Abstract
BackgroundThe recalcitrance of lignocellulosic materials is a major limitation for their conversion into fermentable sugars. Lignin depletion in new cultivars or transgenic plants has been identified as a way to diminish this recalcitrance. In this study, we assessed the success of a sugarcane breeding program in selecting sugarcane plants with low lignin content, and report the chemical composition and agronomic characteristics of eleven experimental hybrids and two reference samples. The enzymatic digestion of untreated and chemically delignified samples was evaluated to advance the performance of the sugarcane residue (bagasse) in cellulosic-ethanol production processes.ResultsThe ranges for the percentages of glucan, hemicellulose, lignin, and extractive (based on oven-dry biomass) of the experimental hybrids and reference samples were 38% to 43%, 25% to 32%, 17% to 24%, and 1.6% to 7.5%, respectively. The samples with the smallest amounts of lignin did not produce the largest amounts of total polysaccharides. Instead, a variable increase in the mass of a number of components, including extractives, seemed to compensate for the reduction in lignin content. Hydroxycinnamic acids accounted for a significant part of the aromatic compounds in the samples, with p-coumaric acid predominating, whereas ferulic acid was present only in low amounts. Hydroxycinnamic acids with ester linkage to the hemicelluloses varied from 2.3% to 3.6%. The percentage of total hydroxycinnamic acids (including the fraction linked to lignin through ether linkages) varied from 5.0% to 9.2%, and correlated to some extent with the lignin content. These clones released up to 31% of glucose after 72 hours of digestion with commercial cellulases, whereas chemically delignified samples led to cellulose conversion values of more than 80%. However, plants with lower lignin content required less delignification to reach higher efficiencies of cellulose conversion during the enzymatic treatment.ConclusionSome of the experimental sugarcane hybrids did have the combined characteristics of high biomass and high sucrose production with low lignin content. Conversion of glucan to glucose by commercial cellulases was increased in the samples with low lignin content. Chemical delignification further increased the cellulose conversion to values of more than 80%. Thus, plants with lower lignin content required less delignification to reach higher efficiencies of cellulose conversion during the enzymatic treatment.
Highlights
The recalcitrance of lignocellulosic materials is a major limitation for their conversion into fermentable sugars
Hemicellulose content The hemicellulose content was calculated on the basis of the monomeric sugars and acetic acid released after acid hydrolysis
The chromatograms showed no detectable mannose and galactose in the mill bagasse sample, whereas small peaks of galactose were detected in clones 87, 89, and 140, corresponding, respectively, to 0.90%, 0.83%, and 0.71% of the oven-dry mass of the plant material. These data suggest that the hemicellulose content detected in the evaluated sugarcane samples consisted mainly of xylan backbones ramified with arabinose and acetic acid
Summary
The recalcitrance of lignocellulosic materials is a major limitation for their conversion into fermentable sugars. Grabber et al [14] reported that artificially lignified cell walls from maize had their digestibility to ruminal biota decreased as a function of increased lignification, and the authors concluded, on the basis of the use of different lignin precursors, that the engineering of plants for reduced lignification or ferulate-lignin crosslinking improves fiber digestibility to a greater degree than does shifting lignin composition (for example, by selecting high syringyl content in lignified plants) This finding is relevant because in grasses, part of the recalcitrance is associated with the occurrence of lignin in cell walls and with the presence of hydroxycinnamic acids linked primarily to the hemicelluloses [8,14,15]. Part of this variation is often associated with the total lignin concentration and the presence of hydroxycinnamic acid crosslinks in the cell walls [12,15]
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