Ethyl acetate production by the elusive alcohol acetyltransferase from yeast

Usability of volatile organic compounds from exhaled breath compared with those from ruminal fluid, serum, urine, and milk to identify diet-specific metabolite profiles in lactating dairy cows

To investigate dietary influences on the volatilome, the volatile subcategory of the metabolome, we performed a comparative untargeted volatilome analysis of exhaled breath, ruminal fluid, serum, urine, and milk from lactating Holstein cows fed different diets. Thirty-two cows (83.3 ± 31.40 DIM, 30.6 ± 5.03 kg of milk/d) were assigned to 4 diets. The experiment lasted 16 wk. Throughout the experiment, half of the animals were fed a hay-based diet (HD; n = 16), and the other half were fed a silage-based diet (SIL; n = 16). In experimental wk 5 to 12, half of the animals in each group received the control concentrate (CON), and the other half was fed with the CON supplemented with a blend of essential oils (EXP). We hypothesized that the basal diet and the essential oils influence the volatile organic compound (VOC) profiles of the cows through potential changes in ruminal fermentation, digestion, and metabolism (hypothesis 1). Furthermore, we hypothesized that the potential effects of essential oils would have a delayed onset and a carryover effect (hypothesis 2). Every 4 experimental weeks (i.e., in wk 4, wk 8, wk 12, and wk 16), samples of exhaled breath, ruminal fluid, serum, urine, milk, and feed were collected for dynamic headspace extraction and gas chromatographic analysis of VOC in their gaseous phase. Milk yield, milk composition, BW, and feed intake were recorded regularly. Linear mixed models and multivariate and univariate data analyses were performed. The total DMI and basal diet intake was similar between cows fed HD and SIL diets. However, SIL cows consumed less of the concentrate, NDF, and water-soluble carbohydrates and more starch than HD cows. The SIL cows had a higher milk production than the HD cows. No effect was found regarding the concentrate type on feed intake or milk production. Irrespective of diet, 2,957 VOC were detected in the gaseous phase of serum; 2,771 in exhaled breath; 1,016 in urine; 1,001 in milk; and 921 in ruminal fluid. Across the experimental wk 4, 8, 12, and 16, the basal diet altered the VOC profiles of ruminal fluid, urine, and exhaled breath but not those of serum and milk. The concentrate type affected only the VOC profiles of the exhaled breath. Most diet-influenced VOC in the affected biological matrices were identified as dietary components. The experimental week influenced the VOC profiles of all matrices, especially those of exhaled breath. The VOC profile of exhaled breath strongly correlated with that of urine, followed by that of ruminal fluid, milk, and serum. This study provides the first description of diet- and time-specific VOC profiles from the biological matrices of dairy cows. The identified discriminatory VOC seem suitable as markers to discriminate between HD and SIL cows. Exhaled breath may be a promising, sensitive, and less invasive tool to follow diet- and time-related metabolic changes.

On-vine drying (passérillage) improves the quality of "Hutai No. 8" table grape wine: Focusing on phenolics, aromas, color and sensory attributes

On-vine drying (passérillage) was employed for Hutai No.8 table grapes with different water loss rates and that are used for winemaking to alter the qualities of wine. Results showed that on-vine drying increased sugar content, thereby elevating alcohol and glycerol contents in the resulting wine. A moderate drying treatment (D2, 18 % water loss) produced wine with optimal red color characteristics. The D2 wine contained abundant individual phenolics such as caffeic acid, salicylic acid, resveratrol, p-coumaric acid, and proanthocyanidin B1, which exhibited strong positive correlations with color parameters (a⁎, C*ab and △E*ab). Similar trends were observed in aroma compounds, with ethyl acetate, ethyl caproate, diethyl succinate, geraniol, linalool, 4-terpinenol, α-terpineol, and β-ionone contents showing significant increment in D2, aligning with enhanced sensory evaluations of wines. Thus, improvement of wine quality can be achieved through moderate on-vine drying (18 % water loss)，serving as a valuable reference for table grape winemaking.

Manganese biofortification in grapevine by foliar spraying improves volatile profiles of Cabernet Sauvignon grapes and wine sensory traits

Manganese (Mn) is involved in plant metabolism as an enzyme cofactor. However, the role of Mn in the formation of volatile compounds in grapes has rarely been studied. To address this gap, this study explored the effect of foliar Mn application on the aroma traits of grapes and wine. Mn nutrient solutions at different concentrations (0 (CK), 300, 1200, and 2400 mg/L) were sprayed on grapevines in 2017 and 2018 and the volatile compounds, odor activity, and sensory features of grapes and wine were investigated. The results showed that Mn application significantly increased Mn content in grape leaves and fruits at harvest. Compared with CK, the total volatile content of grapes was significantly increased by Mn treatment in both years because of the promotion of the accumulation of alcohols and esters. Particularly, 1200 mg/L Mn treatment resulted in a higher sensory score than CK, especially in terms of intensity, duration, and harmony. Multivariate analysis and odor activity values jointly identified eight volatile compounds (ethyl acetate, phenylethyl acetate, and phenylethyl alcohol, etc.) as key odorants that contribute to the floral and fruity flavors of Mn-treated wine. Overall, this study indicated that a moderate concentration of Mn is beneficial for improving the fragrance characteristics of grapes and wines. The results have implications for micronutrient management of grapevines to improve wine flavor quality.

Integrated microbiomic and metabolomic dynamics of Yi traditional fermented liquor

This study examines the microbial community composition, metabolite characteristics, and the relationship between the two during the fermentation process of Yi traditional fermented liquor. Yi traditional fermented foods have a profound historical and cultural background, with significant ethnic characteristics. As a case in point, Yi traditional fermented liquor is typically prepared using local plants or traditional Chinese herbs as fermentation substrates and undergoes a lengthy fermentation process, resulting in a fermented beverage that is reputed to have beneficial effects on human health. These foods are not only characterised by a distinctive flavor profile, but are also perceived to possess certain health benefits in the context of traditional ethnic medicine and wellness practices. The community composition of bacteria and fungi was analyzed using 16S rRNA and ITS1 sequencing technologies, which revealed that microbial diversity was higher in the early stages of fermentation but gradually decreased as fermentation progressed. A total of 130 major volatile flavor compounds and 26 key metabolites were identified at different stages of fermentation. These included acids, sugars, amino acids and flavonoids, which significantly influence the flavor and nutritional value of the fermented products. The study indicates a significant correlation between specific microbial populations (such as yeasts) and key metabolites (such as flavonoids and amino acids). These findings emphasise the significance of the interplay between microbial communities and metabolites in shaping the quality and efficacy of fermented products. They offer a scientific foundation for optimizing traditional fermented food production processes.

