Discovery and development of a novel short-chain fatty acid ester synthetic biocatalyst under aqueous phase from Monascus purpureus isolated from Baijiu

Influence of two sorghum varieties on metabolic factors, microbial community, and flavor component precursors of strong-flavor Baijiu Zaopei

As the primary raw material for Baijiu brewing, sorghum variety exerts an intricate influence on the taste profile of strong-flavor Baijiu. However, how sorghum variety comprehensively affects Baijiu flavor formation through fermentation by microorganisms and metabolites remains largely unknown. Using 16S&ITS rRNA gene sequencing and non-targeted metabolomics, in this study we comprehensively analyzed the changes in microbial communities and metabolites during fermentation of a glutinous and non-glutinous sorghum variety. The results showed that these varieties significantly affected microbial diversity and community structure, and their interactions, among which, there were particularly complex interactions among bacterial communities, while the effects of "functional differentiation" and "community aggregation" of fungal communities were prominent. Furthermore, three bacterial and nine fungal genera were identified as core microorganisms related to changes in glycerophospholipids during fermentation, that led to a change in ester content, ultimately improving Baijiu quality. These findings provide reference for the selection of brewing materials.

Enhanced production of ethyl caproate in strong-flavor Baijiu through a dual bacterial co-culture system and immobilization on natural luffa sponge

Ethyl caproate, a significant aromatic component in strong-flavor baijiu, is synthesized requiring caproic acid as an essential precursor. In this study, a novel caproic acid-producing bacteria Rummeliibacillus suwonensis J-1 was isolated from pit mud. Subsequently, a dual bacterial co-culture system (DBCS) was successfully established by combining J-1 with the acid-producing bacterium Enterococcus sp. D-1, resulting in a 21-fold increase in yield, which reached 4.41 g/L. A new immobilization strategy was developed, utilizing luffa sponge as a carrier for DBCS to facilitate pit mud-free strong-flavor baijiu production. The findings indicated a substantial increase in the ethyl caproate concentrations, with a 218 % increase, reaching 0.625 g/L. Transcriptomic analysis showed that in the two-bacterial system, crucial genes implicated in the biosynthesis pathway of caproic acid in J-1, including Crt, Scad, Ptb, pdxK, L-cysteine dehydrogenase, and l-serine decarboxylase were significantly upregulated, which enhanced the synthesis of caproic acid. These findings suggest that DBCS may have potential applications in fermentation without pit mud and could potentially enhance quality of strong-flavor baijiu.

Applications of Microextraction Technology for the Analysis of Alcoholic Beverages Quality: Current Perspectives and Future Directions

Alcoholic beverages are loved by the majority of consumers because of their diverse characteristics and rich nutritional value; thus, ensuring their quality is necessary for maintaining the rapid development of the alcoholic beverage industry. Due to trace levels of various quality factors and the complexity of the beverage body matrix, pretreatment is usually required before analysis. Among the many pretreatment methods available, microextraction has attracted increasing attention because it aligns with the development direction of green chemistry. This review surveys advancements in microextraction techniques pertaining to three quality aspects in the most frequently consumed alcoholic beverages: baijiu and huangjiu (spirits) and wine and beer (fermented alcoholic drinks). Furthermore, new directions in their development are discussed.

Molecular cloning, overexpression, characterization, and mechanism explanation of an esterase RasEst3 for ester synthesis under aqueous phase

Fatty acid esters are widely used in fragrance compounds, solvents, lubricants, and biofuels. Enzymatic synthesis of these esters in aqueous phase is an environmentally friendly approach. In this study, an esterase RasEst3 from Rasamsonia emersonii was identified for fatty acid ester synthesis through sequence alignment. The gene encoding RasEst3 was heterologously expressed in Escherichia coli BL21(DE3), and its enzymatic properties were analyzed. The enzyme exhibited optimal activity at pH 3.5 and 30 °C, with a preference for medium-chain substrates. Structurally, RasEst3 contains a lid domain and a catalytic domain, with a catalytic triad composed of Ser146-His227-Asp214. The smaller pocket spatial site resistance and the hydrophobicity of the substrate channel facilitate effective substrate binding to the active center. Site-directed mutagenesis and molecular dynamics simulations revealed that the oxygen anion holes formed by Gly69 and Thr70, along with the π-bond stacking formed by Tyr112 and Tyr145, play crucial roles in catalysis. After removing a loop region from RasEst3, its ethyl octanoate synthesis activity increased by 253.22 % compared to the wild-type enzyme. This study not only clarifies the structure-function relationship of RasEst3 but also provides valuable insights for developing novel biocatalysts in green chemistry.

Identification of characteristic volatiles and metabolomic pathways during the fermentation of red grapefruit by Monascus purpureus using HS-SPME-GC-MS and metabolomics

Fermentation of red grapefruit by Monascus purpureus (M. purpureus) results in complex changes in flavor compounds and metabolic profiles, but the specifics of these alterations are not well understood. This study aimed to investigate the changes in flavor compounds and metabolomic traits during this fermentation process. Using Headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) with non-targeted metabolomics, we analyzed flavor compounds and measured physicochemical indices throughout the fermentation period. We identified 23 volatile flavor metabolites before and after fermentation, focusing on acids, alcohols, and aldehydes, of these, 9 showed an upward effect and 14 showed a downward effect. Key metabolic pathways involved included butyric acid, taurine, and hypotaurine, with notable downregulation of acetone and 1-butanol in the butyric acid pathway. The study reveals that butyric acid-related metabolism influences other pathways such as glycolysis, fatty acid metabolism, and the tricarboxylic acid cycle in M. purpureus. These findings provide insights into the generation of flavor compounds during fermentation and offer a theoretical basis for the industrial production of fermented citrus fruits.

Carbonaceous adsorbents in wastewater treatment: From mechanism to emerging application

Adsorption is of great significance in the water pollution control. Carbonaceous adsorbents, such as carbon quantum dots, carbon nanotubes, graphene, and activated carbons, have long been deployed in sustainable wastewater treatment due to their excellent physical structure and strong interaction with various pollutants; these features allow them to spark greater interest in environmental remediation. Although numerous eye-catch researches on carbon materials in wastewater treatment, there is a lack of comprehensive comparison and summary of the vivid structure-activity-application relationships of different types of carbonaceous adsorbents at the molecular and atomic level. Herein, this review aims to scrutinize and contrast the adsorption mechanisms of carbonaceous adsorbents with different dimensions, analyzing the qualitative differences in adsorption capacity from microscopic perspectives, structural diversity caused by preparation methods, and environmental external factors affecting adsorption occurrence. Then, a quantitatively in-depth critical appraisal of traditional and emerging contaminants in wastewater treatment using carbonaceous adsorbents, and innovative strategies for enhancing their adsorption capacity are discussed. Finally, in the context of growing imposed circularity and zero waste wishes, this review offers some promising insights for carbonaceous adsorbents in achieving sustainable wastewater treatment.

