| Pickle Chinese cabbage is one of the local characteristic foods in Northeast,China.The raw material is Chinese cabbage and is fermented by microorganisms.During the fermentation process,the Chinese cabbage has not been sterilized,which consequently result in abundant microbial diversity,various metabolites as well as complex internal and interaction between biotic and abiotic factors.The quality and flavor of pickle Chinese cabbage is directly determined by the metabolites produced by microbial especially bacterial population.In this research,Lactobacillus paracasei HD1.7,isolated from the liquid sample during pickled Chinese cabbage fermentation,was used as a starter culture in cabbage fermentation.A microbial ecological model of pickle Chinese cabbage fermentation was constructed.The control model,i.e.,natural fermentation of pickle Chinese cabbage was set without addition of starter cultures.Metabolomic techniques including HPLC(high performance liquid chromatography)and GC-MS(gas chromatogram-mass spectrometry)and genomic techniques such as quantitative real-time PCR and high throughput sequencing were adopted to research the coupling mechanism between bacterial communities and metabolomes features.The results of this study will help us understand process and function of the microbial ecosystem pickle of Chinese cabbage fermentation and provide the basis for quality monitoring and technology improvement of industrial production.Firstly,this present work investigated the feature of L.paracasei HD1.7homo-lactic fermentation,including changes in p H profile,key enzymes and metabolites.It was found that,along with the fermentation the glucose was consumed and generated a large amount of lactic acid and mannitol,whose final concentrations were 16.92±0.59 g/L and 11.47±0 g/L,respectively.The p H value decreased more dramatically and reached 3.39±0.01.Key enzymes of homo-lactic fermentation,PFK(6-phosphofructokinase),PK(pyruvate kinase)and intermediate metabolites increased firstly and then decreased.The total amount of 17 free amino acids were was4043.20±9.04 mg/L.The succession of bacterial communities in both structure and quantity were investigated during pickle Chinese cabbage fermentation.The results showed that in the early stage of fermentation,the control model had fewer bacteria and was dominated by Lactococcus.In the experimental model,L.paracasei HD1.7 dominated the fermentation system as dominant bacteria.Along with the fermentation,the number of bacteria and species richness increased in both systems.In the control model,Lactobacillus has gradually become the dominant genus.The diversity of bacterium increased and the number of Lactococcus and other harmful bacteria reduced.In the experimental model,the diversity of bacterium decreased firstly and then increased and Lactobacillus always maintained dominance.The number of OUT(operational taxonomic unit)was significantly smaller in the experimental model compared to that in the control model.The results indicated that the strains L.paracasei HD1.7 as starter culture reduced the number of OTU and cut down the diversity of species of pickle Chinese cabbage.The variation of metabolites was monitored during pickle Chinese cabbage fermentation.The results demonstrated that,there were significant differences in the kind and abundance of metabolites between the different systems and the different fermentation stages.Along with the fermentation,the content of organic acids increased,and the experimental model was higher than that of the control model.The final concentration of lactic acid in control model and experimental model was 8.55±0.29 and9.15±0.04 g/L,respectively.Organic acid concentration increased as fermentation progressed inversely,relating to the decrease in p H values.The final p H value was3.26±0.01 and 3.24±0.01 in control model and experimental model,respectively.The content of Vc revealed a decreased trend with the prolonging of fermentation and was significantly higher in experimental model(terminal concentration 455.93±2.82 mg/kg)compared to that in the control model(terminal concentration 293.97±1.63 mg/kg)(p<0.0001).The nitrite concentration in control model reached peak value on the third day(19.02±0.52 mg/kg).Contrastingly,the nitrite concentration in experimental model was under 1.00 mg/kg in the whole fermentation.Free amino acids were detected and the results indicated that the terminal concentration of three out of seventeen amino acids,including threonine,glutamic acid and lysine,as well as total amino acid concentration was 116.71±0.15 mg/kg,significantly higher in experimental model compared to that in the control model(104.35±0.12 mg/kg)(p<0.0001)at 25 days of fermentation.In addition,109 flavor components were detected at the end of fermentation,including esters,alkanes,terpenes,benzene,aldehydes,ketones,naphthalene,ethers,sulfur-containing compounds,heterocyclic compounds and alcohols.The relative content were the most of the esters,alkanes and terpene,and the number of flavor components was significantly higher in experimental model.From the type of view,except for seventh day of fermentation,the control model contained 83 kinds of flavor components more than the experimental model(79 kinds),and other fermentation time point,the number of flavor components were more than that in control model.L.paracasei HD1.7 as starter culture fermentation of pickle Chinese cabbage can change the structure and quantity of bacterial community,promote the growth of dominant bacteria and reduce the bacterial diversity.Also,L.paracasei HD1.7 as starter culture can increase a variety of nutrients content,decrease the content of nitrite and promote the formation of flavor components etc.. |
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