2026 Volume 14 Issue 3 Pages 37-63
The demand for natural flavor compounds, or bioflavors, continues to rise as consumers seek clean-label and sustainable ingredients. Microbial fermentation and biosynthesis offer environmentally friendly and scalable alternatives to traditional extraction and chemical synthesis. This review summarizes recent progress in microbial production of bioflavors, focusing on key microorganisms, including Lactobacillus, Saccharomyces, Pseudomonas, and engineered strains of Escherichia coli and Corynebacterium glutamicum. The biosynthetic routes for major flavor classes, including esters, alcohols, aldehydes, ketones, and lactones, are examined with emphasis on metabolic precursors, enzymatic steps, and regulatory controls. Advances in synthetic biology, metabolic engineering, and bioprocess optimization that improve flavor yield, selectivity, and industrial feasibility are also discussed. Remaining challenges related to substrate cost, downstream processing, and regulatory approval are highlighted. Overall, this review provides an updated perspective on microbial bioflavor production and its expanding relevance to the food, beverage, and fragrance sectors.
Flavor is a primary determinant of consumer acceptance in foods and beverages, influencing both sensory perception and purchasing decisions [1, 2]. Traditionally, flavor compounds such as esters, alcohols, aldehydes, and terpenes have been derived from plant and animal sources or synthesized chemically [3, 4]. However, increasing consumer awareness of health and sustainability, combined with regulatory restrictions on synthetic additives, has accelerated the demand for natural, clean-label flavoring agents. In this context, microbial fermentation has emerged as a promising alternative to produce natural flavors, commonly referred to as bioflavors [5, 6].
Bioflavors encompass a wide range of volatile and non-volatile compounds generated by microorganisms either through native metabolic activities or engineered biosynthetic pathways. This approach leverages the inherent metabolic diversity of bacteria, yeasts, and filamentous fungi to convert renewable, low-cost substrates—such as sugars, amino acids, and agro-industrial residues into high-value flavor molecules. Microorganisms including Lactobacillus spp., Saccharomyces cerevisiae, Kluyveromyces marxianus, and Corynebacterium glutamicum are widely recognized for their ability to produce key flavor compounds, including fruity esters, buttery diacetyl, organic acids, and terpenoid derivatives [6, 7, 8].
Recent progress in metabolic engineering, synthetic biology, and fermentation process optimization has significantly enhanced the efficiency, selectivity, and scalability of microbial flavor production. These advances enable precise pathway reconstruction, flux redirection, and strain improvement for the synthesis of complex or unstable flavor compounds that are difficult to obtain through conventional methods [7, 8]. Moreover, the use of microorganisms with Generally Recognized as Safe (GRAS) status strengthens regulatory compliance and facilitates industrial adoption [9, 10].
Despite these advances, current literature on bioflavor production remains largely descriptive, often organized around specific microorganisms or product categories without systematically linking metabolic mechanisms to flavor outcomes. This review addresses this critical gap by introducing a mechanism-driven, integrative framework that explicitly connects central and secondary metabolic pathways such as amino acid catabolism, lipid metabolism, carbohydrate overflow metabolism, and specialized metabolite biosynthesis with distinct classes of flavor compounds across diverse microbial systems. In addition, this work develops a functional mapping of key microbial taxa and their biosynthetic capacities, emphasizing cross-kingdom metabolic interactions that are frequently overlooked in conventional analyses.
Furthermore, this review provides a critical comparison between traditional fermentation processes and engineered microbial platforms, highlighting inherent trade-offs in flavor complexity, process controllability, and industrial scalability. By incorporating insights from omics-based studies and metabolic regulation, this work advances a translational perspective for precision bioflavor design. This integrative approach not only offers conceptual novelty but also establishes a strategic foundation for bridging fundamental metabolic understanding with practical applications in industrial fermentation. Overall, this review comprehensively summarizes recent developments in microbial bioflavor production, including major flavor classes, microbial hosts, biosynthetic mechanisms, and enabling technologies. It also identifies key challenges and future opportunities for integrating bioflavor systems into sustainable food production and advanced flavor formulation strategies.
This review adopted a systematic approach to identify, evaluate, and synthesize literature on microbial fermentation and biosynthetic pathways for bioflavor production. The aim was to capture recent advances in microbial hosts, metabolic routes, biotechnological tools, and industrial applications related to natural flavor synthesis. A structured search was performed across Scopus, PubMed, Web of Science, and Google Scholar for publications from 2000 to 2025. This timeframe was deliberately selected to reflect: (i) key technological milestones, including the rise of omics technologies, metabolic engineering, and synthetic biology since the early 2000s; (ii) increased availability of high-quality and accessible datasets, particularly with the expansion of genomic and metabolomic databases; and (iii) the rapid evolution of bioflavor research from traditional fermentation-based approaches toward precision biosynthesis using engineered microbial systems.
Search terms included combinations of: “bioflavor”, “natural flavor”, “volatile flavor compounds”, “microbial fermentation”, “microbial biosynthesis”, “metabolic engineering”, “synthetic biology”, “yeast flavor production”, “lactic acid bacteria flavor”, and “engineered microbes flavor synthesis”. Additional relevant sources were identified through citation tracking. Inclusion criteria comprised peer-reviewed articles, reviews, and patents on microbial flavor production using natural or engineered strains, with emphasis on food-grade or GRAS organisms. Studies focused exclusively on chemical synthesis, plant extraction, non-food microorganisms, or lacking methodological clarity was excluded.
