Lignocellulosic Ethanol Production: Studies on Sugarcane Bagasse, Paja Brava, Wheat Straw, Quinoa Stalks and Curupaú

Abstract: Popular Abstract in English The world’s resources of fossil fuels are rapidly being depleted, and we now know that the combustion of these fuels leads to the emission of greenhouse gases resulting in climate change. It is therefore imperative that we develop renewable sources of fuel. The production of bioethanol in Brazil is the best example today of the introduction of a renewable fuel on a large scale. This ethanol is obtained from the fermentation of sugar from sugarcane. In fact, all ethanol today is obtained from the fermentation of sugar (from sugarcane or sugar beets), or starch (mainly from corn or wheat). This is sometimes referred to as “first generation” biofuel. The term “second generation” or “advanced biofuels” refers to fuels derived from lignocellulosic biomass, which means that the whole plant is used (with the exception of the grain). This biomass is available in much larger quantities and does not compete with cultivation for food. However, this material is a complex mixture of macromolecules such as cellulose, hemicellulose and lignin, which are not directly fermentable. The overall resistance of lignocellulosic biomass to degradation is also high, or at least much higher than that of starch. Efficient methods are therefore needed to degrade, in particular, the carbohydrate polymers to obtain sugars that can then be fermented to ethanol, or other desired chemical building blocks, by various micro¬organisms. These questions are addressed in this thesis, through studies on chemical and biochemical methods of obtaining sugars from various kinds of biomass, and the fermentation of these sugars by various kinds of yeasts. The materials studied here were “waste” biomass, such as sugarcane bagasse, which is the solid part of the sugarcane remaining after sugar extraction, wheat straw, Anadenanthera colubrina and quinoa stalks. Quinoa is a crop grown on the Bolivian Altiplano, as is paja brava (the “brave” straw). A. colubrina is a South American tropical hardwood. To obtain sugars from these kinds of materials, they first have to be subjected to steam treatment at a high temperature for a few minutes. This breaks down part of the carbohydrates, the hemicellulose part, which is a polymer consisting of many sugars, both six-carbon sugars and five-carbon sugars. Various kinds of catalysts, typically acids, can be used, and in this study SO2 was found to be a good catalyst for many feedstocks. After this pretreatment, enzymes can be used to break down the cellulose in the remaining material. Optimal conditions for pretreatment were determined in this work. The hemicellulosic sugar yields obtained from the lignocellulosic materials studied ranged from 67 to 80% of the theoretical yield, which represents about 15-18 g of xylose per 100 g dry matter for a typical straw material. Studies were also carried out on fermentation, in particular the design of a process allowing co-fermentation of both hexose (six-carbon) and pentose (five-carbon) sugars. Both natural and genetically engineered yeast strains were evaluated. A genetically modified strain of S. cerevisiae was found to be more tolerant to hydrolysates than the natural xylose fermenting yeast P. stipitis, and showed higher ethanol yields (0.40-0.43 g/g) in the Bolivian straw material. Considerable efforts were made to analyse the liquid fraction after pretreatment. In addition to the sugars and the degradation products of the sugars, aromatic compounds were found in the pretreatment liquid. In this work, glycosylated aromatics were also found, which show binding structures in the original biomass linking the lignin to the hemicellulose. In general, lignocellulosic materials studied in this work have a great potential as ethanol feedstocks and other biorefinery applications. In particular, fractionation processes of these materials propose ways to produce valuable building blocks, i.e. glycosylated aromatics. These studies in this thesis are believed to contribution to novel biomass conversion processes.