Metagenomics reveals the formation mechanism of flavor metabolites during the spontaneous fermentation of potherb mustard (Brassica juncea var. multiceps)

Enhanced acid reduction in lactic acid bacteria: Breeding through irradiation-induced mutation and functional assessment

High concentrations of citric acid (CA), malic acid (MA), and tartaric acids (TA) are the primary contributors to the sour taste of fruit and fruit products. However, lactic acid bacteria that are capable of efficiently degrading these organic acids are scarce. Here, three brands of sauerkraut (Xinxi, X; Yuyuan, Y; and Zou Youcai, Z) with various doses of 60Co γ-irradiation could be treated to induce mutations in their associated lactic acid bacteria and then the abilities of the resulting microbial communities to degrade CA, MA, and TA were evaluated. Sauerkraut X treated with 0.4 kGy irradiation demonstrated the greatest ability of acid reduction. Metagenomic analyses of irradiated (0.4 kGy) and non-irradiated bacterial communities from sauerkraut X revealed a slight decrease in microbial diversity due to irradiation, with a substantial decline in the relative abundance of Lactiplantibacillus xiangfangensis. Concurrently, the relative abundance of dominant acid-reducing lactic acid bacteria such as Levilactobacillus brevis, Pediococcus ethanolidurans, and Lentilactobacillus parafarraginis increased. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed an increase in metabolism-related genes after irradiation, indicating that fatty acid synthesis and aspartate metabolism might be key pathways involved in the enhanced degradation of CA, MA, and TA. Analysis using the Carbohydrate-Active enzymes Database (CAZy) database revealed that glycoside hydrolase (GH) and glycosyltransferase (GT) genes were the most abundant carbohydrate-associated enzyme genes in the bacterial community of sauerkraut X. This finding proved that the oligosaccharides and monosaccharides produced by GH and GT might indirectly affect rates of organic acid degradation. Three highly effective acid-reducing lactic acid bacteria from the microbial community of irradiated sauerkraut X a were isolated and identified via 16S rRNA sequencing as Pediococcus ethanolidurans, Levilactobacillus brevis, and Loigolactobacillus coryniformis. The individual strains showed degradation rates as high as 92.02 % for citric acid (Pediococcus ethanolidurans), 83.04 % for malic acid (Levilactobacillus brevis), and 90.33 % for TA (Loigolactobacillus coryniformis). This study provides a theoretical basis and technical support for the development of enhanced microbial strains that can reduce the acid content of fruit materials.

Integration of volatilomics and microbiome diversity reveals key flavor-related metabolic pathways in semi-dried large yellow croaker (Pseudosciaena crocea)

A complex microbial community is critical for developing unique flavors in semi-dried large yellow croaker (Pseudosciaena crocea). Volatilomics analysis identified hexanal, heptanal, nonanal, phenylacetaldehyde, 1-octen-3-ol, and butanoic acid were identified as the key flavor compounds in the fish. Clostridium sensu stricto was the dominant genus, with a relative abundance of 79.78 % after 4 days of air-drying. Validation results showed a positive association between the accumulation of nonanal, phenylacetaldehyde, and butanoic acid and the presence of Clostridium sensu stricto. Significant correlations were also observed between the genera of Lactobacillus and Microbacterium and the key flavor compounds of hexanal and heptanal. Microorganisms contribute to the metabolism of these compounds, primarily through the metabolism of phenylalanine, linoleic acid, linolenic acid, arachidonic acid, and pyruvate. This flavor-regulating role of microorganisms presents them as potential targets for flavor enhancement in traditional aquatic products.

Dynamics of microbial communities and metabolites during the fermentation of Ningxia goji berry wine: An integrated metagenomics and metabolomics approach

Ningxia Goji Berry Wine (NGBW), a traditional Chinese fermented beverage, exhibits complex flavor quality changes during fermentation, the mechanisms of which remain insufficiently elucidated. This study aimed to elucidate the dynamic shifts in physicochemical properties, metabolites, and microbial communities throughout the controlled fermentation process of NGBW. Metabolomic analysis identified 8 key differential volatile metabolites (VOCs) and 406 differential non-volatile metabolites. The enrichment analysis of KEGG metabolic pathways revealed that, during the fermentation of NGBW, ten critical metabolic pathways-Purine metabolism, Glycine, Serine, and Threonine metabolism, Galactose metabolism, and the Citric Acid (TCA) Cycle-play essential roles. Amplicon sequencing indicated that 25 bacterial genera dominated the microbial ecosystem (relative abundance ≥ 0.1 %). Spearman correlation analysis revealed significant associations between 5 core microorganism and flavor compounds, and 25 core microbes with non-volatile metabolites, suggesting their pivotal roles in flavor formation. This study provides a theoretical basis for optimizing the fermentation process and enhancing the flavor quality of NGBW.