Extracted information was categorized by microbial species (e.g., Saccharomyces cerevisiae, Lactobacillus spp., Corynebacterium glutamicum), flavor compound classes (esters, alcohols, aldehydes, ketones, terpenes), biosynthetic pathways, metabolic precursors, biotechnological interventions (e.g., CRISPR, pathway engineering), fermentation conditions, substrates, and applications in food, beverage, and fragrance sectors. The review prioritizes microbial approaches in both in situ fermentations (e.g., dairy, fruit, and vegetable systems) and ex-situ biosynthesis with engineered platforms. Sensory evaluation and economic analyses were not comprehensively covered and are mentioned only where relevant.
Microbial flavor biosynthesis has transitioned from empirical fermentation practices to a strategically engineered biotechnological platform that increasingly rivals, and in specific applications surpasses, conventional plant extraction and chemical synthesis routes [11, 12, 13]. Unlike plant-based extraction, which is inherently constrained by seasonal variability, low target metabolite abundance, and resource-intensive downstream processing, microbial systems offer reproducible production, modular pathway control, and scalable operation under tightly regulated fermentation conditions. Compared with chemical synthesis, microbial biosynthesis provides superior regio- and stereoselectivity while enabling regulatory classification as “natural flavor”, a decisive advantage in clean-label food markets (Figure 1).
As mentioned in Supplementary Table S1, microbial platforms collectively generate an exceptionally broad spectrum of flavor compounds encompassing alcohols, aldehydes, ketones, esters, organic acids, lactones, terpenes, phenolics, sulphur compounds, and pyrazines through well-defined metabolic pathways. Yeast-driven Ehrlich metabolism underpins the formation of higher alcohols and fruity esters. At the same time, lactic acid bacteria dominate the biosynthesis of buttery ketones and organic acids via citrate and pyruvate metabolism. Oleaginous yeasts and filamentous fungi preferentially produce lipid-derived lactones and methyl ketones through β-oxidation. In contrast, phenolic and sulphur-containing flavor notes arise from amino acid and ferulic acid catabolism in specialized bacterial and fungal taxa. This pathway–compound specificity represents a fundamental strength of microbial systems that extraction-based approaches cannot replicate.
A comparative perspective on microbial platforms is essential to understand the functional diversity and strategic positioning of bioflavor production systems. As summarized in Supplementary Table S1, distinct microbial groups exhibit highly differentiated biosynthetic capabilities, metabolic constraints, and levels of industrial maturity, underscoring that no single platform is universally optimal. Instead, flavor biosynthesis outcomes are inherently shaped by the interplay between metabolic architecture, substrate utilization, and process conditions. Lactic acid bacteria (LAB), for instance, are characterized by their strong compatibility with food matrices and their ability to fine-tune flavor profiles through carbohydrate fermentation and amino acid metabolism. However, their relatively limited metabolic diversity and low volatile titers position them as modulators of flavor rather than primary producers of high-impact aroma compounds. In contrast, yeasts particularly Saccharomyces and non-Saccharomyces species demonstrate high biosynthetic capacity for alcohols, esters, and volatile thiols via the Ehrlich pathway and esterification reactions, making them dominant platforms for fruity and floral aroma generation. Nevertheless, their tendency to channel carbon flux toward ethanol highlights a critical trade-off between flavor production and metabolic efficiency.
Filamentous fungi and actinomycetes further expand the accessible flavor space through lipid-derived lactones, methyl ketones, and nitrogen- or sulphur-containing compounds. These organisms possess exceptional enzymatic systems capable of complex substrate transformations, yet their industrial deployment is constrained by slower growth kinetics, regulatory concerns (e.g., mycotoxins), and process variability. Emerging platforms such as microalgae introduce novel opportunities for sustainable flavor biosynthesis, particularly for green and marine notes, although challenges related to low yields and downstream processing remain significant.
Engineered microbial systems, including Escherichia coli and Corynebacterium glutamicum, represent a paradigm shift toward precision bioflavor production. By leveraging synthetic biology tools and modular pathway design, these hosts enable high controllability and targeted synthesis of specific compounds such as esters and terpenoids. However, regulatory constraints associated with genetically modified organisms (GMOs) and product purification challenges continue to limit their widespread adoption in food applications. Finally, co-culture and microbiome-based systems offer a distinct approach by harnessing metabolic interactions and cross-feeding mechanisms to generate complex and layered flavor profiles. While these systems better mimic natural fermentation ecosystems, their limited predictability and controllability highlight the need for advanced modelling and AI-assisted design strategies.
Collectively, the comparative framework presented in Supplementary Table S1 reveals a critical insight: the future of bioflavor production lies not in the dominance of a single microbial platform, but in the strategic integration of multiple systems, guided by mechanistic understanding and application-specific requirements. This perspective reinforces the need for a systems-level approach that combines metabolic engineering, process optimization, and ecological design to unlock the full potential of microbial flavor biosynthesis.
The primary competitive advantage of microbial flavor biosynthesis lies in its tunability. Advances in metabolic engineering and synthetic biology enable precise redirection of carbon flux toward target flavor classes, facilitating high product specificity and reduced impurity profiles [12, 13, 14, 15]. Engineered yeast and bacterial hosts have demonstrated the feasibility of decoupling growth from flavor formation, allowing tailored aroma design rather than indiscriminate metabolite accumulation. However, this tunability introduces new constraints: excessive pathway overexpression imposes metabolic burden, compromises redox balance, and often reduces volumetric productivity at industrial scale. Furthermore, recovery of volatile or hydrophobic flavor compounds from aqueous fermentation broths remains a major techno-economic bottleneck [13, 17, 19, 20].
