Contrasting the microbial community and metabolic profile of three types of light-flavor Daqu

Heterogenetic mechanism in high-temperature Daqu fermentation by traditional craft and mechanical craft: From microbial assembly patterns to metabolism phenotypes

The mechanical process has a widely usage in large-scale high-temperature Daqu (HTD) enterprises, however, the quality of the mechanical HTD is gapped with the HTD by traditional process. Currently, the understanding of the mechanism behind this phenomenon is still over-constrained. To this end, the discrepancies in fermentation parameters, enzymatic characteristics, microbial assembly and succession patterns, metabolic phenotypes were compared between traditional HTD and mechanical HTD in this paper. The results showed that mechanical process altered the temperature ramping procedure, resulting in a delayed appearance of the peak temperature. This alteration shifted the assembly pattern of the initial bacterial community from determinism to stochasticity, while having no impact on the stochastic assembly pattern of the fungal community. Concurrently, mechanical pressing impeded the accumulation of arginase, tetramethylpyrazine, trimethylpyrazine, 2-methoxy-4-vinylphenol, and butyric acid, as the target dissimilarities in metabolism between traditional HTD and mechanical HTD. Pearson correlation analysis combined with the functional prediction further demonstrated that Bacillus, Virgibacillus, Oceanobacillus, Kroppenstedtia, Lactobacillus, and Monascus were mainly contributors to metabolic variances. The Redundancy analysis (RDA) of fermented environmental factors on functional ASVs indicated that high temperature, high acid and low moisture were key positive drivers on the microbial metabolism for the characteristic flavor in HTD. Based on these results, heterogeneous mechanisms between traditional HTD and mechanical HTD were explored, and controllable metabolism targets were as possible strategies to improve the quality of mechanical HTD.

The mechanism of microbial structure and flavor characteristics in Qing-Jiang-flavor Jiupei regulated by different fermentation seasons

The physicochemical characteristics of Jiupei are crucial in regulating the metabolism of microbial communities and the flavor profile of Baijiu during the fermentation process. This study systematically monitored the physicochemical characteristics of Qing-Jiang-flavor Baijiu Jiupei (QJFJ) and analyzed its microbial community structure and flavor compounds. Results indicated that dominant bacteria were significantly enriched in Summer- and Autumn-Jiupei (Spring: Summer: Autumn: Winter = 1.00: 1.40: 1.29: 1.21), while dominant fungi were significantly enriched in Spring- and Autumn-Jiupei (Spring: Summer: Autumn: Winter = 1.45: 1.00: 1.35: 1.31). Sequentially, reducing sugars (day 0), temperature (day 5 - day 10), moisture (day 15), and acidity (day 20 - day 25) in Jiupei affected the succession pattern of the microbial community, regulating the abundance of Saccharomyces, Staphylococcus, Cyberlindnera, and Lactobacillus, individually. Alcohol and acid compounds are considered seasonal differential compounds in QJFJ. This study will provide a theoretical basis for Baijiu production across different seasons.

Impact of microorganisms on key processes of organic acid metabolism during the occurrence and disappearance of paocai pellicle

In vegetable fermentation, pellicle is a common quality deterioration phenomenon. This study investigates the characteristics of glucose, organic acids, amino acids, and biogenic amines during the pellicle occurrence and disappearance of paocai. The results revealed a slight increase in pH of the fermentation system after pellicle occurred, and glucose was the main carbohydrate that microbial activity primary relied on. The microorganisms responsible for pellicle formation consumed organic acids in brine, but the lactic acid in paocai gradually increased and exceeded 25 mg/g. The appearance of pellicle caused a decrease in total free amino acids from 200.390 mg/100 g to 172.079 when pellicle occurred, whereas its impact on biogenic amines was not apparent. Through Kyoto Encyclopedia of Genes and Genomes pathway enrichment of metagenomics sequencing data, screening, and sorting of the key enzymes involved in organic acid metabolism, it was observed that the composition and species of the key microorganisms capable of metabolizing organic acids were more abundant before the appearance of pellicle. When pellicle occurred, lactic acid may be metabolized by Lactobacillus plantarum; in contrast, Lactobacillus and Pichia were associated with citric acid metabolism, and Lactobacillus, Pichia, Saccharomycodes, and Kazachstania were linked to malic acid metabolism. Moreover, Prevotella, Kazachstania, Lactobacillus, Vibrio, and Siphonobacter were implicated in succinic acid metabolism. Additionally, the production of tartaric acid and oxalic acid in paocai and brine resulted from abiotic effects. This knowledge offers a theoretical basis for precise control of paocai fermentation process. PRACTICAL APPLICATION: Our study revealed the specific situation of the metabolites produced by the microorganisms during the pollution and recovery process of pellicle in paocai fermentation, especially the effect of pellicle on the key process of organic acid metabolism. These research results provided theoretical basis for precise control of paocai fermentation.

