Long-term batch brewing accumulates adaptive microbes, which comprehensively produce more flavorful Chinese liquors

Unveiling the microbiota of sauce-flavor Daqu and its relationships with flavors and color during maturation

This study investigated the microbial community in three-color sauce-flavor Daqu (black, yellow, and white) throughout their maturation processes, together with their physicochemical factors, culturable microbes, flavor components, and fermenting vitalities. Results from high-throughput sequencing revealed distinct microbial diversity, with more pronounced variations in bacterial community than in fungal community. Firmicutes and Ascomycota emerged as the most dominant bacterial and fungal phyla, respectively, during maturation. Genus-level analysis identified Kroppenstedia, Virgibacillus, and Bacillus as dominant bacteria in black Daqu, yellow Daqu, and white Daqu, severally, while Thermoascus was shared as the core dominant fungi for these Daqu. Physicochemical factors, particularly acidity, were found to exert a significant impact on microbial community. Kroppenstedtia was the key bacteria influencing the color formation of these Daqu. Furthermore, correlations between dominant microbes and flavor compounds highlighted their role in Daqu quality. Molds (Aspergillus, Rhizomucor, and Rhizopus), excepting Bacillus, played a crucial role in the formation of pyrazine compounds. Consequently, this study offers innovative insights into the microbial perspectives on color and pyrazine formation, establishing a groundwork for future mechanized Daqu production and quality control of sauce-flavor baijiu.

Caproicibacterium argilliputei sp. nov., a novel caproic acid producing anaerobic bacterium isolated from pit clay

An anaerobic, Gram-positive, rod-shaped, motile and spore-forming bacterium, designated strain ZCY20-5T, was isolated from pit clay of Chinese strong-aroma type Baijiu (Chinese liquor). Phylogenetic analyses based on 16S rRNA gene and genome sequences showed that strain ZCY20-5T belonged to the genus Caproicibacterium, family Oscillospiracheae, but it showed low similarity to the type species Caproicibacterium amylolyticum LBM18003T (98.00 %) and Caproicibacterium lactatifermentans LBM19010T (95.67 %). In anaerobic yeast extract medium, growth was observed at 20-45 °C (optimum, 35-40 °C), at pH 4.0-9.0 (optimum, pH 6.5-7.0) and with 0.0-2.0 % NaCl (w/v). The predominant fatty acids were C16 : 0, C14 : 0, C13 3-OH and C16 3-OH, and the major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, three phospholipids of unknown structure containing glucosamine and two unidentified phospholipids. Strain ZCY20-5T exhibited an 81.32 % pairwise average nucleotide identity value, a 78.98 % average amino acid identity value and a 22.30 % digital DNA-DNA hybridization value compared to its closest relative C. amylolyticum LBM18003T. Based on morphological, physiological, biochemical, chemotaxonomic, genotypic and phylogenetic results, strain ZCY20-5T represents a novel species of Caproicibacterium, and the type strain is ZCY20-5T (=MCCC 1A19399T=KCTC 25590T).

Exploring major variable factors influencing flavor and microbial characteristics of upper jiupei

The flavor quality of jiupei gradually decreased with the increase of cellar height. In this study, high-throughput sequencing, metabolomics and HS-SPME-GC-MS techniques were used to explore the mechanism of flavor quality decline in upper jiupei in mud sealed cellars. The results showed the total content of flavor compounds increased from 1947.48 mg/L in top-site to 3855.51 mg/L in bottom of the cellar, and 19 differential flavor compounds were identified based on OPLS-DA, mainly including 12 esters such as ethyl hexanoate, ethyl butyrate, propyl hexanoate, hexyl caproate and 5 other substances such as caprylic acid, decanal and nonaldehyde. Lactobacillus, Prevotella and Methanobacterium were dominant genus of bacteria in all of cellars, while Thermomyces, Aspergillus, Pichia, Trichosporon and Rhizopus were the dominant genera of fungi. Oxygen was the key factor causing the quality heterogeneity of flavor substances and microbial communities in jiupei at different depths. Anaerobic micro-pressure sealed cellars (AMSC) method was developed and applied to jiupei fermentation, the difference in oxygen content between top site (5.90 ± 0.62 %) and bottom of the cellar (4.17 ± 0.75 %) in AMSC was smaller than that in mud sealed cellars, there were no significant differences in flavor substances content between top site and bottom of the cellar, and microbial communities showed no significant differences of the four-layer jiupei. This study provides a theoretical support for improving the flavor quality of upper jiupei.

