Reduced nitrite and biogenic amine concentrations and improved flavor components of Chinese sauerkraut via co-culture of Lactobacillus plantarum and Zygosaccharomyces rouxii

Regulation mechanism of nitrite degradation in Lactobacillus plantarum WU14 mediated by Fnr

Fumarate and nitrate reduction regulatory protein (Fnr)-a global transcriptional regulator-can directly or indirectly regulate many genes in different metabolic pathways at the top of the bacterial transcription regulation network. The present study explored the regulatory mechanism of Fnr-mediated nitrite degradation in Lactobacillus plantarum WU14 through gene transcription and expression analysis of oxygen sensing and nir operon expression regulation by Fnr. The interaction and the mechanism of transcriptional regulation between Fnr and GlnR were also examined under nitrite stress. After Fnr and GlnR purification by glutathione S-transferase tags, they were successfully expressed in Escherichia coli by constructing an expression vector. The results of electrophoresis mobility shift assay and qRT-PCR indicated that Fnr specifically bound to the PglnR and Pnir promoters and regulated the expression of nitrite reductase (Nir) and GlnR. After 6-12 h of culture, the expressions of fnr and nir under anaerobic conditions were higher than under aerobic conditions; the expression of these two genes increased with sodium nitrite (NaNO2) addition during aerobic culture. Overall, the present study indicated that Fnr not only directly participated in the expression of Nir and GlnR but also indirectly regulated the expression of Nir through GlnR regulation.

Effect of salt concentration on the quality and microbial community during pickled peppers fermentation

This work aimed to investigate the effect of salt concentration on the quality and microbial community of pickled peppers during fermentation, and the cross-correlation between microorganisms and quality was also revealed. The results showed that 9 volatile flavor compounds were unique to the low salt concentration group (D group), which also contained higher content of FAA, lactic acid and acetic acid than high salt concentration group (G group). Meanwhile, the samples of D2 group have a better texture properties. Firmicutes, Proteobacteria, Ascomycota, Lactobacillus, Pectobacterium, and Pseudomonas were detected as the main microbial community during the fermentation with different salt concentrations. Furthermore, the correlations analysis results indicated that the salt concentration has a significant effect on the microbial community of pickled peppers (p < 0.001), and Pediococcus, Lactobacillus, Cedecca, Issatchenkia, Pichia, Kazachstania, and Hanseniaspora were significantly correlated with flavors, which played crucial roles in the unique flavor formation of pickled peppers.

Biogenic amines as an indicator of rye leaven quality during production and storage

Rye leaven, the basic constituent of sour rye soup ('żurek' or white borsch), was obtained through three methods of initiating lacto-fermentation of rye flour. Optimal concentrations of NaCl (1.5%) and garlic (0.5%) were selected by utilizing the response surface methodology. During the production and storage of leaven at 10 °C and 20 °C, the secalin proteins of rye flour degraded significantly and the concentration of free amino acids increased, making the rye leaven an environment potentially conducive to the formation of biogenic amines. Putrescine (max. conc: 116.7 mg kg-1) and tyramine (max. conc: 63.4 mg kg-1) were the amines that occurred in the largest amounts in the leavens. The final concentration of histamine (after 150 days of storage) did not exceed 22 mg kg-1. Regardless of the method of initiation of fermentation, the products that contained fewer biogenic amines better retained their sensory characteristics (r ≤ -0.89, p < 0.05) and had a higher number of lactic acid bacteria (r ≤ -0.66, p < 0.05).

Food yeasts: occurrence, functions, and stress tolerance in the brewing of fermented foods

With the rapid development of systems biology technology, there is a deeper understanding of the molecular biological mechanisms and physiological characteristics of microorganisms. Yeasts are widely used in the food industry with their excellent fermentation performances. While due to the complex environments of food production, yeasts have to suffer from various stress factors. Thus, elucidating the stress mechanisms of food yeasts and proposing potential strategies to improve tolerance have been widely concerned. This review summarized the recent signs of progress in the variety, functions, and stress tolerance of food yeasts. Firstly, the main food yeasts occurred in fermented foods, and the taxonomy levels are demonstrated. Then, the main functions of yeasts including aroma enhancer, safety performance enhancer, and fermentation period reducer are discussed. Finally, the stress response mechanisms of yeasts and the strategies to improve the stress tolerance of cells are reviewed. Based on sorting out these related recent researches systematically, we hope that this review can provide help and approaches to further exert the functions of food yeasts and improve food production efficiency.

