One of the key steps in the conversion of lignocellulosic biomass to fermentable sugars is pretreatment. The goal of pretreatment is to alter the biomass macroscopic and micro­scopic size and structure as well as its submicroscopic chemical composition so that enzymatic hydrolysis of the carbohydrate fraction to monomeric sugars can be achieved with greater yield (Figure 3.1; Mosier et a

could also be conducted to disrupt and separate lignin from the hemicellulose component of the lignocellulosic biomass. For pretreatment to be effective, it must meet the fol­lowing criteria: (1) simple to operate and inexpensive; (2) has the ability to expose the cellulose component of biomass and increase its vulnerability to enzymatic attack;

(3) does not degrade hydrolyzed sugars; (4) generates little or no microbial inhibitory products; and (5) must be environmentally compatible. Unfortunately, during pretreatment and hydrolysis of lignocellulosic biomass, degradation and hydrolysis products such as furfural, HMF, syringaldehyde, glucuronic acid, p-coumaric acid, ferulic acid, syringic acid, levulinic acid, and other phenolic compounds may be generated (Figure 3.2b; Martinez et al. 2001; Ezeji et al. 2007a, b). These compounds can inhibit growth of microbes including fermenting microorganisms (Martinez et al. 2001; Ezeji et al. 2007a, b; Ezeji and Blaschek 2008a). During an investigation on the effect of some of the lignocellulosic hydrolysates inhibitors on growth and ABE production by Clostridium beijerinckii 592, ferulic, and p-coumaric acids were found to be potent inhibitors of growth and ABE production (Figures 3.3 and 3.4). Interestingly, glucuronic acid and HMF were not inhibi­tory to the C. beijerinckii 592, but rather were stimulatory to growth and ABE production at concentrations up to 2.0 g/L (Figures 3.3 and 3.4; Ezeji et al. 2007b; Ezeji and Blaschek 2008a). Similar results were obtained when C. beijerinckii BA101 (Ezeji et al. 2007b) and other solventogenic Clostridium species (Ezeji and Blaschek 2008a) were used.

It is important to note the number and amount of inhibitors likely to be generated during pretreatment of biomass may depend on the type and intensity of pretreatment applied. Some of the processes currently used in the pretreatment of lignocellulosic biomass are subse­quently described in this chapter.

-A-Furfura -©-HMF

Syringaldehyde

-0-Coumaricacid

Glucuronicacid

Ferulic acid

Control

0.5

Inhibitors concentration (g/L)

Figure 3.3. Effect of representative lignocellulosic biomass degradation products on the cell growth of Clostridium beijerinckii 592.

Size Reduction

Size reduction is a mechanical pretreatment of lignocellulosic biomass with the objective being reduction of particle size of biomass by a combination of chipping and milling. During size reduction operation, lignocellulosic biomass material such as corn stover is reduced to 5 to 20 mm in size in a straw chopping machine. Following the chopping process is milling, which reduces the particle size further to <0.5 to 2.0 mm diameters using a hammer, ball, or pin mill. The reduction in particle size results to increase in biomass density and improves handling during subsequent pretreatments. More importantly, it leads to an increase of surface area of lignocellulosic biomass, which improves the contact of polymers and enzymes and ultimately, hydrolysis of biomass to fermentable sugars. A particle size reduction below 40 mesh (0.42 mm) has little or no effect on the hydrolysis of or sugar yield from lignocellulosic biomass (Chang and Holtzapple 2000).