| 英文摘要 |
Laomian, also known as sourdough, is made from wheat flour, bran, corn flour or rice flour, which fermented by backslopping and long-term spontaneous. As a complex microbial system, laomian has a great influence on the flavor of fermented food based on the activities of LAB, yeast and few mould. The flavor is mainly composed of odor and taste, and the generation of compounds is closely related to the microbiota. The previous studies indicated that sourdough improved the flavor characteristics of Chinese steamed bread. However, there is a lack of systematic research and overall understanding on the composition of flavor characteristics of CSB fermented with laomian. Shandong sourdough and Henan jiaozi are the representatives of laomian, and the significant differences are found in the physiological and biochemical characteristics due to different processing patterns. It is also unknown whether there is a difference in the composition of microbiota between Shandong sourdough and Henan jiaozi. Further, the relationship between the flavor of products and the microbiota diversity of these two starters has not been reported. In this work, we screened the flavor characteristic compounds of steamed bread fermented with Shandong sourdough and Henan jiaozi, respectively, and constructed the relationship between the microbiota and the flavor compounds of steamed bread; the metabolism of characteristic flavor compounds was also invetigated by reconstructing the core flora of laomian. The main results of this paper are as followed: 1.To determine the flavor characteristic compounds of steamed bread fermented with laomian, 15 dough starters were collected from Shandong and Henan to investigate the volatile and non-volatile compounds of CSBs. The volatile characteristic compounds of CSB fermented with Shandong sourdough were mainly composed of ethyl acetate, hexyl acetate, trans-2-nonenal, trans-octenal, hexanal and 2-pentyl furan as compared to the CSBs fermented with baker's yeast. Similarity, the volatile characteristic compounds of products in Henan's group were composed of ethyl acetate and hexyl acetate. However, there was no significant difference in taste between the two types of steamed bread fermented with sourdough, and the non-volatile characteristic compounds were mainly composed of 20 kinds of flavor substances, including histidine, leucine, gamma-glutamate, gamma-glutamate, cysteine, citric acid and lactic acid. 2.To explore the microbial flora of laomian and the relationship between the microbiota and the flavor compounds of the CSB fermented with soudough, traditional separation technology combined with high-throughput sequencing method were used to analyze the microbial diversity of two tpyes of sourdough. The results showed that the bacterial species of Shandong sourdough were mainly consisted of Lactobacillus sanfranciscensis, Weissella cibaria and a small amount of Lactobacillus fermentum. Similarity, the bacterial species of Henan jiaozi were mainly composed of W. cibaria, Lactobacillus plantarum and Pediococcus pentosace. Some samples also contained a small amount of Lactobacillus pontis, Lactobacillus paralimentarius, Lactobacillus zymae and Lactobacillus helveticus. Saccharomyces and Candida were the main predominant genus in the two tpyes of sourdough, and there was a small amount of Aspergillus and Penicillium in the Henan jiaozi. No significant difference in yeast composition was found among the samples, which were mainly including S. cerevisiae and Candida, and some samples contained few abnormal Wicherhamomyces. The correlation analysis between the microbiota of sourdough and the flavor compounds of CSB fermented with sourdough showed that microbiota had a positive correlation with volatile compounds, but had no significant correlation with non-volatile taste compounds. 3.To evaluate the formation mechanism of volatile characteristic compounds in CSB fermented with sourdough, the relationship between the core species of laomian and the aroma compounds were constructed. The results showed that L. sanfranciscensis and L. fermentum had higher positive correlation coefficients with acetic acid, pentanal, hexanal, trans-2-nonenal, nonanal, trans-2-octenenal, trans-2, 4-decanedienal, 2-pentyl furan, nonanol and l-octen-3-ol. And Saccharomyces cerevisiae had higher positive correlation coefficient with hexanal, while no significant correlation was found between W. cibaria and C. species and volatile characteristic compounds. The sourdough was co-fermented with L. sanfranciscensis, L. fermentum and S. cerevisiae for 5 days, and the overall aroma characteristic of CSB were similar to those of the CSB fermented with Shandong sourdough, and the characteristic volatiles content was basically similar to those of Shandong's group. Based on the metabolomics analysis, it was also found that linoleic acid and palmitoleic acid had a significantly down-regulated in the processing of sourdough steamed bread, and sourdough enhanced the oxidation degree of unsaturated fatty acids, thus increasing the yield of aldehydes and 2- pentyl furans. 4.γ-glutamyl dipeptide, as the characteristic taste compounds of steamed bread, plays an important role in inproving the overall flavor characteristics of CSB. In order to explore the metabolic mechanism of γ-glutamyl dipeptide in LAB, the mutant strain Lactobacillus reuteri LTH5448 was constructed to evaluate the effect of glutamyl-cysteine ligase on the synthesis of γ-glutamyl dipeptide. According to the sequence comparison in the database, most of the glutamyl cysteine ligases were found in the heterofermentation lactobacilli, while only a few were found in the homofermentation lactobacilli, and L. reuteri LTH5448 mainly contained two glutamyl-cysteine ligase coding genes, gcl1 and gcl2. The L. reuteri LTH5448 mutant strain was constructed by double crossover homologous recombination technology. Sourdough fermented with L. reuteri LTH5448 mutant strain showed that gcl1 preferentially selected isoleucine for the regulation of γ-glutamyl isoleucine synthesis, while gel2 preferentially selected cysteine for the regulation of γ-glutamylcysteine synthesis, gcl did not regulate the metabolism and synthesis of most γ-glutamyl dipeptides. The synthesis of γ-glutamyl dipeptides was the result of the interaction between endogenous protease and microbial enzyme. The taste comparison of CSB fermented with wild type strain and mutant strain indicated that γ-glutamyl dipeptide had an important effect on the saltiness and umami of products. Keywords: Sourdough; Flavor; Microbiota; Aroma; Taste;
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