3.2. Fed-batch hydrolysis with cellulase and cellobiase
In ethanol production from lignocellulosic materials, ethanol concentration in fermentation broth should be as high as possible in order to minimize the energy consumption in evaporation and distillation (Wingren et al., 2003), which requires a relatively high initial sugar concentration in hydrolysate. Raising the substrate concentration in batch hydrolysis helps to obtain higher sugar concentration, but also often causes mixing and heat transfer problems due to the rheological properties of a very dense fibrous suspension (Rudolf et al., 2005). While in fed-batch hydrolysis process, such problems could be effectively avoided. Since substrate added is gradually degraded, the viscosity of reaction mixture can be kept at a low level. The results showed that the substrate concentration was increased to 200 g l−1 in fed-batch process, and that the reducing sugar concentration reached 116.3 g l−1 with a hydrolysis yield of 79.5% after 60 h of reaction (Fig. 3). The glucose concentration in hydrolysate reached 95.3 g l−1, suitable for further ethanol fermentation by S. cerevisiae. In addition, the cellulase dosage was reduced to 15 FPU g−1 substrate in fed-batch process. For degradation of equivalent substrate, fed-batch hydrolysis shortened the reaction time and therefore enhanced the productivity greatly compared with batch hydrolysis.