Effect of inoculation with different Eurotium cristatum strains on the microbial communities and volatile organic compounds of Fu brick tea

Eurotium cristatum is the primary fungus in Fu brick tea (FBT) and plays a crucial role in its special flavor. This study investigated the effect of inoculation with different E. cristatum strains (i.e., ZJ, GX, GZ, HN, and SX) on the microbial communities and volatile organic compounds (VOCs) of FBT. A total of 113 VOCs were identified in all samples, with the concentration of VOCs (alcohols, aldehydes, and ketones) significantly higher in GXE FBT than in other samples. The core VOCs of GXE (19), GZE (16), HNE (19), SXE (15), and ZJE (13) FBT were identified using orthogonal partial least squares discriminant analysis and relative odor activity value (ROAV) analysis. Methional (a27), butanal (a41), 1-octen-3-one (a69), and ethyl acetate (a77) were key markers for inoculated FBTs, and 1-octen-3-ol, dimethyl disulfide, and acetoin-M were the specific markers of HNE. Linalool and (E)-2-octenal were particularly prominent in GXE, and isoamyl acetate-D was an important aroma component of GZE. Differences in microbial diversity were observed among the different inoculated fermented FBTs, and E. cristatum inoculation remarkably influenced the richness and diversity of bacterial communities. The VOCs were closely associated with fungi and bacteria, and 19 potentially dominant microorganisms (10 fungal and 9 bacterial genera) correlated with VOCs were identified. Among them, Aspergillus (fungi) and Pseudomonas (bacteria) exerted the greatest role. The FBT inoculated with E. cristatum from ZJ had the highest content of theaflavins and theabrownins, which intensified the red and yellow colors of the tea. E. cristatum greatly decreased the free amino acids and fatty acids, contributing to the aroma formation of FBT. Therefore, inoculating FBT with E. cristatum remarkably influenced the microbial communities and improved its flavor profile. This work provides a theoretical foundation on the role of E. cristatum in the formation and regulation of FBT flavor.

Unraveling the regional environmental ecology dominated baijiu fermentation microbial community succession and associated unique flavor

Chinese baijiu as one of the famous distilled liquor in which fermented in open environments, with various microorganisms (i.e., bacteria, fungi, and yeast) involved in their brewing process, and created corresponding unique flavor. However, the sources of environmentally enriched microbial communities associated with liquor fermentation are still being characterized yet. Given the dependence of microbial growth and reproduction on environmental ecology, it is important to understand the correlation between baijiu fermentation microbial community and surrounding environmental ecology (i.e., temperature, humidity, wind, and precipitation). This study systematically overviewed the sources of microorganisms in the Jiang-flavor-Baijiu fermentation system. The results showed that microorganisms in baijiu brewing (i.e., mold, lactic acid bacteria, and yeast) mainly originated from surrounding environmental matrices, including the air (i.e., Yeast, Streptomyces and Bacillus), soil (i.e., Xanthomonas, Methanococcus and Comamonas) and water (i.e., Flavobacterium, Acinetobacter, and Pseudomonas) via atmospheric transport, raw material transfer and surface runoff. In addition, the unique baijiu fermentation microbial community diversity depends on local geology and meteorological conditions, highlighting that the structural stability and diversity of the microorganisms in the Baijiu brewing process dominated by local environmental ecology. We also explored the regional environmental conditions on the microbial community and found that the unique Jiang-flavor-Baijiu fermentation microbial community diversity depends on local geology and meteorological conditions. The Jiang-flavor-Baijiu workshop is located in the basin of the middle-and low latitude mountainous areas, with sufficient solar irradiation and rainfall, high air humidity, and low wind speed that favor the growth and propagation of Baijiu fermentation microorganisms. Therefore, the obtained conclusions provide new insights unraveling the key factor controlling the unique flavor of Chinese Baijiu, where protecting the ecology of baijiu brewing-regions is fundamental for maintaining the long-term quality of baijiu.

Exploring Community Succession, Assembly Patterns, and Metabolic Functions of Ester-Producing-Related Microbiota during the Production of Nongxiangxing baijiu

Esters are vital flavor compounds in Chinese Nongxiangxing baijiu and greatly affect the quality of baijiu. Microbial communities inhabiting fermented grains (FGs) have a marked impact on esters. However, the specific microorganisms and their assembly patterns remain unclear. This study utilized high-throughput sequencing and a culture-based method to reveal ester-producing microorganisms. A total of 33 esters were detected, including 19 ethyl esters, 9 linear chain esters, and 2 branched chain esters. A correlation analysis indicated that the bacterial genus Lactobacillus (relative abundance in average: 69.05%) and fungal genera Pichia (2.40%), Aspergillus (11.84%), Wickerhamomyces (0.60%), Thermomyces (3.57%), Saccharomycopsis (7.87%), Issatchenkia (0.96%), and Thermoascus (10.83%) were dominant and associated with esters production and their precursors. The numbers of esters positively correlated with them were 1, 17, 3, 2, 1, 1, 1, and 1, respectively. The modified stochasticity ratio (MST) index and Sloan neutral model revealed that bacteria were predominantly governed by deterministic processes while fungal assemblies were more stochastic. Saturnispora silvae and Zygosaccharomyces bailii were isolated and identified with ester synthesis potential. PICRUSt2 analysis showed that fungi in FG had a high potential for synthesizing ethanol, while 14 enzymes related to esters synthesis were all produced by bacteria, especially enzymes catalyzing the synthesis of acyl-CoA. In addition, ester synthesis was mainly catalyzed by carboxylesterase, acylglycerol lipase and triacylglycerol lipase. These findings may provide insights into ester production mechanism and potential strategies to improve the quality of Nongxiangxing baijiu.

Characterizing the Contribution of Strain Specificity to the Microbiota Structure and Metabolites of Muqu and Fresh High-Temperature Daqu

In this study, the differences in physicochemical properties, microbial community structure, and metabolic characteristics between various fortified Muqu and their corresponding high-temperature Daqu (HTD) were investigated using multiphase detection methods. The results demonstrated that the physicochemical properties, community structure, dominant bacterial composition, and metabolic components varied significantly among the different types of fortified HTD. The differences between HTDs became more pronounced when fortified HTD was used as Muqu. Compared to HTD, Muqu exhibited a more complex volatile profile, while HTD contained higher levels of characteristic non-volatile components. The cultivable bacteria count in Muqu was significantly higher than that in HTD, while the cultivable fungi count was slightly lower than that in HTD. The fungal profiles in HTD were primarily associated with starch hydrolysis and ethanol synthesis, while bacterial activity was more prominent in Muqu. Additionally, pyrazine synthesis was mainly attributed to fungi in Muqu and bacteria in HTD. Source Tracker analysis indicated that 8.11% of the bacteria and 26.76% of the fungi originated from Muqu. This study provides a theoretical foundation for the controlled production of HTD, contributing to improvements in its quality and consistency.

