Assessing the molecular structure basis for biomass recalcitrance during dilute acid and hydrothermal pretreatments

Biotechnology for Biofuels

Yunqiao Pu¹  Fan Hu¹  Fang Huang¹  Brian H Davison¹  Arthur J Ragauskas¹ 

[1] BioEnergy Science Center, Oak Ridge, TN, USA

关键词: Structural transformation;    Cellulose structure;    Hydrothermal pretreatment;    Dilute acid pretreatment;    Biomass recalcitrance;   

Others  :  798162
DOI  :  10.1186/1754-6834-6-15

 received in 2012-10-03, accepted in 2013-01-14,  发布年份 2013

【 摘 要 】

The production of cellulosic ethanol from biomass is considered a promising alternative to reliance on diminishing supplies of fossil fuels, providing a sustainable option for fuels production in an environmentally compatible manner. The conversion of lignocellulosic biomass to biofuels through a biological route usually suffers from the intrinsic recalcitrance of biomass owing to the complicated structure of plant cell walls. Currently, a pretreatment step that can effectively reduce biomass recalcitrance is generally required to make the polysaccharide fractions locked in the intricacy of plant cell walls to become more accessible and amenable to enzymatic hydrolysis. Dilute acid and hydrothermal pretreatments are attractive and among the most promising pretreatment technologies that enhance sugar release performance. This review highlights our recent understanding on molecular structure basis for recalcitrance, with emphasis on structural transformation of major biomass biopolymers (i.e., cellulose, hemicellulose, and lignin) related to the reduction of recalcitrance during dilute acid and hydrothermal pretreatments. The effects of these two pretreatments on biomass porosity as well as its contribution on reduced recalcitrance are also discussed.

【 授权许可】

   
2013 Pu et al.; licensee BioMed Central Ltd.

【 图 表 】

Figure 1.

Figure 2.

