Stability of Glucose Oxidase Activity of Aspergillus niger Spores Produced by Solid-State Fermentation and Their Role as Biocatalysts in Bioconversion Reaction

The aim of this work is to demonstrate the role of conidial spores as a reservoir of glucose oxidase and their stability as a biocatalyst in the bioconversion reaction for the production of gluconic acid. Solid-state fermentation (SSF) was carried out in fixed-bed column bioreactor for the production of Aspergillus niger spores. Growth parameters, sporulation and kinetics of gluconic acid production were analysed at different time intervals during the course of SSF. Spores of different age (48–216 h) were used as biocatalysts in the bioconversion reaction. Spores harvested at a later period of SSF (196 h) produced high titres of gluconic acid (30 g/L) in the bioconversion medium when compared to the spores harvested at early (48 h) stages of SSF (2.2 g/L). Spores (harvested at 200 h and stored in freezer for 91 days) exhibited the same glucose oxidase activity and served as an active and stable catalyst when compared to the fresh spores, showing that aging (storage) did not affect enzymes present in the spores, which suggested that the spores acted as an efficient enzyme reservoir. Yields close to 93 % were obtained with 98 g/L of gluconic acid production, corresponding to an average productivity of 1.7 g/(L·h). The stability of the enzyme in the spores and the ability of conidia to be stored for a long time without the loss of activity add specific advantage to the bioconversion process.