Saccharification system

Figure 19. Continuous forward feed steady state single vessel saccharification system...

Cost sensitivity studies have shown that the successful commercialization of cellulase-based processes, such as the conversion of cellulose to fermentable sugars, is highly dependent on the cost of enzyme production (i). Because fungal -D-glucosidase (EC 3.2.1.21) is the most labile enzyme in this system under process conditions (2), and k to efficient saccharification of cellulose, this enzyme was targeted for application of stabilization technology, both through chemical modification and immobilization to solid supports. 

Several microorganisms have been studied with respect to the production of a cellulolytic enzyme system for the saccharification of cellu-losic materials, the most thoroughly investigated organism and best producer of cellulase being Trichoderma viride (I). Recently, good saccharification data have been reported using a strain of Penicillium (2). 

This chapter deals with three aspects of the cellulolytic enzyme system of Thermoactinomyces sp. the location of the CM-cellulase, Avicelase, and / -glucosidase (cellobiase) activities in the culture, the multiplicity of the extracellular enzyme system, and the stability of the different activities as a function of pH, temperature, and time. The results are discussed with reference to saccharification of cellulosic materials. 

Despite the fact that the / -glucosidase is sensitive to high temperatures, the cellulolytic enzyme system of Thermoactinomyces has several advantages that have to be considered in the choice of an enzyme system for saccharifications. 

The enzyme system is compartmentalized into extracellular CM-cellulase and Avicelase activities and cell-associated / -glucosidase activity, which permits optimal proportions of these activities to be used in saccharifications. 

Fungal cellulase enzyme systems capable of efficiently catalyzing the hydrolytic degradation of crystalline cellulose are typically composed of endo-acting cellulases (EGs), exo-acting cellulases (CBHs), and at least one cellobiase (1-6). The CBHs are typically the predominant enzymes, on a mole fraction basis, in such systems (7). Consequently, the CBHs have been the focus of many studies (8). The three-dimensional structure of prototypical CBHs is known (9-12) and their specificities are, in general, well characterized (13,14). However, mechanism-based kinetic analyses of CBH-catalyzed cellulose saccharification are rather limited (15,16). Studies of this latter type are particularly difficult owing to the inherent complexity of native cellulose substrates. 

Fig. 6. Saccharification of Avicel cellulose by T. reesei cellulase without (panel 0) and with (panel 1) Orpinomyces BglA. Concentrations of sugars were measured using an HPLC system (see Materials and Methods) after 16 h of reaction.

Johnson, E. A., Sakajoh, M., Halliwell, G., Madia, A., and Demain, A. L., Saccharification of complex cellu-losic substrates by the cellulase system from Clostridium thermocellum. Appl. Environ. Microbiol. 1982, 43, 1125-1132. 

Starch conversion refers to the process of converting starch into other products. It involves gelatinization, liquefaction, and saccharification. Liquefaction refers to the acid-or enzyme-catalyzed conversion of starch into maltodextrin. Starch, usually from wet milling of com, is pumped in a slurry to the conversion plant, where it undergoes one or more hydrolytic processes to yield mixtures of various carbohydrates in the form of syrups. The kind and amount of the various carbohydrates obtained depend upon the type of hydrolysis system used (acid, acid-enzyme, or enzyme-enzyme), the extent to which the hydrolytic reaction is allowed to proceed, and the type of enzyme(s) used. The fact that most starches consist of two different kinds of polymers... 

Water used in this supercritical state behaves very differently from water under normal pressure and temperare." In such a supercritical state, the water can be expected to act as an acid or base, but by returning the system to ordinary conditions before pyrolysis occurs, glucose and its derivatives could be obtained in water from cellulose. Therefore, supercritical water treatment can be superior to enzymatic saccharification or oidinaiy acid hydrolysis mentioned above, for the chemical conversion of biomass to useful chemicals. 

Most bacteria are incapable of degrading crystalline cellulose since their cellulase systems are incomplete. However, the cellulolytic enzymes produced by some fungi generally involve all three types of enzymes, so they are very useful in the saccharification of renewable cellulosic resources. 

In designing an efficient SSF system for the conversion of cellulose to ethanol, the fermentation temperature should be compatible with the saccharification temperature that is generally between 45 and 55 °C. The optimal temperature for the most commonly available cellulase is about 50 °C. Therefore, the use of high-temperature-tolerant microbes is desirable for the application of the SSF process to ethanol production. Typical industrial ethanol-producing yeast strains are mesophiHc with an optimal fermentation temperature of 30-37°C. Only a few yeast strains that are thermotolerant, as well as good ethanol fermenters, have been described. However, some thermophilic bacterial species are known to produce ethanol from cellulosic-derived carbohydrates [68,69]. 

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