Characterization of microbial communities in strong aromatic liquor fermentation pit muds of different ages assessed by combined DGGE and PLFA analyses

Multi-factor analysis of the quality of cellar mud of Luzhou-flavor liquor in Yibin production area

The aim of this study was to conduct a thorough scientific investigation into the similarities and differences in the quality of the cellars of different Luzhou-flavor liquor wineries in Yibin production area and the reasons for them. This study analyzed cellar mud samples from five wineries in Yibin production area. The analysis of volatile flavor compounds was carried out using headspace solid-phase microextraction and gas chromatography-mass spectrometry. The bacterial and archaeal community structures and their correlations were analyzed by high-throughput sequencing. The study indicates that the Distillery A had the highest levels of ammonium nitrogen and effective phosphorus, Distillery F had the highest humus levels, and Distillery I had the highest pH levels. The community structure of the principal bacterial and archaeal communities in the five subterranean clays exhibited similarity, and all samples were dominated by Firmicutes as the primary bacterial group. However, there was variation in bacterial abundance. The cellar mud also has obvious regional differences, and there are three genera of differentially dominant archaea in the archaea. In summary, significant differences were observed in the physicochemical indexes of bacterial and archaeal abundance across all five samples. These differences led to variations in both the content and composition of volatile constituents.

Denaturing Gradient Gel Electrophoresis Approach for Microbial Shift Analysis in Thermophilic and Mesophilic Anaerobic Digestions

To determine the evolution of microbial community and microbial shift under anaerobic processes, this study investigates the use of denaturing gradient gel electrophoresis (DGGE). In the DGGE, short- and medium-sized DNA fragments are separated based on their melting characteristics, and this technique is used in this study to understand the dominant bacterial community in mesophilic and thermophilic anaerobic digestion processes. Dairy manure is known for emitting greenhouse gases (GHGs) such as methane, and GHG emissions from manure is a biological process that is largely dependent on the manure conditions, microbial community presence in manure, and their functions. Additional efforts are needed to understand the GHG emissions from manure and develop control strategies to minimize the biological GHG emissions from manure. To study the microbial shift during anaerobic processes responsible for GHG emission, we conducted a series of manure anaerobic digestion experiments, and these experiments were conducted in lab-scale reactors operated under various temperature conditions (28 °C, 36 °C, 44 °C, and 52 °C). We examined the third variable region (V3) of the 16S rRNA gene fingerprints of bacterial presence in anaerobic environment by PCR amplification and DGGE separation. Results showed that bacterial community was affected by the temperature conditions and anaerobic incubation time of manure. The microbial community structure of the original manure changed over time during anaerobic processes, and the community composition changed substantially with the temperature of the anaerobic process. At Day 0, the sequence similarity confirmed that most of the bacteria were similar (>95%) to Acinetobacter sp. (strain: ATCC 31012), a Gram-negative bacteria, regardless of temperature conditions. At day 7, the sequence similarity of DNA fragments of reactors (28 °C) was similar to Acinetobacter sp.; however, the DNA fragments of effluent of reactors at 44 °C and 52 °C were similar to Coprothermobacter proteolyticus (strain: DSM 5265) (similarity: 97%) and Tepidimicrobium ferriphilum (strain: DSM 16624) (similarity: 100%), respectively. At day 60, the analysis showed that DNA fragments of effluent of 28 °C reactor were similar to Galbibacter mesophilus (strain: NBRC 10162) (similarity: 87%), and DNA fragments of effluent of 36 °C reactors were similar to Syntrophomonas curvata (strain: GB8-1) (similarity: 91%). In reactors with a relatively higher temperature, the DNA fragments of effluent of 44 °C reactor were similar to Dielma fastidiosa (strain: JC13) (similarity: 86%), and the DNA fragments of effluent of 52 °C reactor were similar to Coprothermobacter proteolyticus (strain: DSM 5265) (similarity: 99%). To authors' knowledge, this is one of the few studies where DGGE-based approach is utilized to study and compare microbial shifts under mesophilic and thermophilic anaerobic digestions of manure simultaneously. While there were challenges in identifying the bands during gradient gel electrophoresis, the joint use of DGGE and sequencing tool can be potentially useful for illustrating and comparing the change in microbial community structure under complex anaerobic processes and functionality of microbes for understanding the consequential GHG emissions from manure.

Analysis of differences in microorganisms and aroma profiles between normal and off-flavor pit mud in Chinese strong-flavor Baijiu

The unique cellar fermentation process of Chinese strong-flavor Baijiu is the reason for its characteristic cellar aroma flavor. The types, abundance, community structure and metabolic activity of microorganisms in the pit mud directly affect the microbial balance in the white spirit production environment, promoting the formation of typical aromas and influencing the quality of CFSB. During the production process, the production of off-flavor in the cellar may occur. The aim of this study is to elucidate the differences in microbiota and flavor between normal pit mud and abnormal pit mud (pit mud with off-flavor). A total of 46 major volatile compounds were identified, and 24 bacterial genera and 21 fungal genera were screened. The esters, acids, and alcohols in the abnormal pit mud were lower than those in the normal pit mud, while the aldehydes were higher. 3-Methyl indole, which has been proven to be responsible for the muddy and musty flavors, was detected in both types of pit mud, and for the first time, high levels of 4-methylanisole was detected in the pit mud. The microbial composition of the two types of pit mud showed significant differences in the bacterial genera of Sporosarcina, Lactobacillus, Garciella, Anaerosalibacter, Lentimicrobium, HN-HF0106, Petrimonas, Clostridium_sensu_stricto_12 and Bacillus, and the fungal genera of Millerozyma, Penicillium, Mortierella, Monascus, Saccharomyces, Issatchenkia, Pithoascus, Pseudallescheria, and Wickerhamomyces. Additionally, we speculate that Sporosarcina is the predominant bacterial genus responsible for the imbalance of microbiota in pit mud.

