Perspectives for the microbial production of ethyl acetate

Alternate solvent for soybean oil extraction based on extractability and membrane solvent recovery

Ethyl acetate, acetone, 2-propanol, 1-propanol, and ethanol were screened among the class 3 category solvents as an alternative to hexane based on operational and occupational safety and bio-renewability potential. All five solvents exhibited higher extractability (22.3 to 23.2%) than hexane (21.5%) with soybean flour. Additionally, there was no significant difference in the fatty acid and triacylglycerol (TAG) composition of the oils extracted using alternate solvents and hexane, indicating the oil quality was not affected. More importantly, ethyl acetate (2.1%) resulted in a marginally higher yield of TAG, while 2-propanol showed a nearly equal yield to hexane. Further, membrane desolventizing was attempted to mitigate the limitations of higher thermal energy requirements. One of the polydimethylsiloxane membranes exhibited good selectivity (TAG rejection 85.8%) and acceptable flux (59.3 L·m-2·h-1) with an ethyl acetate miscella system. Under plant-simulated recirculation conditions, a two-stage membrane process reduced the oil content in permeate to 2.5%. The study revealed that ethyl acetate could potentially replace hexane, considering its higher TAG extractability and suitability for the membrane-augmented solvent recycling process in the extraction plants.

Comparative study on volatile compounds and taste components of various citrus cultivars using electronic sensors, GC-MS, and GC-olfactometry

Various citrus fruits' flavor compounds were analyzed using an electronic sensor (E-sensor), and odor-active compounds were identified using gas chromatography-mass spectrometry-olfactometry (GC-MS-O). In the E-tongue analysis, the intensity of sweetness, saltiness, and bitterness was highest in Citrus unshiu, while sourness and umami were highest in C. setomi. A total of 43 volatile compounds were detected in the E-nose analysis, and the compound with the highest peak area was limonene, a type of terpenoid, which exhibited a prominent peak area in C. unshiu. Principal component analysis between flavor compounds and each sample explained a total variance of 83.15% and led to the classification of three clusters. By GC-MS-O, 32 volatile compounds were detected, with limonene being the most abundant, ranging from 20.28 to 56.21 mg/kg. The odor-active compounds were identified as (E)-2-hexenal, hexanal, α-pinene, β-myrcene, limonene, γ-terpinene, nonanal, and D-carvone, respectively.

Does saponin in quinoa really embody the source of its bitterness?

While it is widely reported that saponins are the main source of the bitter taste in quinoa, this work found that some saponin compounds in quinoa husks elicit an umami response. The saponins were analyzed qualitatively and quantified by mass spectrometry (UPLC-MS). Two quinoa saponin compounds RT 46 (3-O-β-d-glucopyranosyl-(1 → 3)-α-l-arabino-pyranosyl-phytolaccagenic acid 28-O-β-d-gluco-pyranosyl), and RT 53 (3-O-β-d-glucopyranosyl-(1 → 4)-β-d-glucopyranosyl-28-O-hederagenin) were isolated from quinoa husks through separation and purification. According to eletronic tongue, the main taste response for those compounds was umami. It was found that the two quinoa saponins could bind to sweet and umami receptors. Besides saponins, various flavonoids and polyphenols also appeared in the UPLC-MS spectrum of crude saponins. The electronic tongue and sensory evaluation revealed that flavonoids and polyphenols showed obvious bitterness and astringency at very low concentrations. The study inferred that flavonoids and polyphenols are the main compounds that generate quinoa's bitter taste.

Enhancing solubility and bioavailability of octacosanol: Development of a green O/W nanoemulsion synthesis process

Octacosanol, a naturally occurring higher fatty alcohol, possessed numerous biological effects. However, octacosanol limited solubility in water due to its lipophilic nature and large structure, resulting in poor absorption and low bioavailability. To overcome this challenge, we developed a simple, environmentally friendly, and energy-efficient O/W nanoemulsion synthesis process. The nanoemulsion achieved an average droplet size of approximately 30 nm, exhibited excellent dispersibility and stability at room temperature for 60 days, and showcased robust storage properties insensitive to ambient temperature, pH, NaCl, and sucrose. Remarkably, the preparation process of the nanoemulsion maintained the biological activity of octacosanol while demonstrating significantly enhancing antioxidant activity compared to octacosanol suspension. Additionally, the nanoemulsion displayed negligible cytotoxic effects on Caco-2 cells. Significantly, the octacosanol nanoemulsion exhibited a 5.4-fold enhancement in transmembrane transport efficiency when compared to the suspension in Caco-2 cell monolayers. Additionally, in an in vivo experiment, there was a notable 2.9-fold increase in rat intestinal absorption. These findings could provide valuable insights into the development of octacosanol nanoemulsion, supporting its future applications and paving the way for the design of stable nanoemulsion systems for other lipophilic and sparingly soluble substances.

