Biological Conversion of Amino Acids to Higher Alcohols

Organic acids drove the microbiota succession and consequently altered the flavor quality of Laotan Suancai across fermentation rounds: Insights from the microbiome and metabolome

Laotan Suancai, a popular traditional Chinese fermented vegetable, is manufactured in the industry via four fermentation rounds. However, the differences in flavor quality of Laotan Suancai from the four fermentation rounds and the causes of this variation remain unclear. Metabolome analysis indicated that the different content of five taste compounds and 31 aroma compounds caused the differences in flavor quality among the variated fermentation rounds of Laotan Suancai. Amplicon sequencing indicated that the microbial succession exhibited a certain pattern during four fermentation rounds and further analysis unveiled that organic acids drove the microbiota shift to more acid-resistant populations. Spearman correlation analysis highlighted that seven core microbes may be involved in the formation of differential flavor and the corresponding metabolic pathways were reconstructed by function prediction. Our findings offer a novel perspective on comprehending the deterioration of flavor quality across the fermentation rounds of Laotan Suancai.

Sniffing the wine differences: The role of Starmerella bacillaris biofilm-detached cells

This study investigated the impact of 10 strains of Starmerella bacillaris, co-inoculated as planktonic or biofilm-detached cells with Saccharomyces cerevisiae, on the volatilome of a red wine. The wines produced with St. bacillaris biofilm-detached cells exhibited a greater concentration of glycerol and a lower quantity of ethanol than the other wines. Furthermore, these wines exhibited elevated levels of higher alcohols, organic acids, esters, terpenes, and norisoprenoids. Based on the odor activity value and relative odor contribution, isoamyl acetate, ethyl octanoate, ethyl isobutanoate, and methyl decanoate were the main aroma components of wines made with planktonic cells. The main compounds characterizing the wines obtained with biofilm-detached cells were: phenethyl alcohol, β-damascenone, citronellol, β-ionone, and nerol. The sensory analysis revealed that the wines produced with biofilm-detached cells had higher scores for mouth-feel, spicy, floral, and raspberry notes than the others. The present study provides evidence that St. bacillaris biofilm-detached cells released specific volatile compounds in red wines.

A Comparative Study of Microbial Communities, Biogenic Amines, and Volatile Profiles in the Brewing Process of Rice Wines with Hongqu and Xiaoqu as Fermentation Starters

Rice wine is primarily crafted from grains through saccharification and liquification with the help of Qu. Qu plays an important role in the formation of the flavor quality of rice wine. Hongqu and Xiaoqu represent two prevalent varieties of Qu that are typically utilized in the brewing process of rice wine and play a crucial role in its production. In this study, GC, GC-MS, HPLC, and metagenomic sequencing techniques were used to contrast the microbial flora, biogenic amines, and aroma characteristics developed during the fermentation of rice wines, with Hongqu and Xiaoqu being used as initiating agents for the brewing process. The results show that the content of higher alcohols (including n-propanol, isobutanol, 3-methyl-1-butanol, and phenethyl alcohol) in rice wine brewed with Xiaoqu (XQW) was significantly higher than that in rice wine brewed with Hongqu (HQW). Contrarily, the concentration of biogenic amines in HQW surpassed that of XQW by a notable margin, but tyramine was significantly enriched in XQW and not detected in HQW. In addition, a multivariate statistical analysis revealed distinct disparities in the constitution of volatile components between HQW and XQW. Hexanoic acid, ethyl acetate, isoamyl acetate, ethyl caproate, ethyl decanoate, 2-methoxy-4-vinylphenol, etc., were identified as the characteristic aroma-active compounds in HQW and XQW. A microbiome analysis based on metagenomic sequencing showed that HQW and XQW had different dominant microorganisms in the brewing process. Burkholderia, Klebsiella, Leuconostoc, Monascus, and Aspergillus were identified as the primary microbial genera in the HQW fermentation period, while Pediococcus, Enterobacter, Rhizopus, Ascoidea, and Wickerhamomyces were the main microbial genera in the XQW brewing process. A bioinformatics analysis revealed that the concentrations of microbial genes involved in biogenic amines and esters biosynthesis were significantly higher in HQW than those in XQW, while the content of genes relevant to glycolysis, higher alcohol biosynthesis, and fatty acid metabolism was significantly higher in XQW than in HQW, which are the possible reasons for the difference in flavor quality between the two kinds of rice wine from the perspective of microbial functional genes.

Cyberlindnera fabianii, an Uncommon Yeast Responsible for Gluten Bread Spoilage

A single strain of yeast was isolated from industrial gluten bread (GB) purchased from a local supermarket. This strain is responsible for spoilage consisting of white powdery and filamentous colonies due to the fragmentation of hyphae into short lengths (dust-type spots), similar to the spoilage produced by chalk yeasts such as Hyphopichia burtonii, Wickerhamomyces anomalus and Saccharomycopsis fibuligera. The isolated strains were identified initially by traditional methods as Wickerhamomyces anomalus, but with genomic analysis, they were definitively identified as Cyberlindnera fabianii, a rare ascomycetous opportunistic yeast species with low virulence attributes, uncommonly implicated in bread spoilage. However, these results demonstrate that this strain is phenotypically similar to Wi. anomalus. Cy. fabianii grew in GB because of its physicochemical characteristics which included pH 5.34, Aw 0.97 and a moisture of about 50.36. This spoilage was also confirmed by the presence of various compounds typical of yeasts, derived from sugar fermentation and amino acid degradation. These compounds included alcohols (ethanol, 1-propanol, isobutyl alcohol, isoamyl alcohol and n-amyl alcohol), organic acids (acetic and pentanoic acids) and esters (Ethylacetate, n-propil acetate, Ethylbutirrate, Isoamylacetate and Ethylpentanoate), identified in higher concentrations in the spoiled samples than in the unspoiled samples. The concentration of acetic acid was lower only in the spoiled samples, but this effect may be due to the consumption of this compound to produce acetate esters, which predominate in the spoiled samples.

