Changes in Morphology and Chemical Composition of Sugarcane Bagasse After Sequential Acid-Alkali Pretreatment

Researchers from the Universidade de São Paulo (Brazil) report the results of both morphological and chemical-composition changes in sugarcane bagasse after a two-step sequential pretreatment involving the addition of acid and alkali. Pretreatment is usually the first step in the production of biofuel ethanol from lignocellulosic biomass. The process "deconstructs" the plant cell wall structures of the biomass, so that these can be more easily converted to fermentable sugars for ethanol production.

The study is one of only a few investigations which determine the changes in both morphology and chemical composition of lignocellulosic biomass after pretreatment. The purpose of the first stage acid treatment (using 1% dilute sulfuric acid at 121 ^oC, for 40 minutes) was to remove the hemicellulose component in the biomass. The second stage alkali treatment at different concentrations of sodium hydroxide (NaOH) (also at 121 ^oC, for 40 minutes) was to remove the lignin component. The residue after the sequential treatment is the cellulose-rich component of the biomass which can be easily converted (i.e. hydrolysed or saccharified) into ethanol-fermentable sugars.

Results showed that about 96% of the hemicellulose was removed by acid treatment, while 85% of lignin was removed at alkali (NaOH) concentrations of 1% or higher. The cellulose yield under these conditions was 72%. The researchers concluded that the improvement in cellulose conversion (into ethanol-fermentable sugars) was mainly due to delignification. The removal of lignin by the two-step pretreatment method was attributed to: (1) "loss of cohesion between adjacent cell walls that were initially joined by lignin", and (2) destruction inside the cell wall (such as formation of voids) which exposes more cellulose to enzymatic attack. The full study is published in the open-access journal, Biotechnology for Biofuels (URL above).