Strategies and Challenges of Microbiota Regulation in Baijiu Brewing

The traditional Chinese Baijiu brewing process utilizes natural inoculation and open fermentation. The microbial composition and abundance in the microecology of Baijiu brewing often exhibit unstable characteristics, which directly results in fluctuations in Baijiu quality. The microbiota plays a crucial role in determining the quality of Baijiu. Analyzing the driving effect of technology and raw materials on microorganisms. Elucidating the source of core microorganisms and interactions between microorganisms, and finally utilizing single or multiple microorganisms to regulate and intensify the Baijiu fermentation process is an important way to achieve high efficiency and stability in the production of Baijiu. This paper provides a systematic review of the composition and sources of microbiota at different brewing stages. It also analyzes the relationship between raw materials, brewing processes, and brewing microbiota, as well as the steps involved in the implementation of brewing microbiota regulation strategies. In addition, this paper considers the feasibility of using Baijiu flavor as a guide for Baijiu brewing regulation by synthesizing the microbiota, and the challenges involved. This paper is a guide for flavor regulation and quality assurance of Baijiu and also suggests new research directions for regulatory strategies for other fermented foods.

Comparative analysis of sensory features, microbial diversity, and their correlations in light-flavor Daqu from different regions

This study performed a comparative analysis of the sensory and microbial profiles of light-flavor Bijou (LFD) from Taiyuan (Shanxi Province) and Suizhou (Hubei Province) in China. The results of the electronic nose showed that the aromatic substances of the LFD from Taiyuan (TLFD) were significantly higher (p < .05), while alcohol and aldehyde substances were significantly lower (p < .05) compared with the LFD from Suizhou (SLFD). The average response values of sensors W1C (sensitive to aromatic hydrocarbons), W3C (sensitive to amine and aromatic components), W5C (sensitive to olefins, aromatics, and polar molecules), and W2S (sensitive to alcohol and aldehyde compounds) to TLFD were 0.26, 0.33, 0.34, and 7.72, whereas the response values to SLFD were 0.25, 0.32, 0.33, and 8.04, respectively. The electronic tongue results showed that the aftertaste A (bitter aftertaste) and aftertaste B (astringent aftertaste) of the TLFD were significantly higher (p < .05) and umami was significantly lower (p < .05) as compared to the SLFD. The relative intensities of the aftertaste A, aftertaste B, and umami indicators of TLFD were 0.10, -0.008, and -0.22, respectively, while those of SLFD were -0.23, -0.36, and 0.835, respectively. MiSeq high-throughput sequencing results showed that TLFD exhibited lower fungal richness and diversity compared to SLFD. The dominant bacterial genera were mainly Bacillus (58.12%), Kroppenstedtia (10.11%), and Weissella (6.26%), and the dominant fungal genera were Saccharomycopsis (67.53%), Rasamsonia (9.90%), and Thermoascus (7.10%). Streptomyces and Staphylococcus were identified as the key characteristic microorganisms in TLFD, while Kroppenstedtia, Rasamsonia, and Thermoascus were the key characteristic microorganisms in SLFD. Correlation analysis indicated a stronger correlation between microorganisms and sensory characteristics in SLFD samples. This study provides valuable insights into the sensory and microbiological characteristics of LFD from different regions and offers a new perspective for understanding the production of differently flavored light-flavor Baijiu.