Genome-Wide Identification and Biochemical Characterization of Alcohol Acyltransferases for Aroma Generation in Wickerhamomyces subpelliculosus Isolates from Fermented Food

The importance of nonconventional yeasts has increasingly been highlighted, particularly for aroma formation in fermented foods. Here, we performed de novo whole-genome sequencing of Wickerhamomyces subpelliculosus, which produces a variety of volatile flavor compounds, leading to the identification of the alcohol acyltransferase (AATase) family of genes. The genome of W. subpelliculosus contains seven AATase genes, encoding alcohol-O-acetyltransferases (ATFs) and ethanol acetyltransferase 1 (EAT1) for acetate ester formation, along with ethanol hexanoyl transferase 1 (EHT1) for ethyl ester formation. Among five ATF homologues, only WsATF5 showed acetyltransferase activity toward myriocin, a structural analogue of sphingosine. In contrast, heterologous expression of WsEHT1 and WsEAT1 in Saccharomyces cerevisiae promoted the production of ethyl decanoate and ethyl acetate, respectively, supporting their AATase activity. The enzymatic activity analyses revealed the additional alcoholysis activity of WsEAT1 and the thioesterase activity of WsEHT1. Subcellular localization analysis indicated that WsEAT1 was localized in the mitochondria, WsEHT1 in the endoplasmic reticulum and lipid droplets (LDs), and WsATF5 in the LDs. The novel W. subpelliculosus AATases could be usefully applied to produce flavor components in various food industries.

In situ Product Recovery of Microbially Synthesized Ethyl Acetate from the Exhaust Gas of a Bioreactor by Membrane Technology

Ethyl acetate is at present exclusively produced from fossil resources. Microbial synthesis of this ester from sugar-rich waste as an alternative is an aerobic process. Ethyl acetate is highly volatile and therefore stripped with the exhaust gas from the bioreactor which enables in situ product recovery. Previous research on microbial formation of ethyl acetate has focused on the kinetics of ester synthesis and in part on the ester stripping, while the separation of the ester from the exhaust gas has hardly been investigated. A mixed matrix membrane was developed consisting of Silikalite-1 embedded in polydimethylsiloxane which was installed in a radial-symmetrical membrane module. Evaluation of the separation of ethyl acetate was based on the analysis of the composition of the feed and retentate gas by mass spectrometry. The separation efficiency of the membrane was first tested with varied flows of artificial exhaust gas, containing defined amounts of ethyl acetate. A model for describing the separation process was parametrized by the measured data and used to design a real separation experiment. Ethyl acetate produced from delactosed whey permeate by Kluyveromyces marxianus DSM 5422 in a stirred bioreactor gassed with 0.5 vvm air was successfully separated from the exhaust gas by membranes; 93.6% of the stripped ester was separated. Liquid ethyl acetate was recovered by cooling the permeate gas to ‒78°C, whereby 99.75% of the condensed organic compounds were ethyl acetate. This study demonstrates for the first time that microbially produced and stripped ethyl acetate can be effectively separated from the exhaust gas of bioreactors by membrane technology to obtain the ester in high yield and purity.

Enhanced production of isobutyl and isoamyl acetate using Yarrowia lipolytica

Short-chain esters, particularly isobutyl acetate and isoamyl acetate, hold significant industrial value due to their wide-ranging applications in flavors, fragrances, solvents, and biofuels. In this study, we demonstrated the biosynthesis of acetate esters using Yarrowia lipolytica as a host by feeding alcohols to the yeast culture. Initially, we screened for optimal alcohol acyltransferases for ester biosynthesis in Y. lipolytica. Strains of Y. lipolytica expressing atf1 from Saccharomyces cerevisiae, produced 251 or 613 mg/L of isobutyl acetate or of isoamyl acetate, respectively. We found that introducing additional copies of ATF1 enhanced ester production. Furthermore, by increasing the supply of acetyl-CoA and refining the culture conditions, we achieved high production of isoamyl acetate, reaching titers of 3404 mg/L. We expanded our study to include the synthesis of a range of acetate esters, facilitated by enriching the culture medium with various alcohols. This study underscores the versatility and potential of Y. lipolytica in the industrial production of acetate esters.

From β-Carotene to Retinoids: A Review of Microbial Production of Vitamin A

Vitamin A (retinoids) is crucial for human health, with significant demand across the food, pharmaceutical, and animal feed industries. Currently, the market primarily relies on chemical synthesis and natural extraction methods, which face challenges such as low synthesis efficiency and complex extraction processes. Advances in synthetic biology have enabled vitamin A biosynthesis using microbial cell factories, offering a promising and sustainable solution to meet the increasing market demands. This review introduces the key enzymes involved in the biosynthesis of vitamin A from β-carotene, evaluates achievements in vitamin A production using various microbial hosts, and summarizes strategies for optimizing vitamin A biosynthesis. Additionally, we outline the remaining challenges and propose future directions for the biotechnological production of vitamin A.

Towards biotechnological production of bio-based low molecular weight esters: a patent review

Low molecular weight (LMW) esters, like ethyl acetate, methyl formate or butyl acetate, are widespread bulk chemicals in many industries. Each of them is currently produced in huge amounts (millions of tons per year scale) starting from fossil-based feedstock and they are used mainly because of their low toxicity and complete biodegradability. Energy transition is just half of the story on the path of fighting climate change: 45% of the global greenhouse gas emissions are caused by the production and use of all the products, materials and food necessary for modern human life. If the world is to reach its climate goals, there is the need to leave underground a significant proportion of the fossil feedstock and minimize environmental impacts of chemical manufacturing. This is the reason why a lot of efforts have been made to find novel routes for LMW esters production starting from renewable raw materials (e.g. biomasses or off-gases) and exploiting low-impact manufacturing, such as microbial fermentation or enzymatic reactions. This review reports the most significant patents, in the field of white biotechnology, that will hopefully lead to the commercialization of bio-based LMW esters as well as novel strategies, current problems to be solved, newer technologies, and some patent applications aiming at possible future developments.

Transcriptomics to evaluate the influence mechanisms of ethanol on the ester production of Wickerhamomyces anomalus with the induction of lactic acid

Wickerhamomyces anomalus is one of the most important ester-producing strains in Chinese baijiu brewing. Ethanol and lactic acid are the main metabolites produced during baijiu brewing, but their synergistic influence on the growth and ester production of W. anomalus is unclear. Therefore, in this paper, based on the contents of ethanol and lactic acid during Te-flavor baijiu brewing, the effects of different ethanol concentrations (3, 6, and 9% (v/v)) combined with 1% lactic acid on the growth and ester production of W. anomalus NCUF307.1 were studied and their influence mechanisms were analyzed by transcriptomics. The results showed that the growth of W. anomalus NCUF307.1 under the induction of lactic acid was inhibited by ethanol. Although self-repair mechanism of W. anomalus NCUF307.1 induced by lactic acid was initiated at all concentrations of ethanol, resulting in significant up-regulation of genes related to the Genetic Information Processing pathway, such as cell cycle-yeast, meiosis-yeast, DNA replication and other pathways. However, the accumulation of reactive oxygen species and the inhibition of pathways associated with carbohydrate and amino acid metabolism may be the main reason for the inhibition of growth in W. anomalus NCUF307.1. In addition, 3% and 6% ethanol combined with 1% lactic acid could promote the ester production of W. anomalus NCUF307.1, which may be related to the up-regulation of EAT1, ADH5 and TGL5 genes, while the inhibition in 9% ethanol may be related to down-regulation of ATF2, EAT1, ADH2, ADH5, and TGL3 genes.

