Medium-chain carboxylic acids (MCCA) are saturated fatty acids containing between 6 and 12 carbon atoms, including a carboxylic group. MCCA are generally produced from fossil sources by chemical processes or extracted from natural oils. The use of bacterial consortia instead of pure cultures in the elongation process has several advantages, such as raw, nonsterilized substrates. In this study, the microbial community analysis (Illumina MiSeq) was evaluated considering the influence of pH (5 to 7) and temperature (30 °C to 40 °C). Batch tests were conducted in 80 mL bottles using a mixture of winery effluent plus ruminal fluid as inoculum. A synthetic medium consisting of ethanol and acetic acid was utilized. The kinetics of MCCA for each condition was determined. Production of acetic, proponic and valeric acids was observed when microorganisms belonging to the genus Cutibacterium, Veillonella and Aminobacterium were present. However, heptanoate formation was only identified at 30 °C and pH 5 (5.97 g/L) where Mogibacterium and Burkholderia were detected, indicating the preference of acidic pH by such microorganisms. The maximal production of caproic and octanoic acids (13.58 and 6.65 g/L) was found at 35 °C and pH 6 after 21 d. In that case, the community was dominated by Clostridium sensu stricto 1, Clostridium sensu stricto 12, Christenellaceae-R7, Caproiciproducens (Clostridia class), and communities belonging to genus Bacteroide. In conclusion, it was found that the initial pH and temperature significantly influence MCCA production. The presence of Clostridium is fundamental when ethanol is used as an electron donor for MCCA formation. Furthermore, it was observed that initial pH and temperature determine the proliferation of specific microorganisms and, therefore, the metabolic pathways towards the type of MCCA production.