Screening of strains from pickles and evaluation of characteristics of different methods of fast and low salt fermented mustard leaves (Brassica juncea var. multiceps)

The aim of this study was to isolate strains with excellent fermentation performance from pickles, thus enhancing the quality of rapid, low-salt fermented mustard leaves (Brassica juncea var. multiceps) through process optimization and inoculation fermentation. A high-throughput screening method for acid-producing strains was developed, significantly improving screening efficiency. Lactiplantibacillus plantarum CS8 and Saccharomyces cerevisiae CX1, were selected for their superior fermentation performance and used in subsequent fermentation. Four fermentation methods (spontaneous fermentation, optimized spontaneous fermentation, co-fermentation, and two-phase fermentation) were compared for fermenting fresh mustard leaves at 30 °C for 5 days. Compared to spontaneous fermentation, the other methods resulted in lower pH, higher acid production, and reduced nitrite content, thereby enhancing food safety. Significant variations in metabolites (volatiles, organic acids, and free amino acids) were observed among the groups, with the two-phase fermentation method showing the most favorable changes. Sensory evaluation and microbial community analysis further indicated that the two-phase fermentation achieved higher scores for flavor, taste and overall acceptability, while also shortening the fermentation period and improving both flavor and safety. Therefore, inoculation with these two strains using the two-phase fermentation method can efficiently produce high-quality pickle products in a short time. This research contributes to the industrial production of fermented vegetables, enhancing both pickle quality and economic benefits.

Characterizing flavor development in low-salt Chinese horse bean-chili paste through integrated metabolomics and metagenomics

This study utilized metabolomics and metagenomics to investigate the microbial composition and functions in low- and high-salt Chinese horse bean-chili pastes (CHCPs). The results showed that 25 key metabolites were identified to distinguish the flavor attributes between the two samples. Leuconostoc was identified as the dominant microbiota in low-salt CHCP, while Pantoea prevailed in the high-salt CHCP. Compared to traditional high-salt fermentation, low-salt and inoculated fermentation promoted the increase in the relative abundances of Companionlactobacillus, Levilactobacillus, Tetragenococcus, Zygosaccharomyces and Wickerhamiella as well as the enrichment of carbohydrate and amino acid metabolic pathways, which contributed to the enhancement of characteristic flavor compounds. Further metabolic pathway reconstruction elucidated 21 potential microbial genera associated with the formation of key metabolites, such as Leuconostoc, Levilactobacillus, Pantoea, and Pectobacterium. This study may provide insights for optimizing the fermentation process and improving the flavor quality of low-salt CHCP and similar fermentation products. KEYWORDS: Low-salt fermentation Hight-salt fermentation Chinese horse-bean chili paste Flavor formation Metabolomics Metagenomics.

Innovative omics strategies in fermented fruits and vegetables: Unveiling nutritional profiles, microbial diversity, and future prospects

Fermented fruits and vegetables (FFVs) are not only rich in essential nutrients but also contain distinctive flavors, prebiotics, and metabolites. Although omics techniques have gained widespread recognition as an analytical strategy for FFVs, its application still encounters several challenges due to the intricacies of biological systems. This review systematically summarizes the advances, obstacles and prospects of genomics, transcriptomics, proteomics, metabolomics, and multi-omics strategies in FFVs. It is evident that beyond traditional applications, such as the exploration of microbial diversity, protein expression, and metabolic pathways, omics techniques exhibit innovative potential in deciphering stress response mechanisms and uncovering spoilage microorganisms. The adoption of multi-omics strategies is paramount to acquire a multidimensional network fusion, thereby mitigating the limitations of single omics strategies. Although substantial progress has been made, this review underscores the necessity for a comprehensive repository of omics data and the establishment of universal databases to ensure precision in predictions. Furthermore, multidisciplinary integration with other physical or biochemical approaches is imperative, as it enriches our comprehension of this intricate process.

Probiotic Characteristics and the Anti-Inflammatory Effects of Lactiplantibacillus plantarum Z22 Isolated from Naturally Fermented Vegetables

Currently, there is increasing interest in the commercial utilization of probiotics isolated from traditional fermented food products. Therefore, this study aimed to investigate the probiotic potential of Lactiplantibacillus plantarum (L. plantarum) Z22 isolated from naturally fermented mustard. The results suggest that L. plantarum Z22 exhibits good adhesion ability, antibacterial activity, safety, and tolerance to acidic conditions and bile salts. We further determined the anti-inflammatory mechanism and properties of L. plantarum Z22 and found that L. plantarum Z22 could significantly reduce the secretion of pro-inflammatory cytokines, including interleukin-6 (IL-6), interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), and the expression of the pro-inflammatory mediator cyclooxygenase-2 (COX-2) protein in LPS-induced RAW 264.7 cells. In addition, L. plantarum Z22 also effectively inhibited the signaling pathways of nuclear factor κB (NF-κB) and mitogen-activated protein kinases (MAPKs). This effect can be attributed to a decrease in the levels of reactive oxygen species (ROS) and increased heme oxygenase-1 (HO-1) expression. Moreover, whole-genome sequencing revealed that L. plantarum Z22 contains gene-encoding proteins with anti-inflammatory functions, such as beta-glucosidase (BGL) and pyruvate kinase (PK), as well as antioxidant functions, including thioredoxin reductase (TrxR), tyrosine-protein phosphatase, and ATP-dependent intracellular proteases ClpP. In summary, these results indicated that L. plantarum Z22 can serve as a potential candidate probiotic for use in fermented foods such as yogurt (starter cultures), providing a promising strategy for the development of functional foods to prevent chronic diseases.