The potential of native and engineered Clostridia for biomass biorefining

Since their first industrial application in the acetone-butanol-ethanol (ABE) fermentation in the early 1900s, Clostridia have found large application in biomass biorefining. Overall, their fermentation products include organic acids (e.g., acetate, butyrate, lactate), short chain alcohols (e.g., ethanol, n-butanol, isobutanol), diols (e.g., 1,2-propanediol, 1,3-propanediol) and H2 which have several applications such as fuels, building block chemicals, solvents, food and cosmetic additives. Advantageously, several clostridial strains are able to use cheap feedstocks such as lignocellulosic biomass, food waste, glycerol or C1-gases (CO2, CO) which confer them additional potential as key players for the development of processes less dependent from fossil fuels and with reduced greenhouse gas emissions. The present review aims to provide a survey of research progress aimed at developing Clostridium-mediated biomass fermentation processes, especially as regards strain improvement by metabolic engineering.

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

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

Recent developments in enzymatic and microbial biosynthesis of flavor and fragrance molecules

Flavors and fragrances are an important class of specialty chemicals for which interest in biomanufacturing has risen during recent years. These naturally occurring compounds are often amenable to biosynthesis using purified enzyme catalysts or metabolically engineered microbial cells in fermentation processes. In this review, we provide a brief overview of the categories of molecules that have received the greatest interest, both academically and industrially, by examining scholarly publications as well as patent literature. Overall, we seek to highlight innovations in the key reaction steps and microbial hosts used in flavor and fragrance manufacturing.

Repurposing plant hormone receptors as chemically-inducible genetic switches for dynamic regulation in yeast

Precise control of gene expression is critical for optimizing cellular metabolism and improving the production of valuable biochemicals. However, hard-wired approaches to pathway engineering, such as optimizing promoters, can take time and effort. Moreover, limited tools exist for controlling gene regulation in non-conventional hosts. Here, we develop a two-channel chemically-regulated gene expression system for the multi-stress tolerant yeast Kluyveromyces marxianus and use it to tune ethyl acetate production, a native metabolite produced at high titers in this yeast. To achieve this, we repurposed the plant hormone sensing modules (PYR1ABA/HAB1 and PYR1*MANDI/HAB1*) for high dynamic-range gene activation and repression controlled by either abscisic acid (ABA) or mandipropamid (mandi). To redirect metabolic flux towards ethyl acetate biosynthesis, we simultaneously repress pyruvate dehydrogenase (PDA1) and activate pyruvate decarboxylase (PDC1) to enhance ethyl acetate titers. Thus, we have developed new tools for chemically tuning gene expression in K. marxianus and S. cerevisiae that should be deployable across many non-conventional eukaryotic hosts.

CRISPRi-induced transcriptional regulation of IAH1 gene and its influence on volatile compounds profile in Kluyveromyces marxianus DU3

Mezcal is a traditional Mexican distilled beverage, known for its marked organoleptic profile, which is influenced by several factors, such as the fermentation process, where a wide variety of microorganisms are present. Kluyveromyces marxianus is one of the main yeasts isolated from mezcal fermentations and has been associated with ester synthesis, contributing to the flavors and aromas of the beverage. In this study, we employed CRISPR interference (CRISPRi) technology, using dCas9 fused to the Mxi1 repressor factor domain, to down-regulate the expression of the IAH1 gene, encoding for an isoamyl acetate-hydrolyzing esterase, in K. marxianus strain DU3. The constructed CRISPRi plasmid successfully targeted the IAH1 gene, allowing for specific gene expression modulation. Through gene expression analysis, we assessed the impact of IAH1 down-regulation on the metabolic profile of volatile compounds. We also measured the expression of other genes involved in volatile compound biosynthesis, including ATF1, EAT1, ADH1, and ZWF1 by RT-qPCR. Results demonstrated successful down-regulation of IAH1 expression in K. marxianus strain DU3 using the CRISPRi system. The modulation of IAH1 gene expression resulted in alterations in the production of volatile compounds, specifically ethyl acetate, which are important contributors to the beverage's aroma. Changes in the expression levels of other genes involved in ester biosynthesis, suggesting that the knockdown of IAH1 may generate intracellular alterations in the balance of these metabolites, triggering a regulatory response. The application of CRISPRi technology in K. marxianus opens the possibility of targeted modulation of gene expression, metabolic engineering strategies, and synthetic biology in this yeast strain.

Bridging the gap: linking Torulaspora delbrueckii genotypes to fermentation phenotypes and wine aroma

Climate change and consumer preferences are driving innovation in winemaking, with a growing interest in non-Saccharomyces species. Among these, Torulaspora delbrueckii (Td) has gained recognition for its ability to reduce volatile acidity and enhance aromatic complexity in wine. However, knowledge regarding its phenotypic and genomic diversity impacting alcoholic fermentation remains limited. Aiming to elucidate the metabolic differences between Td and Saccharomyces cerevisiae (Sc) and the Td intraspecies diversity, we conducted a comprehensive metabolic characterization of 15 Td strains. This analysis delved beyond standard fermentation parameters (kinetics and major metabolites production) to explore non-conventional aromas and establish genotype-phenotype links. Our findings confirmed that most Td strains produce less acetic acid and more succinate and glycerol than Sc. The overall aromatic profiles of Td strains differed from Sc, exhibiting higher levels of monoterpenes and higher alcohols, while producing less acetate esters, fatty acids, their corresponding ethyl esters, and lactones. Moreover, we identified the absence of genes responsible for specific aroma profiles, such as decreased ethyl esters production, as well as the absence of cell wall genes, which might negatively affect Td performance when compared to Sc. This work highlights the significant diversity within Td and underscores potential links between its genotype and phenotype.

Mechanisms of production and control of acetate esters in yeasts

Acetate esters, such as isoamyl acetate and ethyl acetate, are major aroma components of alcoholic beverages. They are produced through synthesis from acetyl CoA and the corresponding alcohol by alcohol acetyltransferase (AATase) with specific control of reaction factors, including unsaturated fatty acids and precursors, the percentage of nitrogen, and oxygen. However, the mechanisms by which these specific reaction factors affect acetate ester production remain largely unknown. The cellular mechanisms underlying the effects of these factors on acetate ester production were examined by purifying AATase from yeast, characterizing it, and cloning the ATF gene encoding AATase from sake yeast and bottom-fermenting yeast. Genetic and biochemical studies suggested that the decrease in acetate production with the addition of oxygen and unsaturated fatty acids was due to a decrease in enzyme synthesis resulting from transcriptional repression of the ATF1 gene, which is responsible for most of the AATase activity. Furthermore, these results suggest that expression of the ATF1 gene is intricately regulated by a number of transcriptional regulatory genes such as ROX1 and RAP1. Based on these results, the mechanism of ester regulation by oxygen, unsaturated fatty acids and precursors, and ratio of nitrogen source are becoming clearer from a molecular biological point of view. The physiological significance of ester production by yeast is then discussed. In this review, we summarize the studies on AATase, ATF gene, regulation of ester production, and physiological significance of acetate ester.

