Importance of acid or alkali concentration on the removal of xylan and lignin for enzymatic cellulose hydrolysis

• Alkali prevails acid treatment for enzymatic hydrolysis of grass-type insolubles.
• Alkali treatment resulted in losses of xylan with low glucuronic acid substituents.
• Xylan/glucan ratio of acid insolubles determines enzymatic cellulose hydrolysis.
• High xylan solubilisation by acid decreases enzymatic hydrolysis of cellulose.

The effect of hemicellulose and lignin solubilisation by H2SO4 and NaOH catalysed pretreatments was correlated to the extent of subsequent enzymatic cellulose hydrolysis. Three different grass-type feedstocks, palm empty fruit bunch, sugarcane bagasse and barley straw, were investigated. Soluble fractions after catalysis were characterised for mono- and oligosaccharides contents, while the residues were analysed for constituent monosaccharides composition.Alkali pretreatment resulted into extensive lignin removal. This removal resulted in up to 90% (w/w) conversion of glucan into glucose by enzymes. But, the alkaline conditions also provoked up to 50% unwanted xylan losses. Acid pretreatment resulted into solubilisation (70–80% (w/w)) of xylan with almost no losses, while lignin remained. Although moderate xylan solubilisation increased enzymatic cellulose hydrolysis of residual glucan, extensive removal of xylan decreased it. Therefore, under the treatment conditions, the alkali treatments were the most efficient in terms of enzymatic release of xylose and glucose from the insoluble residues.