Unravelling Metabolic Heterogeneity of Chinese Baijiu Fermentation in Age-Gradient Vessels

Fermentation vessels affect the characteristics of food fermentation; however, we lack an approach to identify the biomarkers indicating fermentation. In this study, we applied metabolomics and high-throughput sequencing analysis to reveal the dynamic of metabolites and microbial communities in age-gradient fermentation vessels for baijiu production. Furthermore, we identified 64 metabolites during fermentation, and 19 metabolites significantly varied among the three vessels (p < 0.05). Moreover, the formation of these 19 metabolites were positively correlated with the core microbiota (including Aspergillus, Saccharomyces, Lactobacillus, and Bacillus). In addition, ethyl lactate or ethyl acetate were identified as the biomarkers for indicating the metabolism among age-gradient fermentation vessels by BP-ANN (R2 > 0.40). Therefore, this study combined the biological analysis and predictive model to identify the biomarkers indicating metabolism in different fermentation vessels, and it also provides a potential approach to assess the profiling of food fermentations.

Bacterial Communities Found in Pit-Wall Mud and Factors Driving Their Evolution

Pit-wall mud (PWM) fosters bacterial communities involved in Baijiu production. PWM varies depending on pit age and height. In this study, we explored the bacterial communities in PWM and factors driving their evolution. The abundance and diversity of bacterial communities were low in new PWM (NPWM). In old PWM (OPWM), similar but diverse bacterial communities were observed at different heights. Lactobacillus was the predominant genus in NPWM, and Caproiciproducens, Aminobacterium, Hydrogenispora, Lactobacillus, Petrimonas, Syntrophomonas, and Sedimentibacter were the dominant genera in OPWM. A decrease was noted in the abundance of Lactobacillus, which indicated evolution. Among all the physicochemical properties, pH had the highest degree of interpretation with an R2 value of 0.965. pH also exerted the strongest effect on bacterial communities. The path coefficients of pH on bacterial community diversity and abundance were 0.886 and 0.810, respectively. Caproiciproducens and Clostridium sensu stricto 12 metabolized lactic acid, inhibiting the growth of Lactobacillus at a suitable pH, which led to the maturation of PWM. Our findings enrich the literature on the evolution of bacterial communities in PM and the maturation of PM.

Product spectrum analysis and microbial insights of medium-chain fatty acids production from waste biomass during liquor fermentation process: Effects of substrate concentrations and fermentation modes

Substrate toxicity would limit the upgrading of waste biomass to medium-chain fatty acids (MCFAs). In this work, two fermentation modes of electro-fermentation (EF) and traditional fermentation (TF) with different concentration of liquor fermentation waste (20%, 40%, 60%) were used for MCFAs production as well as mechanism investigation. The highest caproate (4.04 g/L) and butyrate (13.96 g/L) concentrations were obtained by EF at 40% substrate concentration. TF experiments showed that the substrate concentration above 40% severely inhibited ethanol oxidation and products formation. Compared with TF mode, the total substrates consumption and product yields under EF mode were significantly increased by 2.6%-43.5% and 54.0%-83.0%, respectively. Microbial analysis indicated that EF effectively alleviated substrate toxicity and enriched chain elongation bacteria, particularly Clostridium_sensu_stricto 12, thereby promoting ethanol oxidation and products formation. Caproiciproducens tolerated high-concentration substrates to ensure normal lactate metabolism. This study provides a new way to produce MCFAs from high concentration wastewater.

