Shanxi aged vinegar (SAV) is a well-known vinegar produced by traditional solid-state fermentation and has been used in China for thousands of years. However, how microorganisms and their metabolites change along with fermentation is unclear. Here, 454 high-throughput sequencing and denaturing gradient gel electrophoresis were used to investigate the composition of microbial community. Metabolites were further analyzed by gas chromatography–mass spectrometry and high–performance liquid chromatography. Results showed that the composition of bacterial community changed dramatically at different stages of fermentation. The bacterial genera (relative abundance > 0.1%) decreased from 17 in daqu (starter used in starch saccharification) to 2 at the 12th day of alcohol fernemtation (AF). 15 bacterial genera at the 1st day of acetic acid fermentation (AAF) decreased to 4 genera, involving Acetobacter (50.9%), Lactobacillus (47.9%), Komagataeibacter (formerly Gluconacetobacter, 0.7%) and Propionibacterium (0.1%) at the 7th day of AAF. The structure of fungal community was more homogeneous. Saccharomyces and Saccharomycopsis were predominant in AF and AAF. A total of 87 kinds of nonvolatile metabolites were detected. Canonical correspondence analysis showed a significant correlation between the microbiota succession and the formation of metabolites during the fermentation of SAV. This study provides detailed information for the fermentation mechanism of traditional SAV.

Edible fungi fermentation is an effective strategy for enhancing the functional value of traditional foods, while postbiotics offer multiple health benefit

The Daqu used in the alcoholic fermentation of Shanxi aged vinegar (SAV) is technologically related to the medium-temperature Daqu used for Baijiu production, but variation in vinegar Daqu quality can lead to unstable saccharification, alcoholic fermentation and aroma formation. We therefore hypothesized that functional microorganisms selected from Light-Flavor Baijiu Daqu could be combined with isolates from vinegar fermentation to improve the alcoholic fermentation stage of SAV, and that a defined consortium would provide a controllable system in which to test this hypothesis. Twenty culturable microorganisms were isolated from Daqu and Jiupei (fermented grains). Six were preselected on the basis of taxonomic identity, fermentation-related function and intended functional complementarity, namely Saccharomyces cerevisiae, Pichia kudriavzevii, Saccharomycopsis fibuligera, Lactiplantibacillus plantarum, Lentilactobacillus buchneri and Ligilactobacillus amylovorus; their growth, stress tolerance, ethanol production and acid production were then characterized to assess their suitability for consortium construction. Headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC-MS) was used to profile the volatile compounds produced during fermentation of the resulting synthetic microbial consortium (SMC) in a liquid sorghum-juice medium and to compare them with those of industrial alcoholic fermentation (IAF) of fermented grains. All experiments used three independent biological replicates, and concentrations are reported as semi-quantitative 2-octanol equivalents. Several major volatile compounds accumulated earlier in the SMC than in the IAF system: ethanol (0.741 mg/mL) and ethyl acetate (0.331 mg/mL) both peaked on day 3, and the day-3 ethyl acetate concentration in the SMC already exceeded the maximum value reached in the IAF system on day 7 (0.238 mg/mL). The SMC nevertheless produced fewer volatile compounds (30) than the IAF system (45) and lacked long-chain fatty acid ethyl esters, whereas multivariate analysis indicated that its volatile profile became progressively more uniform during the mid-to-late fermentation stages. Because the two systems differ in substrate composition, physical structure and mass transfer, these findings should not be read as evidence that the SMC outperforms industrial solid-state fermentation. Within that constraint, the SMC reproduced selected metabolic features of SAV alcoholic fermentation in a liquid model system and reached maximum concentrations of several major volatile compounds earlier than the industrial system examined, offering a starting point for the rational design of defined starter consortia for traditional vinegar production.