【 参考文献 】

• [1]Ragauskas AJ, Williams CK, Davison BH, Britovsek G, Cairney J, Eckert CA, Frederick WJ Jr, Hallett JP, Leak DJ, Liotta CL, Mielenz JR, Murphy R, Templer R, Tschaplinski T: The path forward for biofuels and biomaterials. Science 2006, 311:484-489.
• [2]Saxena RC, Adhikari DK, Goyal HB: Biomass-based energy fuel through biochemical routes: A review. Renew Sust Energ Rev 2009, 13:156-167.
• [3]Yang B, Wyman CE: Pretreatment: the key to unlocking low-cost cellulosic ethanol. Biofuels Bioprod Biorefin 2008, 2:26-40.
• [4]Pu Y, Kosa M, Kalluri UC, Tuskan GA, Ragauskas AJ: Challenges of the utilization of wood polymers: how can they be overcome? Appl Microbiol Biotechnol 2011, 91:1525-1536.
• [5]Pu Y, Zhang D, Singh PM, Ragauskas AJ: The new forestry biofuels sector. Biofuels Bioprod Biorefin 2008, 2:58-73.
• [6]Chandra RP, Bura R, Mabee WE, Berlin A, Pan X, Saddler JN: Substrate pretreatment: The key to effective enzymatic hydrolysis of lignocellulosics? Adv Biochem Eng Biotechnol 2007, 108:67-93.
• [7]Lawoko M, Henriksson G, Gellerstedt G: Structural differences between the lignin-carbohydrate complexes present in wood and in chemical pulps. Biomacromolecules 2005, 6:3467-3473.
• [8]Ralph J, Lundquist K, Brunow G, Lu F, Kim H, Schatz PF, Marita JM, Hatfield RD, Ralph SA, Christensen JH, Boerjan W: Lignins: Natural polymers from oxidative coupling of 4-hydroxyphenylpropanoids. Phytochem Rev 2004, 3:29-60.
• [9]Himmel ME, Ding SY, Johnson DK, Adney WS, Nimlos MR, Brady JW, Foust TD: Biomass recalcitrance: engineering plants and enzymes for biofuels production. Science 2007, 315:804-807.
• [10]Mosier N, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M, Ladisch M: Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour Technol 2005, 96:673-686.
• [11]Wyman CE, Dale BE, Elander RT, Holtzapple M, Ladisch MR, Lee YY, Mitchinson C, Saddler JN: Comparative sugar recovery and fermentation data following pretreatment of poplar wood by leading technologies. Biotechnol Prog 2009, 25:333-339.
• [12]Davin LB, Lewis NG: Lignin primary structures and dirigent sites. Curr Opin Biotechnol 2005, 16:407-415.
• [13]Studer MH, DeMartini JD, Davis MF, Sykes RW, Davison B, Keller M, Tuskan GA, Wyman CE: Lignin content in natural Populus variants affects sugar release. Proc Natl Acad Sci USA 2011, 108:6300-6305.
• [14]El Hage R, Chrusciel L, Desharnais L, Brosse N: Effect of autohydrolysis of Miscanthus x giganteus on lignin structure and organosolv delignification. Bioresour Technol 2010, 101:9321-9329.
• [15]Silverstein RA, Chen Y, Sharma-Shivappa RR, Boyette MD, Osborne J: A comparison of chemical pretreatment methods for improving saccharification of cotton stalks. Bioresour Technol 2007, 98:3000-3011.
• [16]Liu C, Wyman CE: Partial flow of compressed-hot water through corn stover to enhance hemicellulose sugar recovery and enzymatic digestibility of cellulose. Bioresour Technol 2005, 96:1978-1985.
• [17]Ishizawa C, Jeoh T, Adney W, Himmel M, Johnson D, Davis M: Can delignification decrease cellulose digestibility in acid pretreated corn stover? Cellulose 2009, 16:677-686.
• [18]Kumar L, Chandra R, Chung PA, Saddler J: Can the same steam pretreatment conditions be used for most softwoods to achieve good, enzymatic hydrolysis and sugar yields? Bioresour Technol 2010, 101:7827-7833.
• [19]Kumar L, Chandra R, Chung PA, Saddler J: The lignin present in steam pretreated softwood binds enzymes and limits cellulose accessibility. Bioresour Technol 2012, 103:201-208.