Effects of film-forming Pichia and Candida yeasts on cider and wine as post-fermentation contaminants

Film-forming yeasts are potential sources of defects in alcoholic beverages. The aim of this study is to assess the growth capacity of Pichia and Candida film-forming yeasts in cider and wine and the effects on their chemical composition. Cider, partially and fully fermented wine were inoculated with strains of C. californica, P. fermentans, P. kluyveri, P. kudriavzevii, P. manshurica, and P. membranifaciens to simulate a post-fermentative contamination. The former three species grew only in cider. Pichia manshurica and P. kudriavzevii displayed high viability in wine up to 13.18% (v v-1) ethanol. Significant changes in odour-active molecules from different chemical groups were observed in cider and wine in the inoculated samples, compared to the non-inoculated ones. Cider is more susceptible to contamination by all of the species tested, due to its low alcohol content, while P. membranifaciens, P. manshurica, and P. kudriavzevii are additionally potential spoilage agents of wine. This study highlights the risk of cider and wine contamination by film-forming yeasts. Their impact on aroma profiles depends on their ability to grow and their metabolism. This study contributes to an understanding of the possible physiological and metabolic mechanisms responsible for film formation and chemical changes in alcoholic beverages.

Exogenous and endogenous dsRNAs perceived by plant Dicer-like 4 protein in the RNAi-depleted cellular context

BACKGROUND

In plants, RNase III Dicer-like proteins (DCLs) act as sensors of dsRNAs and process them into short 21- to 24-nucleotide (nt) (s)RNAs. Plant DCL4 is involved in the biogenesis of either functional endogenous or exogenous (i.e. viral) short interfering (si)RNAs, thus playing crucial antiviral roles.

METHODS

In this study we expressed plant DCL4 in Saccharomyces cerevisiae, an RNAi-depleted organism, in which we could highlight the role of dicing as neither Argonautes nor RNA-dependent RNA polymerase is present. We have therefore tested the DCL4 functionality in processing exogenous dsRNA-like substrates, such as a replicase-assisted viral replicon defective-interfering RNA and RNA hairpin substrates, or endogenous antisense transcripts.

RESULTS

DCL4 was shown to be functional in processing dsRNA-like molecules in vitro and in vivo into 21- and 22-nt sRNAs. Conversely, DCL4 did not efficiently process a replicase-assisted viral replicon in vivo, providing evidence that viral RNAs are not accessible to DCL4 in membranes associated in active replication. Worthy of note, in yeast cells expressing DCL4, 21- and 22-nt sRNAs are associated with endogenous loci.

CONCLUSIONS

We provide new keys to interpret what was studied so far on antiviral DCL4 in the host system. The results all together confirm the role of sense/antisense RNA-based regulation of gene expression, expanding the sense/antisense atlas of S. cerevisiae. The results described herein show that S. cerevisiae can provide insights into the functionality of plant dicers and extend the S. cerevisiae tool to new biotechnological applications.

Alcohol acyltransferases for the biosynthesis of esters

Esters are widely used in food, energy, spices, chemical industry, etc., becoming an indispensable part of life. However, their production heavily relies on the fossil energy industry, which presents significant challenges associated with energy shortages and environmental pollution. Consequently, there is an urgent need to identify alternative green methods for ester production. One promising solution is biosynthesis, which offers sustainable and environmentally friendly processes. In ester biosynthesis, alcohol acyltransferases (AATs) catalyze the condensation of acyl-CoAs and alcohols to form esters, enabling the biosynthesis of nearly 100 different kinds of esters, such as ethyl acetate, hexyl acetate, ethyl crotonate, isoamyl acetate, and butyl butyrate. However, low catalytic efficiency and low selectivity of AATs represent the major bottlenecks for the biosynthesis of certain specific esters, which should be addressed with protein molecular engineering approaches before practical biotechnological applications. This review provides an overview of AAT enzymes, including their sequences, structures, active sites, catalytic mechanisms, and metabolic engineering applications. Furthermore, considering the critical role of AATs in determining the final ester products, the current research progresses of AAT modification using protein molecular engineering are also discussed. This review summarized the major challenges and prospects of AAT enzymes in ester biosynthesis.

Temperature-dependent alcohol acyltransferase reactions as the main enzymatic way to produce short-chain (C4-C8) and medium-chain (C9-C13) esters over the whole Daqu-making process

BACKGROUND

The ester-synthesis enzymes influenced by environmental factors during Daqu-making process largely determine the flavor of Chinese liquor, but the main ester-synthesis enzyme and its key influencer remain unclear. Here, the volatile ester profiles over the whole Daqu-making process, under different treatments, for at least 90 days, were carefully analyzed, and the potential ester-synthesis enzymes, as well as their dependently environmental factors, were explored.

RESULTS

In the detected 46 volatile esters, only the short-chain (C4-C8) and medium-chain (C9-C13) ester content obviously changed, as the primary contributor discriminating different samples. Their trends were both consistent with that of the alcohols and the primary metabolism, which included alcohol acyltransferases (AATs) reaction with alcohols and acyl-CoAs as the substrates. Among the potential ester-synthesis enzymes, the typical AAT activity also exhibited the highest correlation with the short- and medium-chain esters (r > 0.78, P < 0.05). The Mantel test between environmental factors and ester production showed that temperature of Daqu was directly correlated with the short-chain esters (r = 0.58, P < 0.01) and AAT activity (r = 0.56, P < 0.01). Further, the short- and medium-chain ester content in Daqu under the treatment nearer to the reported optimal temperature of 40-50 °C of AATs reaction was overall higher than that of the other treatment Daqu.

CONCLUSION

This study revealed that the temperature-dependent AATs reaction was the main enzymatic method producing the short- and medium-chain esters over the whole Daqu-making process. The results could contribute to the flavor improvement of Baijiu. © 2022 Society of Chemical Industry.

Metabolic Engineering of Escherichia coli for the Biosynthesis of 3-Phenylpropionic Acid and 3-Phenylpropyl Acetate

3-Phenylpropionic acid (3PPA) and its derivative 3-phenylpropyl acetate (3PPAAc) are important aromatic compounds with broad applications in the cosmetics and food industries. In this study, we constructed a plasmid-free 3PPA-producing Escherichia coli strain and designed a novel 3PPAAc biosynthetic pathway. A module containing tyrosine ammonia lyase and enoate reductase, evaluated under the control of different promoters, was combined with phenylalanine-overproducing strain E. coli ATCC31884, enabling the plasmid-free de novo production of 218.16 ± 43.62 mg L^-1 3PPA. The feasibility of the pathway was proved by screening four heterologous alcohol acetyltransferases, which catalyzed the transformation of 3-phenylpropyl alcohol into 3PPAAc. Afterward, 94.59 ± 16.25 mg L^-1 3PPAAc was achieved in the engineered E. coli strain. Overall, we have not only demonstrated the potential of de novo synthesis of 3PPAAc in microbes for the first time but also provided a platform for the future of biosynthesis of other aromatic compounds.

