Review paper
Hydrolysis of lignocellulosic materials for ethanol production: a review

https://doi.org/10.1016/S0960-8524(01)00212-7Get rights and content

Abstract

Lignocellulosic biomass can be utilized to produce ethanol, a promising alternative energy source for the limited crude oil. There are mainly two processes involved in the conversion: hydrolysis of cellulose in the lignocellulosic biomass to produce reducing sugars, and fermentation of the sugars to ethanol. The cost of ethanol production from lignocellulosic materials is relatively high based on current technologies, and the main challenges are the low yield and high cost of the hydrolysis process. Considerable research efforts have been made to improve the hydrolysis of lignocellulosic materials. Pretreatment of lignocellulosic materials to remove lignin and hemicellulose can significantly enhance the hydrolysis of cellulose. Optimization of the cellulase enzymes and the enzyme loading can also improve the hydrolysis. Simultaneous saccharification and fermentation effectively removes glucose, which is an inhibitor to cellulase activity, thus increasing the yield and rate of cellulose hydrolysis.

Introduction

Energy consumption has increased steadily over the last century as the world population has grown and more countries have become industrialized. Crude oil has been the major resource to meet the increased energy demand. Campbell and Laherrere (1998) used several different techniques to estimate the current known crude oil reserves and the reserves as yet undiscovered and concluded that the decline in worldwide crude oil production will begin before 2010. They also predicted that annual global oil production would decline from the current 25 billion barrels to approximately 5 billion barrels in 2050. Because the economy in the US and many other nations depends on oil, the consequences of inadequate oil availability could be severe. Therefore, there is a great interest in exploring alternative energy sources.

Unlike fossil fuels, ethanol is a renewable energy source produced through fermentation of sugars. Ethanol is widely used as a partial gasoline replacement in the US. Fuel ethanol that is produced from corn has been used in gasohol or oxygenated fuels since the 1980s. These gasoline fuels contain up to 10% ethanol by volume. As a result, the US transportation sector now consumes about 4540 million liters of ethanol annually, about 1% of the total consumption of gasoline (Wang et al., 1999). Recently, US automobile manufacturers have announced plans to produce significant numbers of flexible-fueled vehicles that can use an ethanol blend – E85 (85% ethanol and 15% gasoline by volume) – alone or in combination with gasoline. Using ethanol-blended fuel for automobiles can significantly reduce petroleum use and exhaust greenhouse gas emission (Wang et al., 1999). Ethanol is also a safer alternative to methyl tertiary butyl ether (MTBE), the most common additive to gasoline used to provide cleaner combustion (McCarthy and Tiemann, 1998). MTBE is a toxic chemical compound and has been found to contaminate groundwater. The US Environmental Protection Agency recently announced the beginning of regulatory action to eliminate MTBE in gasoline (Browner, 2000). However, the cost of ethanol as an energy source is relatively high compared to fossil fuels. A dramatic increase in ethanol production using the current cornstarch-based technology may not be practical because corn production for ethanol will compete for the limited agricultural land needed for food and feed production. A potential source for low-cost ethanol production is to utilize lignocellulosic materials such as crop residues, grasses, sawdust, wood chips, and solid animal waste.

Extensive research has been completed on conversion of lignocellulosic materials to ethanol in the last two decades (Dale et al., 1984; Wright, 1998; Azzam, 1989; Cadoche and López, 1989; Reshamwala et al., 1995; Bjerre et al., 1996; Duff and Murray, 1996). The conversion includes two processes: hydrolysis of cellulose in the lignocellulosic materials to fermentable reducing sugars, and fermentation of the sugars to ethanol. The hydrolysis is usually catalyzed by cellulase enzymes, and the fermentation is carried out by yeasts or bacteria. The factors that have been identified to affect the hydrolysis of cellulose include porosity (accessible surface area) of the waste materials, cellulose fiber crystallinity, and lignin and hemicellulose content (McMillan, 1994). The presence of lignin and hemicellulose makes the access of cellulase enzymes to cellulose difficult, thus reducing the efficiency of the hydrolysis. The contents of cellulose, hemicellulose, and lignin in common agricultural residues are listed in Table 1. Removal of lignin and hemicellulose, reduction of cellulose crystallinity, and increase of porosity in pretreatment processes can significantly improve the hydrolysis (McMillan, 1994).

