Metabolic physiology of aroma-producing Kluyveromyces marxianus

Optimizing bioreactor conditions for Spirulina fermentation by Lactobacillus helveticus and Kluyveromyces marxianus: Impact on chemical & bioactive properties

This study focused on optimizing the production of fermented Spirulina (FS) products using a bioactivity-guided strategy with Lactobacillus helveticus B-4526 and Kluyveromyces marxianus Y-329 in a 3-L bioreactor. Various operating conditions, including aeration rates and pH modes, were tested. While both microorganisms thrived under all conditions, the "cascade" mode, controlling dissolved oxygen, enhanced protein hydrolysis and antioxidant activity, as confirmed by SDS-PAGE and DPPH/TEAC assays, respectively. Screening revealed that "cascade" FS significantly decreased viability of colon cancer cells (HT-29) in a dose-dependent manner, with up to a 72 % reduction. Doses ≤ 500 μg mL-1 of "cascade" FS proved safe and effective in suppressing NO release without compromising cellular viability. Additionally, "cascade" FS exhibited diverse volatile organic compounds and reducing the characteristic "seaweed" aroma. These findings highlight "cascade" FS as a promising alternative food source with improved bioactive properties, urging further exploration of its bioactive compounds, particularly bioactive peptides.

The Main Features and Microbiota Diversity of Fermented Camel Milk

Fermented camel milk, named shubat in Central Asia, is historically and culturally important because it is mainly consumed by Kazakh people who live not only in Kazakhstan but also in close neighboring countries. However, despite its cultural and dietetic significance for this local population, research on its composition and processing technology and the richness of its microflora is relatively scarce. The present review of this product, which is an important beverage in the Kazakh culture, provides up-to-date information regarding its main components and their variability according to different factors, surveys recent changes in the processing technologies for making it using modern techniques, and explores the biodiversity of its microflora. It was reported that the protein, vitamin C, and calcium contents in shubat vary between 1.19 and 5.63%, 28 and 417 mg L-1, and 1.03 and 1.88 g L-1. The lactose content totally disappears. Shubat contains a complex microbial consortium that contributes to its strong reputation for health benefits, but a scientific demonstration of these claims has only been partially achieved.

Characterization of mating type on aroma production and metabolic properties wild Kluyveromyces marxianus yeasts

Kluyveromyces marxianus yeasts represent a valuable industry alternative due to their biotechnological potential to produce aromatic compounds. 2-phenylethanol and 2-phenylethylacetate are significant aromatic compounds widely used in food and cosmetics due to their pleasant odor. Natural obtention of these compounds increases their value, and because of this, bioprocesses such as de novo synthesis has become of great significance. However, the relationship between aromatic compound production and yeast's genetic diversity has yet to be studied. In the present study, the analysis of the genetic diversity in K. marxianus isolated from the natural fermentation of Agave duranguensis for Mezcal elaboration is presented. The results of strains in a haploid and diploid state added to the direct relationship between the mating type locus MAT with metabolic characteristics are studied. Growth rate, assimilate carbohydrates (glucose, lactose, and chicory inulin), and the production of aromatic compounds such as ethyl acetate, isoamyl acetate, isoamyl alcohol, 2-phenylethyl butyrate and phenylethyl propionate and the diversity in terms of the output of 2-phenylethanol and 2-phenylethylacetate by de novo synthesis were determinate, obtaining maximum concentrations of 51.30 and 60.39 mg/L by ITD0049 and ITD 0136 yeasts respectively.

Kluyveromyces as promising yeast cell factories for industrial bioproduction: From bio-functional design to applications

As the two most widely used Kluyveromyces yeast, Kluyveromyces marxianus and K. lactis have gained increasing attention as microbial chassis in biocatalysts, biomanufacturing and the utilization of low-cost raw materials owing to their high suitability to these applications. However, due to slow progress in the development of molecular genetic manipulation tools and synthetic biology strategies, Kluyveromyces yeast cell factories as biological manufacturing platforms have not been fully developed. In this review, we provide a comprehensive overview of the attractive characteristics and applications of Kluyveromyces cell factories, with special emphasis on the development of molecular genetic manipulation tools and systems engineering strategies for synthetic biology. In addition, future avenues in the development of Kluyveromyces cell factories for the utilization of simple carbon compounds as substrates, the dynamic regulation of metabolic pathways, and for rapid directed evolution of robust strains are proposed. We expect that more synthetic systems, synthetic biology tools and metabolic engineering strategies will adapt to and optimize for Kluyveromyces cell factories to achieve green biofabrication of multiple products with higher efficiency.