• [20]Ding SY, Liu Y-S, Zeng Y, Himmel ME, Baker JO, Bayer EA: How does plant cell wall nanoscale architecture correlate with enzymatic digestibility? Science 2012, 338:1055-1060.
• [21]DeMartini JD, Pattathil S, Avci U, Szekalski K, Mazumder K, Hahn MG, Wyman CE: Application of monoclonal antibodies to investigate plant cell wall deconstruction for biofuels production. Energy Environ Sci 2011, 4:4332-4339.
• [22]Sannigrahi P, Ragauskas AJ, Miller SJ: Effects of two-stage dilute acid pretreatment on the structure and composition of lignin and cellulose in Loblolly pine. BioEnergy Res 2008, 1:205-214.
• [23]Samuel R, Pu Y, Raman B, Ragauskas AJ: Structural characterization and comparison of switchgrass ball-milled lignin before and after dilute acid pretreatment. Appl Biochem Biotechnol 2010, 162:62-74.
• [24]Cao S, Pu Y, Studer M, Wyman CL, Ragauskas AJ: Chemical transformations of Populus trichocarpa during dilute acid pretreatment. RSC Advances 2012, 2:10925-10936.
• [25]Sannigrahi P, Kim DH, Jung S, Ragauskas AJ: Pseudo-lignin and pretreatment chemistry. Energy Environ Sci 2011, 4:1306-1310.
• [26]Hu F, Jung S, Ragauskas A: Pseudo-lignin formation and its impact on enzymatic hydrolysis. Bioresour Technol 2012, 117:7-12.
• [27]Selig MJ, Viamajala S, Decker SR, Tucker MP, Himmel ME, Vinzant TB: Deposition of lignin droplets produced during dilute acid pretreatment of maize stems retards enzymatic hydrolysis of cellulose. Biotechnol Prog 2007, 23:1333-1339.
• [28]Palonen H, Tjerneld F, Zacchi G, Tenkanen M: Adsorption of Trichoderma reesei CBH I and EG II and their catalytic domains on steam pretreated softwood and isolated lignin. J Biotechnol 2004, 107:65-72.
• [29]Donohoe BS, Decker SR, Tucker MP, Himmel ME, Vinzant TB: Visualizing lignin coalescence and migration through maize cell walls following thermochemical retreatment. Biotechnol Bioeng 2008, 101:913-925.
• [30]Li H, Pu Y, Kumar R, Ragauskas AJ, Wyman CE: Investigation of lignin deposition on cellulose during hydrothermal pretreatment, its effect on cellulose hydrolysis, and underlying mechanisms. Sheraton, New Orleans: 34th Symposium on Biotechnology for Fuels and Chemicals; 2012.
• [31]Li JB, Henriksson G, Gellerstedt G: Lignin depolymerization/repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour Technol 2007, 98:3061-3068.
• [32]Kumar R, Mago G, Balan V, Wyman CE: Physical and chemical characterizations of corn stover and poplar solids resulting from leading pretreatment technologies. Bioresour Technol 2009, 100:3948-3962.
• [33]Shuai L, Yang Q, Zhu JY, Lu FC, Weimer PJ, Ralph J, Pan XJ: Comparative study of SPORL and dilute-acid pretreatments of spruce for cellulosic ethanol production. Bioresour Technol 2010, 101:3106-3114.
• [34]Leschinsky M, Zuckerstaetter G, Weber HK, Patt R, Sixta H: Effect of autohydrolysis of Eucalyptus globulus wood on lignin structure. Part 2: influence of autohydrolysis intensity. Holzforschung 2008, 62:653-658.
• [35]Jung S, Foston M, Sullards MC, Ragauskas AJ: Surface characterization of dilute acid pretreated Populus deltoides by ToF-SIMS. Energy Fuel 2010, 24:1347-1357.
• [36]Moxley G, Gaspar AR, Higgins D, Xu H: Structural changes of corn stover lignin during acid pretreatment. J Ind Microbiol Biotechnol 2012, 39(9):1289-1299.
• [37]Pu Y, Cao S, Studer M, Ragauskas AJ, Wyman CE: Chemical characterization of poplar after hot water pretreatment. Clearwater Beach, Florida: 32th Symposium on Biotechnology for Fuels and Chemicals; 2010.
