Metabolism And Relationships Among Microbes In Sourdough Steamed Bread

The Chinese traditional Laojiao steamed bread was highly concerned because of its unique flavor.In this study,the microbial diversity in Laojiao dough was studied and two Lactubacillus strains,Lactobacillus plantarum and Lactobacillus brevis and one Saccharomyces cerevisiae strain were screened for single-or mixed-strain experiment.The relationships among the strains and their effects on the main metabolites were further analyzed by measuring the changes in the number of flora,carbon sources,metabolites and volatile flavor compounds during the dough fermentation.The microbial diversity of the Laojiao dough samples in Hebei,Tianjin and Jilin was analyzed by the combination of culture-dependent and culture-independent methodologies.16s rDNA amplicon sequencing results indicated that the bacterial flora of Heibei sample(HB)and Tianjin sample(TJ)were similar,and the species richness was higher than Jilin sample(JL).74 species of bacteria were identified from all samples and Lactobacillus was the dominsant group,accounting for more than 70%of the samples in each group.This result was consistent with that of the traditional isolation and culture techniques,which identified three types of bacteria as Lactobacillus plantarum,Lactobacillus brevis and Bacillus amyloliquefaciens.The dominsant fungus was Saccharomyces cerevisiae.In addition,Candida and Rhizopus oryzae were also isolated.Thirteen lactic acid bacteria(LAB)producing undesirable flavors were excluded by acid production and taste experiments and the screened remaining strains were re-screened using acid-forminsg capacity,in vitro enzyme activity and antibacterial activity.The results showed that there was no significant difference in acid-forminsg capacity among different LAB strains.The in vitro enzyme activity of HB group was the highest(24.409 U/mL on average)and the strains HB4,HB5,TJ11 and JL6 had good antibacterial effects against Gram-positive bacteria and Gram-negative bacteria.Furthermore,thirteen strains of yeast were excluded due to poor performance in gas yield and bad taste.Among the remaining strains,HB9,JL7 and JL10 showed a strong ability to produce alcohol and HB9 had the best acid tolerance.Based on the above results and analysis of bacterial diversity,Lactobacillus plantarum HB5 and Lactobacillus brevis JL11 and Saccharomyces cerevisiae HB9 were selected as representative strains.The mixture proportion was determinsed by mixed experiment design,and the regression model was established for sensory evaluation of steamed bread prepared by different strains.The optimal number ratio of Saccharomyces cerevisiae,Lactobacillus plantarum and Lactobacillus brevis was 5.6:2.7:1.7.The changes in the number of flora,metabolites and flavor compounds during the dough fermentation were investigated.The results indicated that the number of LAB substantially increased,while the number of yeast decreased at the beginning of fermentation,and increased gradually after 5 h.The pH value of the dough group decreased,while the titratable acid(TTA)showed the opposite trend.The content of maltose and glucose increased first and then decreased,while the content of glucose and fructose in the dough containing yeast decreased and even exhausted.Meanwhile,the organic acid content in the dough with mixed LAB and yeast was higher than that of the dough containing only LAB.A total of 82 flavor compounds including alcohols,ketones,esters,aldehydes and aromatic compounds were detected and analyzed by solid-phase microextraction(SPME)and gas chromatography-mass spectrometry(GC-MS)in the dough fermentation process.The greatest contributive compound to the characteristic flavor was ethanol and n-hexanol.Therefore,microbial composition and metabolic activity has an important influence on the formation of flavor compounds.The optimal freeze-dried protective agent formulation was sucrose content 10%and skim milk content 5%(mass fraction).Under the optimal conditions,the survival rates of LAB and yeast were as high as 89.67%and 89.46%,respectively.