Engineering the synthesis of unsaturated fatty acids by introducing desaturase improved the stress tolerance of yeast

BACKGROUND

Yeast is often used to build cell factories to produce various chemicals or nutrient substances, which means the yeast has to encounter stressful environments. Previous research reported that unsaturated fatty acids were closely related to yeast stress resistance. Engineering unsaturated fatty acids may be a viable strategy for enhancing the stress resistance of cells.

RESULTS

In this study, two desaturase genes, OLE1 and FAD2 from Z. rouxii, were overexpressed in S. cerevisiae to determine how unsaturated fatty acids affect cellular stress tolerance of cells. After cloning and plasmid recombination, the recombinant S. cerevisiae cells were constructed. Analysis of membrane fatty acid contents revealed that the recombinant S. cerevisiae with overexpression of OLE1 and FAD2 genes contained higher levels of fatty acids C16:1 (2.77 times), C18:1 (1.51 times) and C18:2 (4.15 times) than the wild-type S. cerevisiae pY15TEF1. In addition, recombinant S. cerevisiae cells were more resistant to multiple stresses, and exhibited improved membrane functionality, including membrane fluidity and integrity.

CONCLUSION

These findings demonstrated that strengthening the expression of desaturases was beneficial to stress tolerance. Overall, this study may provide a suitable means to build a cell factory of industrial yeast cells with high tolerance during biological manufacturing. © 2023 Society of Chemical Industry.

Contribution of microorganisms from pit mud to volatile flavor compound synthesis in fermented grains for nongxiangxing baijiu brewing

BACKGROUND

Nongxiangxing baijiu (NB) is known for its distinct flavor profile, which is attributed to key aroma compounds. The exposed fermentation technique, utilizing daqu and solid-state fermentation in pit muds, plays a crucial role in flavor development. Though previous studies have investigated the impact of microorganisms from pit ?ud and fermented grains on flavor compound production, a comprehensive understanding of microbial functions in the entire pit fermentation system is lacking. Herein, we aimed to explore the role of pit-mud-derived microorganisms in shaping the microbial community and flavor compound synthesis in NB.

RESULTS

There are 76 volatile flavor compounds that have been identified in fermented grains during NB fermentation. The main flavor compounds in NB clustered within the same network module, and 27.27% of microorganisms in the core modules of the fermented grain co-occurrence network originated from pit mud. The relationship between pit mud microorganisms and flavor compounds revealed a significant positive correlation (92%). Notably, Prevotella and Sarocladium were identified as the main contributors to this effect on flavor.

CONCLUSION

Microorganisms originating from pit mud influenced the composition and activity of microorganisms in fermented grains and facilitated the production of flavor compounds in NB. © 2023 Society of Chemical Industry.

Engineering acetyl-CoA metabolism to enhance stress tolerance of yeast by regulating membrane functionality

Zygosaccharomyces rouxii has excellent fermentation performance and good tolerance to osmotic stress. Acetyl-CoA is a crucial intermediate precursor in the central carbon metabolic pathway of yeast. This study investigated the effect of engineering acetyl-CoA metabolism on the membrane functionality and stress tolerance of yeast. Firstly, exogenous supplementation of acetyl-CoA improved the biomass and the ability of unsaturated fatty acid synthesis of Z. rouxii under salt stress. Q-PCR results suggested that the gene ACSS (coding acetyl-CoA synthetase) was significantly up-expressed. Subsequently, the gene ACSS from Z. rouxii was transformed and heterologously expressed in S. cerevisiae. The recombinant cells exhibited better multiple stress (salt, acid, heat, and cold) tolerance, higher fatty acid contents, membrane integrity, and fluidity. Our findings may provide a suitable means to enhance the stress tolerance and fermentation efficiency of yeast under harsh fermentation environments.

Comparative Analysis of Volatile Flavor Compounds in Strongly Flavored Baijiu under Two Different Pit Cap Sealing Processes

The solid-state fermentation process of strongly flavored Baijiu is complicated by the co-fermentation of many different microorganisms in the fermentation pools. The traditional fermentation pools of strong flavor Baijiu are sealed with mud, and this sealed-pit mud is not easy to maintain; therefore, the pit cap is prone to cracks and to caving in. The destruction of the sealed-pit mud may lead to instability in the composition and an abundance of microorganisms in the fermentation process that results in fluctuations of product quality. Thus, the production method of replacing the mud cap with a new steel cap is gradually attracting the attention of scientific and technical workers in the industry. However, so far, there have been relatively few reports on the use of steel lids for sealing pits for fermentation and brewing. In this study, the volatile flavor components of 270 Baijiu samples from mud-sealing and steel-sealing pits of a Chinese Baijiu distillery were studied qualitatively and quantitatively using Gas Chromatography-Mass Spectrometry (Abbreviated as GC-MS). Our statistical methods included Hierarchical Cluster Analysis (Abbreviated as HCA), Principal Component Analysis (Abbreviated as PCA), and Discriminant Analysis (Abbreviated as DA). A statistical analysis was carried out on the yield of strongly flavored Baijiu, and we made a comprehensive evaluation of the Baijiu produced under the two pit-sealing modes with regard to flavor and economic efficiency. The yield of strong flavored Baijiu was 6.7% higher with steel-sealing pits compared with mud-sealing pits. Cluster analysis categorized the strongly flavored Baijiu samples into two categories initially: (1) samples produced using mud-sealing pits and (2) samples using steel-sealing pits. Our analysis also indicated that the 28 compounds used for quantification were selected correctly. Surprising to the experimental staff, the overall score for the steel-sealing pits was greater than that of the mud-sealing pits based on PCA. Using DA, the prediction results were 100% accurate. In summary, through a comparative analysis of the flavor and yield, which are the two main factors that affect the quality of Baijiu in a distillery, and systematic combination at both experimental and theoretical levels, the differences between the Baijiu production by steel-sealing and the traditional mud-sealing were clear. Regardless of the impact of age, the detectable flavor components of Baijiu from the mud-steeling pits were very consistent with those of the steel-sealing pits in terms of richness or concentration. However, steel-sealing pits were significantly superior to mud-sealing pits with respect to output, consistency in quality, and cost (human and economic) savings.

