Determination of the fungal community of pit mud in fermentation cellars for Chinese strong-flavor liquor, using DGGE and Illumina MiSeq sequencing

Coordination between scientific and technological innovation and the high-quality development of Baijiu industry: The coupling and decoupling perspective

The Baijiu industry is a significant contributor to both the food industry and the light industry. Its high tax characteristics effectively promote the sustainable development of the regional economy. First, the evaluation index system of scientific and technological innovation (STI) and high-quality development of Baijiu industry (HQDBI) were constructed. The entropy-improved CRITIC method was used to measure the weights. Second, the coordination relationship and evolution trend of STI and HQDBI were explored using the coupling coordination model and the Tapio decoupling model. Then, the transfer law and key influencing factors were further investigated using the Markov chain and grey correlation, respectively. The main contribution is the dynamic evolution of the coupling and decoupling relationships from the perspective of multiple Baijiu provinces, and deeply depicts the coordination relationship and evolutionary trends of STI and HQDBI. The results show that: the spatial distribution of the coupling coordination degree shows high values in the east-west and low values in the north-south characteristics. In 2021, a pattern of coordinated development in Baijiu provinces has emerged along the Yangtze River basin. The decoupling state is mainly strong decoupling, but it remains poor in Shanxi. The coordination process is unstable and difficult to achieve leapfrog development. Coordination, sustainability and innovation environment have a greater impact on the coordination of subsystems.

Difference Analysis of the Composition of Iron (Hydr)Oxides and Dissolved Organic Matter in Pit Mud of Different Pit Ages in Luzhou Laojiao and Its Implications for the Ripening Process of Pit Mud

Long-term production practice proves that good liquor comes out of the old cellar, and the aged pit mud is very important to the quality of Luzhou-flavor liquor. X-ray diffraction, Fourier transform ion cyclotron resonance mass spectrometry, and infrared spectroscopy were used to investigate the composition characteristics of iron-bearing minerals and dissolved organic matter (DOM) in 2-year, 40-year, and 100-year pit mud and yellow soil (raw materials for making pit mud) of Luzhou Laojiao distillery. The results showed that the contents of total iron and crystalline iron minerals decreased significantly, while the ratio of Fe(II)/Fe(III) and the content of amorphous iron (hydr)oxides increased significantly with increasing cellar age. DOM richness, unsaturation, and aromaticity, as well as lignin/phenolics, polyphenols, and polycyclic aromatics ratios, were enhanced in pit mud. The results of the principal component analysis indicate that changes in the morphology and content of iron-bearing minerals in pit mud were significantly correlated with the changes in DOM molecular components, which is mainly attributed to the different affinities of amorphous iron (hydr)oxides and crystalline iron minerals for the DOM components. The study is important for understanding the evolution pattern of iron-bearing minerals and DOM and their interactions during the aging of pit mud and provides a new way to further understand the influence of aged pit mud on Luzhou-flavor liquor production.

Effects of six commercially available koji (Chinese Xiaoqu) on the production of ethyl acetate, ethyl lactate, and higher alcohols in Chinese Baijiu (distilled spirit) brewing

Commercial koji has been increasingly used in Chinese Baijiu brewing; however, there are only few studies comparing different koji and their relationship with key components of Chinese Baijiu such as ethyl acetate, ethyl lactate, and higher alcohols. Here, we studied six commercially available koji and showed that the microbial communities in the individual koji varied in composition, with Rhizopus, Aspergillus, and Bacillus primarily associated with starch hydrolysis and Saccharomyces mainly associated with alcohol production. In the brewing processes using the six koji, Saccharomyces was undoubtedly the most abundant fungus and Weissella, Bacillus, and Acinetobacter were the predominant bacterial groups. The levels of ethyl acetate, ethyl lactate, and higher alcohols in all brewing processes using the koji exhibited rapid increase in the early stages of fermentation, which stabilized in the later stages, followed by substantial increase after distillation. The results of metagenomic and redundancy analyses of samples taken during the brewing processes indicated that Saccharomyces from the koji was closely related to the production of ethyl acetate, ethyl lactate, and higher alcohols. This study provides a basis for the quality improvement and application of commercial koji.

Investigation on the key factors associated with flavor quality in northern strong aroma type of Baijiu by flavor matrix

The strong aroma type of Baijiu is popular with its balanced flavor. However, the flavor characteristics of strong aroma type of Baijiu in different regions have significant diversities. Among them, the key aroma compounds in northern strong aroma type of Baijiu and the factors affecting the aroma profile and flavor quality are still unclear. In this study, a total of 78 aroma compounds were identified by gas chromatography-olfactometry-mass spectrometry (GC-O-MS). The quantitative analysis was completed and ethyl hexanoate (1003.24-4506.04 mg/L) was the highest. Aroma profile was successfully simulated by reconstitution, and omission test was applied to investigate the effects of the important aroma compounds on the aroma profile. Additionally, the relationship between expression of aroma compounds and sensory characteristics of samples was illustrated by flavor matrix. The overall aroma profile of northern strong aroma type of Baijiu was formed by the key aroma compounds with different aroma expression.

Simulated Fermentation of Strong-Flavor Baijiu through Functional Microbial Combination to Realize the Stable Synthesis of Important Flavor Chemicals

The solid-state fermentation of Baijiu is complicated by the co-fermentation of many microorganisms. The instability of the composition and abundance of the microorganisms in the fermentation process leads to fluctuations of product quality, which is one of the bottleneck problems faced by the Strong-flavor Baijiu industry. In this study, we established a combination of functional microorganisms for the stable fermentation of the main flavor compounds of Baijiu, including medium and long-chain fatty acid ethyl esters such as hexanoic acid, ethyl ester; butanoic acid, ethyl ester; octanoic acid, ethyl ester; acetic acid, ethyl ester; 9,12-octadecadienoic acid, ethyl ester; and decanoic acid, ethyl ester in the fermented grains. Our study investigated the effects of microbial combinations on the fermentation from three aspects: microbial composition, microbial interactions, and microbial association with flavor compounds. The results showed that the added functional microorganisms (Lactobacillus, Clostridium, Caproiciproducens, Saccharomyces, and Aspergillus) became the dominant species in the fermentation system and formed positive interactions with other microorganisms, while the negative interactions between microorganisms were significantly reduced in the fermentation systems that contained both Daqu and functional microorganisms. The redundancy analysis showed that the functional microorganisms (Lactobacillus, Saccharomyces, Clostridium, Cloacibacterium, Chaenothecopsis, Anaerosporobacter, and Sporolactobacillus) showed strong positive correlations with the main flavor compounds (hexanoic acid, ethyl ester; lactic acid, ethyl ester; butanoic acid, ethyl ester; acetic acid, ethyl ester; and octanoic acid, ethyl ester). These results indicated that it was feasible to produce Baijiu with a functional microbial combination, and that this could promote stable Baijiu production.

