In this study, the correlations between microbial communities with physicochemical properties and volatile flavor compounds (VFCs) during the fermentation of traditional tomato sour soup (CTN) are explored. The results of high-throughput sequencing (HTS) of CTN showed that Lacticaseibacillus (28.67%), Enterobacter (12.37%), and Providencia (12.19%) were the dominant bacteria in the first round of fermentation, while Lacticaseibacillus (50.11%), Enterobacter (13.86%), and Providencia (8.61%) were the dominant bacteria in the second round of fermentation. Additionally, the dominant fungi genera of the first fermentation were Pichia (65.89%) and Geotrichum (30.56%), and the dominant fungi genera of the second fermentation were Pichia (73.68%), Geotrichum (13.99%), and Brettanomyces (5.15%). These results indicate that Lacticaseibacillus is one of the main dominant bacteria in CTN. Then, the dominant strain Lacticaseibacillus casei H1 isolated from CTN was used as a culture to ferment tomato sour soup to monitor dynamic changes in the physicochemical properties and VFCs during enhanced fermentation of tomato sour soup (TN). The physicochemical analysis showed that, compared with CTN, the TN group not only produced acid faster but also had an earlier peak of nitrite and a lower height. The results of the GC–IMS analysis showed that the ester and alcohol contents in the TN group were 1.26 times and 1.8 times that of the CTN group, respectively. Using an O2PLS-DA analysis, 11 bacterial genera and 18 fungal genera were identified as the functional core flora of the CTN group flavor production, further verifying the importance of dominant bacteria for the production of VFCs. This study proved that enhanced fermentation not only shortens the fermentation cycle of tomato sour soup, but also significantly improves its flavor quality, which has great value in the industrial production of tomato sour soup and in the development of a vegetable fermentation starter.

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The effect of Lacticaseibacillus casei H1 inoculation fermentation on the quality characteristics and metabolites of tomato sour soup was evaluated to ascertain determine the substances that improve the quality of tomato sour soup. The trends of the titratable acid, organic acid, and fatty acid contents showed that the inoculation of L. casei H1 accelerates the fermentation process. The sensory evaluation indicated that the tomato sour soup fermented by L. casei H1 was more acceptable than the soup from the spontaneously fermented soup. Further analysis of different metabolites and metabolic pathways found that inoculation of L. casei H1 produced more natural antibacterial substances (hydroxyisocaproic acid), essential polyunsaturated fatty acids (linoleic acid, all cis [6, 9, 12]-linolenic acid), and umami amino acids (L-arginine) (p < .05). Therefore, inoculation with L. casei H1 helped to improve the quality of tomato sour soup. These results provide data support for the use of L. casei H1 as an excellent starter for tomato fermentation. Novelty impact statement:• Differences of metabolic composition profiles in spontaneous and inoculation fermentation of tomato sour soup were explored.• Data support was provided for the use of Lacticaseibacillus casei H1 as an excellent starter for tomato sour soup.• The advantages of inoculating the dominant L. casei H1-fermented tomato sour soup were revealed by the differential metabolites and pathway analysis.