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.

The influence of dilution rate and dissolved oxygen concentration on continuous production of 2,5-diketogluconic acid (2,5-DKGA) by Gluconobacter oxydans subsp. melanogenum is described. Under suitable dissolved oxygen conditions, the glucose oxidation rate is independent of variations in dilution rates over a wide range (D < 0.33 h−1). The higher dilution rate, however, leads to a partial oxidation of the substrate and therefore accumulation of more gluconic acid. Contrary to this, a longer residence time (lower dilution rate) was more suitable for better ketogenic activity, thereby facilitating accumulation of 2,5-DKGA in higher yields. A high dissolved oxygen concentration is further shown to positively influence the overall oxidation process through an organized induction of membrane-bound NAD(P)+-independent dehydrogenases.

The critical concentration of dissolved oxygen (DO) that determines the optimum capability of a microorganism depends upon the desired end product. Glucose oxidase (GOD) as a fermentation product is significantly influenced by oxygen in liquid phase. The DO concentration of 100% related to air at 1 bar is shown to be the threshold concentration for a several fold increase in production of the enzyme. By generating periodic sinusoidal DO oscillations during fermentation it is shown that the DO gradient prevailing in airlift loop reactors of industrial scale is advantageous for the induction of GOD. Additionally, the influence of periodic residence of Aspergillus niger at sub-optimal DO concentration on organized export of the enzyme into the medium is demonstrated.

Gluconobacter species are known to oxidise glucose via a direct oxidation pathway which is distinct from the pentose phosphate pathway. In the present communication results of an investigation on the influence of different dissolved oxygen concentrations (DO) on the production of 2,5-diketogluconic acid in batch and chemostat cultures are given. DO of 30% relative to air at 1 bar was found as a threshold level for optimum productivity. The positive influence of continuous availability of dissolved oxygen on the process of rapid glucose oxidation was unambiguously shown as the result of induction of membrane bound dehydrogenases involved in direct glucose oxidation. Furthermore data of scale-down experiments in which the organism was cultivated under oscillations of dissolved oxygen, are given. The influences of such oscillations of DO in the region of the established threshold (30% saturation) were found to result in a prolonged lag phase for growth and product formation. The data obtained in this study revealed critical residence times at low DO that could be employed as a criterion for scale up of this aerobic process.

A control device has been developed for generating oscillations of dissolved oxygen concentration in fermentation media in order to simulate the conditions in a real production-scale fermentor. Dissolved oxygen concentration is controlled by feeding oxygen and nitrogen at gas feed rates which are set permanently by a personal computer using a simple control algorithm. The dynamic model for the design of the controller takes into account time lags due to dead time, gas phase mixing and oxygen mass transfer as well as the delay of the oxygen probe. The control device was successfully used in gluconic acid fermentations of Aspergillus niger.

Production of 2,5-diketogluconate by Gluconobacter oxydans (ATCC9937) in two types of reactors, namely, airlift and stirred fermentors, is described. Accumulation of diketo-acid is shown to be dependent on the pH of the medium. Oxidation of gluconate to 2,5-diketogluconate in Gluconobacter oxydans is described as a specific reaction carried out through an intermediate of 2-ketogluconate rather than 5-ketogluconate. The enzyme profile of the culture depicting two distinct phases, i.e. direct glucose oxidation and gluconate oxidation, is described.

Increased constant dissolved oxygen tension (200% related to air saturation at 1 bar) has a positive influence on the direct oxidation of glucose byGluconobacter melanogenum. This effect is pronounced and apparent by the faster production of 2,5-diketogluconic acid, mediated through induction in activity of gluconic acid dehydrogenase.