Strategy of electron acceptors for ethanol-driven chain elongation from kitchen waste

A two-phase kitchen waste (KW) fermentation was proposed in the current study to enhance medium-chain fatty acids (MCFAs) production from kitchen waste. In particular, effect of acetate to butyrate ratio (ABR) on MCFAs production was investigated which can be regulated by different pH and organic loading during the acidification phase. Medium ABR (1.00) was obtained when pH is 5.5 and organic loading is 20 g VS/L in FW acidification fermentation. Subsequent chain elongation fermentation demonstrated that the highest yield of caproate 9.67 g/L with selectivity of 79 %, and highest ethanol conversion efficiency of 1.11 was achieved in medium ABR system. Microbial community study showed that medium ABR significantly enrich the functional bacteria especially Clostridium kluyveri. The study provides a new method for chain elongation enhancement without addition of other additives in kitchen waste fermentation system and gives a guide for the regulation of the short-chain fatty acids distribution in its acidification phase.

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.

Occurrence, Diversity, and Character of Bacillaceae in the Solid Fermentation Process of Strong Aromatic Liquors

Strong aromatic liquors, also known as strong aromatic Baijiu (SAB) in China, are manufactured by solid fermentation, with a multi-microbe mixing and cooperative fermentation process that uses Daqu as a brewing starter. Bacillaceae have a specific action in food fermentation, such as soybean and wine, and more recent studies have found Bacillaceae play important roles in the SAB making industry. This review describes the diversity, functionality, and influence of Bacillaceae in Daqu, pit mud, Zaopei, Huangshui within making processes of SAB. Furthermore, aromatic flavor components from the Bacillaceae metabolism of SAB are discussed in this review. Ultimately, the resulting improvements and deeper understanding will benefit practical efforts to apply representatives of Bacillaceae in improving the quality of SAB as well as biological control of the micro-ecological environment of brewing.

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.

Liquor Flavour Is Associated With the Physicochemical Property and Microbial Diversity of Fermented Grains in Waxy and Non-waxy Sorghum (Sorghum bicolor) During Fermentation

The fermentation process of Chinese Xifeng liquor involves numerous microbes. However, the sources of microbes in fermented grain and the link between liquor flavour and physicochemical properties and microbial diversity during fermentation still remain unknown. Herein, two waxy (JiNiang 2 [JN-2] and JinNuo 3 [JN-3]) and four non-waxy (JiZa 127 [JZ-127], JinZa 34 [JZ-34], LiaoZa 19 [LZ-19], and JiaXian [JX]) sorghum varieties were selected for the comprehensive analysis of the relationship between liquor flavour and the physicochemical properties and microbial diversity of fermented grains. Results showed that ethyl acetate was the main flavour component of JZ-127, JZ-34, and JX, whereas ethyl lactate was mainly detected in JN-2, JN-3, and LZ-19. Ethyl lactate accounted for half of the ethyl acetate content, and JX exhibited a higher liquor yield than the other sorghum varieties. The fermented grains of waxy sorghum presented higher temperature and reducing sugar contents but lower moisture and starch contents than their non-waxy counterparts during fermentation. We selected JN-3 and JX sorghum varieties to further investigate the microbial changes in the fermented grains. The bacterial diversity gradually reduced, whereas the fungal diversity showed nearly no change in either JN-3 or JX. Lactobacillus was the most abundant bacterial genus, and its level rapidly increased during fermentation. The abundance of Lactobacillus accounted for the total proportion of bacteria in JX, and it was higher than that in JN-3. Saccharomyces was the most abundant fungal genus in JX, but its abundance accounted for a small proportion of fungi in JN-3. Four esters and five alcohols were significantly positively related to Proteobacteria, Bacteroidetes, and Actinobacteria; Alphaproteobacteria, Actinobacteria, and Bacteroidia; Bacillales, Bacteroidales, and Rhodospirillales; and Acetobacter, Pediococcus, and Prevotella_7. This positive relation is in contrast with that observed for Firmicutes, Bacilli, Lactobacillales, and Lactobacillus. Meanwhile, Aspergillus was the only fungal microorganism that showed a significantly negative relation with such compounds (except for butanol and isopentanol). These findings will help in understanding the fermentation mechanism and flavour formation of fermented Xifeng liquor.

