Biofuel production from plant biomass derived sugars

Oct 2006

Ethanol costs could be significantly reduced if cheap plant biomass-based feedstocks could be utilised. Significant efforts are being undertaken in North America and the EU to develop cost effective technology for the hydrolysis of biomass into its constituent sugars. Enzymatic hydrolysis of biomass has been improved under a U.S. Government funded programme by Novozyme and Genencor, while Canadian company, Iogen, has developed an enzymatic hydrolysis and fermentation process with Canadian government grants which, reportedly, is being commercialised.
Currently, the cost of production of bioethanol for fuel is prohibitively high compared with gasoline. This is due, not only to the expensive sugar or starch feedstock utilised, but also to the low production rates and the inability of conventional yeast fermentation to convert the pentose sugars found in plant biomass. Some yeasts have been engineered to utilise pentose sugars but, to date, these strains produce an economically unviable ethanol yield. Other micro-organisms that are able to consume plant biomass based feedstocks also produce low yields of ethanol even though their metabolisms have been improved through genetic modification. Moreover there are pathogenic variants of these organisms and during fermentation the risk of infection by pathogenic bacteria is high. These problems mean that current non-yeast technology requires the use of labour intensive batch operations under aseptic conditions almost trebling the capital cost of fermentation and lowering plant productivity.
Thermophilic bacilli, growing at 60-65°C, are more robust, can utilise all the constituent sugars of biomass and deliver very efficient fermentation processes. By virtue of their ability to grow at temperatures in excess of 60°C, the risk of contamination by other micro-organisms is greatly reduced and the use of continuous culture techniques becomes feasible, resulting in reduced capital and labour costs. However, until now, relatively little has been done on the development of such strains and processes for large-scale manufacture. Most thermophilic Bacillus and Geobacillus species are capable of metabolising a range of sugar substrates, including those that are common components of plant biomass. A few of these strains secrete relatively small quantities of ethanol as one of a number of products of metabolism.
This study focussed on one such strain and developed the genetic manipulation techniques necessary to engineer its metabolism such that the unwanted products, mainly organic acids, were no longer formed and ethanol became the overwhelming product.

By: R. Cripps (TMO Renewables)

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