Abstract
BackgroundThe natural recalcitrance of lignocellulosic plant cell walls resulting from complex arrangement and distribution of heterogeneous components impedes deconstruction of such cell walls. Dilute acid pretreatment (DAP) is an attractive method to overcome the recalcitrant barriers for rendering enzymatic conversion of polysaccharides. In this study, the internodes of Miscanthus × giganteus, a model bioenergy crop, were subjected to DAP to yield a range of samples with altered cell wall structure and chemistry. The consequent morphological and compositional changes and their possible impact on saccharification efficiency were comprehensively investigated. The use of a series of microscopic and microspectroscopic techniques including fluorescence microscopy (FM), transmission electron microscopy (TEM) and confocal Raman microscopy (CRM)) enabled correlative cell wall structural and chemical information to be obtained.ResultsDAP of M. × giganteus resulted in solubilization of arabinoxylan and cross-linking hydroxycinnamic acids in a temperature-dependent manner. The optimized pretreatment (1% H2SO4, 170°C for 30 min) resulted in significant enhancement in the saccharification efficiency (51.20%) of treated samples in 72 h, which amounted to 4.4-fold increase in sugar yield over untreated samples (11.80%). The remarkable improvement could be correlated to a sequence of changes occurring in plant cell walls due to their pretreatment-induced deconstruction, namely, loss in the matrix between neighboring cell walls, selective removal of hemicelluloses, redistribution of phenolic polymers and increased exposure of cellulose. The consequently occurred changes in inner cell wall structure including damaging, increase of porosity and loss of mechanical resistance were also found to enhance enzyme access to cellulose and further sugar yield.ConclusionsDAP is a highly effective process for improving bioconversion of cellulose to glucose by breaking down the rigidity and resistance of cell walls. The combination of the most relevant microscopic and microanalytical techniques employed in this work provided information crucial for evaluating the influence of anatomical and compositional changes on enhanced enzymatic digestibility.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0282-3) contains supplementary material, which is available to authorized users.
Highlights
The natural recalcitrance of lignocellulosic plant cell walls resulting from complex arrangement and distribution of heterogeneous components impedes deconstruction of such cell walls
Plants have evolved a hierarchical structure during cell wall assembly, which is differentiated into several distinct layers: the shared cell corner middle lamella (Ccml) and compound middle lamella (Cml) that function as a glue between adjoining cells, the thinner primary wall (P) and thicker secondary wall (Sw)
The carbohydrate constituents of native samples accounted for 65.78% of the dry matter (DM), which mainly consisted of glucose (40.85% DM, 62.10% total carbohydrates), xylose (21.28% DM, 32.35% total carbohydrates) and arabinose (2.59% DM, 3.94% total carbohydrates), reflecting the two major polysaccharides present
Summary
The natural recalcitrance of lignocellulosic plant cell walls resulting from complex arrangement and distribution of heterogeneous components impedes deconstruction of such cell walls. The internodes of Miscanthus × giganteus, a model bioenergy crop, were subjected to DAP to yield a range of samples with altered cell wall structure and chemistry. The annual solar energy stored in lignocellulosic plant cell walls is nearly ten times that of the total energy used by humans [4]. The lignocellulosics such as woody materials, agricultural and forestry residues and energy crops have a promising role in large-scale production of liquid transportation fuels and other value-added products. The bioconversion is normally hindered by the complex structure and heterogeneous components (cellulose, hemicelluloses, lignin and pectin) distribution in cell walls. The ultrastructural and compositional complexity of lignocellulosic cell walls is a deterrent to fungal and bacterial attacks
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