Combined chemical and enzymic treatments of corn husk lignocellulosics

Abstract

AbstractThe potential for using corn (Zea mays L) husk residues (carbohydrates 827 g kg−1, lignin 66 g kg−1 DM) as a carbohydrate source for the production of soluble sugars by combined chemical pretreatment and enzymic hydrolysis was assessed. Comparative investigations of acidic and alkaline pretreatments on corn husk have shown that pentose‐containing carbohydrates comprised 86–93% of the solubilised fraction. While pretreatments with 1.25 M NaOH at 25.85° C resulted in preferential extraction of hemicellulose having DP; > 12, acid pretreatments 0.51 M H2SO4, 0.51 M H3PO4 at (85° C) resulted in extensive depolymerisation of this polysaccharide. Xylose and low molecular weight carbohydrates were identified as the major products. Subsequent hydrolysis of the solubilised fraction with crude hemicellulase preparations yielded 40.90% fermentable sugars. When NaOH (0.02–1.25 M), H2SO4 and H3PO4 (0.02–0.51 M) were used as pretreatment solvents (25–85° C, 2 h), NaOH was the most effective in increasing the susceptibility of the residual husk towards enzymes, yielding 83–96% reducing sugars. This solvent solubilised up to 60.6% of the lignin and appeared to disintegrate the fibrillar structure of husk.The crystallinity of husk residues increased following the chemical pretreatments and was positively correlated with cellulose content. Enzymic hydrolysis with commercial cellulase preparations proceeded in two stages: a rapid breakdown of amorphous cellulose after which the hydrolysis rate levelled off. Similar biphasic patterns were observed for the pyrolysis temperature of cellulose. Under the most optimal conditions for husk saccharification (pretreatment with 1.25 M NaOH, 25° C, 2 h, followed by enzymic hydrolysis using a mixture of cellulase and cellobiase), 96% of the cellulose‐enriched residues was hydrolysed to reducing sugars. A cellulase preparation from Trichoderma reesei exhibited substantial hemicellulolytic activity and could, therefore, be used as the sole saccharifying enzyme preparation for husk lignocellulosics.