Tao Wang

The energy intensive nature of harvesting microalgae by centrifugation and drying results in a low energy profit ratio for biofuel production from wet microalgae. To address this, microalgal cultures can be flocculated by polymers to reduce energy consumption and avoid centrifugation. Additionally, lipids can be directly extracted from wet microalgae harvested by centrifugation using liquefied dimethyl ether (DME) to reduce energy consumption. In this study, we investigated the lipid extraction from the green alga, Chlorella sorokiniana (C. sorokiniana), flocculated with amphoteric and cationic polymers using liquefied DME, without centrifugation and drying pretreatment. Although the addition of amphoteric polymer did not interfere with lipid extraction by liquefied DME, the amount of lipids extracted by ethanol from flocculated C. sorokiniana decreased. The addition of cationic polymers also interferes with lipid extraction using liquefied DME, chloroform–methanol mixtures, and ethanol. Surface functional group measurements revealed that the amphoteric and cationic polymer flocculants were insoluble in these solvents, remained in the residue, and did not contaminate the extracted lipids. Thus, the amphoteric flocculant KA305BH was found to be effective for lipid extraction with liquefied DME from C. sorokiniana. The proposed approach can facilitate the energy efficient production of biofuel from wet microalgae.

Extraction of lipids and high-value products from highly wet microalgae requires significant energy for the drying pretreatment. In this study, we examined the direct extraction of lipids, β-carotene, and polyphenolic compounds from wet Dunaliella salina using liquefied dimethyl ether (DME), which is effective in lipid extraction for biofuel production. The amount of DME-extracted β-carotene was 7.0 mg/g, which was higher than that obtained from the chloroform–methanol extraction. Moreover, the total phenolic content extracted with DME and its antioxidant capacity were slightly higher than those extracted with chloroform–methanol. DME removed almost all the water and extracted 29.2 wt% of total lipids and 9.7 wt% of fatty acids. More lipids were extracted from wet samples by liquefied DME than by chloroform–methanol extraction. The C/N ratio of lipids extracted with DME was 112.0, higher than that of chloroform–methanol. The high C/N ratio suggests that nitrogen-containing phosphatidylcholines may be less easily extracted by liquefied DME and may be highly selective. However, the ratio of saturated fatty acids was 34.8%, lower than that of chloroform–methanol. Na⁺ and Mg²⁺ in the culture medium were not extracted using DME. Thus, using the extract with DME has both advantages and disadvantages compared to using the extract with chloroform–methanol; however, it has satisfactory extraction properties. DME is expected to be an environment-friendly alternative solvent because it does not require drying, which is necessary for conventional extraction solvents.

Microalgae are a sustainable source for the production of biofuels and bioactive compounds. This review discusses significant research on innovative extraction techniques using dimethyl ether (DME) as a green subcritical fluid. DME, which is characterized by its low boiling point and safety as an organic solvent, exhibits remarkable properties that enable high extraction rates of various active compounds, including lipids and bioactive compounds, from high-water-content microalgae without the need for drying. In this review, the superiority of liquefied DME extraction technology for microalgae over conventional methods is discussed in detail. In addition, we elucidate the extraction mechanism of this technology and address its safety for human health and the environment. This review also covers aspects related to extraction equipment, various applications of different extraction processes, and the estimation and trend analysis of the Hansen solubility parameters. In addition, we anticipate a promising trajectory for the expansion of this technology for the extraction of various resources.