AbstractIn recent years bioethanol (fuel ethanol derived through fermentation) has become the leading alternative to currently utilised liquid transportation fuels, possessing the benefits of being both sustainable and carbon neutral. If this position is to be maintained, it is clear that the feedstocks and processes used during its production have to be a major focus for the future direction of research, not only within the context of bioethanol but all biofuels in general. In terms of future sustainability, it is important that biofuel production should be derived from waste materials which exhibit limited potential for use in other applications. In this manner, food-to-fuel debates can be circumvented.
Lignocellulose represents a readily available biowaste material for biofuel generation. The brewing and distilling industries in particular are uniquely placed to exploit the conversion of lignocellulose to bioethanol, through the utilisation of the millions of tons of spent grains produced annually by distilleries across the globe. Conversion of spent grains (SG) to bioethanol represents one of the most attractive and indeed challenging opportunities for meeting demand for sustainable bioethanol production. However it presents considerable challenges in terms of costs of production and associated energy balances. Enzyme costs are a particularly challenging issue and any technological developments which have potential to increase cellulolysis and decrease cellulase enzyme dosage are of distinct importance. Additionally, current pre-treatment approaches rely heavily of the use of acids during the pre-treatment stage. This generates a variety of compounds which are inhibitory to fermentation (e.g. acetic acid, furfural) and lead to low ethanol yields. Research which can reduce enzyme loading or improve low temperature pre-treatment are of significant importance.
The research described here sought to evaluate the use of ultrasound in the pre-treatment and enzymolysis of spent grains. The use of acid in the pre-treatment and enzymolysis of SG was optimised to provide a bench-mark for novel hydrolysis techniques. Ultrasound (20 kHz) was assessed as a pre-treatment technique in its own right as well as in combination with various oxidising chemicals such as ozone and hydrogen peroxide. Additionally, high frequency ultrasound (>500 kHz) was investigated in terms of its effect upon the activity of the enzymes involved in SG hydrolysis. A number of yeast species (S.cerevisiae, P.stipitis, K.marxianus, P. tannophilus and C. shehatae) were then evaluated for their ability to ferment the mix of five and six carbon sugars liberated during enzymatic hydrolysis of SG.
Ultrasound was found to be effective in enhancing combined ozone and hydrogen peroxide pre-treatment of SG, although not as effective as the use of acid. Additionally, ultrasound was shown to significantly enhance the activity of cellulose, xylanase and - glucosidase, with the magnitude of the increase highly dependent on frequency and output power. The research presented here has expanded knowledge in terms of the effect of ultrasound upon lignocellulose as well as the enzymes involved in its digestion.
|Date of Award||Jun 2013|
|Supervisor||Graeme Walker (Supervisor) & David H. Bremner (Supervisor)|