Examining the impact of Tetragenococcus halophilus, Zygosaccharomyces rouxii, and Starmerella etchellsii on the quality of soy sauce: a comprehensive review of microbial population dynamics in fermentation

Soy sauce is a popular fermented seasoning due to its distinct flavor and rich umami taste. Its traditional production involves two stages: solid-state fermentation and moromi (brine fermentation). During moromi, the dominant microbial population in the soy sauce mash changes, which is called microbial succession and is essential for the formation of soy sauce flavor compounds. Research has identified the sequence of succession, starting with Tetragenococcus halophilus, then Zygosaccharomyces rouxii, and lastly, Starmerella etchellsii. Factors such as the environment, microbial diversity, and interspecies relationships drive this process. Salt and ethanol tolerance influence microbial survival, while nutrients in the soy sauce mash support the cells in resisting external stress. Different microbial strains have varying abilities to survive and respond to external factors during fermentation, which impacts soy sauce quality. In this review, we would examine the factors behind the succession of common microbial populations in the soy sauce mash and explore how microbial succession affects soy sauce quality. The insights gained can help better manage the dynamic changes in microbes during fermentation, leading to improved production efficiency.

Research Progress of Nitrite Metabolism in Fermented Meat Products

Nitrite is a common color and flavor enhancer in fermented meat products, but its secondary amines may transfer to the carcinogen N-nitrosamines. This review focuses on the sources, degradation, limitations, and alteration techniques of nitrite. The transition among NO3− and NO2−, NH4+, and N2 constitutes the balance of nitrogen. Exogenous addition is the most common source of nitrite in fermented meat products, but it can also be produced by contamination and endogenous microbial synthesis. While nitrite is degraded by acids, enzymes, and other metabolites produced by lactic acid bacteria (LAB), four nitrite reductase enzymes play a leading role. At a deeper level, nitrite metabolism is primarily regulated by the genes found in these bacteria. By incorporating antioxidants, chromogenic agents, bacteriostats, LAB, or non-thermal plasma sterilization, the amount of nitrite supplied can be decreased, or even eliminated. Finally, the aim of producing low-nitrite fermented meat products is expected to be achieved.

Hydrogen-Rich Water Can Restrict the Formation of Biogenic Amines in Red Beet Pickles

Fermented foods are considered the main sources of biogenic amines (BAs) in the human diet while lactic acid bacteria (LAB) are the main producers of BAs. Normal water (NW) and hydrogen-rich water (HRW) were used for preparing red beet pickles, i.e., NWP and HRWP, respectively. The formation of BAs, i.e., aromatic amines (tyramine, 2-phenylethylamine), heterocyclic amines (histamine, tryptamine), and aliphatic di-amines (putrescine), was analyzed in both beet slices and brine of NWPs and HRWPs throughout the fermentation stages. Significant differences in redox value (Eh7) between NWP and HRWP brine samples were noticed during the first and last fermentation stages with lower values found for HRWPs. Total mesophilic aerobic bacteria (TMAB), yeast–mold, and LAB counts were higher for HRWPs than NWPs for all fermentation stages. Throughout fermentation stages, the levels of all BAs were lower in HRWPs than those of NWPs, and their levels in brines were higher than those of beets. At the end of fermentation, the levels (mg/kg) of BAs in NWPs and HRWPs were, respectively: tyramine, 72.76 and 61.74 (beet) and 113.49 and 92.67 (brine), 2-phenylethylamine, 48.00 and 40.00 (beet) and 58.01 and 50.19 (brine), histamine, 67.89 and 49.12 (beet) and 91.74 and 70.92 (brine), tryptamine, 93.14 and 77.23 (beet) and 119.00 and 93.11 (brine), putrescine, 81.11 and 63.56 (beet) and 106.75 and 85.93 (brine). Levels of BAs decreased by (%): 15.15 and 18.35 (tyramine), 16.67 and 13.44 (2-phenylethylamine), 27.65 and 22.7 (histamine), 17.09 and 21.76 (tryptamine), and 21.64 and 19.5 (putrescine) for beet and brine, respectively, when HRW was used in pickle preparation instead of NW. The results of this study suggest that the best method for limiting the formation of BAs in pickles is to use HRW in the fermentation phase then replace the fermentation medium with a new acidified and brined HRW followed by a pasteurization process.