Bacterial diversity associated with volatile compound accumulation in pit mud of Chinese strong-flavor baijiu pit

Pit mud quality is a key parameter that impacts the quality of Chinese strong-flavor Baijiu production.This study was developed to explore spatial bacterial community distributions and the relationships between these distributions and the volatile compound accumulation within the pit mud used in the production of Chinese strong-flavor Baijiu. The results revealed Lactobacillus pasteurii and Limnochorda pilosa were found to be the dominant bacteria present in the upper wall, middle wall, and bottom pit mud layers, whereas the Clostridium genus was detectable at high levels in the lower layer of the pit wall and played a role in contributing to the overall aroma and flavor compounds in produced Chinese strong-flavor Baijiu, with Clostridium abundance being strongly correlated with caproic acid, ethyl caproate, ethyl butyrate, and hexanol levels as well as moderately correlated with butyric acid levels. The abundance of the Lactobacillus genus was positively correlated with levels of ethyl lactate, 1-butanol, and 2,3-butanediol. Limnochorda pilosa was closely associated with ethyl acetate levels. Additionally, the correlations between bacterial communities and chemical properties also investigated, and the results demonstrated PO4^3-, total carbon, K⁺, humus, NH₄⁺-N, and Mg²⁺ levels significantly affected the bacterial community structure of pit mud, and they were positively correlated with the relative abundance of Clostridium. Together, these findings can serve as a foundation for future studies exploring the mechanisms whereby volatile compounds accumulate in different pit mud layers, which facilitates the fermentation regulation and pit mud quality improvement of Chinese strong-flavor Baijiu.

Fungal Diversity Profiles in Pit Mud Samples from Chinese Strong-Flavour Liquor Pit

Pit mud, a specific fermented soil, is an essential material for the fermentation of Chinese strong-flavour liquor. However, few studies to date have sought to characterize the spatial profiles of pit mud fungal communities in fermentation cellars from Chinese strong-flavour liquor distilleries. In this analysis, differences in fungal community structures and physicochemical properties in pit mud samples from different spatial positions within fermentation cellars were analyzed, revealing unique characteristic multidimensional pit mud fungal community profiles. Penicillium roqueforti, Pichia kudriavzevii, Aotearoamyces nothofagi, Penicillium robsamsonii, Alternaria arborescens, Trichosporon insectorum, Seltsamia ulmi, Trichosporon coremiiforme, Malassezia restricta were dominant in the pit mud samples form the upper cellar wall, whereas Metarhizium frigidum, Calonectria pseudoreteaudii, Penicillium clavigerum, Fusarium equiseti, Simplicillium chinense, Aspergillus intermedius, Trichosporon coremiiforme, Fusarium circinatum, Alternaria radicina, Aspergillus heterocaryoticus were predominant in the middle cellar wall. Alternaria radicina, Cladosporium chasmanthicola, Alternaria helianthiinficiens, Penicillium argentinense, Antarctomyces psychrotrophicus, and Trichosporon inkin are majorly present in the down cellar wall layer. Bipolaris axonopicola, Ramgea ozimecii, Penicillium argentinense, Calonectria queenslandica, Metarhizium robertsii, and Penicillium roqueforti were identified as the dominant fungi in pit mud samples from the cellar bottom. Additionally, Alternaria destruens and Alternaria doliconidium are present at notably high levels in all layers of pit mud samples. Moisture, pH, PO₄^3-, acetic acid, humus, K⁺, Mg²⁺, Ca²⁺, butyric acid, and caproic acid levels in these different pit mud positions exhibited a rising incremental pattern from the upper wall layer to the bottom layer, whereas lactic acid levels were significantly lower in the bottom pit mud layer relative to these other layers. Moisture, pH, and NH₄⁺-N were identified as the three most significant factors associated with fungal community composition through a redundancy analysis. Overall, these findings may offer a theoretical foundation for future efforts to improve or standardize artificial pit mud.

Effects of Daqu Attributes on Distribution and Assembly Patterns of Microbial Communities and Their Metabolic Function of Artificial Pit Mud

Daqu provides functional microbiota and various nutrients for artificial pit mud (APM) cultivation. However, little is known about whether its attributes affect the microbiome and metabolome of APM. Here, two types of APM were manufactured by adding fortified Daqu (FD) and conventional Daqu (CD); they were comprehensively compared by polyphasic detection methods after being used for two years. The results showed that FD altered the prokaryotic communities rather than the fungal ones, resulting in increased archaea and Clostridium_sensu_stricto_12 and decreased eubacteria and Lactobacillus. Correlation analysis suggested that these variations in community structure promoted the formation of hexanoic acid, butyric acid, and the corresponding ethyl esters, whereas they inhibited that of lactic acid and ethyl lactate and thus improved the flavor quality of the APM. Notably, pH was the main driving factor for the bacterial community variation, and the total acid mediated the balance between the stochastic and the deterministic processes. Furthermore, the results of the network analysis and PICRUSt2 indicated that FD also enhanced the modularity and robustness of the co-occurrence network and the abundance of enzymes related to hexanoic acid and butyric acid production. Our study highlights the importance of Daqu attributes in APM cultivation, which are of great significance for the production of high-quality strong-flavor Baijiu.

