Structural change in wood by brown rot fungi and effect on enzymatic hydrolysis
Highlights
► Ethanol production from softwood is quite complex and requires severe pretreatment. ► Wood digestibility is affected by physical and chemical structural features. ► Fungal pretreatment is liable for breaking down the cellulose crystalline structure. ► Brown rot fungi disrupt lignin–cellulose complexes and remove the hemicellulose. ► A viable alternative is using a biomimetic system combined with chemical pretreatment.
Introduction
Lignocellulosic biomass is particularly abundant in nature and has a high potential for bioconversion. Lignocellulosic biomass offers a sustainable alternative to fossil fuels as source of carbon. Every year, large accumulation of this biomass results not only in deterioration of the environment but, also in loss of potentially valuable materials that can be processed for the production of biofuels, energy, and value-added chemicals [1], [2].
Cellulose, lignin and hemicellulose, the key components of lignocellulosic biomass, are closely associated with each other at the plant cell level. This close association and the partly cellulose crystalline nature, reduces cellulose reactivity towards enzymatic hydrolysis in biomass [3]. Thus, lignocellulose digestibility is affected by physical and chemical structural features [4], [5]. Making pretreatments, in order to break biomass structural features, necessary to enhance biomass digestibility. The effects of the pretreatment have been described as a disruption of the cell-wall matrix including the connection between carbohydrates and lignin, depolymerizing and solubilizing hemicellulose polymers, as well as changing the degree of cellulose crystallinity. These effects make cellulose more accessible to the enzymes that convert carbohydrate polymers into fermentable sugars [6], [7], [8].
In biological pretreatments, wood-destroying microorganisms that attack naturally wood, degrading lignin and holocellulose, are allowed to grow on the biomass, producing lignin–holocellulose complex breaking. The most serious kind of microbiological decay of wood is caused by fungi as they can cause rapid structural failure. Brown rot fungi are the most destructive type of wood decay [9]. These fungi can be naturally breaking down the highly ordered cellulose crystalline structure [9], [10]. Brown rot fungi preferentially degrade wood polysaccharides, and partially oxidize lignin [11]. They degrade holocellulose causing a rapid decrease in degree of polymerization with a low mass loss, and degradation products are produced faster than they are utilized [9], [12]. In the initial step of wood decay, enzymes are known not to be involved due their size that is larger than pores of the cell wall. Brown rot fungi employ a low-molecular-weight biochemical agent, which is capable of penetrating cell-wall pores, depolymerizing cellulose, thus making it accessible to further degradation [13]. Thus, the initial depolymerization is caused by producing small, diffusible, extracellular oxidants (free radicals), operating at a distance from the hyphae [11], [14]. This initial stage of decay involves the action of extracellular Fenton system (Fe2+ + H2O2) to generate hydroxyl radical (OH) [11].
Wood strength decays rapidly due to brown rot fungi initial attack. The purpose in this present study is evaluating the effect of the structural changes in wood by Gloephylum trabeum and Laetoporeus sulphureus on enzymatic hydrolysis of Pinus radiata and Eucalyptus globulus.
Section snippets
Raw material and preparation
P. radiata and E. globulus samples were chipped and screened to approximately 2.0 cm × 2.5 cm × 0.5 cm. The wood chips were air-dried until reaching approximately 10% (w/w) moisture, and then stored in plastic bags until their use. Prior to the biodegradation experiments, wood chips were immersed in water for 36 h and excess water was drained. Moist wood chips were sterilized (121 °C/30 min) and brought to room temperature.
Fungus, inoculum preparation and wood biodegradation
G. trabeum (ATCC 11539) and L. sulphureus (ATCC 52600) were used for treating wood
Chemical characterization
Chemical composition of the E. globulus and P. radiata wood used in this present work are showed in Table 1. Mass losses by the two strains of brown rot fungi assayed are shown in Fig. 1. G. trabeum degraded the chips most quickly, reaching mass losses of 11.7% and 13% after eight weeks for E. globulus and P. radiata, respectively. While, for in chips decayed by L. sulphureus mass losses were only 3% and 4.7% for E. globulus and P. radiata, respectively. The weight loss was calculated by the
Conclusion
Structural changes in wood as hemicelluloses removal and cellulose DP decrease were most significant with G. trabeum that with L. sulphureus, while both fungi shown similar capacity to decrease the cellulose CrI. The highest enzymatic hydrolysis yields were obtained on wood biopretreated with G. trabeum. G. trabeum can be considered as an effective potential fungus for biological pretreatment, since it provides an effective process in breaking the wood structure. A viable alternative to
Acknowledgements
Financial support was received from FONDECYT (grant 3100043) and SENACYT.
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