Challenges and perspectives of quantitative microbiome profiling in food fermentations

Spontaneously fermented foods are consumed and appreciated for thousands of years although they are usually produced with fluctuated productivity and quality, potentially threatening both food safety and food security. To guarantee consistent fermentation productivity and quality, it is essential to control the complex microbiota, the most crucial factor in food fermentations. The prerequisite for the control is to comprehensively understand the structure and function of the microbiota. How to quantify the actual microbiota is of paramount importance. Among various microbial quantitative methods evolved, quantitative microbiome profiling, namely to quantify all microbial taxa by absolute abundance, is the best method to understand the complex microbiota, although it is still at its pioneering stage for food fermentations. Here, we provide an overview of microbial quantitative methods, including the development from conventional methods to the advanced quantitative microbiome profiling, and the application examples of these methods. Moreover, we address potential challenges and perspectives of quantitative microbiome profiling methods, as well as future research needs for the ultimate goal of rational and optimal control of microbiota in spontaneous food fermentations. Our review can serve as reference for the traditional food fermentation sector for stable fermentation productivity, quality and safety.

Interaction and Application of Molds and Yeasts in Chinese Fermented Foods

Fermentation is an ancient food preservation and processing technology with a long history of thousands of years, that is still practiced all over the world. Fermented foods are usually defined as foods or beverages made by controlling the growth of microorganisms and the transformation of raw and auxiliary food components, which provide the human body with many beneficial nutrients or health factors. As fungus widely used in traditional Chinese fermented foods, molds and yeasts play an irreplaceable role in the formation of flavor substances and the production of functional components in fermented foods. The research progress of molds and yeasts in traditional Chinese fermented foods from traditional to modern is reviewed, including the research on the diversity, and population structure of molds and yeasts in fermented foods. The interaction between fermenting mold and yeast and the latest research results and application development prospects of related industries were discussed.

Metabolomics Analysis for Nitrite Degradation by the Metabolites of Limosilactobacillus fermentum RC4

Nitrite (NIT), a commonly used food additive, especially in pickled and cured vegetables and meat products, might cause acute and chronic diseases. Fermentation with lactic acid bacteria (LAB) is an effective method for degrading NIT and improving the flavor of pickled and cured foods. In this study, Limosilactobacillus fermentum (L. fermentum) RC4 with a high NIT degradation ability was found to degrade NIT in a new manner when compared with reported enzymatic and acid degradation, namely, metabolite degradation during fermentation in MRS broth, which shows a synergistic effect with acid to increase NIT degradation. Liquid chromatography-mass spectrometry analysis identified 134 significantly different metabolites, of which 11 metabolites of L. fermentum RC4, namely, γ-aminobutyric acid (GABA), isocitric acid, D-glucose, 3-methylthiopropionic acid (MTP), N-formyl-L-methionine, dimethyl sulfone (MSM), D-ribose, mesaconate, trans-aconitic acid, L-lysine, and carnosine, showed significant NIT degradation effects compared with the control group (MRS broth). Verification experiments showed that adding the above 11 metabolites to 100 mg/L NIT and incubating for 24 h resulted in NIT degradation rates of 5.07%, 4.41%, 6.08%, 16.93%, 5.28%, 2.41%, 0.93%, 18.93%, 12.25%, 6.42%, and 3.21%, respectively. Among these, three metabolites, namely, mesaconate, MTP, and trans-aconitic acid, showed efficient NIT degradation abilities that might be related to the degradation mechanism involving decarboxylation reactions. This is the first systematic study of NIT degradation by LAB, resulting in the identification of a new metabolite degradation pathway and three efficient NIT degradation metabolites.