Metabolic and tolerance engineering of Komagataella phaffii for 2-phenylethanol production through genome-wide scanning

BACKGROUND

2-Phenylethanol (2-PE) is one of the most widely used spices. Recently, 2-PE has also been considered a potential aviation fuel booster. However, the lack of scientific understanding of the 2-PE biosynthetic pathway and the cellular response to 2-PE cytotoxicity are the most important obstacles to the efficient biosynthesis of 2-PE.

RESULTS

Here, metabolic engineering and tolerance engineering strategies were used to improve the production of 2-PE in Komagataella phaffii. First, the endogenous genes encoding the amino acid permease GAP1, aminotransferase AAT2, phenylpyruvate decarboxylase KDC2, and aldehyde dehydrogenase ALD4 involved in the Ehrlich pathway and the 2-PE stress response gene NIT1 in K. phaffii were screened and characterized via comparative transcriptome analysis. Subsequently, metabolic engineering was employed to gradually reconstruct the 2-PE biosynthetic pathway, and the engineered strain S43 was obtained, which produced 2.98 g/L 2-PE in shake flask. Furthermore, transcriptional profiling analyses were utilized to screen for novel potential tolerance elements. Our results demonstrated that cells with knockout of the PDR12 and C4R2I5 genes exhibited a significant increase in 2-PE tolerance. To confirm the practical applications of these results, deletion of the PDR12 and C4R2I5 genes in the hyper 2-PE producing strain S43 dramatically increased the production of 2-PE by 18.12%, and the production was 3.54 g/L.

CONCLUSION

This is the highest production of 2-PE produced by K. phaffii via L-phenylalanine conversion. These identified K. phaffii endogenous elements are highly conserved in other yeast species, suggesting that manipulation of these homologues might be a useful strategy for improving aromatic alcohol production. These results also enrich the understanding of aromatic compound biosynthetic pathways and 2-PE tolerance, and provide new elements and strategies for the synthesis of aromatic compounds by microbial cell factories.

Exploring jujube wine flavor and fermentation mechanisms by HS-SPME-GC-MS and UHPLC-MS metabolomics

The fermentation metabolites significantly influence the quality of jujube wine. However, the dynamics of these metabolites during fermentation are not well understood. In this study, a total of 107 volatile and 1758 non-volatile compounds were identified using a flavor-directed research strategy and non-targeted metabolomics. The increase in esters and alcohols during fermentation shifted the aroma from grassy, mushroomy, and earthy to a floral and fruity flavor in the jujube wine. Leucine and phenylalanine were notably enriched during fermentation, potentially benefiting human health and enriching the flavor of fruit wines. Moreover, pathway analysis identified four key metabolic pathways and two crucial metabolic substrates, pyruvate and l-aspartate. This study provides a theoretical reference for optimizing the fermentation process and enhancing the quality of jujube wine.

Effect of Caproicibacterium lactatifermentans inoculation on the microbial succession and flavor formation of pit mud used in Chinese Baijiu fermentation

Caproicibacterium lactatifermentans is a major caproate-producing bacterium in high-quality pit mud and has an impact on the synthesis of fatty acids during Baijiu fermentation. To develop an effective method for cultivating high-quality pit mud, we explored the role of Caproicibacterium lactatifermentans inoculation. The inoculation resulted in a high level of Caproicibacterium lactatifermentans (29.16%) and fortified pit mud produced abundant fatty acids and ethyl esters in short-term usage. Rare microbes, such as Hazenella coriacea, promoted the production of fatty acids. After long-term usage, changes in physicochemical properties led to a decrease in caproate-producing bacterium, namely Clostridium and Caproicibacterium, and an increase in microbes with limited fatty acid biosynthesis capability, including Proteiniphilum, Fastidiosipila, and Caldicoprobacter. These alterations ultimately led to a decrease in fatty acids and ethyl esters. In summary, Caproicibacterium lactatifermentans inoculation exhibited positive outcomes in obtaining high-quality pit mud. However, the maintenance of functional microbes necessitates further investigation.

Screening ester-producing yeasts to fortify the brewing of rice-flavor Baijiu for enhanced aromas

To enhance the aromas in Guangdong rice-flavor Baijiu, ester-producing yeast was selected to fortify Baijiu brewing. Among eight kinds of ester-producing yeasts selected, Saccharomyces cerevisiae CM15 (CM15) that showed both the stronger ability to utilize substrates to produce esters and the excellent tolerance to industrially relevant stress factors was chosen. When CM15 was synergistically fermented with six kinds of Kojis from distilleries of rice-flavor liquor in Guangdong, the enhanced total esters had happened to the liquors brewing with the fortified four kinds of Kojis, especially with Koji F. When Koji F was fortified with CM15, the resultant Baijiu showed a higher esters proportion and a lower higher alcohol ratio than that of Baijiu brewed only with Koji F, with the content of ethyl acetate and ethyl lactate increasing by 25% and 214%, respectively. This study suggested that CM15 can be used as a functional microorganism to fortify Baijiu brewing, which might also be suitable for other traditional fermented foods.

