Structural and chemical properties of grass lignocelluloses related to conversion for biofuels

Authors: Anderson, William - Bill; Akin, Danny

Submitted to: Journal of Industrial Microbiology and Biotechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2007
Publication Date: 5/1/2008
Citation: Anderson, W.F., Akin, D.E. 2008. Structural and chemical properties of grass lignocelluloses related to conversion for biofuels. Journal of Industrial Microbiology & Biotechnology. 35:355-366.

Interpretive Summary: Forage grasses are identified as a major contributor as a resource for biofuels in the cellulose-to-ethanol strategy. Grasses like all lignocelluloses, however, are limited by aromatics that protect the sugars and prevent bioconversion. Pretreatment and/or modification is required to mitigate the influence of aromatics for the grass carbohydrates to be fermented into ethanol. This paper addresses the structural and chemical limitations to bioconversion in grass lignocelluloses and proposes biological methods, including plant breeding and enzymes, to improve their utilization as biofuel. Based on identified structural and chemical factors, this work proposes an environmentally friendly framework to improve the availability of sugars in grass lignocellulose and to extract aromatics for high-value co-products to assuage the high conversion costs.

Technical Abstract: Grass lignocelluloses, such as those in corn and switchgrass, are a major resource in the emerging cellulose-to-ethanol strategy for biofuels. The potential bioconversion of carbohydrates in this potential resource, however, is limited by the associated aromatic constituents within the grass fiber. These aromatics include both lignins, which are phenylpropanoid units of various types, and low molecular weight phenolic acids. Structural and chemical studies over the years have identified the location and apparent limitation to fiber degradation imposed by a variety of these aromatic barriers. For example, coniferyl lignin appears to be the most effective limitation to biodegradation, existing in xylem cells of vascular tissues. On the other hand, cell walls with syringyl lignin, e.g., leaf sclerenchyma, are often less recalcitrant. Ferulic and p-coumaric acids that are esterified to hemicellulosic sugars constitute a major limitation to biodegradation in non-lignified cell walls in grass fibers, especially warm season species. Non-chemical methods to improve bioconversion of the lignocelluloses through modification of aromatics include: 1) use of lignin-degrading white rot fungi, 2) pretreatment with phenolic acid esterases, and 3) plant breeding. In addition to availability of carbohydrates for fermentation, separation and collection of ferulic and p-coumaric acids could provide value-added co-products to improve the economics of bioconversion.