High solids processing of biomass slurries provides the following benefits: maximized product concentration in the fermentable sugar stream, reduced water usage, and reduced reactor size. However, high solids processing poses mixing and heat-transfer problems above about 15% for pretreated corn stover solids. High solids slurries exhibit high viscosities and require high power consumption in conventional stirred tanks because they must be run at high rotational speeds to maintain proper mixing. An 8 L scraped surface bioreactor (SSBR) is employed here for enzymatic saccharification experiments to handle high solids loading and as a means for scale-up from laboratory-scale shake flasks. The scraping action of the blades keeps the reactor surface clear, which improves the heat-transfer characteristics. The horizontal rotation of the shaft and blades provides mixing and prevents particle settling much more effectively than in conventional stirred tanks, even at very low rotational speeds. The reactor is designed to be easily scaleable to pilot plant or production scale and operates at a specific power consumption of 0.56 kW/m(3) or less, which is well below the typical power requirement range of industrial-scale reactors. The role of the viscosity of biomass slurries in power consumption of the reactor is presented. Insoluble solids are seen to be the dominant factor affecting viscosity when the remaining insoluble solids level is higher than about 12%, regardless of the initial solids concentration. Below this level, viscosity characteristics are attributed to multiple factors in addition to insoluble solids, such as solids composition, solids morphology, and liquid-phase composition. The efficiency of the saccharification reaction is defined as sugar released per unit energy input. Tests were performed for initial solids loadings between 10 and 25%, with results showing that the efficiency factor is the highest for 20% initial solids concentrations. This efficiency factor can be used to optimize future design and processing strategies.