Formation of gluconic acid at low pH-values by free and immobilized Aspergillus niger cells during citric acid fermentation

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Summary

A recently developed immobilization method, characterized by the adsorption of the mycelia onto a glass-carrier in a fixed-bed reactor, was applied for citric acid production by Aspergillus niger ATCC 9142, and compared with conventional culture techniques.

In a fixed-bed reactor and in a stirred fermenter a rapid gluconic acid production started immediately after nitrate exhaustion, though the pH was below 2.5 During a second production phase a comparatively small amount of citric acid was formed.

In surface and shaken-flask cultures nearly no gluconic acid could be found, whereas citric acid yields were significantly higher than in the fixed-bed reactor and in the stirred fermenter.

Manganese (0.8×10−7 Mol×dm−3 after 6 days incubation) from the stainless steel parts of the vessel seemed to be responsible for both gluconic acid production and small citric acid yields in the stirred fermenter and in the fixed-bed reactor.