An optimized reverse β-oxidation pathway to produce selected medium-chain fatty acids in Saccharomyces cerevisiae

BACKGROUND

Medium-chain fatty acids are molecules with applications in different industries and with growing demand. However, the current methods for their extraction are not environmentally sustainable. The reverse β-oxidation pathway is an energy-efficient pathway that produces medium-chain fatty acids in microorganisms, and its use in Saccharomyces cerevisiae, a broadly used industrial microorganism, is desired. However, the application of this pathway in this organism has so far either led to low titers or to the predominant production of short-chain fatty acids.

RESULTS

We genetically engineered Saccharomyces cerevisiae to produce the medium-chain fatty acids hexanoic and octanoic acid using novel variants of the reverse β-oxidation pathway. We first knocked out glycerolphosphate dehydrogenase GPD2 in an alcohol dehydrogenases knock-out strain (△adh1-5) to increase the NADH availability for the pathway, which significantly increased the production of butyric acid (78 mg/L) and hexanoic acid (2 mg/L) when the pathway was expressed from a plasmid with BktB as thiolase. Then, we tested different enzymes for the subsequent pathway reactions: the 3-hydroxyacyl-CoA dehydrogenase PaaH1 increased hexanoic acid production to 33 mg/L, and the expression of enoyl-CoA hydratases Crt2 or Ech was critical to producing octanoic acid, reaching titers of 40 mg/L in both cases. In all cases, Ter from Treponema denticola was the preferred trans-enoyl-CoA reductase. The titers of hexanoic acid and octanoic acid were further increased to almost 75 mg/L and 60 mg/L, respectively, when the pathway expression cassette was integrated into the genome and the fermentation was performed in a highly buffered YPD medium. We also co-expressed a butyryl-CoA pathway variant to increase the butyryl-CoA pool and support the chain extension. However, this mainly increased the titers of butyric acid and only slightly increased that of hexanoic acid. Finally, we also tested the deletion of two potential medium-chain acyl-CoA depleting reactions catalyzed by the thioesterase Tes1 and the medium-chain fatty acyl CoA synthase Faa2. However, their deletion did not affect the production titers.

CONCLUSIONS

By engineering the NADH metabolism and testing different reverse β-oxidation pathway variants, we extended the product spectrum and obtained the highest titers of octanoic acid and hexanoic acid reported in S. cerevisiae. Product toxicity and enzyme specificity must be addressed for the industrial application of the pathway in this organism.

Stochastic Processes Drive the Assembly and Metabolite Profiles of Keystone Taxa during Chinese Strong-Flavor Baijiu Fermentation

Multispecies communities participate in the fermentation of Chinese strong-flavor Baijiu (CSFB), and the metabolic activity of the dominant and keystone taxa is key to the flavor quality of the final product. However, their roles in metabolic function and assembly processes are still not fully understood. Here, we identified the variations in the metabolic profiles of dominant and keystone taxa and characterized their community assembly using 16S rRNA and internal transcribed spacer (ITS) gene amplicon and metatranscriptome sequencing. We demonstrate that CSFB fermentations with distinct metabolic profiles display distinct microbial community compositions and microbial network complexities and stabilities. We then identified the dominant taxa (Limosilactobacillus fermentum, Kazachstania africana, Saccharomyces cerevisiae, and Pichia kudriavzevii) and the keystone ecological cluster (module 0, affiliated mainly with Thermoascus aurantiacus, Weissella confusa, and Aspergillus amstelodami) that cause changes in metabolic profiles. Moreover, we highlight that the alpha diversity of keystone taxa contributes to changes in metabolic profiles, whereas dominant taxa exert their influence on metabolic profiles by virtue of their relative abundance. Additionally, our results based on the normalized stochasticity ratio (NST) index and the neutral model revealed that stochastic and deterministic processes together shaped CSFB microbial community assemblies. Stochasticity and environmental selection structure the keystone and dominant taxa differently. This study provides new insights into understanding the relationships between microbial communities and their metabolic functions. IMPORTANCE From an ecological perspective, keystone taxa in microbial networks with high connectivity have crucial roles in community assembly and function. We used CSFB fermentation as a model system to study the ecological functions of dominant and keystone taxa at the metabolic level. We show that both dominant taxa (e.g., those taxa that have the highest relative abundances) and keystone taxa (e.g., those taxa with the most cooccurrences) affected the resulting flavor profiles. Moreover, our findings established that stochastic processes were dominant in shaping the communities of keystone taxa during CSFB fermentation. This result is striking as it suggests that although the controlled conditions in the fermentor can determine the dominant taxa, the uncontrolled rare keystone taxa in the microbial community can alter the resulting flavor profiles. This important insight is vital for the development of potential manipulation strategies to improve the quality of CSFB through the regulation of keystone species.

Utilization of delactosed whey permeate for the synthesis of ethyl acetate with Kluyveromyces marxianus

Ethyl acetate is an important organic solvent and currently produced from fossil carbon resources. Microbial synthesis of this ester from sugar-rich waste could be an interesting alternative. Therefore, synthesis of ethyl acetate by Kluyveromyces marxinanus DSM 5422 from delactosed whey permeate (DWP) was studied in an aerated stirred bioreactor at 40 °C. DWP is mainly composed of residual lactose and minerals. The minerals inhibited yeast growth, as witnessed by an increased lag period, a reduced growth rate, and an extended process duration. All experiments were therefore carried out with diluted DWP. In a series of batch experiments, the pH of iron-deficient DWP medium varied between 4.8 and 5.9. The pH of the cultivation medium significantly influenced cell growth and product syntheses, with the highest ethyl acetate yield of 0.347 g g^-1 and lowest by-product formation achieved at pH 5.1. It is likely that this effect is due to pH-dependent iron chelation, which affects the iron bioavailability and the intracellular iron content, thus affecting growth and metabolite synthesis. The viability of yeast cells was always high despite the harsh conditions in DWP medium, which enabled extended usage of the biomass in repeated-batch and fed-batch cultivations. These two culture techniques increased the volume of DWP processed per time by 32 and 84% for the repeated-batch and the fed-batch cultivation, respectively, without a drop of the ester yield. KEY POINTS: • Delactosed whey permeate was converted to ethyl acetate with a high rate and yield. • The formation of ethyl acetate in DWP medium at iron limitation is pH-dependent. • Highly active yeasts from batch processes enabled extension as fed and repeated batch.

Impact of Assimilable Nitrogen Supplementation on Saccharomyces cerevisiae Metabolic Response and Aromatic Profile of Moschofilero Wine

The concentration of nitrogen in must is critical to yeast fermentation efficiency and wine aroma profile. The present work determined the effect of the amount of yeast assimilable nitrogen (YAN) on fermentation kinetics, aroma production, and gene expression patterns of the wine yeast Saccharomyces cerevisiae. Fermentations were performed under two different YAN concentrations of must. Acetate esters, linalool, and nerol appeared to be clearly affected by the different YAN levels. Real-time-PCR results revealed that the genes involved in ethyl and acetate esters production recorded, in general, higher transcript levels under high nitrogen supplementation. In addition, an up-regulation of the BGL2 and EXG1 genes, which are related to terpenes production, was observed in the case of high nitrogen content and it is well corresponded to the terpenol concentration found. Our study revealed the impact of nitrogen supplementation on yeast metabolism and its importance to adjust wine's aromatic composition and sensory profile.