Continuous medium chain carboxylic acids production from excess sludge by granular chain-elongation process

Short chain carboxylic acids (SCCAs) production is one of the primary ways to recycle excess sludge (ES). However, the high cost for the SCCAs separation/extraction due to its complete miscibility in water hinders the practical application of SCCAs and the popularization of this recycling way. To overcome this barrier, this study performed an emerging chain elongation (CE) technology to upgrade the SCCAs-rich sludge fermentation broth into the highly hydrophobic medium chain carboxylic acids (MCCAs). In a continuous expanded granule sludge bed (EGSB) reactor, a maximal MCCAs yield of 67.39 % and the corresponding concentration of 9.80 g COD/L (224.97 mM C/L) were achieved. By supplying CO₂ at a loading rate of 2 [Formula: see text] to lower the hydrogen partial pressure, the ethanol utilization rate and the resulting MCCAs yield were further improved. In addition, three branched-MCCAs including iso-caproate, iso-heptylate, and iso-caprylate were obtained the first time from waste biomass with the average proportions of 6.17 %, 3.65 %, and 0.8 %, respectively. The branched-MCCAs came from the CE of branched-SCCAs. The granule sludges performing CE were mainly consisted of rod-shaped cells, and dominated by Clostridium sensu stricto and Clostridium IV. This study is expected to lay a foundation for recycling ES to MCCAs.

Application of Wickerhamomyces anomalus in Simulated Solid-State Fermentation for Baijiu Production: Changes of Microbial Community Structure and Flavor Metabolism

Wickerhamomyces anomalus is conducive to the synthesis of ester compounds in brewing the Chinese liquor Baijiu; esters are crucial for the quality of Baijiu. In this study, simulated solid-state fermentation for Baijiu production was used to explore whether artificial addition of W. anomalus could improve the flavor substance in Baijiu, and the underlying mechanisms. Two experimental groups were studied, in which W. anomalus Y3604 (Group A) and YF1503 (Group B) were added, respectively; in the control group (Group C), no W. anomalus was added. Adding strain Y3604 increased the content of esters in fermentation samples, especially ethyl acetate and ethyl caproate, and reduced the content of higher alcohols. Adding strain YF1503 had little effect on the ester content but decreased the content of higher alcohols. The diversity and abundance of prokaryotic genera in Group A and B samples were similar, but there were some differences compared with Group C. The correlations of genera in Group A or B samples were simple compared with group C. Although the predominant eukaryotic genera in the three groups were consistent, the abundance of each gene varied among groups. Based on our findings, bioaugmentation of Baijiu fermentation with W. anomalus will change the ethyl acetate content and cause changes in the levels of other flavor substances. We suggest that the changes in flavor substances caused by the addition of W. anomalus are mainly due to changes in the microbial community structure that result from the addition of W. anomalus.

Evolving the core microbial community in pit mud based on bioturbation of fortified Daqu

Directional stress is an effective measure to change the community structure and improve the bioactivity of pit mud (PM). In this study, the addition of fortified Daqu to artificial PM (APM) was intended to disturb the microbial community and further affect metabolites. To evaluate the effect of fortified Daqu on culturing APM, the microbial communities of APM with or without the addition of fortified Daqu were investigated by fluorescence in situ hybridization and Illumina MiSeq. The results indicated that microbes (Clostridium sp., Clostridium kluyveri, hydrogenotrophic methanogens, and acetotrophic methanogens) related to the production of key aroma compounds increased notably when fortified Daqu was added. In particular, the hydrogenotrophic and acetotrophic methanogens increased by 6.19- and 4.63-fold after 30 days of culture. Subsequently, metabolites (organic acids, volatile compounds) were also analyzed by HPLC (high-performance liquid chromatography) and HS-SPME-GC-MS (headspace solid phase microextraction - gas chromatography - mass spectrometry). The results showed that the content of butyric acid and hexanoic acid was significantly higher when fortified Daqu was added to APM. In addition, the proportion of esters and phenols was also higher than in APM without fortified Daqu. A survey of the microbial compositions of APMs with or without added fortified Daqu indicated that the microbial community evolves into a functional community favoring liquor brewing. We have developed a novel process by disturbing the community diversity.

