Novel biorefining systems for photosynthetic microorganism

This thesis describes research done on novel biorefining systems for microalgae. Following the principles of Green Chemistry, the environmental impact of biomass processing can best be minimized by utilizing many components of the biomass, extracting them with innovative solvents and by getting maximum utility out of any solvent used. Switchable Hydrophilicity Solvents (SHSs) are a new class of solvents able to change their nature from hydrophobic to hydrophilic and vice-versa. In this work a SHS has been used to carry out the fragmentation and extraction of both hydrophobic and hydrophilic molecules from microalgal biomass in the two opposite-hydrophilicity states that the SHS can exhibit, thus bringing about a quite substantial simplification in the biorefining of the matrix and increasing the “useful effect” of the solvent itself that performs a second, complementary biomass extraction task during the regeneration stage. The proposed concept of “Circular Extraction” scheme has been validated with the N,N-dimethyl-cyclohexylamine on the oleaginous microalga Scenedesmus dimorphus, showing an extraction efficiency in the backward-mode of 52 % for proteins, 50 % for carbohydrates, and 93 % for lipids. In the second part of this work, the PCH (1,2-propanediol, choline chloride, water 1:1:1) Natural Deep Eutectic Solvent (NaDES) was used to treat microalgal biomass and carry out the extraction of cellular components, such as lipids, proteins, carbohydrates and photosynthetic pigments (chlorophylls and carotenoids) from the biomass itself. NaDES, i.e. mixtures formed by natural primary metabolites present in all organisms form intermolecular hydrogen bonds and, when mixed in a certain ratio, change their state from solid to liquid forming a eutectic system. Experiments were carried out based on different contact time between biomass and PCH: 24 and 72 hours, with and without pre-treatment with ultrasound. Biomass was shaken together with the PCH solvent in the presence of glass beads to promote the extraction efficiency. The analysis of the extract composition was carried out spectrophotometrically for pigments (chlorophylls and carotenoids), with biochemical assays for proteins and carbohydrates and gravimetrically for the determination of lipids. The results showed the ability of PCH, coupled with the mechanical destruction of cell walls, to solubilize a wide range of polar biomolecules at room temperature, precisely the 49 % of proteins, 46 % of carbohydrates, 15 % of neutral lipids, 16 % of chlorophylls and 32 % of carotenoids. In the last part of the experimental work is introduced and demonstrated the first SHS ever described in the open literature which is based on a natural deep eutectic solvent (NaDES). The innovative solvent system that is disclosed therein is based on natural metabolites, that are intrinsically safe, and on water solution of a hydrophilicity-switching compound whose toxicity is practically nil, therefore also duplicating this as the first finding of a practically bio-safe switchable hydrophilicity solvent. Together, the two findings mark a significant step toward the identification of a destination-neutral biorefining tool for the obtainment of biologic fractions from biomasses with a wide range of potential market applications. We demonstrated the potential of this novel system for fractionating proteins-, carbohydrates- and lipids-containing biomass by following the Circular Extraction biorefining scheme on microalgae. The present finding is quite a substantial step toward intrinsic safety in biomass processing.