Microbial Communities In Traditional Sourdoughs From Different Areas Of China And Metabolic Activity Of Dominant Microorganism

Sourdough is the traditional starter culture for making Chinese steamed bread, a staple diet in China, with a long history of about1700years. Recently, Chinese sourdough has been given renewed attention to its traditional flavor and organoleptic quality since new market demands require rich flavor, unique characteristic and improved health benefits, which are lack in the steamed breads fermented by baker’yeast. Although China has plenty of sourdoughs, this research is still at a very early stage. This dissertation systematically studied the microbial communities in Chinese traditional sourdoughs collected from different aeras of China, the microbioral communities and metabolites were monitored periodically to investigate the fermentation process of steamed bread, and the volatile components of steamed bread were determined by different techniques.Bacterial communities in Chinese sourdoughs were analyzed by culture-dependent and culture-independent methods. Based on the culture-dependent method,6bacteria were identified, namely Lactobacillus plantarum, Enterococcus faecium, Bacillus subtilis, Lactobacillus casei, Achromobacter sp., Saccharomonospora saliphila. PCR-DGGE was used as a culture-independent method, suggesting that5bacteria species were detected, namely Lactococcus garvieae, Lactobacillus plantarum, Lactobacillus alimentarius, Lactobacillus sanfranciscensis, Bacillus subtilis. Culturing results showed Lactobacillus plantarum was the frequently isolated species, while PCR-DGGE analysis suggested Lactobacillus sanfranciscensis was the dominated LAB, Lactobacillus alimentarius and Lactococcus garvieae were the sub-dominant one. Lc. garvieae and Bacillus subtilis were first found as a prevalent bacterium species in sourdough microbiota. There were35genus identified by high-throughtput sequencing method, the predominant genus were Lactobacillus, Leuconostoc, Weissella.Fungal communities in Chinese sourdoughs were analyzed by culture-dependent and PCR-DGGE methods.7yeasts species, Saccharomyces cerevisiae, Pichia anomala, Saccharomycopsis fibuligera, Issatchenkia orientalis, Candida guilliermondii, Rhodotorula aurantiaca, Candida glabrata, were isolated as well as one mold Rhizopus oryzae. PCR-DGGE results suggested that7fungi species were detected, namely Cyberlindnera jadinii, Wickerhamomyces anomalus, Saccharomyces cerevisiae, Candida tropicalis, Torulaspora delbrueckii, Candida humilis, and Rhizopus oryzae. Both approaches revealed that Saccharomyces cerevisiae was the predominant yeast, and Saccharomyces cerevisiae, Wickerhamomyces anomalus and Rhizopus oryzae were identified by two methods.The microbioral communities and metabolites were monitored periodically to investigate during the fermentation of making steamed bread. Yeast population increased significantly at the beginning and then keeped stable, while the LAB increased firstly and then decreased. At the first fermentation stage, yeast colonies in Hn sample increased from6.66log cfu/g to8.76log cfu/g, in the meanwhile, LAB increased from7.82log cfu/g to8.91log cfu/g. However, at the second stage, yeast colonies increased in Sx and Hr samples, while keeped stable in Hn, Hf, Gs samples. Except Hn sample, LAB colonies decreased significantly in other samples. Results of PCR-DGGE showed that changes of microbial communities were similar for yeast and LAB. The predominant species increased and some species disappeared during the fermentation process. pH values were declined during fermentation process, while TTA values showed opposite trend of changing. Changes of glucose and amino acid contents increased at initial stage and then decreased. Lactic acid and acetic acid contents increased gradually along with the fermentation.Three techniques, SDE, SPME, Purge&trap, for the volatile compounds of steamed bread were established. For the SDE method,19components were identified, that is,12aldehydes,3alcohols,3ketones and2-pentylfuran;21components were identified by SPME method,8aldehydes,7alcohols,2ketones,2acids,2-pentylfuran and naphthalene; however, only12compounds were isolated by purge&trap method,5alcohols,4aldehydes, acetic acid, hexyl acetate,2-pentylfuran and naphthalene. The above three methods were compared with each other. Finally, the SDE and SPME methods were used for the analysis of volatiles in Chinese steamed bread.Then fermentation characteristics and volatile components in steamed bread maked by Saccharomyces cerevisiae were studied. Hr-1with highest fermentation activity was screened from13isolated yeast strains. Changes of pH and TTA were more relatively flater than traditional sourdough samples; changes of glucose contents decreased during the fermentation process; SEM results showed that starch granules were damaged or deformed, while changes of protein were relatively significant, there were more filamentous structures than the dough without fermentation;13volatile compounds were identified by SDE method, most of components were aldehydes and ketones,15volatile compounds were identified by SPME method, aldehydes and alcohols were main compounds.