Chitosan and inulin synergized with Lactiplantibacillus plantarum LPP95 to improve the quality characteristics of low-salt pickled tuber mustard

Low-salt pickled vegetables are in line with a healthier diet, yet ensuring consistent quality of such products is challenging. In this study, low-salt tuber mustard pickles fermented with Lactiplantibacillus plantarum LPP95 in the presence of chitosan and inulin were analyzed over a 30-day period, and quality changes were evaluated. Total acid productions along with high bacterial counts (106 CFU/mL) were observed in the initial 20 days during indoor storage temperature, in which the reduced fiber aperture was found significantly lead to an increase in crispness (16.94 ± 1.87 N) and the maintenance of a low nitrate content (1.23 ± 0.01 mg/kg). Moreover, the combined pickling treatment resulted in higher malic acid content, lower tartaric acid content, and a decrease in the content of bitter amino acids (e.g., isoleucine and leucine), thus leading to an increase in the proportion of sweet amino acids. Additionally, combined pickling led to the production of unique volatile flavor compounds, especially the distinct spicy flavor compounds isothiocyanates. Moreover, the combined pickling treatment resulted in an increase in the abundance of Lactiplantibacillus and promoted microbial diversity within the fermentation system. Thus, the synergistic effect among chitosan, inulin, and L. plantarum LPP95 significantly enhanced the quality of pickles. The study offers a promising strategy to standardize the quality of low-salt fermented vegetables.

Organic acids drove the microbiota succession and consequently altered the flavor quality of Laotan Suancai across fermentation rounds: Insights from the microbiome and metabolome

Laotan Suancai, a popular traditional Chinese fermented vegetable, is manufactured in the industry via four fermentation rounds. However, the differences in flavor quality of Laotan Suancai from the four fermentation rounds and the causes of this variation remain unclear. Metabolome analysis indicated that the different content of five taste compounds and 31 aroma compounds caused the differences in flavor quality among the variated fermentation rounds of Laotan Suancai. Amplicon sequencing indicated that the microbial succession exhibited a certain pattern during four fermentation rounds and further analysis unveiled that organic acids drove the microbiota shift to more acid-resistant populations. Spearman correlation analysis highlighted that seven core microbes may be involved in the formation of differential flavor and the corresponding metabolic pathways were reconstructed by function prediction. Our findings offer a novel perspective on comprehending the deterioration of flavor quality across the fermentation rounds of Laotan Suancai.

Metabolomics and HS-SPME-GC-MS-based analysis of quality succession patterns and flavor characteristics changes during the fermentation of Lycium barbarum and Polygonatum cyrtonema compound wine

This work set out to investigate how the physicochemical markers, volatiles, and metabolomic characteristics of mixed fermented the fermentation of Lycium barbarum and Polygonatum cyrtonema compound wine (LPCW) from S. cerevisine RW and D. hansenii AS2.45 changed over the course of fermentation. HS-SPME-GC-MS combined with non-targeted metabolomics was used to follow up and monitor the fermentation process of LPCW. In total, 43 volatile chemical substances, mostly alcohols, esters, acids, carbonyl compounds, etc., were discovered in LPCW. After 30 days of fermentation, phenylethyl alcohol had increased to 3045.83 g/mL, giving off a rose-like fresh scent. The biosynthesis of valine, leucine, and isoleucine as well as the metabolism of alanine, aspartic acid, and glutamic acid were the major routes that led to the identification of 1385 non-volatile components in total. This study offers a theoretical foundation for industrial development and advances our knowledge of the fundamental mechanism underlying flavor generation during LPCW fermentation.

Integrated metatranscriptomics and metabolomics reveal microbial succession and flavor formation mechanisms during the spontaneous fermentation of Laotan Suancai

Laotan Suancai, a Chinese traditional fermented vegetable, possesses a unique flavor that depends on the fermentative microbiota. However, the drivers of microbial succession and the correlation between flavor and active microbiota remain unclear. A total of 21 characteristic flavor metabolites were identified in Laotan Suancai by metabolomics, including 8 sulfides, 6 terpenes, 3 organic acids, 2 isothiocyanates, 1 ester, and 1 pyrazine. Metatranscriptome analysis revealed variations in the active microbiota at different stages of fermentation, and further analysis indicated that organic acids were the primary drivers of microbial succession. Additionally, we reconstructed the metabolic network responsible for the formation of characteristic flavor compounds and identified Companilactobacillus alimentarius, Weissella cibaria, Lactiplantibacillus plantarum, and Loigolactobacillus coryniformis as the core functional microbes involved in flavor development. This study contributed to profoundly understanding the relationship between the active microbiota and flavor quality formation, as well as the targeted selection of starters with flavor regulation abilities.

