Optimising butyrate and lactate yields fermenting xylose in a mixed culture system
Abstract
Despite being the second most abundant monomer in the earth, xylose fermentation has not been studied as thoroughly as glucose fermentation. This work studies the pH effect in xylose fermentation, focusing on lactic and butyric acid production as medium chain fatty acids (MCFA) precursors. Continuous reactor operation showed that changes within a slightly acidic pH range (pH 4.5 to pH 6) affect to VFA production but not their distribution, being butyric and acetic acid the main products regardless the pH value, while lactic acid was only detected when xylose or butyric acid concentrations dropped. In contrast, xylose conversion was negatively affected by low pH values. The use of lactic acid as substrate was assessed in continuous and batch reactors, being consumed and mainly converted into butyric acid.
Full text
• Xylose consumption is complete at pH 6. • Butyric acid yield is maximised at pH 5. • A steady lactic acid production could not be reached. • Lactic acid was only produced when butyric acid concentration dropped. • Co-fermenting xylose and lactate promote butyric acid yield. J. Iglesias-Riobó, M. Mauricio-Iglesias, M. Carballa Optimising butyrate and lactate yields fermenting xylose in a mixed culture system CRETUS, Department of Chemical Engineering, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain [email protected] INTRODUCTION CONTINUOUS REACTOR CONCLUSIONS Medium chain fatty acids (MCFA), produced from volatile fatty acids (VFA) and an electron donor, such as lactate, are promising alternatives for carbon recovery (Spirito et al., 2014). To obtain simultaneously a suitable VFA composition and an electron donor for chain elongation, pH stands as one of the most important parameters, since it affects the fermentation bioenergetics and therefore the product spectra (González Cabaleiro et al., 2015; Regueira et al., 2020). When dealing with one of the most abundant source of organic carbon, such as lignocellulosic waste streams, there are still significant gaps in literature. This study analyses the pH effect in xylose mixed-culture fermentation targeting lactic and butyric acid as MCFA precursors. González-Cabaleiro, R., Lema, J.M., Rodríguez, J., 2015.Metabolic energy-based modelling explains product yielding in anaerobic mixed culture fermentations.PLoS One 10.https://doi.org/10.1371/journal.pone.0126739. Regueira, A., Bevilacqua, R., Lema, J.M., Carballa, M., Mauricio-Iglesias, M., 2020. A metabolic model for targeted volatile fatty acids production by cofermentation of carbohydrates and proteins.Bioresour Technol 298. https://doi.org/10.1016/j.biortech.2019.122535 Spirito, C.M., Richter, H., Rabaey, K., Stams, A.J.M., Angenent, L.T., 2014.Chain elongation in anaerobic reactor microbiomes to recover resources from waste.Curr Opin Biotechnol. https://doi.org/10.1016/j.copbio.2014.01.003 This research was funded by Spanish Government through the CELL4CHEM project (PCI2021-121989, ERACoBioTech 3rd call). The authors belong to a Galician Competitive Research Group (ED431C-2021/37). N2 sparging pH control Mixed culture fermentation Influent - Xylose - Mineral medium Effluent - Non-consumed xylose - Intermediates (lactic acid, ethanol, etc.) - Volatile fatty acids Operational conditions T: 37 ºC pH 6 Butyrate test (XB) 8 g COD Xylose/L 4 g COD Butyrate/L Lactate test (XL) 8 g COD Xylose/L 4 g COD Lactate/L Blank test (X) 8 g COD Xylose/L 0 1 2 3 4 5 6 Acetic acid Butyric acid Caproic acid Lactic acid Xylose Biomass Concentration (g COD/L) pH 4.5 pH 5 pH 6 •Uncomplete xylose conversion at pH 4.5. •Butyric acid concentration is optimised at pH 5. •Lactic acid appears intermittently during the operation. •The biomass concentration did not suffer major changes. ACKNOWLEDGEMENTS BATCH ASSAYS Operational conditions T: 37 ºC HRT: 1 day OLR: 12 g COD/L·d pH: 4.5 – 6 LACTATE ADDITION IN CONTINUOUS REACTOR 0 1 2 3 4 5 6 7 Acetic acid Butyric acid Lactic acid Succinic acid Xylose Biomass Concentration (g COD/L) pH 5 pH 5 + 2 g/L lactate •50 % of lactate is consumed. •Butyric acid concentration raised a 19 %. •Lactic acid is less energetically efficient than xylose as substrate. Lactic and butyric acid presence affected negatively to xylose conversion rate 0 2 4 6 8 10 12 14 012 24 36 48 60 72 84 96 108 120 132 144 Concentration (g COD/L) Time (h) 8 g Xylose/L (X) 0 2 4 6 8 10 12 14 012 24 36 48 60 72 84 96 108 120 132 144 Concentration (g COD/L) Time (h) 8 g Xylose/L + 4 g Butyrate/L (XB) 0 2 4 6 8 10 12 14 012 24 36 48 60 72 84 96 108 120 132 144 Concentration (g COD/L) Time (h) 8 g Xylose/L + 4 g Lactate/L (XL) Butyric acid concentration is not inhibitory in process Lactate addition promoted butyric acid production