Steam explosion of two hardwood species was carried out with and without addition of sulfuric acid (H2SO4 0.5%, p/v) as a pretreatment catalyst. In general, wood chips of Eucalyptus viminalis Labill were shown to be more amenable to pretreatment than chips derived from bolds of Mimosa scabrella Benth (bracatinga). This was apparent from all pretreatment parameters tested including the overall recovery yields of pretreated fractions, carbohydrates (pentoses and hexoses) recovered as water-solubles, yield of dehydration by-products and lignin susceptibility to acid hydrolysis. There was no evidence for complete deacetylation of both wood species during pretreatment and lignin appeared to undergo extensive acid hydrolysis at higher pretreatment severities. Steam treatment at 205ºC for 5 min without addition of an acid catalyst was shown to be uncapable of removing the hemicellulose component from bracatinga chips, as determined by chemical analysis of the steam-treated water-insoluble fractions. Nearly 30% of the hemicellulose (xylan) found in bracatinga remained unhydrolysed after pretreatment, whereas more than 90% of this component could be removed from eucalypt chips under the same pretreatment conditions. Likewise, pretreatment of eucalypt chips resulted in a more extensive solubilization of glucans (cellulose) by acid hydrolysis. Addition of dilute H2SO4 as a pretreatment catalyst generally increased the recovery yield of fermentable sugars in the water-soluble fractions and this effect was more pronounced for the pretreatment of bracatinga chips. Steam-treated substrates produced from bracatinga were also less accessible to enzymatic hydrolysis than those produced from eucalypt chips, regardless of the use of an acid catalyst.
Eucalyptus viminalis; Mimosa scabrella; steam explosion; pretreatment; bioconversion
