Enhancement of enzymatic susceptibility of lignocellulosic wastes by microwave irradiation

Abstract

Microwave irradiation in the presence of water provides a new pretreatment method for enzymatic saccharification of lignocellulosic wastes. When three representative lignocellulosic wastes, sugar cane bagasse, rice straw, and rice hulls were treated with microwave energy, acidity and furfural production increased with increasing temperature and reached 0.30-1.13 meq and 0.07-1.63%, respectively, at 227/sup 0/C. No substantial change in crystallinity of the cellulose was detected by short heating (< 8 min) below 230/sup 0/C. Hemicellulose and lignin, however, suffered acid-catalized autohydrolysis. The reducing sugar production started at 180-190/sup 0/C and showed a maximum (11-27%) at about 230/sup 0/C, and the amount of the water-soluble components and degraded lignin extractable with 90% dioxane or methylalcohol were prominent above 180/sup 0/C but showed no maximum up to 240/sup 0/C. Aqueous dioxane removed 1.3-2.5 times more lignin than methyl alcohol above 220/sup 0/C. The enzymatic susceptibility of all the lignocellulosic wastes were markedly improved by microwave pretreatment above 160/sup 0/C and showed a maximum at 223-228/sup 0/C, independent of their source. The maximal percentage of reducing sugars was 77-84% of the polysaccharide present in the original lignocellulosic wastes. The increase in available surface of cellulose due to degradation of encrusting lignin and hemicellulose is suggested to be a rate-determining factor for enzymatic saccharification of cellulose in lignocellulosic wastes because of the stability of crystallinity of cellulose toward microwave irradiation. 32 references, 6 figures, 4 tables.