Evaluation of the flocculent strainZymomonas mobilis Z1-81 for the production of sorbitol and gluconic acid (original) (raw)

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-8-lactonase (GL). By using the flocculent strain Z. rnobilis 21-81, mechanical methods for cell concentration are unnecessary. Flocculation of Z. mobilis 21-81 is seriously affected by both stirring and initial glucose concentration (SO). Under mild agitation conditions, with So = 100 g. I-' , up to 76% of the final biomass was recovered by settling. At any impeller speed tested, similar specific GFOWGL activities (8.4 to 9.5 U. g-I cell) were measured. When So was increased to 144 and 210 g * l-l, however, enzyme activities above 14 U g-' were found. Ideal pH and temperature values for GFOWGL, in whole Z. mobilis 21-81 cells, are 6.4 and 39 "C, respectively. The K,,, and the theoretical V,, are 250 g glucose .1-' and 26.6 g gluconic acid. g-I cell. h-I. With the settled biomass, bioconversion of 300 g .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-' cell. h-'. Zymomonas mobilis produces glucono-a-lactone and sorbitol from glucose and fructose in reactions catalyzed by the intracellular enzyme glucose-fructose oxidoreductase (GFOR) (EC 1.1.99). In the sequence, glucono-6-lactone is hydrolyzed to form gluconic acid by glucono-6-lactonase (GL) (EC 3.1.1.17) (ZACHARIOU and SCOPES 1986). Based on those enzymes, the biological production of sorbitol and gluconic acid from an equimolar mixture fructose/glucose was tested (CHUN and ROGERS 1988, REHR et al. 1991). These processes were carried out with cells of 2. mobilis, previously grown, concentrated by centrifugation, and permeabilized to prevent any metabolic activity and assure high yields. According to SILVEIRA et al. (1994), good yields and productivities are also found in batch runs with non-permeabilized 2. mobilis cells, when an initial glucose plus fructose concentration of 600 g a 1-I is used. To bypass the centrifugation step in a possible industrial process, the utilization of a flocculent strain of 2. mobilis seems to be an interesting alternative. In this case, the concentrated cell mass would be obtained by natural settling. Thus, the main objective of this paper was to evaluate the effect of glucose concentration and agitation on growth, flocculation, and GFOWGL activity of the flocculent strain. Z. mobilis 21-81. Furthermore, the enzymes contained in the settled biomass were examined with respect to some kinetic and physicochemical parameters. At last, the ability of non-permeabilized cells to convert efficiently glucose and fructose to gluconic acid and sorbitol was studied. Materials and methods Microorganism: Zyrnomonas mobilis 21-81 was provided by the Instituto Oswaldo G. de Lima of the Universidade Federal de Pernambuco (Recife, Brazil). The strain has been maintained in liquid medium at 4 "C.