Mechanistic Modelling of Biomass Growth, Glucose Consumption and Ethanol Production by Kluyveromyces marxianus in Batch Fermentation

This paper presents results concerning mechanistic modeling to describe the dynamics and interactions between biomass growth, glucose consumption and ethanol production in batch culture fermentation by Kluyveromyces marxianus (K. marxianus). The mathematical model was formulated based on the biological assumptions underlying each variable and is given by a set of three coupled nonlinear first-order Ordinary Differential Equations. The model has ten parameters, and their values were fitted from the experimental data of 17 K. marxianus strains by means of a computational algorithm design in Matlab. The latter allowed us to determine that seven of these parameters share the same value among all the strains, while three parameters concerning biomass maximum growth rate, and ethanol production due to biomass and glucose had specific values for each strain. These values are presented with their corresponding standard error and 95% confidence interval. The goodness of fit of our system was evaluated both qualitatively by in silico experimentation and quantitative by means of the coefficient of determination and the Akaike Information Criterion. Results regarding the fitting capabilities were compared with the classic model given by the logistic, Pirt, and Luedeking-Piret Equations. Further, nonlinear theories were applied to investigate local and global dynamics of the system, the Localization of Compact Invariant Sets Method was applied to determine the so-called localizing domain, i.e., lower and upper bounds for each variable; whilst Lyapunov's stability theories allowed to establish sufficient conditions to ensure asymptotic stability in the nonnegative octant, i.e., R+,03. Finally, the predictive ability of our mechanistic model was explored through several numerical simulations with expected results according to microbiology literature on batch fermentation.

Metabolic engineering of Kluyveromyces marxianus for biomass-based applications

Kluyveromyces marxianus ATCC 26,548 was cultivated in aerobic chemostats with [1-13C] and [U-13C] glucose as carbon source under three different growth conditions (0.10, 0.25, and 0.5 h-1) to evaluate metabolic fluxes. Carbon balances closed always within 97-102%. Growth was carbon limited, and the cell yield on glucose was the same. The extracellular side-product formation was very low, totaling 0.0008 C-mol C-mol-1 substrate at 0.5 h-1. The intracellular flux ratios did not show significant variation for metabolic flux analysis from labelling and biomass composition and metabolic flux ratio analysis from labelling. The observed strictly oxidative metabolism and the stability of the metabolism in terms of fluxes even at high growth rates, without triggering out the synthesis of by-products, is an extremely desired condition that underlines the potential of K. marxianus for biotechnological biomass-related applications and the comprehension of the metabolic pools and pathways is an important step to engineering this organism.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03324-x.

Spoilage Potential of Contaminating Yeast Species Kluyveromyces marxianus, Pichia kudriavzevii and Torulaspora delbrueckii during Cold Storage of Skyr

This study investigated the spoilage potential of yeast strains Kluyveromyces marxianus (Km1, Km2 and Km3), Pichia kudriavzevii Pk1 and Torulaspora delbrueckii Td1 grown in skyr in cold storage. Yeast strains were isolated from skyr and identified by sequencing of the 26S rRNA gene. K. marxianus yeasts were grown in skyr to high numbers, generating large amounts of volatile organic compounds (VOC) associated with off-flavours, among them were alcohols (3-methyl-1-butanol, 2-methyl-1-propanol and 1-hexanol), esters (ethyl acetate and 3-methylbutyl acetate) and aldehydes (hexanal, methylbutanal and methylpropanal). Growth of P. kudriavzevii Pk1 led to moderate increases in several alcohols and esters (mostly, 3-methyl-1-butanol and ethyl acetate), whereas only minor shifts in VOCs were associated with T. delbrueckii Td2. The levels of the key aroma compounds, diacetyl and acetoin, were significantly decreased by all K. marxianus strains and P. kudriavzevii Pk1. In contrast to the other yeast species, K. marxianus was able to utilize lactose, producing ethanol and carbon dioxide. Based on the overall results, K. marxianus was characterised by the highest spoilage potential. The study revealed the differences between the yeast species in fermentative and spoilage activities, and clarified the role of yeast metabolites for off-flavour formation and quality defects in skyr during cold storage.