The Weissella and Periweissella genera: up-to-date taxonomy, ecology, safety, biotechnological, and probiotic potential

Bacteria belonging to the genera Weissella and Periweissella are lactic acid bacteria, which emerged in the last decades for their probiotic and biotechnological potential. In 2015, an article reviewing the scientific literature till that date on the taxonomy, ecology, and biotechnological potential of the Weissella genus was published. Since then, the number of studies on this genus has increased enormously, several novel species have been discovered, the taxonomy of the genus underwent changes and new insights into the safety, and biotechnological and probiotic potential of weissellas and periweissellas could be gained. Here, we provide an updated overview (from 2015 until today) of the taxonomy, ecology, safety, biotechnological, and probiotic potential of these lactic acid bacteria.

Meta-analysis of microbiomes reveals metagenomic features of fermented vegetables

An insightful exploration of the fermented vegetable microbiome is the key to improving food quality and sustainability. Based on 57 fermented vegetable samples from China, Ireland, the UK, and Germany retrieved from public genome databases, we conducted a high-resolution meta-analysis of the fermented vegetable microbiomes. There were significant differences in the microbiota composition and functional pathway diversity of the tested samples, as reflected by the differences in their geographical origins. Metagenomic analysis also revealed the metagenomic features of carbohydrate-active enzymes and antibiotic resistance genes in the fermented vegetable metagenomes. Five putative new species were detected by recovering 221 metagenome-assembled genomes belonging to the genera Rubrobacteraceae, Bifidobacteriaceae, and Ruminococcaceae. Our results provide new ecological insights into the implications of fermented vegetable microbiota composition and functional potential and highlight the importance of high-resolution metagenomic analysis to further investigate the fermented food microbiome.

Study on the Characteristics of Vacuum-Bagged Fermentation of Apo Pickle and Visualization Array Analysis of the Fermentation Process

Apo pickle is a fermented food with a long edible history in the Jiangnan region of China. Traditionally, plastic bottles are used as Apo pickle's fermentation containers, and artificial bottling costs are high. The goal of this study is to compare the fermentation effects of Apo pickle fermented under low pressure in a vacuum bag (VBA) and Apo pickle fermented under normal pressure in plastic bottles (TBA) to determine the feasibility of fermenting Apo pickle in a vacuum bag rather than a plastic bottle, thereby lowering production costs. At the same time, a gas-sensitive colorimetric sensor array (CSA) was developed to distinguish different fermentation stages of Apo pickle. The results revealed that the main genera in the initial and final phases of Apo pickle fermentation were Weissella and Lactobacillus, unaffected by fermentation containers. At the same fermentation time, the abundance of Lactobacillus and the content of flavor substances in VBA were higher, and the fermentation speed of VBA was faster at 0-15 d, so a vacuum bag could be used instead of a plastic bottle. The CSA could discriminate between different fermentation procedures of Apo pickles with an accuracy rate of 93.8%. Its principle is similar to that of an electronic nose. It has the advantages of convenience, rapidity, and no need for professional equipment, so it can be used as a new method to judge the fermentation degree of apo pickle.

Metagenomic insights into the microbiota involved in lactate and butyrate production and manipulating their synthesis in alfalfa silage

AIMS

Lactate and butyrate are important indicators of silage quality. However, the microorganisms and mechanisms responsible for lactate and butyrate production in silage are not well documented.

METHODS AND RESULTS

whole-metagenomic sequencing was used to analyse metabolic pathways, microbiota composition, functional genes, and their contributions to lactate and butyrate production in alfalfa silage with (SA) and without (CK) sucrose addition. Carbon metabolism was the most abundant metabolic pathway. We identified 11 and 2 functional genes associated with lactate and butyrate metabolism, respectively. Among them, D-lactate dehydrogenase (ldhA) and L-lactate dehydrogenase (ldhB) were most important for the transition between D/L-lactate and pyruvate and were primarily related to Lactobacillus in the SA group. The genes encoding L-lactate dehydrogenase (lldD), which decomposes lactate, were the most abundant and primarily associated with Enterobacter cloacae. Butyrate-related genes, mainly encoding butyryl-CoA: acetate CoA-transferase (but), were predominantly associated with Klebsiella oxytoca and Escherichia coli in the CK group.

CONCLUSIONS

Enterobacteriaceae and Lactobacillaceae were mainly responsible for butyrate and lactate formation, respectively.

Evaluation and comparison of physicochemical properties, volatile substances, and microbial communities of leaf mustard (Brassica juncea var. multiceps) under natural and inoculated fermentation