The microbial diversity and flavour metabolism of Chinese strong flavour Baijiu: a review

Strong flavour Baijiu is widely consumed in China and is produced by the fermentation of grains using microbial starters. However, a comprehensive understanding of the diversity and metabolic characteristics of microbial communities involved in the solid-state fermentation of Baijiu is important for determining the relationship between microbial composition, flavour metabolism and understanding Baijiu fermentation conditions. Although studies have examined the metabolic pathways and impact of major processes on flavour compounds in strong flavour Baijiu, aspects of the fermentation process remain unexplored. In this review, methods are discussed for the optimisation of microbial diversity in strong flavour Baijiu and associated effects on the flavour of Baijiu. Recent studies are reviewed on starters (Daqu), fermented grains (Jiupei), and pit mud together with the effects of microbial composition on the quality of strong flavour Baijiu. The challenges of Baijiu research and production are discussed, including the role of the microbial diversity of Daqu and Jiupei in the flavour composition of strong flavour Baijiu. This review contributes to the current understanding of processing strong flavour Baijiu and serves as a reference for screening flavour related microorganisms, which is valuable for improving the quality of strong flavour 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.

Metabolomics analysis of salt tolerance of Zygosaccharomyces rouxii and guided exogenous fatty acid addition for improved salt tolerance

BACKGROUND

Zygosaccharomyces rouxii plays an irreplaceable role in the manufacture of traditional fermented foods, which are produced in a high-salt environment. However, there is little research on strategies for improving salt tolerance of Z. rouxii.

RESULTS

In this study, metabolomics was used to reveal the changes in intracellular metabolites under salt stress, and the results show that most of the carbohydrate contents decreased, the contents of xanthohumol and glycerol increased (fold change 4.07 and 5.35, respectively), while the contents of galactinol, xylitol and d-threitol decreased (fold change -9.43, -5.83 and -3.59, respectively). In addition, the content of four amino acids and six organic acids decreased, while that of the ten nucleotides increased. Notably, except for stearic acid (C18:0), all fatty acid contents increased. Guided by the metabolomics results, the effect of addition of seven exogenous fatty acids (C12:0, C14:0, C16:0, C18:0, C16:1, C18:1, and C18:2) on the salt tolerance of Z. rouxii was analyzed, and the results suggested that four exogenous fatty acids (C12:0, C16:0, C16:1, and C18:1) can increase the biomass yield and maximum growth rate. Physiological analyses demonstrated that exogenous fatty acids could regulate the distribution of fatty acids in the cell membrane, increase the degree of unsaturation, improve membrane fluidity, and maintain cell integrity, morphology and surface roughness.

CONCLUSION

These results are applicable to revealing the metabolic mechanisms of Z. rouxii under salt stress and screening potential protective agents to improve stress resistance by adding exogenous fatty acids. © 2022 Society of Chemical Industry.

Chinese Baijiu: The Perfect Works of Microorganisms

Chinese Baijiu is one of the famous distilled liquor series with unique flavors in the world. Under the open environment, Chinese Baijiu was produced by two solid-state fermentation processes: jiuqu making and baijiu making. Chinese Baijiu can be divided into different types according to the production area, production process, starter type, and product flavor. Chinese Baijiu contains rich flavor components, such as esters and organic acids. The formation of these flavor substances is inseparable from the metabolism and interaction of different microorganisms, and thus, microorganisms play a leading role in the fermentation process of Chinese Baijiu. Bacteria, yeasts, and molds are the microorganisms involved in the brewing process of Chinese Baijiu, and they originate from various sources, such as the production environment, production workers, and jiuqu. This article reviews the typical flavor substances of different types of Chinese Baijiu, the types of microorganisms involved in the brewing process, and their functions. Methods that use microbial technology to enhance the flavor of baijiu, and for detecting flavor substances in baijiu were also introduced. This review systematically summarizes the role and application of Chinese Baijiu flavor components and microorganisms in baijiu brewing and provides data support for understanding Chinese Baijiu and further improving its quality.

The microbial communities in Zaopeis , free amino acids in raw liquor, and their correlations for Wuliangye‐flavor raw liquor production

Wuliangye‐flavor liquor (WLFL), a specific Chinese traditional liquor, one of the major type of global distilled spirits, offers a unique flavor system acquired across thousands of years of development. Free amino acids (FAAs), as major health factors, are considered to be primarily derive from the hydrolysis of protein from the Zaopeis (ZPs) by microbial populations during fermentation. Here, we investigated the changes of microbial communities in ZPs and FAAs in raw liquor (RL) directly related ZPs from different ages of WLFL fermentation pits by phospholipid fatty acid fingerprint (PLFA) and high performance liquid chromatography method. Results indicated that the total PLFAs of 20‐ and 50‐year ZPs were significantly higher (p < .05) than 1‐ and 5‐year ZPs. Gram‐positive bacteria (G⁺), anaerobic bacteria, and fungi were dominant in the ZPs. Furthermore, the total of FAAs content was highly increased (p < .05) in RLs of aged fermentation pit (20‐ and 50‐year, 24.86–30.23 mg/g, 28.73–37.15 mg/g) compared with young (1‐ and 5‐year, 20.97–26.46 mg/g, 17.83–28.70 mg/g), while, the different ages of RLs could be distinguished according to 9 biomarkers of FAAs (Variable importance in projection, VIP >1; p < .05), including tyrosine, aspartic acid, leucine, glutamic acid, citrulline, alanine, proline, glycine, and valine. Particularly, the biomarkers of FAAs were positively correlated with gram‐negative bacteria (G⁻) and fungi, but negatively correlated with G⁺. This is the first report to uncover the microbial communities in zaopeis, free amino acids in RL, and their correlations for Wuliangye‐flavor raw liquor production.