Regulatory acceptance and sensory performance in real food matrices further shape industrial implementation. While non-GMO, naturally fermented flavors are broadly accepted, products derived from genetically modified microorganisms face region-specific regulatory barriers, particularly in the European Union and parts of Asia. Precision fermentation strategies employing GRAS organisms, adaptive laboratory evolution, or genome editing without foreign DNA insertion increasingly represent a pragmatic compromise between innovation and regulatory feasibility. Importantly, flavor efficacy cannot be inferred solely from chemical identity or titer. Matrix interactions with lipids, proteins, and polysaccharides strongly modulate aroma release and perception, necessitating the integration of sensory science with microbial and process engineering [21, 22, 23, 24].
Collectively, microbial flavor biosynthesis should no longer be viewed as a universal replacement for chemical synthesis or plant extraction, but rather as a complementary, application-specific technology. Future competitiveness will depend on integrating pathway-level engineering with strain robustness, process intensification, and sensory-driven design frameworks, transforming microbial platforms from flavor producers into predictive, industrially reliable aroma engineering systems [25, 26, 27].
Compared with conventional approaches, microbial fermentation occupies a strategic middle ground between plant extraction and chemical synthesis. Seasonal variability, low compound yields, and high land and solvent requirements constrain plant-based extraction. At the same time, chemical synthesis, although cost-efficient and scalable, often suffers from poor stereoselectivity and increasing consumer resistance to “artificial” labels. Microbial fermentation offers a distinct advantage by enabling the production of flavor compounds with high structural specificity under controlled, reproducible conditions, while retaining regulatory eligibility for “natural flavor” claims. However, this advantage is contingent on precise metabolic control and process optimization, positioning microbial systems not merely as alternative sources, but as tuneable bio factories whose value lies in their balance between natural authenticity and industrial controllability.
Overall, microbial flavor biosynthesis is not a one-size-fits-all strategy but depends on the intrinsic metabolic capacity of each microbial group and its alignment with specific flavor targets and applications. Understanding how different microbial taxa generates distinct aroma compounds at the pathway and enzyme levels is therefore critical. The next section focuses on microbial taxa and the biosynthetic mechanisms that underpin their flavor-producing potential.
Each microbial taxon exhibits characteristic biosynthetic strengths and constraints shaped by central carbon flux, amino acid metabolism, lipid transformation capacity, and secondary metabolite production. Lactic acid bacteria primarily act as flavor modulators within complex matrices; yeasts function as high-output producers of volatile esters and alcohols; actinomycetes and filamentous fungi expand flavor diversity through secondary metabolism; and microalgae offer emerging, sustainability-driven flavor spaces. The following subsections (Sections 4.1–4.5) therefore integrate microbial taxonomy with biosynthetic mechanisms, flavor functionality, and application relevance.
4.1 Lactic acid bacteriaLactic acid bacteria (LAB) are central contributors to flavor development in dairy, vegetable, meat, and cereal fermentations, where they function primarily as fine-tuners rather than dominant producers of high-intensity aroma compounds. Their flavor-forming capacity arises from the coordinated metabolism of carbohydrates, amino acids, and, to a lesser extent, lipids, with outcomes strongly influenced by strain-specific enzymatic repertoires and fermentation conditions such as pH, temperature, salt concentration, and nutrient availability [28, 29]. Carbohydrate metabolism proceeds via glycolysis in homofermentative LAB or via the phosphoketolase pathway in heterofermentative LAB, yielding lactic acid as the principal product (Supplementary Table S2). Heterofermentative LAB additionally produce acetic acid, ethanol, CO₂, and other organic acids, thereby increasing flavor complexity and contributing to acidity-driven sensory balance [19, 30].
4.2 ActinomycetesActinomycetes, particularly Streptomyces species, represent a distinct flavor platform characterized by extensive secondary metabolism and the capacity to generate structurally complex aroma compounds. Unlike LAB and yeasts, whose flavor profiles are largely dictated by primary metabolism, actinomycetes produce a broad spectrum of volatile organic compounds (VOCs) including pyrazines, terpenes, aldehydes, ketones, lactones, and sulphur- and nitrogen-containing compounds via specialized biosynthetic pathways (Supplementary Table S3). Pyrazines, responsible for nutty, roasted, and cocoa-like notes, are formed from amino acid and sugar precursors through NRPS-independent and Maillard-like reactions. Amino acid catabolism further yields Strecker aldehydes and sulphur volatiles, such as methanethiol and dimethyl trisulfide, which contribute meaty and umami-like aromas [42, 43]. Terpenoid biosynthesis proceeds primarily via the MEP pathway, supplying IPP and DMAPP precursors for the formation of mono- and sesquiterpenes via terpene synthases [44, 45]. Actinomycetes also produce lactones via fatty acid β-oxidation and polyketide-derived compounds such as geosmin and 2-methylisoborneol, which impart earthy and musty notes.
While these compounds can enhance complexity at low concentrations, they may also constitute off-flavors, highlighting the need for precise pathway control [44, 46, 47, 48, 49]. Flavor production is highly sensitive to aeration, substrate composition, and regulatory networks controlled by global transcription factors (e.g., adpA and afsR) [47, 50]. Their large biosynthetic gene clusters make actinomycetes attractive targets for metabolic engineering, heterologous expression, and pathway refactoring. Although regulatory hurdles and process complexity currently limit widespread food application, actinomycetes represent a high-value platform for savory and umami-oriented flavors in seasonings, fermented sauces, and flavor additives [46, 50].