• [38]Ziebell A, Gracom K, Katahira R, Chen F, Pu Y, Ragauskas AJ, Dixon RA, Davis M: Increase in 4-coumaryl alcohol units during lignification in alfalfa (Medicago sativa) alters the extractability and molecular weight of lignin. J Biol Chem 2010, 285:38961-38968.
• [39]Davison BH, Drescher SR, Tuskan GA, Davis MF, Nghiem NP: Variation of S/G ratio and lignin content in a Populus family influences the release of xylose by dilute acid hydrolysis. Appl Biochem Biotechnol 2006, 130:427-435.
• [40]Chang VS, Holtzapple MT: Fundamental factors affecting biomass enzymatic reactivity. Appl Biochem Biotechnol 2000, 84–6:5-37.
• [41]Dien BS, Sarath G, Pedersen JF, Sattler SE, Chen H, Funnell-Harris DL, Nichols NN, Cotta MA: Improved sugar conversion and ethanol yield for forage sorghum (Sorghum bicolor L. Moench) lines with reduced lignin contents. Bioenergy Res 2009, 2:153-164.
• [42]Voelker SL, Lachenbruch B, Meinzer FC, Jourdes M, Ki C, Patten AM, Davin LB, Lewis NG, Tuskan GA, Gunter L, et al.: Antisense down-regulation of 4CL expression alters lignification, tree growth, and saccharification potential of field-grown poplar. Plant Physiol 2010, 154:874-886.
• [43]Chen F, Dixon RA: Lignin modification improves fermentable sugar yields for biofuel production. Nat Biotechnol 2007, 25:759-761.
• [44]Pu Y, Chen F, Ziebell A, Davison BH, Ragauskas AJ: NMR characterization of C3H and HCT down-regulated alfalfa lignin. Bioenerg Res 2009, 2:198-208.
• [45]Shen H, He X, Poovaiah CR, Wuddineh WA, Ma J, Mann DG, Wang H, Jackson L, Tang Y, Stewart CN, Chen F, Dixon RA: Functional characterization of the switchgrass (Panicum virgatum) R2R3-MYB transcription factor PvMYB4 for improvement of lignocellulosic feedstocks. New Phytol 2012, 193:121-136.
• [46]Boerjan W, Ralph J, Baucher M: Lignin biosynthesis. Ann Rev Plant Biol 2003, 54:519-546.
• [47]Chen L, Auh C-K, Dowling P, Bell J, Chen F, Hopkins A, Dixon RA, Wang Z-Y: Improved forage digestibility of tall fescue (Festuca arundinacea) by transgenic down-regulation of cinnamyl alcohol dehydrogenase. Plant Biotechnol J 2003, 1:437-449.
• [48]Fu C, Mielenz JR, Xiao X, Ge Y, Hamilton CY, Rodriguez M Jr, Chen F, Foston M, Ragauskas A, Bouton J, Dixon RA, Wang Z-Y: Genetic manipulation of lignin reduces recalcitrance and improves ethanol production from switchgrass. Proc Natl Acad Sci 2011, 108:3803-3808.
• [49]Vanholme R, Morreel K, Darrah C, Oyarce P, Grabber JH, Ralph J, Boerjan W: Metabolic engineering of novel lignin in biomass crops. New Phytol 2012, 196:978-1000.
• [50]Ralph J, Kim H, Lu F, Grabber JH, Leplé J-C, Berrio-Sierra J, Mir Derikvand M, Jouanin L, Boerjan W, Lapierre C: Identification of the structure and origin of a thioacidolysis marker compound for ferulic acid incorporation into angiosperm lignins (and an indicator for cinnamoyl CoA reductase deficiency). Plant J 2008, 53:368-379.
• [51]Elumalai S, Tobimatsu Y, Grabber JH, Pan X, Ralph J: Epigallocatechin gallate incorporation into lignin enhances the alkaline delignification and enzymatic saccharification of cell walls. Biotechnol Biofuels 2012, 5:59. BioMed Central Full Text
• [52]Eudes A, George A, Mukerjee P, Kim JS, Pollet B, Benke PI, Yang F, Mitra P, Sun L, Çetinkol ÖP, et al.: Biosynthesis and incorporation of side-chain-truncated lignin monomers to reduce lignin polymerization and enhance saccharification. Plant Biotechnol J 2012, 10:609-620.