Analysis of the Microbial Community Structure and Volatile Metabolites of JIUYAO in Fangxian, China

JIUYAO is an important saccharification starter in the production of huangjiu and is also an important source of flavor. In this study, the microbial community structure of JIUYAO from Fangxian was studied by high-throughput sequencing (HTS) technology for the first time. The volatile flavor compounds of the JIUYAO metabolites were also analyzed by headspace solid-phase microextraction combined with full two-dimensional gas chromatography-mass spectrometry (HS-SPME-GC×GC/MS) for the first time. The results showed that there were 15 dominant bacterial genera, including Weissella, Pediococcus, unclasssified_k_norank_d_Bacteria, Lactobacillus, Leuconostoc, etc. Thirteen species of dominant fungi included Wickerhamomyces, Saccharomycopsis, Rhizopus, etc. The different samples of JIUYAO were similar in their microbial species, but the number of species was significantly different. A total of 191 volatile flavor compounds (VFCs) were detected, among which esters, alcohols, acids, and alkenes were the main flavor compounds, and 21 terpenoids were also detected. In addition, the functional prediction of micro-organisms in JIUYAO revealed that global and overview maps, amino acid metabolism, and carbohydrate metabolism were the dominant categories. Through correlation analysis, 538 potential correlations between the dominant micro-organisms and the different flavor compounds were obtained. This study revealed the interactions between the micro-organisms and the volatile metabolites in JIUYAO, which provided reliable data for the analysis of the microbial community structure of Fangxian JIUYAO and provided theoretical support for the quality evaluation of JIUYAO.

Dynamic Transcriptome Analysis Reveals Transcription Factors Involved in the Synthesis of Ethyl Acetate in Aroma-Producing Yeast

Ethyl acetate is an important flavor element that is a vital component of baijiu. To date, the transcription factors that can help identify the molecular mechanisms involved in the synthesis of ethyl acetate have not been studied. In the present study, we sequenced and assembled the Wickerhamomyces anomalus strain YF1503 transcriptomes to identify transcription factors. We identified 307 transcription factors in YF1503 using high-throughput RNA sequencing. Some transcription factors, such as C2H2, bHLH, MYB, and bZIP, were up-regulated, and these might play a role in ethyl acetate synthesis. According to the trend of ethyl acetate content, heat map results and STEM, twelve genes were selected for verification of expression levels using quantitative real-time PCR. This dynamic transcriptome analysis presents fundamental information on the transcription factors and pathways that are involved in the synthesis of ethyl acetate in aroma-producing yeast. Of significant interest is the discovery of the roles of various transcription factor genes in the synthesis of ethyl acetate.

Metabolic engineering of Clostridium autoethanogenum for ethyl acetate production from CO

Background

Ethyl acetate is a bulk chemical traditionally produced via energy intensive chemical esterification. Microbial production of this compound offers promise as a more sustainable alternative process. So far, efforts have focused on using sugar-based feedstocks for microbial ester production, but extension to one-carbon substrates, such as CO and CO2/H2, is desirable. Acetogens present a promising microbial platform for the production of ethyl esters from these one-carbon substrates.

Results

We engineered the acetogen C. autoethanogenum to produce ethyl acetate from CO by heterologous expression of an alcohol acetyltransferase (AAT), which catalyzes the formation of ethyl acetate from acetyl-CoA and ethanol. Two AATs, Eat1 from Kluyveromyces marxianus and Atf1 from Saccharomyces cerevisiae, were expressed in C. autoethanogenum. Strains expressing Atf1 produced up to 0.2 mM ethyl acetate. Ethyl acetate production was barely detectable (< 0.01 mM) for strains expressing Eat1. Supplementation of ethanol was investigated as potential boost for ethyl acetate production but resulted only in a 1.5-fold increase (0.3 mM ethyl acetate). Besides ethyl acetate, C. autoethanogenum expressing Atf1 could produce 4.5 mM of butyl acetate when 20 mM butanol was supplemented to the growth medium.

Conclusions

This work offers for the first time a proof-of-principle that autotrophic short chain ester production from C1-carbon feedstocks is possible and offers leads on how this approach can be optimized in the future.

Nature's Most Fruitful Threesome: The Relationship between Yeasts, Insects, and Angiosperms

The importance of insects for angiosperm pollination is widely recognized. In fact, approximately 90% of all plant species benefit from animal-mediated pollination. However, only recently, a third part player in this story has been properly acknowledged. Microorganisms inhabiting floral nectar, among which yeasts have a prominent role, can ferment glucose, fructose, sucrose, and/or other carbon sources in this habitat. As a result of their metabolism, nectar yeasts produce diverse volatile organic compounds (VOCs) and other valuable metabolites. Notably, some VOCs of yeast origin can influence insects' foraging behavior, e.g., by attracting them to flowers (although repelling effects have also been reported). Moreover, when insects feed on nectar, they also ingest yeast cells, which provide them with nutrients and protect them from pathogenic microorganisms. In return, insects serve yeasts as transportation and a safer habitat during winter when floral nectar is absent. From the plant's point of view, the result is flowers being pollinated. From humanity's perspective, this ecological relationship may also be highly profitable. Therefore, prospecting nectar-inhabiting yeasts for VOC production is of major biotechnological interest. Substances such as acetaldehyde, ethyl acetate, ethyl butyrate, and isobutanol have been reported in yeast volatomes, and they account for a global market of approximately USD 15 billion. In this scenario, the present review addresses the ecological, environmental, and biotechnological outlooks of this three-party mutualism, aiming to encourage researchers worldwide to dig into this field.

Characterization of an Aspergillus niger for Efficient Fatty Acid Ethyl Ester Synthesis in Aqueous Phase and the Molecular Mechanism

Fatty acid ethyl esters are important flavor chemicals in strong-flavor baijiu. Microorganisms are the main contributors to ester synthesis during baijiu manufacture. However, the ester synthesis was unstable between batches. This was owing to a limited knowledge of the mechanisms for ester synthesis by microorganisms. In this work, a fatty acid ethyl ester synthesizing Aspergillus niger strain CGMCC (China General Microbiological Culture Collection) 3.4309 was identified. The conversion ratios of ethyl valerate, ethyl caproate, ethyl caprylate, and ethyl caprate were 7.87, 29.20, 94.80, and 85.20%, respectively, under the optimized conditions. A comparison of transcriptomes under the initial and optimized ester synthetic conditions indicated that 23 genes were upregulated in transcription level and encoded enzymes with potential abilities for ester synthesis. Eleven of the enzymes were expressed, and three of them, numbered An605, An1097, and An3131, showed the ability to catalyze fatty acid ethyl ester synthesis under aqueous phase, with capric acid as the preferred substrate. The possible enzymatic catalytic mechanism was proposed based on homology modeling and molecular docking. This study reported for the first time that A. niger showed the ability to efficiently catalyze the synthesis of short- and medium-chain fatty acid ethyl esters in aqueous phase, identified the key enzymes, and analyzed the basic enzymatic properties. This is helpful to promote the application of related microorganisms and enzyme resources in the baijiu industry.

