Co-production of lactic acid and short-chain fatty acids from fruit and vegetable wastes by repeated-batch fermentation

A large number of fruit and vegetable wastes have been produced in the world each year. Improper disposal can cause resource waste and environmental pollution. The platform chemicals (such as medium-chain fatty acids, MCFAs) from fruit and vegetable waste) have been the important direction in promoting the high production value of anaerobic fermentation. In general, MCFAs can be produced under the chain elongation process, where the short-chain fatty acids (SCFAs) and lactic acid are essential carbon sources to act as electron acceptors and donors, respectively. Therefore, both SCFAs and lactic acid can be obtained during the acidification of fruit and vegetable waste for the subsequent synthesis of MCFAs. It is very necessary to meet the requirements of application and commercialization for stable and long-term continuous fermentation. This study aims to investigate the feasibility of repeated-batch fermentation for the co-production of lactic acid and SCFAs from fruit and vegetable waste. A systematic analysis was made to explore the effects of replacement ratios and feed total solid (TS) concentrations on acidification. Furthermore, the microbial community structure was determined to correlate the product distribution and microbial community, in order to explore the influencing mechanism of replacement ratio and feed TS concentration. The results showed that the replacement ratio and feed TS concentration of repeated-batch fermentation were regulated to promote productivity and the lactic acid/SCFAs ratio. The maximum productivity of acidification product was obtained at a feed TS concentration of 8%, which were (2.36±0.17), (3.63±0.20), and (5.25±0.25) g/(L·d), respectively, at replacement ratios of 30%, 50%, and 70%. The maximum productivity of the acidification product was obtained in (4.55±0.17) g/(L·d) at a feed TS concentration of 6% and a replacement ratio of 90%. The product composition analysis demonstrated that the main products were obtained as acetic acid and butyric acid at a replacement ratio of 30%, which comprised more than 60% and 20% of the total, respectively. The lactic acid was the main product (>70%) at replacement ratios of 50%, 70%, and 90%. The highest proportion of lactic acid (78%) was obtained at a replacement ratio of 70% and a feed TS concentration of 8%. In addition, the ratio of lactic acid/SCFAs ranged from 3±0.1 to 6±0.3 at replacement ratios of 50%, 70%, and 90%, respectively, while carbon source concentrations ranged from (335±16) to (985±29) mmol C/L. According to the productivity of the acidification product, the ratio of lactic acid/SCFAs and the carbon source concentration, a replacement ratio of 70%, and a feed TS concentration of 8% were relatively more favorable for further production of MCFAs. The productivity, carbon source concentration, and lactic acid/SCFAs ratio were (5.25±0.25) g/(L·d), (985±29) mmol C/L, and (5±0.3), respectively. Microbial community analysis revealed that lactic acid bacteria such as Lactobacillus and Enterococcus were dominant in the bacteria at high replacement ratios and high feed TS concentrations. The co-produced lactic acid and SCFAs after heterolactic fermentation can be expected to serve as electron donors and acceptors for MCFAs synthesis. Furthermore, SCFAs synthesis bacteria can predominate at low replacement ratios. Their primary byproducts of SCFAs can be utilized as electron acceptors for the synthesis of MCFAs. The present study demonstrated a potential strategy to recover high-value products from fruit and vegetable waste.