in the fatty acid profile of microbial oils with saturated and polyunsaturated acids in smaller amounts, depending on culture conditions and medium composition. The best-known sterol of microbial origin is ergosterol, which can be present in the form of esters and free sterol. The highest content of this compound has been reported for yeasts of the genera _Saccharomyces_, _Metschikowia_, _Kluyveronuyes_, _Pichia_ and _Torulaspora_[14; 15].  
The aim of the present study was a multifaceted evaluation of the concept of batch culture in comparison with fed-batch culture modes in terms of the growth and the efficiency of microbial oil accumulation by _Yarrowia lipolytica_ yeast. The ability of SCOs accumulation in yeast cells over time has been analyzed, depending on culture conditions i.e., medium supplementation, to highlight the potential of yeast biomass as a source of bioactive components, the fatty acid profile and sterols content in extracted oil samples were also determined. Moreover, the oxidative stability of the extracted oils was studied.