Post-incubation pH impacts the lipid extraction assisted by Pulsed Electric Fields (PEF) from wet biomass of Auxenochlorella protothecoides

Pulsed Electric Fields (PEF) have been largely investigated as a promising technology for the selective extraction of valuable compounds from biological cells. This approach comes from the well-known ability of PEF to permeabilize lipid membranes and thus facilitating mass transfer from inside to outside of the cells. PEF application on oleaginous microorganisms such as microalgae has been in the spotlight as a pretreatment for lipid extraction for biodiesel usage. For years, research has predominantly been focused on the optimization of PEF parameters, however, intrinsic microbial parameters and the external factors also can be critical in the downstream process during and after PEF application. Therefore, an overall optimization of the PEF-assisted lipid extraction from microalgae is still necessary in order to be a competitive strategy. In this context, an evaluation of the impact of concentration of the microalgae biomass along with the influence of the medium pH on the final lipid yield was carried out.
The microalgae Auxenochlorella prothotecoides grown under mixotrophic conditions was used in this study. The lipid yield and electroporation level of PEF-treated A. ... mehrprotothecoides were determined at a medium pH of 3.0 and 5.0 under variation of the pre- or post-PEF incubation time and for split-dose treatments. Low energetic PEF treatments with 40 kV/ cm and 1 μs pulses at 9.6 and 19.2 kJ/L were applied either in batch mode or in continuous flow.
The concentration of the microalgae biomass (from 10 to 100 gdw/L), typically performed in order to reduce operational cost, resulted in a progressive variation of the pH going up to 2 points of difference during the 20 h after harvesting, even when the resuspensions were made in pH-buffer media. Conversely, under unconcentrated conditions the pH of microalgae suspension was maintained. Furthermore, when PEF treatments were applied to microalgae biomass concentrated to 100 gdw/L, the treated suspension increased its pH by around 2 points difference to control samples within 6 hours after PEF processing. Therefore, subsequent experiments were performed under non-concentrated conditions in order to avoid pH variability.
Results showed that under non-concentrated conditions (10 gdw/L), pre-PEF incubation did not influence the degree of electroporation or the lipid extraction yield in all the pH media studied. However, post-PEF incubation significantly increased the number of electroporated cells (> 60%) in medium pH 3.0, while no change was observed at pH 5.0. On the other hand, split-dose PEF treatments at pH 5.0 caused significantly higher electroporation levels and lipid extraction yields than equivalent single-dose treatments. Therefore, these results showed that medium pH is critical in the final electroporation and lipid extraction yields of A. protothecoides and should be considered in further studies. Moreover, the biomass concentration effect on the pH fluctuation could explain the incubation effect observed in previous studies. In any case, the possibility of adjusting the pH of the media during the downstream processing of microalgae by PEF for lipid extraction could provide interesting opportunities for further optimization, which might contribute to its future applicability.