Fermentation and Cultivation Technology for Improved Ethanol Production from Lignocellulose

Abstract: The work presented in this thesis has been aiming at improving ethanol production from lignocellulose. Ethanol from renewable resources is receiving renewed attention due to apprehensions of global warming and dwindling oil reserves. Lignocellulose constitutes a potential raw material source for large scale ethanol production. To produce ethanol from lignocellulose, the hemicellulose and cellulose components have to be hydrolyzed to saccharides by acid or enzymatic hydrolysis. Lignocellulose hydrolysates often contain inhibitors that affect the fermentation negatively and cause a decrease in ethanol productivity. Since ethanol is a low-value product it is crucial that the production process is relatively simple, robust and efficient. The overall aim of the current work was to further improve fermentation of lignocellulose hydrolysates by modifying existing and developing new fermentation and cultivation methods. The lignocellulosic materials evaluated in the current work were spruce and sugar cane bagasse. Spruce is an abundant feedstock in Sweden, whereas sugar-cane bagasse is an abundant agricultural residue, available in large quantities in the tropical and subtropical zone. It was shown that by improving fermentation techniques and developing methods for on-site yeast production, significantly improved fermentation of lignocellulose hydrolysates at industrially applicable conditions could be achieved. An existing fed-batch method for controlled fermentation of toxic hydrolysates was further developed to allow for scale-up. It was demonstrated that fast and efficient fermentation of an inhibitory dilute-acid hydrolysate from spruce in 20 L-scale could be obtained by applying a feed-rate control that used the total gas flow from the reactor as input-signal. It was shown that precultivation of yeast on hemicellulose hydrolysate significantly improved its inhibitor tolerance and made it possible to significantly increase the content of water insoluble solids in SSF. Ethanol concentrations above 5 % (v/v) were attained in simultaneous saccharification and fermentation of steam-pretreated spruce by using yeast adapted to the hydrolysate. More efficient utilization of the different saccharides in simultaneous saccharification and fermentation of sugar cane bagasse could be obtained by using xylose-fermenting yeasts. Both recombinant Saccharomyces cerevisiae and naturally xylose-fermenting yeast were evaluated. Even though high xylose conversion was obtained with both yeasts, S. cerevisiae proved much more suitable for fermentation at industrial conditions.