The interaction between Lactobacillus plantarum SC-5 and its biogenic amine formation with different salt concentrations in Chinese Dongbei Suancai

Dongbei Suancai (DBSC) - a Chinese cabbage-based sauerkraut is a traditional fermented food which is popular in Asian countries. The biogenic amines that are usually generated during spontaneous fermentation have raised public health concern, while inoculation technology may solve this problem. In the current research, the biogenic amines, as well as their interactions with the microbial community in DBSC inoculated with Lactobacillus plantarum SC-5 or spontaneously fermented without inoculation were systematically investigated throughout 60 d fermentation. High-performance liquid chromatography analysis showed that the predominant biogenic amines in DBSC including putrescine, tyramine, spermidine, cadaverine and histamine increased during fermentation. Inoculated DBSC had a significantly lower content of total biogenic amines than the spontaneously fermented DBSC (216.72-237.33 mg/kg vs. 234.62-266.81 mg/kg) during 60 days' fermentation (P < 0.05). High throughput sequencing based on 16S rDNA identified 70 species in the bacterial community belonging to 7 genera of lactic acid bacteria, of which Lactobacillus, Leuconostoc and Lactococcus were dominant. Furthermore, six common genera of bacteria were positively correlated with biogenic amines based on Spearman's rank correlation test. Notably, the abundance of Lactobacillus plantarum SC-5 was negatively correlated with the content of biogenic amines in DBSC. In conclusion, inoculation of the proper starter like Lactobacillus plantarum SC-5 can reduce total biogenic amines in DBSC possibly by modifying the microbial communities in the fermented sauerkraut, which provides practical guidance for industrial production of high quality DBSC.

Improving the quality of Suancai by inoculating with Lactobacillus plantarum and Pediococcus pentosaceus

The quality characteristics of Suancai fermented with Lactobacillus plantarum CGMCC No.20193 (Lb. plantarum) and Pediococcus pentosaceus CGMCC No. 20192 (P. pentosaceus) were investigated. Their inoculation affected the bacterial communities revealed by Pacbio Sequel platform. After fermentation, the dominant phylum and genus in inoculation and spontaneous fermented Suancai were Firmicutes and Lactobacillus. Compared with single inoculation, the co-inoculation of Lb. plantarum and P. pentosaceus had a higher bacterial diversity. The Suancai co-inoculated with Lb. plantarum and P. pentosaceus had a more similar VCs profile with spontaneous fermented Suancai. The inoculation of Lb. plantarum and P. pentosaceus increased the content of organic acids, such as lactate, acetate, citrate, succinate, malate and tartrate. The most amino acids content in Suancai fermented with Lb. plantarum and P. pentosaceus were higher than that in spontaneous fermented Suancai. Compared single inoculation, the Suancai co-inoculated with Lb. plantarum and P. pentosaceus had a higher similarity of organoleptic tastes with spontaneous fermented Suancai. These results may facilitate the understanding of the starters' effects on the Suancai fermentation and the selection of applicable starters to manipulate the flavor.

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.

Microbiota Succession and Chemical Composition Involved in the Radish Fermentation Process in Different Containers

Traditional Chinese fermented vegetables are a type of brine-salted fermented vegetable product. During the spontaneous fermentation, various compounds are produced, degraded, and converted, influencing the quality of the fermented pickle. To ascertain the effect of different containers on the fermentation process of the pickles, this study investigated the bacterial diversity and the chemical composition characteristics of the pickle (radish) fermented in commonly used containers including glass jars (GL), porcelain jars (PO), and plastic jars (PL). The correlation between chemical compounds and microbial community was further analyzed. The changes in pH values suggested that PL may facilitate the quickest fermentation of the pickles, while the process in PO progressed at the lowest rate. The PL brine samples contained higher levels of lactic acid and threonine, while more abundant volatile chemical compounds were evident in PO. The container materials had no significant influence on the microbial structure, wherein Lactobacillus was the absolute dominant genus in all containers. But container material did have an effect on the abundance of specific genus, such as Lactococcus and Pediococcus. The correlation between these major genera was also analyzed and gene function prediction indicated that the top three pathways were: carbohydrate metabolism, amino acid metabolism, and energy metabolism. Lactobacillus negatively correlated with methionine, tyrosine, lysine, and arginine, but positively correlated with ammonia, and lactic acid and acetic acid both just correlated with Pediococcus. This study provides new insights into the microbiota succession and chemical compounds involved in the vegetable fermentation.

