| Vegetable wastes(VWs)have high water content,rich nutrients,and are easily degraded by microorganisms,which are ideal substrates for anaerobic fermentation.Converting short-chain fatty acids(SCFAs)produced by anaerobic fermentation into hexanoic acid with higher added value based on carbon chain elongation is an efficient and environmentally friendly way of resource recovery and utilization.Insufficient electron donors,complex metabolic pathways and by-products in fermentation systems lead to low yields and difficult separation,which are currently urgent issues in the biological synthesis of hexanoic acid.In view of this,in this paper,using Chinese cabbage waste(CCW)as raw material,lactic acid pre-fermentation was used to provide more electron donors for hexanoic acid synthesis.The effects of pH on product distribution,microbial community structure,and lactic acid metabolism during the chain elongation stage of hexanoic acid production were studied,and the mechanism of their effects was preliminarily explored;The effects of adding different sources of electron donor lactic acid on the yield of hexanoic acid were compared in order to provide a theoretical basis for effectively improving the electron conversion rate of lactic acid and the yield of hexanoic acid.The main research contents and results are as follows:(1)The effects of the addition of Lactobacillus and pH regulation on the anaerobic fermentation characteristics of CCW.Compared with natural fermentation,the addition of Lactobacillus had no significant effect on the product composition and lactic acid yield of CCW anaerobic fermentation;the lactic acid yields of the pH 4.5 and control groups were34.39% and 32.83%,respectively,and the selectivity of lactic acid was above 80%;the system converted to ethanol type fermentation at pH 4.0,with ethanol accounting for 83.00%of the total product,while butyric acid was the main product at pH 5.5;pH 6.5 was favorable for hydrolysis and acidification of substrate,but the product composition was complex.(2)Effect of pH on lactate metabolic pathways and hexanoic acid production.Lactobacillus and Caproiciproducens at pH 5.5were the key genus driving lactate chain elongation for hexanoic acid synthesis,mainly through the fatty acid biosynthesis(FAB)pathway.pH 6.0 increased microbial diversity and complex composition of fermentation products.The relative abundance of Prevotella 6,Succiniclasticum and Blautia increased,promoting the reverse β-oxidation(RBO)and propionate metabolism pathways.The lack of trans-Hex-2-enoyl-Co A reductase(EC:1.3.1.38)functional gene in the fermentation system resulted in the disruption of the RBO pathway and butyric acid accumulation.The yield of hexanoic acid at pH 5.5 was 5.10±0.01 g COD/L,which was 38.59%% and 4.51% higher than that at pH 5.0 and at pH 6.0,respectively.The initial pH of the uncontrol pH group was only 3.82,which severely inhibited the chain elongation reaction.(3)Effect of supplementary addition of different lactate sources on hexanoic acid synthesis.The concentration of hexanoic acid in the added lactic acid fermentation broth(PFB)group and the added lactic acid solution(LA)group increased by 143.93% and 33.47%respectively.The hexanoic acid selectivity was significantly increased in the PFB group,accounting for 37.37% of the total product,with the hexanoic acid yield of 7.00 ± 0.29 g COD/L and the electronic efficiency was 187.5% higher than that of the LA group.The Rikenellaceae＿RC9＿gut＿group and Caproiciproducens in the PFB group system were the dominant microorganisms for hydrolysis and hexanoic acid synthesis,respectively..The relative abundance of Caproiciproducens in the LA group was 0.82%and the hexanoic acid yield was reduced by 72.83%. |
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