Study of the anaerobic decomposition of valeric acid using variable stoichiometry and thermodynamic control

The stoichiometry of biological decomposition reactions of many organic compounds depends on the environmental conditions and the relative concentrations of reactants and products, making that, often the calculation models based on constant stoichiometric coefficients are not able to describe experimental values. This is the case of proteins in anaerobic medium, for which an effective and realistic method for expressing the dynamics of aminoacids reactions has not been found yet. A methodology that has begun to be explored is to raise the reactions using variable stoichiometric coefficients, depending on thermodynamic relations expressing the energy available for the microorganisms growth. Since the reactions of aminoacids in anaerobic environment gives volatile fatty acids (VFA) and H2, a preliminary step to raise variable stoichiometry reactions of aminoacids is to clearly define the subsequent reactions related to its products using the same methodology, that is, to estimate a variable stoichiometry depending of thermodynamic variables. The aim of this work has been to study the biological decomposition reaction in anaerobic environment of valeric acid, one of the VFA produced. This acid produces propionate, acetate and hydrogen, propionate produces acetate and hydrogen, and the methane is produced finally from the last two compounds. The system is composed by four microorganism populations, catalysing four different interrelated biological reactions. The method applied has been to estimate the change of the Gibbs free energy of every reaction depending on the concentrations of reactants and products, and to determine the energy available for the microorganisms growth, which is variable along time during the system dynamics evolution. Variable maximum growth constant for every microorganism has been estimated based on the derived variable stoichiometry, obtaining a kinetic model with thermodynamic control. Published data related to batch experiments designed to study the kinetics of valerate decomposition ...