Production of cellulase on mixtures of xylose and cellulose in a fed-batch process.

Cellulase production by the RUT-C30 mutant of the fungus Triclzoderma reesei on mixtures of xylose and cellulose was studied in a fed-batch system. An initial mixture of 30 g/L xylose and 20 g/L cellulose and an intermittent feeding rate of 5 g/L/day xylose and 15 g/L/day cellulose (total of 20 g/L/day) produced the highest titer of enzyme activity of 12.5 IFPU/mL, 57% higher than for a batch system with similar mixed substrate. The total productivity for this feeding strategy was 45.4 IFPU/L/h, a 25% improvement compared to a batch system with similar mixed feed. The study also showed that xylose can be used to replace cellulose in both the initial mixture and the intermittent feed without affecting the maximal enzyme activity and productivity as compared to cellulose alone. However, substituting xylose for more than 25% of the cellulose in the intermittent feed suppressed enzyme production. Although the yield of cellulase in IFPU/g of carbon utilized is less for fed-batch than for a batch process, the fed-batch system achieves higher overall productivities, and any substrate left after enzyme production can be used in subsequent saccharification to sugars for ethanol production. In the past decade, enzymatic hydrolysis of cellulose has received attention for renewable production of fuels from cellulosics because of the high selectivity of enzymatic hydrolysis compared to acid hydrolysis. In enzymatic hydrolysis, the substrate can in principle be converted completely to sugars, which are clean, and no extra process is required to remove impurities and toxic materials before fermentation. The sugars from hydrolysis can be used in production of liquid fuels or chemicals through fermentation. In enzymatic hydrolysis, the cost of producing hydrolytic enzymes constitutes a major portion of total production cost, and according to Wright,’ this cost can constitute approximately 25% of the overall ethanol fuel cost. To improve the economics of the enzymatic hydrolysis process, research has been focused on several areas including improving the strains of microorganisms, selecting an effective mode of operation, and optimizing the operating conditions. Currently, cellulase enzymes are produced by growing different mutants of T. reesei such as RUT-C30 and RLP-37 on c e l l ~ l o s e . ~ ~ However, the growth of the fungus on cellulose is slow. If the growth time could be re-