Optimization of Sugar Yield and Input Energy By Step-Wise Reduction of Agitate Rate During Lignocellulose Hydrolysis

Reaching the high sugar concentration and inputting the low energy during enzymatic hydrolysis of lignocellulosic biomass is crucial for the production of bioethanol and other chemicals. To achieve this goal, the initial high solid loadings and total low mixing energy consume are required in the hydrolysis step. The less mass transfer resistance resulted from high mixing power would be beneficial to obtain high sugar concentration in the short time [1], but the shear deactivation of cellulase seems to reduce the hydrolysis efficiency, especially at the extra high agitate rate [2]. Therefore, it is a key task to find the optimal agitate rate at different hydrolysis time scale at a given solid loadings for producing the high sugar concentration in the low cost.

In this paper, we investigated the effect of agitate rates on the hydrolysis of lignocellulose and found a new mixing strategy to optimize the hydrolysis process. Firstly, the shear deactivation of cellulase was studied under different agitate conditions. Then the time-course of glucose concentration was examined under different agitate rates at various solid loadings to find the optimal agitate condition for the hydrolysis process. Moreover, for the whole hydrolysis process, the agitate condition was suggested to be a step-wise reduction according to the changes of glucose concentration. The main ideas and conclusions are summarized as follows.

(1)    A significant cellulase deactivation for pure enzyme solution occurred after 24 h at the agitate rate of 150RPM, leading to the decreases in glucose concentration in the latter hydrolysis step. The deactivation extents of a total cellulase mixture were related with the incubating time and mixing intensity [3]. At low solid loadings, a little increased glucose yield were found after 24h hydrolysis under agitate rate of 150 rpm than that under static condition; whereas a significant increase on the glucose yield could be seen at high solid loadings (10%) after the agitate rate increased from 0 to 150 rpm.

(2)    The optimal agitate rate at the given solid loading was examined to increase hydrolysis efficiency. At low solid loadings, there was only a little increase in the glucose concentration at 150 rpm than that at the static state after 72h hydrolysis, whereas a significant decreased glucose concentration was found at 200 rpm due to the serious inactivation of enzymes. At high solid loadings, there was a greater increase in glucose yields under agitate condition (150 rpm and 100 rpm) than those without mechanical agitation. The results revealed that extra high agitate rate have a serious negative effect on the cellulose hydrolysis due to the cellulase deactivation. However, the appropriate agitate rate would lead to higher glucose concentration, especially at high solid loadings.

(3)    The hydrolysis of corncob were further optimized by using the step-wise reduction of the agitate rate. Adequate mixing is required to ensure sufficient contact between the substrate and enzymes and to promote heat and mass transfer within the reaction vessel. But it was not necessary to agitate continuously at the high rate for achieve the high sugar concentration [4]. The optimized mixing form could not only benefit the enzymatic hydrolysis process by reducing the energy consumption and limiting the cellulase deactivation, but also provide a new mixing form for the hydrolysis of lignocellulose at high solid loadings.

The authors acknowledge the financial supports received from the National Natural Science Foundation of China (No. 21276192 and 20976125), Open Funding Project of the State Key Laboratory of Bioreactor Engineering, and the Ministry of Education (Grant Nos. NCET-11-0372, 20110032130004 and B06006).

References

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2.            Gunjikar, T.P., S.B. Sawant, and J.B. Joshi, Shear deactivation of cellulase, exoglucanase, endoglucanase, and b-glucosidase in a mechanically agitated reactor. Biotechnology Progress, 2001. 17(6): p. 1166-1168.

3.            Ye, Z., K.M. Hatfield, and R. Eric Berson, Deactivation of individual cellulase components. Bioresource Technology, 2012. 106: p. 133-137.

4.            Ingesson, H., et al., The effect of shaking regime on the rate and extent of enzymatic hydrolysis of cellulose. Journal of Biotechnology, 2001. 88(2): p. 177-182.