Analyzing the differences and correlations between key metabolites and dominant microorganisms in different regions of Daqu based on off-target metabolomics and high-throughput sequencing

Daqu is usually produced in an open environment, which makes its quality unstable. The microbial community of Daqu largely determines its quality. Therefore, in order to improve the fermentation characteristics of Daqu, samples were collected from Jinsha County (MT1), Xishui County (MT2), and Maotai Town (MT3) in Guizhou Province to explore the microbial diversity of Daqu and its impact on Daqu's metabolites.Off-target metabolomics was used to detect metabolites, and high-throughput sequencing was used to detect microorganisms. Metabolomics results revealed that MT1 and MT2 had the highest relative fatty acid content, whereas MT3 had the highest organooxygen compound content. Principal component analysis and partial least squares discriminant analysis revealed significant differences in the metabolites among the three groups, followed by the identification of 33 differential metabolites (key metabolites) filtered using the criteria of variable importance in projection >1 and p < 0.001. According to the microbiological results, Proteobacteria was the dominant bacteria phylum in three samples. Gammaproteobacteria was the dominant class in MT1(26.84 %) and MT2(36.54 %), MT3 is Alphaproteobacteria(25.66 %). And Caulobacteraceae was the dominant family per the abundance analysis, MTI was 24.32 %, MT2 and MT3 were 33.71 % and 24.40 % respectively. Three samples dominant fungi phylum were Ascomycota, and dominant fungi family were Thermoascaceae. Pseudomonas showed a significant positive connection with various fatty acyls, according to correlation analyses between dominant microorganisms (genus level) and key metabolites. Fatty acyls and Thermomyces showed a positive correlation, but Thermoascus had the reverse relation. These findings offer a theoretical framework for future studies on the impact of metabolites on Baijiu quality during fermentation.

Influences of thioalcohols on the release of aromas in sesame-flavor baijiu

This study investigated the interactions between 2-furylmethanethiol, benzenemethanethiol, and 18 skeletal aroma-active compounds as well as four aroma notes in sesame-flavor baijiu based on the Feller Additive Model, the Odor Activity Value (OAV) Approach, and the Sigma-Tau (σ-τ) plots. In addition, a predictive model for the interactions between 2-furylmethanethiol and esters was developed, and the determinants of the interaction results in complex systems were explored. The results reveal that both thioalcohols interacted with the skeletal aroma-active compounds in a similar trend, where 2-furylmethanethiol tends to enhance the release of fruit and acid aroma. Moreover, the intensity of the thiols and their intensity ratio to the notes were the determinants of the interaction results in the multivariate blended system, with the lower the concentration of the thiols, the closer the ratio was to 1, and the more likely that additive interactions would take place. Predictive modeling showed that 2-furylmethanethiols were more likely to have additive or synergistic effects with esters when the olfactory thresholds of the esters were between 75.86 and 199.53 μg/L. Conversely, masking effects were more likely.

Lipases for targeted industrial applications, focusing on the development of biotechnologically significant aspects: A comprehensive review of recent trends in protein engineering

Lipases are remarkable biocatalysts, adept at catalyzing the breakdown of diverse compounds into glycerol, fatty acids, and mono- and di-glycerides via hydrolysis. Beyond this, they facilitate esterification, transesterification, alcoholysis, acidolysis, and more, making them versatile in industrial applications. In industrial processes, lipases that exhibit high stability are favored as they can withstand harsh conditions. However, most native lipases are unable to endure adverse conditions, making them unsuitable for industrial use. Protein engineering proves to be a potent technology in the development of lipases that can function effectively under challenging conditions and fulfill criteria for various industrial processes. This review concentrated on new trends in protein engineering to enhance the diversity of lipase genes and employed in silico methods for predicting and comprehensively analyzing target mutations in lipases. Additionally, key molecular factors associated with industrial characteristics of lipases, including thermostability, solvent tolerance, catalytic activity, and substrate preference have been elucidated. The present review delved into how industrial traits can be enhanced through directed evolution (epPCR, gene shuffling), rational design (FRESCO, ASR), combined engineering strategies (i.e. CAST, ISM, and FRISM) as protein engineering methodologies in contexts of biodiesel production, food processing, and applications of detergent, pharmaceutics, and plastic degradation.

Synthesis of Isoamyl Fatty Acid Ester, a Flavor Compound, by Immobilized Rhodococcus Cutinase

Isoamyl fatty acid esters (IAFEs) are widely used as fruity flavor compounds in the food industry. In this study, various IAFEs were synthesized from isoamyl alcohol and various fatty acids using a cutinase enzyme (Rcut) derived from Rhodococcus bacteria. Rcut was immobilized on methacrylate divinylbenzene beads and used to synthesize isoamyl acetate, butyrate, hexanoate, octanoate, and decanoate. Among them, Rcut synthesized isoamyl butyrate (IAB) most efficiently. Docking model studies showed that butyric acid was the most suitable substrate in terms of binding energy and distance from the active site serine (Ser114) γ-oxygen. Up to 250 mM of IAB was synthesized by adjusting reaction conditions such as substrate concentration, reaction temperature, and reaction time. When the enzyme reaction was performed by reusing the immobilized enzyme, the enzyme activity was maintained at least six times. These results demonstrate that the immobilized Rcut enzyme can be used in the food industry to synthesize a variety of fruity flavor compounds, including IAB.

The carbohydrate metabolism and expression of carbohydrate-active enzyme genes in Aspergillus luchuensis fermentation of tea leaves

Introduction

Carbohydrates, which make up 20 to 25% of tea beverages, are responsible for their flavor and bioactivity. Carbohydrates of pu-erh tea change during microbial fermentation and require further research. In this study, we examined the carbohydrate metabolism and expression of carbohydrate-active enzyme genes during the fermentation of tea leaves with Aspergillus luchuensis.

Methods

Widely targeted metabolomics analysis, high-performance anion-exchange chromatography measurements, and transcriptomics were used in this study.

Results

After fermentation, the levels of soluble sugar, hemicellulose, lignin, eight monosaccharides, and seven sugar alcohols increased. Meanwhile, the relative contents of polysaccharides, D-sorbitol, D-glucose, and cellulose decreased. High expression of 40 genes encoding 16 carbohydrate enzymes was observed during fermentation (FPKM>10). These genes encode L-iditol 2-dehydrogenase, pectinesterase, polygalacturonase, α-amylase, glucoamylase, endoglucanase, β-glucosidase, β-galactosidase, α-galactosidase, α-glucosidase, and glucose-6-phosphate isomerase, among others.

Discussion

These enzymes are known to break down polysaccharides and cell wall cellulose, increasing the content of monosaccharides and soluble sugars.

