Effects of salinity on the synthesis of 3-methylthiopropanol, 2-phenylethanol, and isoamyl acetate in Zygosaccharomyces rouxii and Z. rouxii 3-2

Screening of a Saccharomyces cerevisiae Strain with High 3-Methylthio-1-Propanol Yield and Optimization of Its Fermentation Conditions

3-Methylthio-1-propanol (3-Met) is an important flavor compound in various alcoholic beverages such as Baijiu and Huangjiu. To maintain the content of 3-Met in these alcoholic beverages, it is necessary to screen a micro-organism with high yield of 3-Met from the brewing environment. In this study, the ability of yeast strains from the Baijiu brewing to produce 3-Met was analyzed, aiming to obtain yeast with high-yield 3-Met, and its fermentation conditions were optimized. Firstly, 39 yeast strains were screened using 3-Met conversion medium. The results showed that the majority of the strains from Baijiu brewing sources could produce 3-Met, and nearly half of the strains produced more than 0.5 g/L of 3-Met. Among these, yeast F10404, Y03401, and Y8#01, produced more than 1.0 g/L of 3-Met, with yeast Y03401 producing the highest amount at 1.30 g/L. Through morphological observation, physiological and biochemical analysis, and molecular biological identification, it was confirmed that yeast Y03401 was a Saccharomyces cerevisiae. Subsequently, the optimal fermentation conditions for 3-Met production by this yeast were obtained through single-factor designs, Plackett-Burman test, steepest ascent path design and response surface methodology. When the glucose concentration was 60 g/L, yeast extract concentration was 0.8 g/L, L-methionine concentration was 3.8 g/L, initial pH was 4, incubation time was 63 h, inoculum size was 1.6%, shaking speed was 150 rpm, loading volume was 50 mL/250 mL, and temperature was 26 °C, the content of 3-Met produced by S. cerevisiae Y03401 reached a high level of 3.66 g/L. It was also noteworthy that, in contrast to other study findings, this yeast was able to create substantial amounts of 3-Met even in the absence of L-methionine precursor. Based on the clear genome of S. cerevisiae and its characteristics in 3-Met production, S. cerevisiae Y03401 had broad prospects for application in alcoholic beverages such as Baijiu.

Effects of Environmental Stresses on Synthesis of 2-Phenylethanol and IAA by Enterobacter sp. CGMCC 5087

2-Phenylethanol (2-PE) and indole-3-acetic acid (IAA) are important secondary metabolites produced by microorganisms, and their production are closely linked to the growth state of microorganisms and environmental factors. Enterobacter CGMCC 5087 can produce both 2-PE and IAA depending on α-ketoacid decarboxylase KDC4427. This study aimed to investigate the effects of different environment factors including osmotic pressure, temperature, and pH on the synthesis of 2-PE and IAA in Enterobacter sp. CGMCC 5087. The bacteria exhibited an enhanced capacity for 2-PE synthesis while not affecting IAA synthesis under 5% NaCl and pH 4.5 stress conditions. In an environment with pH 9.5, the synthesis capacity of 2-PE remained unchanged while the synthesis capacity of IAA decreased. The synthesis ability of 2-PE was enhanced with an increase in temperature within the range of 25 °C to 37 °C, while the synthesis capacity of IAA was not affected significantly. Additionally, the expression of KDC4427 varied under stress conditions. Under 5% NaCl stress and decreased temperature, expression of the KDC4427 gene was increased. However, altering pH did not result in significant differences in gene expression levels, while elevated temperature caused a decrease in gene expression. Furthermore, molecular docking and molecular dynamics simulations suggested that these conditions may induce fluctuation in the geometry shape of binding cavity, binding energy, and especially the dαC-C- value, which played key roles in affecting the enzyme activity. These results provide insights and strategies for the synthesis of metabolic products 2-PE and IAA in bacterial fermentation, even under unfavorable conditions.

Unraveling the functional consequences of a novel germline missense mutation (R38C) in the yeast model of succinate dehydrogenase subunit B: insights into neurodegenerative disorders

This study explores the implications of a novel germline missense mutation (R38C) in the succinate dehydrogenase (SDH) subunit B, which has been linked to neurodegenerative diseases. The mutation was identified from the SDH mutation database and corresponds to the SDH2R32C allele, mirroring the human SDHBR38C mutation. By subjecting the mutant yeast model to hydrogen peroxide (H2O2) stress, simulating oxidative stress, we observed heightened sensitivity to oxidative conditions. Quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) analysis revealed significant regulation (p < 0.05) of genes associated with antioxidant systems and energy metabolism. Through gas chromatography-mass spectrometry (GC-MS) analysis, we examined yeast cell metabolites under oxidative stress, uncovering insights into the potential protective role of o-vanillin. This study elucidates the biological mechanisms underlying cellular oxidative stress responses, offering valuable insights into its repercussions. These findings shed light on innovative avenues for addressing neurodegenerative diseases, potentially revolutionizing therapeutic strategies.

