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 since they must be run at high rotational speeds to maintain proper mixing. An 8 liter scraped surface bio-reactor (SSBR) is employed here for enzymatic saccharification experiments in order to handle high solids loading and as a means for scale-up from lab-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. The role of the viscosity of biomass slurries in power consumption of the reactor is presented. The efficiency of the saccharification reaction is defined as sugar released per unit energy input. Tests were performed for 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.