Section snippets

Pretreatment of lignocellulosic materials

The effect of pretreatment of lignocellulosic materials has been recognized for a long time (McMillan, 1994). The purpose of the pretreatment is to remove lignin and hemicellulose, reduce cellulose crystallinity, and increase the porosity of the materials. Pretreatment must meet the following requirements: (1) improve the formation of sugars or the ability to subsequently form sugars by enzymatic hydrolysis; (2) avoid the degradation or loss of carbohydrate; (3) avoid the formation of

Enzymatic hydrolysis of cellulose

Enzymatic hydrolysis of cellulose is carried out by cellulase enzymes which are highly specific (Béguin and Aubert, 1994). The products of the hydrolysis are usually reducing sugars including glucose. Utility cost of enzymatic hydrolysis is low compared to acid or alkaline hydrolysis because enzyme hydrolysis is usually conducted at mild conditions (pH 4.8 and temperature 45–50 °C) and does not have a corrosion problem (Duff and Murray, 1996). Both bacteria and fungi can produce cellulases for

Improving enzymatic hydrolysis

The factors that affect the enzymatic hydrolysis of cellulose include substrates, cellulase activity, and reaction conditions (temperature, pH, as well as other parameters). To improve the yield and rate of the enzymatic hydrolysis, research has focused on optimizing the hydrolysis process and enhancing cellulase activity (Cantwell et al., 1988; Durand et al., 1988; Orpin, 1988).

Future prospects

The US fuel ethanol industry produced more than 6.2 billion liters of ethanol in 2000, most of which was produced from corn (MacDonald et al., 2001). However, an increase of ethanol production from corn will compete for the limited land against corn-based food and feed production. The price of corn was estimated to increase by $1.20–2.00/ton for every 2.5 million tonnes of corn used to make ethanol (Elander and Putsche, 1996). On the other hand, there is a huge amount of low-value or waste

References (107)

  • A Saxena et al.

    Simultaneous saccharification and fermentation of waste newspaper to ethanol

    Bioresour. Technol.

    (1992)
  • F Shafizadeh et al.

    Thermal degradation of 2-deoxy-d-arabino-hexonic acid and 3-deoxy-d-ribo-hexono-1,4-lactone

    Carbohyd. Res.

    (1975)
  • R.P Tengerdy et al.

    Increasing the feed value of forestry waste by ammonia freeze explosion treatment

    Biol. Wastes

    (1988)
  • R.W Thring et al.

    Recovery of a solvolytic lignin: effects of spent liquor/acid volume ration, acid concentration and temperature

    Biomass

    (1990)
  • P.F Vidal et al.

    Ozonolysis of lignin – improvement of in vitro digestibility of poplar sawdust

    Biomass

    (1988)
  • E.Y Vlasenko et al.

    Enzymatic hydrolysis of pretreated rice straw

    Bioresour. Technol.

    (1997)
  • A.E Wheals et al.

    Fuel ethanol after 25 years

    Trends Biotechnol.

    (1999)
  • D.E Akin et al.

    Alterations in structure, chemistry, and biodegradability of grass lignocellulose treated with the white rot fungi Ceriporiopsis subvermispora ad Cyathus stercoreus

    Appl. Environ. Microbiol.

    (1995)
  • P Ander et al.

    Selective degradation of wood components by white-rot fungi

    Physiol. Plant.

    (1977)
  • S Aziz et al.

    Organosolv pulping – a review

    Tappi. J.

    (1989)
  • A.M Azzam

    Pretreatment of cane bagasse with alkaline hydrogen peroxide for enzymatic hydrolysis of cellulose and ethanol fermentation

    J. Environ. Sci. Health. B.

    (1989)
  • J.O Baker et al.

    A new thermostable endoglucanase, Acidothermus cellulolyticus E1: synergism with Trichoderma reesei CBH1 and comparison to Thermomonospora fusca E5

    Appl. Biochem. Biotechnol.

    (1994)
  • I Ballesteros et al.

    Selection of thermotolerant yeasts for simultaneous saccharification and fermentation (SSF) of cellulose to ethanol

    Appl. Biochem. Biotechnol.

    (1991)
  • G Beldman et al.

    Synergism in cellulose hydrolysis by endoglucanases and exoglucanases purified from Trichoderma viride

    Biotechnol. Bioeng.

