Principles and Challenges Involved in the Enzymatic Hydrolysis of Cellulosic Materials at High Total Solids

The enzymatic hydrolysis of plant polysaccharides is a key unit operation for the production of cellulosic ethanol. However, hydrolysates with sugar concentrations as high as 180–200 g L−1 (18–20 °Brix) must be produced for a successful ethanol fermentation and this can only be achieved at high total solids. With this, a significant decrease in both capital and production costs is achieved. In addition, less water is needed, effluent generation is minimized and the cost of wastewater treatment is reduced. At high total solids, the rheology of biomass slurries exhibits large apparent viscosities and yield stresses that increase with the volume fraction of the insoluble solids, creating both mass and heat transfer limitations at various levels within the body of the fiber suspension. In this context, this chapter reviews the most recent developments in enzymatic hydrolysis for the production of high sugar concentrations using high total solids and low enzyme loadings. This chapter also reviews several strategies to overcome these rheological problems, such as fed-batch feeding and the addition of chemical additives that are able to decrease the effects of extremely high initial viscosities, thus facilitating the substrate liquefaction while decreasing the unproductive binding of enzymes. Lastly, a brief discussion is given about the impact of different impellers setups on the hydrolysis rate, since adequate mixing capacity and low energy consumption are key factors in designing bioreactors for lignocellulose processing.