Comparison of direct glucose oxidation by Gluconobacter oxydans subsp. suboxydans and Aspergillus niger in a pilot scale airlift reactor

Abstract

Abstract Gluconobacter spp. and Aspergillus niger alike are able to directly oxidize glucose to gluconic acid. A wide difference in KM, O2 of the two involved enzymes (glucose oxidase and glucose dehydrogenase) exists. Comparison of the oxidation process was made by cultivating these organisms in an airlift reactor (9.25 m tall) that generates a wide gradient in dissolved oxygen (DO) due to hydrostatic pressure and liquid circulation (DO concentration of 200% to 90% of saturation with air at 1 bar pressure). During the fermentations a minimum DO concentration of 10% at a gradient of 130% to 10% was observed. However, it was found that local dissolved oxygen concentrations neither affected growth nor product formation by the two cultures; short residence time for the culture to pass through the lowest DO concentration in one circulation loop is possibly the reason. However, in Aspergillus niger the formation of glucose oxidase seems to be positively influenced by variations in local dissolved oxygen concentration. Thus a constant high oxygen supply is not necessary on an industrial scale; a periodic high DO concentration which always prevails in tower shaped fermentors (like airlift loop reactors) is sufficient to induce enzyme synthesis.