Contribution of endo‐ and exocellular glucose oxidase to gluconic acid production at increased dissolved oxygen concentrations

For gluconic acid fermentation with Aspergillus niger oxygen‐enriched air was used to increase dissolved oxygen concentration during the production phase from c. 30% to 100% saturation related to air at 1 bar. At the highest dissolved oxygen concentration the activity of glucose oxidase increased more than three‐fold. At the same time, the gluconic acid production rate increased by a factor of two. During the fermentations, glucose oxidase was present endocellularly as well as exocellularly; the presence of this enzyme in the broth was due to a controlled secretion. It was found that the two fractions of the enzyme showed different activities in their contribution to overall acid production. Due to a limitation of oxygen diffusion into the mycelial pellets, activity of endocellular enzyme was reduced by 50–80%. However, since the major part of the enzyme was present in the pellets the action of endocellular glucose oxidase is not negligible. This latter factor must be taken into consideration when optimizing bioreactor design.