Spores of Aspergillus niger as reservoir of glucose oxidase synthesized during solid-state fermentation and their use as catalyst in gluconic acid production

Abstract

To exploit conidiospores of Aspergillus niger as a vector for glucose oxidase extraction from solid media, and their direct use as biocatalyst in the bioconversion of glucose to gluconic acid. Spores of A. niger (200 h old) were shown to fully retain all the glucose oxidase synthesized by the mycelium during solid-state fermentation (SSF). They acted as catalyst and carried out the bioconversion reaction effectively, provided they were permeabilized by freezing and thawing. Glucose oxidase activity was found retained in the spores even after repeated washings. Average rate of reaction was 1.5 g l(-1) h(-1) with 102 g l(-1) of gluconic acid produced out of 100 g l(-1) glucose consumed after approx. 100 h reaction, which corresponded to a molar yield close to 93%. These results were obtained with permeabilized spores in the presence of a germination inhibitor, sodium azide. Spores of A. niger served as efficient catalyst in the model bioconversion reaction after permeabilization. To our knowledge, this is the first detailed study on the ability of A. niger spores to act as reservoir of enzyme synthesized during SSF without its release into solid media. Use of this material served as an innovative concept for enzyme extraction and purification from a solid medium. Moreover, this approach could compete efficiently with the conventional use of mycelial form of the fungus in gluconic acid production.