Increase ethyl acetate production in Saccharomyces cerevisiae by genetic engineering of ethyl acetate metabolic pathway

Precision fermentation for improving the quality, flavor, safety, and sustainability of foods

Precision fermentation involves the rewiring of metabolic pathways in generally recognized as safe microorganisms, fermentation scale-up, and downstream processing to produce food ingredients from abundant and inexpensive substrates. Using precise genome editing of food-fermenting microorganisms, precision fermentation can also produce fermented foods with more desirable properties. These genetic tools allow for the manipulation of flavors and nutritional content in fermented foods, the economic production of functional food ingredients, and the sustainable production of otherwise-costly macronutrients. By introducing the metabolic designs, genetic modifications, and resulting products of engineered microorganisms developed through academic and industrial research, this review aims to provide insights into the potentials and challenges of precision fermentation for the economic, safe, and sustainable production of foods.

Analysis of ester-producing performance in high-yield ethyl hexanoate yeast and the effect on metabolites in bio- enhanced Daqu, the starter for Baijiu and other traditional fermented foods

AIMS

Ethyl hexanoate, one of the key flavor compounds in strong-flavor Baijiu. To improve the content of ethyl hexanoate in strong-flavor Baijiu, a functional strain with high yield of ethyl hexanoate was screened and its ester-producing performance was studied.

METHODS AND RESULTS

Upon identification, the strain was classified as Candida sp. and designated as ZY002. Under optimal fermentation conditions, the content of ethyl hexanoate synthesized by ZY002 can be as high as 170.56 mg L-1. A fermentation test was carried out using the ZY002 strain bioaugmented Daqu to verify the role of the strain applied to Baijiu brewing. It was found that strain ZY002 could not only improve the moisture and alcohol contents of fermented grains but also diminish the presence of reducing sugar and crude starch. Furthermore, it notably amplified the abundance of flavor compounds.

CONCLUSION

In this study, Candida sp. ZY002 with a high yield of ethyl hexanoate provided high-quality strain resources for the actual industrial production of Baijiu.

Metabolic engineering of Saccharomyces cerevisiae for the synthesis of valuable chemicals

In the twenty first century, biotechnology offers great opportunities and solutions to climate change mitigation, energy and food security and resource efficiency. The use of metabolic engineering to modify microorganisms for producing industrially significant chemicals is developing and becoming a trend. As a famous, generally recognized as a safe (GRAS) model microorganism, Saccharomyces cerevisiae is widely used due to its excellent operational convenience and high fermentation efficiency. This review summarizes recent advancements in the field of using metabolic engineering strategies to construct engineered S. cerevisiae over the past ten years. Five different types of compounds are classified by their metabolites, and the modified metabolic pathways and strategies are summarized and discussed independently. This review may provide guidance for future metabolic engineering efforts toward such compounds and analogues. Additionally, the limitations of S. cerevisiae as a cell factory and its future trends are comprehensively discussed.

Unravelling Metabolic Heterogeneity of Chinese Baijiu Fermentation in Age-Gradient Vessels

Fermentation vessels affect the characteristics of food fermentation; however, we lack an approach to identify the biomarkers indicating fermentation. In this study, we applied metabolomics and high-throughput sequencing analysis to reveal the dynamic of metabolites and microbial communities in age-gradient fermentation vessels for baijiu production. Furthermore, we identified 64 metabolites during fermentation, and 19 metabolites significantly varied among the three vessels (p < 0.05). Moreover, the formation of these 19 metabolites were positively correlated with the core microbiota (including Aspergillus, Saccharomyces, Lactobacillus, and Bacillus). In addition, ethyl lactate or ethyl acetate were identified as the biomarkers for indicating the metabolism among age-gradient fermentation vessels by BP-ANN (R2 > 0.40). Therefore, this study combined the biological analysis and predictive model to identify the biomarkers indicating metabolism in different fermentation vessels, and it also provides a potential approach to assess the profiling of food fermentations.

Effective hydrolysis for waste plant biomass impacts sustainable fuel and reduced air pollution generation: A comprehensive review

Among various natural biowastes availability in the environment, agricultural residues showed great impacts. It is due to huge availability and cheap carbon source, creating big challenges for their utility and systematic reduction. Objective of this review is to address the waste biomass availability and huge quantities issues and also put effort to minimize this nutrient load via biotransforming into value-added products. Different wastes (organic/inorganic) generation with their negative issues are due to numbers of developmental and social activities, reported. Currently, various efforts are found for these wastes minimization via generation of different types of value-added products (biogas, bioH₂, alcoholic fuel, organic acids and others products) and these wastes in municipal cities are also reported with production of advanced biofuels as promising outcomes. For hydrolysis of complex organic resources including lignocellulosic biomasses, physicochemical, structural or compositional changes are needed that aid in conversion into sugar and organic compounds such as biofuels. So, efficient and effective pretreatment processes selection (physical, biological, chemical or combined one) is critical to achieve these hydrolysis goals and resultant cellulose or hemicellulose components can be accessible by biological catalysis. These can achieve final hydrolysis and fermentative or monomer sugars. And later, synthesis of fuels or value-added products during microbial fermentation or biotransformation processes can be achieved. This review discusses pretreatment techniques for improved hydrolysis for fermentative sugar with emphasis on reduced quantities of toxic compounds (furfural compound) in hydrolyzed biomasses. Minimum deterioration fuel economy also reported with production of different bioproducts including biofuels. Additionally, impacts of toxic products and gasses emission are also discussed with their minimization.