Exploring the heterogeneity of community and function and correspondence of "species-enzymes" among three types of Daqu with different fermentation peak-temperature via high-throughput sequencing and metagenomics

The enzyme activity of Daqu is an important prerequisite for defining it as a Baijiu starter. However, little is known about the functional species related to enzymes in different types of Daqu at the metagenomic level. Therefore, we analyzed the differences in enzymatic properties, microbial composition and metabolic function of three types of Daqu, namely high-, medium- and low-temperature Daqus (HTD, MTD and LTD), by combining chemical feature and multi-dimensional sequencing. The results showed that both liquefaction, saccharification, fermentation and esterification powers were remarkably weaker in HTD compared to MTD and LTD. Totally, 30 bacterial and 5 fungal phyla were identified and significant differences in community structures were also observed among samples, with Brevibacterium/Microascus, Pseudomonas, and Lactobacillus/Saccharomycopsis identified as biomarkers for HTD, MTD and LTD, respectively. Additionally, the importance of deterministic assembly in bacterial communities was proportional to the fermentation peak-temperature, while stochastic assembly dominated in fungal ones. Metagenomics analysis indicated eukaryota (>80 %, mainly Ascomycota) predominated in HTD and MTD while bacteria (54.3 %, mainly Actinobacteriota) were more abundant in LTD. However, the functional profiles and pathways of MTD and LTD were more similar, and the synthesis and metabolism of carbohydrates and amino acids were the crucial biological functions of all samples. Finally, the relationship between species and enzymes in different samples was constructed and the functional species in LTD and MTD were more diverse than HTD, which elucidated the functional species associated with enzyme activity in each type of Daqu. These results will greatly enrich our understanding of the core functional species in three typical Daqu, which provide available information for rational regulation of Daqu quality and the Baijiu fermentation.

Microbial communities and their correlation with flavor compound formation during the mechanized production of light-flavor Baijiu

Light-flavor Baijiu fermentation is a typical spontaneous solid-state fermentation process fueled by a variety of microorganisms. Mechanized processes have been increasingly employed in Baijiu production to replace traditional manual operation processes, however, the microbiological and physicochemical dynamics in mechanized processes remain largely unknown. Here, we investigated the microbial community succession and flavor compound formation during a whole mechanized fermentation process of light-flavor Baijiu using the conventional dilution plating method, PacBio single-molecule real-time (SMRT) sequencing and headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. The results showed that largely different fungal and bacterial communities were involved in the soaking and fermentation processes. A clear succession from Pantoea agglomerans to Bacillus (B.) smithii and B. coagulans in dominant bacterial species and from Cladosporium exasperatum to Saccharomyces cerevisiae and Lichtheimia ramosa in dominant fungal species occurred in the soaking processes. In the fermentation process, the most dominant bacterial species was shifted from Pantoea agglomerans to Lactobacillus (La.) acetotolerans and the most dominant fungal species were shifted from Lichtheimia ramose and Rhizopus arrhizus to Saccharomyces cerevisiae. The bacterial and fungal species positively associated with acidity and the formation of ethanol and different flavor compounds were specified. The microbial species exhibited strong co-occurrence or co-exclusion relationships were also identified. The results are helpful for the improvement of mechanized fermentation process of light-flavor Baijiu production.

Metagenomic analyses reveal microbial communities and functional differences between Daqu from seven provinces

Microbial communities perform the brewing function in Daqu. Macrogenomics and PICRUST II analyses revealed the differences in microbes and metabolic functions among Daqu from the seven Baijiu-producing provinces. Jiang-flavored Daqu (Guizhou, Shandong, and Hubei provinces) generally forms an aroma-producing functional microbiota with Kroppenstedtia, Bacillus, Thermoascus, Virgibacillus, and Thermomyces as the core, which promotes the metabolism of various amino acids and aroma compounds. Light-flavored Daqu (Shanxi Province) enriched the Saccharomycopsis, Saccharomyces, and lactic acid bacteria (LAB) microbiota through low-temperature fermentation. These microbes can synthesize alcohol and lactic acid but inhibit amino acid metabolism within the Light-flavored Daqu. Bifidobacterium and Saccharomycopsis were dominant in the Tao-flavored Daqu (Henan province). This unique microbial structure is beneficial for pyruvate fermentation to lactate. Research also found that Strong-flavored Daqu from Jiangsu and Sichuan provinces differed significantly. The microbial communities and metabolic pathways within Jiangsu Daqu were similar to those within Jiang-flavored Daqu, but Sichuan Daqu was dominated by Thermoascus, LAB, and Thermoactinomyces. In addition, Spearman correlation analysis indicated that Kroppenstedtia, Bacillus, and Thermomyces were not only positively related to flavor metabolism but also negatively correlated with Saccharomycopsis. This research will help establish a systematic understanding of the microbial community and functional characteristics in Daqu.