Comparison of physicochemical parameters, microbial community composition and flavor substances during mechanical and traditional brewing process of Jiang-flavor baijiu

Mechanized, automated and intelligent brewing is an important trend of innovation and transition in Jiang-flavor baijiu industry. In this study, physicochemical parameters, microbial community composition and flavor substances during 3rd round heap fermentation between mechanical and traditional workshop were investigated and compared based on traditional culturable methods, high-throughput sequencing technology and gas chromatography analysis. The dominant bacterial and fungal genera were consistent between the two workshops, but mechanized brewing had a significant impact on the composition of fungal communities. Rhodococcus and Monascus were special genera in mechanical workshop. The interaction relationship between physicochemical parameters and dominant microorganisms in mechanized workshop was different from traditional workshop as well. This study provided a scientific basis for further analyzing the mechanism of mechanized brewing of Jiang-flavor baijiu.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10068-023-01483-y.

N-Doping CQDs as an Efficient Fluorescence Probe Based on Dynamic Quenching for Determination of Copper Ions and Alcohol Sensing in Baijiu

To address an accurate detection of heavy metal ions in Baijiu production, a nitrogen-doping carbon quantum dots (N-CQDs) was prepared by hydrothermal method from citric acid and urea. The as-prepared N-CQDs had an average particle size of 2.74 nm, and a large number of functional groups (amino, carbonyl group, etc.) attached on its surface, which obtained a 9.6% of quantum yield (QY) with relatively high and stable fluorescence performance. As a fluorescent sensor, the fluorescence of N-CQDs at 380 nm excitation wavelength could be quenched quantitatively by adding Cu2+, due to the dynamic quenching of electron transfer caused by the binding of amine groups and Cu2+, which showed excellent sensitivity and selectivity to Cu2+ in the range of 0.5-5 μM with a detection limit (LOD) of 0.032 μM. In addition, the N-CQDs as well as could be applied to quantitative determine alcohol content in the range of 10-80 V/V% depending on the fluorescence enhancement. Upon the experiment, the fluorescent mechanism was studied by Molecular dynamics (MD) simulations, which demonstrated that solvent effect played an influential role on sensing alcohol content in Baijiu. Overall, the work provided a theoretically guide for the design of fluorescence sensors to monitor heavy metal ion in liquid drinks and sense alcohol content.

Temporal Profile of the Microbial Community and Volatile Compounds in the Third-Round Fermentation of Sauce-Flavor baijiu in the Beijing Region

Sauce-flavor baijiu produced in the Beijing and Guizhou regions has regional characteristic flavors, but the differences in flavor compounds and reasons for their formation remain unclear. The sauce-flavor baijiu brewing process involves several rounds of fermentation. In this study, we investigated the temporal distribution of microbial communities and flavor substances during the third round of sauce-flavor baijiu fermentation in the Beijing region, and we then compared and analyzed the differences of flavor substances and microorganisms in the fermented grains of sauce-flavor baijiu in the Beijing and Guizhou regions. It was found that 10 bacterial genera and 10 fungal genera were dominant in the fermented grains. The acidity of the fermented grains had a significant driving effect on the microbial community succession. A total of 81 volatile compounds were identified and quantified in the fermented grains, of which esters and alcohols were relatively abundant. The differences in 30 microbial community compositions and their resulting differences in terms of the fermentation parameters of fermented grains are responsible for the differences in the profiles of flavor compounds between sauce-flavor baijiu produced in the Beijing and Guizhou regions.

Decoding the key compounds responsible for the empty cup aroma of soy sauce aroma type baijiu

The empty cup aroma in soy sauce aroma type baijiu (SSB) is distinct, but the specific compounds responsible for its unique aroma remain unknown. The aroma characteristics of SSB and the empty cup were investigated using molecular sensory science. Fifty-three and 27 aroma active compounds were identified in SSB and empty cup aroma, respectively. AEDA of the empty cup showed ethyl 3-phenylpropanoate, phenylethyl alcohol, sotolon, p-cresol, and 2,3-dimethyl-5-ethyl pyrazine could be the most important aroma contributors to the empty cup aroma due to their high FD values. Sotolon, characterized by its seasoning-like and herbal aroma, was identified as a crucial aroma compound for the empty cup aroma for the first time. Lactic acid was found to decrease the olfactory threshold of sotolon markedly in both 53% ethanol water solution and empty cup, promoting the contribution of sotolon to the empty cup aroma.

Comparison of the Correlations of Microbial Community and Volatile Compounds between Pit-Mud and Fermented Grains of Compound-Flavor Baijiu

The microbial composition and volatile components of fermented grains (FG) and pit mud (PM) are crucial for the quality and flavor of compound-flavor baijiu (CFB). The physicochemical indices, culturable microorganisms, microbial communities, and volatile components of FG and PM were analyzed and correlated in our research. Considering FG and PM, amplicon sequencing was used to analyze the microbial community and the volatile components were detected by headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME). For FG, redundancy analysis and correlation perfume Circos were used to clarify the correlations between the dominant microbial community and volatile components. The results showed that Aspergillus, Pichia, and Rhizopus were the main fungal microflora in FG and PM, whereas Lactobacillus and Bacillus were the dominant bacteria in FG, and Methanosarcina and Clostridium sensu stricto 12 were the dominant bacteria in the PM. The microbial community and volatile compounds in the CB sampled from the bottom layers of the FG were greatly affected by those in the PM. There were 32 common volatile components in CB and PM. For FG, most of the volatile components were highly correlated with Lactobacillus, Bacillus, Aspergillus, Pichia, and Monascus, which includes alcohols, acids and esters. This study reveals correlations between microbial composition, volatile components, and the interplay of FG and PM, which are conducive to optimizing the fermentation process and improving the quality of CFB base.

Characterization of key aroma compounds in gray sufu fermented using Leuconostoc mesenteroides subsp. Mesenteroides F24 as a starter culture

Gray sufu is a traditional fermented bean product with strong flavor in China, but traditional fermentation methods often lead to its off-flavor. This study was performed to investigate the flavor quality characteristics of gray sufu fermented using L. mesenteroides F24. Results showed 220 volatile compounds in gray sufu, among which alcohols and esters were the main volatiles. Inoculation with L. mesenteroides F24 considerably affected the contents of flavor substances in gray sufu and substantially increased the main flavor compounds. In addition, 29 kinds of key volatile compounds were identified by analyzing the ROAVs. Four unique key flavor substances were found in gray sufu inoculated with L. mesenteroides F24. This study is the first report on the feasibility of L. mesenteroides F24 as a promising starter culture to improve the flavor quality of gray sufu. The results provide a theoretical basis for improving the processing and quality control of gray sufu.