Mitigation of microbial nitrogen-derived metabolic hazards as a driver for safer alcoholic beverage choices: An evidence-based review and future perspectives

Alcoholic beverages have been enjoyed worldwide as hedonistic commodities for thousands of years. The unique quality and flavor are attributed to the rich microbiota and nutritional materials involved in fermentation. However, the metabolism of these microbiota can also introduce toxic compounds into foods. Nitrogen-derived metabolic hazards (NMH) are toxic metabolic hazards produced by microorganisms metabolizing nitrogen sources that can contaminate alcoholic beverages during fermentation and processing. NMH contamination poses a risk to dietary safety and human health without effective preventive strategies. Existing literature has primarily focused on investigating the causes of NMH formation, detection methods, and abatement techniques for NMH in fermentation end-products. Devising effective process regulation strategies represents a major challenge for the alcoholic beverage industry considering our current lack of understanding regarding the processes whereby NMH are generated, real-time and online detection, and the high degradation rate after NMH formation. This review summarizes the types and mechanisms of nitrogenous hazard contamination, the potential risk points, and the analytical techniques to detect NMH contamination. We discussed the changing patterns of NMH contamination and effective strategies to prevent contamination at different stages in the production of alcoholic beverages. Moreover, we also discussed the advanced technologies and methods to control NMH contamination in alcoholic beverages based on intelligent monitoring, synthetic ecology, and computational assistance. Overall, this review highlights the risks of NMH contamination during alcoholic beverage production and proposes promising strategies that could be adopted to eliminate the risk of NMH contamination.

Potential Risks of Tebuconazole during Wine Fermentation at the Enantiomer Level Based on Multiomics Analysis

The enantioselectivity and potential risks of tebuconazole enantiomers (R-tebuconazole and S-tebuconazole) in wine fermentation were investigated in this study using Cabernet Sauvignon grapes. Tebuconazole was mainly degraded during the alcoholic fermentation stage, and no obvious transformation between R-tebuconazole and S-tebuconazole was observed. Selective degradation between these two enantiomers occurred, with R-tebuconazole degrading faster than S-tebuconazole. The residual tebuconazole inhibits glucose metabolism and the unsaturated fatty acid formation in the wine fermentation system and inhibits gene expression in the late phase of Saccharomycetales, affecting its cell wall formation. Overall, the findings highlight that R-tebuconazole exhibited a higher risk than S-tebuconazole in these processes. These insights are potentially exploitable to understand chiral pesticides at the enantiomer level using multiomics technology in food-processing systems.

Biosynthesis pathways of expanding carbon chains for producing advanced biofuels

Because the thermodynamic property is closer to gasoline, advanced biofuels (C ≥ 6) are appealing for replacing non-renewable fossil fuels using biosynthesis method that has presented a promising approach. Synthesizing advanced biofuels (C ≥ 6), in general, requires the expansion of carbon chains from three carbon atoms to more than six carbon atoms. Despite some specific biosynthesis pathways that have been developed in recent years, adequate summary is still lacking on how to obtain an effective metabolic pathway. Review of biosynthesis pathways for expanding carbon chains will be conducive to selecting, optimizing and discovering novel synthetic route to obtain new advanced biofuels. Herein, we first highlighted challenges on expanding carbon chains, followed by presentation of two biosynthesis strategies and review of three different types of biosynthesis pathways of carbon chain expansion for synthesizing advanced biofuels. Finally, we provided an outlook for the introduction of gene-editing technology in the development of new biosynthesis pathways of carbon chain expansion.

Control of Beany Flavor from Soybean Protein Raw Material in Plant-Based Meat Analog Processing

The development of plant-based meat analogs is currently hindered by the beany flavor generated by raw soybean protein and extrusion processing. Wide concern has led to extensive research on the generation and control of this unwanted flavor, as an understanding of its formation in raw protein and extrusion processing and methods through which to control its retention and release are of great significance for obtaining ideal flavor and maximizing food quality. This study examines the formation of beany flavor during extrusion processing as well as the influence of interaction between soybean protein and beany flavor compounds on the retention and release of the undesirable flavor. This paper discusses ways to maximize control over the formation of beany flavor during the drying and storage of raw materials and methods to reduce beany flavor in products by adjusting extrusion parameters. The degree of interaction between soybean protein and beany compounds was found to be dependent on conditions such as heat treatment and ultrasonic treatment. Finally, future research directions are proposed and prospected. This paper thus provides a reference for the control of beany flavor during the processing, storage, and extrusion of soybean raw materials used in the fast-growing plant-based meat analog industry.

Black soldier fly (Hermetia illucens) larvae as potential feedstock for the biodiesel production: Recent advances and challenges

Black soldier fly larvae (BSFL) Hermetia illucens is fastest growing and most promising insect species especially recommended to bring high-fat content as 5th generation bioenergy. The fat content can be fully optimized during the life-cycle of the BSFL through various organic dietary supplements and environmental conditions. Enriched fat can be obtained during the larval stages of the BSF. The presence of high saturated and unsaturated fatty acids in their body helps to produce 70 % of extractable oil which can be converted into biodiesel through transesterification. The first-generation biodiesel process mainly depends on catalytic transesterification, however, BSFL had 94 % of biodiesel production through non-catalytic transesterification. This increases the sustainability of producing biodiesel with less energy input in the process line. Other carbon emitting factors involved in the rearing of BSFL are less than the other biodiesel feedstocks including microalgae, cooking oil, and non-edible oil. Therefore, this review is focused on evaluating the optimum dietary source to produce fatty acid rich larvae and larval growth to accumulate C16-18 fatty acids in larger amounts from agro food waste. The process of optimization and biorefining of lipids using novel techniques have been discussed herein. The sustainability impact was evaluated from the cultivation to biodiesel conversion with greenhouse gas emissions scores in the entire life-cycle of process flow. The state-of-the-art in connecting circular bioeconomy loop in the search for bioenergy was meticulously covered.