Effects of Different Daqu on Microbial Community Domestication and Metabolites in Nongxiang Baijiu Brewing Microecosystem

The quality and yield of the fresh Baijiu mainly depend on the activity of pit mud (PM) and the quality of Daqu. However, the cultivation of PM is a long-term process, and high-quality Daqu can change the community structure of fermented grain (FG) and accelerate the evolution of PM communities. The present research aimed to investigate the four different types of Daqu on the FG-fermenting microbial community structure and metabolites and their interphase interactions with PM. These results show that Kroppenstedtia in the bacterial community of Taikong Daqu (TK) was positively correlated with ethyl caproate, which significantly increased the content of FG volatile metabolites, especially lipid components, and facilitated the accelerated evolution of Methanobacteriales and Methanosarcinales in PM. Bacillus has a high relative abundance in Qianghua Daqu (QH), which shows obvious advantages to improving the alcoholic strength of FG and contributing to increasing the abundance of Methanomicrobiales in PM. Qianghua and traditional-mixed Daqu (HH) have a similar bacterial composition to QH and a similar fungal composition to traditional Daqu (DZ), and thus also showed the advantage of increased yield, but the volatile flavor metabolites produced were not as dominant as DZ. β-diversity analysis showed that in TK fermentation systems, FG is more likely to domesticate the structure of PM microorganisms. These results indicated that the interaction between microbial communities in Baijiu fermentation niches was significantly influenced by different Daqu. It can not only enhance the key volatiles in FG but also accelerate the evolving direction of the community in PM. Daqu fortified by functional genera or microbiota can evolve a community structure more suitable for Baijiu fermentation. The microbiota composition and interaction between the communities in both Daqu and PM significantly impacts the yield and quality of the base liquor.

Flavor mystery of Chinese traditional fermented baijiu: The great contribution of ester compounds

Chinese traditional fermented baijiu is a famous alcoholic beverage with unique flavor. Despite its consumption for millennia, the flavor mystery behind baijiu is still unclear. Studies indicate that esters are the most important flavor substances, and bring health benefits. However, the aroma contribution and formation mechanism of esters still need to be clarified to reveal the flavor profile of baijiu. This review systematically summarizes all the 510 esters and finds 9 ethyl esters contribute greatly to the flavor of baijiu. The 508 different microbial species that have been identified affect the synthesis of esters through fatty acid and amino acid metabolism. The determination of minimum functional microbial groups and the analysis of their metabolic characteristics are crucial to reveal the mechanism of formation of baijiu flavor, and ensure the reproducible formation of flavor substances.

The Depth-Depended Fungal Diversity and Non-depth-Depended Aroma Profiles of Pit Mud for Strong-Flavor Baijiu

Microorganisms in pit mud are the essential factor determining the style of strong flavor Baijiu. The spatial distribution characteristics of fungal communities and aroma in the pit mud for strong flavor Baijiu from Xinjiang, China, were investigated using Illumina MiSeq high-throughput sequencing and electronic nose technology. A total of 138 fungal genera affiliated with 10 fungal phyla were identified from 27 pit mud samples; of these, Saccharomycopsis, Aspergillus, and Apiotrichum were the core fungal communities, and Aspergillus and Apiotrichum were the hubs that maintain the structural stability of fungal communities in pit mud. The fungal richness and diversity, as well as aroma of pit mud, showed no significant spatial heterogeneity, but divergences in pit mud at different depths were mainly in pH, total acid, and high abundance fungi. Moisture, NH4 +, and lactate were the main physicochemical factors involved in the maintenance of fungal stability and quality in pit mud, whereas pH had only a weak effect on fungi in pit mud. In addition, the fungal communities of pit mud were not significantly associated with the aroma. The results of this study provide a foundation for exploring the functional microorganisms and dissecting the brewing mechanism of strong flavor Baijiu in Xinjiang, and also contributes to the improvement of pit mud quality by bioaugmentation and controlling environmental physicochemical factors.

Aminipila luticellarii sp. nov., an anaerobic bacterium isolated from the pit mud of strong aromatic Chinese liquor, and emended description of the genus Aminipila

A novel mesophilic, aerotolerant anaerobic bacterium, designated JN-18^T, was isolated from the pit mud of a strong aromatic Chinese liquor. According to a 16S rRNA gene sequence analysis, it had the highest sequence similarity to Aminipila butyrica DSM 103574^T (95.69%). The G+C content of its genomic DNA was 43.39 mol%. The cells were Gram-stain-negative, slightly curved rods with flagella. Optimum growth was observed at 37 °C, pH 6.5 and without extra addition of NaCl. Strain JN-18^Tutilized amino acids (l-alanine, l-arginine, l-asparagine, l-lysine, l-methionine, l-serine and l-threonine), malate and pyruvate, and used l-arginine and l-lysine to produce acetate, butyrate, H₂, and CO₂. The major cellular fatty acids of strain JN-18^T were C_14:0, C_16:0 DMA and C_18:1 cis-9 DMA. The carbohydrate composition of the cell wall predominantly included galactose, glucose and rhamnose. Based on its phylogenetic, phenotypic, physiological and biochemical characteristics, strain JN-18^T was classified as a representative of a novel species within the genus Aminipila, for which the name Aminipila luticellarii sp. nov. is proposed. The type strain is JN-18^T (=CCAM 412^T=JCM 39126^T).

Analysis of the difference between aged and degenerated pit mud microbiome in fermentation cellars for Chinese Luzhou-flavor baijiu by metatranscriptomics

BACKGROUD

Chinese Luzhou-flavor baijiu (LFB) was fermented in an underground cellar, and the bottom and side of the cellar were covered with pit muds (PMs), where the metabolic activity of the microorganisms had a significant effect on the LFB quality. PMs can be divided into aged pit mud (AP) and degenerated pit mud (DP), thus, the qualities of LFB generated from AP and DP were different. In this essay, metatranscriptomics method was applied to illustrate the differences of the two PMs, as well as to search out the pivotal microorganisms and genes influencing the quality of LFB.

RESULTS

Archaea, Clostridium and some thermophilic microorganisms might bring significant effect in AP, while the active eukaryota and Anaeromyxobacter would cause degeneration in PM. Also, the metabolism of carbohydrate and amino acid were more active in AP. What is more, carbohydrate, amino acid and their derivant can produce important organic acids via the activity of the microorganisms in PMs. There were eight critical enzymes noticed in the organic acids metabolic pathway, which were more actively expressed in AP, demonstrating active expression of the critical genes related to organic acid metabolism could have a positive effect on LFB quality.

