[Digestion] cellulosic ethanol process

[Duncan Martin]

Dear all that

Bear in mind that finding a 'magic' organism capable of direct conversion of 
cellulose to ethanol is only 1% of the battle, so let's not get too excited!

Researchers' press releases tended to be based on: (a) a compelling need to 
announce the most trivial advance as a breakthrough and (b) a pretty 
comprehensive ignorance of the realities and economics of process 
engineering.

Cellulose is a "difficult" substrate, resistant to microbial breakdown, 
partly because it is highly insoluble. That is why plants evolved it.

The degradation of cellulose in nature is commonly effected by complex 
consortia of microorganisms -- which often need to include the relatively 
few fungi that can degrade the lignin which cellulose is often wrapped up 
in. Consequently, the decay of cellulosic materials is typically slow -- it 
can take years in the case of woody (lignin-rich) materials.

It is therefore likely that the rate of conversion from our magic organism 
will be too low to be of any commercial use.

Another problem is tolerance to product inhibition.  Ethanol is toxic, to 
microorganisms, as it is to us. The yeasts we use in conventional 
fermentation have an unusual natural ability to tolerate high levels of 
ethanol, enhanced by selective breeding in the capable hands of generations 
of brewers, winemakers and distillers.

Consequently, our magic organism might only be capable of generating a very 
dilute solution of alcohol, so that the energy required to distill it to any 
useful concentration as a fuel would be greater. (There was similar 
excitement among researchers some 30 years ago over the prospect of using 
the bacterium Zymonas mobilis to produce ethanol from soluble substrates. 
However, it could not produce high concentrations and, as far as I am aware, 
has never been used in a commercial process.)

While I am writing, could I ask contributors to avoid the use of local names 
for plants etc? (When I read about digesting cat tails, I am tempted to 
suggest digesting the whole cat!) I presume the cattail is a plant.  Is it a 
weed, as its colourful name suggests?  Or a crop? What is it used for?

Duncan J Martin

Centre Councillor
Republic of Ireland Centre
Chartered Institution of Wastes Management