4.3 YeastYeasts, particularly Saccharomyces cerevisiae, function as high-output flavor producers in fermented foods and beverages, including bread, beer, wine, cocoa, and emerging plant-based systems. Their flavor-generating capacity arises from the efficient conversion of sugars, amino acids, and lipids into alcohols, esters, organic acids, and sulphur compounds through tightly interconnected metabolic networks (Table 1) [54, 55].
| Flavor Compound | Precursors | Metabolic Pathway | Yeast Species Involved | Sensory Characteristics | Food/Beverage Applications |
|---|---|---|---|---|---|
| Ethanol | Glucose, other sugars | Alcoholic fermentation (glycolysis → pyruvate → ethanol) | Saccharomyces cerevisiae | Alcoholic, warming | Wine, Beer, Bread, Kombucha |
| Carbon dioxide (CO₂) | Glucose | By-product of alcoholic fermentation | Saccharomyces cerevisiae, Brettanomyces bruxellensis | Effervescence | Bread leavening, Beer, Sparkling wine |
| Acetaldehyde | Pyruvate | Intermediate in ethanol formation | Saccharomyces. cerevisiae, Pichia spp. | Pungent, green apple-like | Wine (undesirable at high levels), Bread |
| Higher alcohols | Amino acids (e.g., leucine, valine) | Ehrlich pathway (amino acid catabolism) | Saccharomyces cerevisiae, Torulaspora delbrueckii | Solvent-like, fruity, fusel | Beer, Wine, Spirits |
| Esters (e.g., ethyl acetate, isoamyl acetate) | Alcohols + Acids | Enzymatic esterification | Saccharomyces. cerevisiae, Kloeckera apiculata | Fruity, banana, and pear | Wine, Beer, Cider |
| Acetic acid | Acetaldehyde, ethanol | Oxidation of ethanol or overflow metabolism | Saccharomyces cerevisiae, Brettanomyces spp. | Vinegar-like, sharp | Kombucha, Wine (can be off-flavor) |
| Sulphur compounds | Methionine, cysteine | Sulphur amino acid metabolism | Saccharomyces. cerevisiae, Hanseniaspora uvarum | Rotten egg, cabbage (H₂S), or savoury | Wine, Beer, Bread (mostly undesirable) |
| Diacetyl | Pyruvate via acetolactate | By-product of valine synthesis | Saccharomyces cerevisiae, Brettanomyces spp. | Buttery, creamy | Beer (undesirable at high levels) |
| Phenolic compounds (e.g., 4-vinylguaiacol) | Ferulic acid | Phenolic acid decarboxylation | Brettanomyces, Saccharomyces cerevisiae (POF+ strains) | Clove-like, spicy | Wheat beer, Belgian ales |
| Fatty acids (e.g., octanoic acid) | Acetyl-CoA | Fatty acid synthesis and degradation | Saccharomyces cerevisiae | Soapy, cheesy at high levels | Beer, Wine (off-flavor in excess) |
| Volatile thiols | Cysteine conjugates | β-lyase and carbon–sulphur bond cleavage | Saccharomyces cerevisiae, Torulaspora spp. | Passionfruit, grapefruit | Wine (e.g., Sauvignon Blanc) |
Sources: [40, 48, 49, 52, 53, 54, 55]
Alcoholic fermentation converts sugars into ethanol and CO₂, while secondary metabolites such as glycerol, acetaldehyde, and succinic acid shape mouthfeel and background flavor. The Ehrlich pathway plays a central role in aroma formation by converting branched-chain and aromatic amino acids into fusel alcohols, which are subsequently esterified by alcohol acetyltransferases (Atf1, Atf2) to produce fruity acetate esters [56]. Lipid metabolism further contributes to the production of medium-chain fatty acids and ethyl esters associated with floral and tropical notes [52, 54].
Yeast sulphur metabolism generates volatile thiols and sulphides that contribute to complexity at low levels but cause off-flavors when poorly controlled. Non-Saccharomyces yeasts expand aroma diversity by modifying terpenoids and phenolic precursors, particularly in wine and spontaneous fermentations [52, 53]. From an industrial standpoint, yeasts offer scalability, genetic tractability, and compatibility with precision fermentation. Advances in metabolic engineering enable pathway enhancement or suppression, positioning yeasts as versatile platforms for customized flavor design beyond traditional alcoholic products [54, 55, 56, 57].
4.4 Filamentous FungiFilamentous fungi such as Aspergillus, Penicillium, Rhizopus, and Mucor combine strong enzymatic secretion with secondary metabolism, enabling extensive substrate conversion and flavor diversification. Their capacity to hydrolyze carbohydrates, proteins, and lipids underpins flavor development in solid-state and submerged fermentations (Supplementary Table S4), including soy sauce, miso, tempeh, and cheese [58, 59, 60, 61, 62, 63, 64]. Fungal lipases and esterases drive the formation of methyl ketones, secondary alcohols, and lactones, while amino acid catabolism yields fusel alcohols, aldehydes, and sulphur compounds. Secondary metabolites, including terpenoids, phenols, and polyketides, further contribute to aged, roasted, and umami-associated notes. Fungi are increasingly used as clean-label flavor cell factories, particularly in plant-based foods, where they generate dairy- and meat-like sensory profiles.
