Caproic acid production from food waste using indigenous microbiota: Performance and mechanisms

Carbon dioxide as key player in chain elongation and growth of Clostridium kluyveri: Insights from batch and bioreactor studies

Chain elongation technology allows medium-chain fatty acids (MCFAs) production as an alternative to fossil resources. Clostridium kluyveri generates n-caproate primarily from ethanol and acetate, presumably requiring CO2 for growth. Here, the impact of CO2 on C. kluyveri was explored. Bottle studies revealed the bacterium's adaptability to low CO2 levels, even in conditions with minimal dissolved NaHCO3 (0.0003 M) and unfavorable pH (below 6) under 1 bar CO2. Bioreactor investigations demonstrated a direct correlation between CO2 availability and bacterial growth. The highest n-caproate production (11.0 g/L) with 90.1 % selectivity was achieved in a bioreactor with continuous CO2 supply at 3 mL/min. Additional bottle experiments pressurized with 1 bar CO2 and varying ethanol:acetate ratios (1:1, 2:1, 4:1) also confirmed CO2 consumption by C. kluyveri. However, increasing the ethanol:acetate ratio did not enhance n-caproate selectivity, likely due to overly acidic pH conditions. These findings provide insights into chain-elongators responses under diverse conditions.

Biogas slurry recirculation regulates food waste fermentation: Effects and mechanisms

Regularly adding biogas slurry into fermentation reactors is an effective way to enhance hydrogen or methane production. However, how this method affects the production of valuable organic acids and alcohols is still being determined. This study investigated the effects of different addition ratios on semi-continuous fermentation reactors using food waste as a substrate. The results showed that an addition ratio of 0.2 increased lactic acid production by 30% with a yield of 0.38 ± 0.01 g/g VS, while a ratio of 0.4 resulted in mixed acid fermentation dominated by n-butyric acid (0.07 ± 0.01 g/g VS) and n-caproic acid (0.06 ± 0.00 g/g VS). The introduction of Bifidobacteriaceae by biogas slurry played a crucial role in increasing lactic acid production. In contrast, exclusive medium-chain fatty acid producers enhanced the synthesis of caproic acid and heptanoic acid via the reverse β-oxidation pathway. Mechanism analyses suggested that microbial community structure and activity, substrate hydrolysis, and cell membrane transport system and structure changed to varying degrees after adding biogas slurry.