CONCLUSION

This study identified specific differences in active microorganisms, active expressed genes and the expression levels of key genes in vital metabolic pathway between AP and DP. Which may be the actual reason for the differences in the quality of LFB made from different PMs. Mastering these results will provide assistance to improve the quality of LFB. © 2021 Society of Chemical Industry.

Furfural and organic acid targeted carbon dot sensor array for the accurate identification of Chinese baijiu

Baijiu quality control has always been a major challenge for researchers. In this paper, taking furfural which is closely related to baijiu brewing process and organic acids related to baijiu fermentation process and microorganism types as the main discriminating factors, a carbon dot (CDs) sensor array targeting furfural and organic acids was constructed to identify 41 kinds of baijiu. Through the fluorescence response investigation of CDs synthesized by isomers of benzenediol, aminophenol, and phenylenediamine to different baijiu, two CDs synthesized by meta-benzene substitutions containing -NH₂ were selected to build a fluorescence sensor array. Due to the aggregation-induced enhancement effect between furfural and the CDs, and the protonation of organic acid and the CDs, different fluorescence changes were observed, the sensor array combined with partial least squares regression could quantitatively analyze furfural and organic acids. What is more, semi-quantitative analysis of furfural and lactic acid in baijiu was performed. Owing to the interaction of the two CDs with furfural and organic acids in baijiu, the sensor array could accurately identify different baijiu through linear discriminant analysis. This sensor array has potential applications in the quantitative analysis of flavor substances in other alcoholic beverages, moreover, this method could provide a quick response and practical tool for real-time quality control monitoring in the baijiu industry.

Cooperation within the microbial consortia of fermented grains and pit mud drives organic acid synthesis in strong-flavor Baijiu production

Mud cellars have long been used as anaerobic bioreactors for the fermentation of Chinese strong-flavor Baijiu, where starchy raw materials (mainly sorghum) are metabolized to ethanol and various flavor compounds by multi-species microorganisms. Jiupei (fermented grains) and pit mud are two spatially linked microbial habitats in the mud cellar, yet their metabolic division of labor remains unclear. Here, we investigated the changes in environmental variables (e.g., temperature, oxygen, pH), key metabolites (e.g., ethanol, organic acids) and microbial communities in jiupei and pit mud during fermentation. Jiupei (low pH, high ethanol) and pit mud (neutral pH) provided two habitats with distinctly different environmental conditions for microbial growth. Lactic acid accumulated in jiupei, while butyric and hexanoic acids were mainly produced by microbes inhabiting the pit mud. Biomass analysis using quantitative real-time PCR showed that bacteria dominated the microbial consortia during fermentation, moreover cluster and principal coordinate analysis (PCoA) analysis showed that the bacterial communities of jiupei and pit mud were significantly divergent. The bacterial community diversity of jiupei decreased significantly during the fermentation process, and was relatively stable in pit mud. Lactobacillus dominated the jiupei bacterial community, and its relative abundance reached 98.0% at the end of fermentation. Clostridia (relative abundance: 42.9-85.5%) was the most abundant bacteria in pit mud, mainly distributed in the genus Hydrogenispora (5.3-68.4%). Fungal communities of jiupei and pit mud showed a similar succession pattern, and Kazachstania, Aspergillus and Thermoascus were the predominant genera. PICRUSt analysis demonstrated that enzymes participating in the biosynthesis of acetic and lactic acid were mainly enriched in jiupei samples, while the bacterial community in the pit mud displayed greater potential for butyric and hexanoic acid synthesis. Assays from an in vitro simulated fermentation further validated the roles of jiupei microbiota in acetic and lactic acid production, and these acids were subsequently metabolized to butyric and hexanoic acid by the pit mud microbiota. This work has demonstrated the synergistic cooperation between the microbial communities of jiupei and pit mud for the representative flavor formation of strong-flavor Baijiu.

Metagenome and analysis of metabolic potential of the microbial community in pit mud used for Chinese strong-flavor liquor production

Complex microbiomes of pit mud (PM) play significant roles in imbuing flavors and qualities of Chinese strong-flavor liquor (CSFL) during fermentation. However, understanding both of the taxonomic and functional diversity of the whole microorganisms in PM still remain a major challenge. Here, PM microbiomes were investigated based on metagenomic sequencing, assembly and binning. Metagenomic data revealed that Euryarchaeota was the predominant phylum, followed by Firmicutes, Proteobacteria, Bacteroidetes and Actinobacteria. For further functional exploration, 703 metagenome-assembled genomes (MAGs), including 304 novel strains, 197 novel species, and 94 novel genera were reconstructed. Three primary groups of Firmicutes (n = 406), Euryarchaeota (n = 130) and Bacteroidetes (n = 74), particularly genus of them Syntrophomonas, Thermacetogenium and Clostridium, methanogens (Methanobacterium, Methanoculleus, and Methanosarcina), Proteiniphilum and Prevotella, contained most of metabolic potential genes. Additionally, Chloroflexi was firstly reported to have potential to be involved in the caproic acid (CA) production. Bacteroidetes could be the key phylum to synthesize terpenes, and Armatimonadetes, Firmicutes, Ignavibacteriae and Verrucomicrobia may possess the same metabolic potential as well. Overall, this study will significantly improve our understanding of the diverse PM microbiome and help guide the future exploration of microbial resources for modifying PM fermentation processes.

Daqu microbiota exhibits species-specific and periodic succession features in Chinese baijiu fermentation process

Daqu, a brick-shaped product spontaneously fermented under an open environment, has been regarded as the starter of fermentation, raw enzyme preparation and raw materials for baijiu production. However, its contribution in baijiu fermentation has not been fully elaborated yet. Here, the effects of daqu microbiota on baijiu fermentation were investigated under both field-scale and lab-scale conditions. In field-scale baijiu fermentation, the dominant daqu microbes (average relative abundance>10.0%), including unclassified_Leuconostocaceae, Thermoascus, and Thermomyces, tended to dominate the early stage (0-7 d). However, the rare daqu microbes (average relative abundance <0.1%, e.g., Kazachstania) tended to dominate the middle and late stages (11-40 d). In addition, some genera showed differences in species diversity between daqu and fermented grains. The average relative abundance of Lactobacillus was over 75% during baijiu fermentation, and most of them were affiliated with Lactobacillus acetotolerans, while Lactobacillus crustorum dominated the Lactobacillus OTUs in daqu. The similar patterns were also observed during lab-scale baijiu fermentation. The results of function prediction showed the enriched metabolic pathways were associated with glycolysis and long-chain fatty acid esters in baijiu fermentation. These results improved the understanding of daqu microbiota function during baijiu fermentation and provided a basic theory to support the regulation of baijiu production.