Effect of co-culture with Tetragenococcus halophilus on the physiological characterization and transcription profiling of Zygosaccharomyces rouxii

Zygosaccharomyces rouxii and Tetragenococcus halophilus are widely existed and play vital roles during the manufacture of fermented foods such as soy sauce. The aim of this study was to elucidate the effect of T. halophilus CGMCC 3792 on the physiological characterizations and transcription profiling of Z. rouxii CGMCC 3791. Salt tolerance analysis revealed that co-culture with T. halophilus enhanced the salt tolerance of Z. rouxii during salt stress. Analysis of the volatile compounds revealed that co-culture reduced the level of 1-butanol, improved the level of octanoic acid which all were produced by T. halophilus and reduced the level of phenylethyl alcohol produced by Z. rouxii. The presence of Z. rouxii decreased the contents of 3,4-dimethylbenzaldehyde and acetic acid produced by T. halophilus. In addition, co-culture improved the content of benzyl alcohol significantly. Analysis of membrane fatty acid showed that co-culture improved the content of palmitic (C16:0) and stearic (C18:0) acids in cells of Z. rouxii, and reduced the contents of myristic (C14:0), palmitoleic acid (C16:1) and oleic acid (C18:1). In order to further explore the interactions between the two strains, RNA-seq technology was used to investigate the effect of co-culture with T. halophilus on the transcription profiling of Z. rouxii. By comparing cells incubated in co-culture group with cells incubated in single-culture group, a total of 967 genes were considered as differentially expressed genes (DEGs). Among the DEGs, 72 genes were up-regulated, while 895 genes were down-regulated. These DEGs took party in various activities in cells of Z. rouxii, and the result showed co-culture with T. halophilus had a positive effect on proteolysis, the attachment of a cell to another cell, extracellular protein accumulation, energy metabolism, and a negative effect on oxidative phosphorylation, small molecular substances metabolism, DNA replication and repair, and transcription in cells of Z. rouxii. Results presented in this study may contribute to further understand the interactions between Zygosaccharomyces rouxii and Tetragenococcus halophilus.

Dynamic changes of bacterial communities and nitrite character during northeastern Chinese sauerkraut fermentation

Northeastern Chinese sauerkraut is a well-known traditional fermented vegetable in China. Incomplete identification of the microorganisms' (bacteria in spontaneous fermentation) diversity and accumulation of nitrite make it difficult to normalize the fermentation process and product qualities of northeastern Chinese sauerkraut. Conventional culturing and polymerase chain reaction-denaturing gradient gel electrophoresis methods were combined to describe microbial structure and diversity. Lactobacillus, Leuconostoc, Enterobacter, Accumulibacter, Thermotoga, Pseudomonas, Clostridium, Rahnella and Citrobacter were predominant microorganisms in different fermentation periods. The pH value and nitrite concentration presented a certain relevance to the amount of lactic acid bacteria. Lactobacillus and Leuconostoc had the ability to decrease nitrite by inhibiting nitrate-reducing bacteria such as Enterobacter. Therefore, Northeastern Chinese sauerkraut should not be eaten until 4 weeks of fermentation for the safety and quality of fermented foods. Northeastern Chinese sauerkraut is rich in lactic acid bacteria, which demonstrate its ability as an excellent probiotic for applications in functional foods.

Selection of Taste Markers Related to Lactic Acid Bacteria Microflora Metabolism for Chinese Traditional Paocai: A Gas Chromatography-Mass Spectrometry-Based Metabolomics Approach

Traditional paocai brine (PB) is continuously propagated by back-slopping and contains numerous lactic acid bacteria (LAB) strains. Although PB is important for the quality of paocai (Chinese sauerkraut), the taste features, taste-related compounds of PB-paocai and the effects of LAB communities from PB on the taste compounds remain unclear. An electronic tongue was used to evaluate the taste features of 13 PB-paocai samples. Umami, saltiness, bitterness, sweetness, and aftertaste astringency were the main taste features of PB-paocai. A total of 14 compounds were identified as discriminant taste markers for PB-paocai via gas chromatography-mass spectrometry (GC-MS)-based multimarker profiling. A LAB co-culture (Lactobacillus plantarum, Lactobacillus buchneri, and Pediococcus ethanoliduran) from PB could significantly increase glutamic acid (umami), sucrose (sweetness), glycine (sweetness), lactic acid (sourness), and γ-aminobutyric acid in PB-paocai, which would endow it with important flavor features. Such features could then facilitate starter screening and fermentation optimization to produce paocai-related foods with better nutritional and sensory qualities.