Bioethanol

Spotlight
Received: 17 September 2010 Revised: 07 December 2010 Accepted: 23 December 2010 Published online in Wiley Online Library:

(wileyonlinelibrary.com) DOI 10.1002/jsfa.4455

Fuel from plant cell walls: recent developments in second generation bioethanol research
Charis Cook∗ and Alessandra Devoto
Abstract
As bioethanol from sugarcane and wheat falls out of favour due to concernsabout food security, research is ongoing into genetically engineering model plants and microorganisms to find the optimum cell wall structure for the ultimate second generation bioethanol crop. Charis Cook and Alessandra Devoto highlight here the progress made to tailor the plant cell wall to improve the accessibility of cellulose by acting on the regulation, the structure or the relativecomposition of other cell wall components to ultimately improve saccharification efficiency. They also consider possible side effects of cell wall modification and focus on the latest advances made to improve the efficiency of digestion of lignocellulosic materials by cell wall degrading microorganisms. c 2011 Society of Chemical Industry

rich. Wheat and sugarcane2 are widely used for both food andbioethanol. They are both high maintenance, needing good quality farmland. Concerns about the global impact of this has resulted in the controversial ‘food vs fuel’ debate that surrounds bioethanol use.3

SECOND GENERATION BIOETHANOL
Not all sugar in plants is in starchy seeds or fruits. All plants also contain sugars locked into the cellulose microfibrils that, together with hemicelluloses and lignin,make up cell walls (reviewed by Somerville4 ) This water repellent lignocellulosic material is an ideal source of glucose, as well as other monosaccharide residues, for bioethanol production as plants such as trees and grasses are rich in lignocellulose and will grow on marginal land that is not suitable for food crops.5,6 Many waste products are also rich in lignocelluloses.4 Cell walls haveprotective and structural functions making them resistant to degradation. Cellulose microfibrils wrap around the cell to form the framework of cell wall.7 Hemicellulose cross-links the cellulose microfibrils via hydrogen bonds, increasing the strength of the cellulose frame.8 In primary cell walls, such as in leaves and fruit, pectin is the third main component of the cell wall. Secondary cell wallsinstead contain the waterproof phenolic macromolecule lignin (reviewed by Popper9 ). The lignin and hemicellulose network surrounding cellulose is an effective defence against physical and biological attacks in vivo.10 Consequently, humans must use physical, chemical and biological means to extract cellulose. Total carbon emissions from generating

Figure 1. Tobacco, alfalfa and poplar: do thesemodel plants hold the answer to efficient biofuel production?.

BIOETHANOL PAST AND PRESENT
Bioethanol is the fermentation product of plant biomass. Alone among renewable energy sources, bioethanol, biodiesel and biobutanol are liquid fuels and a smooth change from currently used fossil fuel to biofuel can occur with few modifications required to infrastructure and engine mechanics.1 Bioenergy cropsand food crops share the same prerequisite: they must be energy
∗

Correspondenceto:Charis Cook,Biological Sciences,Royal Holloway,University of London, Egham Hill, Egham, Surrey TW20 0EX, UK. E-mail: PTBA156@live.rhul.ac.uk

1729

Biological Sciences, Royal Holloway, University of London, Egham Hill, Egham, Surrey TW20 0EX, UK

J Sci Food Agric 2011; 91: 1729–1732

www.soci.org

c2011 Society of Chemical Industry

www.soci.org and burning lignocellulosic ethanol are therefore comparable to the burning of fossil fuels.11 Physical pretreatment of biomass aims to remove or reduce the protective lignin and hemicellulose layer surrounding the cellulose microfibrils. These pretreatments are expensive due to the high temperatures that are required and have toxic by-products...