Volatile Aroma Compound Production Is Affected by Growth Rate in S. cerevisiae

The initial growth rate of a yeast strain is a key parameter in the production of fermented beverages. Fast growth is linked with higher fermentative capacity and results in less slow and stuck fermentations unable to reach the expected final gravity. As concentrations of metabolites are in a constant state of flux, quantitative data on how growth rate affects the production of aromatic compounds becomes an important factor for brewers. Chemostats allow to set and keep a specific dilution rate throughout the fermentation and are ideal system to study the effect of growth on aroma production. In this study, we ran chemostats alongside batch and fed-batch cultures, compared volatile profiles detected at different growth rates, and identified those affected by the different feeding profiles. Specifically, we quantified six abundant aroma compounds produced in anaerobic glucose-limited continuous cultivations of S. cerevisiae at different dilution rates. We found that volatile production was affected by the growth rate in four out of six compounds assayed, with higher alcohols and esters following opposite trends. Batch and fed-batch fermentations were devised to study the extent by which the final concentration of volatile compounds is influenced by glucose availability. Compared with the batch system, fed-batch fermentations, where the yeast growth was artificially limited by a slow constant release of nutrients in the media, resulted in a significant increase in concentration of higher alcohols, mirroring the results obtained in continuous fermentations. This study paves the way to further process development optimization for the production of fermented beverages. IMPORTANCE The production of fermentation beverages will need to quickly adapt to changes in both the climate and customer demands, requiring the development of new strains and processes. Breakthroughs in the field are hindered by the limited knowledge on the interplay between physiology and aroma compound production in yeast. No quantitative data on how growth rate affects aroma profile is available in the literature to guide optimization of the complex flavors in fermented beverages. In this study, we exploited the chemostat system, alongside with batch and fed-batch cultures, to compare volatile profiles at different growth rates. We identified the aromatic compounds affected by the different feeding profiles and nutrient limitations. Moreover, we uncovered the correlation between yeast growth, esters, and higher alcohols production. This study showcases the potential of the application of feeding profiles for the manipulation of aroma in the craft beverage industry.

Effect of Fuel Preheating on Engine Characteristics of Waste Animal Fat-Oil Biodiesel in Compression Ignition Engine

The present study aims at understanding the effects of fuel preheating on engine characteristics of waste animal fat-oil (WAF-O) biodiesel in a single-cylinder CI engine, with the preheating technique proposed as an effective means for enhancing the fuel properties. To understand the effects of the preheated fuel, the WAF-O biodiesel was preheated at 60, 80, 100 and 120 °C and tested along with neat diesel and unheated WAF-O biodiesel. For this purpose, biodiesel was produced from different animal wastes by means of KOH-assisted ethanol-based transesterification, reporting its maximum yield as 96.37 ± 1.8%, with significant distribution of unsaturated oleic acid, saturated palmitic acid and stearic acid. Upon evaluating its fuel characteristics as per ASTM D6751 standards, a rise in preheating temperature by 1 °C reduced the density and kinematic viscosity of WAF-O biodiesel by 0.383 kg/m3 and 0.025 mm2/s, respectively, and was explained by the weakening of intermolecular forces between its fatty acid ester molecules. Preheated samples reported superior combustion characteristics by exhibiting increased in-cylinder pressure (2.24%, on average) and heat release rates in addition to their shortened ignition delay (1−4 °CA). Furthermore, preheating of WAF-O biodiesel reduced its specific fuel consumption and increased its brake thermal efficiency by 7.86% (on average) and 9.23% (on average), respectively. However, higher preheating temperatures (>120 °C) resulted in increased fuel consumption owing to its varied flow characteristics. In addition to the changes in combustion characteristics, preheating WAF-O bio-diesel also resulted in reduced carbon monoxide, nitrous oxide and hydrocarbon emission by 13.88%, 7.21% and 26.94%, respectively, and increased carbon dioxide emission by 7.58%. Summing up, the enhancements in overall engine characteristics of preheated samples were accounted for by their improvised fuel injection characteristics due to their reduced density and viscosity, which ensured for their effective combustion.

Rapamycin enhanced the production of 2-phenylethanol during whole-cell bioconversion by yeast

2-Phenylethanol (2-PE), a higher alcohol with a rose-like odor, has been widely utilized in food, perfume, and beverages. Saccharomyces cerevisiae is one of the most promising microorganisms for the biosynthesis of natural 2-PE. However, the growth of S. cerevisiae is generally inhibited by 2-PE, which makes its production in yeast cell factories challenging. Here, the whole-cell bioconversion was used to avert growth inhibition, leading to an increase in the concentration and productivity of 2-PE. Moreover, rapamycin (Rap) addition further improved the efficiency of 2-PE synthesis. The concentration of 2-PE (2.20 g/L) was 1.68-fold higher than that in the absence of Rap during the whole-cell bioconversion by S. cerevisiae BY4741. RT-qPCR results showed that Rap addition increased the transcription of ARO9, ARO10, ADH2, GAP1, ARO80, GLN3, and GDH2. When the GLN3 was knocked out, the transcriptional levels of the genes were dramatically decreased, and the concentration of 2-PE significantly decreased to 0.21 g/L. The results indicated that Rap enhanced the flux of the Ehrlich pathway, and Gln3 exerted a central role in the regulation of Rap. Furthermore, commercial yeast (S. cerevisiae FY202001) was selected to verify the applicability of Rap. In the presence of Rap, 3.67 g/L 2-PE was obtained by whole-cell bioconversion in flask, which was increased by 9% than that in the absence of Rap. Finally, the 2-PE titer reached 4.93 g/L by whole-cell bioconversion in a 5 L bioreactor, with a yield of 84 mol% from L-phenylalanine and a productivity of 0.103 g/L h, which was far higher than that of the currently reported in S. cerevisiae. These findings provided a new idea for the efficient synthesis of 2-PE. KEY POINTS: • Whole-cell bioconversion was used to produce 2-PE. • The regulation of the Ehrlich pathway by Rap provides a theoretical basis for developing an effective yeast cell factory to produce 2-PE. • The 2-PE productivity of 0.103 g/L h is far higher than that of the currently reported in S. cerevisiae .