Fungal community diversity and fermentation characteristics in regional varieties of traditional fermentation starters for Hong Qu glutinous rice wine

Hong Qu glutinous rice wine (HQ wine) is a traditional alcoholic beverage produced in China by fermenting cooked rice using a fermentation starter prepared with the fungus Monascus purpureus. This starter (Hong Qu, HQ) is made empirically by open spontaneous fermentation that is hard to control and standardize, resulting in inconsistent wine quality. This study investigates representative HQ samples from a large geographic region. It explores fungal microbiome compositions, identifies characteristic differences important for the production of various HQ wine styles, and reveals the key fungi responsible for HQ wine fermentation characteristics. The source of the HQ inoculum was found to be the main factor influencing fungal community composition and diversity, followed by processing technology and geographical distribution. Linear discriminant analysis effect size (LEfSe) uncovered 14 genera as potential biomarkers to distinguish regional varieties of HQ. Significant differences were also found in fermentation characteristics such as liquefying power (LP), saccharifying power (SP), fermenting power (FP), total acid content (TA) and liquor-producing power (LPP). The key fungi responsible for LP (5 genera), SP (3 genera), FP (1 genera), LPP (4 genera), and TA (4 genera) were determined using redundancy correlation analysis. Finally, Spearman's correlation analysis indicated that LPP shows a strong positive correlation with FP and LP, while TA displays a strong negative correlation with FP. The results of this study may be utilized to prepare consistently high quality, next-generation HQ by better controlling fungal community structures, and to design fermentation processes for HQ wines with desirable oenological characteristics.

Bacterial diversity and flavor profile of Zha-Chili, a traditional fermented food in China

Zha-chili is a traditional Chinese fermented food with special flavor, which is often used as an appetizer in condiments and an important energy source. The final quality of zha-chili is closely related to its microbial community structure. However, the differences of bacterial diversity in zha-chili from different regions and how bacterial species affect zha-chili fermentation process and flavor quality have not been reported. In this study, the bacterial diversity and flavor quality of zha-chili samples from different regions were analyzed using Illumina Miseq high-throughput sequencing, electronic nose and electronic tongue technology. Twenty-three bacterial phyla and 665 bacterial genera were identified in all zha-chili samples. Firmicutes, Proteobacteria and Actinobacteria were the dominant bacterial phyla in zha-chili samples, while Lactobacillus, Pseudomonas, Pediococcus, Weissella and Staphylococcus were the dominant bacterial genera. The bacterial community structure of zha-chili samples from different regions was significantly diverse (p < 0.05). The flavor of zha-chili samples also varied in different regions, and the discrepancy of taste was much greater than that of aroma. Moreover, there were significant correlations (p < 0.05) between 6 dominant bacterial genera and 8 flavor indicators (3 aroma indicators, 5 taste indicators). In addition, the results of microbiome phenotypes prediction by BugBase and bacterial functional potential prediction using PICRUSt showed that eight out of nine predicted phenotypic functions of zha-chili samples from different regions were significantly different (p < 0.05), bacterial metabolism was vigorous in the zha-chili samples, and Lactobacillus was the dominant bacterial genus involved in metabolism during fermentation.

Characterization of Microbial Diversity and Community Structure in Fermentation Pit Mud of Different Ages for Production of Strong-Aroma Baijiu

In the traditional fermentation process of strong-aroma Baijiu, a fermentation pit mud (FPM) provides many genera of microorganisms for fermentation. However, the functional microorganisms that have an important effect on the quality of Baijiu and their changes with the age of fermentation pit (FP) are poorly understood. Herein, the Roche 454 pyrosequencing technique and a phospholipid fatty-acid analysis were employed to reveal the structure and diversity of prokaryotic communities in FPM samples that have been aged for 5, 30, and 100 years. The results revealed an increase in total prokaryotic biomass with an FP age; however, Shannon’s diversity index decreased significantly (p < 0.01). These results suggested that a unique microbial community structure evolved with uninterrupted use of the FP. The number of functional microorganisms, which could produce the flavor compounds of strong-aroma Baijiu, increased with the FP age. Among them, Clostridium and Ruminococcaceae are microorganisms that directly produce caproic acid. The increase of their relative abundance in the FPM might have improved the quality of strong-aroma Baijiu. Syntrophomonas, Methanobacterium, and Methanocorpusculum might also be beneficial to caproic acid production. They are not directly involved but provide possible environmental factors for caproic acid production. Overall, our study results indicated that an uninterrupted use of the FP shapes the particular microbial community structure in the FPM. This research provides scientific support for the concept that the aged FP yields a high-quality Baijiu.

Basic flavor types and component characteristics of Chinese traditional liquors: A review

Chinese traditional liquor, a major type of global distilled spirits, offers a unique flavor system acquired across thousands of years of development. Owing to the various raw brewing materials, types of koji, fermentation vessels, and processes used during liquor production, significant differences can occur in the content of flavor chemical components, such as esters, alcohols, aromatics, ketones, nitrogen compounds, acids, and aldehydes in the resulting liquor. Therefore, the liquor can be characterized on the basis of four basic flavors: sauce-, strong-, light-, and rice-aroma, and eight derivative flavors: feng-, sesame-, chi-, te-, mixed-, laobaigan-, herbal-, and fuyu-aroma. In this review, we describe the production and development process of Chinese traditional liquor in detail; summarize the flavor types, flavor chemical composition characteristics, and research progress related to this liquor; and discuss the influence of trace chemical components on liquor flavor, with the aim of laying a theoretical foundation for stabilizing the quality and increasing the yield of traditional liquor.