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.

Strategies for optimizing acetyl-CoA formation from glucose in bacteria

Acetyl CoA is an important precursor for various chemicals. We provide a metabolic engineering guideline for the production of acetyl-CoA and other end products from a bacterial chassis. Among 13 pathways that produce acetyl-CoA from glucose, 11 lose carbon in the process, and two do not. The first 11 use the Embden-Meyerhof-Parnas (EMP) pathway to produce redox cofactors and gain or lose ATP. The other two pathways function via phosphoketolase with net consumption of ATP, so they must therefore be combined with one of the 11 glycolytic pathways or auxiliary pathways. Optimization of these pathways can maximize the theoretical acetyl-CoA yield, thereby minimizing the overall cost of subsequent acetyl-CoA-derived molecules. Other strategies for generating hyper-producer strains are also addressed.

Analysis of Saccharification Products of High-Concentration Glutinous Rice Fermentation by Rhizopus nigricans Q3 and Alcoholic Fermentation of Saccharomyces cerevisiae GY-1

A two-stage process was used to prepare rice alcohol, i.e., saccharification of glutinous rice by Rhizopus nigricans Q3, followed by Saccharomyces cerevisiae's fermentation. Rhizopus nigricans Q3 was cultured during the saccharification stage, and Saccharomyces cerevisiae GY-1 was added in the fermentation stage. Total sugar content and reducing sugar content in these two stages were analyzed. The relationship between the production proportion and consumption of the reducing sugar in the saccharification interval was analyzed using reducing sugar indices. It is an important rule that the high-concentration syrup and oligosaccharides prepared by glutinous rice could reach 42°Bx and 250 mg/mL by high-concentration fermentation in the growth stage of R. nigricans Q3.

Metabolic Engineering of Saccharomyces cerevisiae for Ethyl Acetate Biosynthesis

Ethyl acetate can be synthesized from acetyl-CoA and ethanol via a reaction by alcohol acetyltransferases (AATase) in yeast. In order to increase the yield of acetyl-CoA, different terminators were used to optimize the expressions of acetyl-CoA synthetase (ACS1/2) and aldehyde dehydrogenase (ALD6) to increase the contents of acetyl-CoA in Saccharomyces cerevisiae. ATF1 coding AATase was coexpressed in expression cassettes of ACS1/ACS2 and ALD6 to promote the carbon flux toward ethyl acetate from acetyl-CoA. Further to improve ethyl acetate production, four heterologous AATase including HuvEAT1 (Hanseniaspora uvarum), KamEAT1 (Kluyveromyces marxianus), VAAT (wild strawberry), and AeAT9 (kiwifruit) were introduced. Subsequently mitochondrial transport and utilization of pyruvate and acetyl-CoA were impeded to increase the ethyl acetate accumulation in cytoplasm. Under the optimal fermentation conditions, the engineered strain of PGAeΔPOR2 produced 1.69 g/L ethyl acetate, which was the highest value reported to date by metabolic engineering methods.

Perspectives for the microbial production of ethyl acetate

Ethyl acetate is one of the short-chain esters and widely used in the food, beverage, and solvent areas. The ethyl acetate production currently proceeds through unsustainable and energy intensive processes, which are based on natural gas and crude oil. Microbial conversion of biomass-derived sugars into ethyl acetate may provide a sustainable alternative. In this review, the perspectives of bio-catalyzing ethanol and acetic acid to ethyl acetate using lipases in vitro was introduced. Besides, the crucial elements for high yield of ethyl acetate in fermentation was expounded. Also, metabolic engineering in yeasts to product ethyl acetate in vivo using alcohol acyl transferases (AAT) was discussed. KEY POINTS: •The accumulation of acetyl-CoA is crucial for synthesizing ethyl acetate in vivo; AAT-mediated metabolic engineering could efficiently improve ethyl acetate production.

Towards renewable flavors, fragrances, and beyond

Esters constitute a large space of unique molecules with broad range of applications as flavors, fragrances, pharmaceuticals, cosmetics, green solvents, and advanced biofuels. Global demand of natural esters in food, household cleaner, personal care, and perfume industries is increasing while the ester supply from natural sources has been limited. Development of novel microbial cell factories for ester production from renewable feedstocks can potentially provide an alternative and sustainable source of natural esters and hence help fulfill growing demand. Here, we highlight recent advances in microbial production of esters and provide perspectives for improving its economic feasibility. As the field matures, microbial ester production platforms will enable renewable and sustainable production of flavors and fragrances, and open new market opportunities beyond what nature can offer.