| The Chinese traditional fermented dough is a complex biochemical system,and the type and content of flavor substances is an important factor affecting its quality.Besides raw materials and fermentation methods,the production of dough flavor substances is more important due to the role of microbial fermentation.The mixed fermentation system composed of yeasts,bacteria and molds can produce sugars,alcohols,acids,esters and CO2 by fermentation of flour,so as to produce suitable taste and flavor of Chinese steamed bread.Some studies have shown that the internal microbial flora and metabolites were gradually changing during dough fermentation.After this dynamic change process,the unique texture and flavor of fermented flour products were finally formed.However,the research on the changes of microbial flora and flavor metabolites was still lacking.Henan is a big province of flour products manufacturing in China.The traditional fermented dough has a long history,wide distribution,numerous species and large quantity in Henan Province.However,the differences of microbial community structure in different regions and terrain environments are not clear,and the interaction between dominant strains of dough is still unknown.In this study,we have identified and analyzed the microorganisms of traditional fermented dough in different terrain(plain,mountain and basin)from Henan area,studied the relationship between the dynamic changes of microbial community and flavor substances in dough fermentation,and studied the promotion effect and mechanism of lactic acid bacteria on yeast by compounding core strains.The main results of this study are as follows:1.In order to make clear the microbial community composition of traditional fermented dough in different terrain environment of Henan Province,the culturedependent method was used to analyze the bacterial and fungal flora in 32 traditional fermented doughs.Lactic acid bacteria are the main bacteria,of which Pediococcus pentosaceus is the dominant species in the three terrain condition samples,especialy in the basin samples;there are more Lactobacillus crustorum and Lactobacillus plantarum in plain samples;Lactobacillus paralimentarius is the second dominant bacteria in mountain samples,and some samples also contain Lactobacillus brevis,Lactobacillus mindensis and Weisseria cibaria.The main fungi were yeast,among which Saccharomyces cerevisiae and Saccharomycopsis fibuligera were the dominant species.Saccharomyces cerevisiae accounted for a high abundance in 32 traditional fermented doughs,while Saccharomycopsis fibuligera was widely distributed in basin samples.In some mountain samples,there were Issatchenkia orientalis,Candida tropicalis and Pichia kudriavzevil.2.High-throughput sequencing combined with gas chromatography-mass spectrometry(GC-MS)was used to study the dynamic changes of microbial community during dough fermentation process,and the differences of microbial community structure in different fermentation stages and their effects on the formation of flavor compounds were analyzed.The 131 kinds of volatile aroma compounds were identified in the fermentation process of traditional fermented doughs,and these compounds changed with the fermentation process.The composition of volatile aroma compounds was different in three fermentation stages.In the first stage(0 h),alcohols and aldehydes were the main components.In the second stage(3 h),the types and relative contents of alcohols and esters were increased,and the relative contents of acids were decreased,which was the key stage for the formation of aroma substances in dough fermentation.In the third stage(9 h),the metabolism and accumulation of various flavor compounds were maximized,and the relative content of acids increased.These differences in volatile flavor compounds are the key factors causing the difference of dough aroma components,and also the main reason affecting the flavor specificity of traditional fermented steamed bread.The correlation analysis showed that Lactobacillus had a high positive correlation coefficient with phenylacetaldehyde,phenylethanol,benzaldehyde and ?-nonolactone,but negatively correlated with 2-ethylhexanal,decanal,caproic acid,ethyl caproate,etc.Pediococcus was positively correlated with hexyl acetate,2-pentylfuran,(E)-2-heptenenal,heptanol and ethyl lactate,and negatively correlation with ?-hexylcinnamaldehyde,nonanal,2-ethylhexanal and decanal.Acetobacter were positively correlated with hexanoic acid,ethyl decanoate,(E)-2-heptenenal,ethyl caproate,2-ethylhexanal,and negatively correlated with benzaldehyde,?-nonactone and 1-octen-3-ol.Yeast was mainly correlated with phenylacetaldehyde,2-pentylfuran,(E)-2-octenal,(E)-2-heptenealdehyde,limonene,heptanoic acid,ethyl caprylate,hexyl formate and ethyl decanoate.3.To explore the effect of lactic acid bacteria on yeast in dough fermentation,the lactic acid bacteria and yeast of traditional fermented dough were recombined to study the effect of compound strains on the quality of steamed bread.In the process of mixed fermentation of Lactobacillus and yeast,the number of yeasts increased significantly by 2-10 times and lactic acid bacteria increased by 1.5-20 times after dough fermentation,among which Lactobacillus fermentum,Lactobacillus crustorum and Pediococcus acidilactici increased significantly.Lactobacillus fermentum can significantly promote the proliferation of yeast.After adding lactic acid bacteria,the dough produced more lactic acid,and the p H value decreased,especially for Pediococcus pentosaceus and Lactobacillus fermentum.Acetic acid,tartaric acid and succinic acid were the main acid substances in the test dough and steamed bread.After adding Pediococcus pentosaceus,the specific volume of steamed bread increased,the hardness decreased,and the gas production in the dough increased significantly.From the overall evaluation of sensory evaluation,the score of steamed bread fermented by Pediococcus pentosaceus was the highest,which improved the quality of steamed bread,and was preferred by the test subjects.4.In order to further evaluate the interaction mechanism between Saccharomyces cerevisiae and Pediococcus pentosaceus during dough fermentation.The gene expression level of S.cerevisiae under co-fermentation of S.cerevisiae and Pediococcus pentosaceus was analyzed by transcriptomics technology,and the promotion effect of P.pentosaceus on yeast metabolism was analyzed from the perspective of differential expression of carbohydrate,amino acid and flavor related genes.The results of transcriptome sequencing showed that 383 differentially expressed genes were identified in Saccharomyces cerevisiae after the addition of P.pentosus(P < 0.05).KEGG pathway functional enrichment analysis showed that these genes were mainly involved in amino acid metabolism,glucose metabolism and fatty acid metabolism.In the process of co-fermentation,P.pentosaceus could promote the genes expression of Saccharomyces cerevisiae related to glycolysis and TCA cycle(PFK27,PDC1,PDA1 and CIT2).So as to improve the ability of flavor substance metabolism and gas(CO2)production in dough.The up expression genes of Ehrlich pathway(ARO9,GDH1 and PDC1)in Saccharomyces cerevisiae indicated that P.pentosus could improve the amino acid metabolism of S.cerevisiae and promote the synthesis of flavor compounds during fermentation.These result were further verified by fluorescence quantitative PCR.During the fermentation process of flour products,carbohydrate metabolism produced energy,CO2 and intermediate metabolites,which promoted yeast fermentation and dough gas production,and amino acid metabolism contributed to the production of flavor substances in flour products. |
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