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Optimization of the Simultaneous Saccharification and Fermentation of an Insoluble Substrate into Microbial Metabolites

There is great interest and potential in the biological utilization of widely available insoluble substrates, such as cellulose, for the production of useful microbial metabolites. In particular, a simultaneous saccharification and fermentation is promising because it can overcome the strong reaction inhibition caused by the intermediate end product in the saccharification reaction. FIGURE 1 shows the reaction sequence for the direct bioconversion of insoluble cellulose carried out as a simultaneous saccharification and fermentation (SSF). Cellulose hydrolysis has been simplified to two reactions: first, cellulose solubilization, which includes adsorption of the cellulases (E2) on the insoluble substrate, and second, conversion of cellobiose to glucose. Interactions are also shown; the strongest is cellobiose inhibition of the first reaction, but glucose inhibition is also important. Inhibition by the microbial product (metabolite) may also exist. The viability of a direct simultaneous bioconversion is also influenced by the compatibility of the enzyme and product formation systems, as determined by temperature and pH optima and the possibility of enhanced cell lysis by the external glucanases. In the enzymatic hydrolysis of cellulose for accumulation of glucose, it has been shown that an adequate balance of the participating cellulolytic enzymes is a necessity; in particular, the addition of a high cellobiase activity is required for high yield of fermentable glucose.' A crucial question then is the determination of the appropriate cellulolytic activities relative to the needs of the metabolite-producing microorganism for a number of important reasons. Some of these are shown in TABLE 1, among them is the determination of enzymes needed to be cloned in recombinant strains. This paper discusses our present work on the stages to be followed in developing a strategy for optimization of a simultaneous direct bioconversion. They include development of a generalized, simple mathematical model and its use to analyze rate limitations; compatibility studies between the enzymatic and microbial systems; initial results in the development of a strategy for the overall optimization of the SSF; and finally a look into the possible merits and limitations of a continuous process for such a system.