Hydrothermal pre-treatment coupled with urea solubilisation enables efficient protein extraction from microalgae

Microalgae-based bulk production, especially for biofuel, is not yet economically feasible. One way to improve financial performance of such systems is to maximise value through a biorefinery approach (e.g. co-production streams, waste minimisation). This can be implemented via extraction of valuable intracellular compounds upon cell disruption; however both low-cost, energy efficient, and scalable cell disruption techniques, and green chemicals are required. Waste heat is a readily available resource in most production plants, and can be used to power a hydrothermal pre-treatment (HTP) step to disrupt the cells. In this study, we use HTP (120-180 degrees C, 5-30 min) to disrupt Chlorella sp. cell walls. We perform a subsequent water-miscible solvent and/or urea extraction (UX) to solubilise and recover proteins. The maximum extraction efficiency of 62% was achieved after HTP at 180 degrees C for 10 min. Regardless of the reaction time, 120 degrees C was found to be ineffective for protein extraction. The efficiency can be further increased to 84% if a filter-washing step with acetone and urea is implemented. Most of the recovered proteins are between 8 to 14 kDa across all temperature ranges, suggesting that extraction efficiency depended both on partial protein hydrolysis and on accessibility to solvent (i.e. cell wall degradation, solid dispersion, membrane solubilisation). The most significant amino acid losses on a per mole basis were lysine (22%) and arginine (14%). The production of flexible and high-value protein-based products via HTP-UX may enable a circular and sustainable production step for microalgal biorefineries, while improving the downstream fuel processes through nitrogen heteroatom reduction.