Abstract
Switchgrass was pretreated with sodium hydroxide (NaOH) at various concentrations and pretreatment times to investigate how delignification caused by NaOH affects its lignin chemistry. NaOH resulted in significant delignification ranging from 44.0 to 84.6% depending on pretreatment intensity. While there was no significant glucan loss due to NaOH pretreatment, higher NaOH concentrations removed xylan by up to 28.3%. Nitrobenzene oxidation (NBO) was used to study changes in lignin chemistry, and indicated that at higher NaOH concentrations, the amount of 4-hydroxygenzaldehyde (Hy) degraded from p-hydroxyphenyl propanol (H) lignin units was significantly reduced (p < 0.05). However, amounts of syringic (SA) and vanillic (VA) acids generated from syringyl (S) and guaiacyl (G) degradation were greater at higher NaOH concentration. S/G ratio (=0.62 raw switchgrass) did not significantly (p > 0.05) change with 15 min pretreatment, but it increased to 0.75 and 0.72, respectively, with 30 and 60 min pretreatments (p < 0.05). Increase in NaOH concentration did not significantly (p > 0.05) change S/G ratio, but H/G ratio (=0.48 raw switchgrass) decreased significantly to 0.14 regardless of pretreatment times. Overall, the H unit was found to be more susceptible to NaOH than S and G unit monolignols. Though changes in lignin chemistry due to NaOH concentration were observed, their impact on cellulolytic enzyme action during hydrolysis could not be fully understood. Further studies on lignin isolation may help to determine how these changes in lignin chemistry by NaOH impact cellulolytic enzymes.
Highlights
Lignocellulosic biomass, a resource which is believed to be sustainable and widely available, is mainly composed of cellulose, hemicellulose, and lignin
Lignin is a biosynthesized phenolic compound formed by the oxidative coupling of p-hydroxycinnamyl alcohols and related compounds like guaiacyl (G), syringyl (S), and p-phydroxyphenyl (H) propanol monolignols cross-linked by a variety of bonds. β-O-4-linked aryl ether bonds form non-condensed linkages which are relatively easier to cleave compared to condensed linkages like carbon-carbon bonds [1,2,3,4,5]
Enzymatic hydrolysis was performed to determine if changes in lignin chemistry of switchgrass—as affected by pretreatment intensity—have an impact on fermentable sugar production
Summary
Lignocellulosic biomass, a resource which is believed to be sustainable and widely available, is mainly composed of cellulose, hemicellulose, and lignin. There is little knowledge on how pretreatment intensity affects the lignin chemistry of herbaceous grasses and whether those changes can influence subsequent fermentable sugar production via enzymatic hydrolysis. Switchgrass (Pancium virgatum) is a warm-season perennial grass that has been identified as a significant bioresource for second-generation bioethanol production because of its high carbohydrate content, rapid growth, and excellent biomass yield with relatively less water and nitrogen inputs. It is well-adapted to various soil and climate conditions [11,12,13,14,15]. Enzymatic hydrolysis was performed to determine if changes in lignin chemistry of switchgrass—as affected by pretreatment intensity—have an impact on fermentable sugar production
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