Due to the uncontrolled fermentation process and unstable quality of naturally fermented leaf mustard, inoculated fermentation is receiving more attention. Here, the physicochemical properties, volatile compounds, and microbial community in leaf mustard under natural fermentation (NF) and inoculated fermentation (IF) were analyzed and compared. The contents of total acid, crude fiber, and nitrite of leaf mustard were measured. Headspace-solid phase microextraction-gas chromatography-mass spectrometry and orthogonal projection on latent structure-discriminant analysis were used to analyze the differences of volatile compounds in NF and IF leaf mustard. Moreover, Illumina MiSeq high-throughput sequencing technology was employed to reveal the composition of microbiota. The results showed that the nitrite content in leaf mustard after IF (3.69 mg/kg) was significantly lower than that after NF (4.43 mg/kg). A total of 31 and 25 kinds of volatile components were identified in IF and NF, respectively. Among the detected compounds, 11 compounds caused the differences between IF and NF leaf mustard. The results of inter-group difference analysis showed that there were significant differences in fungal flora between IF and NF samples. Saccharomycetes, Kazachstania, and Ascomycota were the landmark microorganisms in IF leaf mustard and the landmark microorganisms in NF were Mortierellomycota, Sordariomycetes, and Eurotiomycetes. The abundance of probiotics (such as Lactobacillus) in IF leaf mustard (51.22%) was higher than that in NF (35.20%) and the abundance of harmful molds (such as Mortierella and Aspergillus) was opposite. Therefore, IF leaf mustard showed the potential to reduce the content of nitrite and harmful molds and increase the beneficial volatile compounds and probiotics. PRACTICAL APPLICATION: Leaf mustard of inoculated fermentation (IF) showed better fermented characteristics than natural fermentation in terms of lower nitrite content, greater beneficial volatile substances, and better potential for increasing probiotics and reducing harmful molds. These results provided a theoretical basis for IF leaf mustard and contributed to the industrial production of fermented leaf mustard.

Characterization of Functional Microorganisms in Representative Traditional Fermented Dongcai from Different Regions of China

Dongcai is loved for its delicious flavor and nutritional value. The microorganisms in Dongcai play a vital role in their flavor, quality, and safety, and the microbial communities of Dongcai vary greatly from region to region. However, it remains unknown what the predominant microorganisms are in different traditional Dongcai and how they affect its flavor. The objective of this study is to explore the microbial diversity of traditional fermented Dongcai in three representative Chinese regions (Tianjin, Sichuan, and Guangzhou) and further assess their microbial functions. The microbial diversity of fermented Dongcai in Guangdong has the lowest diversity compared to fermented Dongcai in Sichuan, which has the highest. The distribution of the main genera of fermented Dongcai varies from region to region, but Carnimonas, Staphylococcus, Pseudomonas, Sphingomonas, Burkholderia-Caballeronia-Paraburkholderia, and Rhodococcus are the dominant genera in common. In addition, halophilic bacteria (HAB, i.e., Halomonas Bacillus, Virgibacillus, etc.) and lactic acid bacteria (LAB, i.e., Weissella and Lactobacillus) are also highly abundant. Of these, Burkholderia-Caballeronia-Paraburkholderia, Rhodococcus, Sphingomonas, Ralstonia, and Chromohalobacter are dominant in the Sichuan samples. In the Tianjin samples, Lactobacillus, Weissella, Virgibacillus, Enterobacter, Klebsiella, and Pseudomonas are the most abundant. Predictions of microbial metabolic function reveal that carbohydrates, amino acids, polyketides, lipids, and other secondary metabolites are abundantly available for biosynthesis. In addition, the different flavors of the three types of Dongcai may be due to the fact that the abundance of HAB and LAB shows a significant positive correlation with the amounts of important metabolites (e.g., salt, acid, amino nitrogen, and sugar). These results contribute to our understanding of the link between the distinctive flavors of different types of Dongcai and the microorganisms they contain and will also provide a reference for the relationship between microbial communities and flavor substances in semi-fermented pickles.

Investigation on fresh bamboo replacing rice husks for brewing light-aroma Baijiu

Husks are the main source of bran and furfural flavor in traditional Chinese light-aroma Baijiu, but they negatively affect its smell and taste. Here, bran husks were replaced with fresh bamboo to brew light-aroma Baijiu. Flavor components in Jiupei and Baijiu were detected through headspace solid-phase microextraction with gas chromatography-mass spectrometry, and physicochemical properties were assessed; flavor results were obtained from correlation, principal component, and cluster analyses. Starch and reducing sugar content in Jiupei negatively correlated with moisture, alcohol content, and acidity. Fresh bamboo reduced furfural from bran husks in Jiupei by 88.5% and increased alcohol distillation by 51%; it also improved starch efficiency (5%). Surprisingly, isovanillin was found to be present in Baijiu. Total Baijiu yield (57% ± 2.01%) was attained when crushed bamboo size was 1.5 cm × 0.3 cm × 0.3 cm. This study supports the use of fresh bamboo (an eco-friendly alternative for husks) in brewing light-aroma Baijiu. PRACTICAL APPLICATION: The use of fresh bamboo as a replacement for rice husks in brewing light-aroma Baijiu was investigated. It attenuated the chaff taste in light-aroma Baijiu and increased the liquor yield. Surprisingly, isovanillin was also present in the base Baijiu, and it added to the fragrance. This study not only supports the use of bamboo as an auxiliary material for brewing light-aroma Baijiu but also provides a reference for brewing light-aroma Baijiu with alternative auxiliary materials.