Deep sequencing reveals changes in prokaryotic taxonomy and functional diversity of pit muds in different distilleries of China

Purpose

The microbial community in the pit mud correlated closely with the quality of the final product of Chinese strong-flavored Baijiu (CSFB). However, environmental conditions and brewing processes can vary by region and distilleries. This may lead to differences in microbial composition and function in pit mud. Therefore, revealing the features of the pit mud microbial community structure and functions of different distilleries will provide key information for understanding the diversity and difference of microbes in the brewing of CSFB, which will be beneficial for the improvement of the quality of pit mud and CSFB in the future.

Methods and results

Illumina MiSeq sequencing of 16S rRNA gene amplicons was used to analyze the similarities and differences in microbial community structure and function in pit muds of different distilleries located in Shihezi (Xinjiang), Xiangyang (Hubei), and Yibin (Sichuan). At the genus level, Clostridium, Lactobacillus, Aminobacterium, Petrimonas, Syntrophomonas, Methanoculleus, Syntrophaceticus, Sedimentibacter, Caloramator, Ruminococcus, Bacillus, Methanosarcina, and Garciella were the dominated genera of pit muds. There were great differences in the composition of microorganisms in pit muds used by different distilleries. The significantly enriched prokaryotic microbiotas of pit muds collected in the distilleries of Xiangyang were mainly affiliated with Bacillus, Lactobacillus, and Croceifilum, and the relative abundance of methanogens, such as Methanomicrobia and Methanobacteria, were only significantly enriched in the pit mud collected from the distilleries of Yibin (P < 0.05). Functional analysis indicated that the difference of microbial composition in pit mud will further lead to significant differences in various metabolic functions.

Conclusion

The compositions and functions of dominant microorganisms in pit mud used for the production of CSFB by different enterprises across regions in China were greatly different, and there was a close relationship between the compositions and functions of microorganisms in pit mud. Therefore, it may be an effective method to improve CSFB fermentation processes by directionally regulating the microbial community functions of pit mud using specific strains.

Microbial community spatial structures in Luzhou-flavored liquor pit muds with different brewing materials

Background

Although studies have shown that Bacteroidetes, Clostridiales, and Lactobacillales are the main components of the microbial community in pit mud during the brewing of Luzhou-flavored liquor, little is known about the effect of brewing materials on spatial structures of this microbiome.

Methods

High-throughput sequencing of the V4-V5 region of prokaryotic 16S rRNA gene was performed to analyze the microbial community diversity and spatial heterogeneity in Luzhou-flavored liquor pit muds with different brewing ingredients. The structural characteristics and heterogeneous spatial distribution of the pit mud microbial communities were examined using bioinformatics and multivariate statistical analysis methods.

Results

Our results showed that Euryarchaeota, Actinobacteria, Bacteroidetes, Chlorobi, Chloroflexi, Firmicutes, Proteobacteria, Synergistetes, Tenericutes, and WWE1 were the dominant phyla in the pit mud microbiome. The Shannon and Simpson indices of the pit mud microbiome with three grains (M3G) in the upper layer were significantly lower than those in middle layer and bottom, whereas those of the pit mud microbiome with five grains (M5G) in bottom were significantly lower than those in middle layer (p < 0.05). There were significant differences in the microbial community compositions between the pit muds with different brewing ingredients and locations in the same pit (p < 0.05). T78 of Anaerolinaceae, Butyrivibrio, Dehalobacter_Syntrophobotulus, Desulfosporosinus, Asteroleplasma, and vadinCA02 of Synergistaceae were significantly enriched in M3G, whereas Prevotella, Vagococcus, Caldicoprobacter, Butyrivibrio, Coprococcus, Dorea, Sporanaerobacter, Tepidimicrobium, TissierellaSoehngenia, RFN20 of Erysipelotrichaceae, Sutterella, 125ds10 of Alteromonadales, Vibrio, and Sphaerochaeta were significantly enriched in M5G. This study provides a theoretical basis for exploring the influence of brewing ingredients in pit muds on the production of Luzhou-flavored liquor and the specific influence of pit mud microorganisms in different locations on liquor production.

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.

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.

Incorporation of Exogenous Fatty Acids Enhances the Salt Tolerance of Food Yeast Zygosaccharomyces rouxii

Fatty acids have great effects on the maintenance of the cell membrane structure, cell viability, and cell metabolisms. In this study, we sought to elucidate the effects of exogenous fatty acids on the salt tolerance of food yeast Zygosaccharomyces rouxii. Results showed that Z. rouxii can grow by using exogenous fatty acids (C12:0, C14:0, C16:0, C16:1, C18:0, C18:1, and C18:2) as the sole carbon source. Four fatty acids (C12:0, C16:0, C16:1, and C18:1) can improve the salt tolerance of cells, enhance the formation of the cell biofilm, regulate the chemical compositions, restore growth in the presence of cerulenin, regulate the contents of membrane fatty acids, and control the expression of key genes in the fatty acid metabolism. Our results reveal that Z. rouxii can synthesize membrane fatty acids from exogenous fatty acids and the supplementation of these fatty acids can override the need for de novo fatty acid biosynthesis.

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.