Microbial Communities and Flavor Compounds during the Fermentation of Traditional Hong Qu Glutinous Rice Wine

As a traditional Chinese rice wine, Hong Qu glutinous rice wine (HQW) is popular among consumers due to its unique flavor. However, its quality changes during fermentation, and the potential relationships between flavor and microbes have not been systematically researched. In this work, physicochemical properties (pH, total sugar, alcohol, amino acid nitrogen), flavor compounds (organic acids, free amino acids, and volatile compounds), and microbial communities were investigated. The results revealed that Pantoea, Lactiplantibacillus, Lactobacillus, Leuconostoc, and Weissella predominated the bacterial genera, and Monascus was the predominant fungal genus. Organic acids, free amino acids, and key volatile compounds (esters and alcohols) significantly increased during fermentation. The correlations analysis showed that Lactiplantibacillus was closely associated with flavor compounds formation. This study deepens our understanding of the roles of microorganisms in flavor formation on traditional HQW fermentation.

Comparative Investigation of the Physicochemical Properties of Chars Produced by Hydrothermal Carbonization, Pyrolysis, and Microwave-Induced Pyrolysis of Food Waste

This work presents a comparative study of the physicochemical properties of chars derived by three thermochemical pathways, namely: hydrothermal carbonization, HTC (at 180, 200 and 220 °C), pyrolysis, PY, (at 500, 600 and 700 °C) and microwave assisted pyrolysis, MW (at 300, 450 and 600 W). The mass yield of HTC samples showed a decrease (78.7 to 26.7%) as the HTC temperature increased from 180 to 220 °C. A similar decreasing trend in the mass yield was also observed after PY (28.45 to 26.67%) and MW (56.45 to 22.44%) of the food waste mixture from 500 to 700 °C and 300 to 600 W, respectively. The calorific value analysis shows that the best among the chars prepared by three different heating methods may be ranked according to the decreasing value of the heating value as: PY500, MW300, and HTC180. Similarly, a decreasing trend in H/C values was observed as: PY500 (0.887), MW300 (0.306), and HTC180 (0.013). The scanning electron microscope (SEM) analyses revealed that the structure of the three chars was distinct due to the different temperature gradients provided by the thermochemical processes. The results clearly show that the suitable temperature for the HTC and PY of food waste was 180 °C and 500 °C, respectively, while the suitable power for the MW of food waste was 300 W.

Characterization of the flavor and nutritional value of coconut water vinegar based on metabolomics

Tender Coconut water is popular for its deliciousness and nutrition. Mature coconut water, usually discarded as waste in the coconut kernel-based food industry due to its unpleasant flavor, was used as a raw material to make vinegar by liquid-state fermentation. The compounds in fresh coconut water with high odor activity values (OAVs) were isovaleric acid and acetic acid, with pungent sour tastes. The compounds with high OAVs in aged coconut water vinegar were phenylethyl acetate, isoamyl acetate and benzaldehyde, with almond, banana or pear-like aromas. Coconut water vinegar was rich in essential amino acids, especially phenylalanine. Through pathway analysis, seventeen key metabolic pathways and three key metabolic substrates (aspartate, glutamate and pyruvate) were found. According to sensory evaluation, the aged vinegar tastes better. Coconut water vinegar is delicious and nutritious, so reprocessing mature coconut water into vinegar is an appropriate way to reuse waste coconut water.

High-throughput integrated pretreatment strategies to convert high-solid loading microalgae into high-concentration biofuels

The commercial feasibility of energy-efficient conversion of highly concentrated microalgal suspensions to produce high-titer biofuels is a major bottleneck due to high energy consumption. Herein, high-titer biofuels (bioethanol, higher-alcohols, and biodiesel) were generated from carbohydrate-rich Chlamydomonas mexicana and lipid-rich Chlamydomonas pitschmannii biomass through energy-saving microwave pretreatment, successive fermentation, and transesterification. Microwave pretreatment needed low specific energy (4.2 MJ/kg) for 100 g/L of microalgal suspension. Proposed sustainable integrated pretreatments method achieved unprecedented total conversion efficiency (67%) and highest biomass utilization (87%) of C. pitschmannii (100 g/L) with high yields of bioethanol (0.48 g-ethanol/g-carbohydrates), higher-alcohols (0.44 g-higher-alcohols/g-proteins), and biodiesel (0.90 g-biodiesel/g-lipids). Transmission electron microscopy showed the changes in the microalgal cellular integrity before and after sequential fermentations. Energy-efficient integrated pretreatments enhanced the extraction efficiency and whole utilization of high-concentration microalgae to generate high-titer biofuels with minimum waste production.

