4.7. Lipid Extraction Utilizing Green Solvents

The conventional technologies of oil recovery from plant seeds (e.g., oilseeds for vegetable oil extraction) use solvent extraction, most commonly with n-hexane for its attributes such as nonpolar nature, low latent heat of vaporization (330 kJ/kg), which facilitates the easy recovery after the extraction, and high solubility of oil [31]. However, using n-hexane as a solvent has led to several consequences such as toxicity, air pollution, and harmfulness that prompted looking for alternative options.

The chloroform/methanol-based solvent used in the traditional Folch and Bligh and Dyer methods efficiently extracts lipids from plants, animals, and microbes. However, researchers have tested other safer solvents due to health and environmental concerns. Among the green solvents, supercritical CO₂, plant-derived terpenes (e.g., D-limonene, p-cymene, and α-pinene), ionic liquids (non-aqueous salt solution) are emerging [31]. With the help of Conductor-like Screening Model for realistic Solvatation (COSMO-RS), Breil et al. [73] selected ethanol and ethyl acetate as potential substitution of methanol and chloroform for the extraction of lipids from yeast (Yarrowia lipolytica IFP29). Moreover, ethanol, ethyl acetate, isopropanol, and n-propanol are good alternatives as they are categorized as class 3 solvents that have a lower risk to human health and have no negative genotoxicity and long term carcinogenicity [63]. Moreover, ethanol is one of the cleanest among these solvents, considering the renewability and availability as a food-grade solvent, and being cheaper than other solvents [63].

Lin et al. [59] observed that ethyl acetate and ethanol at 2:1 or 1:1 ratios (v/v) provide a similar yield of lipids (comparison with chloroform/methanol, 2:1, v/v) from fresh egg yolk, boiled yolk, yolk powder, and raw animal tissues. Probst et al. [109] demonstrated that cyclopentyl methyl ether is an alternative solvent to chloroform and can efficiently extract triacylglycerols from yeast, Lipomyces starkeyi. De Jesus et al. [65] tested the traditional methods with green solvents 2-methyl tetrahydrofuran (2-MeTHF) and cyclopentyl methyl ether (CPME) for the extraction of lipids from wet microalgae biomass of Chlorella pyrenoidosa. In results, extractions using traditional Bligh and Dyer methods and Folch showed significantly higher yield (113.5–115.1 mg lipids/g biomass), followed by Hara and Radin [16] method (108.66 mg lipids/g biomass). Among the green solvents, the 2-MeTHF/isoamyl alcohol/water system used in the Bligh and Dyer method provided the highest (83.2%) yield of lipids (compared to chloroform/methanol). However, the estimated cost of solvents was a minimum for the Hara and Radin methodology (hexane/isopropanol solvent mixture), costing US$ 167.00/kg of fatty acids. While extraction using the 2-MeTHF/isoamyl alcohol/water system cost 30 times higher (US $4500.00). These observations suggest that based on cost, green solvents are uncompetitive in comparison to fossil-based solvents. However, in the future, the higher production of green solvents may reduce the cost.