Dynamic changes of volatile compounds and bacterial diversity during fourth to seventh rounds of Chinese soy sauce aroma liquor

Chinese soy sauce aroma liquor (CSSL) is a famous Baijiu. Multiple rounds of fermentation, the characteristic of CSSL processing, contributes to the differences in the quality of the liquor of different rounds. In this study, the grains on cooled, stacked, and fermented stages of 4th to 7th rounds were taken, of which the environmental factors, bacterial diversity, and volatile compounds were comprehensively analyzed. Lactobacillaceae, Bacillaceae, Thermoactinomycetaceae, and Enterobacteriaceae were the top four families, of which Lactobacillaceae dominated the fermented stage of each round. Principal component analysis (PCA) and principal coordinate analysis (PCoA) supported the popular view that the liquors of 3rd to 5th rounds possess the best quality. Lactobacillaceae is an extremely critical bacterium for CSSL fermentation. This study provides comprehensive understanding regarding the dynamic changes in fermented grains during the 4th to 7th rounds, which could help to improve the processing technology of CSSL.

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.

Correlation analysis between aroma components and microbial communities in Wuliangye-flavor raw liquor based on HS-SPME/LLME-GC-MS and PLFA

As the main raw material for commercial Wuliangye-flavor liquor, Wuliangye-flavor raw liquor (WFRL) plays an important role in the formation of flavor components. Aroma components and microbial community diversity of 4 WFRLs produced by the same fermenting cellar were investigated by headspace solid phase microextraction, liquid-liquid microextraction combined gas chromatography-mass spectrometry and phospholipid fatty acid fingerprint method. Correlations between aroma components and microbial community of WFRL were disclosed by multivariate statistical analysis techniques. Results indicated that a total of 75 aroma components were identified, including 39 esters, 11 alcohols, 9 acids, 7 aldehyde, 5 ketones and 4 others, whereas esters were dominant for the samples (67.49%). Meanwhile, Gram-positive bacteria (G+) and aerobe in the fermenting cellar increased gradually from top to bottom, compared with Gram-negative bacteria (G-), anaerobe and fungi showed the opposite trend. Furthermore, 4 WFRLs could be distinguished according to 13 differential aroma components (VIP > 1, P < 0.05). Whereas, the differential aroma components were positively correlated with G-, anaerobe and fungi, but negatively correlated with G+ and aerobe. In particular, 4 key esters including ethyl caproate, ethyl acetate, ethyl butyrate and ethyl lactate that determine the grade of WFRL were positively correlated with anaerobe and fungi. Therefore, the results can be used as a reference to understand the correlation between aroma components and microbial communities in different quality grades of WFRL.

Heat preadaptation improved the ability of Zygosaccharomyces rouxii to salt stress: a combined physiological and transcriptomic analysis

Zygosaccharomyces rouxii plays important roles in the brewing process of fermented foods such as soy sauce, where salt stress is a frequently encountered condition. In this study, effect of heat preadaptation on salt tolerance of Z. rouxii and the protective mechanisms underlying heat preadaptation were investigated based on physiological and transcriptomic analyses. Results showed that cells subjected to heat preadaptation (37 °C, 90 min) prior to salt stress aroused many physiological responses, including maintaining cell surface smooth and intracellular pH level, increasing Na⁺/K⁺-ATPase activity. Cells subjected to heat preadaptation increased the amounts of unsaturated fatty acids (palmitoleic C16:1, oleic C18:1, linoleic C18:2) and decreased the amounts of saturated fatty acids (palmitic C16:0, stearic C18:0) which caused the unsaturation degree (unsaturated/saturated = U/S ratio) increased by 2.4 times when compared with cells without preadaptation under salt stress. Besides, salt stress led to increase in contents of 5 amino acids (valine, proline, threonine, glycine, and tyrosine) and decrease of 2 amino acids (serine and lysine). When comparing the cells pre-exposed to heat preadaptation followed by challenged with salt stress and the cells without preadaptation under salt stress, the serine, threonine, and lysine contents increased significantly. RNA sequencing revealed that the metabolic level of glycolysis by Z. rouxii was weakened, while the metabolic levels of the pentose phosphate pathway and the riboflavin were enhanced in cells during heat preadaptation. Results presented in this study may contribute to understand the bases of adaptive responses in Z. rouxii and rationalize its exploitation in industrial processes.Key points• Heat preadaptation can improve high salinity tolerance of Z. rouxii.• Combined physiological and transcriptomic analyses of heat preadaptation mechanisms.• Provide theoretical support for the application of Z. rouxii.

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.

Detection of Viable and Total Bacterial Community in the Pit Mud of Chinese Strong-Flavor Liquor Using Propidium Monoazide Combined With Quantitative PCR and 16S rRNA Gene Sequencing

Microbiota in the pit mud (PM) plays a crucial role in the production of Chinese strong-flavor liquor (CSFL), the most popular distilled liquor in China. However, previous studies used total microbes, instead of viable ones, for the characterization of the microbial community in this environment. In this study, we used propidium monoazide (PMA) combined with quantitative polymerase chain reaction (qPCR) and 16S rRNA gene sequencing to verify the effect of non-viablee bacteria on the characterization of PM bacteria. After PMA concentration optimization, 50 μM PMA was chosen to pretreat 5 and 20 years PMs. The qPCR results showed that there were 50.78 and 71.84% of non-viable bacteria in the 5-year PM and 20-year PM, respectively. Both copy numbers of total bacteria and viable bacteria were significantly higher in 20-year PM than those in 5-year PM. Nevertheless, in terms of bacterial diversity and composition analyses at the operational taxonomic unit (OTU), phylum, class, and genus levels, 16S rRNA gene sequencing results displayed no significant differences between total bacteria and viable bacteria in both PM types. In conclusion, it is necessary for non-viable bacteria to be considered in determining absolute biomass of bacteria in PM, but not necessary in the analysis of diversity and composition of PM bacteria. To the best of our knowledge, our study is the first attempt to analyze viable bacteria in the PM of CSFL and provides useful information on how to accurately characterize a microbial community in a PM environment.

