In-Plant Cellulase Production
In spite of all the recent advances in cellulase production, the high cost of cellulase is one of the major hindrances to make the second — generation ethanol cost competitive with gasoline. An alternative approach for reducing costs with cellulolytic enzymes is to obtain them by dedicated production (i. e., in-plant production), developing custom-made cellulase preparations for saccharification of a particular type of biomass used in the plant. Additionally, production of cellulases is within the context of biorefinery and circumvents their high prices, providing a great motivation to develop the present work. Several cellulosic ethanol pilot plants have experimented with the idea of production of their own cellulase. In one recent experiment, Maeda and coworkers studied the cellulase production by Penicillium funiculosum using pretreated sugarcane bagasse as a carbon source for cellulase production, and its application in the hydrolysis of sugarcane bagasse for ethanol production by fed-batch operation [46]. This study aimed to produce a cellulase blend and to evaluate its application in a simultaneous saccharification and fermentation (SSF) process. First sugarcane bagasse was subjected to pretreatments; the residual solid fraction was named sugarcane bagasse partially delignified cellulignin (PDC), and was used for enzyme production and ethanol fermentation. The enzyme production was performed in a bioreactor with two inoculum concentrations (5 and 10% v/v). The fermentation inoculated with higher inoculum size reduced the time for maximum enzyme production (from 72 to 48 h). The use of a higher inoculum size (10% v/v) resulted in increased enzyme titer and volumetric productivity. The volumetric productivity (U/L h) of cellulases in 5 and 10% inoculum are shown in Table 6.2.
The increase in cellulase activity in the medium was concomitant with an increase in the production of protein that was secreted by the microorganism. The kinetic profiles of protein production in bioreactors with 5% v/v (A) and 10%v/v (B) of pre-inoculum are shown in Figure 6.4.
Furthermore, the produced cellulase blend was evaluated for its stability at 37°C, operation temperature of the simultaneous SSF process, and at 50°C, optimum temperature of cellulase blend activity. In this study, Maeda et al. reported that the cellulolytic preparation was stable for at least 300 h at both 37°C and 50°C. The ethanol production was carried out by PDC fed-batch SSF process,
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Figure 6.4 Kinetic profiles of protein production in bioreactors with 5% v/v (A) and 10% v/v (B) of pre-inoculum. (Reprinted with permission from reference [46]; copyright 2013 Elsevier). |
using the onsite cellulase blend. The feeding strategy circumvented the classic problems of diffusion limitations by diminishing the presence of a high solid:liquid ratio at any time, resulting in high ethanol concentration at the end of the process (100 g/L), which corresponded to a fermentation efficiency of 78% of the maximum obtainable theoretically. Maeda and coworkers claimed that these experiments led to the production of 380 L of ethanol per ton of sugarcane bagasse partially delignified cellulignin (PDC) [46].