Cross-flow filtration for the recovery of lipids from microalgae aqueous extracts: Membrane selection and performances
Graphical abstract
Introduction
The biorefinery of renewable resources like microalgae offers great opportunities for substituting biomolecules (lipids, proteins, carbohydrates, and antioxidants) for traditional raw materials in various industry sectors, such as food/nutrition and animal feed, cosmetics, pharmacy, energy and green chemistry. Such strategies necessitate innovative choices of soft and energy-efficient processes to guarantee the integrity of fragile molecules and develop eco-friendly production. For large-scale production (food, energy and green chemistry), a wet processing of biomass is proposed, which avoids expensive drying steps and reduces solvent use [1], [2], [3], [4], [5]. However, the energetically efficient extraction of biomolecules at low cost on an industrial scale is not yet mature [6], [7], [8], [9], [10], [11]. Wet biomass treatment includes (1) harvesting, (2) cell disruption step to release the valuable biochemicals in the aqueous phase, (3) fractionation steps (extraction, concentration and purification).
The integration of membrane processes into the downstream processing of the microalgae involves the harvesting and the concentration of microalgae [12], [13], [14], [15], [16], [17], [18], but membrane filtration is also a promising clean separation process for the fractionation steps [19], [20], [21], [22], [23], [24], [25], [26].
In this work we focus on the recovery and concentration of lipids from Parachlorella kessleri, cultivated in starving conditions to enhance lipid production. Lipid recovery from microalgae has mainly been carried out with supercritical CO on dried matter or by solvent extraction [27], [28], [20]. In this study the membrane processes are developed. After grinding of the microalgae and centrifugation, the supernatant contains large quantities of the lipids, dispersed in water [29]. Clavijo Rivera et al. [29] demonstrated that after bead milling and centrifugation of P. kessleri, the supernatant contains emulsified lipids in the aqueous phase. This lipid phase contains neutral lipids (triglycerides and free fatty acids) and polar lipids (phospho and glycolipids). The purpose of this work is to study the recovery and concentration of these valuable compounds by membrane processes.
Membrane filtration has previously been used for the recovery of emulsified lipids in effluents from different industries (food, petroleum, metallurgy, etc.), mostly using micro or ultrafiltration membranes [30], [31], [32]. The lipids are mixed with surfactants which strongly influence the separation [33]. The lipids separation from the aqueous phase implies concentration of the lipids on the one hand, and/or destabilization of the water–oil interface to induce coalescence on the other. The concentration of the lipids into the retentate necessitates hydrophilic membranes to limit the fouling of the membrane by organic matter. These membranes can be made of cellulose, polyacrylonitrile (PAN), polyethersulfone (PES), hydrophilated polyvinylidene fluoride (PVDF) [34], [35], [36], [37], [38]. New hydrophilic membranes are also being developed [39], [40], [41]. Coalescence is enhanced with hydrophobic membranes (PVDF, polytetrafluoroethylene PTFE, polypropylene PP) [35], [34], [42].
However, the fractionation of disrupted microalgae has rarely been investigated [43], [21], [22], [23], [25], [24]. Prediction and control of the membrane filtration in complex mixtures with macromolecules, organic and mineral compounds, is known to be difficult.