Temperature Driven Membrane Lipid Adaptation in Glacial Psychrophilic Bacteria

Bacteria inhabiting non-polar glaciers are exposed to large variations in temperature, which significantly affects the fluidity of bacterial cell membranes. In order to maintain normal functions of the cell membranes, psychrophilic bacteria adapt by changing the composition of cell membrane fatty acids. However, information on the exact pattern of cell membrane adaptability in non-polar low-temperature habitats is scarce. In the present study, 42 bacterial strains were isolated from the Ghulmet, Ghulkin, and Hopar glaciers of the Hunza Valley in the Karakoram Mountain Range, Pakistan and their cell membrane fatty acid distributions studied, using gas chromatography/mass spectrometry (GC-MS) for the analysis of fatty acid methyl esters (FAMEs) liberated by acid-catalyzed methanolysis. Furthermore, Gram-negative and Gram-positive groups were grown under different temperature settings (5, 15, 25, and 35°C) in order to determine the effect of temperature on cell membrane (CM) fatty acid distribution. The analyses identified the major groups of cell membrane fatty acids (FA) as straight-chain monounsaturated fatty acids (n-MUFAs) and branched fatty acids (br-FAs), accounting for more than 70% of the fatty acids analyzed. The distribution of br-FAs and n-FAs in bacterial cell membranes was significantly affected by temperature, with the level of br-FAs decreasing relative to n-FAs with increasing temperature. Notably, the production of polyunsaturated fatty acids (PUFAs) was only seen at lower temperatures. This study contributes to understanding, for the first time, the role of br-FAs in the maintenance of cell membrane fluidity of bacteria inhabiting non-polar habitats.

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.

Physicochemical properties and microbial community dynamics during Chinese horse bean-chili-paste fermentation, revealed by culture-dependent and culture-independent approaches

The production of Chinese horse bean-chili-paste (CHCP) involves three fermentation phases: chili-to-moromi fermentation (CF) phase, horse bean-to-meju fermentation (HF) phase and moromi-meju mixed fermentation (MF) phase. To understand the microbial dynamics among these three phases and the potential roles of viable microbes for fermentation, microbial community dynamics was investigated by using culture-dependent and culture-independent methods. Furthermore, the capacities of enzyme-producing of the isolates were determined. During the CF phase, reducing sugar content increased from 3.1% to 3.49%, while pH declined from 4.85 to 4.5. The protein content in the HF phase and MF phase reduced sharply from 22.23% to 10.29% and 4.39%-1.19%, respectively. Bacillus sp., Staphylococcus sp., Oceanobacillus sp., Candida sp., Zygosaccharomyces sp. and Aspergillus sp. dominated the CF phase, while Bacillus sp., Candida sp. and Zygosaccharomyces sp. were the dominant microorganisms in both the HF and MF phases. B. amyloliquefaciens, B. methylotrophicus, B. subtilis, B. licheniformis and A. oryzae possessed strong capacities of producing enzymes, i.e. α-amylase, cellulase and xylanase, acid protease and leucine aminopeptidase, and could make a great contribution to CHCP fermentation.

Zygosaccharomyces rouxii Combats Salt Stress by Maintaining Cell Membrane Structure and Functionality

Zygosaccharomyces rouxii is an important yeast that is required in the food fermentation process due to its high salt tolerance. In this study, the responses and resistance strategies of Z. rouxii against salt stress were investigated by performing physiological analysis at membrane level. The results showed that under salt stress, cell integrity was destroyed, and the cell wall was ruptured, which was accompanied by intracellular substance spillover. With an increase of salt concentrations, intracellular Na⁺ content increased slightly, whereas intracellular K⁺ content decreased significantly, which caused the increase of the intracellular Na⁺/K⁺ ratio. In addition, in response to salt stress, the activity of Na⁺/K⁺-ATPase increased from 0.54 to 2.14 μmol/mg protein, and the ergosterol content increased to 2.42-fold to maintain membrane stability. Analysis of cell membrane fluidity and fatty acid composition showed that cell membrane fluidity decreased and unsaturated fatty acid proportions increased, leading to a 101.21% rise in the unsaturated/saturated fatty acid ratio. The results presented in this study offer guidance in understanding the salt tolerance mechanism of Z. rouxii, and in developing new strategies to increase the industrial utilization of this species under salt stress.

Prokaryotic communities in multidimensional bottom-pit-mud from old and young pits used for the production of Chinese Strong-Flavor Baijiu

The pit mud (PM) prokaryotic communities with obvious difference between old and young PM is essential for solid-fermentation of Chinese Strong-Flavor Baijiu. The bottom-PM (BPM) is considered more important. In this study, the multidimensional prokaryotic communities of old and young BPMs were investigated. The old BPM presented stratified difference within the depth of 0-7 cm, especially, the surface 0-1 cm was characteristic of dominant Caproiciproducens (34.79%). The young BPM showed significant difference between quarter/center and deep corner (1-7 cm), the former were characteristic of abundant Lactobacillus (12.80%-42.72%), while the deep corner was distinctive of dominant Caproiciproducens (17.85%-64.45%). The lactic acid, pH and soluble Ca²⁺ were considered as the 3 most significant environmental factors through redundancy analysis (RDA). This study may help illuminate the BPM aging process, and allow the future artificial regulation of young BPM.

