Pullulanase reversion reactions

Dextrose yield, however, can be increased by conducting saccharification at a lower solids level where the reverse reaction is minimized. For instance, dextrose yields of 98.8, 98.2, 97.5, and 96.9% dry basis can be achieved at solids levels of 10, 15, 20, and 25%, respectively (10). Low solids operation, however, is not used commercially owing to problems associated with microbial contamination and cost of water removal. Dextrose level can be increased by 0.5—1.5% at normal reaction solids by using an enzyme such as pullulanase (11) or a B. megaterium amylase (12) in conjunction with... 

Approximately 0.5% of most of the starches is still not hydrolyzed to glucose. This could be attributed to the densely branched portion of the starch which is not susceptible to enzyme attack due to steric hindrance. However, earlier studies showed that further increases in the amount of pullulanase resulted in an even lower conversion, suggesting enzyme inhibition which would be due to the molecular size of the pullulanase. Alternatively, the 0.5% unhydrolyzed portion could be products from the reverse reaction catalyzed by amyloglucosidase. 

The optimal pH of pullulanase is around 5.0 for the hydrolysis reaction, whereas the optimal pH of pullulanase for the reverse synthesis reaction is aroimd 4.0-4.5, as shown in Fig. 4.2. 

Only high substrate concentration is favorable for the reverse synthesis reaction of pullulanase (Fig. 4.3). But when the substrate concentration is higher than 80%, the conversion rate of Mal-jS-CD starts to decrease. It might be because the viscosity of the reaction system becomes too high, which is harmful for the enzymatic reaction, when increasing the substrate concentration to above 80% [6]. 

According to Bo Cui, the overall optimal conditions of the production of Mal-/3-CD by the reverse synthesis reaction of pullulanase, obtained by orthogonal experimental design are temperature of 70°C, pH of 4.2, substrate ratio of 20 and enzyme amount 300 U/g /3-CD [6]. 

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