Combined effects of fermentation starters and environmental factors on the microbial community assembly and flavor formation of Zhenjiang aromatic vinegar.

Microbial ecosystems of fermented foods are largely interfered by human activities in myriad ways. The aim of this study was to illuminate the impacts of various starters and environmental variables on the fermentation process of Zhenjiang aromatic vinegar (ZAV), one of the four representative cereal vinegars in China. The effects of environmental variables (e.g., ethanol, total acidity, temperature) and starters (e.g., jiuqu, maiqu, seed pei) on the profiles of microbiome and metabolome (e.g., organic acids, amino acids and volatiles) during fermentation process of ZAV were analyzed. Amongst the four fermentation stages, acetic acid fermentation was the main stage for the accumulation of flavor substances, and subsequently, the contents of acids (mainly acetic, lactic and citric acids) and volatile metabolites (e.g., 2,3-butanedione, acetoin, etc.) continued to enrich in sealed fermentation stage. Principal coordinate analysis (PCoA) and analysis of similarities (ANOSIM) showed that the fungal and bacterial community structures of four fermentation stages were significantly different. As for bacterial community, the dominant OTUs with average relative abundance over 10% in at least one fermentation stage were assigned to the genera Acetilactobacillus, Acetobacter, Acinetobacter, Aeromonas, Lactobacillus, and Pseudomonas. The dominant fungal populations in each fermentation stage were obviously divergent, including Wickerhamomyces, Saccharomyces, Alternaria, Fusarium, etc. SourceTracker analysis demonstrated that jiuqu and seed pei provided microorganisms to initiate starch saccharification and acetic acid fermentation stages, respectively, and maiqu was mainly the donor of enzymes in alcohol fermentation. Spearman correlation coefficients revealed positive relationships between fungal community and various flavor metabolites, indicating the essential role of fungi in the flavor formation of ZAV. This study systematically reveals the effects of fermentation starters and environmental variables on vinegar production and deepens the understanding of the traditional production craft.

Constructing a Defined Starter for Multispecies Vinegar Fermentation via Evaluation of the Vitality and Dominance of Functional Microbes in an Autochthonous Starter.

Mature vinegar culture has usually been used as a type of autochthonous starter to rapidly initiate the next batch of acetic acid fermentation (AAF) and maintain the batch-to-batch uniformity of AAF in the production of traditional cereal vinegar. However, the vitality and dominance of functional microbes in autochthonous starters remain unclear, which hinders further improvement of fermentation yield and production. Here, based on metagenomic (MG), metatranscriptomic (MT), and 16S rRNA gene sequencings, 11 bacterial operational taxonomic units (OTUs) with significant metabolic activity (MT/MG ratio >1) and dominance (relative abundance >1%) were targeted in the autochthonous vinegar starter, all of which were assigned to 4 species (Acetobacter pasteurianus, Lactobacillus acetotolerans, L. helveticus, Acetilactobacillus jinshanensis). Then, we evaluated the successions and interactions of these 11 bacterial OTUs at different AAF stages. Last, a defined starter was constructed with 4 core species isolated from the autochthonous starter (A. pasteurianus, L. acetotolerans, L. helveticus, Ac. jinshanensis). The defined starter culture could rapidly initiate the AAF in a sterile or unsterilized environment, and similar dynamics of metabolites (ethanol, titratable acidity, acetic acid, lactic acid, and volatile compounds) and environmental indexes (temperature, pH) of fermentation were observed as compared with that of autochthonous starter (P > 0.05). This work provides a method to construct a defined microbiota from a complex system while preserving its metabolic function. IMPORTANCE Complex microorganisms are beneficial to the flavor formation in natural food fermentation, but they also pose challenges to the mass production of standardized products. It is attractive to construct a defined starter to rapidly initiate fermentation process and significantly improve fermentation yield. This study provides a comprehensive understanding of vital and dominant species in the autochthonous vinegar starter via multi-omics, and designs a defined microbial community for the efficient fermentation of cereal vinegar.

Komagataeibacter europaeus improves community stability and function in solid-state cereal vinegar fermentation ecosystem: Non-abundant species plays important role.

Solid-state fermentation of Chinese traditional cereal vinegar is a complex and retractable ecosystem with multi-species involved, including few abundant and many non-abundant species. However, the roles of non-abundant species in vinegar fermentation remain unknown. Here, we studied the assembly and co-occurrence patterns for abundant and non-abundant bacterial sub-communities using Zhenjiang aromatic vinegar fermentation as a model system. Our results showed that the change of reducing sugar and total titratable acid were the main driving forces for the assembly of abundant and non-abundant sub-communities, respectively. The non-abundant sub-community was more sensitive to the environmental variation of acetic acid fermentation (AAF) process. Integrated co-occurrence network revealed that non-abundant sub-communities occupied most of the nodes in the network, which play fundamental roles in network stability. Importantly, non-abundant species-Komagataeibacter europaeus, showed the highest value of degree in the co-occurrence network, implying its importance for the metabolic function and resilience of the microbial community. Bioaugmentation of K. europaeus JNP1 verified that it can effectively modulate bacterial composition and improve the robustness of co-occurrence network in situ, accompanied by (i) increased acetic acid content (14.78%) and decreased reducing sugar content (40.38%); and (ii) increased the gene numbers of phosphogluconate dehydratase (212.24%) and aldehyde dehydrogenase (192.31%). Overall, the results showed that non-abundant bacteria could be used to regulate the desired metabolic function of the community, and might play an important ecological significance in traditional fermented foods.

Effect of Lactobacillus plantarum enriched with organic/inorganic selenium on the quality and microbial communities of fermented pickles.

Lactobacillus enriched with organic/inorganic selenium and pickles fermented with the Lactobacillus plantarum R were prepared. The results showed that selenium-enriched Lactobacillus plantarum R enhanced the antioxidant capacity, inhibition rate of advanced glycation end-products (AGEs), nitrite degradation, and the organic acid production of fermented pickles, while Lactobacillus plantarum R enriched with inorganic selenium (R-Se-IN) showed the best performance. Twenty-three aroma-active substances and seven characteristic compounds were detected in the R-Se-IN group. Moreover, the bacterial community result revealed that Lactococcus, Lactobacillus, and Leuconostoc were predominant in the R-Se-IN group, while the other groups contained Enterobacter, Halomonas, and Klebsiella. Furthermore, the correlations between environmental factors, differential flavor substances, and microbial communities were explored based on multivariate statistical analysis. These results indicate that the addition of Lactobacillus plantarum R enriched with organic/inorganic selenium influenced the environmental factors, differential flavor substances, and microbial communities of the fermented pickles.