Thioesterase enzyme families: Functions, structures, and mechanisms

Thioesterases are enzymes that hydrolyze thioester bonds in numerous biochemical pathways, for example in fatty acid synthesis. This work reports known functions, structures, and mechanisms of updated thioesterase enzyme families, which are classified into 35 families based on sequence similarity. Each thioesterase family is based on at least one experimentally characterized enzyme, and most families have enzymes that have been crystallized and their tertiary structure resolved. Classifying thioesterases into families allows to predict tertiary structures and infer catalytic residues and mechanisms of all sequences in a family, which is particularly useful because the majority of known protein sequence have no experimental characterization. Phylogenetic analysis of experimentally characterized thioesterases that have structures with the two main structural folds reveal convergent and divergent evolution. Based on tertiary structure superimposition, catalytic residues are predicted.

Metabolic engineering of Escherichia coli for efficient biosynthesis of butyl acetate

Background

Butyl acetate is a versatile compound that is widely used in the chemical and food industry. The conventional butyl acetate synthesis via Fischer esterification of butanol and acetic acid using catalytic strong acids under high temperature is not environmentally benign. Alternative lipase-catalyzed ester formation requires a significant amount of organic solvent which also presents another environmental challenge. Therefore, a microbial cell factory capable of producing butyl acetate through fermentation of renewable resources would provide a greener approach to butyl acetate production.

Result

Here, we developed a metabolically engineered strain of Escherichia coli that efficiently converts glucose to butyl acetate. A modified Clostridium CoA-dependent butanol production pathway was used to synthesize butanol which was then condensed with acetyl-CoA through an alcohol acetyltransferase. Optimization of alcohol acetyltransferase expression and redox balance with auto-inducible fermentative controlled gene expression led to an effective titer of 22.8 ± 1.8 g/L butyl acetate produced in a bench-top bioreactor.

Conclusion

Building on the well-developed Clostridium CoA-dependent butanol biosynthetic pathway, expression of an alcohol acetyltransferase converts the butanol produced into butyl acetate. The results from this study provided a strain of E. coli capable of directly producing butyl acetate from renewable resources at ambient conditions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01755-y.

The Influence of Delayed Sealing and Repeated Air Ingress during the Storage of Maize Silage on Fermentation Patterns, Yeast Development and Aerobic Stability

This study investigates the effects of delayed sealing and repeated air ingress on the formation of primary fermentation products and other volatile organic compounds (VOC), the development of yeasts and the aerobic stability (ASTA) of maize (26.8% dry matter, DM). After packing, the silos were sealed either promptly or with a delay of 24 h, with repeated air ingress after 27, 55 and 135 days of storage. Losses of DM, fermentation pattern, including VOC, yeast numbers and aerobic stability, were determined 6 times during storage for 142 days. Yeast numbers markedly increased during the first three fermentation days, with the effect being much stronger in silage sealed with a delay than in promptly sealed silage (log10 cfu/g FM 7.27 vs. 5.88, p < 0.002). Simultaneously, the concentrations of ethanol and ethyl esters and DM losses increased. The DM losses were closely correlated with the total concentrations of alcohols and acetic acid (delay: R2 = 0.71, p < 0.001; prompt: R2 = 0.91, p < 0.001, respectively). The repeated air ingress for 24 h during storage after completion of the main fermentation phase had only a minor effect on fermentation pattern, VOC formation and DM losses. The relationship between the counts of total yeasts and lactate-assimilating yeasts (LAY) was very strong (R2 = 0.995, p < 0.001), and LAY numbers were shown to be largely responsible for aerobic instability (R2 = 0.752, p < 0.001). This trial proved the detrimental effects of air on silage fermentation with delayed sealing to be much more deleterious than repeated short-term air ingress after about one month of storage.

Repurposing brewery contaminant yeast as production strains for low-alcohol beer fermentation

A number of fungal isolates were recently obtained from a survey of the microbiota of multiple breweries and brewery products. Here, we sought to explore whether any of these brewery contaminants could be repurposed for beneficial use in beer fermentations, with particular focus on low-alcohol beer. There were 56 yeast strains first screened for the utilization of different carbon sources, ability to ferment brewer's wort, and formation of desirable aroma compounds. A number of strains appeared maltose-negative and produced desirable aromas without obvious off-flavours. These were selected for further scaled-up wort fermentations. The selected strains efficiently reduced wort aldehydes during fermentation, thus eliminating undesirable wort-like off-flavours, and produced a diverse volatile aroma profile. Two strains, Trigonopsis cantarellii and Candida sojae, together with a commercial Saccharomycodes ludwigii reference strain, were selected for 30-L-scale wort fermentations based on aroma profile and similarity to a commercial reference beer during sensory analysis using projective mapping. Both strains performed comparably to the commercial reference, and the T. cantarellii strain in particular, produced low amounts of off-flavours and a significantly higher amount of the desirable monoterpene alcohol trans-geraniol. The strain was also sensitive to common food preservatives and antifungal compounds and unable to grow at 37°C, suggesting it is relatively easily controllable in the brewery, and appears to have low risk of pathogenicity. This study shows how the natural brewery microbiota can be exploited as a source of non-conventional yeasts for low-alcohol beer production.

Volatile Compounds in Monovarietal Wines of Two Amarone Della Valpolicella Terroirs: Chemical and Sensory Impact of Grape Variety and Origin, Yeast Strain and Spontaneous Fermentation

Aroma profiles of withered Corvina and Corvinone wines from two different Valpolicella terroirs were investigated in relationship to yeast strain and use of spontaneous fermentation. The results indicated that volatile chemical differences between wines were mainly driven by grape origin, which was associated with distinctive compositional profiles. Wine content in terpenes, norisoprenoids, benzenoids and C₆ alcohols, as well as some fermentative esters, were indeed significantly affected by grape origin. Conversely, yeast strain influence was mainly associated with fermentation-derived esters. Sensory analysis, besides confirming the major role of grape origin as driver of wine differentiation, indicated that spontaneous fermentations reduced the sensory differences associated with grape origin and variety, mainly due to high content of acetic acid and ethyl acetate.

Co-production of hydrogen and ethyl acetate in Escherichia coli

BACKGROUND

Ethyl acetate (C₄H₈O₂) and hydrogen (H₂) are industrially relevant compounds that preferably are produced via sustainable, non-petrochemical production processes. Both compounds are volatile and can be produced by Escherichia coli before. However, relatively low yields for hydrogen are obtained and a mix of by-products renders the sole production of hydrogen by micro-organisms unfeasible. High yields for ethyl acetate have been achieved, but accumulation of formate remained an undesired but inevitable obstacle. Coupling ethyl acetate production to the conversion of formate into H₂ may offer an interesting solution to both drawbacks. Ethyl acetate production requires equimolar amounts of ethanol and acetyl-CoA, which enables a redox neutral fermentation, without the need for production of by-products, other than hydrogen and CO₂.

