Fermentation of lignocellulosic material : a study on bacterial contamination and yeast physiology
Progressive depletion of oil and conventional fossil fuels, an increased energy demand and an increased struggle for national energy security has led to the development of alternative biobased fuels, bioethanol being one of them. Bioethanol can be made from many different raw materials, and based upon this are classified as 1st generation and 2nd generation. This research work dealt with 2nd generation bioethanol using lignocellulosics as raw material. Lignocellulosic material is an abundant, non-edible raw material that can be converted to ethanol using the yeast Saccharomyces cerevisiae. Due to the recalcitrant nature of lignocellulosics, it is a harsh substrate to ferment. Despite this, bacterial contaminations do occur. This research work aimed to identify potential methods to combat bacterial contamination in industrial fermentations and to identify physiological reactions of the yeast Saccharomyces cerevisiae upon fermentation of lignocellulosic material. The raw material used was spruce processed in a commercial biorefinery and a demonstration plant; spent sulphite liquor; and high-gravity dilute-acid spruce hydrolysate, respectively. Two potential methods to combat bacterial contaminations were identified. The cultivation procedure of a pitching agent was proven to influence bacterial concentration and is suggested as a potential antimicrobial activity. Treatment with sodium chloride and ethanol also turned out to selectively support the viability of yeast and reduce the number of bacterial cells. As lignocellulosics are rich in metabolic inhibitors, additional antimicrobial activities may be deleterious for the ethanol production potential of the yeast. By comparing physiological effects and the level of energy of two strains of S. cerevisiae, in the two substrates mentioned above, it was found that a commercially-available strain developed for industrial fuel ethanol production performed poorly in fermentations of spent sulphite liquor while a strain originally harvested from a spent sulphite liquor-based fermentation performed well both short-term and long-term. Using high-gravity dilute-acid spruce hydrolysate changed the scenario, demonstrating the importance of strain selection.