Enhanced and Selective Lipid Extraction from the Microalga P. tricornutum by Dimethyl Carbonate and Supercritical CO2 Using Deep Eutectic Solvents and Microwaves as Pretreatment

Microalgae are promising alternative sources of several bioactive compounds that are useful for human applications. However, lipids are traditionally extracted with toxic organic solvents (e.g., mixtures of chloroform and methanol or hexane). In this work, we develop a new lipid extraction protocol for obtaining a fatty-acids-rich extract from the diatom Phaeodactylum tricornutum. Deep eutectic solvents (DESs) and microwaves (MWs) were investigated as pretreatments for environmentally friendly solvent extractions using dimethyl carbonate (DMC) and supercritical CO2 (scCO(2)). Pretreatments with various DESs formed by choline chloride (ChCl) and different hydrogen-bond donors (oxalic acid, levulinic acid, urea, ethylene glycol, and sorbitol) were tested in combination with DMC extraction. DESs formed by ChCl and carboxylic acids gave the best results, increasing both the selectivity and the total fatty acid (TFA) extraction yield of DMC (by 16% and 80%, respectively). DESs combined with MW heating followed by DMC extraction allowed a TFA yield and fatty acid profile comparable to those of the traditional Bligh and Dyer extraction method to be reached, along with a much better selectivity (88% vs 35%). This pretreatment was also demonstrated to significantly improve the extraction efficiency of scCO(2), increasing the TFA yield by a factor of 20 and providing highly purified triglyceride extracts.