Hexanoic Acid Production From Chinese Cabbage Waste By Anaerobic Fermentation With Mixed Culture

With the commercial circulation of vegetables and the improvement of consumers’ requirements for vegetable quality,about 30% of vegetable are discarded or buried every year,resulting in serious environmental pollution and waste of resources.The transformation of vegetable waste into medium chain fatty acids represented by hexanoic acid by anaerobic fermentation is a high-value transformation approach with broad application prospects.Based on batch anaerobic fermentation,the effects of pH value and the source of electron donor ethanol on the performance of hexanoic acid production from Chinese cabbage waste(CCW)were investigated.Through the analysis of volatile fatty acid composition and microbial community structure changes in the fermentation process,the mechanism of hexanoic acid production by anaerobic fermentation was preliminarily discussed.The main results are as follows:(1)Effects of pH values on acid production efficiency and microbial community structure evolution via anaerobic fermentation of CCW.The results showed that the concentration of total VFAs(TVFAs)was highest at pH 6.0,which was 20241.4 mg COD/L,followed by pH7.0.Ethanol,acetate and butyrate were dominant under the acidic condition.The main products at pH 7.0 were acetate,propionate,and butyrate.At pH 6.0,ethanol,acetate and butyrate were rapidly produced during the initial stage.The hexanoic acid concentration increased quickly from day 6 due to the chain extension between ethanol and butyrate,and was 4,885.1 mg COD/L on day 8,accounting for 30.4% of the TVFA.Through the identification of key microorganisms in the mixed bacteria by high-throughput sequencing technology,it was found that Bacteroidia and Clostridia were dominant at pH 6.0 and the uncontrolled pH,respectively.The production of hexanoic acid was suspected to be related to the symbiosis between Clostridium IV,Candida,Dialister and Ruminococcus.(2)Effect of exogenous ethanol electron donor on the production efficiency of hexanoic acid by anaerobic fermentation of CCW.Under the condition of constant pH 6.0,batch anaerobic fermentation was carried out with CCW as substrate.When the electron donor ethanol of the fermentation system was consumed,hexanoic acid was continuously produced by adding ethanol.The results showed that adding ethanol at a high concentration at one time would inhibit the formation of hexanoic acid;adding ethanol gradually in batches,the concentration of hexanoic acid reached 13665.6 mg COD/L when the ratio of cumulative ethanol addition to acetic acid was 2.3:1,accounting for 44.9%of TVFAs,the yield reached38.3%,and the fermentation type was changed from butyric acid to hexanoic acid.(3)Effect of yeast pre-fermentation on hexanoic acid production by anaerobic fermentation of CCW.The self-generated electron donor ethanol in the anaerobic fermentation of CCW was realized by inoculating yeast,which provided the basis for the production of hexanoic acid,and the effects of different inoculation methods on the efficiency of hexanoic acid production were investigated.The results showed that the ethanol type conversion of substrate could be well realized by inoculating yeast.The maximum concentration of ethanol in the reactor after yeast pretreatment reached 14816.1 mg COD/L,the concentration and yield of hexanoic acid were improved,reached 7750.3 mg COD/L and 22.7%,respectively.In addition to the reverse β-oxidation pathway characterized by the consumption of ethanol and butyrate,the fatty acid biosynthesis pathway characterized by the consumption of ethanol and aceate also occupies an important role.The structure of bacterial community in the two reactors of yeast pre-fermentation and synchronous fermentation was similar,mainly Firmicutes,Proteobacteria and Bacteroidetes,while the control group without yeast was mainly Firmicutes and Proteobacteria,and the functional bacteria producing ethanol and hexanoic acid were different.The functional microorganisms that play the role of hydrolysis to produce acid and ethanol provide the basis for the subsequent production of hexanoic acid.In the yeast pretreated reactor,Clostridium kluyveri dominated the production of hexanoic acid.