Metaproteomics insights into traditional fermented foods and beverages

Traditional fermented foods and beverages (TFFB) are important dietary components. Multi-omics techniques have been applied to all aspects of TFFB research to clarify the composition and nutritional value of TFFB, and to reveal the microbial community, microbial interactions, fermentative kinetics, and metabolic profiles during the fermentation process of TFFB. Because of the advantages of metaproteomics in providing functional information, this technology has increasingly been used in research to assess the functional diversity of microbial communities. Metaproteomics is gradually gaining attention in the field of TFFB research because it can reveal the nature of microorganism function at the protein level. This paper reviews the common methods of metaproteomics applied in TFFB research; systematically summarizes the results of metaproteomics research on TFFB, such as sauces, wines, fermented tea, cheese, and fermented fish; and compares the differences in conclusions reached through metaproteomics versus other omics methods. Metaproteomics has great advantages in revealing the microbial functions in TFFB and the interaction between the materials and microbial community. In the future, metaproteomics should be further applied to the study of functional protein markers and protein interaction in TFFB; multi-omics technology requires further integration to reveal the molecular nature of TFFB fermentation.

Culturing Bacteria From Fermentation Pit Muds of Baijiu With Culturomics and Amplicon-Based Metagenomic Approaches

The Baijiu-making microbiota has an important role in the alcohol production, flavor, and character of Baijiu. 16S rRNA gene sequencing revolutionized the understanding of Baijiu-making microbiota. In this study, nine phyla, 23 classes, 49 orders, 99 families, and 201 genera were detected in pit muds (PMs) by 16S rRNA gene sequencing. Firmicutes and Bacteroidetes predominated (>99%). At the order level, Clostridiales, Bacteroidales, and Bacillales predominated (>92%). At the genus level, Hydrogenispora, Petrimonas, Proteiniphilum, and Sedimentibacter predominated. The pure culture of Baijiu-making prokaryotes was essential to elucidating the role of these microbes in the fermentation of Baijiu. According to the theory of microbial culturomics, a culturing approach with multiple culture conditions was adopted, combining 16S rRNA gene sequencing. We identified 215 prokaryotic strains, which were assigned to 66 species, 41 genera, four phyla, and 19 potential new species. Gas conditions were key factors in culturomics. In addition, culturomics significantly increased the number of species isolated from the fermentation PM compared with previous reports. With culturomics, the diversity spectrum of culturable bacteria in the PM was increased 273.33% at the genus level. This study confirms the complementary role of culturomics in the exploration of complex microbiota.

Analyzing bacterial community in pit mud of Yibin Baijiu in China using high throughput sequencing

“Yibin Baijiu” (YB) is a special Chinese strong-aroma Baijiu (CSAB) that originated in Yibin, a city in western China. YB is fermented in cellars lined with pit mud (PM), the microbiota in which may affect YB quality. In this study, high throughput sequencing of the 16S rRNA gene was used to demonstrate the bacterial community structure and diversity in PM of YB. In addition, the physicochemical characteristics of PM were also analyzed, including moisture content, pH, and available phosphorous, ammonia nitrogen, and humic acid levels. Results showed that Firmicutes was the dominant phylum in all PM samples with abundance > 70.0%, followed by Euryarchaeota (11.3%), Bacteroidetes (6.5%), Synergistetes (3.0%), Actinobacteria (1.4%), and Proteobacteria (1.2%). Furthermore, 14 different genera with average relative abundance of > 1% were detected. The Chao1 and Shannon indexes did not vary significantly between the sub-layer and middle-layer PM (P > 0.05). However, Linear discriminant analysis Effect Size (LEfSe) analysis showed that the relative abundance of Lactobacillus in the sub-layer PM was significantly higher than in middle-layer PM. pH differed significantly (P < 0.05) between the two groups. Canonical correspondence analysis revealed that bacterial community in PM correlated significantly with available phosphorous content and pH. Our study provides basic data for further elucidating the diversity of microbiota in the PM of YB and the potential mechanism of Baijiu production.

Major Metabolites and Microbial Community of Fermented Black Glutinous Rice Wine With Different Starters

Black glutinous rice wine (BGRW) is a traditional Chinese rice wine that is brewed using multiple strains. However, the roles of these microorganisms, particularly their contributions to aroma formation, are poorly understood. Accordingly, the main goal of this study was to determine the microbial communities and major metabolites of different traditional fermentation starters. Anshun (AS) starter and Xingyi (XY) starter were used for BGRW to provide insight into their potential contributions to the variation in flavor and aroma. High-throughput sequencing of the microbial community using the Illumina MiSeq platform revealed significant differences during fermentation between the two starter groups. Pediococcus, Leuconostoc, and Bacillus were the dominant bacterial genera in the AS group, whereas Leuconostoc, Pediococcus, and Gluconobacter were the dominant genera in the XY group. In addition, Rhizopus, Saccharomyces, and Saccharomycopsis were the predominant fungal genera detected in both samples. The major metabolites in the two groups were identified by high-performance liquid chromatography and headspace-solid-phase microextraction gas chromatography–mass spectrometry. A total of seven organic acids along with 47 (AS) and 43 (XY) volatile metabolites were detected, among which lactic acid was the primary organic acid, and esters were the largest group in both types of wine. Principal components analysis further revealed significant differences in the dynamic succession of metabolites between the two samples. Correlation analysis showed that 22 and 17 microorganisms were strongly correlated with the production of major metabolites in AS and XY, respectively. Among them, Pediococcus, Leuconostoc, Lactobacillus, Lactococcus, and Streptococcus were shown to play crucial roles in metabolite synthesis. Overall, this study can provide a valuable resource for the further development and utilization of starters to improve the aromatic quality of BGRW.

