Enhanced production of propionic acid through acidic hydrolysis by choice of inoculum

BACKGROUND: In this study, the enhancement of propionic acid production from a model feedstock mimicking kitchen waste was investigated. For that purpose, two operational runs of a semicontinuous anaerobic hydrolysis reactor were carried out at pH 6.0 ± 0.1 and mesophilic (30 °C) temperature. Two different types of inocula, a mixed microbial culture selected over 24 months for growth on cellulose and a culture contained in goat cheese were compared. RESULTS: The results show that the goat cheese inoculum was significantly more efficient for propionic acid (PA) production. The highest propionic acid concentration achieved amounted to 139 mmol L⁻¹ at a yield of 23.3 mg g⁻¹ volatile solids (VS), which was 55% greater than what was achieved with the mixed culture. Furthermore, it was observed that propionic acid production was enhanced by a combination of high hydraulic retention time (HRT) with low organic loading rate (OLR), ensuring sufficient time for complete processing of the complex organic substrates. The fermentation could be kept in a stable process of propionic acid production at HRT of 20 days and a rather low OLR of 11.1 g L⁻¹ day⁻¹ VS. CONCLUSION: Our results give a better understanding of PA production in semicontinuous mode, applying optimized process parameters and selecting the adequate microbial community for inoculation. This study provides important information for the improvement of PA production from complex substrates for future industrial application.