Michael Träger

Comparison of airlift and stirred reactors for fermentation with Aspergillus niger

Abstract The design and fabrication of a simple laboratory airlift fermentor are described. This reactor excels in reliability and low power requirement. Its efficiency has been proved for the production of gluconic acid by Aspergillus niger. As in the case of bubble columns, the desired morphology of A. niger, i.e pellet growth, is obtained in the laboratory airlift fermentor as well as in a pilot scale airlift fermentor of 260 l working volume. Thus, pellet formation occurred under rather low stress conditions as compared to stirred tanks where high agitation conditions are expected to be required to achieve similar results.

DIKETO-GLUCONATE FERMENTATION BY GLUCONOBACTER OXYDANS

Abstract 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.

Pilot-scale citric acid production with Aspergillus niger under several conditions

Abstract Citric acid fermentations with Aspergillus niger in stirred tank reactors (500 l and 2.6 m3) were carried out using cane juice, cane molasses and a synthetic medium. Highest yields were obtained with the cane juice and synthetic medium. The rate of stirring had a pronounced influence on the morphology of A. niger.

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

Abstract Gluconobacter spp. and Aspergillus niger alike are able to directly oxidize glucose to gluconic acid. A wide difference in K M, 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.

Influence of constant and oscillating dissolved oxygen concentration on Aspergillus niger

Abstract 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.

Device for fermentations with oscillations of dissolved oxygen

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.