4.5 MicroalgaeMicroalgae represent a distinct microbial platform for flavor biosynthesis, driven primarily by lipid oxidation, carotenoid cleavage, and sulphur amino acid metabolism rather than classical fermentative routes (Table 2). Lipoxygenase-mediated fatty acid degradation and carotenoid cleavage dioxygenases generate green, marine, floral, and umami-associated volatiles such as hexenal, 1-octen-3-ol, dimethyl sulphide, and β-ionone, positioning microalgae as complementary to LAB and yeasts rather than direct competitors [65, 66, 67, 68, 69, 70]. From a process perspective, flavor formation is tightly coupled to photosynthetic activity, oxidative stress, and lipid accumulation, making production highly sensitive to light, oxygen, and nutrient availability. Although these conditions can enhance aroma formation, they often compromise biomass productivity and yield low volatile titers, limiting scalability and downstream efficiency.
| Flavor Compound | Precursor(s) | Biosynthetic Pathway | Microalgae Species Involved | Sensory Characteristics | Food Applications |
|---|---|---|---|---|---|
| Dimethyl sulphide (DMS) | Dimethylsulfoniopropionate (DMSP) | Cleavage of DMSP by DMSP-lyase | Phaeodactylum tricornutum, Isochrysis galbana | Sulphurous, marine, umami | Seafood seasoning, marine umami enhancers |
| Cis-3-hexenal | Linolenic acid | Lipoxygenase (LOX) pathway | Chlorella vulgaris, Nannochloropsis spp. | Green, grassy, fresh | Flavor enhancement in beverages or smoothies |
| 1-Octen-3-ol | Linoleic acid | Lipoxygenase cleavage | Tetraselmis spp., Chlorella spp. | Mushroom-like, earthy | Soups, sauces, and umami-rich formulations |
| β-Ionone | Carotenoids (β-carotene) | Oxidative cleavage of carotenoids | Dunaliella salina, Haematococcus pluvialis | Floral, fruity, violet-like | Natural flavoring for dairy/desserts |
| Hexanal | Linoleic acid | LOX pathway, fatty acid oxidation | Chlorella vulgaris, Spirulina platensis | Green, leafy, sharp | Flavorant in snacks, functional seasonings |
| Aldehydes (e.g., trans-2-hexenal) | Linolenic acid | Lipid peroxidation, enzymatic oxidation | Nannochloropsis, Tetraselmis | Fresh, cucumber-like | Salad dressings, algae-based dips |
| Terpenes (e.g., limonene, linalool) | Isoprenoid precursors (IPP, DMAPP) | MEP pathway (methylerythritol phosphate) | Botryococcus braunii, Chlorella spp. | Citrus, floral, herbal | Beverages, functional chewing gums |
| Ketones (e.g., β-ionone, geranylacetone) | Carotenoids | Oxidative degradation | Dunaliella salina, Haematococcus spp. | Fruity, floral, sweet | Natural additives in confectionery |
| Fatty acids (volatile) | Lipids (PUFAs) | Lipid hydrolysis and oxidation | Nannochloropsis Sp., Chlorella, Spirulina | Fishy, fatty, savoury | Used cautiously in seafood or savoury blends |
| Umami compounds (e.g., glutamic acid) | Proteins/peptides | Proteolysis and deamination | Chlorella vulgaris, Spirulina platensis | Savoury, brothy, umami | Plant-based soups, bouillon, and seasonings |
Sources : [40, 63, 64, 65, 66, 67, 68]
Nevertheless, omics-guided strain selection, modulation of LOX and MEP pathways, and hybrid phototrophic–heterotrophic cultivation strategies are beginning to decouple flavor biosynthesis from growth penalties. Combined with sustainability advantages and regulatory acceptance of non-GMO strains, microalgae offer strong potential for nuanced flavor modulation and functional food applications rather than high-intensity aroma production.
Microalgae have emerged as promising platforms for bioflavor production due to their diverse metabolic capabilities and ability to synthesize a wide range of volatile organic compounds. Among these, lipoxygenase (LOX)-mediated pathways play a central role in the formation of C6–C9 aldehydes, alcohols, and their derivatives, which are key contributors to characteristic aroma profiles. LOX-derived volatiles are often associated with desirable green and marine-like notes, making them valuable in applications requiring fresh or plant-like sensory attributes, such as in vegetable-based foods, seafood analogues, and functional beverages.
However, the formation of LOX-derived compounds also presents notable challenges. Under certain conditions, excessive accumulation of these volatiles can lead to off-flavors, commonly described as grassy, beany, or oxidized notes, which may reduce product acceptability. These undesirable attributes are often linked to lipid oxidation processes, particularly in systems with high polyunsaturated fatty acid content, as is typical for many microalgal species.
The balance between desirable and undesirable aroma formation depends on multiple factors, including microalgal species, cultivation conditions, lipid composition, and downstream processing parameters. For instance, stress conditions such as light intensity, nutrient limitation, or oxidative stress can enhance LOX activity and subsequently alter volatile profiles. Therefore, careful optimization of cultivation and processing conditions is essential to modulate LOX pathways and achieve targeted flavor outcomes. Overall, while LOX-derived volatiles represent a valuable route for generating natural flavor compounds from microalgae, their dual role highlights the importance of controlled bioprocessing strategies to maximize desirable sensory characteristics while minimizing off-flavor formation.
Recent advances in microbial bioflavor research highlight that different compound classes are governed by distinct metabolic constraints, process sensitivities, and scalability trade-offs, rather than merely by the availability of biosynthetic pathways. Understanding these differences is essential for selecting appropriate microbial hosts and fermentation strategies for industrial flavor production.