Comparative study on physicochemical properties, microbial composition, and the volatile component of different light flavor Daqu

Baijiu, a type of liquor, is known for its pure fragrance and softness. Its unique style is attributed to the complex microbial flora and flavor precursors found in Daqu. In order to elaborate the nature of light flavor Daqu to guide the baijiu production, four Daqu samples (DQ1, DQ2, DQ3, and DQ4) from Shanxi province were analyzed to determine their microbial structure, physicochemical properties, and volatile flavors using high-throughout put seqencing and headspace solid-phase microextraction/gas chromatography-mass spectrometry method in this study. The findings indicated that there were no noticeable variations in the water content and esterase activity of the four Daqu. However, the DQ2 sample had a higher acidity value and saccharifying enzyme activity, whereas DQ3 had the highest protease activity. The microbial community structure of the four Daqu was similar, with Lactobacillus and Streptophyta as the dominant bacteria, but the abundance of bacteria was different among the four Daqu. Issachenkia was a common dominant fungus genus in all samples. Rhizopus and Lichtemia were higher in DQ1 and DQ2, while Torulaspora, Aspergillus, and Candida were more prevalent in DQ4. A total of 27 volatile components were detected in the four Daqu, including esters, alcohols, ketones, aldehydes, and acids. DQ2 had the most volatile components and ethyl lactate and ethyl acetate were the most significant esters in the four samples. In conclusion, the physicochemical indicators of the four light flavor Daqu had distinct differences. There were significant variations in the abundance of bacteria and fungi, leading to differences in the volatile component content. These research findings can serve as a theoretical foundation for blending different light flavors Daqu and hold great significance in enhancing the quality of baijiu.

Contrasting the microbial community and non-volatile metabolites involved in ester synthesis between Qing-flavor Daqu and Nong-flavor Daqu

Daqu, a fermentation starter, was important source of key flavors of Chinese Baijiu. The quality of Chinese Baijiu could be significantly affected by the ester-synthesis microorganisms. In order to clarify the microbial community that promoted the ester formation in Daqu, the dynamic changes of microbial community and non-volatile profiles of Qing-flavor Daqu and Nong-flavor Daqu samples through the whole making process were investigated by Illumina MiSeq platform and liquid chromatograph-mass spectrometry (LC-MS). The non-volatile compounds related to ester synthesis were identified by comparing with ester synthesis pathway and partial least squares discriminant analysis (PLS-DA). Correlations between microbial community and non-volatile metabolites involved in ester synthesis of two types of Daqu were disclosed by Pearson correlation analysis. Results showed that a total of 50 key compounds involved in ester synthesis were identified and 25 primary functional microorganisms were screened in 39 samples. Among them, in Qing-flavor Daqu, the top three primary functional microorganisms that had strong correlations with ester-formation precursors were Lactobacillus, Pantoea, and Sphingomonas; Lactobacillus and Pantoea had significantly positive interactions with various microorganisms, but Sphingomonas did not interact with others. In Nong-flavor Daqu, the top three primary functional microorganisms that had strong correlations with ester-formation precursors were Candida, Apiotrichum, and Cutaneotrichosporon. Candida showed strong positive correlation with other microorganisms, whereas Apiotrichum and Cutaneotrichosporon had no interaction with other microorganisms. The study could help our understanding of the microbial metabolism process in Daqu and provided a scientific basis for a controllable and feasible fermentation system.