Improvement of physicochemical characteristics, flavor profiles and functional properties in Chinese radishes via spontaneous fermentation after drying

Spontaneously dried-fermented radishes have been consumed in China for hundreds of years and are usually fermented for a long time to acquire high quality. In this study, the spontaneously dried-fermented radishes with short-term manufacturing periods were made from five different varieties of radishes that grew in the same environment. In addition, the physicochemical characteristics (i.e., moisture content, soluble solid, and pH value), flavor profiles (i.e., free amino acids, organic acids, and volatile compounds), and functional properties (i.e., total phenolics content, total flavonoids content, sulforaphane content, and γ-aminobutyric acid [GABA] content) of these five raw radishes and spontaneously dried-fermented radishes were analyzed and compared. In detail, the content of volatile and nonvolatile compounds increased, especially in oxalic acid, succinic acid, and umami free amino acids. Furthermore, functional components, such as sulforaphane and GABA, were also enriched via spontaneous fermentation after drying. In addition, the results of principal component analysis, hierarchical clustering analysis, and redundancy analysis showed that there were significant discrepancies appeared when raw radishes were processed via spontaneous fermentation or not. These results suggested that the process of spontaneous fermentation after drying may contribute to improving the quality of fresh radishes. Notably, radishes with red skin and flesh were regarded as exceptional varieties for processing, because of the preferable flavor profiles and affluent functional substances via spontaneous fermentation after drying. Therefore, these findings could deliver a systematical insight into developing processed radishes with high quality. PRACTICAL APPLICATION: The spontaneously dried-fermented radishes were manufactured through the process of spontaneous fermentation after drying, which acquired tasty and healthy characteristics by accumulating the volatile and nonvolatile compounds as well as the functional components, like total phenolics, total flavonoids, sulforaphane, and γ-aminobutyric acid. Importantly, because of the excellent processing properties, the radishes with red skin and flesh could be more appropriate to produce spontaneously dried-fermented radishes. Our findings may provide a practical strategy for developing vegetable relishes with superb flavor profiles and good functional properties in pickled vegetables.

Reduced formation of biogenic amines in low-salt Zhacai via fermentation under CO2-modified atmosphere

Reducing sodium salt content in traditional fermented vegetables and developing low-salt fermented products have attracted increasing attention.However, low-salt fermented vegetables are prone to accumulate toxic biogenic amines (BAs) caused by the undesirable metabolism of spoilage microorganisms. This study aimed to investigate the impact of a CO₂-modified atmosphere (MA) approach to the fermentation of low-salt Zhacai and the accumulation of BAs. The results show CO₂-MA effectively suppressed the production of excessive BAs in low-salt Zhacai, as evidenced by a decrease in the total BA content from 63.66 to 161.41 mg/ kg under natural air conditions to 1.88-24.76 mg/ kg under CO₂-MA. Overall, the mechanism of hindering BA formation was closely related to the change in the microbial community and the downregulation of BA-producing enzymes. Lactic acid bacteria, including Lactiplantibacillus plantarum, Weissella spp., and Pediococcus spp., were enriched under CO₂-MA, whereas amine-producing microorganisms (e.g., Halomonas spp., Psychrobacter spp., Corynebacterium spp., and Levilactobacillus brevis) were greatly inhibited. Moreover, metagenomic analysis revealed that genes encoding amino acid decarboxylase, amine deiminase, and amine synthase were downregulated, which could be the fundamental reason for BA reduction. This study provides an alternative method for reducing BA production in fermented food.

Flavor production in fermented chayote inoculated with lactic acid bacteria strains: Genomics and metabolomics based analysis

In this study, genomics and metabolomics were combined to reveal possible bio-synthetic pathways of core flavor compounds in pickled chayote via lactic acid bacteria (LAB) fermentation. The Lactiplantibacillus plantarum, Levilactobacillus brevis, and Lacticaseibacillus paracasei were selected as core LAB strains with better flavor-producing ability for chayote fermentation. The genomic results showed L. plantarum contained the largest number of metabolism annotated genes, while L. brevis had the fewest. Besides, the largest number of volatile compounds was detected in chayote fermented by L. plantarum, followed by L. brevis and L. paracasei. Some unique odor-active compounds (aldehydes, esters, and alcohols) and taste-active compounds (amino acids and dipeptides) were produced by different LAB strains. Accordingly, phenylalanine metabolic pathway (M00360), amino acid metabolic decomposition pathway (the Ehrlich pathway) and the anabolic pathway (the Harris pathway), and fatty acid biosynthesis pathway (M00061) were the main biosynthesis pathway involved in the flavor formation via LAB fermentation.

Two sides of the same coin: Meta-analysis uncovered the potential benefits and risks of traditional fermented foods at a large geographical scale

Traditional fermented foods, which are well-known microbial resources, are also bright national cultural inheritances. Recently, traditional fermented foods have received great attention due to their potential probiotic properties. Based on shotgun metagenomic sequencing data, we analyzed the microbial diversity, taxonomic composition, metabolic pathways, and the potential benefits and risks of fermented foods through a meta-analysis including 179 selected samples, as well as our own sequencing data collected from Hainan Province, China. As expected, raw materials, regions (differentiated by climatic zones), and substrates were the main driving forces for the microbial diversity and taxonomic composition of traditional fermented foods. Interestingly, a higher content of beneficial bacteria but a low biomass of opportunistic pathogens and antibiotic resistance genes were observed in the fermented dairy products, indicating that fermented dairy products are the most beneficial and reliable fermented foods. In contrast, despite the high microbial diversity found in the fermented soy products, their consumption risk was still high due to the enrichment of opportunistic pathogens and transferable antibiotic resistance genes. Overall, we provided the most comprehensive assessment of the microbiome of fermented food to date and generated a new view of its potential benefits and risks related to human health.