    (1988)
  • D Ben-Ghedalia et al.

    The effect of combined chemical and enzyme treatment on the saccharification and in vitro digestion rate of wheat straw

    Biotechnol. Bioeng.

    (1981)
  • D Ben-Ghedalia et al.

    Chemical treatments for increasing the digestibility of cotton straw

    J. Agric. Sci.

    (1983)
  • V.S Bisaria

    Bioprocessing of Agro-residues to glucose and chemicals

  • A.B Bjerre et al.

    Pretreatment of wheat straw using combined wet oxidation and alkaline hydrolysis resulting in convertible cellulose and hemicellulose

    Biotechnol. Bioeng.

    (1996)
  • R.A Blanchette

    Delignification by wood-decay fungi

    Annu. Rev. Phytopathol.

    (1991)
  • P.J Blotkamp et al.

    Enzymatic hydrolysis of cellulose and simultaneous fermentation to alcohol. Biochemical engineering: renewable sources of energy and chemical feedstocks

    AIChE Symp. Series

    (1978)
  • K Boominathan et al.

    cAMP-mediated differential regulation of lignin peroxidase and manganese-dependent peroxidase production in the white-rot basidiomycete Phanerochaete chrysosporium

    Proc. Natl. Acad. Sci. (USA)

    (1992)
  • A.H Brennan et al.

    High temperature acid hydrolysis of biomass using an engineering-scale plug flow reactor: result of low solids testing

    Biotechnol. Bioeng. Symp.

    (1986)
  • Browner, C., 2000. Remarks as prepared for delivery to press conference on March 20, 2000. Available from...
  • D.R Cahela et al.

    Modeling of percolation process in hemicellulose hydrolysis

    Biotechnol. Bioeng.

    (1983)
  • C.J Campbell et al.

    The end of cheap oil

    Sci. Am.

    (1998)
  • B.A Cantwell et al.

    Molecular cloning of bacillus β-glucanases

  • M Castanon et al.

    Effects of the surfactant Tween 80 on enzymatic hydrolysis of newspaper

    Biotechnol. Bioeng.

    (1981)
  • H.L Chum et al.

    Organosolv pretreatment for enzymatic hydrolysis of poplars: 1. enzyme hydrolysis of cellulosic residues

    Biotechnol. Bioeng.

    (1988)
  • T.A Clark et al.

    Steam explosion of the soft-wood Pinus radiata with sulphur dioxide addition. I. Process optimization

    J. Wood Chem. Technol.

    (1987)
  • A.O Converse et al.

    A model for enzyme adsorption and hydrolysis of microcrystalline cellulose with slow deactivation of the adsorbed enzyme

    Biotechnol. Bioeng.

    (1988)
  • A.O Converse et al.

    Kinetics of thermochemical pretreatment of lignocellulosic materials

    Appl. Biochem. Biotechnol.

    (1989)
  • M.P Coughlan et al.

    Comparative biochemistry of fungal and bacterial cellulolytic enzyme system

  • Z.Y Dai et al.

    Improved plant-based production of E1 endoglucanase using potato: expression optimization and tissue targeting

    Mol. Breed.

    (2000)
  • B.E Dale et al.

    A freeze-explosion technique for increasing cellulose hydrolysis

    Biotechnol. Bioeng. Symp.

    (1982)
  • B.E Dale et al.

    Fermentation of lignocellulosic materials treated by ammonia freeze-explosion

    Dev. Ind. Microbiol.

    (1984)
  • T Dewes et al.

    Composition and Microbial degradability in the soil of farmyard manure from ecologically-managed farms

    Biol. Agric. Hortic.

    (1998)
  • B.S Dien et al.

    Development of new ethanologenic Escherichia coli strains for fermentation of lignocellulosic biomass

    Appl. Biochem. Biotechnol.

    (2000)
  • Dipardo, J., 2000. Outlook for biomass ethanol production. Energy information administration. Available from...
  • H Durand et al.

    Classical and molecular genetics applied to Trichoderma reesei for the selection of improved cellulolytic industrial strains

  • R.T Elander et al.

    Ethanol from corn: technology and economics

  • Cited by (5002)

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    Paper No. BAE 2000-08 of the Journal Series of the Department of Biological and Agricultural Engineering, North Carolina State University, Raleigh, NC 27695-7625, USA.

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