Fermentation of undetoxified dilute acid lignocellulose hydrolysate for fuel ethanol production
Important aspects of ethanol production from undetoxified dilute acid lignocellulose hydrolysate are covered in this thesis, which primarily focuses on the use of Saccharomyces cerevisiae (Baker's yeast) as the biocatalyst. Nine different strains of S. cerevisiae were compared for fermentation of dilute acid lignocellulose hydrolysate with batch and fed-batch methodology and the one found to be best, ATCC 96581, was used in further studies involving S. cerevisiae.
Cultures of this strain could stay viable during extended continuous cultivations with low cell growth, achieved with cell recirculation, and consumed more than 99% of the available glucose. However, the ethanol yield was not significantly affected neither by excessive nitrogen limitation nor microaerobic conditions. Importantly, microaerobicity stabilised cell survival during these cumbersome conditions.
Cell retention was also applied to ferment dilute acid lignocellulose hydrolysate in a continuous reactor, which increased the cell concentration and improved both the utilisation of hexoses and the intracellular conversion of at least one inhibitory compound in the hydrolysate (5-hydroxymethyl furfural). Cell retention by filtration, as a method to improve fermentation of lignocellulose hydrolysate supplemented with wheat hydrolysate, ammonium sulphate and biotin, was furthermore compared to alternative methods of cell retention: immobilisation and sedimentation. All cell retention methods were useful for improving hexose utilisation at a dilution rate of 0.10 h-1, but washout occurred in all systems except for the immobilised system when the dilution rate was increased to 0.20 h-1.
In a study on M. indicus, an 'alternative' fermenting organism, which can ferment also pentoses, it was shown that this organism is suitable for fed-batch fermentation of dilute acid lignocellulose hydrolysate. With an initial dilution rate of 0.20 h-1, the sugar consumption and the ethanol yield were satisfactory. Moreover, the troublesome filamentous growth of M. indicus was circumvented.