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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Collaborative Specialization

Environment and Sustainability

Supervisor

Rehmann, Lars

Abstract

Microalgae have gained interest as sources of renewable lipids in the biofuel sector due to their ability to sequester carbon dioxide into triacylglycerol (TAG), a biodiesel feedstock. However, industrial-scale production of microalgae exclusively for fuel production is limited by technical and economic challenges. Some marine microalgae can accumulate large amounts of polyunsaturated fatty acids (PUFAs) such as docosahexaenoic acid (DHA) and other unsaturated fatty acids, which are high-value compounds linked to the prevention of various cardiovascular diseases. This thesis therefore examines the extraction of lipids and DHA from two microorganisms (Chlorella vulgaris, a model organism for lipid production, and Thraustochytrium sp., an industrially relevant DHA producer).

Ionic liquids (ILs) have been shown to assist in cell disruption and lipid extraction from microalgae. A recently described ionic liquids were therefore synthesized and characterized for lipid extraction. It was used in comparison with commercially available ILs throughout this thesis. Initially, lipid extraction was evaluated from dry samples of the model organism C. vulgaris, followed by the processing of wet extraction from fresh algae samples. Treating wet biomass is technically more challenging, but a necessity, as complete dewatering and drying in economically not feasible. To further enhance the IL-mediated extraction and cell disruption the previously evaluated process was combined with electrolysis. Electrolytic pre-treatment of microalgae can disrupt the cell wall, aiding in the release of intracellular lipids. ILs have high electric conductivity and hence synergies were expected when combining these two treatments. The obtained extraction efficiency was up to 44.53 ± 2.38% for the combined process compared to 29 ± xx % with ILs only and 1.52 ± 0.74% with electrolysis only.

Extraction technologies developed with C. vulgaris were subsequently transferred to extract value-added lipids fromThraustochytrium sp. (T18) which possesses a high polyunsaturated fatty acids (PUFAs) content, with a specifically large percentage of omega-3 fatty acids such as docosahexaenoic acid (DHA). Two ILs were assessed for their ability to facilitate the extraction of PUFAs from dry and fresh Thraustochytrium sp. (T18) cultures. The results show that ILs could facilitate the extraction of over 90% (w/w) of the available oils from dried T18 biomass and still maintain an extraction efficiency of around 80% (w/w) when using wet slurry.

Subsequently, the extra18 using the synthesized IL was optimized using a central composite design (CCD) and response surface methodology (RSM). The total extraction efficiency was up to around 97% of DHA-rich oil, showing that ionic liquid based methods might be suitable to process marine micro-algae with value-added lipid composition.

Summary for Lay Audience

Microalgae, as sources for various valuable products including biodiesel, ω-3 fatty acids, astaxanthin, phenolics, etc., have been regarded as renewable and sustainable solutions to mounting energy demands and environmental issues. However, some conventional extraction processes of these products from microalgae are time- and energy-consuming, such as mechanical and biological techniques. Ionic liquids have been widely shown to pretreat and disrupt cells to assist lipid extraction in a short period time around one hour. Thus, ionic liquid based extraction processes were investigated in this work, focusing on the extraction of lipids for biodiesel and DHA production from two microorganisms Chlorella vulgaris and Thraustochytrium sp., respectively.

In this work, some ionic liquids were screened for the above two kinds of microalgae. Extracting lipids from wet biomass is more challenging than from dry biomass due to the eliminations of dewatering and drying steps. Initially, dry samples of Chlorella vulgaris were evaluated and followed by wet extraction from fresh algae samples. The extraction efficiencies of dry microalgae are around 75%. Also, electrolysis pretreatment, aiming in aiding the release of lipids from cells, was added to pretreat the fresh samples, where the synergic effect of ionic liquid and electrolysis was expected. The extraction efficiency was up to around 45% for the combined pretreatment compared to around 29% with ILs and around 2% with electrolysis.

The developed extraction technologies were subsequently applied to extract value-added lipids from Thraustochytrium sp.(T18) with a high content of ω-3 fatty acids such as DHA. The results show that the extraction efficiency was around 80% (w/w) when using wet slurry. A central composite design was used to optimize the extraction efficiency when extracting from the dry T18, and the extraction efficiency was found around 97%. The results show that ionic liquid based methods are capable of efficiently assisting value-added lipid extraction from marine microalgae.

In summary, Ionic liquid based extraction process has high potential to efficiently gain valuable materials from various microalgae with relatively low energy and time consumption.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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