medium-chain carboxylic acids, caproic acid, mixed cultures, cassava wastewater.

The carbon chain elongation process appears as an opportunity for the biological production of products with high added value, such as medium chain carboxylic acids (MCCAs). Among MCCAs, caproic acid has several applications in the market, such as antimicrobials (pharmaceutical industry), additives (animal feed), and chemical products (production of flavorings, biofuels, and bioplastics). The biological production of caproic acid, on larger scales (pilot and industrial), still presents different challenges, such as: the complex structure of microbiomes, the presence of competitive metabolic pathways, the toxicity of the undissociated acids, in addition to the viability of in-line extraction techniques. Thus, the objective of this work was to evaluate the chain elongation process aiming at the production of caproic acid, through the use of different inoculums (ruminal fluid, anaerobic granular sludge, and cassava wastewater), separately or in the consortium, from two experiments with three batches (B-I, B-II, and B-III) each. For that purpose, reactors in different scales (0.610 L reactors – Article 1 and 7 L anaerobic reactor – Article 2) were employed, using acetic acid and ethanol as substrates. The highest concentrations of caproic acid in article 1 were accepted in B-III, according to the following inocula: 10.1 g.L-1 for anaerobic granular sludge, 9.6 g.L-1 for cassava wastewater and 9.1 g.L-1 for ruminal fluid. Microbial analyses revealed a greater dominance of the phylum Firmicutes and the genus Clostridium sensu stricto 12 in the reactors, thus justifying the high generation of caproic acid. In article 2, using an anaerobic reactor with 7 L and mixed inoculum (anaerobic granular sludge and cassava wastewater), the following maximum productions of caproic acid were achieved: 7.0 g.L-1 at the end of B-II and after its pH adjustment, 7.8 g.L-1. Microbial analyses indicated the dominance of MCCA producing microorganisms, such as Caproiciproducens, C. sensu stricto 12, Enterococcus, and Rummeliibacillus along the three batches. The two experiments carried out reached, from the cost benefit analysis, that it is necessary to replace chemical inhibitors (2-BES) and synthetic substrates due to their high costs, to guarantee the economic viability of the gain in the carbon chain.