Identification of key residues for efficient glucose transport by the hexose transporter CgHxt4 in high sugar fermentation yeast Candida glycerinogenes

Efficient hexose transporters are essential for the development of industrial yeast strains with high fermentation performance. We previously identified a hexose transporter, CgHxt4, with excellent sugar uptake performance at ultra-high glucose concentrations (200 g/L) in the high sugar fermenting yeast C. glycerinogenes. To understand the working mechanism of this transporter, we constructed 87 mutants and examined their glucose uptake performance. The results revealed that five residues (N321, N322, F325, G426, and P427) are essential for the efficient glucose transport of CgHxt4. Subsequently, we focused our analysis on the roles of N321 and P427. Specifically, N321 and P427 are likely to play a role in glucose coordination and conformational flexibility, respectively. Our results help to expand the application potential of this transporter and provide insights into the working mechanism of yeast hexose transporter. KEY POINTS: • Five residues, transmembrane segments 7 and 10, were found to be essential for CgHxt4. • N321 and P427 are likely to play a role in glucose coordination and conformational flexibility, respectively. • Chimeric CgHxt5.4TM7 significantly enhanced the performance of CgHxt5.

Protein biomethanation: insight into the microbial nexus

Biomethanation of carbohydrates (e.g., lignocellulosic biomass) and lipids (e.g., waste oils) has been well studied. However, investigations on the biomethanation of protein-rich biowastes (PRBs) and associated microbial communities have not been reported. This review summarizes the challenges in the metabolic process of anaerobic digestion of PRBs and the microbial instability associated with it. We discuss the diversity of bacterial and archaeal communities via metagenomics under PRB mono- and codigestion. A stable community structure with enhanced metabolic activity is a core factor in PRB biomethanation. The application of strategies such as codigestion of PRBs with carbon-rich biomass and microbial stimulation/augmentation would make PRB biomethanation more feasible.

GAT1 Gene, the GATA Transcription Activator, Regulates the Production of Higher Alcohol during Wheat Beer Fermentation by Saccharomyces cerevisiae

Uncoordinated carbon-nitrogen ratio in raw materials will lead to excessive contents of higher alcohols in alcoholic beverages. The effect of GAT1 gene, the GATA transcription activator, on higher alcohol biosynthesis was investigated to clarify the mechanism of Saccharomyces cerevisiae regulating higher alcohol metabolism under high concentrations of free amino nitrogen (FAN). The availability of FAN by strain SDT1K with a GAT1 double-copy deletion was 28.31% lower than that of parent strain S17, and the yield of higher alcohols was 33.91% lower. The transcript levels of the downstream target genes of GAT1 and higher alcohol production in the double-copy deletion mutant suggested that a part of the effect of GAT1 deletion on higher alcohol production was the downregulation of GAP1, ARO9, and ARO10. This study shows that GATA factors can effectively regulate the metabolism of higher alcohols in S. cerevisiae and provides valuable insights into higher alcohol biosynthesis, showing great significance for the wheat beer industry.

Yeast Fermentation at Low Temperatures: Adaptation to Changing Environmental Conditions and Formation of Volatile Compounds

Yeast plays a key role in the production of fermented foods and beverages, such as bread, wine, and other alcoholic beverages. They are able to produce and release from the fermentation environment large numbers of volatile organic compounds (VOCs). This is the reason for the great interest in the possibility of adapting these microorganisms to fermentation at reduced temperatures. By doing this, it would be possible to obtain better sensory profiles of the final products. It can reduce the addition of artificial flavors and enhancements to food products and influence other important factors of fermented food production. Here, we reviewed the genetic and physiological mechanisms by which yeasts adapt to low temperatures. Next, we discussed the importance of VOCs for the food industry, their biosynthesis, and the most common volatiles in fermented foods and described the beneficial impact of decreased temperature as a factor that contributes to improving the composition of the sensory profiles of fermented foods.

Identification of Core Regulatory Genes and Metabolic Pathways for the n-Propanol Synthesis in Saccharomyces cerevisiae

The n-propanol produced by Saccharomyces cerevisiae has a remarkable effect on the taste and flavor of Chinese Baijiu. The n-propanol metabolism-related genes were deleted to evaluate the role in the synthesis of n-propanol to ascertain the key genes and pathways for the production of n-propanol by S. cerevisiae. The results showed that CYS3, GLY1, ALD6, PDC1, ADH5, and YML082W were the key genes affecting the n-propanol metabolism in yeast. The n-propanol concentrations of α5ΔGLY1, α5ΔCYS3, and α5ΔALD6 increased by 121.75, 22.75, and 17.78%, respectively, compared with α5. The n-propanol content of α5ΔPDC1, α5ΔADH5, and α5ΔYML082W decreased by 24.98, 8.35, and 8.44%, respectively, compared with α5. The contents of intermediate metabolites were measured, and results showed that the mutual transformation of glycine and threonine in the threonine pathway and the formation of propanal from 2-ketobutyrate were the core pathways for the formation of n-propanol. Additionally, YML082W played important role in the synthesis of n-propanol by directly producing 2-ketobutyric acid through l-homoserine. This study provided valuable insights into the n-propanol synthesis in S. cerevisiae and the theoretical basis for future optimization of yeast strains in Baijiu making.

iTRAQ-based proteomic analysis reveals the molecule mechanism of reducing higher alcohols in Chinese rice wine by nitrogen compensation

Purpose

Higher alcohol is a by-product of the fermentation of wine, and its content is one of the most important parameters that affect and are used to appraise the final quality of Chinese rice wine. Ammonium compensation is an efficient and convenient method to reduce the content of higher alcohols, but the molecule mechanism is poorly understood. Therefore, an iTRAQ-based proteomic analysis was designed to reveal the proteomic changes of Saccharomyces cerevisiae to elucidate the molecular mechanism of ammonium compensation in reducing the content of higher alcohols.