Synergistic Fermentation with Functional Microorganisms Improves Safety and Quality of Traditional Chinese Fermented Foods

Traditional fermented foods are favored by people around the world for their positive health and taste advantages. Many of the fermented foods, including Chinese traditional fermented foods, are produced through mixed-culture fermentation. Apart from reducing the formation of harmful compounds such as ethyl carbamate (EC) and biogenic amines (BAs) during food fermentation, it is also difficult to precisely control and regulate the fermentation process based on the control of environmental conditions alone, due to the complex microbiota and an unclarified fermentation mechanism. In this review, key microorganisms involved in Chinese fermented foods such as baijiu, soy sauce, and vinegar production are elaborated, and relations between microbial composition and the aroma or quality of food are discussed. This review focuses on the interpretation of functions and roles of beneficial (functional) microorganisms that participate in food fermentation and the discussion of the possibilities of the synergistic use of functional microorganisms to improve the safety and quality of Chinese fermented foods. Conducting work toward the isolation of beneficial microorganisms is a challenge for modern food fermentation technology. Thus, methods for the isolation and mutagenesis of functional microbial strains for synergistic food fermentation are summarized. Finally, the limitations and future prospects of the use of functional microorganisms in traditional Chinese fermented foods are reviewed. This review provides an overview of the applications of synergistic fermentation with functional microorganisms in the improvement of the safety or sensory qualities of fermented foods.

Effect of fermentation methods on the quality and in vitro antioxidant properties of Lycium barbarum and Polygonatum cyrtonema compound wine

Lycium barbarum and Polygonatum cyrtonema are known for their medicinal, edible, and ornamental properties. The sensory indices of the novel high-quality L. barbarum and P. cyrtonema compound wine (LPCW) fermented by Saccharomyces cerevisiae RW and Debaryomyces hansenii AS2.45 under different inoculation methods were analyzed. The alcohol content of the LPCW ranged from 3.88 to 4.75 % under three mixed inoculations. The total saponin and total polysaccharide contents in LPCW inoculated with D. hansenii first and S. cerevisiae after 24 h were 4.39 mg/mL and 0.21 mg/mL, respectively. Ethyl butyrate, citronellol, and 3-(methylthio) propanol were unique metabolites of D. hansenii. 4-Methoxybenzoic acid was the core product of brewing of by S. cerevisiae. Except for wine inoculated with S. cerevisiae only, the acceptability scores of all the LPCW samples were higher than 7.3. Our data provided the foundation for the development and application of medicinal and food homologous substances in food fermentation.

Screening of Yeasts Isolated from Baijiu Environments for Producing 3-Methylthio-1-propanol and Optimizing Production Conditions

3-Methylthio-1-propanol (3-Met) is widely used as a flavoring substance and an essential aroma ingredient in many foods. Producing 3-Met by microbial transformation is green and eco-friendly. In the present study, one strain, YHM-G, which produced a high level of 3-Met, was isolated from the Baijiu-producing environment. Strain YHM-G was identified as Hyphopichia burtonii according to its morphological properties, physiological and biochemical characteristics, and ribosomal large subunit 26S rRNA gene D1/D2 domain sequence analysis. The optimal conditions for 3-Met production by YHM-G were obtained by single factor design, Plackett-Burman design, steepest ascent path design and response surface methodology as follows: 42.7 g/L glucose, pH 6, 0.9 g/L yeast extract, 6 g/L L-methionine (L-Met), culture temperature 28 °C, shaking speed 210 rpm, loading volume 50 mL/250 mL, inoculum size 0.5% (v/v), culturing period 48 h and 2.5 g/L Tween-80. Under these optimal conditions, the 3-Met production by strain YHM-G was 3.16 g/L, a value 88.1% higher than that before optimization. Strain YHM-G can also produce a variety of flavor compounds that are important for many foods. This strain thus has the potential to increase the abundance of 3-Met in some fermented foods and enhance their aroma profiles.

