Analysis of microbial community and the characterization of Aspergillus flavus in Liuyang Douchi during fermentation

Studying the release of soybean taste-active peptides prepared by proteases derived from different fungi through Peptidomics

This study investigated the release of taste-active peptides from soybean isolate catalyzed by proteases from Aspergillus flavus strains. The frequent presence of Leu at the C-terminus and the hydrophilic amino acids at the N-terminus related to bitterness and umami taste of hydrolysates, respectively. Detailly, H40650 demonstrating high umami, low bitterness, and a high peptide yield. During the hydrolysis of H40650, umami taste changes were associated with the content of peptides <1 kDa and 28 umami peptides, while bitterness was linked to the content of peptides <3 kDa and 30 bitter peptides. Additionally, most taste-active peptides were derived from the 7S globulin α subunit and 11S globulin G2. The protease produced by CGMCC 40650 has marked specificity for the region of 281-333 amino acids in protein P0D15, which contained 24.53 % umami amino acids. These findings could offer new insights into the preparation of taste-active peptides.

Exploring the Core Functional Microbiota Related to Flavor Compounds in Douchi from the Sichuan-Chongqing Region

Douchi is a traditional Chinese fermented soybean product. In the Sichuan-Chongqing region, Mucor-type douchi was particularly famous for its distinctive flavor. Nevertheless, the association between microorganisms and douchi flavor is still poorly understood. In this study, high-throughput sequencing, amino acid analysis, and gas chromatography-mass spectrometry (GC-MS) were used to investigate the bacterial and fungal profiles as well as the flavor compounds (sixteen amino acids and one-hundred volatile flavor compounds) of seven different types of douchi. High levels of glutamic and aspartic acids were observed. Microbial analysis found that Bacillus, Tetragenococcus, Weissella, Aspergillus, Mucor, and Penicillium were the prime microorganisms. In total, 100 volatile components were detected; however, none of them was common to all the douchi products, although most volatile components had the aromas of flowers, fruits, caramel, and cocoa. An analysis of the flavor compounds was conducted using two-way orthogonal partial least-squares discriminant analysis (O2PLS-DA). Based on the analysis, it was found that Glu had negative correlations with most microorganisms, and Aspergillus had positive correlations with 2-pentylfuran and phenylacetaldehyde. This study provides a theoretical foundation for the regulation and enhancement of douchi flavor.

Analysis of differential metabolites in Liuyang douchi at different fermentation stages based on untargeted metabolomics approach

The quality and flavor of Liuyang Douchi are usually closely related to the metabolites compostion. This work described the metabolic profiles of Liuyang douchi during fermentation. Obvious hydrolysis of carbohydrates, proteins and slight lipids degradation were observed. Notably, the qu-making and pile-fermentation stage of douchi could be easily distinguished according to their metabolites profile, and pile-fermentation stage showed the most abundant metabolites. Specifically, organic acid, such as succinic acid and lactic acid, accumulated during pile-fermentation, as well as amino acids and derivatives. Especially glutamate (Glu), which contributed to the umami taste, increased form 0.82 mg/g to 15.90 mg/g after fermentation. Meanwhile, metabolisms related to amino acids were also the main enrichment metabolic pathways. Among them, some flavor compunds such as phenylacetaldehyde might drived from phenylalanine metabolism. These results could provide a new understanding on the metabolic characteristics during Liuyang douchi fermentation.

Microbiology of fermented soy foods in Asia: Can we learn lessons for production of plant cheese analogues?

The food industry is facing the challenge of creating innovative, nutritious, and flavored plant-based products, due to consumer's increasing demand for the health and environmental sustainability. Fermentation as a unique and effective tool plays an important role in the innovation of food products. Traditional fermented soy foods are popular in many Asian and African countries as nutritious, digestible and flavorful daily staples or condiments. They are produced by specific microorganisms with the unique fermentation process in which microorganisms convert the ingredients of whole soybean or soybean curd to flavorful and functional molecules. This review provides an overview on traditional fermented food produced from soy, including douchi, natto, tempeh, and sufu as well as stinky tofu, including the background of these products, the manufacturing process, and the microbial diversity involved in fermentation procedures as well as flavor volatiles that were identified in the final products. The contribution of microbes to the quality of these five fermented soy foods is discussed, with the comparison to the role of cheese ripening microorganisms in cheese flavor formation. This communication aims to summarize the microbiology of fermented soy foods in Asia, evoking innovative ideas for the development of new plant-based fermented foods especially plant-based cheese analogues.