Exploring the controllability of the Baijiu fermentation process with microbiota orientation

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

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.

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.

Recent Advances in Greener Asymmetric Organocatalysis Using Bio-Based Solvents

Efficient synthetic methods that avoid the extensive use of hazardous reagents and solvents, as well as harsh reaction conditions, have become paramount in the field of organic synthesis. Organocatalysis is notably one of the best tools in building chemical bonds between carbons and carbon-heteroatoms; however, most examples still employ toxic volatile organic solvents. Although a portfolio of greener solvents is now commercially available, only ethyl alcohol, ethyl acetate, 2-methyltetrahydrofuran, supercritical carbon dioxide, ethyl lactate, and diethyl carbonate have been explored with chiral organocatalysts. In this review, the application of these bio-based solvents in asymmetric organocatalytic methods reported in the last decade is discussed, highlighting the proposed mechanism pathway for the transformations.

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.

“Key Factor” for Baijiu Quality: Research Progress on Acid Substances in Baijiu

Baijiu is the national liquor of China, which has lasted in China for more than 2000 years. Abundant raw materials, multi-strain co-fermentation, and complex processes make the secrets of baijiu flavor and taste still not fully explored. Acid substances not only have a great influence on the flavor and taste of baijiu, but also have certain functions. Therefore, this paper provides a systematic review for the reported acid substances, especially for their contribution to the flavor and functional quality of baijiu. Based on previous studies, this paper puts forward a conjecture, a suggestion, and a point of view, namely: the conjecture of “whether acid substances can be used as ‘key factor’ for baijiu quality “; the suggestion of “the focus of research on acid substances in baijiu should be transferred to evaluating their contribution to the taste of baijiu”; and the view of “acid substances are ‘regulators’ in the fermentation process of baijiu”. It is worth thinking about whether acid substances can be used as the key factors of baijiu to be studied and confirmed by practice in the future. It is hoped that the systematic review of acid substances in baijiu in this paper can contribute to further in-depth and systematic research on baijiu by researchers in the future.

Ethyl acetate produced by Hanseniaspora uvarum is a potential biocontrol agent against tomato fruit rot caused by Phytophthora nicotianae

In this study, an oomycete strain FQ01 of Phytophthora nicotianae, which could cause destructive postharvest disease, was isolated. At present, chemical fungicides are the main reagents used for controlling Phytophthora diseases. It is necessary to find new control techniques that are environmentally friendly. The biocontrol activity of Hanseniaspora uvarum MP1861 against P. nicotianae FQ01 was therefore investigated. Our results revealed that the volatile organic compounds (VOCs) released by the yeast strain MP1861 could inhibit the development of P. nicotianae FQ01. The major component of the VOCs produced by the yeast strain MP1861 was identified to be ethyl acetate (70.8%). Biocontrol experiments showed that Phytophthora disease in tomato fruit could be reduced by 95.8% after the yeast VOCs treatment. Furthermore, ethyl acetate inhibited the mycelial growth of the oomycete strain FQ01, and damaged the pathogen cell membrane. This paper describes the pioneering utilization of the yeast strain MP1861 for biocontrol of postharvest fruit rot in tomato caused by P. nicotianae.

The Sensorial and Chemical Changes in Beer Brewed with Yeast Genetically Modified to Release Polyfunctional Thiols from Malt and Hops