Giorno et al. [43] studied triglyceride separation from a wet sonicated biomass. They used ultrafiltration cellulose membranes with 30 and 100 kDa molecular weight cutoffs and achieved water permeation flux between 22 and 27 L h m with a transmembrane pressure (TMP) of 0.14–0.2 bar for concentration experiments. Montalescot et al. [23] also studied the wet biomass fractionation after bead milling or high pressure disruption. The author used ceramic membranes with a porous diameter of between 50 nm and 1.4 m. Unlike with Giorno, the lipid transmission was less than 4%. A strong retention of proteins and carbohydrates was also mentioned. Following centrifugation, the filtration of the supernatant allowed a better transmission of carbohydrates. Lorente et al. [25] compared cross-flow and dynamic filtration with a vibratory shear enhanced processing (VSEP) for the concentration of lipids from a steam-exploded biomass before solvent extraction. They tested several membranes and selected two: a PES 5 kDa and a PVDF 100 kDa. They used a transmembrane pressure equal to 5 bar and managed to concentrate the lipids with a permeate flux between 7 and 25 L h m. The authors recorded a strong reduction (50–60%) in the solvent volume needed for lipid extraction after filtration. Safi et al. [24] tested membrane filtration to recover proteins from disrupted cells. They compared high pressure disruption (HPD) and enzymatic cell disruption combined with ultrafiltration and diafiltration. They tested PES membranes with different molecular weight cut offs (MWCO = 300, 500 and 1000 kDa). Strong fouling was noted for the largest MWCO. The best permeation flux was achieved with PES 300 kDa (30–40 L. m), at 2 bar. The best total protein recovery was obtained with enzymatic hydrolysis and ultrafiltration/diafiltration (24%) but the proteins were denatured, whereas the HPD led to lower protein recovery (17%) but the native structure seemed to be preserved.
The above authors encountered difficulties in fractionating of a complex medium coming from disrupted cells and membrane fouling. The composition of microalgae extracts depends on culture batches, grinding, clarification and storage conditions prior to filtration [29]. This variability in the characteristics of the filtration feed solution may lead to different interactions between molecules in the liquid phase and also with the membrane. Consequently, differences in the feed solution are expected to influence filtration performances and hinder the comparison of results for accurate selection of an adapted membrane, and operating conditions for the separation process. For this reason, it would be helpful to use mixtures with well-known characteristics.
In this work, a model solution was formulated, based on the analysis of the lipid fraction from P. kessleri aqueous extracts [29]. This solution was then filtered to evaluate the performances (retention, flux and cleanability) of PAN, PES and PVDF membranes to concentrate lipids. The most appropriate material and conditions were then selected and verified on real microalgae fractions.
Section snippets
Parachlorella kessleri culture, harvesting and storage
The strain P. kessleri was cultivated in autotrophic conditions using a BBM medium. The first step was to inoculate the pre-culture in a bubble column photobioreactor (PBR) with 15 L of normal BBM medium (0.75 g L NaNO). After 10 days, the culture reached a stationary growth phase and a modified BBM medium (0.23 g L NaNO) was gradually added to reach 100 L in nitrogen starvation conditions. The PBR was aerated with 0.5–1 L min CO and 5 L min filtered air. The pH was maintained at 7.5
Model solution formulation
The comparison of high-pressure liquid chromatography (HPLC) profiles of triglycerides from the vegetable oils mixture and the real microalgae extracts is presented in Fig. 1. The analysis shows a good concordance of the triglycerides of both products with some differences in proportion. This vegetable oil mixture was used to formulate the model solution. The lipid phase should have the same sensitivity to temperature and therefore the same viscoelasticity properties as microalgae lipids. This
Conclusions
This work proposes membrane cross-flow filtration for the recovery of lipids from starving P. kessleri aqueous extracts obtained after milling and centrifugation. Selection of the appropriate membrane for the lipids concentration necessitates a mixture with a controlled composition, so a model solution was formulated. The composition was based on the analysis of real products. The use of this model solution allowed selection of a PAN 500 kDa membrane for filtering real microalgae extracts. The
Conflict of interest
None.
Acknowledgments
The authors would like to thank Guillaume Roelens, Delphine Drouin (GEPEA), Marie Cueff, Jordan Tallec (GEPEA, Algosolis) and Laurence Lavenant (INRA BIA) for their active participation to the preparation of the microalgae extracts and to their analysis. This work was supported by CNRS, France (interdisciplinary project Algues-Molécules-Territoire, 2014-2017), GIS Europôle Mer, France (Sciences et Ingénieries Marines, 2016-2017), ADEME (2017-2020) and the Challenge Food For Tomorrow/Cap Aliment
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