Insight into the autochthonous lactic acid bacteria as starter culture for improving the quality of Sichuan radish paocai: Changes in microbial diversity and metabolic profiles

Paocai is a traditional Chinese fermented vegetable product popular in Asian countries. Recently, functional starters were used to control the fermentation process and improve the quality of paocai. In this study, three autochthonous lactic acid bacteria including Lactiplantibacillus plantarum LB6, Lactiplantibacillus pentosus LB3, and Weissella cibaria W51 were selected as starters and the effect of the starters on the fermentation of paocai was investigated. The results suggested that the inoculated fermentation led to a lower nitrite peak and more pronounced changes in pH and total titratable acid in the early stage of fermentation, compared with natural fermentation. Analysis of the flavor compounds indicated that the total content of volatile organic compounds of paocai through natural fermentation was significantly lower than that in inoculated fermentation. As for free amino acids, in the early stage of fermentation, the types and contents of free amino acids in the inoculated fermentation paocai were higher than those in the blank group. In the later stage of fermentation, the contents of amino acids representing umami and sweet tastes were also higher than those in the blank group. The bacterial community analysis showed that Lactobacillus and Lactococcus were the dominant bacteria in both inoculated fermentation and natural fermentation. Then, the correlations among physicochemical properties, microbial community and flavor compounds were revealed, and it was found that the dominant bacteria such as Lactococcus, Leuconostoc, Lactobacillus and Weissella displayed a considerable impact on the physical and chemical properties and flavor of paocai. In addition, the metabolic pathways involved in flavor formation and the abundance of related enzymes were elucidated. The abundance of enzymes involved in generating prephenic acid, 2-methylbutanoic acid, L-lactic acid, D-lactic acid, butanoic acid, etc., and in the pathway of producing flavor substances (His, Met, ethyl hexanoate, etc.) was up-regulated in the inoculated fermentation. Results presented in this study may provide a reference for the development of paocai starters and further guidance for the flavor improvement of Sichuan paocai.