Kinetic and stoichiometric model for the computer-aided design of protein fermentation into volatile fatty acids

Abstract Mixed culture fermentation to produce volatile fatty acids stands as a solid option for valorising protein-rich wastes, such as those from the (agro)industry. However, the current knowledge about this process is limited and there are no appropriate tools for designing the production of volatile fatty acids from proteinaceous wastes. Available models fail to reproduce how the process stoichiometry varies with the substrate composition and environmental conditions. Likewise, existing kinetic parameters were not estimated in anaerobic fermentative conditions and cannot be used reliably for designing fermentation processes. Therefore, in this work, a model for the production of volatile fatty acids from proteins in mixed-culture fermentations was developed. The model incorporates a variable volatile fatty acids production stoichiometry that depends on the operational conditions and the protein. To reflect the commonly observed incomplete protein consumption in fermentative conditions, the model considers that protein acidification process attains a thermodynamic equilibrium with its products. To calibrate the model, targeted designed fermentation experiments were carried out with casein and gelatine, which were chosen due to their different amino acid composition as well as their relevance in industrial organic wastes. The model reproduces with accuracy the experimental data on the fermentation of two proteins under different environmental conditions, which indicates that protein acidification is well described. The model developed in this work together with the proposed simulation framework proved to be an effective tool for selecting the optimal design parameters leading to highly selective and productive processes and for the early-stage design of processes valorising protein-rich wastes into chemicals.