Methods

The iTRAQ proteomic analysis method was used to analyze a blank group and an experimental group with an exogenous addition of 200 mg/L (NH4)2HPO4 during inoculation. The extracted intracellular proteins were processed by liquid chromatography-mass spectrometry and identified using bioinformatics tools. Real-time quantitative polymerase chain reaction was used to verify the gene expression of differentially expressed proteins.

Results

About 4062 proteins, including 123 upregulated and 88 downregulated proteins, were identified by iTRAQ-based proteomic analysis. GO and KEGG analysis uncovered that significant proteins were concentrated during carbohydrate metabolism, such as carbon metabolism, glyoxylate, and dicarboxylate metabolism, pyruvate metabolism, and the nitrogen metabolism, such as amino acid synthesis and catabolism pathway. In accordance with the trend of differential protein regulation in the central carbon metabolism pathway and the analysis of carbon metabolic flux, a possible regulatory model was proposed and verified, in which ammonium compensation facilitated glucose consumption, regulated metabolic flow direction into tricarboxylic acid, and further led to a decrease in higher alcohols. The results of RT-qPCR confirmed the authenticity of the proteomic analysis results at the level of gene.

Conclusion

Ammonium assimilation promoted by ammonium compensation regulated the intracellular carbon metabolism of S. cerevisiae and affected the distribution of metabolic flux. The carbon flow that should have gone to the synthesis pathway of higher alcohols was reversed to the TCA cycle, thereby decreasing the content of higher alcohols. These findings may contribute to an improved understanding of the molecular mechanism for the decrease in higher alcohol content through ammonium compensation.

Metabolic Detoxification of 2-Oxobutyrate by Remodeling Escherichia coli Acetate Bypass

2-Oxobutyrate (2-OBA), as a toxic metabolic intermediate, generally arrests the cell growth of most microorganisms and blocks the biosynthesis of target metabolites. In this study, we demonstrated that using the acetate bypass to replace the pyruvate dehydrogenase complex (PDHc) in Escherichia coli could recharge the intracellular acetyl-CoA pool to alleviate the metabolic toxicity of 2-OBA. Furthermore, based on the crystal structure of pyruvate oxidase (PoxB), two candidate residues in the substrate-binding pocket of PoxB were predicted by computational simulation. Site-directed saturation mutagenesis was performed to attenuate 2-OBA-binding affinity, and one of the variants, PoxB^F112W, exhibited a 20-fold activity ratio of pyruvate/2-OBA in substrate selectivity. PoxB^F112W was employed to remodel the acetate bypass in E. coli, resulting in l-threonine (a precursor of 2-OBA) biosynthesis with minimal inhibition from 2-OBA. After metabolic detoxification of 2-OBA, the supplies of intracellular acetyl-CoA and NADPH (nicotinamide adenine dinucleotide phosphate) used for l-threonine biosynthesis were restored. Therefore, 2-OBA is the substitute for pyruvate to engage in enzymatic reactions and disturbs pyruvate metabolism. Our study makes a straightforward explanation of the 2-OBA toxicity mechanism and gives an effective approach for its metabolic detoxification.

Genetic engineering of an industrial yeast Candida glycerinogenes for efficient production of 2-phenylethanol

Microbial cell factories offer an economic approach for synthesizing "natural'" aromatic flavor compounds. During their fermentation process, the inefficient synthesis pathway and product cytotoxicity are the major barriers to the high-level production. This study combined metabolic engineering and tolerance engineering strategies to maximize the valuable rose-smell 2-phenylethanol (2-PE) production in Candida glycerinogenes, a GRAS diploid industrial yeast. Firstly, 2-PE metabolic networks involved in Ehrlich pathway were stepwise rewired using metabolic engineering, including the following: (1) overexpressing L-phenylalanine permease Aap9 enhanced precursor uptake; (2) overexpressing enzymes (aminotransferase Aro9 and decarboxylase Aro10) of Ehrlich pathway increased catalytic efficiency; and (3) disrupting the formation of by-product phenylacetate catalyzed by Ald2 and Ald3 maximized the metabolic flux toward 2-PE. Then, tolerance engineering was applied by overexpression of a stress-inducible gene SLC1 in the metabolically engineered strain to further enhance 2-PE production. Combining these two approaches finally resulted in 5.0 g/L 2-PE in shake flasks, with productivity reaching 0.21 g/L/h, which were increased by 38.9% and 177% compared with those of the non-engineered strain, respectively. The 2-PE yield of this engineered strain was 0.71 g/g L-phenylalanine, corresponding to 95.9% of theoretical yield. This study provides a reference to efficiently engineering of microbial cell factories for other valuable aromatic compounds. KEY POINTS: • Metabolic engineering improved 2-PE biosynthesis. • Tolerance engineering alleviated product inhibition, contributing to 2-PE production. • The best strain produced 5.0 g/L 2-PE with 0.959 mol/mol yield and high productivity.

Changes in the Composition of Methylxanthines, Polyphenols, and Volatiles and Sensory Profiles of Cocoa Beans from the Sul 1 Genotype Affected by Fermentation

The Sul 1 cacao (Theobroma cacao) genotype has become more popular among cocoa producers due to its resistance toward vascular streak dieback, a devastating dieback disease in cacao plants. Sul 1 cocoa beans were subjected to prolonged fermentation of 10 days (240 h). Changes in the composition of methylxanthines, polyphenols, and volatiles and sensory profiles of the cocoa beans as affected by fermentation were analyzed. The first 48 h of fermentation significantly determined the composition of the methylxanthines and polyphenols in the cocoa beans. A prolonged fermentation (>96 h) period did not further reduce the contents of methylxanthines and polyphenols in the cocoa beans. Important volatiles characteristic of the cocoa/chocolate flavor were mostly developed after 48 h of fermentation. The fermentation for 72 h retained considerable amounts of methylxanthines and polyphenols in the beans while producing cocoa mass with good sensory profiles.