Exploring the controllability of the Baijiu fermentation process with microbiota orientation

Product quality and stability improvement is important for development of the Baijiu industry. Generally, Baijiu brewing is carried out in a spontaneous fermentation system mediated by microbiota. Thus, complexity and instability are major features. Due to the insufficient understanding of the mechanism for producing Baijiu, the precise control of the fermentation progress has still not been realized, ultimately affecting product quality and stability. The flavor of Baijiu is the most important factor in determining its quality and is formed by microbiota under the driving force of various physicochemical parameters, such as moisture, acidity, and temperature. Therefore, exploring the association among microbiota (core), physicochemical factors (reference) and flavor compounds (target) has become a key point to clarify the formation mechanism for the flavor quality of Baijiu. Daqu fermentation and liquor fermentation are the two major stages of Baijiu brewing. Daqu, distillers' grains, and pit mud, as the most important fermentation substrates of the microbiota respectively, provide a large number of functional microorganisms related to the flavor components. To this end, we reviewed the relevant research progress of microbiota diversity in different fermentation substrates and the interaction mechanisms among microbiota, physicochemical parameters, and flavor components in this paper. Moreover, a research hypothesis of precise control of the Baijiu fermentation process by building fermentation models based on this is proposed. The key point for this idea is the identification of core microbiota closely associated with the formation of key flavor components by multi-omics technology and the acquisition of culturable strains. With this foundation, fermentation models suitable for different brewing environments will be established by constructing synthetic microbiota, designing mathematical models, and determining key fermentation model parameters. The ultimate goal will be to effectively improve the quality and stability of Baijiu products through model regulation.

Unveiling innovation in aroma attribute evaluation of Niulanshan Baijiu: An advanced exploration of two different processing methods via food sensory omics and penalty analysis

Niulanshan Baijiu (NLS), a notable variety of Baijiu known for its light flavor and extensive historical legacy, was subjected to a comparative analysis using two different processes: Hunzheng Xucha (HX) and Qingzheng Qingcha (QQ). The study combined sensory-oriented flavor analysis and penalty analysis to assess the differences between the two processes. Aroma compounds in NLS were extracted using liquid-liquid extraction and headspace solid phase microextraction. Gas chromatography-olfactometry-mass spectrometry was employed to identify 46 aroma-active compounds, including the first-time discovery of ethyl isohexanoate and 2,4-nonadienal in NLS. Quantification of 35 compounds with odor activity value (OAV) ≥ 1 was achieved using internal standard curve methods. Sensory assessments by a cohort of 111 participants highlighted the preference for HX-NLS in terms of flavor, while QQ-NLS exhibited a sour-Chen aroma that required improvement. The study further revealed the significant impact of acetic acid, butyric acid, hexanoic acid, octanoic acid, and 3-methylbutanal on the sour-Chen aroma in liquor.

Comprehensive genomic and metabolomic analysis revealed the physiological characteristics and pickle like odor compounds metabolic pathways of Bacillus amyloliquefaciens ZZ7 isolated from fermented grains of Maotai-flavor baijiu

Pickle like odor (PLO) is one of the main defective flavors of Maotai flavor baijiu (MFB). Understanding and controlling the PLO compounds producing strains not only solves the problem of PLO from the source, but also ensures the high-quality production of MFB. However, the relevant research on PLO compounds producing strains has not been reported in MFB. In this study, we identified a Bacillus amyloliquefaciens ZZ7 with high yield of PLO compounds in the fermented grains of MFB, and measured its physiological characteristics. It produces 627 volatile compounds and 1,507 non-volatile compounds. There are 7 volatile sulfur compounds that cause the PLO, the content of dimethyl disulfide, dimethyl trisulfide, and dimethyl sulfur is relatively high, accounting for 89.43% of the total volatile sulfur compounds. The genome size of B. amyloliquefaciens ZZ7 is 3,902,720 bp with a GC content of 46.09%, and a total of 3,948 protein coding genes were predicted. Moreover, the functional annotation of coding genes and an assessment of the metabolic pathways were performed by genome annotation, showing it has strong ability to transport and metabolize amino acids and carbohydrates. Comprehensive genomic and metabolomic analysis, the metabolic pathway of PLO compounds of B. amyloliquefaciens ZZ7 was revealed, which mainly involves 12 enzymes including sulfate adenylyltransferase, cysteine synthase, cystathionine γ-synthase, etc. This work provides biological information support at both genetic and metabolic levels for the mechanism of B. amyloliquefaciens ZZ7 to synthesize PLO compounds, and provides a direction for the subsequent genetic modification of ZZ7 to solve PLO from the source in the MFB.

Insights into the influence of physicochemical parameters on the microbial community and volatile compounds during the ultra-long fermentation of compound-flavor Baijiu

Introduction

While the variation in physicochemical parameters, microbial communities, metabolism, composition, and the proportion of volatile components in fermented grains (FG) affect final Baijiu quality, their complex interactions during the ultra-long fermentation of compound-flavor Baijiu (CFB) are still poorly understood.

Methods

In this study, amplicon sequencing was used to analyze the microbial community, and headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was used to analyze the volatile components in FG during ultra-long fermentation of CFB. The relationships between the dominant microbial communities, physicochemical parameters, and volatile components were analyzed using redundancy analysis and network analysis.

Results

During ultra-long fermentation, bacterial diversity was initially higher than during the mid and late stages. Fungal diversity in the mid stages was higher than that initially and later in the process. A total of 88 volatile components, including six alcohols, 43 esters, eight aldehydes and ketones, 13 acids, and 18 other compounds were detected in FG. Starch and reducing sugars in FG strongly affected the composition and function of bacterial and fungal communities. However, acidity had little effect on the composition and function of the bacterial flora. Lactobacillus, Bacillus, Weissella, and Pichia were the core microbial genera involved in metabolizing the volatile components of FG.

Discussion

We provide insights into the relationships and influences among the dominant microbial communities, physicochemical parameters, and volatile components during ultra-long fermentation of CFB. These insights help clarify the fermentation mechanisms of solid-state fermentation Baijiu (SFB) and control and improve the aroma quality of CFB.

Chinese Baijiu and Whisky: Research Reservoirs for Flavor and Functional Food

Baijiu is a traditional spirit with high reputation in the Chinese community, and whisky, on the other hand, is a renowned spirit in Western culture, with both contributing a major proportion to the consumption and revenue in the global spirit market. Interestingly, starting with similar raw materials, such as grains, diverse production methods lead to different organoleptic profiles. In addition, such enormous attention they attract renders them as a crucial part in food and the related industry. Therefore, great efforts are made in improving product quality and optimizing production processes, such as flavor enhancement, facility development, and deep utilization of byproducts. Given the huge impacts and great involvements of these spirits in the general food industry, research focusing on either spirit is of referential significance for other relevant fields. With the aim of facilitating such collaboration, this review discusses the current research status, in a comparative manner, of both spirits in respect to key production processes-oriented sensory and flavor analysis, deep utilization of byproducts, and spirit-derived functional food investigations. Finally, the internal correlations based on the abovementioned criteria are identified, with research prospects proposed.

Roles of sulfur-containing compounds in fermented beverages with 2-furfurylthiol as a case example

Aroma is a critical component of the flavor and quality of beverages. Among the volatile chemicals responsible for fragrance perception, sulfur compounds are unique odorants due to their extremely low odor threshold. Although trace amounts of sulfur compounds can enhance the flavor profile of beverages, they can lead to off-odors. Sulfur compounds can be formed via Maillard reaction and microbial metabolism, imparting coffee aroma and altering the flavor of beverages. In order to increase the understanding of sulfur compounds in the field of food flavor, 2-furfurylthiol (FFT) was chosen as a representative to discuss the current status of their generation, sensory impact, enrichment, analytical methods, formation mechanisms, aroma deterioration, and aroma regulation. FFT is comprehensively reviewed, and the main beverages of interest are typically baijiu, beer, wine, and coffee. Challenges and recommendations for FFT are also discussed, including analytical methods and mechanisms of formation, interactions between FFT and other compounds, and the development of specific materials to extend the duration of aroma after release.