• [53]Garrote G, Kabel MA, Schols HA, Falque E, Dominguez H, Parajo JC: Effects of Eucalyptus globulus wood autohydrolysis conditions on the reaction products. J Agri Food Chem 2007, 55:9006-9013.
• [54]Vegas R, Kabel M, Schols HA, Alonso JL, Parajó JC: Hydrothermal processing of rice husks: effects of severity on product distribution. J Chem Technol Biotechnol 2008, 83:965-972.
• [55]Kumar L, Chandra R, Saddler J: Influence of steam pretreatment severity on post-treatments used to enhance the enzymatic hydrolysis of pretreated softwoods at low enzyme loadings. Biotechnol Bioeng 2011, 108:2300-2311.
• [56]Kumar L, Tooyserkani Z, Sokhansanj S, Saddler JN: Does densification influence the steam pretreatment and enzymatic hydrolysis of softwoods to sugars? Bioresour Technol 2012, 121:190-198.
• [57]Marzialetti T, Olarte MBV, Sievers C, Hoskins TJC, Agrawal PK, Jones CW: Dilute acid hydrolysis of Loblolly pine: A comprehensive approach. Ind Eng Chem Res 2008, 47:7131-7140.
• [58]Sun Y, Cheng JJ: Dilute acid pretreatment of rye straw and bermudagrass for ethanol production. Bioresour Technol 2005, 96:1599-1606.
• [59]Kabel MA, Bos G, Zeevalking J, Voragen AG, Schols HA: Effect of pretreatment severity on xylan solubility and enzymatic breakdown of the remaining cellulose from wheat straw. Bioresour Technol 2007, 98:2034-2042.
• [60]Adel AM, Abd El-Wahab ZH, Ibrahim AA, Al-Shemy MT: Characterization of microcrystalline cellulose prepared from lignocellulosic materials. Part I. Acid catalyzed hydrolysis. Bioresour Technol 2010, 101:4446-4455.
• [61]Chen XW, Lawoko M, van Heiningen A: Kinetics and mechanism of autohydrolysis of hardwoods. Bioresour Technol 2010, 101(20):7812-7819.
• [62]Hu F, Jung S, Ragauskas A: Impact of Pseudolignin versus Dilute Acid-Pretreated Lignin on Enzymatic Hydrolysis of Cellulose. ACS Sus Chem Eng 2012, 1:62-65.
• [63]Liu CG, Wyman CE: The effect of flow rate of compressed hot water on xylan, lignin, and total mass removal from corn stover. Ind Eng Chem Res 2003, 42:5409-5416.
• [64]Liu CG, Wyman CE: The effect of flow rate of very dilute sulfuric acid on xylan, lignin, and total mass removal from corn stover. Ind Eng Chem Res 2004, 43:2781-2788.
• [65]Yang B, Wyman CE: Characterization of the degree of polymerization of xylooligomers produced by flowthrough hydrolysis of pure xylan and corn stover with water. Bioresour Technol 2008, 99:5756-5762.
• [66]Yang B, Wyman CE: Dilute acid and autohydrolysis pretreatment. Meth Mol Biol 2009, 581:103-114.
• [67]Zhang J, Tang M, Viikari L: Xylans inhibit enzymatic hydrolysis of lignocellulosic materials by cellulases. Bioresour Technol 2012, 121C:8-12.
• [68]Ishizawa CI, Davis MF, Schell DF, Johnson DK: Porosity and its effect on the digestibility of dilute sulfuric acid pretreated corn stover. J Agric Food Chem 2007, 55:2575-2581.
• [69]Kumar R, Wyman CE: Effect of enzyme supplementation at moderate cellulase loadings on initial glucose and xylose release from corn stover solids pretreated by leading technologies. Biotechnol Bioeng 2009, 102:457-467.
• [70]Qing Q, Yang B, Wyman CE: Xylooligomers are strong inhibitors of cellulose hydrolysis by enzymes. Bioresour Technol 2010, 101(24):9624-9630.
• [71]Qing Q, Wyman CE: Supplementation with xylanase and beta-xylosidase to reduce xylo-oligomer and xylan inhibition of enzymatic hydrolysis of cellulose and pretreated corn stover. Biotechnol Biofuels 2011, 4:18-29. BioMed Central Full Text