Maltose-Negative Yeast in Non-Alcoholic and Low-Alcoholic Beer Production

Although beer is a widely used beverage in many cultures, there is a need for a new drinking alternative in the face of rising issues such as health concerns or weight problems. However, non-alcoholic and low-alcoholic beers (NABLAB) still have some sensory problems that have not been fully remedied today, such as “wort-like”/”potato-like” flavours or a lack of aroma. These defects are due to the lack of alcohol (and the lack of the aldehyde-reducing effect of alcohol fermentation), as well as production techniques. The use of new yeast strains that cannot ferment maltose—the foremost sugar in the wort—is highly promising to produce a more palatable and sustainable NABLAB product because production with these yeast strains can be performed with standard brewery equipment. In the scientific literature, it is clear that interest in the production of NABLAB has increased recently, and experiments have been carried out with maltose-negative yeast strains isolated from many different environments. This study describes maltose-negative yeasts and their aromatic potential for the production of NABLAB by comprehensively examining recent academic studies.

The same genetic regulation strategy produces inconsistent effects in different Saccharomyces cerevisiae strains for 2-phenylethanol production

A CRISPR/Cas9 system with gene editing efficiency of 100% in the industrial diploid Saccharomyces cerevisiae CWY-132 strain for 2-phenylethanol (2-PE) production was constructed. The effect of deletion of acetyltransferase gene ATF1 in the Ehrlich pathway on 2-PE synthesis was studied for the first time in S. cerevisiae. Laboratory and industrial strains were compared for the deletion effect of ATF1 and acetaldehyde dehydrogenase genes ALD2 and ALD3 involved in competing branches of the Ehrlich pathway on the 2-PE titer. The results showed that in 2-PE low-yielding haploid strain PK-2C, the ATF1∆ mutant produced 2-PE of 0.45 g/L, an increase of 114%, whereas in CWY-132, the 2-PE yield of ATF1∆ decreased significantly from 3.50 to 0.83 g/L. In PK-2C, the 2-PE yield of ALD2∆ increased from 0.21 to 1.20 g/L, whereas in CWY-132, it decreased from 3.50 to 3.02 and 2.93 g/L in ALD2∆ and ALD3∆ mutants, respectively, and to 1.65 g/L in ALD2∆ALD3∆. These results indicate that the same genetic manipulation strategy used for strains with different 2-PE yield backgrounds produces significantly different or even opposite effects. Moreover, we found that a supply of NADH or GSH increased the 2-PE production in S. cerevisiae. The correlation between the synthesis of 2-PE and ethanol was also revealed, and the tolerance of cells to 2-PE and ethanol was suggested to be a key limiting factor for further increase of 2-PE production in high-yielding strains. KEY POINTS: • Deletion of genes competing for 2-PE synthesis produces different effects in S. cerevisiae strains. • The ATF1∆, ALD2∆, or ALD3∆ increased 2-PE production in laboratory strains but not industrial strains. • The supply of NADH or GSH increased the titer of 2-PE in S. cerevisiae.

Genome-based selection and application of food-grade microbes for chickpea milk fermentation towards increased L-lysine content, elimination of indigestible sugars, and improved flavour

Background

Plant-based milk alternatives are more popular than ever, and chickpea-based milks are among the most commercially relevant products. Unfortunately, limited nutritional value because of low levels of the essential amino acid l-lysine, low digestibility and unpleasant taste are challenges that must be addressed to improve product quality and meet consumer expectations.

Results

Using in-silico screening and food safety classifications, 31 strains were selected as potential l-lysine producers from approximately 2,500 potential candidates. Beneficially, 30% of the isolates significantly accumulated amino acids (up to 1.4 mM) during chickpea milk fermentation, increasing the natural level by up to 43%. The best-performing strains, B. amyloliquefaciens NCC 156 and L. paracasei subsp. paracasei NCC 2511, were tested further. De novo lysine biosynthesis was demonstrated in both strains by ¹³C metabolic pathway analysis. Spiking small amounts of citrate into the fermentation significantly activated l-lysine biosynthesis in NCC 156 and stimulated growth. Both microbes revealed additional benefits in eliminating indigestible sugars such as stachyose and raffinose and converting off-flavour aldehydes into the corresponding alcohols and acids with fruity and sweet notes.

Conclusions

B. amyloliquefaciens NCC 156 and L. paracasei subsp. paracasei NCC 2511 emerged as multi-benefit microbes for chickpea milk fermentation with strong potential for industrial processing of the plant material. Given the high number of l-lysine-producing isolates identified in silico, this concept appears promising to support strain selection for food fermentation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01595-2.

