Development of a consolidated bioprocess for the production of citric acid using Aspergillus niger as biocatalyst

The industrial production of citric acid, an ingredient in beverages, pharmaceuticals, and cosmetics, is based on microbial fermentation of glucose or sucrose. Given the elevated cost of these sugars, lignocellulosic biomass is emerging as a cost-effective and environmentally friendly feedstock for sustainable bioprocesses. However, fermentation of lignocellulosic materials requires that they are first broken down enzymatically. This can be achieved by the filamentous fungus Aspergillus niger, which has the ability to secrete hydrolytic enzymes and to produce citric acid. Here, we investigated the production of citric acid using a consolidated bioprocess, in which all conversion steps - from the solid substrate to the final product - occurred in a single process stage. The press cake derived from a perennial ryegrass (Lolium perenne) was used as substrate and glucose or the remaining press juice were utilized as an additional carbon source. Aspergillus niger produced citrate successfully only when the press cake was supplemented with press juice (2.1 +/- 0.0 g kgDM-1) and especially glucose (84.7 +/- 0.3 g kgDM-1). Confocal laser scanning microscopy revealed differences in fungal mycelia based on the carbon source supplemented. Overall, the results indicate the successful implementation of solid-state fermentation for the sustainable production of citric acid by A. niger fed on press cake.