Bioinoculants play a significant role in shaping the rhizospheric microbial community: a field study with Cajanus cajan

The present study is an attempt to understand the impact of bioinoculants, Azotobacter chroococcum (A), Bacillus megaterium (B), Pseudomonas fluorescens (P), on (a) soil and plant nutrient status, (b) total resident and active bacterial communities, and (c) genes and transcripts involved in nitrogen cycle, during cultivation of Cajanus cajan. In terms of available macro- and micro-nutrients, triple inoculation of the bioinoculants (ABP) competed well with chemical fertilizer (CF). Their 'non-target' effects were assessed in terms of the abundance and activity of the resident bacterial community by employing denaturing gradient gel electrophoresis (DGGE). The resident bacterial community (16S rRNA gene) was stable, while the active fraction (16S rRNA transcripts) was influenced (in terms of abundance) by the treatments. Quantification of the genes and transcripts involved in N cycle by qPCR revealed an increase in the transcripts of nifH in the soil treated with ABP over CF, with an enhancement of 3.36- and 1.57- fold at flowering and maturity stages of plant growth, respectively. The bioinoculants shaped the resident microflora towards a more beneficial community, which helped in increasing soil N turnover and hence, soil fertility as a whole.

Bioturbation effect of fortified Daqu on microbial community and flavor metabolite in Chinese strong-flavor liquor brewing microecosystem

Traditional spontaneous fermentation in microecosystem with microbial successions and environmental variables results in inconsistent quality of fermented foods. We therefore propose the directional bioturbation for microbiota regulation and metabolites production in food fermentation. Here, we revealed the bioturbation effect of fortified Daqu on microbial community based on taxonomic composition, co-occurrence network, and metabolic potential, using Chinese strong-flavor liquor fermentation as a microecosystem. According to principal coordinate analysis, microbial communities were obviously influenced by the bioturbation of fortified Daqu. More specifically, bioturbation increased the abundances of Caproiciproducens, Clostridium, Aspergillus, Candida, Methanobacterium, and Methanosarcina, while decreased that of Lactobacillus. Meanwhile, higher abundances of most genes that encoding enzymes involved in interspecies hydrogen transfer between hexanoic acid bacteria and methanogens were observed in the bioturbated ecosystem by PICRUSt approach. Additionally, co-occurrence analysis showed that bioturbation increased the diversity and complexity of interspecies interactions in microecosystem, which contributed to higher production of flavor metabolites such as hexanoic acid, ethyl hexanoate, and hexyl hexanoate. These results indicated that the bioturbation of fortified Daqu is feasible for flavor metabolism by interspecies interactions of functional microbiota in liquor fermentation. Taken together, it is of great importance for regulating Chinese liquor and even other foods fermentation by bioturbation.

Contrasting bacterial community structure in artificial pit mud-starter cultures of different qualities: a complex biological mixture for Chinese strong-flavor Baijiu production

The complex starter culture for artificial pit mud (APMSC) hosts a wide variety of microbial communities that play a crucial role in Chinese strong-flavor Baijiu production. Based on its organoleptic properties, the quality of APMSC can be divided into normal and inferior quality grades. However, the relationship between the APMSC microbial community and APMSC quality is poorly understood. In this study, the bacterial community structure in normal and inferior APMSC derived from two different production batches was analyzed using denaturing gradient gel electrophoresis and Illumina MiSeq sequencing. Highly similar patterns of bacterial diversity and community structure were observed in the APMSC samples of the same quality, and a significant higher bacterial species diversity (Shannon index and Chao1) was detected in the normal compared to the inferior APMSC samples. Fifteen genera were detected in the APMSC samples, and seven (Caproiciproducens, Clostridium, Lactobacillus, Bacillus, Pediococcus, Rummeliibacillus, and Sporolactobacillus) were dominant, accounting for 92.12-99.89% of total abundance. Furthermore, the bacterial communities in the normal and inferior APMSC had significantly different structure and function. The normal APMSC was characterized by abundant Caproiciproducens and Clostridium and high caproic and butyric acid contents. In contrast, the inferior APMSC was overrepresented by Lactobacillus and Bacillus and lactic and acetic acids. This study may help clarify the key microbes sustaining APMSC ecosystem stability and functionality, and guide future improvements in APMSC production.

Effect of Fermentation Processing on the Flavor of Baijiu

Baijiu, otherwise known as the national liquor of China, is a well-known distilled spirit that has been produced for over 2000 years. Baijiu is a clear and transparent fermented alcoholic beverage containing more than 1870 volatile compounds, including esters, alcohols, aromatics, ketones, heterocycles, nitrogenous compounds, acids, aldehydes, terpenes, sulfur compounds, acetals, and lactones. This perspective describes the baijiu production process, summarizes recent progress in the development and study of baijiu flavor chemistry, discusses the effect of fermentation processing on baijiu flavor, and presents novel development trends in the baijiu industry in the dual directions of flavor and health. Furthermore, the direction of research and development for modernization in the baijiu industry is discussed and proposed.

Comparison of Bacterial Diversity Between Two Traditional Starters and the Round-Koji-Maker Starter for Traditional Cantonese Chi-Flavor Liquor Brewing

Xiaoqu is a traditional fermentation starter that is used for Chinese liquor production. Although microorganisms in the starters are closely associated with the quality and flavor of liquor, knowledge of the microbiota in xiaoqu is still far from complete, let alone the starters produced by new processes. Here, Illumina MiSeq high-throughput sequencing was applied to study bacterial composition in three types of xiaoqu used in Cantonese soybean-flavor (Chi-flavor) liquor, namely two traditional starters (Jiu Bing and Bing Wan) and a Round-Koji-maker starter (San qu) produced by the automatic starter-making disk machine. The results showed bacterial diversity in traditional starters was similar and higher than that in the Round-Koji-maker starter. Lactobacillus and Pediococcus were the dominant genera in all starters, while other different dominant genera also existed in different starters, which were Weissella, Acetobacter, and Gluconobacter for Jiu Bing, Weissella for Bing Wan, and Bacillus, Acetobacter, Acinetobacter and Klebsiella for San qu, respectively. Meanwhile, Cytophagaceae, one particular microbial family, and some pathogens including Klebsiella, Cronobacter, and Enterobacter were also found in San qu, indicating the automatic starter-making disk machine should be ameliorated before applied into industrial production. These results enriched our knowledge on xiaoqu-related microorganisms and might be helpful in industrial Chi-flavor liquor production and the development of fermentation technology.

