Biological Lignin Degradation,Lignolytic Fungi,Biopulping Consortium,Biochemical Pulping,Tensile Strength,Burst Strength

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Home >> Industrial and Microbial Biotechnology >> Microbes and Microbial Genomics for Industry >> Biological Lignin Degradation

Your Ad Here	Biological lignin degradation In the pulping process, degradation of lignin can be achieved through treatment with microbes, of which lignolytic fungi are the most important. A biological step can be integrated in pulping process, both in chemical pulping as well as in mechanical pulping. Three ways have been suggested for this purpose, which are outlined. It may be seen that the biological step may be a pre-treatment or a post-defibration, to remove lignin. However, the use of a biological step in lignin degradation, is still at the level of research and experimentation. Its use at industrial scale is seen as a distinct possibility due to successful results already obtained in several experiments.
To support research in this area, a Biopulping Consortium (funded by 20 companies) was established in USA in 1987. Pulping process leading to ligning degradation
Biochemical pulping. In has already been shown that a treatment of aspen chips (aspen is a wood; wood is received by pulp mills in two forms, logs or chips, the latter being more popular) with Phanerochaete chrysosporiumbefore craft pulping (sulphate process) gives improved tensile strength (resistance to rupture by a force parallel to sheet) and burst strength (resistance to rupture by a force perpendicular to sheet). But it leads to decreased tear strength (resistance to elongation under transverse shear), brightness and yield. Brightness may be improved later by bleaching. More research and experimentation is needed before biochemical pulping becomes a reality and used in paper industry.
Biomechanical pulping. Mechanical pulping involves two steps: (i) primary refining or defibration and (ii) secondary refining. Biological step may be introduced before or after the defibration step, and both have some advantages. Treatment of coarse pulp (obtained after defibration) with lignolytic fungi (e.g. Trametes yersicolor, P. chrysosporium, Pleurotus ostreatus)has been shown to give increased tensile strength, and may reduce energy requirement for secondary refining. Pre-treatment also leads to increased pulp strength. For this purpose wood chips (3-8 mm thick) are supplemented with a nutrient solution and sterilized by autoclaving. They are then inoculated with the mycelium of the fungus, pre-grown in agar or liquid meduim. The inoculated chips are incubated at high humidity and at a temperature (25-40˚C) optimal for growth of the specific fungus being used.
	Aeration is achieved by diffusion or forced ventilation sufficient to replenish oxygen. The chips may be agitated intermittently for distribution of inoculum and moisture. Within 2-6 weeks, the fungus colonizes the wood leading to softening and swelling of the fibre walls. This treatment may lead to substantial energy savings and paper strength improvement. Different lignolytic fungi used for this purpose include the following: Tremetes versicolor, Dichomitus squalens, Pholiota mutabilis, Phlebia subserialis, P. brevispora, P. tremellosa, Phanerochaete chrysosporium, and Ceriporiopsis subvermispora.

Before biological pulping is used in industry, considerable research in the following area would be needed and is being undertaken: (i) More lignolytic fungi (there are 2000 known species of lignolytic fungi) need to be tried, and search needs to be made for better strains, which act quickly, have high competitive ability and do not darken the wood. (ii) Fermentation technology needs to be developed, which includes optimization of conditions for fungal activity, designing of a reactor and scaling up of the fermentation step. (iii) Mechanism of lignin degradation by fungi needs to be studied. Energy savings and strength improvement generally do not correlate closely with bulk lignin removal or even with each other, and a better understanding of the mechanism may really help.

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