Feasibility of multi-hydrolytic enzymes production from optimized grape pomace residues and wheat bran mixture using Aspergillus niger in an integrated citric acid-enzymes production process

[Display omitted] •Hydrolytic enzyme activities were maximized in WP′/RP/WB mixtures by mixture design.•A. niger recycled mycelium from citric acid fermentation was the inoculum for SSF.•WB addition induced the production of the target enzymes.•The optimum ternary feedstock mixture was composed of 15% WP′, 15% RP, 70% WB.•Enzymes production strengthens plant profitability and local bioeconomy. In this study, the potential of the co-generation of hydrolytic enzymes in the biorefinery plant for citric acid fermentation was investigated. Aspergillus niger B60 mycelium along with the solid residue after the recovery of sugars from white pomace (WP′) were recycled from citric acid fermentation. A mixture design was used to determine the optimum ternary feedstock mixture composed of WP′ (15%), red grape pomace (15%) and wheat bran (70%) that produced the target enzymes with high activities, which were compared to those from pure feedstocks. Maximum carboxymethyl cellulase, polygalacturonase, amylase, xylanase and acid protease activities obtained through solid-state fermentation (120 h, 30 °C) of the feedstock mixture were 668 IU/g, 3,151 IU/g, 1,099 IU/g, 579 IU/g and 204 IU/g (dry mass basis), respectively. The system was successfully simulated in SuperPro Designer. Results showed that the enzymes production process serves as the main contributor to the profitability of the biorefinery plant.