Based on the results of our micro-titer plate screenings, we studied the influence of copper in the batch cultivation of Aspergillus niger strain AB 1.13 CAD pyr + 4.1. Itaconic acid production level was proved to be positively related with copper concentration in the medium. Moreover, the best itaconic acid producing condition was medium 12 with 0.02 mM Cu2+, which
yielded a 2.5 folds increase in itaconic acid production compared to the reference medium (Figure 3) [4]. Although the percentage of itaconic acid (20 %) among the total organic acids is lower in comparison to our previous study (80 %), the production level is two folds higher. In this respect, it was interesting to note that A. niger was the most copper tolerant among 11 fungal species especially under low pH conditions [23], allowing the use of copper for enhancing itaconic acid production in this species. In our cultivations, copper increment had no toxic effect on biomass growth. Besides, copper ions could increase the production of citric acid as well. The research group of Haq found 0.015 mM of Cu2+ could increase the productivity of citric acid by A. niger [24,25]. Similarly in our research with A. niger strain AB 1.13 CAD pyr + 4.1, highest citric acid production was observed using 0.01 mM Cu2+.