Spatiotemporal accumulation differences of volatile compounds and bacteria metabolizing pickle like odor compounds during stacking fermentation of Maotai-flavor baijiu

Pickle like odor (PLO) is undesirable in Maotai-flavor baijiu; however, its formation mechanism is unclear. Furthermore, there is a lack of understanding of the spatiotemporal accumulation of volatile compounds (including PLO compounds, PLOC) and of the microorganisms responsible for the production of PLOC during stacking fermentation. In this study, we analyzed the spatiotemporal distribution differences of 132 volatile compounds in piled fermented grains. PLOC (n = 5) were higher in pile surface than in pile center, reaching their highest levels at 6th and 5th rounds, respectively. The microorganisms in pile center were more conducive to the formation of alcohols, while those in the pile surface more promoted the synthesis of esters. Rhodococcus and Zygosaccharomyces promoted the formation of PLOC. Acetobacter was negatively correlated with the content of sulfur compounds by promoting their conversion into non-volatile sulfur compounds, thereby reducing the content of PLOC. This study provides information on the spatiotemporal differences of volatile compounds (especially PLOC) in piled fermented grains and identified the microorganisms that produce PLOC.

Investigation on the interaction between 1,3-dimethyltrisulfide and aroma-active compounds in sesame-flavor baijiu by Feller Additive Model, Odor Activity Value and Partition Coefficient

The interaction between 1,3-dimethyltrisulfide and aroma-active compounds in sesame-flavor baijiu was evaluated by Feller's additive model and Odor Activity Value Approach, and the reason for the interaction can promote the release of fruity and caramel aromas of ethyl caproate, ethyl was explored by the Partition Coefficient Approach. The interaction results indicated that 1,3-dimethyltrisulfide caprate and furan-2-ylmethanol. Others showed masking effect. The Partition Coefficient showed that the effect of 1,3-dimethyltrisulfide on the volatility of esters was one of the reasons for the interaction affecting the flavor perception, and the volatility of ethyl esters with longer carbon chains at high phase ratio (PRs) is more likely to be promoted. Besides, the prediction model was initially proposed: y = 2.0112 ln(x) + 0.1461, which indicated that esters with the olfactory threshold lower than 33.80 μg/L are more likely to have positive effects with 1,3-dimethyltrisulfide, the negative effect is more likely to occur conversely.

Two novel Planococcus species isolated from baijiu pit mud with potential application in brewing

Two novel Gram-positive bacteria, designated strains REN8^T and REN14^T, were isolated from baijiu pit mud in Sichuan Province, China. REN8^T achieved the best growth at 37°C, a pH of 8.0, and a NaCl concentration of 2%, while REN14^T displayed optimal growth at 37°C, a pH of 6.0, and a NaCl concentration of 1%. 16S rRNA and genomic phylogenetic analysis showed that REN8^T and REN14^T were clustered with the genus Planococcus. The genomic DNA G + C contents of REN8^T and REN14^T were 46.7 and 45.1 mol%, respectively. The dDDH and ANI values were 24.5 and 80.43% between REN8^T and P. salinarum (the most closely related type strain) and 25.1 and 82.42% between REN14^T and P. soli (the most closely related type strain). Genomic analysis showed that several carbohydrate-active enzymes and secondary metabolite gene clusters existed in REN8^T and REN14^T. Chemotaxonomic characteristics of REN8^T and REN14^T included major fatty acids, predominant menaquinones, and polar lipids, all of which were consistent with the genus Planococcus. Based on the polyphasic taxonomic method, these two strains represent two novel species of the genus Planococcus; the name Planococcus beigongshangi sp. nov. is proposed for the type strain REN8^T (=JCM 33964^T = GDMCC 1.2213^T), and the name Planococcus beijingensis sp. nov. is proposed for the type strain REN14^T (=JCM 34410^T = GDMCC 1.2209^T). The addition of REN8^T and REN14^T might improve the quality of huangjiu by considerably increasing the amino acid nitrogen content and acidity and decreasing the bioamine content, with no significant change in alcohol content.

Dietary Supplementation with Mono-Lactate Glyceride Enhances Intestinal Function of Weaned Piglets

Mono-lactate glyceride (LG) is a short-chain fatty acid ester. It has been shown that short-chain fatty acid esters play an important role in maintaining intestinal structure and function. The aim of this study is to investigate the effects of mono-lactate glyceride on growth performance and intestinal morphology and function in weaned piglets. Sixteen 21-day-old weaned piglets of similar weight were distributed arbitrarily to two treatments: The control group (basal diet) and the LG group (basal diet + 0.6% mono-lactate glyceride). The experiment lasted for 21 days. On day 21 of the trial, piglets were weighed, and blood and intestinal samples were collected for further analysis. Results showed that dietary supplementation with 0.6% mono-lactate glyceride decreased (p < 0.05) the diarrhea rate and the contents of malondialdehyde and hydrogen peroxide in the ileum and jejunum and increased (p < 0.05) the expression of intestinal tight junction protein (Occludin) and the activities of superoxide dismutase and catalase in the ileum and colon. In addition, mono-lactate glyceride supplementation could enhance intestinal mucosal growth by increasing (p < 0.05) the mRNA levels of extracellular regulated protein kinases, promote intestinal mucosal water and nutrient transport and lipid metabolism by increasing (p < 0.05) the mRNA levels of b^0,+ amino acid transporter, aquaporin 3, aquaporin 10, gap junction protein alpha 1, intestinal fatty acid-binding protein, and lipoprotein lipase, enhance antiviral and immune function by increasing (p < 0.05) the mRNA levels of nuclear factor kappa-B, interferon-β, mucovirus resistance protein II, 2'-5'-oligoadenylate synthetase-like, interferon-γ, C-C motif chemokine ligand 2, and toll-like receptor 4, and enhance antioxidant capacity by increasing (p < 0.05) the mRNA levels of NF-E2-related factor 2 and glutathione S-transferase omega 2 and decreasing (p < 0.05) the mRNA level of NADPH oxidase 2. These results suggested that dietary supplementation with mono-lactate glyceride could decrease the diarrhea rate by improving intestinal antioxidant capacity, intestinal mucosal barrier, intestinal immune defense function, and intestinal mucosal water and nutrient transport. Collectively, dietary supplementation with 0.6% mono-lactate glyceride improved the intestinal function of weaned piglets.

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.

