(582cn) 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

1.            Palmqvist,
B., M. Wiman, and G. Lid¨¦n, Effect of mixing on enzymatic hydrolysis of
steam-pretreated spruce: a quantitative analysis of conversion and power
consumption. Biotechnology for Biofuels, 2011. 4(1): p. 10.

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.