Fig. 4B showed the sugar yield from different pre-hydrolysis time (24, 48, 72 and 80 h) with 20% solid loading. As can be observed, the sugar yield was increased with the pre-hydrolysis time at the optimum temperature. The glucose and xylose yield of 72 h pre-hydrolysis was 8.5% and 4.0%, respectively, increased by 41.6% and 14.3% compared with that of 24 h pre-hydrolysis. However, the ethanol productivity was not increased with longer pre-hydrolysis time (the data was not shown in this paper). It was indicated that short time pre-hydrolysis has positive impact on the ethanol production, while long time has negative impact. The negative influence might be caused by sugar inhibition [83]. Albertyn et al. has reported that the high glucose concentration would lead to high osmotic stress for yeast cells, and production of glycerol to adapt to high osmotic stress, thus resulting in lower ethanol yield [84]. After 24 h of pre-hydrolysis with 20% solid loading, the reducing sugar concentration nearly reached 11% (glucose 7.0% and xylose 3.5%), and viscosity of the solid–liquid mixture prior to the addition of the microorganism could reduce S. cerevisiae IPE005 and facilitate conversion sugar to ethanol in the sequent SSCF. The industrial results of 400 m3 SSCF reactor for 48 h were shown in Table 5. The industrial operation results indicated that the maximum ethanol yield decreased 4.15%, which was 72.3% of the theoretical yield. The overall time of hydrolysis and fermentation process was 72 h, which was reduced by more than 24 h compared with SHF. Thus, ethanol productivity was increased by PHSSCF, which was important for the commercialization of lignocellulosic bioethanol.
With the addition of additives, reducing sugar yield was much higher than without additives (Fig. 5). It indicated that the additives improved enzymatic hydrolysis [74]. At high enzyme loading (20.0 FPU/g substrate), the final reducing sugar yield was increased by 17.4% with the additives. At low enzyme loading (15.0 FPU/g substrate), the final reducing sugar yield with the additives was nearly to that of enzyme loading of 20.0 FPU/g substrate without additives. Thus, the enzyme loading was reduced by 25.0% with synergistic enzymatic hydrolysis system compared with that single cellulase addition. Techno-economical calculations have indicated that a 50% reduction of enzyme loading was beneficial if the yield decreases less than 6–7% and required residence time is not increased by more than 30% [85]. Therefore, the enzymatic hydrolysis was improved with synergistic enzymatic hydrolysis system in industrial tank of 400 m3.