Cellulose continuous saccharification

Figure 17. Semi-continuous saccharification of modified cellulose...

Typically, the SSF process is carried out in a CSTR reactor in batch mode. Under these reaction conditions, the fermentation product, ethanol, exerts its effect not only on microbes but also on saccharification. To overcome this problem, and to improve the efficiency of ethanol production from cellulose, the continuous removal of end-product during ethanol production would have advantages. With this type of process application, the SSF process can be operated in a fed-batch mode. Fed-batch operation is similar to continuous operation except the fermentation broth is retained in the fermentor at all times whereas the solid substrate is continuously fed into the fermentor [73]. Another method is to continuously remove ethanol during the SSF process (see Sect. 2.1.3). 

The present study is aimed at investigating the increase in the susceptibility of cellulose towards cellulase in order to increase both rate and yield of sugar. It demonstrates the possibility of regenerating enzyme digested-resistant cellulose into highly susceptible form. It also describes a stable model reaction system for semi- or fully-continuous system for saccharification of cellulose into glucose using untreated culture filtrates of Trichoderma viride. 

Figure 18. Relationship between substrate concentration and saccharification rate of modified cellulose (SF-HM, SF-MH) in semi-continuous agitated reactors...

In the preceding methods, the enzyme is confined to small droplets, capsules, or inert carrier. Enlargement of a capsule to the size of a fermenter is theoretically possible, and this principle has been applied with practical modifications. The enzyme is free in solution as in the batch procedures, but an ultrafilter serves to separate the reaction products from the enzyme and substrate. Substrate is continuously pumped into the system, and product is removed by ultrafiltration to provide a continuous system. This method is applicable only to systems where the substrate is a large or an insoluble substance, so that it - with the enzyme - are retained inside the membrane. Successful demonstration of experimental runs on cellulose saccharification (Chose and Kostick, 1969) and on starch hydrolysis (Butterworth et al., 1969) have been made. Success depends upon the availability of suitable membranes and practical application on their cost. 

Since cellulose hydrolysis and fermentation of the sugar solution are carried out continuously in the same tank and glucose is continuously fermented, the feedback inhibition to cellulose due to the increase of glucose concentration is avoided. From a technological aspect, this method simplifies the equipment needed, saves production time, and improves the production efficiency. But there are some inhibitory factors, such as the inhibition of xylose and incoordination of the saccharification and fermentation times (Oscar and Carlos 2007). 

Griggs AJ, Stickel JJ, Lischeske JJ. (2012a). A mechanistic model for enzymatic saccharification of cellulose using continuous distribution kinetics I depolymerization by EGI and CBHI. Biotechnol Bioeng, 109(3), 665-675. 

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