The biotransformation of hop aroma, particularly by the cysteine S-conjugate beta-lyase enzyme (CSL), has been a recent topic of tremendous interest among brewing scientists and within the brewing community. During a process often referred to as biotransformation, yeast-encoded enzymes convert flavorless precursor molecules found in barley and hops into volatile thiols that impart a variety of desirable flavors and aromas in beer. Two volatile thiols of particular interest are 3-mercaptohexan-1-ol (3MH) and its acetate ester, 3-mercaptohexyl acetate (3MHA), which impart guava and passionfruit flavors, respectively. In this study, a parental Saccharomyces cerevisiae brewing strain that displayed low thiol biotransformation activity was genetically manipulated (GM) to substantially increase its thiol biotransformation potential. Construction of this GM strain involved integration of a gene encoding a highly active CSL enzyme that converts thiol precursors into the volatile thiol, 3MH. Three additional strains were subsequently developed, each of which paired CSL expression with expression of an alcohol acyltransferase (AAT) gene. It was hypothesized that expression of an AAT in conjunction with CSL would increase production of 3MHA. Fermentation performance, sensory characteristics, and 3MH/3MHA production were evaluated for these four GM strains and their non-GM parent in 1.5hL fermentations using 100% barley malt wort hopped at low levels with Cascade hops. No significant deviations in fermentation performance (time to attenuation, final gravity, alcohol content, wort fermentability) or finished beer chemistry were observed between the GM strains and the parent strain with the exception of the speed of vicinal diketones reduction post-fermentation, which was quicker for the GM strains. The GM strains produced beer that had up to 73-fold and 8-fold higher 3MH and 3MHA concentrations than the parent strain, achieving concentrations that were up to 79-fold greater than their sensory detection thresholds. The beers were described as intensely tropical and fruity, and were associated with guava, passionfruit, mango, pineapple and sweaty aromas. These experiments demonstrate the potential of genetic modification to dramatically enhance yeast biotransformation ability without creating off flavors or affecting fermentation performance.

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

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

Recent trends in microbial flavour Compounds: A review on Chemistry, synthesis mechanism and their application in food

Aroma and flavour represent the key components of food that improves the organoleptic characteristics of food and enhances the acceptability of food to consumers. Commercial manufacturing of aromatic and flavouring compounds is from the industry's microbial source, but since time immemorial, its concept has been behind human practices. The interest in microbial flavour compounds has developed in the past several decades because of its sustainable way to supply natural additives for the food processing sector. There are also numerous health benefits from microbial bioprocess products, ranging from antibiotics to fermented functional foods. This review discusses recent developments and advancements in many microbial aromatic and flavouring compounds, their biosynthesis and production by diverse types of microorganisms, their use in the food industry, and a brief overview of their health benefits for customers.

One-Pot Ethyl Acetate Production from Ethanol Photooxidation on Rutile TiO2(110): Strong Photon Energy Dependence

Ethyl acetate (EA) production from sequential ethanol (EtOH) photooxidation on a rutile(R)-TiO₂(110) surface has been investigated by the temperature-programmed desorption (TPD) method at 355 and 266 nm. Significant EA product is detected under 266 nm irradiation, which is most likely to be formed via cross-coupling of primary dissociation products, aldehyde (CH₃CHO) and ethoxy groups. On the contrary, EA formation at 355 nm is negligible. In addition, the initial rate of EA formation from EtOH at 266 nm is nearly 2 orders of magnitude faster than that at 355 nm. Quantitative analysis suggests that EA formation from sequential EtOH photooxidation on R-TiO₂(110) is strongly dependent on photon energy or the energy of hot holes. This experimental result raises doubt about the traditional photocatalysis model on TiO₂ where charge carriers relax to their respective band edges prior to charge transfer to adsorbates during the photocatalytic process, leading to no dependence on photon energy in TiO₂ photocatalysis.

Screening lager yeast with higher ethyl-acetate production by adaptive laboratory evolution in high concentration of acetic acid

Ethyl-acetate is important for the flavor and aroma of the alcoholic beverages, therefore, there have been extensive efforts toward increasing its production by engineering yeast strains. In this study, we reported a new approach to breed non-genetic modified producing yeast strain with higher ethyl-acetate production for beer brewing. First, we demonstrated the positive effect of higher acetic acid concentration on inducing the expression of acetyl-CoA synthetase (ACS). Then, we applied adaptive laboratory evolution method to evolve strain with higher expression level of ACS. As a result, we obtained several evolved strains with increased ACS expression level as well as ethyl-acetate production. In 3 L scale fermentation, the optimal strain EA60 synthesized more ethyl-acetate than M14 at the same time point. At the end of fermentation, the ethyl-acetate production in EA60 was 21.4% higher than M14, while the other flavor components except for acetic acid were changed in a moderate degree, indicating this strain had a bright prospect in industrial application. Moreover, this study also indicated that ACS1 played a more important role in increasing the acetic acid tolerance of yeast, while ACS2 contributed to the synthesis of cytosol acetyl-CoA, thereby facilitating the production of ethyl-acetate during fermentation.