abstract
The fossil resources are declining while the requirements of modern lifestyle for energy and materials are growing. Hence, the search for sustainable alternatives to produce fuels and chemicals from non-fossil feedstocks is increasing. Among all biofuels, ethanol is currently being industrially produced from sugar-containing biomass such as sugarcane and corn. The use of these raw-materials, belonging to human and animal feeding, resulted in the rise of prices of food all over the world and, consequently, in social disturbance. The use of industrial by-products, raw-materials outside the food chain, with polysaccharides hydrolysed to fermentable sugars, is an attractive prospect for future biotechnologies. In this context, spent sulphite liquors (SSLs), by-products from the pulp and paper industry, are promising feedstocks for bioprocessing. The composition of SSLs depends on the type of wood used by the pulp and paper industry (softwoods, hardwoods or mixture of both). Hardwood spent sulphite liquor (HSSL) is a by-product from the pulp and paper industry, rich in pentoses, which is not fully exploited for bioprocessing. The sustainable fermentation of pentoses into bioethanol is a challenge to overcome since not all the microorganisms are able to use these sugars. Scheffersomyces stipitis is one of the most efficient yeast to naturally ferment pentoses to ethanol. However, besides sugars (35-45 g.L-1), HSSL contains microbial inhibitors that limit the possibility of its bioprocessing. Therefore, the main purpose of this work was the production of bioethanol by S. stipitis from HSSL of Eucalypt globulus. To accomplish this objective two different strategies were studied. The first one was the bio-detoxification of HSSL with the filamentous fungus Paecilomyces variotii, known for growing in polluted residues. Two fermentative approaches were compared, a single batch and a sequential batch reactor (SBR). Biological treatment of HSSL to remove microbial inhibitors was more efficient in the SBR. P. variotti was able to assimilate acetic acid as well as low molecular weight phenolics such as, gallic acid and pyrogallol, recognized yeast inhibitors. This bio-detoxified HSSL was subjected to a successful fermentation by S. stipitis, attaining a maximum ethanol concentration of 2.36 g.L−1 with a yield of 0.17 g.g−1. Moreover, the biomass produced by P. variotii is a potential source of protein and other nutrients for animal feeding. Hence, SCP production by P. variotii from HSSL was studied using a SBR with and without mixed salts supplementation. The best approach for SCP production was the SBR without salts addition. The biomass produced presented 82.8 % of protein with 6 essential amino acids and 1.1 % of DNA. Therefore the produced SCP could be considered a good candidate for animal feeding and, eventually, human nutrition. This is a major advantage for a biorefinary approach, since this bio-detoxification process and the SCP production can be integrated with bioethanol production by S. stipitis. The second strategy to produce bioethanol was to improve the tolerance of S. stipitis in order to utilize the xylose present in HSSL without the removal of inhibitory compounds. A continuous reactor with increasing HSSL concentrations, between 20 % and 60 % (v/v) was operated during 382 generations of HSSL, at a dilution rate of 0.20 h-1. The resulting adapted population (POP) showed improved fermentation behaviour in 60 % HSSL when compared with the parental strain (PAR). POP achieved a maximum ethanol concentration of 6.93 g.L-1, with a maximum ethanol yield of 0.26 g.g-1. It was also showed that POP could assimilate dissolved lignin oligomers and tannins probably through activating oxidative pathways. Moreover, preculturing PAR in HSSL improved its tolerance towards the HSSL inhibitors and also the yeast fermentation ability. Nevertheless, preculturing POP in HSSL, redirected its metabolism to the assimilation of inhibitors, reducing the ethanol production.
subject category
PROGRAMA DOUTORAL EM ENGENHARIA QUÍMICA (3º CICLO)
authors
Susana Raquel de Sousa Pereira
our authors
Susana Raquel de Sousa Pereira
PhD Studentsupervisors
Ana Maria Rebelo Barreto Xavier; Luísa Seuanes Serafim
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