Comparison of microbial communities and physiochemical characteristics of two traditionally fermented vegetables

Investigating the core microbiota and its influencing factors in traditional Chinese pickles

Pickles are a type of traditional fermented food in Northeast China that exhibit a broad variety of preparations, flavors and microbial components. Despite their widespread consumption, the core microorganisms in various traditional pickles and the precise impact of ecological variables on the microbiota remains obscure. The present study aims to unravel the microbial diversity in different pickle types collected from household (12 samples) and industrial (10 samples) sources. Among these 22 samples tested, differences were observed in total acid, amino acid nitrogen, nitrite, and salt content. Firmicutes and Ascomycota emerged as the predominant microbial phyla as observed by Illumina MiSeq sequencing. Amongst these, the commonly encountered microorganisms were Lactobacillus, Weissella and yeast. Comparative analysis based on non-metric multidimensional scaling (NMDS), showed that the microbial community in the pickles was affected by external conditions such as major ingredients and manufacturing process. Correlation analysis further showed that the resident core microorganisms in pickles could adapt to the changing internal fermentation environment. The insights gained from this study further our understanding of traditional fermented foods and can be used to guide the isolation of excellent fermented strains.

Comparative Analysis of Traditional and Modern Fermentation for Xuecai and Correlations Between Volatile Flavor Compounds and Bacterial Community

Differences in flavor compounds and bacterial communities of Xuecai by traditional and modern fermentation are poorly understood. Allyl isothiocyanate (E9), ethyl acetate (E1), 3-butenenitrile (N1), phenol (P1), ethanol (A1), and 3-(2,6,6-trimethyl-1-cyclohexen-1-yl) acrylaldehyde (L11) were the main flavor compounds that differed between Xuecai produced by traditional and modern fermentation. Among these compounds, the contents of N1 and E9 were higher in modern fermentation Xuecai. Traditional fermentation Xuecai possessed higher contents of A1, P1, E1, and L11. High-throughput sequencing showed that Lactobacillus-related genera was the most abundant genus (50%) in modern fermentation Xuecai. However, in traditional fermentation Xuecai, Halanaerobium (29.06%) and Halomonas (12.96%) were the dominant genera. Halophilic bacteria (HB) positively contribute to the flavor of Xuecai. Carbohydrate metabolism and amino acid metabolism were the most abundant pathways associated with the bacterial communities of the Xuecai. This indicated that Xuecai flavor formation is mainly dependent on protein and carbohydrate degradation. This study provides a novel insight that HB may be important for flavor formation of Xuecai.

Biochemical changes and microbial community dynamics during spontaneous fermentation of Zhacai, a traditional pickled mustard tuber from China

Zhacai is a traditional fermented vegetable that has been consumed in China for centuries. It is currently manufactured by spontaneous fermentation and therefore mostly relies on the activities of autochthonous microorganisms. Here, we characterized microbial community dynamics and associated biochemical changes in 12% salted Zhacai during a 90-day spontaneous fermentation process using high-throughput sequencing and chromatography-based approaches to identify associations between microorganisms and fermentation characteristics. Amplicon sequencing targeting bacterial 16S rRNA genes revealed that bacterial communities were dominated by halophilic bacteria (HAB, i.e., Halomonas and Idiomarina) and lactic acid bacteria (LAB, i.e., Lactobacillus-related genera and Weissella) after 30 days of fermentation. In addition, the relative abundances of the fungal genera Debaryomyces, Sterigmatomyces, and Sporidiobolus increased as fermentation progressed. Concomitantly, pH decreased while titratable acidity increased during fermentation, along with associated variation in biochemical profiles. Overall, the levels of organic acids (i.e., lactic and acetic acid), free amino acids (i.e., alanine, lysine, and glutamic acid), and volatiles (i.e., alcohols, esters, aldehydes, and ketones) increased in mature Zhacai. In addition, the abundances of Lactobacillus-related species, Halomonas spp., Idiomarina loihiensis, as well as that of the yeast Debaryomyces hansenii, were strongly correlated with increased concentrations of organic acids, amino acids, biogenic amines, and volatiles. This study provides new detailed insights into the succession of microbial communities and their potential roles in Zhacai fermentation.

Role of lactic acid bacteria in flavor development in traditional Chinese fermented foods: A review

Traditional Chinese fermented foods are favored by consumers due to their unique flavor, texture and nutritional values. A large number of microorganisms participate in the process of fermentation, especially lactic acid bacteria (LAB), which are present in almost all fermented foods and contribute to flavor development. The formation process of flavor is complex and involves the biochemical conversion of various food components. It is very important to fully understand the conversion process to direct the flavor formation in foods. A comprehensive link between the LAB community and the flavor formation in traditional Chinese fermented foods is reviewed. The main mechanisms involved in the flavor formation dominated by LAB are carbohydrate metabolism, proteolysis and amino acid catabolism, and lipolysis and fatty acid metabolism. This review highlights some useful novel approaches for flavor enhancement, including the application of functional starter cultures and metabolic engineering, which may provide significant advances toward improving the flavor of fermented foods for a promising market.