Profiling the effects of physicochemical indexes on the microbial diversity and its aroma substances in pit mud

Microbial diversity of pit mud (PM) plays a significant role in Baijiu's flavour. Here we explored the microbial community structures and aroma substances of Wenwang Winery with high-throughput sequencing coupling with headspace solid-phase microextraction-gas chromatography-mass spectrometry. We discovered that the odorant was mainly derived from 14 aroma compounds because of their OAVs ≥ 1 (OAV, the ratio of substance concentration to aroma threshold; s, on behalf of the plural), such as ethyl hexanoate (2438), ethyl octanoate (975), caproic acid (52) and etc. Moreover we also revealed that Lactobacillaceae (97·08%) was the mainly bacterial microbial community in 2-year-old PM, companied by the primarily fungi including Aspergillaceae (55·45%), Unclassified Ascomycota (11·13%) and Dipodascaceae (5·72%). Compared with the 2-year-old PM, bacterial floras in 20-year-old PM and 30-year-old PM were more abundant (i.e. Dysgonomonadaceae, Clostridium and Synerggstaceas), while no fungi were detected. Besides, the physicochemical analysis showed that the content of Lactobacillaceae was inversely associated with moisture, pH and ammonia nitrogen. By further Spearman's correlation coefficient analysis, we verified that the content of Lactobacillaceae was positively correlated with ethyl hexanoate, while negatively correlated with ethyl octanoate and caproic acid. Meanwhile, ethyl octanoate and caproic acid were positively correlated with most flora including Ruminococcaceae, Dysgonomonadaceae and Clostridiacea, which were related to physicochemical indexes. This work demonstrates promise for adjusting the physicochemical indexes of PM to affect the micro-organisms and aroma, which may provide a reference for the production of high-quality Baijiu.

Open microbiome dominated by Clostridium and Eubacterium converts methanol into i-butyrate and n-butyrate

Isobutyrate (i-butyrate) is a versatile platform chemical, whose acid form is used as a precursor of plastic and emulsifier. It can be produced microbially either using genetically engineered organisms or via microbiomes, in the latter case starting from methanol and short-chain carboxylates. This opens the opportunity to produce i-butyrate from non-sterile feedstocks. Little is known on the ecology and process conditions leading to i-butyrate production. In this study, we steered i-butyrate production in a bioreactor fed with methanol and acetate under various conditions, achieving maximum i-butyrate productivity of 5.0 mM day^-1, with a concurrent production of n-butyrate of 7.9 mM day^-1. The production of i-butyrate was reversibly inhibited by methanogenic inhibitor 2-bromoethanesulfonate. The microbial community data revealed the co-dominance of two major OTUs during co-production of i-butyrate and n-butyrate in two distinctive phases throughout a period of 54 days and 28 days, respectively. The cross-comparison of product profile with microbial community composition suggests that the relative abundance of Clostridium sp. over Eubacterium sp. is correlated with i-butyrate productivity over n-butyrate productivity.

Co-occurrence patterns among prokaryotes across an age gradient in pit mud of Chinese strong-flavor liquor

It is widely believed that the quality and characteristics of Chinese strong-flavor liquor (CSFL) are closely related to the age of the pit mud; CSFL produced from older pit mud tastes better. This study aimed to investigate the alteration and interaction of prokaryotic communities across an age gradient in pit mud. Prokaryotic microbes in different-aged pit mud (1, 6, and 10 years old) were analyzed by Illumina MiSeq sequencing of the 16S rRNA gene. Analysis of the 16S rRNA gene indicated that the prokaryotic community was significantly altered with pit mud age. There was a significant increase in the genera Methanosarcina, Methanobacterium, and Aminobacterium with increased age of pit mud, while the genus Lactobacillus showed a significant decreasing trend. Network analysis demonstrated that both synergetic co-occurrence and niche competition were dominated by 68 prokaryotic genera. These genera formed 10 hubs of co-occurrence patterns, mainly under the phyla Firmicutes, Euryarchaeota, and Bacteroidetes, playing important roles on ecosystem stability of the pit mud. Environmental variables (pH, NH₄⁺, available P, available K, and Ca²⁺) correlated significantly with prokaryotic community assembly. The interaction of prokaryotic communities in the pit mud ecosystem and the relationship among prokaryotic communities and environmental factors contribute to the higher quality of the pit mud in older fermentation pits.

