10-10: Galacturonic acids hinder the enzymatic hydrolysis of green crops

Pectic substances, abundant in the cell wall and middle lamella, are complex acid polysaccharides with a backbone of galacturonic acid residues. The side chains of pectin consist of arabinose, galactose and xylose. The solubility of pectins depends on cross-chain bonding by calcium and on the pH used. Many cellulose rich crops contain significant amounts of pectin. It can be assumed that the location and structure of pectin hinders the accessibility of lignocellulolytic enzymes and thus, the hydrolysis of pectin would enhance the conversion of recalcitrant crops.

 In this study, fiber hemp was investigated as a potential green energy crop. Galacturonic acid amounts to almost 6% of the raw material, and comprises about 10% of total carbohydrates. As could be expected, fresh hemp was hydrolyzed fairly poorly with commercial cellulases, but the hydrolysis yield was increased by 10% when pectinases were added. For comparison, hemp was also treated with 1% NaOH at 120ºC. All uronic acids were dissolved during the alkaline treatment. After this, the enzymatic hydrolysability of the treated hemp was increased significantly. It was remarkable that especially the conversion of xylose was significantly enhanced as compared to the fresh material. Mild alkali conditions during the ensiling treatment by the addition of urea were studied as well. After the storage, minor removal of pectin was observed followed by enhanced enzymatic hydrolysis, as compared to the fresh material. Thus, optimizing the ensilage conditions and enrichment of the enzyme mixture with pectinases can significantly improve the conversion processes of annual green crops.