# Nutritional and Medicinal Properties of Microbial Oil
## 4 Extraction of Microbial Oil
The authors of those studies showed that a pressure of 1500 bar and five passes, provided by mixing using a high-speed disperser, allows maximum cell disruption, comparable to the total oil recovery reached with the dry route when the yeast biomass is lyophilized and subjected to \(n\)-hexane extraction [57]. Other alternative techniques based on cell electroporation-- high-pulsed electric fields (HPEF), high-voltage electrical discharges (HVED), mechanical expression (ME), and moderate-pulsed electric fields--assist the mechanical expression (MPEF-ME) seem to be less efficient. HPH, ME, and MPEF-ME induced changes in the content of some fatty acids [58; 59]. What should be mentioned is that many authors increase lipid recovery by using a combination of techniques, e.g., bead milling and HPH [52]. Finally, the softest method of cell disintegration is ultrasound (US)-based technology related to the cavitation phenomena, which is the formation of vapor bubbles and their implosion near the cell surface and free radical formation. The majority of investigations with the US deal with the laboratory level, and the power used varies even up to 2800 W [59]. Usually, the highest lipid contents are achieved using ultrasound-assisted extraction coupled with [PERSON]'s, bead milling, or [PERSON]'s techniques [60]. Some nonmechanical methods, e.g., acid and base digestions or osmotic shock, have been also tested [40]. A small number of papers deal with supercritical carbon dioxide (SC-CO\({}_{2}\)) extraction. [PERSON] et al. [61] used ethanol-macerated yeast cells of _Y. lipolytica_, resulting in 15 mg of yeast oil per g of solvent used. The results showed that oil recovery is more efficient when adding a pretreatment to the extraction procedure, and none of the techniques tested were efficient in recovering all of the oil contained in the yeast [57; 61]. The main methods used to disrupt microalgae cells on a large scale are bead milling, high-speed homogenization, and high-pressure homogenization; however, ultrasounds, microwave treatments, enzymatic lysis, and pulsed electric field have the potential for scale-up and have been already applied for lipid extraction from _Chorella vulgaris_ (enzymatic lysis [62]) and _Nannochloropsis_ sp. (HPH and HSH) [63; 64]. Microwave treatment hasbeen especially dedicated to lipid recovery and investigated for _Chloerella_ sp., _Notoc_ sp., _Synechocystis, Scenedesmus obliquus_, and _Botryococcus_[65]. The challenges for designing high-yield oil extraction methods from algae are highly variable ranging for the cell wall composition. The bottlenecks cited in the literature due to the high energy demand are still downstream processing in the microalgae commercial applications in biorefineries [53; 65]. It is also claimed that algae must be dried (rotary or spray-dried) prior to being fed into a press to obtain the lipids [66]. Some authors claim that a better understanding of the cell wall composition and knowledge of the cell wall chemistry would provide information to aid in the design of better protocols for oil extraction, including genetic engineering techniques. Cell wall modification displays a potentially promising approach to improve both the harvesting of oleaginous yeast biomass and cell susceptibility to lysis. The target genes for further modifications are glucanase, chitinase, and cross-linking enzymes expressed in the cell wall of the oleaginous strain [38]. A mutant of _Trichosporon_ has been recently isolated, possessing a modified cell wall, which acquired the ability to generate cell aggregates that are more easily separable from the culture broth by centrifugation [67]. To sum up, many papers claim that the novel extraction processes of oil from wet biomass or the use of emerging technologies can considerably reduce the energy required for drying; however, the majority of them are limited to a small laboratory scale, a need for unavailable commercial-scale instruments, or are still not as good as conventional solvent extraction.