Structural and Functional Changes in Prokaryotic Communities in Artificial Pit Mud during Chinese Baijiu Production

Strong-flavor baijiu (SFB) accounts for more than 70% of all Chinese liquor production. In the Chinese baijiu brewing industry, artificial pit mud (APM) has been widely used since the 1960s to construct fermentation cellars for production of high-quality SFB. To gain insights at the systems level into the mechanisms driving APM prokaryotic taxonomic and functional dynamics and into how this variation is connected with high-quality SFB production, we performed the first combined metagenomic, metaproteomic, and metabolomic analyses of this brewing microecosystem. Together, the multi-omics approach enabled us to develop a more complete picture of the changing metabolic processes occurring in APM microbial communities during high-quality SFB production, which will be helpful for further optimization of APM culture technique and improvement of SFB quality.

The Chinese alcoholic beverage strong-flavor baijiu (SFB) gets its characteristic flavor during fermentation in cellars lined with pit mud. Microbes in the pit mud produce key precursors of flavor esters. The maturation time of natural pit mud of over 20 years has promoted attempts to produce artificial pit mud (APM) with a shorter maturation time. However, knowledge about the molecular basis of APM microbial dynamics and associated functional variation during SFB brewing is limited, and the role of this variability in high-quality SFB production remains poorly understood. We studied APM maturation in new cellars until the fourth brewing batch using 16S rRNA gene amplicon sequencing, quantitative PCR, metaproteomics, and metabolomics techniques. A total of 36 prokaryotic classes and 195 genera were detected. Bacilli and Clostridia dominated consistently, and the relative abundance of Bacilli decreased along with the APM maturation. Even though both amplicon sequencing and quantitative PCR showed increased abundance of Clostridia, the levels of most of the Clostridium proteins were similar in both the first- and fourth-batch APM samples. Six genera correlated with eight or more major flavor compounds in SFB samples. Functional prediction suggested that the prokaryotic communities in the fourth-batch APM samples were actively engaged in organic acid metabolism, and the detected higher concentrations of proteins and metabolites in the corresponding metabolic pathways supported the prediction. This multi-omics approach captured changes in the abundances of specific microbial species, proteins, and metabolites during APM maturation, which are of great significance for the optimization of APM culture technique.

IMPORTANCE Strong-flavor baijiu (SFB) accounts for more than 70% of all Chinese liquor production. In the Chinese baijiu brewing industry, artificial pit mud (APM) has been widely used since the 1960s to construct fermentation cellars for production of high-quality SFB. To gain insights at the systems level into the mechanisms driving APM prokaryotic taxonomic and functional dynamics and into how this variation is connected with high-quality SFB production, we performed the first combined metagenomic, metaproteomic, and metabolomic analyses of this brewing microecosystem. Together, the multi-omics approach enabled us to develop a more complete picture of the changing metabolic processes occurring in APM microbial communities during high-quality SFB production, which will be helpful for further optimization of APM culture technique and improvement of SFB quality.

Zooming in on Butyrate-Producing Clostridial Consortia in the Fermented Grains of Baijiu via Gene Sequence-Guided Microbial Isolation

Butyrate, one of the key aroma compounds in Luzhou-flavor baijiu, is synthesized through two alternative pathways: butyrate kinase (buk) and butyryl-CoA: acetate CoA-transferase (but). A lack of knowledge of butyrate-producing microorganisms hinders our ability to understand the flavor formation mechanism of baijiu. Here, temporal dynamics of microbial metabolic profiling in fermented grains (FG) was explored via PICRUSt based on 16S rRNA gene sequences. We found Bacilli and Bacteroidia were the major potential butyrate producers in buk pathway at the beginning of fermentation, while later Clostridia dominated the two pathways. Clone library analysis also revealed that Clostridia (~73% OTUs) was predominant in buk pathway throughout fermentation, followed by Bacilli and Bacteroidia, and but pathway was merely possessed by Clostridia. Afterward, Clostridia-specific 16S rRNA gene sequencing demonstrated Clostridium might be the major butyrate-producing genus in two pathways, which was subsequently evaluated using culture approach. Seventeen Clostridium species were isolated from FG based on 16S rRNA gene sequence-guided medium prediction method. Profiles of short-chain fatty acids and but and buk genes in these species demonstrated phylogenetic and functional diversities of butyrate-producing Clostridium in FG. These findings add to illustrate the diversity of potential butyrate producers during brewing and provide a workflow for targeting functional microbes in complex microbial community.

