Zinc oxide nanoparticle-based citric acid production using Aspergillus brasiliensis: process optimization, kinetics and mechanisms

Abstract The current study optimized simultaneous nanoparticle-based saccharification and citric acid (CA) production from pre-treated potato peel waste using Aspergillus brasiliensis . The model gave a high coefficient of determination ( R 2 ) (0.929) and predicted optimum pre-treatment conditions of 0.05 wt%, 19.85%, 32.5 °C and 2.03 for ZnO nanoparticle (NP) concentration, solid loading, temperature, and pH respectively. The validated process resulted in A. brasiliensis biomass and CA concentration of 1.54 g/L and 19.85 g/L, respectively. This was 1.19 and 1.38-fold higher compared to the control experiment, respectively. Interestingly, the kinetic assessment also revealed increase (2.67-fold) in maximum specific growth rate ( µ max ) and maximum potential CA concentration ( P m ) (1.07-fold) in the ZnO nanoparticle-based system. Potential of catalytic micro-environment and steady Zn 2+ release in the growth medium is the most probable mechanism of ZnO NP triggering of A. brasiliensis for high specific growth rate and CA productivity. Findings from this study could facilitate the implementation of nanoparticle catalysed waste-based CA bioprocessing that might improve waste management and lower CA production cost, in keeping with the waste management, environmental sustainability and food nexus towards developing a circular bioeconomy.