Effect of structural features on enzyme digestibility of corn stover Sehoon Kim, Mark T. Holtzapple * Department of Chemical Engineering, Texas A&M University, College Station, TX 77843-3122, USA Received 28 March 2005; accepted 29 March 2005 Available online 14 June 2005 Abstract Corn stover was pretreated with excess calcium hydroxide (0.5 g Ca(OH) 2 /g raw biomass) in non-oxidative and oxidative con- ditions at 25, 35, 45, and 55 °C. The enzymatic digestibility of lime-treated corn stover was affected by the change of structural fea- tures (acetylation, lignification, and crystallization) resulting from the treatment. Extensive delignification required oxidative treatment and additional consumption of lime (up to 0.17 g Ca(OH) 2 /g biomass). Deacetylation reached a plateau within 1 week and there were no significant differences between non-oxidative and oxidative con- ditions at 55 °C; both conditions removed approximately 90% of the acetyl groups in 1 week at all temperatures studied. Delignification highly depended on temperature and the presence of oxygen. Lignin and hemicellulose were selectively removed (or solubilized), but cellulose was not affected by lime pretreatment in mild temperatures (25–55 °C), even though corn stover was contacted with alkali for a long time, 16 weeks. The degree of crystallinity slightly increased from 43% to 60% with delignification because amorphous components (lignin, hemi- cellulose) were removed. However, the increased crystallinity did not negatively affect the 3-d sugar yield of enzymatic hydrolysis. Oxidative lime pretreatment lowered the acetyl and lignin contents to obtain high digestibility, regardless of crystallinity. The non-linear models for 3-d hydrolysis yields of glucan (Y g ), xylan (Y x ), and holocellulose (Y gx ) were empirically established as a function of the residual lignin (L) for the corn stover pretreated with lime and air. Ó 2005 Published by Elsevier Ltd. Keywords: Corn stover; Lime pretreatment; Structural features; Acetyl; Lignin; Crystallinity 1. Introduction The enzymatic hydrolysis of lime-treated biomass is affected by structural features resulting from the treat- ment. Key structural features that affect digestibility are the extent of acetylation, lignification, and crystallization. Lime pretreatment removes amorphous substances (e.g., lignin and hemicellulose), which increases the crys- tallinity index. Chang and Holtzapple (2000) reported correlations between enzymatic digestibility and three structural factors: lignin content, crystallinity, and acetyl content. They concluded that (1) extensive delignifica- tion is sufficient to obtain high digestibility regardless of acetyl content and crystallinity; (2) delignification and deacetylation remove parallel barriers to enzymatic hydrolysis; and (3) crystallinity significantly affects initial hydrolysis rates but has less effect on ultimate sugar yields. These results indicate that an effective lignocellu- lose treatment process should remove all the acetyl groups and reduce the lignin content to about 10% in the treated biomass. Further lignin reduction incurs an extra cost; therefore, it is not justified for enzymatic hydrolysis. Lee and Fan (1982) reported that the rate of enzymatic hydrolysis depends on enzyme adsorp- tion and the effectiveness of the adsorbed enzymes, instead of the diffusive mass transfer of enzyme. Lignin removal increases enzyme effectiveness by eliminating 0960-8524/$ - see front matter Ó 2005 Published by Elsevier Ltd. doi:10.1016/j.biortech.2005.03.040 * Corresponding author. Tel.: +1 409 845 9708; fax: +1 409 845 6446. E-mail address: m-holtzapple@tamu.edu (M.T. Holtzapple). Bioresource Technology 97 (2006) 583–591