Effects of fortification of Daqu with various yeasts on microbial community structure and flavor metabolism

Fortification of Daqu with isolated functional strains can influence the metabolic activity of the microbial community, and thus alter the flavors of the Baijiu produced with Daqu as a fermentation starter. Here, we analyzed the microbial community dynamics of, and volatile compound production by, Daqu fortified respectively with three high-yield ethyl caproate-producing yeasts (Saccharomyces cerevisiae Y7#09, Hyphopichia burtonii F12507 and Clavispora lusitaniae YX3307), or with a mixture of these three strains, during the fermentation of Baijiu. The microbial community was investigated using Illumina HiSeq technology. Three bacterial genera (Bacillus, Lactobacillus and Enterobacter) and four fungal genera (Pichia, Clavispora, Saccharomyces and Saccharomycopsis) were dominant in the microbial communities. The volatile compounds were examined by gas chromatography-mass spectrometry. Forty-one flavor compounds were detected in all samples, including seven alcohols, 26 esters and four aldehydes. In particular, an increase in ethyl caproate content was associated with Daqu fortified with S. cerevisiae Y7#09, C. lusitaniae YX3307, or the mixed inoculum. The ester content of these fortified Daqu was higher in the later stage of the fermentation than that in unfortified Daqu, or in Daqu fortified with H. burtonii F12507. Our results show that fortification of Daqu with aroma-producing yeast strains influenced the microbial community composition in the Daqu and affected its metabolic activity. Overall, this study reveals the features of fortified Daqu microbial communities in different phases and improves understanding of the relationships between aroma-producing yeast and the metabolic activity of microbial communities in Baijiu production.

Interactive effect of carbon source with influent COD/N on nitrogen removal and microbial community structure in subsurface flow constructed wetlands

Carbon source and influent COD/N (chemical oxygen demand: total nitrogen) pose distinct effects on nitrogen removal efficiency and microbial community structure of constructed wetlands. To investigate the interactive effect of carbon source with COD/N on nitrogen removal and microbial community structure in subsurface flow constructed wetlands, glucose (C₆H₁₂O₆) and sodium acetate (C₂H₃NaO₂) were used to determine five COD/N ratios in nine groups of constructed wetlands divided into glucose constructed wetlands and sodium acetate constructed wetlands. Results showed that efficiency in COD removal increased with COD/N, and peak value reached 92.7%. Interactive effect of carbon source with COD/N on system pH and ammonium removal was notably significant. Differences in ammonium removal performance between treatments were achieved by the variation of influent COD/N ratio and the change of system pH resulted from different carbon sources, and the result suggested that glucose was a better choice at high COD/N ratio. System microbial community structure was significantly affected by carbon source, influent COD/N ratio and their interaction. Microbial biomass in constructed wetlands significantly increased with increasing COD/N ratio. Higher density and diversity of fungus were observed in glucose constructed wetlands, particularly at COD/N ratio of 7 and 10.

Microbial communities associated with barley growing in an oil sands reclamation area in Alberta, Canada

Microbial communities that colonize the plant rhizosphere and the root interior can ameliorate plant stress and promote growth. These plant–microbe associations are being investigated to assist in reclamation soils in northern Alberta. This study assessed the diversity of bacterial species associated with barley plants growing at different cover managements and slope positions in an oil sands reclamation area. Phospholipid fatty acid analysis of the microbial communities indicated that both cover type and slope, in addition to soil total and organic carbon, NH4+, and organic matter, were significant determinants of microbial community composition. However, analysis of denaturing gel gradient electrophoresis (DGGE) banding patterns revealed that while most bulk and rhizosphere soils differentiated by cover management, no clustering was observed in endophytes. In addition, techniques to assess culture-dependent endophytic bacteria revealed a dominance of the class Gammaproteobacteria, in which Enterobacteriaceae (44%), Xanthomonaceae (30%), and Pseudomonaceae (26%) were the most abundant families in this class. Several endophytic isolates also matched those from DGGE profiles. The results of this study suggest that plants growing on oil sands reclamation covers host a wide range of bacterial endophytes, which should be assessed as to their potential to assist plant establishment and growth at such sites.

Complete Genome Sequence of Clostridium kluyveri JZZ Applied in Chinese Strong-Flavor Liquor Production

Chinese strong-flavor liquor (CSFL), accounting for more than 70% of both Chinese liquor production and sales, was produced by complex fermentation with pit mud. Clostridium kluyveri, an important species coexisted with other microorganisms in fermentation pit mud (FPM), could produce caproic acid, which was subsequently converted to the key CSFL flavor substance ethyl caproate. In this study, we present the first complete genome sequence of C. kluyveri isolated from FPM. Clostridium kluyveri JZZ contains one circular chromosome and one circular plasmid with length of 4,454,353 and 58,581 bp, respectively. 4158 protein-coding genes were predicted and 2792 genes could be assigned with COG categories. It possesses the pathway predicted for biosynthesis of caproic acid with ethanol. Compared to other two C. kluyveri genomes, JZZ consists of longer chromosome with multiple gene rearrangements, and contains more genes involved in defense mechanisms, as well as DNA replication, recombination, and repair. Meanwhile, JZZ contains fewer genes involved in secondary metabolites biosynthesis, transport, and catabolism, including genes encoding Polyketide Synthases/Non-ribosomal Peptide Synthetases. Additionally, JZZ possesses 960 unique genes with relatively aggregating in defense mechanisms and transcription. Our study will be available for further research about C. kluyveri isolated from FPM, and will also facilitate the genetic engineering to increase biofuel production and improve fragrance flavor of CSFL.

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.