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.

Draft Genome Sequence of the Butanoic Acid-Producing Bacterium Clostridium luticellarii DSM 29923, Used for Strong Aromatic Chinese Liquor Production

The strictly anaerobic, Gram-positive bacterium Clostridium luticellarii, which has straight or slightly curved rod-shaped cells, polar endospores, and peritrichous flagella, is used for the production of strong aromatic Chinese liquors. C. luticellarii is able to produce butanoic acid. The draft genome sequence consists of 3.757 Mbp, including 3,632 predicted protein-encoding genes.

Diversity, Function, and Application of Clostridium in Chinese Strong Flavor Baijiu Ecosystem: A Review

Baijiu is a Chinese traditional distilled liquor with an annual yield over 13.12 million tons. Strong flavor baijiu (SFB) also called Luzhou-flavor liquor, takes account for > 70% of the total baijiu produced. SFB is produced by an open solid fermentation process with a complex microbial ecosystem. Clostridium is one of the most important microorganisms for the formation of the main flavor compounds of SFB, such as ethyl caproate. In this paper, we review current research progress on the Clostridium in the SFB ecosystem, focusing on the species diversity, physiological and metabolic features along with interspecies interactions. Systems biology approaches for the study of Clostridium from SFB ecosystems were discussed and explored. Furthermore, current applications of Clostridium in SFB production were discussed.

PRACTICAL APPLICATION

Strong flavor baijiu (SFB) accounts for more than 70% of total yield of Chinese baijiu, which exists for hundreds of years. Clostridium is common in SFB ecosystem and identified to be one of main contributors of flavor compounds in SFB. Study on the Clostridium from SFB ecosystem is not only helpful for the understanding of flavor compounds formation mechanism, but also the improvement of SFB quality. This study focuses on the current researches on the Clostridium species in SFB ecosystem, including the species diversity, physiological and metabolic features, and applications.

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.

Insights into the microbial diversity and community dynamics of Chinese traditional fermented foods from using high-throughput sequencing approaches

Chinese traditional fermented foods have a very long history dating back thousands of years and have become an indispensable part of Chinese dietary culture. A plethora of research has been conducted to unravel the composition and dynamics of microbial consortia associated with Chinese traditional fermented foods using culture-dependent as well as culture-independent methods, like different high-throughput sequencing (HTS) techniques. These HTS techniques enable us to understand the relationship between a food product and its microbes to a greater extent than ever before. Considering the importance of Chinese traditional fermented products, the objective of this paper is to review the diversity and dynamics of microbiota in Chinese traditional fermented foods revealed by HTS approaches.

Illuminating Anaerobic Microbial Community and Cooccurrence Patterns across a Quality Gradient in Chinese Liquor Fermentation Pit Muds

Fermentation pit mud, an important reservoir of diverse anaerobic microorganisms, is essential for Chinese strong-aroma liquor production. Pit mud quality, according to its sensory characteristics, can be divided into three grades: degraded, normal, and high quality. However, the relationship between pit mud microbial community and pit mud quality is poorly understood, as are microbial associations within the pit mud ecosystem. Here, microbial communities at these grades were compared using Illumina MiSeq sequencing of the variable region V4 of the 16S rRNA gene. Our results revealed that the pit mud microbial community was correlated with its quality and environmental factors. Species richness, biodiversity, and relative and/or absolute abundances of Clostridia,Clostridium kluyveri, Bacteroidia, and Methanobacteria significantly increased, with corresponding increases in levels of pH, NH4 (+), and available phosphorus, from degraded to high-quality pit muds, while levels of Lactobacillus, dissolved organic carbon, and lactate significantly decreased, with normal samples in between. Furthermore, 271 pairs of significant and robust correlations (cooccurrence and negative) were identified from 76 genera using network analysis. Thirteen hubs of cooccurrence patterns, mainly under the Clostridia,Bacteroidia,Methanobacteria, and Methanomicrobia, may play important roles in pit mud ecosystem stability, which may be destroyed with rapidly increased levels of lactic acid bacteria (Lactobacillus,Pediococcus, and Streptococcus). This study may help clarify the relationships among microbial community, environmental conditions, and pit mud quality, allow the improvement of pit mud quality by using bioaugmentation and controlling environmental factors, and shed more light on the ecological rules guiding community assembly in pit mud.