Utilization of a styrene-derived pathway for 2-phenylethanol production in budding yeast

2-Phenylethanol (2-PE) is an important flavor ingredient and is widely applied in the fields of food, cosmetics, and pharmaceuticals. Despite that Saccharomyces cerevisiae has the ability to naturally synthesize 2-PE via the Ehrlich pathway, de novo synthesis of 2-PE in high titer still remains a huge challenge. In this study, a non-native styrene degradation pathway was introduced into S. cerevisiae, which represents the first time to demonstrate the functional expression of "styrene-derived" 2-PE synthesis in yeast. Using a host strain engineered with L-phenylalanine (L-Phe) overproduction, the heterologous 2-PE pathway coupled with endogenous Ehrlich pathway produced 233 mg/L 2-PE under shake flasks. Additionally, we further engineered the permease transporters to improve the intracellular L-Phe availability, and further improved the 2-PE titer to 680 mg/L. Taken together, our work represents one of the pioneering reports to explore "styrene-derived" pathway in S. cerevisiae. The synthetic yeast described here might be used as a platform for the future development of next-generation high-yielding 2-PE yeast strains.Key Points• A styrene-derived pathway was established in yeast for 2-phenylethanol productions; membrane-associated styrene oxide isomerase was functional in yeast.• Transporter engineering to improve the L-phenylalanine importation with enhanced 2-phenylethanol productions.

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.

Yeast-driven whey biorefining to produce value-added aroma, flavor, and antioxidant compounds: technologies, challenges, and alternatives

Whey is a liquid residue generated during the production of cheese and yogurt. It has a pH between 3.9 and 5.6, and a high chemical oxygen demand (COD), from 60 to 80 g/L. Whey contains lactose, proteins, and minerals. Globally, approximately 50% of the whey generated is untreated and is released directly into the environment, which represents an environmental risk. To overcome whey management problems, conventional thermo-physical valorization treatments have been explored, which are complex, costly and energy-intensive. As an alternative, whey fermentation processes employing bacteria, fungi and yeast are economical and promising methods. Among them, yeast fermentation creates value-added products such as antimicrobials, biofuels, aromas, flavors, and antioxidants with no need for previous conditioning of the whey, such as hydrolysis of the lactose, prior to whey biorefining. The biorefining concept applied to whey is discussed using chemical and biological transformation pathways, showing their pluses and minuses, such as technical drawbacks. The main challenges and solutions for the production of fusel alcohols, specifically for 2-phenylethanol, are also discussed in this review.

2-Phenylethanol and 2-phenylethylacetate production by nonconventional yeasts using tequila vinasses as a substrate

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Yeast species influenced the de novo synthesis of 2-phenylethylacetate.

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Inhibitory compounds showed a strong influence on cell growth and 2-phenylethylacetate production for the evaluated yeasts.

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More than a 50 % reduction in the chemical and biochemical oxygen demand was achieved by yeast fermentation.

Vinasses from the tequila industry are wastewaters with highly elevated organic loads. Therefore, to obtain value-added products by yeast fermentations, such as 2-phenylethanol (2-PE) and 2-phenylethylacetate (2-PEA), could be interesting for industrial applications from tequila vinasses. In this study, four yeasts species (Wickerhamomyces anomalus, Candida glabrata, Candida utilis, and Candida parapsilosis) were evaluated with two different chemically defined media and tequila vinasses. Differences in the aroma compounds production were observed depending on the medium and yeast species used. In tequila vinasses, the highest concentration (65 mg/L) of 2-PEA was reached by C. glabrata, the inhibitory compounds decreased biomass production and synthesis of 2-PEA, and biochemical and chemical oxygen demands were reduced by more than 50 %. Tequila vinasses were suitable for the production of 2-phenylethylacetate by the shikimate pathway. A metabolic network was developed to obtain a guideline to improve 2-PE and 2-PEA production using flux balance analysis (FBA).

Traceability of Functional Volatile Compounds Generated on Inoculated Cocoa Fermentation and Its Potential Health Benefits

Microbial communities are responsible for the unique functional properties of chocolate. During microbial growth, several antimicrobial and antioxidant metabolites are produced and can influence human wellbeing. In the last decades, the use of starter cultures in cocoa fermentation has been pushed to improve nutritional value, quality, and the overall product safety. However, it must be noted that unpredictable changes in cocoa flavor have been reported between the different strains from the same species used as a starter, causing a loss of desirable notes and flavors. Thus, the importance of an accurate selection of the starter cultures based on the biogenic effect to complement and optimize chocolate quality has become a major interest for the chocolate industry. This paper aimed to review the microbial communities identified from spontaneous cocoa fermentations and focused on the yeast starter strains used in cocoa beans and their sensorial and flavor profile. The potential compounds that could have health-promoting benefits like limonene, benzaldehyde, 2-phenylethanol, 2-methylbutanal, phenylacetaldehyde, and 2-phenylethyl acetate were also evaluated as their presence remained constant after roasting. Further research is needed to highlight the future perspectives of microbial volatile compounds as biomarkers to warrant food quality and safety.

