The Effect of Iron on Gluconic Acid Production by Aureobasidium pullulans

New processes have been previously described for the continuous and discontinuous production of gluconic acid by Aureobasidium pullulans (de bary) Arnaud. Little is known about the regulatory mechanisms of gluconic acid production by A. pullulans. The response of growth and gluconic acid metabolism to a variable profile of iron concentrations was studied with A. pullulans in batch and chemostat cultures. A surprisingly high optimum N-dependent iron ion concentration in the feed medium, in the range between 0.5 mM and 3.0 mM Fe (optimum 1-2 mM), was found to be particular requirement for economically profitable continuous production of gluconic acid with 3 g/l NH4Cl. Increased iron concentration promoted growth on defined glucose medium. 223.3 g/l gluconic acid were continuously produced at a formation rate of the generic product (Rj) of 16.8 g/(l h) and a specific gluconic acid productivity (mp) of 2.5 g/(g h) at 13 h residence time (RT) with 1mM iron, compared with 182 g/l reached at 0.1 mM. The product selectivity (product yield based on glucose) increased continuously by raising iron concentration following a saturation curve, reaching a maximum of about 98% (mol/mol) at 2 mM Fe and 76.2% conversion, compared with only 84.3% determined at 0.1 mM. The process is not obligatory growth limiting or growth related and residual nitrogen was found in all of continuous experiments, e.g. 197 mg/l of nitrogen at 0.1 mM and 201 mg/l at 2 mM of iron.