5.1 EstersEsters such as ethyl acetate, isoamyl acetate, and ethyl butyrate are key aroma compounds in fermented foods and beverages. They are mainly produced by Saccharomyces cerevisiae and Kluyveromyces marxianus through alcohol acyltransferase (AAT)-catalyzed reactions that combine alcohols with acyl-CoA precursors [27]. Their synthesis is tightly linked to carbon and nitrogen metabolism and is strongly influenced by fermentation conditions, including pH, temperature, and oxygen availability. While metabolic engineering has enabled targeted control of ester formation by modulating AAT activity, precursor supply, and ester hydrolysis [8], excessive pathway amplification can impose a metabolic burden and compromise strain robustness.
Overexpression of ATF1 or ATF2 in Saccharomyces cerevisiae substantially increases acetate ester production; however, this strategy frequently shifts the primary bottleneck from enzyme capacity toward precursor availability, intracellular redox balance, and global metabolic stability. Importantly, many studies implicitly assume that increased intracellular ester synthesis directly translates into enhanced flavor output. In practice, ester volatility, oxygen-dependent hydrolysis, and rapid stripping during aerated fermentation often offset genetic improvements, particularly at scale. This persistent gap between pathway-level optimization and process-level performance highlights the need to integrate metabolic engineering with fermentation design.
Synthetic biology approaches that support heterologous ester pathways in hosts such as Escherichia coli and Yarrowia lipolytica expand the range of fermentative and non-fermentative platforms for bioester synthesis [71]. Nevertheless, ester instability in aqueous systems and challenges in downstream recovery remain major constraints. Consequently, microbial ester biosynthesis is most effective when deployed in integrated fermentation–recovery systems tailored to specific food matrices such as fruit-based products, dairy fermentations, kombucha, and plant-based beverages [11].
5.2 TerpenesTerpenes such as limonene, linalool, and geraniol are major aroma compounds derived from the isoprenoid precursors IPP and DMAPP via the mevalonate (MVA) or methylerythritol phosphate (MEP) pathway. Reconstruction of these pathways in microbial hosts, particularly Escherichia coli and Saccharomyces cerevisiae, has enabled controlled terpene biosynthesis through the introduction of plant terpene synthase genes and balancing of precursor supply [13, 72, 73, 74, 75]. However, terpene production frequently reveals a mismatch between biosynthetic flux and host tolerance, as intermediate accumulation and product toxicity often limit achievable titers.
Most terpene engineering strategies prioritize increasing precursor flux but often overlook terpene-induced membrane stress, intracellular sequestration, and impaired cell integrity. As a result, productivity is often constrained not by pathway capacity but by host physiology. Engineering efforts are therefore increasingly directed toward mitigating metabolic stress through tuning ERG20 flux, improving cofactor availability, and managing intracellular terpene partitioning.
Because terpenes are highly hydrophobic, they tend to accumulate in membranes or form emulsions in fermentation broths, complicating downstream recovery. Accordingly, in situ extraction, two-phase fermentation, and secretion engineering have emerged as essential complements to pathway engineering. For food-related applications, GRAS hosts such as Corynebacterium glutamicum and Bacillus subtilis are gaining prominence due to their safety profiles and process robustness [72, 74] underscoring that regulatory acceptance and process stability are as critical as metabolic performance.
5.3 Lactones and ketonesLactones and ketones contribute fruity, creamy, nutty, and caramel notes and are increasingly produced via microbial routes as sustainable alternatives to chemical synthesis [76, 77, 78]. Yeasts and lactic acid bacteria, including Lactobacillus, Lactococcus, and Yarrowia lipolytica, generate these compounds primarily through fatty acid metabolism and peroxisomal β-oxidation. Yeasts convert hydroxy fatty acids into γ- and δ-lactones via sequential oxidation and cyclisation, whereas bacteria form methyl ketones from fatty acyl-CoA intermediates through enzymes such as OleA [79, 80, 81]. Although microbial lactone and ketone biosynthesis offer superior stereochemical control compared to chemical synthesis, many studies underestimate the complexity of redox balance, oxygen availability, and intracellular compartmentalization required for consistent production. Peroxisomal activity, cofactor regeneration, and metabolic cross-talk collectively shape yield and enantiomeric purity, often limiting reproducibility beyond laboratory conditions.
The microbial production of raspberry ketone exemplifies how metabolic engineering can overcome the limitations of low natural abundance and inefficient chemical synthesis. Engineered E. coli and S. cerevisiae redirect carbon flux from glucose through the shikimate pathway to tyrosine and subsequently to raspberry ketone via PAL/TAL-mediated reactions and benzalacetone synthase activity [78, 81, 82, 83]. While feedback-resistant enzymes and deregulated aromatic amino acid biosynthesis have improved pathway efficiency, productivity remains sensitive to regulatory complexity and metabolic stress, indicating that future advances will depend on system-level optimization rather than linear pathway extension.
5.4 Amino acid derivativesAmino acid derivatives arise from enzymatic and non-enzymatic reactions during fermentation, aging, and heat processing, contributing key taste attributes including umami, bitterness, sweetness, and kokumi, as well as volatile aromas [84]. Their sensory impact is governed not only by chemical identity but also by interactions with food matrices and processing conditions. Umami perception primarily derives from glutamate, aspartate, and umami-active peptides, whereas bitterness is associated with hydrophobic and branched-chain amino acids.