Elevated production of the aromatic fragrance molecule, 2-phenylethanol, using Metschnikowia pulcherrima through both de novo and ex novo conversion in batch and continuous modes

BACKGROUND

2-phenylethanol (2PE) is a fragrance molecule predominantly used in perfumes and the food industry. It can be made from petrochemicals inexpensively, however, this is unsuitable for most food applications. Currently, the main method of production for the bio-derived compound is to extract the trace amounts found in rose petals, which is extremely costly. Potentially fermentation could provide an inexpensive, naturally sourced, alternative.

RESULTS

In this investigation, 2PE was produced from the yeast Metschnikowia pulcherrima, optimised in flasks before scaling to 2 L batch and continuous operation. 2PE can be produced in high titres under de novo process conditions with up to 1500 mg L-1 achieved in a 2 L stirred bioreactor. This is the highest reported de novo titre to date, and achieved through high sugar loadings coupled with low nitrogen conditions. The process successfully ran in continuous mode also, with a concentration of 650 mg L-1 of 2PE being maintained. The 2PE production was further increased by the ex novo conversion of phenylalanine and semi-continuous solid phase extraction from the supernatant. Under optimal conditions 14 000 mg L-1 of 2PE was produced.

CONCLUSIONS

The work presented here offers a novel route to naturally sourced 2PE through a scalable fermentation with a robust yeast highly suited to industrial biotechnology. © 2018 The Authors. Journal of Chemical Technology & Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Kluyveromyces marxianus yeast enables the production of low FODMAP whole wheat breads

There is evidence that a diet low in Fermentable Oligo-, Di-, Monosaccharides And Polyols (FODMAPs) alleviates symptoms in approx. 70% of the patients suffering from irritable bowel syndrome. Through fructans, wheat containing products are a major source of FODMAPs in the western diet. Although fructans are partially degraded during dough fermentation by Saccharomyces cerevisiae invertase, wheat bread contains notable fructan levels. In this study, it was shown that Kluyveromyces marxianus strain CBS6014 can degrade more than 90% of the fructans initially present in wheat whole meal during bread making, which can be attributed to its high inulinase activity. As K. marxianus CBS6014 was not able to consume maltose during fermentation, alternative sugars (sucrose) or glucose releasing enzymes (amyloglucosidase) had to be included in the bread making recipe to ensure sufficient production of CO₂ and high bread quality. Five volatile flavor compounds were produced in significantly different levels when K. marxianus CBS6014 was used as starter culture compared with the conventional S. cerevisiae bakery strain. These differences were, however, not detected when sensory analysis of the crumb was performed. This study demonstrates the potential of inulinase-producing K. marxianus strains for the production of (whole meal) breads low in FODMAPs.

Model-based biotechnological potential analysis of Kluyveromyces marxianus central metabolism

The non-conventional yeast Kluyveromyces marxianus is an emerging industrial producer for many biotechnological processes. Here, we show the application of a biomass-linked stoichiometric model of central metabolism that is experimentally validated, and mass and charge balanced for assessing the carbon conversion efficiency of wild type and modified K. marxianus. Pairs of substrates (lactose, glucose, inulin, xylose) and products (ethanol, acetate, lactate, glycerol, ethyl acetate, succinate, glutamate, phenylethanol and phenylalanine) are examined by various modelling and optimisation methods. Our model reveals the organism's potential for industrial application and metabolic engineering. Modelling results imply that the aeration regime can be used as a tool to optimise product yield and flux distribution in K. marxianus. Also rebalancing NADH and NADPH utilisation can be used to improve the efficiency of substrate conversion. Xylose is identified as a biotechnologically promising substrate for K. marxianus.

Metabolic engineering of Escherichia coli for production of 2-Phenylethylacetate from L-phenylalanine

In order to meet the need of consumer preferences for natural flavor compounds, microbial synthesis method has become a very attractive alternative to the chemical production. The 2‐phenylethanol (2‐PE) and its ester 2‐phenylethylacetate (2‐PEAc) are two extremely important flavor compounds with a rose‐like odor. In recent years, Escherichia coli and yeast have been metabolically engineered to produce 2‐PE. However, a metabolic engineering approach for 2‐PEAc production is rare. Here, we designed and expressed a 2‐PEAc biosynthetic pathway in E. coli. This pathway comprised four steps: aminotransferase (ARO8) for transamination of L‐phenylalanine to phenylpyruvate, 2‐keto acid decarboxylase KDC for the decarboxylation of the phenylpyruvate to phenylacetaldehyde, aldehyde reductase YjgB for the reduction of phenylacetaldehyde to 2‐PE, alcohol acetyltransferase ATF1 for the esterification of 2‐PE to 2‐PEAc. Using the engineered E. coli strain for shake flasks cultivation with 1 g/L L‐phenylalanine, we achieved co‐production of 268 mg/L 2‐PEAc and 277 mg/L 2‐PE. Our results suggest that approximately 65% of L‐phenylalanine was utilized toward 2‐PEAc and 2‐PE biosynthesis and thus demonstrate potential industrial applicability of this microbial platform.