Clostridium luticellarii sp. nov., isolated from a mud cellar used for producing strong aromatic liquors

A strictly anaerobic, Gram-stain-positive bacterium, designated FW431T, was isolated from a mud cellar used for producing strong aromatic Chinese liquors. The strain was able to produce butanoic acid, an important component of the aroma style of Chinese liquors. Cells of strain FW431T were straight or slightly curved rods with a polar endospore and peritrichous flagella. The major cellular fatty acids (>10 % of the total) were C16 : 0, C18 : 1ω9c and C18 : 0. Biolog assays indicated that the strain preferably metabolizes palatinose, l-fucose, β-hydroxybutyric acid, l-rhamnose and α-ketobutyric acid among 95 carbon sources tested. FW431T was related most closely to Clostridium ljungdahlii DSM 13528T and Clostridium kluyveri DSM 555T based on 16S rRNA gene sequence similarities of 95.0 and 94.2 %, respectively. The DNA G+C content of the genomic DNA was 44.4 mol%. Based on the evidence presented here, FW431T ( = CGMCC 1.5201T = KCTC 15519T) is proposed as the type strain of a novel species, Clostridium luticellarii sp. nov.

Identification and quantification of the caproic acid-producing bacterium Clostridium kluyveri in the fermentation of pit mud used for Chinese strong-aroma type liquor production

Chinese strong-aroma type liquor (CSAL) is a popular distilled alcoholic beverage in China. It is produced by a complex fermentation process that is conducted in pits in the ground. Ethyl caproate is a key flavor compound in CSAL and is thought to originate from caproic acid produced by Clostridia inhabiting the fermentation pit mud. However, the particular species of Clostridium associated with this production are poorly understood and problematic to quantify by culturing. In this study, a total of 28 closest relatives including 15 Clostridia and 8 Bacilli species in pit muds from three CSAL distilleries, were detected by culture-dependent and -independent methods. Among them, Clostridium kluyveri was identified as the main producer of caproic acid. One representative strain C. kluyveri N6 could produce caproic, butyric and octanoic acids and their corresponding ethyl esters, contributing significantly to CSAL flavor. A real time quantitative PCR assay of C. kluyveri in pit muds developed showed that a concentration of 1.79×10(7) 16S rRNA gene copies/g pit mud in LZ-old pit was approximately six times higher than that in HLM and YH pits and sixty times higher than that in LZ-new pit respectively. This method can be used to improve the management of pit mud microbiology and its impact on CSAL quality.

Proteomic and high-throughput analysis of protein expression and microbial diversity of microbes from 30- and 300-year pit muds of Chinese Luzhou-flavor liquor

Luzhou-flavor liquor is fermented based on the metabolism of special microbial communities in pit. In this study, total proteins and DNAs of microbes from 30- and 300-year pit muds were firstly extracted. Meanwhile, an efficient approach for protein extraction with increased protein content was optimized. iTRAQ-based proteomic was then applied to investigate the aroma-forming functional protein expression of microbes from the samples. Furthermore, high-throughput sequencing of 16S rDNA was employed to reveal microbial diversity. We comparatively identified 63 proteins of aroma-forming functional microbes in these samples, and found that 59 of these proteins were highly expressed in the 300-year pit mud. Those aroma-forming functional proteins were found to be involved in methanogenesis, as well as the formation of caproic acid and butyric acid during the liquor fermentation. High-throughput sequencing revealed that the microbes most commonly found in both samples were members of phylum Firmicutes (by 97% sequence similarity), both of which, along with another common Methanobacterium, were important components of aroma-forming functional colonies in the pit muds for the brewing of Chinese liquor. The findings in this study afford us new insight into the different protein expression levels and microbial communities in two pit muds.