Effects of post-fermentation on the flavor compounds formation in red sour soup

Red Sour Soup (RSS) is a traditional fermented food in China. After two rounds of fermentation, sour soup has a mellow flavor. However, the microbial composition and flavor formation processes in post-fermentation in RSS are unclear. This study investigates the bacteria composition of RSS during the post-fermentation stage (0–180 days) using high-throughput sequencing. The results show that lactic acid bacteria (LAB) are dominant during the post-fermentation process, and their abundance gradually increases with fermentation time. Additionally, gas chromatography-mass spectrometry was used to detect volatile flavor compounds in the post-fermentation process. Seventy-seven volatile flavor compounds were identified, including 24 esters, 14 terpenes, 9 aromatic hydrocarbons, 9 alkanes, 6 heterocyclic compounds, 3 alcohols, 3 acids, 3 ketones, 2 phenols, 2 aldehydes, 1 amine, and 1 other. Esters and aromatic hydrocarbons are the main volatile compounds in RSS during the post-fermentation process. Orthogonal partial least squares screening and correlation analysis derived several significant correlations, including 48 pairs of positive correlations and 19 pairs of negative correlations. Among them, Acetobacter spp., Clostridium spp. and Sporolactobacillus spp. have 15, 14, 20 significant correlation pairs, respectively, and are considered the most important bacterial genera post-fermentation. Volatile substances become abundant with increasing fermentation time. LAB are excessive after more than 120 days but cause a drastic reduction in volatile ester levels. Thus, the post-fermentation time should be restricted to 120 days, which retains the highest concentrations of volatile esters in RSS. Overall, these findings provide a theoretical basis to determine an optimal post-fermentation time duration, and identify essential bacteria for manufacturing high-quality starter material to shorten the RSS post-fermentation processing time.

GC-TOF-MS-Based Non-Targeted Metabolomic Analysis of Differential Metabolites in Chinese Ultra-Long-Term Industrially Fermented Kohlrabi and Their Associated Metabolic Pathways

Fermented kohlrabi is a very popular side dish in China. Chinese kohlrabies industrially fermented for 0 years (0Y), 5 years (5Y), and 10 years (10Y) were employed and analyzed by non-targeted metabolomics based on GC-TOF-MS, and the differential metabolites were screened using multivariate statistical analysis techniques, including principal component analysis (PCA) and orthogonal partial least squares discrimination analysis (OPLS-DA). The results showed that 47, 38, and 33 differential metabolites were identified in the three treatment groups of 0Y and 5Y (A1), 0Y and 10Y (A2), and 5Y and 10Y (A3), respectively (VIP > 1, p < 0.05). The metabolites were mainly carbohydrates, amino acids, and organic acids. Furthermore, 13 differential metabolites were screened from the three groups, including L-glutamic acid, L-aspartic acid, γ-aminobutyric acid, and other compounds. Four metabolic pathways termed alanine, aspartate, and glutamate metabolism, arginine biosynthesis, arginine and proline metabolism, and glycolysis/gluconeogenesis were the most significant pathways correlated with the differential metabolites, as analyzed according to the Kyoto Encyclopedia of Genes and Genomes (KEGG). The odors for the three ultra-long-term industrially fermented kohlrabies were significantly different, as detected by E-nose. The present work describes the changes in metabolites between different ultra-long-term industrially fermented kohlrabies and the associated metabolic pathways, providing a theoretical basis for the targeted regulation of characteristic metabolite biosynthesis in Chinese fermented kohlrabi.

Flavour Generation during Lactic Acid Fermentation of Brassica Vegetables—Literature Review

Fermentation is a method of food preservation that has been used for centuries. Lactic acid fermentation, apart from extending the shelf-life of vegetables, affects significantly the flavour of food products. In this review, the formation of flavour, including both taste and aroma, in fermented Brassica vegetables is summarized. The flavour-active compounds are generated in various metabolic pathways from many precursors present in raw materials used for fermentation. In Brassica vegetables, a unique group of chemicals, namely glucosinolates, is present, which significantly influence the flavour of fermented products. In this summary, we took a closer look at the flavour of two of the most commonly eaten worldwide fermented Brassica products, which are sauerkraut and kimchi. Finally, the needs and directions for future studies were addressed.

