# Nutritional and Medicinal Properties of Microbial Oil
## 4 Extraction of Microbial Oil  
The reality of using SCOs is far from being achieved, due to the high cost of their recovery from biomass. The costs can be lowered by the usage of a waste carbon source in the culture medium and by a reduction in the operational costs of SCO extraction, which may be accomplished by mechanical methods, chemical and enzymatic methods, or a combination of them [46; 47]. Based on the current literature, there is no extraction method that is 100% effective in yielding oils derived from microorganisms. Still, there is a lot of research describing examples of their use. Different biomass treatments (wet, oven-dried, and freeze-dried) can be applied before cell lysis. It is claimed that dry biomass extraction is more efficient than wet biomass extraction for lipid recovery in solvent-based extraction processes. A decreased amount of water can maximize the mass transfer and decrease the formation of emulsion. However, the biomass drying step before the extraction is economically costly for large-scale applications [48; 49]. A study conducted by [PERSON] et al. [49] showed that adopting wet extraction practices reduces the energy required for extraction by almost 60%, providing further evidence of its energy-saving benefits. The most effective solvent historically used to extract oil from wet materials at ambient conditions is chloroform, as shown in [PERSON]'s and [PERSON]'s methods. Chloroform cannot be used for commercial use,because it is highly toxic and carcinogenic [49]. The safest and, nowadays, most popular way to ensure that all of the cellular lipids are extracted is to employ a ternary solvent composition including a polar as well as a non-polar solvent [50]. Recently, an alternative, green solvent, cyclopentyl methyl ether (CPME), was successfully used in biphasic systems of CPME:water:alcohol to extract the lipids from the wet cells of the oleaginous yeast _Lipomyces starkeyi_[51]. [PERSON] et al. [50] used the greener solvent pair ethyl acetate/ethanol with wet sample yeast _Y. lipolytica_, followed by the addition of water and ethyl acetate 1:2 (\(v/v\)), for the separation of aqueous and organic phases. Among the 41 tested solvents, isoamyl acetate was selected as the most appropriate \"green\" solvent, maximizing the lipid extraction compared to \(n\)-hexane for _Y. lipolytica_ in the studies provided by [PERSON] et al. [52]. The process of extracting lipids from oleaginous yeasts is often hindered by the resistance of the cell wall, the limited accessibility of the lipids, and the difficulty of mass transfer. The cost-effectiveness of cell disruption is related to energy consumption, the time to obtain the reasonable effect, the consumables, the product quality, and the required labor intensity [53; 54]. Alternative pre-treatments could be freezing/defrosting, cold drying, bead milling, and microwave treatment. These methods can help to make the lipid structure more accessible to the solvents, thereby reducing the biggest limitation of the process--the diffusion of the solvent into the raw material [54]. [PERSON] et al. [47] evaluated liquid nitrogen pre-treated biomass and maceration, followed by ultrasonication extraction, as the treatment with the highest percentage of disrupted cells and the highest oil yield of _Y. lipolytica_ QU21 and _Meyerozyma guillermondii_ BI281A. Some investigations point out that the lipid yield decreased with increasing pressure, and low pressure (200 MPa) collapsed the cells, while high pressure (400 MPa) created protrusions on the cell wall and the cell fragments spread into the environment [55]. High-pressure homogenization (HPH) fragments cells via shear stress, cavitation, turbulence, and friction [56], and has recently been extensively investigated for oil recovery in the wet biomass of _Y. lipolytica_ yeast.