Submitted to: Applied Biochemistry and Biotechnology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/14/2006
Publication Date: 1/15/2007
Citation: Akin, D.E. 2007. Grass lignocellulose: strategies to overcome recalcitrance. Applied Biochemistry and Biotechnology. 136-140:3-15. 2007. Interpretive Summary: A priority for alternative energy is the use of lignocellulose from agricultural crops, such as grasses. Lignocelluloses, however, must be pre-treated to obtain sugars for conversion to ethanol. Previous research was reviewed on the structure of grass lignocellulose, factors limiting biodegradation, and environmentally friendly means to improve their bioconversion to useful products. Results are important in showing the use of environmentally friendly means to upgrade the value of agricultural residues and the identification of potential value-added co-products.
Technical Abstract: Grass lignocelluloses are limited in bioconversion by aromatic constituents, which include both lignins and phenolic acids esters. Histochemistry, ultraviolet absorption microspectrophotometry, and response to microorganisms and specific enzymes have been used to determine the significance of aromatics toward recalcitrance. Coniferyl lignin appears to be the most effective limitation to biodegradation, existing in xylem cells of vascular tissues; cell walls with syringyl lignin, e.g., leaf sclerenchyma, are less recalcitrant. Esterified phenolic acids, i.e., ferulic and p-coumaric acids, often constitute a major chemical limitation in non-lignified cell walls to biodegradation in grasses, especially warm season species. Methods to improve biodegradability through modification of aromatics include: plant breeding, use of lignin-degrading white rot fungi, and addition of esterases. Plant breeding for new cultivars has been especially effective for nutritionally-improved forages, e.g., bermudagrasses. In laboratory studies, selective white rot fungi that lack cellulases delignified the lignocellulosic materials and improved fermentation of residual carbohydrates. Phenolic acid esterases released p-coumaric and ferulic acids for potential co-products, improved the available sugars for fermentation, and improved biodegradation. The separation and removal of the aromatic components for co-products, while enhancing the availability of sugars for bioconversion, could improve the economics of bioconversion.