RESULTS

We engineered Escherichia coli towards improved conversion of formate into H₂ and CO₂ by inactivating the formate hydrogen lyase repressor (hycA), both uptake hydrogenases (hyaAB, hybBC) and/or overexpressing the hydrogen formate lyase activator (fhlA), in an acetate kinase (ackA) and lactate dehydrogenase (ldhA)-deficient background strain. Initially 10 strains, with increasing number of modifications were evaluated in anaerobic serum bottles with respect to growth. Four reference strains ΔldhAΔackA, ΔldhAΔackA p3-fhlA, ΔldhAΔackAΔhycAΔhyaABΔhybBC and ΔldhAΔackAΔhycAΔhyaABΔhybBC p3-fhlA were further equipped with a plasmid carrying the heterologous ethanol acyltransferase (Eat1) from Wickerhamomyces anomalus and analyzed with respect to their ethyl acetate and hydrogen co-production capacity. Anaerobic co-production of hydrogen and ethyl acetate via Eat1 was achieved in 1.5-L pH-controlled bioreactors. The cultivation was performed at 30 °C in modified M9 medium with glucose as the sole carbon source. Anaerobic conditions and gas stripping were established by supplying N₂ gas.

CONCLUSIONS

We showed that the engineered strains co-produced ethyl acetate and hydrogen to yields exceeding 70% of the pathway maximum for ethyl acetate and hydrogen, and propose in situ product removal via gas stripping as efficient technique to isolate the products of interest.

Probing specificities of alcohol acyltransferases for designer ester biosynthesis with a high-throughput microbial screening platform

Alcohol acyltransferases (AATs) enables microbial biosynthesis of a large space of esters by condensing an alcohol and an acyl-CoA. However, substrate promiscuity of AATs prevents microbial biosynthesis of designer esters with high selectivity. Here, we developed a high-throughput microbial screening platform that facilitates rapid identification of AATs for designer ester biosynthesis. First, we established a microplate-based culturing technique with in situ fermentation and extraction of esters. We validated its capability in rapid profiling of the alcohol substrate specificity of 20 chloramphenicol acetyltransferase variants derived from Staphylococcus aureus (CAT_Sa ) for microbial biosynthesis of acetate esters with various exogeneous alcohol supply. By coupling the microplate-based culturing technique with a previously established colorimetric assay, we developed a high-throughput microbial screening platform for AATs. We demonstrated that this platform could not only probe the alcohol substrate specificity of both native and engineered AATs but also identify the beneficial mutations in engineered AATs for enhanced ester synthesis. We anticipate the high-throughput microbial screening platform provides a useful tool to identify novel wildtype and engineered AATs that have important roles in nature and industrial biocatalysis for designer bioester production.

Packing a punch: understanding how flavours are produced in lager fermentations

Beer is one of the most popular beverages in the world and it has an irreplaceable place in culture. Although invented later than ale, lager beers dominate the current market. Many factors relating to the appearance (colour, clarity and foam stability) and sensory characters (flavour, taste and aroma) of beer, and other psychological determinants affect consumers' perception of the product and defines its drinkability. This review takes a wholistic approach to scrutinise flavour generation in the brewing process, focusing particularly on the contribution of the raw ingredients and the yeasts to the final flavour profiles of lager beers. In addition, we examine current developments to improve lager beer flavour profiles for the modern consumers.

Microbial production of butyl butyrate: from single strain to cognate consortium

Butyl butyrate (BB) is an important chemical with versatile applications in beverage, food and cosmetics industries. Since chemical synthesis of BB may cause adverse impacts on the environment, biotechnology is an emerging alternative approach for microbial esters biosynthesis. BB can be synthesized by using a single Clostridium strain natively producing butanol or butyrate, with exogenously supplemented butyrate or butanol, in the presence of lipase. Recently, E. coli strains have been engineered to produce BB, but the titer and yield remained very low. This review highlighted a new trend of developing cognate microbial consortium for BB production and associated challenges, and end up with new prospects for further improvement for microbial BB biosynthesis.

Developing Clostridia as Cell Factories for Short- and Medium-Chain Ester Production

Short- and medium-chain volatile esters with flavors and fruity fragrances, such as ethyl acetate, butyl acetate, and butyl butyrate, are usually value-added in brewing, food, and pharmacy. The esters can be naturally produced by some microorganisms. As ester-forming reactions are increasingly deeply understood, it is possible to produce esters in non-natural but more potential hosts. Clostridia are a group of important industrial microorganisms since they can produce a variety of volatile organic acids and alcohols with high titers, especially butanol and butyric acid through the CoA-dependent carbon chain elongation pathway. This implies sufficient supplies of acyl-CoA, organic acids, and alcohols in cells, which are precursors for ester production. Besides, some Clostridia could utilize lignocellulosic biomass, industrial off-gas, or crude glycerol to produce other branched or straight-chain alcohols and acids. Therefore, Clostridia offer great potential to be engineered to produce short- and medium-chain volatile esters. In the review, the efforts to produce esters from Clostridia via in vitro lipase-mediated catalysis and in vivo alcohol acyltransferase (AAT)-mediated reaction are comprehensively revisited. Besides, the advantageous characteristics of several Clostridia and clostridial consortia for bio-ester production and the driving force of synthetic biology to clostridial chassis development are also discussed. It is believed that synthetic biotechnology should enable the future development of more effective Clostridia for ester production.

Molecular characterization of the Saccharomycopsis fibuligera ATF genes, encoding alcohol acetyltransferase for volatile acetate ester formation

Aroma ester components produced by fermenting yeast cells via alcohol acetyltransferase (AATase)-catalyzed intracellular reactions are responsible for the fruity character of fermented alcoholic beverages, such as beer and wine. Acetate esters are reportedly produced at relatively high concentrations by non-Saccharomyces species. Here, we identified 12 ATF orthologues (SfATFs) encoding putative AATases, in the diploid genome of Saccharomycopsis fibuligera KJJ81, an isolate from wheat-based Nuruk in Korea. The identified SfATF proteins (SfAtfp) display low sequence identities with S. cerevisiae Atf1p (between 13.3 and 27.0%). All SfAtfp identified, except SfAtf(A)4p and SfAtf(B)4p, contained the activation domain (HXXXD) conserved in other Atf proteins. Culture supernatant analysis using headspace gas chromatography mass spectrometry confirmed that the recombinant S. cerevisiae strains expressing SfAtf(A)2p, SfAtf(B)2p, and SfAtf(B)6p produced high levels of isoamyl and phenethyl acetates. The volatile aroma profiles generated by the SfAtf proteins were distinctive from that of S. cerevisiae Atf1p, implying difference in the substrate preference. Cellular localization analysis using GFP fusion revealed the localization of SfAtf proteins proximal to the lipid particles, consistent with the presence of amphipathic helices at their N- and C-termini. This is the first report that systematically characterizes the S. fibuligera ATF genes encoding functional AATases responsible for acetate ester formation using higher alcohols as substrate, demonstrating their biotechnological potential for volatile ester production.