Energy-efficient pretreatments for the enhanced conversion of microalgal biomass to biofuels

The physiological properties, including biochemical composition and cell wall thickness, of microalgal species have a remarkable effect on the pretreatment of biomass and its further conversion to biofuels. In the present study, multiple biofuels (bioethanol, higher alcohols (C3-C5), and biodiesel) were produced using energy-efficient microwave pretreatment, successive carbohydrate/protein fermentation, and lipid transesterification from three microalgal strains (Pseudochlorella sp., Chlamydomonas mexicana, and Chlamydomonas pitschmannii). The microwave pretreatment method required the lowest specific energy (5 MJ/kg) compared to ultrasound pretreatment. The proposed integrated approach achieved high conversion efficiency (46%) and maximum biomass utilization (93%) of C. mexicana with improved yields of bioethanol (0.46 g-ethanol/g-carbohydrates), higher alcohols (0.44 g-higher alcohols/g-proteins), and biodiesel (0.74 g-biodiesel/g-lipids). This study suggests that the application of an appropriate pretreatment method for microalgal strains having different physiological properties is essential for improving the extraction efficiency and conversion of biomass to biofuels with less waste production.

Biocomponent-based microalgal transformations into biofuels during the pretreatment and fermentation process

Microalgal cell wall integrity and composition have a significant impact on the fermentation process and biofuel recovery. In this study, various biofuels (bioethanol, higher alcohols (C3-C5), and biodiesel) were produced by the fermentation of carbohydrates and proteins, and transesterification of lipids from three different microalgal strains (Pseudochlorella sp., Chlamydomonas mexicana, and Chlamydomonas pitschmannii), each possessing different proportions of bioconstituents (carbohydrates, proteins, and lipids). Changes in the cell wall structure and thickness were observed before and after fermentation using transmission electron microscopy. Pseudochlorella sp. showed the highest yields of bioethanol (0.45 g-ethanol/g-carbohydrates), higher alcohols (0.44 g-higher alcohols/g-proteins), and biodiesel (0.55 g-biodiesel/g-lipids), which consequently revealed a maximum energy recovery (42%) from whole constituents. This study suggests that different physiological properties, including cell wall thickness and the proportion of bioconstituents in microalgae, could have a significant impact on the pretreatment and fermentation efficiencies for biofuels production.

Protein-based biorefining driven by nitrogen-responsive transcriptional machinery

BACKGROUND

Protein-based bioconversion has been demonstrated as a sustainable approach to produce higher alcohols and ammonia fertilizers. However, owing to the switchover from transcription mediated by the bacterial RNA polymerase σ⁷⁰ to that mediated by alternative σ factors, the biofuel production driven by σ⁷⁰-dependent promoters declines rapidly once cells enter the stationary phase or encounter stresses. To enhance biofuel production, in this study the growth phase-independent and nitrogen-responsive transcriptional machinery mediated by the σ⁵⁴ is exploited to drive robust protein-to-fuel conversion.

RESULTS

We demonstrated that disrupting the Escherichia coli ammonia assimilation pathways driven by glutamate dehydrogenase and glutamine synthetase could sustain the activity of σ⁵⁴-mediated transcription under ammonia-accumulating conditions. In addition, two σ⁵⁴-dependent promoters, argTp and glnAp2, were identified as suitable candidates for driving pathway expression. Using these promoters, biofuel production from proteins was shown to persist to the stationary phase, with the net production in the stationary phase being 1.7-fold higher than that derived from the optimal reported σ⁷⁰-dependent promoter P _LlacO₁. Biofuel production reaching levels 1.3- to 3.4-fold higher than those of the σ⁷⁰-dependent promoters was also achieved by argTp and glnAp2 under stressed conditions. Moreover, the σ⁵⁴-dependent promoters realized more rapid and stable production than that of σ⁷⁰-dependent promoters during fed-batch fermentation, producing up to 4.78 g L ^- 1 of total biofuels.

CONCLUSIONS

These results suggested that the nitrogen-responsive transcriptional machinery offers the potential to decouple production from growth, highlighting this system as a novel candidate to realize growth phase-independent and stress-resistant biofuel production.

Evaluation of Infrared Radiation Combined with Hot Air Convection for Energy-Efficient Drying of Biomass

Cost-effective biomass drying is a key challenge for energy recovery from biomass by direct combustion, gasification, and pyrolysis. The aim of the present study was to optimize the process of biomass drying using hot air convection (HA), infrared (IR), and combined drying systems (IR-HA). The specific energy consumption (SEC) decreased significantly by increasing the drying temperature using convective drying, but higher air velocities increased the SEC. Similarly, increasing air velocity in the infrared dryer resulted in a significant increase in SEC. The lowest SEC was recorded at 7.8 MJ/kg at an air velocity of 0.5 m/s and an IR intensity of 0.30 W/cm2, while a maximum SEC (20.7 MJ/kg) was observed at 1.0 m/s and 0.15 W/cm2. However, a significant reduction in the SEC was noticed in the combined drying system. A minimum SEC of 3.8 MJ/kg was recorded using the combined infrared-hot air convection (IR-HA) drying system, which was 91.7% and 51.7% lower than convective and IR dryers, respectively. The present study suggested a combination of IR and hot air convection at 60 °C, 0.3 W/cm2 and 0.5 m/s as optimum conditions for efficient drying of biomass with a high water content.