Tetracycline residue alters profile of lactic acid bacterial communities and metabolites of ginger pickle during spontaneous fermentation

Antibiotic residue, an emerging contaminant, has been an increasing concern worldwide in vegetables. However, the focus is still limited to its accumulation in vegetables and its adverse health effect on humans, with little mention of its impact on the vegetable process. Fermentation is an important method of the vegetable process. Tetracycline (TC) is the most widely used antibiotic, and its residue is often higher than other antibiotics in ginger. Therefore, current research was to characterize how the TC residue affected the lacticacidbacterial (LAB) communities and metabolites during the salted fermentation of ginger. The TC residue, organic acids, volatile compounds and LABs were analyzed during the fermentation, and the correlations between the LABs and compounds were also revealed. The results suggested that the hetero-lactic fermentation did not happen under the TC residue although the TC residue decreased from 4 mg/kg to 2.56 mg/kg in salt brine of ginger fermentation. Meanwhile, TC residue affected the occurrence, propagation and succession of LABs. The LAB biomarkers shifted to Lab. parafarraginis, Lab. buchneri and Lab. kisonensis under TC residue. Responded to LAB communities, the organic acids and volatile compounds were also markedly changed under the TC residue. And the important volatile compound variables shifted to citronellol, allylsenevol, geranyl acetate, vinylstearylether, isothiocyanic acid phenethyl, 3-octanol, geraniol, bingpian, citral and camphor. The transformation of LAB biomarkers induced by TC residue was the main cause of the change of important volatile compound variables, but interestingly, not all of them had significant positive or negative correlations. These results indicated that the antibiotic residue has an adverse ecological effect on the vegetable fermentation process.Therefore, the antibiotic residue should be listed as a quality control index in fermented vegetable raw materials.

Metagenomics Reveals the Microbial Community Responsible for Producing Biogenic Amines During Mustard [Brassica juncea (L.)] Fermentation

Biogenic amines (BAs) are considered potential hazards produced during fermented food processing, and the production of BAs is closely related to microbial metabolism. In this work, the changes of BA content were analyzed during mustard fermentation, and microbes and gene abundance responsible for producing BAs were revealed by metagenomic analyses. The results showed that cadaverine, putrescine, tyramine, and histamine were generated during mustard fermentation, which mainly accumulate in the first 6 days of fermentation. According to the metagenome sequencing, the predominant genus was Bacillus (64.78%), followed by Lactobacillus (11.67%), Weissella (8.88%), and Leuconostoc (1.71%) in the initial fermentation stage (second day), while Lactobacillus (76.03%) became the most dominant genus in the late stage. In addition, the gene abundance of BA production enzymes was the highest in the second day and decreased continuously as fermentation progressed. By tracking the source of the enzyme in the KEGG database, both Bacillus and Delftia closely correlated to the generation of putrescine. Besides, Bacillus also correlated to the generation of tyramine and spermidine, and Delftia also correlated to the generation of cadaverine and spermine. In the processes of fermentation, the pH of fermented mustard showed slower decrease compared with other similar fermented vegetables, which may allow Bacillus to grow at high levels before the pH &lt;4. This study reveals the change of BA content and microbes involved in BA formation during mustard fermentation.

Impact of chiral tebuconazole on the flavor components and color attributes of Merlot and Cabernet Sauvignon wines at the enantiomeric level

The impact of chiral tebuconazole on the flavor and appearance of Merlot and Cabernet Sauvignon wines were systematically studied. Gas chromatography-ion mobility spectrometry and headspace-solid phase microextraction coupled with gas chromatography mass spectrometry qualitatively and quantitatively identified the flavor components, and a photographic colorimeter was used for color attribute analysis. Tebuconazole enantiomers had different effects on the flavor and appearance of young wines, especially R-tebuconazole. The flavor differences were mainly manifested in fruity and floral characteristics of the wine due to changes in the concentrations of acids, alcohols, and esters; R-tebuconazole alters the concentrations of key flavor compounds to the greatest extent. Tebuconazole treatment changes the color of young wines, with the final red shade of wine being control group > rac-tebuconazole ≥ S-tebuconazole > R-tebuconazole. Since chiral tebuconazole negatively alters wine, grapes treated with chiral pesticides should be subject to stricter quality control during processing.

Metabolic Footprint Analysis of Volatile Organic Compounds by Gas Chromatography-Ion Mobility Spectrometry to Discriminate Mandarin Fish (Siniperca chuatsi) at Different Fermentation Stages

Chinese fermented mandarin fish (Siniperca chuatsi) have unique aroma characteristics that are appreciated by local consumers. In this study, electronic nose (E-nose) and gas chromatography–ion mobility spectrometry analyses were combined to establish a volatile fingerprint of fermented mandarin fish during fermentation. Clear separation of the data allowed mandarin fish samples at different fermentation stages to be distinguishing using E-nose analysis. Forty-three volatile organic compounds were identified during fermentation. Additionally, partial least squares discrimination analysis was performed to screen for different VOC metabolites in the fermented mandarin fish; the levels of six VOCs changed significantly during fermentation (variable importance in projection >1; p < 0.05). Three VOCs, i.e., hexanal-D, nonanal, and limonene were identified as potential biomarkers for fermentation. This study provided a theoretical basis for flavor real-time monitoring and quality control of traditional mandarin fish fermentation.