Differential Characteristics of the Metabolic Profiles and Microbial Community between Superior and Normal Grades of Nongxiangxing-daqu.

Nongxiangxing-daqu (NXDQ), as a saccharification and fermentation agent, directly affects the flavor and yield of fresh Nongxiangxing Baijiu (NXBJ). The difference in fermentation temperature owing to the artificial turning operation leads to the formation of superior (S) and normal (N) grades of NXDQ. Here, aiming to explore the discriminant characteristics of two grades of NXDQ, we studied the physicochemical properties, volatile compounds and microbial communities using HS-SPME-GC/MS and high-throughput sequencing technology. The NXDQ grades presented different physicochemical properties. Staphylococcus, Weissella, Lactobacillus and Thermoascus were dominant in the S grade (S-NXDQ), while Bacillus, Thermoactinomyces and Aspergillus were predominant in the N grade (N-NXDQ). Higher alcohols, aldehydes and ketones positively correlated with the bacterial biomarkers could be used as metabolic biomarkers for N-NXDQ; the S-NXDQ had a higher abundance of key enzymes involved in lactic acid and ethanol fermentation, while N-NXDQ had a higher abundance of key enzymes involved in amino acid synthesis and long-chain fatty acid and lipid metabolism. N-NXDQ and S-NXDQ had different microbial and metabolic biomarkers. These findings provide insight into the discriminant characteristics of different grades of NXDQ, a theoretical basis for rational evaluation of NXDQ, and effective information for quality improvement of daqu.

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.

Gene Mining and Flavour Metabolism Analyses of Wickerhamomyces anomalus Y-1 Isolated From a Chinese Liquor Fermentation Starter.

Luzhou-flavoured liquor is one of Chinese most popular distilled liquors. Hundreds of flavoured components have been detected from this liquor, with esters as its primary flavouring substance. Among these esters, ethyl hexanoate was the main component. As an essential functional microbe that produces ethyl hexanoate, yeast is an important functional microorganism that produces ethyl hexanoate. The synthesis of ethyl hexanoate in yeast mainly involves the lipase/esterase synthesis pathway, alcohol transferase pathway and alcohol dehydrogenase pathway. In this study, whole-genome sequencing of W. anomalus Y-1 isolated from a Chinese liquor fermentation starter, a fermented wheat starter containing brewing microorganisms, was carried out using the Illumina HiSeq X Ten platform. The sequence had a length of 15,127,803 bp with 34.56% GC content, encoding 7,024 CDS sequences, 69 tRNAs and 1 rRNA. Then, genome annotation was performed using three high-quality databases, namely, COG, KEGG and GO databases. The annotation results showed that the ko7019 pathway of gene 6,340 contained the Eht1p enzyme, which was considered a putative acyltransferase similar to Eeb1p and had 51.57% homology with two known medium-chain fatty acid ethyl ester synthases, namely, Eht1 and Eeb1. Ethyl hexanoate in W. anomalus was found to be synthesised through the alcohol acyltransferase pathway, while acyl-coenzyme A and alcohol were synthesised under the catalytic action of Eht1p. The results of this study are beneficial to the exploration of key genes of ester synthesis and provide reference for the improvement of liquor flavoured.

Physical and chemical properties, structural characterization and nutritional analysis of kefir yoghurt.

Scanning electron microscopy (SEM), Confocal laser scanning microscopy (CLSM) and low field nuclear magnetic resonance (LF-NMR) were used to analyse the relationship between the chemical, texture, rheology, microstructure and water distribution of kefir (yeast, acetic acid bacteria and Lactobacillus plantarum) yoghurt fermented by mixed bacteria and L. plantarum L1 fermented yoghurt. This work was conducted to prepare a real champagne yoghurt and explore the difference between it and ordinary yoghurt. The nutritional evaluation of the two treatment groups was carried out by amino acid analysis, and the volatile flavour substances of the two treatment groups were detected by solid phase microextraction (SPME)-gas chromatograph (GC)-mass spectrometry (MS). Results showed that the addition of acetic acid bacteria and yeast increased the water content of kefir, resulting in a decrease in its water-holding rate. Moreover, the increase in acidity weakened the connection between the protein networks, the flocculent protein structure was not more densely stacked than the L1 group, and the internal bonds were unstable. The rheological results showed that the apparent viscosity decreased faster with the increase in shear force. The CLSM and LF-NMR showed that the hydration and degree of freedom of kefir yoghurt protein decreased, resulting in an increased protein network density. The SEM showed that the cross-linking between kefir casein clusters was considerably tight to form small chains, the pore distribution was uneven, and a weak cheese structure was formed. In addition, the volatile flavour substances in the kefir group increased the phenylethyl alcohol, isobutanol, and isoamyl alcohol compared with those in the L1 group, with a slight refreshing taste brought by alcohol and special soft malt alcohol aroma and rose aroma not found in ordinary yoghurt, which was more in line with the characteristics and taste of traditional kefir champagne yoghurt. Graphical Abstract.

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

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