Mating of 2 Laboratory Strains Resulted in Enhanced Production of 2-Phenylethanol by Biotransformation of L-Phenylalanine

2-Phenylethanol (2-PE) is an aromatic alcohol with a rosy scent which is widely used in the food, fragrance, and cosmetic industries. Promising sources of natural 2-PE are microorganisms, especially yeasts, which can produce 2-PE by biosynthesis and biotransformation. Thus, the first challenging goal in the development of biotechnological production of 2-PE is searching for highly productive yeast strains. In the present work, 5 laboratory <i>Saccharomyces cerevisiae</i> strains were tested for the production of 2-PE. Thereafter, 2 of them were hybridized by a mating procedure and, as a result, a new diploid, <i>S. cerevisiae</i> AM1-d, was selected. Within the 72-h batch culture in a medium containing 5 g/L of <smlcap>L</smlcap>-phenylalanine, AM1-d produced 3.83 g/L of 2-PE in a shaking flask. In this way, we managed to select the diploid <i>S. cerevisiae</i> AM1-d strain, showing a 3- and 5-fold increase in 2-PE production in comparison to parental strains. Remarkably, the enhanced production of 2-PE by the hybrid of 2 yeast laboratory strains is demonstrated here for the first time.

Identification of yeast and bacteria involved in the mezcal fermentation of Agave salmiana

AIMS

To identify the yeast and bacteria present in the mezcal fermentation from Agave salmiana.

METHODS AND RESULTS

The restriction and sequence analysis of the amplified region, between 18S and 28S rDNA and 16S rDNA genes, were used for the identification of yeast and bacteria, respectively. Eleven different micro-organisms were identified in the mezcal fermentation. Three of them were the following yeast: Clavispora lusitaniae, Pichia fermentans and Kluyveromyces marxianus. The bacteria found were Zymomonas mobilis subsp. mobilis and Zymomonas mobilis subsp. pomaceae, Weissella cibaria, Weissella paramesenteroides, Lactobacillus pontis, Lactobacillus kefiri, Lactobacillus plantarum and Lactobacillus farraginis.

CONCLUSIONS

The phylogenetic analysis of 16S rDNA and ITS sequences showed that microbial diversity present in mezcal is dominated by bacteria, mainly lactic acid bacteria species and Zymomonas mobilis. Pichia fermentans and K. marxianus could be micro-organisms with high potential for the production of some volatile compounds in mezcal.

SIGNIFICANCE AND IMPACT OF THE STUDY

We identified the community of bacteria and yeast present in mezcal fermentation from Agave salmiana.

Analysis of beers from an 1840s' shipwreck

Two bottles of beer from an about 170-year-old shipwreck (M1 Fö 403.3) near the Åland Islands in the Baltic Sea were analyzed. Hop components and their degradation compounds showed that the bottles contained two different beers, one more strongly hopped than the other. The hops used contained higher levels of β-acids than modern varieties and were added before the worts were boiled, converting α-acids to iso-α-acids and β-acids to hulupones. High levels of organic acids, carbonyl compounds, and glucose indicated extensive bacterial and enzyme activity during aging. However, concentrations of yeast-derived flavor compounds were similar to those of modern beers, except that 3-methylbutyl acetate was unusually low in both beers and 2-phenylethanol and possibly 2-phenylethyl acetate were unusually high in one beer. Concentrations of phenolic compounds were similar to those in modern lagers and ales.

Cell factory applications of the yeast Kluyveromyces marxianus for the biotechnological production of natural flavour and fragrance molecules

Kluyveromyces marxianus is emerging as a new platform organism for the production of flavour and fragrance (F&F) compounds. This food-grade yeast has advantageous traits, such as thermotolerance and rapid growth, that make it attractive for cell factory applications. The major impediment to its development has been limited fundamental knowledge of its genetics and physiology, but this is rapidly changing. K. marxianus produces a wide array of volatile molecules and contributes to the flavour of a range of different fermented beverages. Advantage is now being taken of this to develop strains for the production of metabolites such as 2-phenylethanol and ethyl acetate. Strains that were selected from initial screens were used to optimize processes for production of these F&F molecules. Most developments have focused on optimizing growth conditions and the fermentation process, including product removal, with future advancement likely to involve development of new strains through the application of evolutionary or rational engineering strategies. This is being facilitated by new genomic and molecular tools. Furthermore, synthetic biology offers a route to introduce new biosynthetic pathways into this yeast for F&F production. Consumer demand for biologically-synthesized molecules for use in foods and other products creates an opportunity to exploit the unique potential of K. marxianus for this cell factory application.