A prevailing limitation in amino acid–derived flavor research is the tendency to equate increased metabolite concentration with enhanced sensory perception. In reality, umami and kokumi intensities depend on post-biosynthetic modification and receptor-level interactions, particularly activation of calcium-sensing receptors by γ-glutamyl peptides (GGP). This highlights a critical disconnect between metabolic output and sensory response that must be addressed in flavor biotechnology.
Proteases and peptidases hydrolyze proteins into free amino acids and peptides that shape taste profiles [85, 86, 87]. Enzymes such as glutaminase, glutamate dehydrogenase, glutathione synthetase, and γ-glutamyl transpeptidase (GGT) coordinate glutamate enrichment and γ-glutamyl peptide (GGP) formation, central to kokumi perception [88, 89, 90, 91]. These amino acids originate from core metabolic pathways, including the TCA cycle and the shikimate pathway [92, 93, 94, 95, 96, 97, 98, 99, 100, 101], which explains the strong linkage between central metabolism and flavor outcomes.
In Bacillus spp., glutamate is polymerized to γ-polyglutamic acid (γ-PGA) by the PgsB–PgsC–PgsA–PgsE complex, with the L/D-glutamate ratio influencing polymer properties and functionality [102, 103, 104, 105, 106, 107, 108, 109, 110]. Overall, amino acid derivative formation reflects coordinated regulation of proteolysis, amino acid biosynthesis, and transpeptidation, positioning this flavor class as particularly amenable to systems-level strain selection, fermentation control, and integration with sensory science [111, 112, 113, 114].
Submerged fermentation (SmF) is the dominant industrial platform for bioflavor production due to its high process controllability, reproducibility, and compatibility with modern bioreactors. Cultivation in liquid media enables precise regulation of pH, temperature, aeration, agitation, and nutrient supply, which strongly influence flavor yield and the formation of volatile metabolites [13, 14]. SmF supports a wide range of bioflavor-producing microorganisms, including LAB, yeasts, filamentous fungi, and engineered bacteria. Importantly, SmF allows metabolic pathways to be steered through environmental control; for example, Lactococcus lactis can be directed toward diacetyl production by modulating pH and aeration to promote oxidative decarboxylation of α-acetolactate, while nitrogen limitation in Saccharomyces cerevisiae enhances Ehrlich-pathway-derived higher alcohols and esters [20, 23].
Various operational modes are employed in SmF. Batch fermentation is suitable for small-volume or specialty flavors; fed-batch strategies sustain metabolic activity while preventing catabolite repression through controlled substrate feeding; and continuous fermentation, though less common for complex aroma mixtures, offers steady-state production for single-target compounds [25, 26]. The selection of carbon and nitrogen sources critically shapes aroma profiles: amino acid supplementation (e.g., leucine, phenylalanine) enhances ester and fusel alcohol formation, lipid-rich substrates favor lactone synthesis via lipolysis and β-oxidation, and intracellular redox balance (NADH/NAD⁺ ratios) modulates multiple volatile-generating reactions [27, 71].
However, a recurring limitation of SmF lies in the implicit assumption that pathway optimization alone guarantees improved flavor yield. In reality, volatile stripping during aeration, product dilution in the aqueous phase, and oxygen-sensitive degradation frequently offset genetic and metabolic gains. SmF also accommodates co-culture systems that generate more complex aroma profiles; LAB–yeast combinations, for instance, integrate diacetyl formation with ester synthesis, producing multidimensional flavor bouquets suited to dairy and beverage applications. Advances such as dissolved oxygen control, pH-stat operation, and in situ product recovery (ISPR) mitigate product inhibition and volatilization losses [4, 6]. Despite challenges including foaming, shear stress, and downstream recovery demands, the integration of metabolic engineering with refined SmF strategies continues to reinforce SmF as a foundational technology for scalable and sustainable bioflavor biosynthesis [7, 9].
6.2 Solid-state fermentation (SSF)Solid-state fermentation (SSF) has re-emerged as a complementary platform to submerged systems, particularly for producing complex bioflavors using filamentous fungi, selected LAB, and yeasts adapted to low-water environments. Growth on moist solid substrates with minimal free water generates microenvironments that resemble natural ecological niches, enhancing volatile retention and reducing contamination risks [10, 13]. Low-moisture conditions often intensify secondary metabolism and stress-associated pathways, thereby increasing aroma complexity. For example, Aspergillus oryzae cultivated on rice or wheat bran produces elevated lactones and aldehydes through intensified lipase and β-oxidation activity, while Rhizopus spp. generate esters and ketones via coordinated esterase, alcohol dehydrogenase, and oxidase activity [5, 16].
SSF systems typically employ tray fermenters, packed-bed reactors, or rotating drums, each presenting trade-offs in oxygen transfer, heat dissipation, and moisture control. Tray and packed-bed systems are cost-effective for small- to medium-scale operations, whereas rotating drums provide improved mixing for larger-scale processes. Substrate selection is central to SSF performance; agro-industrial residues such as wheat bran, rice husk, and fruit peels serve as both nutrient sources and structural matrices, aligning SSF with waste valorization and circular bioeconomy principles [23, 27].
Nevertheless, SSF scalability remains constrained by limited real-time monitoring and difficulties maintaining homogeneous temperature, moisture, and aeration. Localized overheating can suppress microbial activity or unpredictably shift aroma profiles, underscoring that SSF flavor outcomes are inherently more heterogeneous than those of SmF. Emerging solutions, including improved heat and mass transfer modelling, online sensing technologies, and semi-continuous SSF configurations, are beginning to address these challenges [35, 115]. Overall, SSF offers high flavor concentration and enhanced aromatic complexity and, when coupled with strain engineering and enzyme supplementation, represents a powerful complement to submerged fermentation rather than a direct replacement [13, 29].
