4.3.1

Sorbitol/Gluconate Production

The production of sorbitol by Z. mobilis when grown on sucrose or a mixture of glucose and fructose has been reported earlier by several groups [94,95]. In subsequent studies on the mechanism of sorbitol production, an enzyme complex was identified by Leigh et al. [96] which was capable of oxidizing glucose to gluconic acid concomitant with the reduction of fructose to sor­bitol. This enzyme was described as a glucose-fructose oxidoreductase with a tightly coupled (non-dialyzable) co-factor identified as NADP [97]. The mechanism for sorbitol/gluconic acid production and the associated enzymes are shown in Fig. 8 with the pathway from gluconate to ethanol not being functional if cells of Z. mobilis are fully permeabilized. As shown in Fig. 8, the possibility exists also of producing a mixture of sorbitol and gluconolactone if gluconolactonase activity is deleted.

Kinetic studies have been reported for a 60% sugar solution (300 gL-1 glucose and 300 g L-1 fructose) using toluene-treated permeabilized cells of Z. mobilis in which a sorbitol concentration of 290 g L-1 and a gluconic acid concentration of 283 gL-1 were achieved after 15 h in a batch process [98]. A continuous process with immobilized cells was developed with only a small loss of enzyme activity (less than 5%) evident after 120 h. With a strongly basic anion exchange resin and a buffer system at pH = 9.0, good separation of sorbitol and gluconic acid was achieved. Subsequent studies using immo-

4.3.2