Lignocellulosic residues valorization using a combined approach of sub-supercritical fluids and biotechnology

Abstract

An integrated processed combined sub-, supercritical fluids and biotechnology was proposed to create a valorization chain for the lignocellulosic residues. Hot compressed water was evaluated as an alternative medium to perform the simultaneous extraction of added value compounds directly available in the lignocellulosic residues such as phenolic compounds as well as the hydrolysis of the main structural components, cellulose, hemicellulose and lignin. Different lignocellulosic residues such as recycled paper mill sludge, grape pomace and apple pomace were studied. The main goal of the hydrolysis was to obtained aqueous solutions rich in sugars which can be further used as alternative carbon source to oleaginous yeasts growth which are capable of accumulating lipids and carotenoids. As final stage of this proposed process, the use of alternative oil sources was evaluated by using an integrated continuous process, which combined supercritical carbon dioxide and enzymes. Therefore, this valorization chain will be able to generate form lignocellulosic residue, three added value compounds: lipids, carotenoids and biodiesel by combining for the first time several alternative solvents, bioengineering and biotechnology. Recycled paper mill sludge (RPS) was used as lignocellulosic residue model to evaluate the potential of using HCW to perform the hydrolysis of the three main components, cellulose, hemicellulose and lignin. Two different operational modes were tested: batch and semi-continuous flow reactors. From a detailed process analysis including the evaluation of different operational parameters, target products recovery and the devolvement of detailed mass balances, the semi-continuous model proved to be the most fitted process design, with a higher recovery of water soluble compounds, mainly monosaccharides, avoiding at the same time the formation of degradation compounds. Grape pomace was used to evaluate the potential of using a HCW treatment, which simultaneous extracts the phenolic compounds directly available in the residue as well as the hydrolysis of its main structures in order to obtain a solution rich in sugars. HCW proved to have the potential to be used as a solvent for the extraction of added-value compounds as well as a reactant for the hydrolysis. The extraction of antioxidants using HCW presented yields which are 10 times higher than those using conventional solvents. Additionally, HCW hydrolysis allows fast recovery of up to 60% of structural carbohydrates of grape pomace mainly in the form of oligosaccharides, which can be further digested by enzymes or used directly by microorganisms such as Rhodotorula babjevae yeast. This yeast exhibited an OD 2 to 5 fold higher than obtained with model mixtures, demonstrating, for the first time, that HCW-treated agricultural residues affords suitable carbon source for the growth of non-Saccharomyces yeast. Five different oleaginous yeasts strains were tested to demonstrate the suitability of apple pomace hydrolysates material as carbon source not only for growth but also for lipids and carotenoids production. Good performances in terms of growth, lipids and carotenoids accumulation were observed without performing any growth operational conditions optimization. Alternative oil sources, such as waste cooking oil, was used as raw material to produce an alternative biofuel- biodiesel applying a combined integrated continuous process using SC-CO2 and a continuous enzymatic flow reactor. It was made the proof of concept by achieving a product with >96 % of FAME using a ScCO2-based process as described, whether the starting material is virgin or waste cooking sunflower oil.