Extraction of lipids from wet microalga Auxenochlorella protothecoides using pulsed electric field treatment and ethanol-hexane blends

Abstract

Pulsed Electric Field (PEF) treatment was used as pre-treatment on the microalgae strain Auxenochlorella protothecoides (A.p.) prior to organic solvent extraction of lipids. Experiments were performed on fresh biomass from mixotrophic or autotrophic culture which both had an evaluated lipid content of 30-35 % of cell dry weight. Lipid yield was determined gravimetrically and compared to the reference lipid content assessed by bead-milling and subsequent Soxhlet extraction. The biomass was concentrated at 10 % w/w solids prior to PEFtreatment and further dewatered afterwards to approximately 25 % w/w before extraction. PEFtreatment with an energy input of 1.5 MJ per kilogram of dry matter induced electropermeabilisation of the microalgae cells detected by the increase of the conductivity of the microalgae supernatant. This greatly increased the lipid yield upon subsequent monophasic solvent extraction. A mixture of Water/Ethanol/Hexane 1:18:7.3 vol/vol/vol enabled to recover 92 %, and 72 %, of the evaluated lipid content of mixotrophically, and autotrophically respectively, grown A.p., after two hours of extraction. Recovery increased to 97 %, and 90 % respectively, after 20 hours of extraction. The same extraction system on untreated biomass yielded maximum 10 % of lipid content. The highest yields were obtained with 80 mL of solvent for 1 g dry biomass but solvent volume could be reduced by a factor two in case of mixotrophically grown microalgae. However, the solvent:biomass ratio still remains high, and includes a water-miscible solvent, ethanol. Total lipid extraction was confirmed by nile red staining of residual biomass combined with fluorescence microscopy imaging and flow cytometry. Gas chromatography analyses of extracted lipids after transesterification revealed that PEF-treatment did not alter their fatty acid composition. Overall PEF-treatment shows promising features for upscaling especially in a biorefinery concept since it avoids potentially harmfull temperature increase and small debris problematic for further processing.

Incubation time after pulsed electric field treatment of microalgae enhances the efficiency of extraction processes and enables the reduction of specific treatment energy

“…The lipid extraction protocol was fully described in (Silve et al, 2018). In brief, 5 mL of the microalgae suspension at 100 gDW/L were centrifuged, supernatant was disregarded and the wet microalgae pellet was resuspended in a hexane-ethanol blend in order to reach a Extraction yields were determined gravimetrically in a precision balance.…”

“…The biomass was bead-milled (Mixer mill, MM400, Retsch, Haan, Germany) and then extracted in a Soxhlet apparatus (Behrotest Kompact-Apparatur KEX 30, Behr Labortechnik) using hexane as an extraction solvent. Details are given in (Silve et al, 2018). For each microalgae batch, lipid content was evaluated in triplicates.…”

“…Additionally, PEF-treatment applied prior to lipid extraction with Ethanol-Hexane blends, enabled to reach almost total lipid extraction while yields from non-treated biomass were close to zero (Silve et al, 2018). Most experiments were performed with a fixed energy input of 150 kJ/L and a biomass concentration of 100g/L i.e.…”

“…211¬7a obtained from SAG, Culture Collection of algae, Göttingen, Germany. Axenic cultures were maintained in glass cultivation flasks in a modified Wu medium (detailed composition can be found in(Silve et al, 2018)). Experiments were performed on microalgae cultivated either mixotrophically in cultivation flasks containing glucose as a carbon source or autotrophically in 25L photo-bioreactors bubbled with air enriched at 3% of CO2 as the only carbon source.…”

“…Experiments were performed on microalgae cultivated either mixotrophically in cultivation flasks containing glucose as a carbon source or autotrophically in 25L photo-bioreactors bubbled with air enriched at 3% of CO2 as the only carbon source. The details of cultivation including medium composition can be found in(Silve et al, 2018). After harvesting, the microalgae suspension was concentrated by centrifugation at 3000 g using a Sigma 8k centrifuge (Sigma Laborzentrifugen GmbH, Osterode am Harz, Germany), with a swinging-bucket rotor.…”