Succession and Diversity of Microbial Flora during the Fermentation of Douchi and Their Effects on the Formation of Characteristic Aroma

This study aims to understand the development and succession of the microbial community during the production of traditional Aspergillus-type Douchi as well as their effects on the formation and variation of characteristic aroma compounds. High-throughput sequencing technology, solid-phase microextraction, gas chromatography-mass spectrometry, and Spearman correlation analysis were conducted to study the changes in the microbial community and characteristic flavor during the fermentation process. Aspergillus spp. was dominant in the early stage of fermentation, whereas Staphylococcus spp., Bacillus spp., and Millerozyma spp. became dominant later. At the early stage, the main flavor compounds were characteristic soy-derived alcohols and aldehydes, mainly 1-hexanol, 1-octen-3-ol, and nonanal. In the later stage, phenol, 2-methoxy-, and 3-octanone were formed. Correlation analysis showed that six bacterial genera and nine fungal genera were significantly correlated with the main volatile components, with higher correlation coefficients, occurring on fungi rather than bacteria. Alcohols and aldehydes were highly correlated with the relative abundance of bacteria, while that of yeast species such as Millerozyma spp., Kodamaea spp., and Candida spp. was positively correlated with decanal, 3-octanol, 2-methoxy-phenol, 4-ethyl-phenol, 3-octanone, and phenol. The novelty of this work lies in the molds that were dominant in the pre-fermentation stage, whereas the yeasts increased rapidly in the post-fermentation stage. This change was also an important reason for the formation of the special flavor of Douchi. Correlation analysis of fungi and flavor substances was more relevant than that of bacteria. As a foundation of our future focus, this work will potentially lead to improved quality of Douchi and shortening the production cycle by enriching the abundance of key microbes.

Rise and metabolic roles of Vibrio during the fermentation of crab paste

Microbial community may systematically promote the development of fermentation process of foods. Traditional fermentation is a spontaneous natural process that determines a unique nutritional characteristic of crab paste of Portunus trituberculatus, However, rare information is available regarding the development pattern and metabolic role of bacterial community during the fermentation of crab paste. Here, using a 16S rRNA gene amplicon sequencing technology, we investigated dynamics of bacterial community and its relationship with metabolites during the fermentation of crab paste. The results showed that bacterial community changed dynamically with the fermentation of crab paste which highlighted by consistently decreased α-diversity and overwhelming dominance of Vibrio at the later days of fermentation. Vibrio had a positive correlation with trimethylamine, hypoxanthine, formate, and alanine while a negative correlation with inosine and adenosine diphosphate. In contrast, most of other bacterial indicators had a reverse correlation with these metabolites. Moreover, Vibrio presented an improved function potential in the formation of the significantly increased metabolites. These findings demonstrate that the inexorable rise of Vibrio not only drives the indicator OTUs turnover in the bacterial community but also has incriminated the quality of crab paste from fresh to perished.

Unraveling the effects of sulfamethoxazole on the composition of gut microbiota and immune responses in Stichopus variegatus

The aim of this work was to reveal the changes in gut microbiota composition and immune responses of sea cucumber (Stichopus variegatus) after being affected by different doses of sulfamethoxazole. In this study, the bacterial 16S rRNA of gut microbiota were analyzed by high-throughput sequencing, and the activities of immune enzymes [lysozyme (LZM), phenoloxidase (PO), alkaline phosphatase (AKP), and acid phosphatase (ACP)] in the gut of S. variegatus were determined. The results showed that the gut microbiota presented a lower richness in the antibiotic treatment groups compared with the control group, and there were significant differences among the dominant bacteria of different concentration treatments. At the genus level, the abundance of Escherichia, Exiguobacterium, Acinetobacter, Pseudomonas, and Thalassotalea were significantly decreased in the 3 mg/L treatment group, while Vibrio was significantly increased. Furthermore, the 6 mg/L treatment group had less effect on these intestinal dominant bacteria, especially Vibrio. The changes in relative abundance of Vibrio at the species level indicated that lower concentrations of sulfamethoxazole could enhance the enrichment of Vibrio mediterranei and Vibrio fortis in S. variegatus more than higher concentrations of sulfamethoxazole. Meanwhile, the 3 mg/L treatment group significantly increased the activities of PO, AKP, and ACP, and decreased the activity of LZM. These results suggested that lower doses of sulfamethoxazole have a greater effect on the gut microbiota composition and immune responses in S. variegatus and may increase the risk of host infection.

Microbial succession and its correlation with the dynamics of volatile compounds involved in fermented minced peppers

Fermented minced peppers are a traditional fermented food that has a unique flavor due to various microbial communities involved in fermentation. Understanding the changes in microbial communities and volatile components of fermented minced peppers is particularly important to unveil the formation of unique flavor of fermented peppers. In this study, the microbial communities and volatile compounds in fermented minced pepper was analyzed by high-throughput sequencing and GC-MS, as well as their underlying correlations were also established. Results indicated that 17 genera were identified as dominant microorganisms in the fermentation of minced pepper, accompanied by the detection of 64 volatile compounds. Further hierarchical clustering analysis (HCA) displayed that dynamic change of volatile metabolites were involved in the fermentation process, where alkane volatile components were mainly generated in the early stage (3-5 days), and alcohols volatile components were in the middle stage (7-17 days), while ester volatile components were mainly produced in both the early stage (3-5 days) and last stage (17-20 days). Bidirectional orthogonal partial least squares (O2PLS) analysis revealed that 11 genera were core functional microorganisms of fermented minced pepper. Cladosporium and Hansenpora were significantly correlated with the formation of 9 and 6 volatiles, respectively. These findings provide new insights into aroma profile variation of fermented minced peppers and underlying mechanism of characteristic aroma formation during fermentation.