• [72]Chen X, Shekiro J, Franden MA, Wang W, Zhang M, Kuhn E, Johnson DK, Tucker MP: The impacts of deacetylation prior to dilute acid pretreatment on the bioethanol process. Biotechnol Biofuels 2012, 5:8-21. BioMed Central Full Text
• [73]Pan XJ, Gilkes N, Saddler JN: Effect of acetyl groups on enzymatic hydrolysis of cellulosic substrates. Holzforschung 2006, 60:398-401.
• [74]Selig MJ, Adney WS, Himmel ME, Decker SR: The impact of cell wall acetylation on corn stover hydrolysis by cellulolytic and xylanolytic enzymes. Cellulose 2009, 16:711-722.
• [75]Chen XW, Lawoko M, van Heiningen A: Kinetics and mechanism of autohydrolysis of hardwoods. Bioresour Technol 2010, 101:7812-7819.
• [76]Chen X, Shekiro J, Elander R, Tucker M: Improved xylan hydrolysis of corn stover by deacetylation with high solids dilute acid pretreatment. Ind Eng Chem Res 2012, 51:70-76.
• [77]Klemm D, Heublein B, Fink HP, Bohn A: Cellulose: fascinating biopolymer and sustainable raw material. Angew Chem 2005, 44:3358-3393.
• [78]Thygesen A, Oddershede J, Lilholt H, Thomsen AB, Stahl K: On the determination of crystallinity and cellulose content in plant fibers. Cellulose 2005, 12(6):563-576.
• [79]Atalla RH, VanderHart DL: Native cellulose: a composite of two distinct crystalline forms. Science 1984, 223(4633):283-285.
• [80]Larsson PT, Hult EL, Wickholm K, Pettersson E, Iversen T: CP/MAS 13C-NMR spectroscopy applied to structure and interaction studies on cellulose I. Solid State Nucl Magn Reson 1999, 15(1):31-40.
• [81]Stephens CH, Whitmore PM, Morris HR, Bier ME: Hydrolysis of the amorphous cellulose in cotton-based paper. Biomacromolecules 2008, 9:1093-1099.
• [82]Foston M, Ragauskas AJ: Changes in lignocellulosic supramolecular and ultrastructure during dilute acid pretreatment of Populus and switchgrass. Biomass Bioenerg 2010, 34:1885-1895.
• [83]Yu Y, Wu HW: Significant differences in the hydrolysis behavior of amorphous and crystalline portions within microcrystalline cellulose in hot-compressed water. Ind Eng Chem Res 2010, 49:3902-3909.
• [84]Hsu TC, Guo GL, Chen WH, Hwang WS: Effect of dilute acid pretreatment of rice straw on structural properties and enzymatic hydrolysis. Bioresource Technol 2010, 101:4907-4913.
• [85]Samuel R, Pu Y, Foston M, Ragauskas AJ: Solid-state NMR characterization of switchgrass cellulose after dilute acid pretreatment. Biofuels 2010, 1(1):85-90.
• [86]Xiao LP, Sun ZJ, Shi ZJ, Xu F, Sun RC: Impact of hot compressed water pretreatment on the structural changes of woody biomass for bioethanol production. Bioresources 2011, 6(2):1576-1598.
• [87]Lee JM, Jameel H, Venditti RA: A comparison of the autohydrolysis and ammonia fiber explosion (AFEX) pretreatments on the subsequent enzymatic hydrolysis of coastal Bermuda grass. Bioresource Technol 2010, 101:5449-5458.
• [88]Segal L, Creely JJ, Martin AE Jr, Conrad CM: An empirical method for estimating the degree of crystallinity of native cellulose using the X-ray diffractometer. Text Res J 1959, 29:786-794.
• [89]Park S, Baker JO, Himmel ME, Parilla PA, Johnson DK: Cellulose crystallinity index: measurement techniques and their impact on interpreting cellulase performance. Biotechnol Biofuels 2010, 3:10. BioMed Central Full Text
• [90]Chundawat SPS, Beckham GT, Himmel ME, Dale BE: Deconstruction of lignocellulosic biomass to fuels and chemicals. Annu Rev Chem Biomol Eng 2011, 2:121-145.
