AbstractBACKGROUNDCost‐effective processing of lignocellulosic biomass into sugar derived products, such as biofuels or biochemicals, needs to be performed at high water insoluble solid (WIS) loading. However, the difficult rheology of such materials presents significant challenges. The aim of this study was to investigate if a Cel5A endoglucanase can be used to reduce the viscosity of two types of pretreated softwood: steam pretreated Scots pine and sulfite pretreated Norway spruce.RESULTSThe viscosity of steam pretreated pine increased (by more than 60%) during the first 20 min of enzymatic hydrolysis, followed by a gradual decrease. A slightly lower viscosity during prolonged hydrolysis could be obtained by replacing 25% of the protein in Cellic CTec3 with the Cel5A endoglucanase. Very different results were obtained with sulfite pretreated spruce. The viscosity of this material was rapidly reduced by either CTec3 or the Cel5A endoglucanase, without a transient initial increase in viscosity. Even very low doses of Cel5A (0.1 mg protein per g glucan) decreased the viscosity of sulfite pretreated spruce 30‐fold within 6 h.CONCLUSIONLow endoglucanase doses can be used to reduce the viscosity of sulfite pretreated softwood, whereas the viscosity of steam pretreated softwood is less affected by endoglucanase activity. © 2018 Society of Chemical Industry