Production of citric acid from fermentation solution by cultivation <i>Aspergillus niger-1</i> strain on an extract obtained from topinambur (<i>Helianthus tuberosus</i>) artichoke tubers

Background: Citric acid is an organic hydroxycarboxylic tribasic acid. It is widely distributed in plants: in berries, citrus fruits, for example, in lemons, up to 9% of dry weight. Citric acid is used in many industrial processes, but it is most frequently used as a flavoring ingredient and preservative in food products. Because of its beneficial antioxidant properties, citric acid helps control acidity and aids in digestion. Citric acid is traditionally produced by microbiological synthesis using beet molasses as the primary raw material during the metabolism of the producer strain Aspergillus niger-1. Objective: Nowadays, the microbial synthesis of citric acid also depends on the utilization of readily available and reasonably priced products as raw materials. This study reports on the biosynthesis of citric acid by the producer strain Aspergillus niger-1 on an extract derived from the tubers of the Jerusalem artichoke plant that contains inulin. Currently, the synthesis of citric acid with microbes is also associated with the use of readily available products as raw materials. This study describes the biosynthesis of citric acid during the growth of the producer strain Aspergillus niger-1 on an extract derived from Jerusalem artichoke tubers and containing inulin. Methods: To prepare the extract, water was added to the crushed Jerusalem artichoke tubers in a ratio of 1:2. The inulin content in the extract was determined spectrophotometrically at a wavelength of 490 nm. Fungi were grown on a shaker for submerged cultivation for seven days. HPLC and neutralization methods were used to determine the concentration of citric acid in the samples. Results: After a series of treatments, 2.0 liters of aqueous extract were obtained from crushed Jerusalem artichoke (Helianthus tuberosus) tubers, which contained about 6.5–7.0% inulin. On the fifth day of culture development, the fungus fermentation solution had the highest concentration of citric acid (5.8%), of which 4.3% citric acid was found in the supernatant. Ultimately, 3.5 g of milky-white crystalline powder was isolated from the supernatant, the yield of which was 60.3% of the amount of citric acid in the fermentation solution. Conclusion: To date, the aqueous extract from Jerusalem artichoke tubers has not been utilized as a carbon source for the microbiological synthesis of citric acid. Due to its high inulin content, resistance to environmental stressors, and minimal agricultural requirements, we proposed studying the possibility of obtaining citric acid from inulin-containing raw materials—Topinambur tubers (Helianthus tuberosus)—using microorganisms. Novelty of the study: The scientific novelty lies in obtaining citric acid from an aqueous extract from tubers of topinambur, which is an alternative to the classic method of obtaining citric acid from beet molasses. Since this is a multi-stage process, molasses is an expensive raw material. Keywords: Topinambur (Helianthus tuberosus), inulin-containing extract, Aspergillus niger, fermentation solution, supernatant, citric acid powder, food additives