Lipid extraction from fresh Nannochloropsis oceanica using semi-hydrophobic eutectic solvents

Conventional lipid extraction from microalgae involves energy-intensive pretreatments and the use of non-renewable organic solvents. Eutectic solvents (ES), a new class of designer solvents, hold the potential to improve lipid extraction. Hydrophilic ES have been reported to impair the cell wall of microalgae, bypassing the need for pretreatments. However, other hydrophobic solvents were still required as extraction medium. Recently, ES imidazole and hexanoic acid was discovered to exhibit tuneable hydrophobicity, i.e., dissolving both water and lipids depending on their molar composition. In this work, we evaluated the feasibility of imidazole/hexanoic ES as a single solvent for lipid extraction from intact wet and dried microalga Nannochloropsis oceanica. Single-factor multilevel design of experiments is used to evaluate the yield under different conditions (ES composition, temperature, time, solvent/biomass ratio, and water content). Interestingly, the extractions from wet algae paste were higher than dried biomass, reaching a comparable yield to the traditional chloroform/methanol method. From wet biomass, >80 % lipids were extracted by the imidazole/hexanoic acid ES (15:85 mol/mol) at 50 °C within 2 h. Whereas, the extraction yield of dry biomass was lower, reaching only 65 % even after 12 h under the same condition. Supplementation of water during the dry extraction resulted in the same yield as the wet extraction. This research demonstrated that ES can be used to replace non-renewable organic solvents without the need of using mechanical disruption and can be applied directly on wet biomass.