Evaluation of the flocculent strain Zymomonas mobilis Z1-81 for the production of sorbitol and gluconic acid

Abstract

The biological production of sorbitol and gluconic acid from glucose and fructose is normally performed with previously grown and concentrated Zymomonas mobilis suspensions, in reactions catalyzed by the enzymes glucose-fructose oxidoreductase (GFOR) and glucono-δ-lactonase (GL). By using the flocculent strain Z. mobilis Z1-81, mechanical methods for cell concentration are unnecessary. Flocculation of Z. mobilis Z1-81 is seriously affected by both stirring and initial glucose concentration (S0). Under mild agitation conditions, with S0 = 100 g · 1-1, up to 76% of the final biomass was recovered by settling. At any impeller speed tested, similar specific GFOR/GL activities (8.4 to 9.5 U · g-1 cell) were measured. When S0 was increased to 144 and 210g · 1-1, however, enzyme activities above 14 U · g-1 were found. Ideal pH and temperature values for GFOR/GL, in whole Z. mobilis Z1-81 cells, are 6.4 and 39 °C, respectively. The K(m) and the theoretical V(max) are 250 g glucose · 1-1 and 26.6 g gluconic acid · g-1 cell · h-1. With the settled biomass, bioconversion of 300 g · 1-1 each of glucose and fructose led to gluconic acid and sorbitol yields between 91 and 92% of the theoretical maximum, with specific productivities of 1.09 and 1.00 g product · g-1 cell · h-1.