Directing product formation by mixed culture fermentation

Abstract

Our society is dealing with the shortage of fossil fuels and chemical feedstocks and, on the other hand, with the increase of wastes generated by municipalities, agriculture and industries. Biobased industry, the conversion of renewable resources or wastes to chemicals and fuels, by microbial fermentations or enzymes has been receiving increasingly attention. The aim is to develop new technologies, increase efficiencies and reduce the costs in fermentation, bioconversion, and in downstream processing. A great challenge is to develop fermentation technology that employs the underutilized biomass residues, often considered as wastes. The use of open mixed cultures can be an option to convert these waste streams into bulk chemicals. Based on natural inocula with a high microbial diversity, open mixed cultures are capable of dealing with substrate mixtures of variable composition under non-sterile conditions. Despite the high number of species, selection occurs for a limited number of microorganisms that best adapt to the imposed conditions. The big challenge of mixed culture biotechnology is to design the operational conditions that will select for the desired metabolic conversion. The aim of this project was to study the impact of the operational conditions on mixed culture fermentation: on the product spectrum and on the microbial population established. The variables studied were: pH, carbon source, different influent substrate concentrations and mixtures of substrates. To this end, experiments were performed in a continuous operated bioreactor; and the results were interpreted considering the available knowledge on pure culture fermentation and theoretical predictions, based on thermodynamic calculations.