Physiological and transcriptomic analyses revealed the change of main flavor substance of Zygosaccharomyces rouxii under salt treatment

Zygosaccharomyces rouxii was a highly salt-tolerant yeast, playing an important role in soy sauce fermentation. Previous studies reported that Z. rouxii under salt treatment produces better fermented food. However, the detailed change of main flavor substance was not clear. In this study, the physiological and transcriptomic analyses of Z. rouxii under salt treatment was investigated. The results revealed the high salt tolerance of Z. rouxii. Analysis of physiological data showed that the proportion of unsaturated fatty acids was significantly increased with the increment of salt concentrations. The analysis of organic acids showed that the content of succinic acid was significantly higher in the salt-treated Z. rouxii while oxalic acid was only identified at the 18% salt concentration-treated group. Results of volatile substances analysis showed that concentrations of 3-methyl-1-butanol and phenylethyl alcohol were significantly increased with the increment of salt concentrations. A comparison of transcriptome data showed that the genes involved in the TCA cycle and the linoleic acid synthesis process exhibited different expressions, which is consistent with the results of physiological data. This study helps to understand the change of main flavor substance of Z. rouxii under salt treatment and guide their applications in the high salt liquid state fermentation of the soy sauce.

The revenge of Zygosaccharomyces yeasts in food biotechnology and applied microbiology

Non-conventional yeasts refer to a huge and still poorly explored group of species alternative to the well-known model organism Saccharomyces cerevisiae. Among them, Zygosaccharomyces rouxii and the sister species Zygosaccharomyces bailii are infamous for spoiling food and beverages even in presence of several food preservatives. On the other hand, their capability to cope with a wide range of process conditions makes these yeasts very attractive factories (the so-called "ZygoFactories") for bio-converting substrates poorly permissive for the growth of other species. In balsamic vinegar Z. rouxii is the main yeast responsible for converting highly concentrated sugars into ethanol, with a preference for fructose over glucose (a trait called fructophily). Z. rouxii has also attracted much attention for the ability to release important flavor compounds, such as fusel alcohols and the derivatives of 4-hydroxyfuranone, which markedly contribute to fragrant and smoky aroma in soy sauce. While Z. rouxii was successfully proposed in brewing for producing low ethanol beer, Z. bailii is promising for lactic acid and bioethanol production. Recently, several research efforts exploited omics tools to pinpoint the genetic bases of distinctive traits in "ZygoFactories", like fructophily, tolerance to high concentrations of sugars, lactic acid and salt. Here, I provided an overview of Zygosaccharomyces industrially relevant phenotypes and summarized the most recent findings in disclosing their genetic bases. I suggest that the increasing number of genomes available for Z. rouxii and other Zygosaccharomyces relatives, combined with recently developed genetic engineering toolkits, will boost the applications of these yeasts in biotechnology and applied microbiology.

Obtainment, selection and characterization of a mutant strain of Kluyveromyces marxianus that displays improved production of 2-phenylethanol and enhanced DAHP synthase activity

AIMS

Yeasts produce 2-phenylethanol (2-PE) from sugars via de novo synthesis; however, its synthesis is limited due to feedback inhibition on the isofunctional 3-deoxy-d-arabino-heptulosonate-7-phosphate (DAHP) synthases (Aro3p and Aro4p). This work aimed to select Kluyveromyces marxianus mutant strains with improved capacity to produce 2-PE from sugars.

METHODS AND RESULTS

Kluyveromyces marxianus CCT 7735 mutant strains were selected from UV irradiation coupled with screening of p-fluoro-dl-phenylalanine (PFP) tolerant strains on culture medium without l-Phe addition. Most of them produced 2-PE titres higher than the parental strain and the Km_PFP41 mutant strain stood out for displaying the highest 2-PE specific production rate. Moreover it showed higher activity of DAHP synthase than the parental strain. We sequenced both ARO3 and ARO4 genes of Km_PFP41 mutant and identified mutations in ARO4 which caused changes in both size and conformation of the Aro4p. These changes seem to be associated with the enhanced activity of DAHP synthase and improved production of 2-PE exhibited by that mutant strain.

CONCLUSIONS

The Km_PFP41 mutant strain presented improved 2-PE production via de novo synthesis and enhanced DAHP synthase activity.

SIGNIFICANCE AND IMPACT OF THE STUDY

The mutant strain obtained in this work may be exploited as a yeast cell factory for high-level synthesis of 2-PE.