Microbial Community Affects Daqu Quality and the Production of Ethanol and Flavor Compounds in Baijiu Fermentation

Daqu is a traditional starter for Baijiu fermentation and is produced by spontaneous fermentation of ground and moistened barley or wheat. The quality of Daqu is traditionally evaluated based on physicochemical and subjective sensory parameters without microbiological analysis. Here, we compared the physicochemical characteristics of qualified (QD) and inferior (ID) Daqu, their microbial communities based on plate counting and PacBio SMRT sequencing of rRNA gene libraries, and their impacts on Baijiu fermentation. The results showed that the glucoamylase and α-amylase activities of QD were significantly higher than those of ID. The counts of yeasts and relative abundances of functional microbes, especially the amylolytic bacterium Bacillus licheniformis and fungi Saccharomycopsis fibuligera and Lichtheimia ramosa, were significantly higher in QD than in ID. The laboratory-scale Baijiu fermentation tests showed that the relative abundances of the amylolytic microbes were higher in the QD than the ID fermentation set, resulting in more efficient fermentation, as indicated by more weight loss and higher moisture content in the former. Consequently, more glycerol, acetic acid, ethanol, and other volatile compounds were produced in the QD than in the ID fermentation set. The results suggest that Daqu quality is determined by, and can be evaluated based on, its microbial community.

Complete Genome Sequencing and Bacteriocin Functional Characterization of Pediococcus ethanolidurans CP201 from Daqu

This study aims to sequence the whole genome of Pediococcus ethanolidurans CP201 isolated from Daqu and determine the anti-corrosion ability of bacteriocins on chicken breast. The whole genome sequence information of P. ethanolidurans CP201 was analyzed, and its gene structure and function were explored. It was found that gene1164 had annotations in the NR, Pfam, and Swiss-Prot databases, and was related to bacteriocins. The exogenous expression of the bacteriocin gene Pediocin PE-201 was analyzed based on the pET-21b vector and the host BL21, and the corresponding bacteriocin was successfully expressed under the induction of IPTG. After purification by NI-NTA column, enterokinase treatment, membrane dialysis concentration treatment, and SDS-PAGE electrophoresis, the molecular weight was about 6.5 kDa and the purity was above 90%. By applying different concentrations of bacteriocin to chicken breast with different levels of contamination, the control of pathogenic bacteria, the ordinary contamination level (OC) group, and the high contamination level (MC) group could be completely achieved with 25 mg/L bacteriocin. In conclusion, the bacteriocin produced by the newly isolated CP201 can be applied to the preservation of meat products to prevent the risk of food-borne diseases.

Bacteria and filamentous fungi running a relay race in Daqu fermentation enable macromolecular degradation and flavor substance formation

As the saccharifying and fermentative agent, medium-temperature Daqu (MT-Daqu) plays an irreplaceable role in the production of strong-flavor Baijiu. Numerous studies have focused on the microbial community structure and potential functional microorganisms, however, little is known about the succession of active microbial community and the formation mechanism of community function during MT-Daqu fermentation. In this study, we presented an integrated analysis of metagenomics, metatranscriptomics, and metabonomics covering the whole fermentation process of MT-Daqu to reveal the active microorganisms and their participations in metabolic networks. The results showed that dynamic of metabolites were time-specific, and the metabolites and co-expressed active unigenes were further classified into four clusters according to their accumulation patterns, with members within each cluster displaying a uniform and clear pattern of abundance across fermentation. Based on KEGG enrichment analysis in co-expression clusters and succession of active microbial community, we revealed that Limosilactobacillus, Staphylococcus, Pichia, Rhizopus, and Lichtheimia were metabolically active members at the early stage, and their metabolic activities were conducive to releasing abundant energy to drive multiple basal metabolisms such as carbohydrates and amino acids. Thereafter, during the high temperature period and at the end of fermentation, multiple heat-resistant filamentous fungi were transcriptionally active populations, and they acted as both the saccharifying agents and flavor compound producers, especially aromatic compounds, suggesting their crucial contribution to enzymatic activity and aroma of mature MT-Daqu. Our findings revealed the succession and metabolic functions of the active microbial community, providing a deeper understanding of their contribution to MT-Daqu ecosystem.

