Cellulose Accessibility Determines the Rate of Enzymatic Hydrolysis of Steam-pretreated Spruce

Spruce chips steam-pretreated at various conditions, according to a central composite design, were used for investigating the influence of pretreatment conditions on enzymatic hydrolysis, accounting for the individual effects of pretreatment temperature (194-220 °C), time (3-11 min) and sulfur dioxide uptake (0.7-2.5%). The materials were analyzed for several surface characteristics, including IR absorption, enzyme adsorption capacity, total surface area, cellulosic surface area, and cellulosic pore sizes. This work showed a clear correlation between rate of enzymatic hydrolysis and specific surface area. Although the lignin content of the particle surface increased at higher pretreatment temperature and residence time, the initial rate of enzymatic hydrolysis increased. Enzyme adsorption measurements and staining methods revealed that the higher rate of hydrolysis of these materials was due to increased accessibility of the cellulose. An accessible cellulose fraction is thus more important than a low surface lignin content for the enzymatic hydrolysis of steam-pretreated spruce.