Lignin: breaking down the cell wall for bioenergy
Fungus helps scientists understand the mechanism of how to break down lignin to access the structural sugars for ethanol production.
Plant cell walls contain cellulose, hemicellulose and lignin. Cellulose and hemicellulose are made up of polymers of five and six carbon sugars. These structural sugars contain the energy, when converted to ethanol, that can be burned in a gasoline engine. Lignin is the protector of cellulose and hemicellulose. It acts like a bubble wrap, protecting them from degradation. The biggest challenge – and expense – with making cellulosic ethanol is unwrapping this protective structure (lignin) so that enzymes can access the long chains (polymers) of sugar in cellulose and hemicellulose.
White rot fungus is found in the environment and has been found to play a role in degradation of wood in forests. White rot fungi use an enzyme called laccase to break down lignin. Researchers at the University of Georgia are studying how this fungus works and if there are natural properties that can be used or mimicked in the process of making cellulosic ethanol.
Laccase uses mediators to oxidize and reduce lignin (break it down). Four mediators in laccase have been discovered and tested for their ability to degrade lignin and cellulose. To make ethanol, we want to break down the lignin without destroying cellulose. Violuric acid (one of the four mediators) breaks down 38 percent of lignin without hurting cellulose.
Violuric acid was tested at various concentrations on switchgrass and sweet sorghum feedstocks. More research needs to be done to determine the optimum concentration of violuric acid for each type of feedstock. Variations from one type of feedstock to another, or even variations from one growing season to another, could have impacts on the efficiency of this method in a commercial biorefinery.
The key to making cellulosic ethanol at commercial scales lies in figuring out how to unwrap lignin in the cell wall. Cell walls from different feedstocks are all wrapped a little bit differently. If all cell walls were wrapped the same, it would be easier to devise a solution. Research like this is needed to move the cellulosic ethanol industry forward.