Mechanism of Enhancing Pyrazines in Daqu via Inoculating Bacillus licheniformis with Strains Specificity

Despite the importance of pyrazines in Baijiu flavor, inoculating functional strains to increase the contents of pyrazine in Daqu and how those interact with endogenic communities is not well characterized. The effects of inoculating Bacillus licheniformis with similar metabolic capacity on pyrazine and community structure were assessed in the Daqu complex system and compared with traditional Daqu. The fortification strategy increased the volatile metabolite content of Daqu by 52.40% and the pyrazine content by 655.99%. Meanwhile, results revealed that the pyrazine content in Daqu inoculated isolate J-49 was 2.35-7.41 times higher than isolate J-41. Both isolates have the almost same capability of 2,3-butanediol, a key precursor of pyrazine, in pure cultured systems. Since the membrane fatty acids of isolate J-49 contain unsaturated fatty acids, it enhances the response-ability to withstand complex environmental pressure, resulting in higher pyrazine content. PICRUSt2 suggested that the increase in pyrazine was related to the enzyme expression of nitrogen metabolism significantly increasing, which led to the enrichment of NH4⁺ and 2,3-butanediol (which increased by 615.89%). These results based on multi-dimensional approaches revealed the effect of functional bacteria enhancement on the attribution of Daqu, laid a methodological foundation regulating the microbial community structure and enhanced the target products by functional strains.

Integrated multi-omics approaches to understand microbiome assembly in Jiuqu, a mixed-culture starter

The use of Jiuqu as a saccharifying and fermenting starter in the production of fermented foods is a very old biotechnological process that can be traced back to ancient times. Jiuqu harbors a hub of microbial communities, in which prokaryotes and eukaryotes cohabit, interact, and communicate. However, the spontaneous fermentation based on empirical processing hardly guarantees the stable assembly of the microbiome and a standardized quality of Jiuqu. This review describes the state of the art, limitations, and challenges towards the application of traditional and omics-based technology to study the Jiuqu microbiome and highlights the need for integrating meta-omics data. In addition, we review the varieties of Jiuqu and their production processes, with particular attention to factors shaping the microbiota of Jiuqu. Then, the potentials of integrated omics approaches used in Jiuqu research are examined in order to understand the assembly of the microbiome and improve the quality of the products. A variety of different approaches, including molecular and mass spectrometry-based techniques, have led to scientific advances in the analysis of the complex ecosystem of Jiuqu. To date, the extensive research on Jiuqu has mainly focused on the microbial community diversity, flavor profiles, and biochemical characteristics. An integrative approach to large-scale omics datasets and cultivated microbiota has great potential for understanding the interrelation of the Jiuqu microbiome. Further research on the Jiuqu microbiome may explain the inherent property of compositional stability and stable performance of a complex microbiota coping with environmental perturbations and provide important insights to reconstruct synthetic microbiota and develop modern intelligent manufacturing procedures for Jiuqu.

Comparative evaluation of the microbial diversity and metabolite profiles of Japanese-style and Cantonese-style soy sauce fermentation

Microorganisms play essential roles in flavor formation during soy sauce fermentation. Different soy sauce fermentation types significantly affect flavor formation. However, comparisons of microbial communities and metabolites between different fermentation types have been little studied. Here, we investigated variation in microbial communities, metabolite profiles, and metabolic pathways during Japanese-type (JP) and Cantonese-type (CP) fermentation. Free amino acids and volatile compound profiles varied significantly between fermentation types, with JP samples containing higher contents of esters (39.84%; p &lt; 0.05), alcohols (44.70%; p &lt; 0.05) in the 120 d fermentation samples. Volatile compound profiles varied significantly between fermentation types, with JP samples containing higher contents of esters, alcohols, and free amino acids (p &lt; 0.05). Metagenomic analysis indicated that both JP and CP communities were dominated by Tetragenococcus, Staphylococcus, Weissella (bacteria), and Aspergillus (fungi), but the two communities varied differently over time. Tetragenococcus drastically increased in abundance throughout the fermentation (from 0.02 to 59.2%) in JP fermentation, whereas Tetragenococcus (36.7%) and Staphylococcus (29.7%) dominated at 120 d of fermentation in CP fermentation. Metagenomic functional profiles revealed that the abundances of most genes involved with carbohydrate, amino acid, and lipid metabolism exhibited significant differences between fermentation types (p &lt; 0.05) during the middle to late fermentation stages. Furthermore, predicted metabolic pathways for volatile substance biosynthesis differed between JP and CP fermentation, likely explaining the differences in flavor metabolite profiles. In addition, most of the genes associated with flavor generation were affiliated with Tetragenococcus, Weissella, Staphylococcus, Bacillus, and Aspergillus, suggesting that these microbes play important roles in flavor production during soy sauce fermentation. This study significantly improves our understanding of microbial functions and their metabolic roles in flavor formation during different soy sauce fermentation processes.