• [91]Håkansson H, Ahlgren P, Germgård U: The degree of disorder in hardwood kraft pulps studied by means of LODP. Cellulose 2005, 12:327-335.
• [92]Battista OA: Hydrolysis and crystallization of cellulose. Ind Eng Chem Res 1950, 42(3):502-507.
• [93]Battista OA, Coppick S: Hydrolysis of native versus regenerated cellulose structures. Test Res J 1947, 17(8):419-422.
• [94]Nickerson RF, Habrle JA: Cellulose intercrystalline structure: study by hydrolytic methods. Ind Eng Chem Res 1947, 39(11):1507-1512.
• [95]Huang F, Ragauskas AJ: Dilute H2SO4 and SO2 pretreatments of Loblolly pine wood residue for bioethanol production. Ind Biotechnol 2012, 8(1):22-30.
• [96]Jahan MS, Muna SP: Studies on the macromolecular components of nonwood available in Bangladesh. Ind Crop Prod 2009, 30:344-350.
• [97]Hubbell CA, Ragauskas AJ: Effect of acid-chlorite delignification on cellulose degree of polymerization. Bioresource Technol 2010, 101(19):7410-7415.
• [98]Hallac BB, Sannigrahi P, Pu Y, Ray M, Murphy RJ, Ragauskas AJ: Effect of ethanol organosolv pre-treatment on enzymatic hydrolysis of Buddleja davidii stem biomass. Ind Eng Chem Res 2010, 49:1467-1472.
• [99]Martínez JM, Reguant J, Montero MÁ, Montané D, Salvadó J, Farriol X: Hydrolytic pre-treatment of softwood and almond shells, Degree of polymerization and enzymatic digestibility of the cellulose fraction. Ind Eng Chem Res 2007, 36:688-696.
• [100]Pan X, Xie D, Kang KY, Yoon SL, Saddler JN: Effect of organosolv ethanol pre-treatment variables on physical characteristics of hybrid poplar substrates. Appl Biochem Biotechnol 2007, 136–140:367-377.
• [101]Pan X, Xie D, Yu RW, Saddler JN: The bioconversion of mountain pine beetle-killed lodgepole pine to fuel ethanol using the organosolv process. Biotechnol Bioeng 2008, 101:39-48.
• [102]Grethlein HE: The effect of pore size distribution on the rate of enzymatic hydrolysis of cellulosic substrates. Nat Biotechnol 1985, 3(2):155-160.
• [103]Yang B, Wyman CE: BSA treatment to enhance enzymatic hydrolysis of cellulose in lignin containing substrates. Biotechnol Bioeng 2006, 94:611-617.
• [104]Zeng M, Mosier NS, Huang CP, Sherman DM, Ladisch MR: Microscopic examination of changes of plant cell structure in corn stover due to hot water pretreatment and enzymatic hydrolysis. Biotechnol Bioeng 2007, 97:265-278.
• [105]Zeng M, Ximenes E, Ladisch MR, Mosier NS, Vermerris W, Huang CP, Sherman DM: Tissue-specific biomass recalcitrance in corn stover pretreated with liquid hot-water: SEM imaging (part 2). Biotechnol Bioeng 2012, 109:398-404.
• [106]Hsu TC, Guo GL, Chen WH, Hwang WS: Effect of dilute acid pretreatment of rice straw on structural properties and enzymatic hydrolysis. Bioresour Technol 2010, 101:4907-4913.
• [107]Foston M, Ragauskas AJ: Changes in the structure of the cellulose fiber wall during dilute acid pretreatment in Populus studied by 1H and 2H NMR. Energy Fuel 2010, 24:5677-5685.
• [108]Chen W-H, Tu Y-J, Sheen H-K: Disruption of sugarcane bagasse lignocellulosic structure by means of dilute sulfuric acid pretreatment with microwave-assisted heating. Appl Energy 2011, 88:2726-2734.
• [109]Yu C-T, Chen W-H, Men L-C, Hwang W-S: Microscopic structure features changes of rice straw treated by boiled acid solution. Ind Crops Prod 2009, 29:308-315.
• [110]Chen W-H, Tu Y-J, Sheen H-K: Impact of dilute acid pretreatment on the structure of bagasse for bioethanol production. Int J Energy Res 2010, 34:265-274.