Remember the melting witch in The Wizard of Oz?
What if corn stalks, for instance, could be induced to “melt” from tough biomass into a sugary puddle? That is, to breakdown their cellulosic biomass into a solution of glucose and other sugars.
If so, it would be much more cost effective to use corn biomass as a source of raw material to make biofuels than is currently the case.
This is because lignocellulosic biomass is very difficult to enzymatically digest in the laboratory as a preliminary step to the production of cellulosic ethanol.
Typically, such plant material is cooked in sulfuric acid to render it suitable for commercial conversion to ethanol.
Thus, despite the fact that cellulosic biomass is by far the most abundant source of material for biofuel production, it is currently not cost effective because of the extensive pretreatment required to make it useful.
But what if digestive enzymes were embedded within the plant biomass itself, to be activated only by special conditions?
“Plant biomass” consists mainly of plant cell wall material, chiefly cellulose (see pie chart right). As cellulose and the other plant cell wall structural components are the plant’s chief line of defense against herbivorous insects and microbes, this is tough stuff.
Some bacteria and fungi, however, have evolved to produce enzymes such as cellulases that can digest plant cell walls. (We saw here last week that even plants have genes that code for cellulase.)
Some especially effective plant biomass-digesting microbes live in the guts of some insects (termites) and animals (cows). Indeed, cell wall-digesting enzymes obtained from these microbes can be used in the laboratory to breakdown – albeit slowly – plant biomass in vitro.
One of the reasons this process is slow is that the enzymes must digest the material from the outside in. But what if these enzymes could digest plant cell walls from the inside out?
Some recent research is aimed at doing just that. This idea has also attracted interest from the private sector (see PDF file describing such a project here).
Basically, the idea works like this. Microbial genes coding for potent cell wall-digesting enzymes are incorporated into transgenic corn, for example. These enzymes are designed to be deposited in the plant cell walls. But the key to this plan is to make sure these enzymes are inactive until activated by conditions typically not experienced naturally, such as temperatures above 140o F. After corn stalks, for example, are harvested, they then are heated to such temperatures, which activates the digestive enzymes embedded in the biomass itself.
Using cellulosic biomass for biofuel production instead of corn kernels, for example, would help to ease pressure on food prices.
Bottom line: Until lignocellulosic material can be cheaply rendered appropriate for fermentation to ethanol, biofuels will remain an impractical solution to America’s current energy crisis.