Comparative analysis of the microbiotas and physicochemical properties inside and outside medium-temperature Daqu during the fermentation and storage

Medium-temperature Daqu (MT-Daqu), a saccharification-fermentation agent and aroma-producing agent, is used to produce Chinese strong-flavor Baijiu. Many related studies have been published; however, less is known about microbial community and quality properties inside and outside the MT-Daqu from fermentation to storage. Here, along with determining the physicochemical index, the microbial community of MT-Daqu was investigated using both culture-dependent and culture-independent methods during 31 days of fermentation and 4 months of storage. Volatile compounds of mature MT-Daqu were analyzed using headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC–MS). The results indicated obvious variation in the microbial community due to the changes in environmental conditions, and the physicochemical indices shifted from fluctuations in the fermentation period to relative stability after storage for 3 months. Moreover, the microbial counts and physicochemical indices of the inner layers of MT-Daqu differed from those of the outer layers. The dominant communities, including the bacterial phyla Firmicutes, Proteobacteria, and Actinobacteria and the fungal phyla Ascomycota and Mucoromycota, showed different abundances in the two parts of the mature MT-Daqu, and different microbial communities were enriched in both parts. Additionally, pyrazines and alcohols were the most abundant volatile aroma compounds in the mature MT-Daqu.

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.

Characterization of Microbial Shifts during the Production and Ripening of Raw Ewe Milk-Derived Idiazabal Cheese by High-Throughput Sequencing

Simple Summary

Idiazabal is a traditional cheese produced from raw ewe milk in the Basque Country (Southwestern Europe). The sensory properties of raw milk cheeses have been attributed, among other factors, to microbial shifts that occur during the production and ripening processes. In this study, we used high-throughput sequencing technologies to investigate the microbiota of Latxa ewe raw milk and the dynamics during cheese production and ripening processes. The microbiota of raw milk was composed of lactic acid bacteria (LAB), environmental bacteria and non-desirable bacteria. Throughout the cheese making and ripening processes, the growth of LAB was promoted, whereas that of non-desirable and environmental bacteria was inhibited. Moreover, some genera not reported previously in raw ewe milk were detected and clear differences were observed in the bacterial composition of raw milk and cheese among producers, in relation to LAB and environmental or non-desirable bacteria, some of which could be attributed to the production of flavour related compounds.

Abstract

In this study, we used high-throughput sequencing technologies (sequencing of V3–V4 hypervariable regions of 16S rRNA gene) to investigate for the first time the microbiota of Latxa ewe raw milk and the bacterial shifts that occur during the production and ripening of Idiazabal cheese. Results revealed several bacterial genera not reported previously in raw ewe milk and cheese, such as Buttiauxella and Obesumbacterium. Both the cheese making and ripening processes had a significant impact on bacterial communities. Overall, the growth of lactic acid bacteria (LAB) (Lactococcus, Lactobacillus, Leuconostoc, Enterococcus, Streptococcus and Carnobacterium) was promoted, whereas that of non-desirable and environmental bacteria was inhibited (such as Pseudomonas and Clostridium). However, considerable differences were observed among producers. It is noteworthy that the starter LAB (Lactococcus) predominated up to 30 or 60 days of ripening and then, the growth of non-starter LAB (Lactobacillus, Leuconostoc, Enterococcus and Streptococcus) was promoted. Moreover, in some cases, bacteria related to the production of volatile compounds (such as Hafnia, Brevibacterium and Psychrobacter) also showed notable abundance during the first few weeks of ripening. Overall, the results of this study enhance our understanding of microbial shifts that occur during the production and ripening of a raw ewe milk-derived cheese (Idiazabal), and could indicate that the practices adopted by producers have a great impact on the microbiota and final quality of this cheese.