Rajai H. Atalla, John W. Brady, James F. Matthews, Shi-You Ding, and Michael E. Himmel

6.1 Introduction

In this chapter, we focus on the accessibility of native celluloses in plant cell walls to hy­drolytic agents and the manner in which accessibility is modified by dehydration and thermal chemical pretreatments. The primary barrier to enzymatic hydrolysis of celluloses in living plants is their encrustation with lignin. To overcome this barrier, it is necessary to remove the lignin chemically or to fragment the tissue to expose the cellulose; that is, the cell wall must be deconstructed chemically or mechanically. Chemical deconstruction is usually carried out at high temperatures. Mechanical deconstruction is also usually carried out at elevated temperatures to facilitate removal of hemicelluloses, if the primary objective is hydrolysis of the cellulose to glucose. It is therefore important to assess the effects of temperature on the state of aggregation of cellulose. Past studies have shown that temperature elevation almost always results in tighter aggregation of the cellulose chains in the microfibrils in a manner that reduces their accessibility to hydrolytic agents. It is important, therefore, to understand the state of aggregation in the living plant prior to dehydration and to better understand the transformations that arise as a consequence of dehydration in the course of pretreatment of plant biomass.

Microscopic evidence suggests that the microfibrils and nanofibrils of cellulose in higher plants possess a long-period helical twist in their native state. Though the microscopic ev­idence has revealed the twist in bacterial and algal celluloses, recent theoretical analyses indicate that higher plant celluloses also possess a helical twist that is more pronounced because of its shorter period, and this indeed has been confirmed through atomic force microscope (AFM) imaging. It is important, therefore, to review the evidence regarding the helical twist characteristic of the native state and consider how the highly ordered biolog­ical structures, which are species — and tissue-specific, are transformed as a consequence of dehydration into states that are less species — and tissue-specific. We will also consider the driving forces responsible for the dehydration. Finally, we will consider how pretreatment processes might be modified to preserve as much as possible the inherent accessibility of celluloses in their native state.

Biomass Recalcitrance: Deconstructing the Plant Cell Wall for Bioenergy. Edited by Michael. E. Himmel © 2008 Blackwell Publishing Ltd. ISBN: 978-1-405-16360-6