Effect of the environment microbiota on the flavour of light-flavour Baijiu during spontaneous fermentation

Light-flavour Baijiu is a type of Chinese liquor with a pure and mild flavour produced by traditional spontaneous solid-state fermentation. The flavour of this liquor has been found to vary in the different periods of annual production. To explore the factors affecting flavour, the microbiota of the surrounding environment, starter and fermentation process in different periods were investigated. Results showed that the ester content and acidity of light-flavour Baijiu were significantly lower when annual production was resumed after a summer break. HCA plot of volatile flavour profile and bacterial PCoA results indicated that the differences occurred at later stages, mainly due to different structures of Lactobacillus. Correlation analysis by O2PLS indicated that Lactobacillus positively correlated with esters. Species-level analysis showed that the lack of L. acetotolerans on the surface of the jar might cause a lag in fermentation and lower ester content. Thereafter, L. acetotolerans was revived during fermentation and enriched on the surface of the jar, which promoted ester formation. As important sources of L. acetotolerans, the air and fermentation jars played a critical role during fermentation. Therefore, this systematic study on environmental microbial ecology is valuable for quality control and to explore environmental microbiota functions during spontaneous fermentation.

Biochemical characterisation and dominance of different hydrolases in different types of Daqu - a Chinese industrial fermentation starter

BACKGROUND

Daqu is a fermentative saccharification agent that is used to initiate fermentation in the production of Chinese liquor and vinegar. This study investigated the differences of amylase, protease and esterase in dominance of different types of Daqu, which can be useful for quality control and flavor improvement of Daqu production by enzyme technology.

RESULTS

Hydrolase activities in different Daqu samples were compared by principal component analysis (PCA). Based on protein electrophoresis and ¹ H NMR spectroscopy, the protein patterns and metabolites in Daqu were further analysed. The results indicated that the highest amylase activities and diversities were found in low/medium-temperature of Daqu which had light-flavour and strong-flavour. Proteases play a significant role in determining the quality of high-temperature Daqu samples which had a sauce-flavour. Furthermore, the contributions of esterase to both strong and sauce flavour development in high-temperature Daqu are similar.

CONCLUSION

Results from the present work showed that differences in amylase, protease and esterase play a leading role in different types of Daqu, which can be useful for quality control and technology development of Daqu. © 2017 Society of Chemical Industry.

Prokaryotic diversity and biochemical properties in aging artificial pit mud used for the production of Chinese strong flavor liquor

At present, artificial pit mud (APM) is widely used in Chinese liquor-making industry and plays a particular role in the production of Chinese strong flavor liquor (CSFL). However, APM frequently ages during fermentation, thus becoming unsuitable for sustainable use due to its low-quality. The reasons behind, and results of, APM aging during the production of CSFL are not yet understood. Sequencing the V4 region of the 16S rRNA gene shows that prokaryotic diversity is significantly decreased (Shannon's diversity index, P < 0.01) and community composition is distinctly changed (from 1197 to 865 OTUs) in aging APM. On the phylum level, the increase of Firmicutes and decrease of Proteobacteria are the main consequences of APM aging during the production of CSFL. The counting of cultivatable bacteria confirmed that there was a large increase in Lactobacilli and aerobic spore-forming bacteria in aging low-quality APM (more than twofold). Unexpectedly, the total number of caproic acid-producing bacteria, mainly Clostridia, did not change significantly between the two kinds of APM. Furthermore, biochemical analysis indicates that the pH and the levels of NH₄⁺ and K⁺ are decreased in aging low-quality APM (P < 0.01). The results obtained in this study support the possibility that environmental factors (pH, nutrients) induce the decrease of prokaryotic diversity, and the changed community composition influences the environmental properties. Therefore, through interfering with the cycle, APM aging can be controlled potentially by adjustment of environmental factors and/or supplementation of diminished or missed microorganisms.

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.

The process-related dynamics of microbial community during a simulated fermentation of Chinese strong-flavored liquor

BACKGROUND

Famous Chinese strong-flavored liquor (CSFL) is brewed by microbial consortia in a special fermentation pit (FT). However, the fermentation process was not fully understood owing to the complicate community structure and metabolism. In this study, the process-related dynamics of microbial communities and main flavor compounds during the 70-day fermentation process were investigated in a simulated fermentation system.

RESULTS

A three-phase model was proposed to characterize the process of the CSFL fermentation. (i) In the early fermentation period (1-23 days), glucose was produced from macromolecular carbohydrates (e.g., starch). The prokaryotic diversity decreased significantly. The Lactobacillaceae gradually predominated in the prokaryotic community. In contrast, the eukaryotic diversity rose remarkably in this stage. Thermoascus, Aspergillus, Rhizopus and unidentified Saccharomycetales were dominant eukaryotic members. (ii) In the middle fermentation period (23-48 days), glucose concentration decreased while lactate acid and ethanol increased significantly. Prokaryotic community was almost dominated by the Lactobacillus, while eukaryotic community was mainly comprised of Thermoascus, Emericella and Aspergillus. (iii) In the later fermentation period (48-70 days), the concentrations of ethyl esters, especially ethyl caproate, increased remarkably.

CONCLUSIONS

The CSFL fermentation could undergo three stages: saccharification, glycolysis and esterification. Saccharomycetales, Monascus, and Rhizopus were positively correlated to glucose concentration (P < 0.05), highlighting their important roles in the starch saccharification. The Lactobacillaceae, Bacilli, Botryotinia, Aspergillus, unidentified Pleosporales and Capnodiales contributed to the glycolysis and esterification, because they were positively correlated to most organic acids and ethyl esters (P < 0.05). Additionally, four genera, including Emericella, Suillus, Mortierella and Botryotinia, that likely played key roles in fermentation, were observed firstly. This study observed comprehensive dynamics of microbial communities during the CSFL fermentation, and it further revealed the correlations between some crucial microorganisms and flavoring chemicals (FCs). The results from this study help to design effective strategies to manipulate microbial consortia for fermentation process optimization in the CSFL brew practice.