Effects of biofilm and co-culture with Bacillus velezensis on the synthesis of esters in the strong flavor Baijiu

Biofilm plays an important role in resisting the adverse environment, improving the taste and texture, and promoting the synthesis of flavor substances. However, to date, the findings on the effect of biofilm and dominating bacteria Bacillus on the ester synthesis in the Baijiu field have been largely lacked. Therefore, the objectives of the present study were to primarily isolate biofilm-producing microbes in the fermented grains, evaluate the stress tolerance capacity, and unveil the effect of biofilm and co-culture with Bacillus on the ester synthesis in the strong flavor Baijiu. Results indicated that after isolation and evaluation of stress-tolerance capacity, bacterial strain BG-5 and yeast strains YM-21 and YL-10 were demonstrated as mediate or strong biofilm-producing microbes and were identified as Bacillus velezensis, Saccharomycopsis fibuligera, and Zygosaccharomyces bailii, respectively. Solid phase microextraction/gas chromatography-mass spectrometer indicated that biofilm could enhance the diversity of esters while reduce the contents of ester. The scanning electron microscopy showed an inhibitory effect of B. velezensis on the growth of S. fibuligera, further restraining the production of esters. Taken together, both biofilm and B. velezensis influence the ester synthesis process. The present study is the first to reveal the biofilm-producing microorganisms in fermented grains and to preliminarily investigate the effect of biofilm on the ester synthesis in the Baijiu field.

Comparative proteome and volatile metabolome analysis of Aspergillus oryzae 3.042 and Aspergillus sojae 3.495 during koji fermentation

Aspergillus oryzae 3.042 and Aspergillus sojae 3.495 are crucial starters for fermented soybean foods since their abundant secreted enzymes. This study aimed to compare the differences in protein secretion between A. oryzae 3.042 and A. sojae 3.495 during the soy sauce koji fermentation and the effect on volatile metabolites to understand the fermentation characteristics of the strains better. Label-free proteomics detected 210 differentially expressed proteins (DEPs) enriched in amino acid metabolism and protein folding, sorting and degradation pathways. Subsequently, extracellular enzyme analysis showed that three peptidases, including peptide hydrolase, dipeptidyl aminopeptidase, and peptidase S41, were up-regulated in A. sojae 3.495. Seven carbohydrases, including α-galactosidase, endo-arabinase, β-glucosidase, α-galactosidase, α-glucuronidase, arabinan-endo 1,5-α-l-arabinase, and endo-1,4-β-xylanase were up-regulated in A. oryzae 3.042, contributing to the difference in enzyme activity. Significantly different extracellular enzymes influenced the content of volatile alcohols, aldehydes and esters such as (R, R)-2,3-butanediol, 1-hexanol, hexanal, decanal, ethyl l-lactate and methyl myristate in both strains, which affected the type of aroma of koji. Overall, this study revealed the differences in molecular mechanisms between A. oryzae 3.042 and A. sojae 3.495 under solid-state fermentation, providing a reference for targeted enhancement strains.

Synthesis of Short-Chain Alkyl Butyrate through Esterification Reaction Using Immobilized Rhodococcus Cutinase and Analysis of Substrate Specificity through Molecular Docking

Alkyl butyrate with fruity flavor is known as an important additive in the food industry. We synthesized various alkyl butyrates from various fatty alcohol and butyric acid using immobilized Rhodococcus cutinase (Rcut). Esterification reaction was performed in a non-aqueous system including heptane, isooctane, hexane, and cyclohexane. As a result of performing the alkyl butyrate synthesis reaction using alcohols of various chain lengths, it was found that the preference for the alcohol substrate had the following order: C6 > C4 > C8 > C10 > C2. Through molecular docking analysis, it was found that the greater the hydrophobicity of alcohol, the higher the accessibility to the active site of the enzyme. However, since the number of torsions increased as the chain length increased, it became difficult for the hydroxyl oxygen of the alcohol to access the ^γO of serine at the enzyme active site. These molecular docking results were consistent with substrate preference results of the Rcut enzyme. The Rcut maintained the synthesis efficiency at least for 5 days in isooctane solvent. We synthesized as much as 452 mM butyl butyrate by adding 100 mM substrate daily for 5 days and performing the reaction. These results show that Rcut is an efficient enzyme for producing alkyl butyrate used in the food industry.

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

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

Molecular mechanism of LIP05 derived from Monascus purpureus YJX-8 for synthesizing fatty acid ethyl esters under aqueous phase

Fatty acid ethyl esters are important flavor chemicals in strong-flavor Baijiu. Monascus purpureus YJX-8 is recognized as an important microorganism for ester synthesis in the fermentation process. Enzyme LIP05 from YJX-8 can efficiently catalyze the synthesis of fatty acid ethyl esters under aqueous phase, but the key catalytic sites affecting esterification were unclear. The present work combined homology modeling, molecular dynamics simulation, molecular docking and site-directed mutation to analyze the catalytic mechanism of LIP05. Protein structure modeling indicated LIP05 belonged to α/β fold hydrolase, contained a lid domain and a core catalytic pocket with conserved catalytic triad Ser150-His215-Asp202, and the oxyanion hole composed of Gly73 and Thr74. Ile30 and Leu37 of the lid domain were found to affect substrate specificity. The π-bond stacking between Tyr116 and Tyr149 played an important role in stabilizing the catalytic active center of LIP05. Tyr116 and Ile204 determined the substrate spectrum by composing the substrate-entrance channel. Residues Leu83, Ile204, Ile211 and Leu216 were involved in forming the hydrophobic substrate-binding pocket through steric hindrance and hydrophobic interaction. The catalytic mechanism for esterification in aqueous phase of LIP05 was proposed and provided a reference for clarifying the synthesis of fatty acid ethyl esters during the fermentation process of strong-flavor Baijiu.

Revealing the Changes in Compounds When Producing Strong-Flavor Daqu by Statistical and Instrumental Analysis

Daqu is not only a crucial starter in the production of baijiu, but it is also an important source of flavoring substances, so maintaining a stable quality is an important part of improving the quality of baijiu. Nonetheless, since the production of daqu is still a natural fermentation process, which is influenced by seasonal factors, the rapid testing of daqu quality is a problem that must be solved. In this study, headspace solid-phase microextraction technology (HS-SPME) was used to explore the volatile components in daqu, and a total of 115 volatile components were extracted. By constructing an untargeted statistical model, the variation in volatile compounds in dissimilar production processes of daqu was studied, and the differences between different maturation stages and the correlations between volatile compounds were analyzed. Subsequently, six compounds, including ethyl acetate, ethanol, phenylethanol, (R,R)-2,3-butanediol, ethyl caproate, and 2,3-butanediol, were further screened out by partial least squares discrimination analysis (PLS-DA), and the symbolic combination of daqu’s maturity was speedily judged in accordance with the changes in marker compound concentrations to lay the foundation for the mechanization of baijiu production.

