Sugarcane bagasse serves as source of renewable energy for production of biofuels and chemicals. The conversion process involves combinations of physical pretreatment and thermo-assisted chemical pretreatment, which disrupts the structure of the lignocellulosic complex architecture.
Two most common thermo-assisted chemical pretreatment used are the dilute acid catalyzed and dilute alkali catalyzed treatments. As the former leads to fermentation inhibitors, such as furans and organic acids, which require an additional detoxification step, dilute alkali catalyzed pretreatments, such as thermo-assisted sodium hydroxide pretreatments, dissolve lignin and hemicellulose, leaving only small amounts of inhibitors in hemi-cellulosic residues.
However, little is known on how sodium hydroxide interacts with lignin and to which extent remaining lignin or hemicellulose obtained after pretreatments affects enzymatic hydrolysis of cellulose-rich residues.
Researchers from Wageningen University in The Netherlands studied the effect of sodium hydroxide pretreatments of sugarcane bagasse (SCB) on insoluble remaining lignin structures. SCB pretreatments at different dosages of sodium hydroxide were compared at different temperatures and residence times. The study is published in Bioresource Technology.
Lignin is composed of 4-phenylpropanoid units based on degree of methoxylation of the benzene ring as p-coumaryl alcohol (H-units), coniferyl alcohol (G-units) and sinapyl alcohol (S-units) in which their proportion varies with plant type biomass. A lignin-carbohydrate complex is formed when lignin is interlinked with xylan. The most common linkages found within lignin are β-O-4, β-5, β-β, 5-5 and 5-O-4.
The authors hypothesized that ester linkages between H-units and xylan are cleaved at 4% of NaOH (pH > 8.6), provoking removal of xylan but not lignin and 9% NaOH pretreatments (pH > 10) are hypothesized to cleave β-O-4 linkages resulting in release of non-core lignin phenolics such as ferulates and coumarates.
Hence, alkaline pretreatment was performed on sugarcane bagasse, with varying amount of sodium hydroxide added, temperature and residence time. The samples were neutralized after pretreatment using acetic acid, later centrifuged and separated into a wet residue and supernatant before being analyzed for carbohydrate content and composition and for lignin by pyrolysis GC/MS.
The wet neutralized residues were subjected to enzymatic hydrolysis and the enzyme digests were analyzed for glucose and xylose contents using high-performance anion-exchange chromatography with pulsed amperometric detection.
After alkaline pretreatments of sugarcane bagasse, results show that as pretreatment conditions become more severe, less xylan remained in the residue. During pretreatment the pH became lower for most samples with reasons being that xylan became converted to small organic acids.
The residual xylan recovery correlated well with the enzymatic conversion of glucan to glucose after sodium hydroxide pretreatments. Higher normalized yield of lignin resulted to lower enzymatic conversion of glucan to glucose in the residues, but not as pronounced as the former.
Results from characterization of alkaline treated sugarcane bagasse residues confirmed the authors’ hypothesis. At 4% sodium hydroxide, xylan yields were lower than 70% and ester linkages are known to be cleaved at pH>8.6 correlating to decreased p-coumarate levels while ferulate levels remained constant in residual lignin. At 9% sodium hydroxide pretreatments, both p-coumarates and ferulates levels were lower in the remaining residues than at 0 or 4% sodium hydroxide, from which can be deduced that β-0-4 linkages are cleaved resulting in a release of non-core lignin phenolics such as ferulates and couramates.
This study showed that pretreatments at 4% and 9% of added sodium hydroxide resulted in residues with decreased levels of ester linked p-coumarates and ferulates, respectively.
‘Figure Source: Pauline Harmsen – Senior Scientist, Wageningen University and Research, The Netherlands’
Murciano Martínez, P., Punt, A.M., Kabel, M.A., Gruppen, A.H. Deconstruction of lignin linked p-coumarates, ferulates and xylan by NaOH enhances the enzymatic conversion of glucan, Bioresource Technology 216 (2016) 44-51.
Wageningen University, Laboratory of Food Chemistry, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands.
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