Steam-explosion of olive stones: hemicellulose solubilization and enhancement of enzymatic hydrolysis of cellulose

Olive stones (whole stones and seed husks in fragments) were processed by steam-explosion under different experimental conditions of temperature and time, 200–236°C for 2–4 min, with or without previous acid impregnation with 0.1% H2SO4 (w/w). This paper examines the solubilization of hemicelluloses and their molecular weight distribution. The subsequent enzymatic hydrolysis of the solid residue, using a preparation of cellulase, was also studied. The maximum yield of the pentosan recovered in the water solution was 63% pentose in the starting material for seed husk treated at 200°C for 2 min (log R0 3.24) prior to acid-impregnation, or at 215°C for 2 min (log R0 3.69) without acid, compared to 39% of the potential yield for whole stones pre-impregnated with acid under more severe conditions (at log R0=4.07). This indicates that the autohydrolysis of hemicellulose in seed husks when compared to whole stones is enhanced. The molecular weight distribution of profile sugars showed that the depolymerization of hemicelluloses is a function of the severity of the treatment. Steam-explosion improved the accessibility of the cellulose and increased the enzymatic hydrolysis yield after steam-explosion with respect to material without steam explosion (ball-milled material), although little increase in the extent of saccharification occurred when the alkali-soluble lignin was removed. Only when the substrate was post-treated with Na-chlorite was the enzymatic hydrolysis improved, the water-insoluble residue being almost completely hydrolyzed in 8 h of incubation.