Comparative metabolomic-based metabolic mechanism hypothesis for microbial mixed cultures utilizing cane molasses wastewater for higher 2-phenylethanol production

The mixed microbes coculture method in cane molasses wastewater (CMW) was adopted to produce 2-phenylethanol (2-PE). Comparative metabolomics combined with multivariate statistical analysis was performed to profile the differences of overall intracellular metabolites concentration for the mixed microbes cocultured under two different fermentation conditions with low and high 2-PE production. In total 102 intracellular metabolites were identified, and 17 of them involved in six pathways were responsible for 2-PE biosynthesis. After further analysis of metabolites and verification by feeding experiment, an overall metabolic mechanism hypothesis for the microbial mixed cultures (MMC) utilizing CMW for higher 2-PE production was presented. The results demonstrated that the branches of intracellular pyruvate metabolic flux, as well as the flux of phenylalanine, tyrosine, tryptophan, glutamate, proline, leucine, threonine, and oleic acid, were closely related to 2-PE production and cell growth, which provided theoretical guidance for domestication and selection of species as well as medium optimization for MMC metabolizing CMW to enhance 2-PE yield.

Improving 2-phenylethanol and 6-pentyl-α-pyrone production with fungi by microparticle-enhanced cultivation (MPEC)

Trichoderma atroviride IMI 206040 synthesizes the coconut lactone 6-pentyl-α-pyrone (6-PAP) de novo and Aspergillus niger DSM 821 produces the rose-like flavour compound 2-phenylethanol (2-PE) from the precursor l-phenylalanine. Here, microparticles of different chemical composition and nominal particle diameter in the range 5-250 µm were added to shake-flask cultures of both fungi to investigate the particles' effect on product formation. Maximum 2-PE concentration increased by a factor of 1.3 to 1430 mg/l with the addition of 2% w/v talc (40 µm diameter). Maximum 6-PAP concentration increased by a factor of 2 to 40 mg/l with the addition of 2% w/v iron (II, III) oxide. The influence of ions leaching out of the particles was investigated by cultivating the fungi in leached particle medium. For the first time, the positive effect of the microparticle-enhanced cultivation (MPEC) technique on the microbial production of volatile metabolites, here flavour compounds from submerged fungal cultures, is demonstrated. The effect is strain- and particle-specific.

Monitoring of quorum-sensing molecules during minifermentation studies in wine yeast

At high cell density or under low nutrient conditions, yeasts collectively adapt their metabolism by secreting aromatic alcohols in what is known as quorum sensing. However, the mechanisms and role of quorum sensing in yeast are poorly understood, and the methodology behind this process is not well established. This paper describes an effective approach to study quorum sensing in yeast fermentations. The separation, detection, and quantification of the putative quorum-sensing molecules 2-phenylethanol, tryptophol, and tyrosol have been optimized on a simple HPLC-based system. With the use of a phenyl HPLC column and a fluorescence detector, the sensitivity of the system was significantly increased. This allowed extraction and concentration procedures to be eliminated and the process to be scaled down to 2 mL minifermentations. Additionally, an innovative method for rapid viable-cell counting is presented. This study forms the basis for detailed studies in kinetics and regulation of quorum sensing in yeast fermentation.

Growth of the yeast Kluyveromyces marxianus CBS 6556 on different sugar combinations as sole carbon and energy source

