Effect of Wood Particle Size on Fungal Growth in a Model Biomechanical Pulping Process

The pretreatment of aspen wood chips with white-rot fungus has been evaluated as a way of making biomechanical pulp. Our study addressed (1) whether wood particle size (chip size) affects the growth pattern of the attacking organism, and (2) whether the difference in particle size between chips and coarse pulp is related to the availability of wood polymers to the fungus. We qualitatively evaluated the growth of Phanerochaete chrysosporium BKM-F-1767 on aspen wood using standard industrial 6- and 19-mm chips and coarse refiner mechanical pulp. Scanning electron microscopy revealed a slight increase in the number of hyphae in the 19-mm chips compared to that in the 6-mm chips, but no major morphological differences in cellulose or lignin loss. Dense aerial hyphal growth occurred around the chips, but not around the coarse pulp. The fungus appeared to attack the coarse pulp from both outside and within the fiber wall. Hyphae within both the middle lamella and the cell lumina attacked the cell walls. The fungus eroded the chip cell walls and their constituents primarily from the wood cell lumen outward. After only 3 weeks of fungal treatment, both chips and coarse pulp showed marked localized cell-wall thinning and fragmentation as well as generalized swelling and relaxing of the normally rigid cell-wall structure. We conclude that particle size has only a minor effect on fungal growth on wood under conditions such as those likely to be used in a commercial biopulping process

Characterization of Palo Podrido, a Natural Process of Delignification in Wood

Chemical and morphological changes of incipient to advanced stages of palo podrido, an extensively delignified wood, and other types of white rot decay found in the temperate forests of southern Chile were investigated. Palo podrido is a general term for white rot decay that is either selective or nonselective for the removal of lignin, whereas palo blanco describes the white decayed wood that has advanced stages of delignification. Selective delignification occurs mainly in trunks of Eucryphia cordifolia and Nothofagus dombeyi, which have the lowest lignin content and whose lignins have the largest amount of β-aryl ether bonds and the highest syringyl/guaiacyl ratio of all the native woods included in this study. A Ganoderma species was the main white rot fungus associated with the decay. The structural changes in lignin during the white rot degradation were examined by thioacidolysis, which revealed that the β-aryl ether-linked syringyl units were more specifically degraded than the guaiacyl ones, particularly in the case of selective delignification. Ultrastructural studies showed that the delignification process was diffuse throughout the cell wall. Lignin was first removed from the secondary wall nearest the lumen and then throughout the secondary wall toward the middle lamella. The middle lamella and cell corners were the last areas to be degraded. Black manganese deposits were found in some, but not all, selectively delignified samples. In advanced stages of delignification, almost pure cellulose could be found, although with a reduced degree of polymerization. Cellulolytic enzymes appeared to be responsible for depolymerization. A high brightness and an easy refining capacity were found in an unbleached pulp made from selectively delignified N. dombeyi wood. Its low viscosity, however, resulted in poor resistance properties of the pulp. The last stage of degradation (i.e., decomposition of cellulose-rich secondary wall layers) resulted in a gelatinlike substance. Ultrastructural and chemical analyses of this substance showed the matrix to have no microfibrillar structure characteristic of woody cell walls but to still be rich in glucan