Deep eutectic solvents for the extraction of fatty acids from microalgae biomass: Recovery of omega-3 eicosapentaenoic acid

Microalgae are a vast group of autotrophic microorganisms whose metabolic diversity makes them a natural source of valuable organic compounds such as lipids, carbohydrates, proteins, vitamins, and bioactive molecules. Several microalgae species contain notable amounts of polyunsaturated fatty acids, particularly eicosapentaenoic acid (EPA), which is an important alpha-linolenic acid derivative for human health. Conventional methods are considered effective at recovering total lipids from microalgae, however, they imply the use of large volumes of organic solvents such as methanol and chloroform, which are toxic and pose environmental risks. Thus, it is necessary to find new methods involving sustainable and green extracting phases. Deep eutectic solvents (DES) are renewable compounds often formed, but not exclusively, by quaternary ammonium salts and non-hydrated metal halides. Due to their availability, low cost, biodegradability, and environmental friendliness, DES are a promising alternative to organic solvents in extraction processes. This work assesses the efficiency of several DES phases for the extraction of fatty acids from the microalgae Nannochloropsis gaditana with a special interest in the recovery of EPA. The tested phases include mixtures containing choline chloride, lactic acid, ethylene glycol, and sodium acetate. Their performances were compared to those provided by conventional methods based on the use of organic solvents. Specifically, an in-situ transesterification process based on methanol with 10 %v/v of HCl was optimized in terms of temperature, time, and catalyst amount to be used as a reference. The results show that several of the tested eutectics such as choline chloride-ethylene glycol were capable of matching and even outperforming the best results obtained for EPA, with 104 % of extracted EPA methyl ester as the percentage of the mass obtained with HCl-methanol. The extraction capacity of DES was also improved by microalgae biomass pretreatment using ultrasonic and NaCl-based methods in a further stage. In the case of EPA extraction, and under optimal conditions, DES were capable of recovering over 18 % more quantity than the obtained with HCl-methanol. These results demonstrate that DES are effective at both recovering total fatty acids from pretreated biomass and at selectively recovering EPA using both unpretreated and pretreated biomass.