The yeast Kluyveromyces marxianus has been pointed out as a promising microorganism for a variety of industrial bioprocesses. Although genetic tools have been developed for this yeast and different potential applications have been investigated, quantitative physiological studies have rarely been reported. Here, we report and discuss the growth, substrate consumption, metabolite formation, and respiratory parameters of K. marxianus CBS 6556 during aerobic batch bioreactor cultivations, using a defined medium with different sugars as sole carbon and energy source, at 30 and 37 °C. Cultivations were carried out both on single sugars and on binary sugar mixtures. Carbon balances closed within 95 to 101 % in all experiments. Biomass and CO2 were the main products of cell metabolism, whereas by-products were always present in very low proportion (<3 % of the carbon consumed), as long as full aerobiosis was guaranteed. On all sugars tested as sole carbon and energy source (glucose, fructose, sucrose, lactose, and galactose), the maximum specific growth rate remained between 0.39 and 0.49 h(-1), except for galactose at 37 °C, which only supported growth at 0.31 h(-1). Different growth behaviors were observed on the binary sugar mixtures investigated (glucose and lactose, glucose and galactose, lactose and galactose, glucose and fructose, galactose and fructose, fructose and lactose), and the observations were in agreement with previously published data on the sugar transport systems in K. marxianus. We conclude that K. marxianus CBS 6556 does not present any special nutritional requirements; grows well in the range of 30 to 37 °C on different sugars; is capable of growing on sugar mixtures in a shorter period of time than Saccharomyces cerevisiae, which is interesting from an industrial point of view; and deviates tiny amounts of carbon towards metabolite formation, as long as full aerobiosis is maintained.

Metabolic response of Pseudomonas putida during redox biocatalysis in the presence of a second octanol phase

A key limitation of whole-cell redox biocatalysis for the production of valuable, specifically functionalized products is substrate/product toxicity, which can potentially be overcome by using solvent-tolerant micro-organisms. To investigate the inter-relationship of solvent tolerance and energy-dependent biocatalysis, we established a model system for biocatalysis in the presence of toxic low logP(ow) solvents: recombinant solvent-tolerant Pseudomonas putida DOT-T1E catalyzing the stereospecific epoxidation of styrene in an aqueous/octanol two-liquid phase reaction medium. Using (13)C tracer based metabolic flux analysis, we investigated the central carbon and energy metabolism and quantified the NAD(P)H regeneration rate in the presence of toxic solvents and during redox biocatalysis, which both drastically increased the energy demands of solvent-tolerant P. putida. According to the driven by demand concept, the NAD(P)H regeneration rate was increased up to eightfold by two mechanisms: (a) an increase in glucose uptake rate without secretion of metabolic side products, and (b) reduced biomass formation. However, in the presence of octanol, only approximately 1% of the maximally observed NAD(P)H regeneration rate could be exploited for styrene epoxidation, of which the rate was more than threefold lower compared with operation with a non-toxic solvent. This points to a high energy and redox cofactor demand for cell maintenance, which limits redox biocatalysis in the presence of octanol. An estimated upper bound for the NAD(P)H regeneration rate available for biocatalysis suggests that cofactor availability does not limit redox biocatalysis under optimized conditions, for example, in the absence of toxic solvent, and illustrates the high metabolic capacity of solvent-tolerant P. putida. This study shows that solvent-tolerant P. putida have the remarkable ability to compensate for high energy demands by boosting their energy metabolism to levels up to an order of magnitude higher than those observed during unlimited growth.

1H nuclear magnetic resonance-based metabolomic characterization of wines by grape varieties and production areas

(1)H NMR spectroscopy was used to investigate the metabolic differences in wines produced from different grape varieties and different regions. A significant separation among wines from Campbell Early, Cabernet Sauvignon, and Shiraz grapes was observed using principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA). The metabolites contributing to the separation were assigned to be 2,3-butanediol, lactate, acetate, proline, succinate, malate, glycerol, tartarate, glucose, and phenolic compounds by PCA and PLS-DA loading plots. Wines produced from Cabernet Sauvignon grapes harvested in the continental areas of Australia, France, and California were also separated. PLS-DA loading plots revealed that the level of proline in Californian Cabernet Sauvignon wines was higher than that in Australian and French Cabernet Sauvignon, Australian Shiraz, and Korean Campbell Early wines, showing that the chemical composition of the grape berries varies with the variety and growing area. This study highlights the applicability of NMR-based metabolomics with multivariate statistical data sets in determining wine quality and product origin.

Appropriate sampling for intracellular amino acid analysis in five phylogenetically different yeasts

Methanol quenching and fast filtration, the two most common sampling protocols in microbial metabolome analysis, were validated for intracellular amino acid analysis in phylogenetically different yeast strains comprising Saccharomyces cerevisiae, Kluyveromyces marxianus, Pichia pastoris, Schizosaccharomyces pombe and Zygosaccharomyces bailii. With only few exceptions for selected amino acids, all yeasts exhibited negligible metabolite leakage during quenching with 60% cold buffered methanol. Slightly higher leakage was observed with increasing methanol content in the quenching solution. Fast filtration resulted in identical levels for intracellular amino acids in all strains tested. The results clearly demonstrate the validity of both approaches for leakage-free sampling of amino acids in yeast.