Metaproteomics of Microbiota in Naturally Fermented Soybean Paste, Da-jiang

Da-jiang is a typical traditional fermented soybean product in China. At present, the proteins in da-jiang are needed to be explored. The composition and species of microbial proteins in traditional fermented da-jiang were analyzed by metaproteomics based on sodium dodecyl sulfonate-polyacrylamide gel electrophoresis (SDS-PAGE) and liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). The results showed that the number and variety of microbial proteins in the traditional fermented da-jiang from different regions were different. The production site influences the fermentation in da-jiang. Then we analyzed the functions of the microbial proteins identified in da-jiang, and found that they were mainly involved in the process of protein synthesis, glycometabolism and nucleic acid synthesis. In addtion, we compared the proteins composition in different da-jiang. There are 51 common proteins of naturally fermented da-jiang, and 25 common microbial sources. The main commonly microbial sources of fungal proteins are Saccharomyces cerevisiae and Schizosaccharomyces; the main commonly microbial sources of bacterial proteins are Enterococcus faecalis, Leuconostoc mesenteroides, Acinetobacter baumannii, and Bacillus subtilis. These common microbes play the predominant role in da-jiang fermentation. The present results help us to understand the fermentation of da-jiang and improve the quality and safety of final products in the future.

PRACTICAL APPLICATION

The study illustrated metaproteome of microbiota in traditional fermented soybean paste, da-jiang, by sodium dodecyl sulfonate-polyacrylamide gel electrophoresis (SDS-PAGE) and liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS). A method of extracting metaproteome from microbiota in da-jiang was attempted. The findings help to understand the fermentation of da-jiang and improve the quality and safety of da-jiang in fermented industry.

Diversity and Function of Microbial Community in Chinese Strong-Flavor Baijiu Ecosystem: A Review

Strong flavor baijiu (SFB), also called Luzhou-flavor liquor, is the most popular Chinese baijiu. It is manufactured via solid fermentation, with daqu as the starter. Microbial diversity of the SFB ecosystem and the synergistic effects of the enzymes and compounds produced by them are responsible for the special flavor and mouthfeel of SFB. The present review covers research studies focused on microbial community analysis of the SFB ecosystem, including the culturable microorganisms, their metabolic functions, microbial community diversity and their interactions. The review specifically emphasizes on the most recently conducted culture-independent analysis of SFB microbial community diversity. Furthermore, the possible application of systems biology approaches for elucidating the molecular mechanisms of SFB production were also reviewed and prospected.

Deep sequencing reveals high bacterial diversity and phylogenetic novelty in pit mud from Luzhou Laojiao cellars for Chinese strong-flavor Baijiu

The pit mud (PM) in fermentation cellar is a complex ecosystem that hosts diverse microbial communities that contribute to the production of Chinese strong-flavor Baijiu (CSFB). However, the microbial ecology of PM, particularly the extent of their phylogenetic novelty remains poorly understood. Here we conducted Illumina MiSeq sequencing to explore the diversity and novelty patterns of PM bacterial communities from Luzhou Laojiao cellars in use for 40 and 400years. High diversity indices were found in the PM with 16 phyla and 105 genera. Interestingly, the compositions of dominant genera of the PM were significantly different than that reported previously for PM sampled from other geographic sites, suggesting greater microbial diversity of PM. The dominant genus of Caproiciproducens, a caproic acid-producing bacterium, is the first reported for Chinese Baijiu production. Our results demonstrate that the PM hosts a large number of novel taxa, with 26% of the total OTUs (operational taxonomic units) distant to cultured counterparts. The class Clostridia within Firmicutes presented the highest proportion of novel OTUs. Most novel OTUs were initially isolated from diverse environments, the most abundant of which came from Chinese Baijiu brewing ecosystems, highlighting the huge culturing gap within the PM, but at the same time suggesting the importance of these OTUs in CSFB production. The data presented in this study significantly increases the number of bacteria known to be associated with CSFB production and should help guide the future exploration of microbial resources for biotechnological applications.

Evaluation of different PCR primers for denaturing gradient gel electrophoresis (DGGE) analysis of fungal community structure in traditional fermentation starters used for Hong Qu glutinous rice wine

Denaturing gradient gel electrophoresis (DGGE) has become a widely used tool to examine microbial community structure. However, when DGGE is applied to evaluate the fungal community of traditional fermentation starters, the choice of hypervariable ribosomal RNA gene regions is still controversial. In the current study, several previously published fungal PCR primer sets were compared and evaluated using PCR-DGGE, with the purpose of screening a suitable primer set to study the fungal community of traditional fermentation starters for Hong Qu glutinous rice wine. Firstly, different primer sets were used to amplify different hypervariable regions from pure fungal cultures. Except NS1/FR1+ and ITS1fGC/ITS4, other primer sets (NL1+/LS2R, NL3A/NL4GC, FF390/FR1+, NS1/GCFung, NS3+/YM951r and ITS1fGC/ITS2r) amplified the target DNA sequences successfully. Secondly, the selected primer sets were further evaluated based on their resolution to distinguish different fungal cultures through DGGE fingerprints. Three primer sets (NL1+/LS2R, NS1/GCFung and ITS1fGC/ITS2r) were finally selected for investigating the fungal community structure of different traditional fermentation starters for Hong Qu glutinous rice wine. The internal transcribed spacer (ITS) region amplified by ITS1fGC/ITS2r, which is more hypervariable than the 18S rRNA gene and 26S rRNA gene, provides an excellent tool to separate amplification products of different fungal species. Results indicated that PCR-DGGE profile using ITS1fGC/ITS2r showed more abundant fungal species than that using NL1+/LS2R and NS1/GCFung. Therefore, ITS1fGC/ITS2r is the most suitable primer set for PCR-DGGE analysis of fungal community structure in traditional fermentation starters for Hong Qu glutinous rice wine. DGGE profiles based on ITS1fGC/ITS2r revealed the presence of twenty-four fungal species in traditional fermentation starter. A significant difference of fungal community can be observed directly from DGGE fingerprints and principal component analysis. The statistical analysis results based on the band intensities of fungal DGGE profile showed that Saccharomyces cerevisiae, Saccharomycopsis fibuligera, Rhizopus oryzae, Monascus purpureus and Aspergillus niger were the dominant fungal species. In conclusion, the comparison of several primer sets for fungal PCR-DGGE would be useful to enrich our knowledge of the fungal community structures associated with traditional fermentation starters, which may facilitate the development of better starter cultures for manufacturing Chinese Hong Qu glutinous rice wine.