1. Field of the Invention
This invention relates to the biological modification of the lignocellulosic components of gramineous agricultural plant materials for the purpose of making the cellulose more available for use in a variety of applications. For instance, the digestibility of lignocellulosic grasses by cattle is limited to only that portion of the cellulose which is not physically bound by the lignin component. Also, with the current trend toward the production of alcohol as a renewable energy resource, the feasibility of employing lignocellulosic residues as sources for fermentable sugars hinges on the amount of free cellulose which can be enzymatically hydrolyzed to a glucose. Typically, up to about 50% of the cellulose in natural, grassy plant tissue is rendered unavailable for such uses by the lignin associated therewith.
2. Description of the Prior Art
Studies of the degradation of lignocellulosic materials have revealed that the lignin component can be depleted disproportionately faster than the polysaccharides when acted on by some white-rot fungi [Kirk, T. K., Phytopathology 63: 1504-1507 (1973)]. An approach to optimizing the ability of such organisms to preferentially decompose the lignin in wood materials is taught by Eriksson et al., U.S. Pat. No. 3,962,033 and P. Ander et al., Material and Organismen, Beiheft 3: 129-140, which apply a procedure for mutating spores of mycelia of wild strain white-rot fungi, and selecting the mutants having the desired degradative properties. While Eriksson teaches that these organisms can also be used for reducing the lignin content of certain grassy materials (e.g., straw and bagasse), their extent of selectivity in degrading the components of these materials is unreported. The chemical structure of grassy lignins is characteristically different from that of the woody lignins, and an organism adapted for preferentially degrading lignocellulose in one, typically will not manifest the same degree of preferentiality in the other.
Zadrazil [European J. Appl. Microbiol. 4: 273-281 (1977)] observed that a variety of basidiomycetes, including Pleurotus cornucopiae, Pleurotus sp. (Florida isolate), and Stropharia rugosannulata would generally increase the digestibility of wheat straw after about 20 days of cultivation. Kaneshiro [Dev. Ind. Microbiol. 18: 591-598 (1977)] reports preliminary results that the basidiomycetes Pleurotus ostreatus has potential for preferentially degrading the lignin content of feedlot waste fiber and wheat straw. In one fermentation experiment, the lignin content in a mixture of the fiber and straw was decreased from 12% to 7% with only slight depletion of the cellulose.