Production of pigments by Monascus ruber CCT0302 in culture media containing maltose as substrate

The aim of this study was to show how maltose production residues can be used to obtain natural pigments by Monascus ruber CCT 3802 in solid and submerged cultures. The microbial growth and the colour and heat stability characteristics of the pigments produced in both solid and submerged media, with different maltose syrup concentrations, were determined. The results showed that the addition of maltose provided significant increases in the velocity of microbial growth and production of red pigments. The highest radial growth velocity of Monascus ruber (0.1053 mm h−1) was obtained when cultivated in a medium containing 5 g L−1 of maltose syrup, corresponding to a 71.7% increase in growth as compared to the growth velocity in the control medium. Using submerged fermentation, the culture medium containing 10 g L−1 of maltose syrup provided the greatest concentrations of red pigments (14.54 AU510nm g−1 dry biomass) with an intense dark red colour, showing that Monascus ruber CCT 3802 had the capacity to assimilate the substrate and produce pigments. The red pigments produced in the cultures showed good heat stability with activation energies of 13.735 Kcal mol−1.

Clostridium btbubcensis BJN0001, a potentially new species isolated from the cellar mud of Chinese strong-flavor baijiu, produces ethanol, acetic acid and butyric acid from glucose

A novel strain, designated BJN0001, was isolated from the cellar mud of Chinese strong-flavor baijiu. The complete genome of strain BJN0001 was 2,688,791 bp and annotated with 2610 genes. Whole-genome similarity metrics such as average nucleotide identity (ANI) of BJN0001 with reference genomes reveals clear species boundaries of < 95% ANI value for species. The DNA-DNA hybridization (DDH) values of BJN0001 with the type species were all lower than 70% DDH value for species. Based on these results, the strain BJN0001 was considered a potentially new species of the genus Clostridium. Meanwhile, the fermentation characteristics indicated that the strain had the capability to convert glucose to ethanol, acetic acid and butyric acid, which could provide basic data for revealing its function in baijiu fermentation.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03271-7.

Fungal Interactions Strengthen the Diversity-Functioning Relationship of Solid-State Fermentation Systems

Traditional fermentation processes are driven by complex fungal microbiomes. However, the exact means by which fungal diversity affects fermentation remains unclear. In this study, we systematically investigated the diversity of a fungal community and its functions during the multibatch Baijiu fermentation process. Metabolomics analysis showed that the metabolic profiles of the Baijiu were enhanced with an increase in the fermentation time, as determined from the characteristic volatile flavors. High-throughput sequencing technology revealed that the major fungal species involved in sauce-flavor Baijiu fermentation are Pichia sp. (41.75%, average relative abundance), Saccharomyces sp. (13.07%), thermophilic species (9.16%), Monascus sp. (6.80%), Aspergillus sp. (4.69%), Schizosaccharomyces sp. (3.76%), Thermomyces sp. (3.74%), and Zygosaccharomyces sp. (1.41%). In addition, the fungal diversity increased as the number of fermentation batches increased. Moreover, the increased fungal diversity contributed to the modularity of the fungal communities, wherein Pichia sp., Torulaspora sp., and Saccharomyces sp. maintained the stability of the fungal community. In addition, metatranscriptomics sequencing technologies were used to reconstruct the key metabolic pathways during fermentation, and it was found that the increased microbial diversity significantly promoted glucose-mediated carbon metabolism. Finally, functional gene analysis showed that functional microorganisms, such as Zygosaccharomyces and Pichia, can enhance fermentation as a result of the high expression of pyruvate decarboxylase and propanol-preferring alcohol dehydrogenase during the metabolism of pyruvate. These results indicate that fungal biodiversity can be exploited to enhance fermentation-based processes via network interactions and metabolism during multiple-batch fermentation. IMPORTANCE Biodiversity and network interactions act simultaneously on the microbial community structure in the Baijiu fermentation process, thereby rendering the microbiome dynamics challenging to manage and predict. Understanding the complex fermentation community and its relationship to community functions is therefore important in the context of developing improved fermentation biotechnology systems. Our work demonstrates that multiple-batch fermentation steps increase microbial diversity and promote community stability. Crucially, the enhanced modularity in the microbial network increases the metabolism of flavor compounds and ethanol. This study highlights the power of biodiversity and network interactions in regulating the function of the microbiome in food fermentation ecosystems.

Production of caproic acid by Rummeliibacillus suwonensis 3B-1 isolated from the pit mud of strong-flavor baijiu

Caproic acid is the precursor of ethyl caproate, the main representative flavor substance of strong-flavor baijiu (SFB). Caproic acid-producing bacteria are considered to be the most important type of acid-producing microorganisms in the pit mud of the SFB ecosystem. In this study, the Rummeliibacillus suwonensis 3B-1 with a high yield of caproic acid (4.064g/L) was screened from SFB pit mud. The genome of the R. suwonensis 3B-1 was sequenced, the total size was found to be 4,117,671bp and a calculated GC content of 35.86%. The caproic acid biosynthesis pathway was identified and analyzed, and it showed that 3B-1 could not only use ethanol, but it could also use glucose and other carbon sources as substrates to produce caproic acid. According to the genome analysis and with an optimized medium, the optimal conditions for caproic acid production were yeast powder at 3g/L, sodium acetate at 15g/L, and 1% biotin at 8mL/100mL. The yield of caproic acid reached 4.627g/L, an increase of 13.9%, which was higher than that of general caproic acid bacteria. This is the first report of the synthesis of caproic acid by R. suwonensis. This strain could be used to produce caproic acid, an artificial pit mud preparation, and/or an enhanced inoculum in the production of SFB.

Study on microbial community of "green-covering" Tuqu and the effect of fortified autochthonous Monascus purpureus on the flavor components of light-aroma-type Baijiu

"Green-covering" Tuqu (TQ), as one of Xiaoqu, is a special fermentative starter (also known as Jiuqu in Chinese) that originated in southern China and is characterized by a layer of green mold covering (Aspergillus clavatus) the surface and (sometimes) with a red heart. It plays a vital role in producing light-aroma-type Baijiu (LATB). However, to date, the microbiota that causes red heart of TQ remain largely unexplored, and it is still unclear how these microbiota influence on the quality of LATB. In this study, two types of TQ, one with a red heart (RH) and another with a non-red heart (NRH), were investigated by high throughput sequencing (HTS) and directional screening of culture-dependent methods. The obtained results revealed the differences in the microbial communities of different TQ and led to the isolation of two species of Monascus. Interestingly, the results of high performance liquid chromatography (HPLC) detection showed that citrinin was not detected, indicating that Monascus isolated from TQ was no safety risk, and the contents of gamma-aminobutyric acid in the fermented grains of RH were higher than that of NRH during the fermentation. Selecting the superior autochthonous Monascus (M1) isolated from the TQ to reinoculate into the TQ-making process, established a stable method for producing the experimental "red heart" Tuqu (ERH), which confirmed that the cause of "red heart" was the growth of Monascus strains. After the lab-scale production test, ERH increased ethyl ester production and reduced higher alcohols production. In addition, Monascus had an inhibitory effect on the growth of Saccharomyces and Aspergillus. This study provides the safe, health-beneficial, and superior fermentation strains and strategies for improving the quality of TQ and LATB.

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.