Differences in the bacterial profiles and physicochemical between natural and inoculated fermentation of vegetables from Shanxi Province

Purpose

Fermented vegetables can be divided into two types, natural fermented and artificially inoculated fermented. By detecting and identifying the changes of bacterial diversity using physical and chemical indicators during natural and inoculation fermentation, we analyzed and determined the dominant bacteria in the fermentation process and revealed the relationship between bacteria and volatile substances.

Methods

We used the Illumina Miseq to sequence the bacteria in fermented vegetable samples at different fermentation periods, and calculated the total number of mesophilic microorganisms and lactic acid bacteria. We used the pH and nitrite to monitor the acidification process. GC-MS was used to determine volatile flavor compounds. Finally, we analyzed the correlation between volatile flavor compounds and bacteria.

Results

Total mesophilic microorganisms and the number of lactic acid bacteria in the inoculated fermentation were higher than the natural fermentation. The bacterial diversity Shannon and Simpson indexes of the natural fermentation, higher than those of inoculated fermentation in 0~7 days, were between 55~71% and 36~45%, respectively. On the 7th day, the proportion ofLactobacillusin the natural fermentation and inoculated fermentation were 53.4% and 90.2%, respectively, which were significantly different.Lactobacilluswas the dominant genus in the fermented vegetables and an important genus to promote the formation of volatile flavors.Lactobacilluswas negatively correlated with two volatile substances (4-[2,2,6-trimethyl-7-oxabicyclo [4.1.0] hept-1-yl]-3-Buten-2-one (K4) and a-Phellandrene (X1)) and played a leading role in the fermentation process.

Conclusions

Results demonstrated that the total number of mesophilic microorganisms and lactic acid bacteria in inoculated fermentation were more than those in natural fermentation. Inoculated fermentation can shorten the fermentation cycle and reduce the content of nitrite. Lactic acid bacteria were the dominant bacteria in fermented vegetables.

Microbial Community Dynamics and Metabolome Changes During Spontaneous Fermentation of Northeast Sauerkraut From Different Households

Sauerkraut, one of the most popular traditional fermented vegetable foods in northern China, has been widely consumed for thousands of years. In this study, the physicochemical characteristics, microbial composition and succession, and metabolome profile were elucidated during the fermentation of traditional northeast sauerkraut sampled from different households. The microbial community structure as determined by high-throughput sequencing (HTS) technology demonstrated that Firmicutes and Proteobacteria were the predominant phyla and Weissella was the most abundant genus in all samples. Except for Weissella, higher relative abundance of Clostridium was observed in #1 sauerkraut, Clostridium and Enterobacter in #2 sauerkraut, and Lactobacillus in #3 sauerkraut, respectively. Meanwhile, Principal component analysis (PCA) revealed significant variances in the volatilome profile among different homemade sauerkraut. Acids and lactones were dominant in the #1 sauerkraut. The #2 sauerkraut had significantly higher contents of alcohols, aldehydes, esters, sulfides, and free amino acids (FAAs). In comparison, higher contents of terpenes and nitriles were found in the #3 sauerkraut. Furthermore, the potential correlations between the microbiota and volatilome profile were explored based on Spearman’s correlation analysis. Positive correlations were found between Clostridium, Enterobacter, Lactobacillus, Leuconostoc, Weissella and most volatile compounds. Pseudomonas, Chloroplast, Rhizobium, Aureimonas, and Sphingomonas were negatively correlated with volatile compounds in sauerkraut. This study provided a comprehensive picture of the dynamics of microbiota and metabolites profile during the fermentation of different homemade northeast sauerkraut. The elucidation of correlation between microbiota and volatile compounds is helpful for guiding future improvement of the fermentation process and manufacturing high-quality sauerkraut.

Impact of fermentation conditions on the diversity of white colony-forming yeast and analysis of metabolite changes by white colony-forming yeast in kimchi

The presence of white colony-forming yeast (WCFY) on kimchi surfaces indicates a reduction in kimchi quality. This study aimed to investigate the effect of different fermentation temperatures (4, 10, and 20 °C) and packaging conditions (open or closed) on WCFY diversity, and the changes of metabolite by the difference of WCFY diversity. Community analysis using high-throughput DNA sequencing revealed that Kazachstania servazzii and K. barnettii were most prevalent in kimchi fermented under closed packaging condition at 4, 10, and 20 °C. In open packaging condition, four species of Candida sake, K. servazzii, K. barnettii, and Tausonia pullulans were the predominant yeast species at 4 °C, and four species of C. sake, K. servazzii, K. barnettii, and Debaryomyces hancenii were predominantly detected at 10 °C. The diversity of the WCFY community was higher under the open rather than the closed packaging condition. However, at all fermentation temperatures, non-volatile metabolite production by the different WCFY communities did not significantly differ between open and closed packaging conditions, whereas glycerol levels in kimchi samples harboring WCFY increased relative to the control (0 day). These results indicate that fermentation temperature and air exposure can alter WCFY diversity on kimchi surface, however, non-volatile metabolite profiles in kimchi soup are not significantly affected by the difference of WCFY diversity caused by packaging conditions. This study furthers the current understanding of the growth of undesirable WCFY in kimchi.