6.3 Bioreactor design and process optimizationBioreactor configuration and process optimization are central determinants of yield, reproducibility, and flavor consistency in microbial bioflavor production (Supplementary Table S5). Because flavor biosynthesis is highly sensitive to environmental conditions, precise control of operational parameters is essential to direct metabolic flux toward desired volatile compounds while suppressing unwanted by-products. In SmF, stirred-tank bioreactors remain the industry standard due to their scalability and the ability to fine-tune aeration, agitation, pH, and temperature.
Oxygen transfer represents a critical control point: microaerophilic conditions in Saccharomyces cerevisiae favor ester synthesis via alcohol acetyltransferases, whereas elevated oxygen levels promote the accumulation of aldehydes and ketones. Agitation must ensure homogeneous mixing without imposing excessive shear stress, particularly for filamentous fungi [19, 33]. pH and temperature directly modulate enzyme kinetics in flavor pathways; for example, maintaining pH 5.0–5.5 enhances ferulic acid conversion to vanillin in Amycolatopsis spp., while temperatures near 30 °C optimize isoamyl acetate synthesis in Saccharomyces cerevisiae. Fermentation mode further shapes aroma outcomes: fed-batch systems allow controlled precursor feeding to sustain biosynthesis without triggering repression, whereas continuous systems, though less suited to complex flavor mixtures, stabilize production of single aroma compounds [32, 37,116].
In situ product recovery (ISPR) strategies, including gas stripping, adsorption resins, and membrane-based separation, are increasingly employed to reduce volatilization losses and alleviate product inhibition, thereby improving effective productivity. In SSF, bioreactor design prioritizes moisture retention, heat removal, and oxygen diffusion within the solid matrix; packed-bed, tray, and rotating-drum systems enable management of these variables through forced aeration, periodic mixing, or substrate turning [28, 74]. Maintaining 60–70% moisture and preventing thermal hotspots are essential for preserving metabolic activity and flavor fidelity.
Crucially, bioreactor design should not be viewed as a passive container for engineered strains but as an active component of flavor biosynthesis. The integration of real-time monitoring (pH, dissolved oxygen, CO₂ evolution, temperature), metabolic modelling, and adaptive control strategies enables dynamic steering of microbial metabolism. Together, these approaches bridge the gap between laboratory optimization and industrial reliability, positioning process engineering as a co-equal driver of innovation in bioflavor production [37, 53]. Despite extensive laboratory-scale success, the industrial translation of microbial bioflavor production remains non-trivial. Scale-up introduces critical bottlenecks, including oxygen transfer limitations, volatile compound stripping, and the trade-off between high product titters and sensory balance. Strategies such as fed-batch cultivation, in situ product removal, and bioreactor design optimization are increasingly required to mitigate aroma loss and metabolic burden. Importantly, fermentation strategies must be tailored not only to maximize yield but also to preserve the complex flavor profiles that distinguish microbial bioflavors from chemically synthesized counterparts, underscoring scalability as a biochemical and sensory challenge.
In situ product recovery (ISPR) strategies, including gas stripping, adsorption resins, and membrane-based separation, are increasingly employed to reduce volatilization losses and alleviate product inhibition, thereby improving effective productivity. In SSF, bioreactor design prioritizes moisture retention, heat removal, and oxygen diffusion within the solid matrix; packed-bed, tray, and rotating-drum systems enable management of these variables through forced aeration, periodic mixing, or substrate turning [28, 74]. Maintaining 60–70% moisture and preventing thermal hotspots are essential for preserving metabolic activity and flavor fidelity.
Crucially, bioreactor design should not be viewed as a passive container for engineered strains but as an active component of flavor biosynthesis. The integration of real-time monitoring (pH, dissolved oxygen, CO₂ evolution, temperature), metabolic modelling, and adaptive control strategies enables dynamic steering of microbial metabolism. Together, these approaches bridge the gap between laboratory optimization and industrial reliability, positioning process engineering as a co-equal driver of innovation in bioflavor production [37, 53]. Despite extensive laboratory-scale success, the industrial translation of microbial bioflavor production remains non-trivial. Scale-up introduces critical bottlenecks, including oxygen transfer limitations, volatile compound stripping, and the trade-off between high product titters and sensory balance. Strategies such as fed-batch cultivation, in situ product removal, and bioreactor design optimization are increasingly required to mitigate aroma loss and metabolic burden. Importantly, fermentation strategies must be tailored not only to maximize yield but also to preserve the complex flavor profiles that distinguish microbial bioflavors from chemically synthesized counterparts, underscoring scalability as a biochemical and sensory challenge.
In addition to laboratory-scale optimization, the transition to industrial-scale fermentation introduces several critical challenges that significantly influence process performance and product quality. One of the primary limitations is the oxygen transfer rate (OTR), particularly in high-cell-density cultures where oxygen demand exceeds supply. Insufficient oxygen transfer can lead to metabolic shifts, reduced product formation, and the accumulation of unwanted by-products [28, 37]. Furthermore, flocculation and foaming phenomena frequently arise during large-scale operations. Flocculation can affect cell distribution and reactor homogeneity, while excessive foaming often caused by proteinaceous compounds and biosurfactants can disrupt mixing, reduce effective working volume, and complicate process control. These factors collectively impact mass transfer efficiency and overall fermentation stability [53, 74].
Another important consideration is the accumulation of microbial by-products, such as organic acids, higher alcohols, or secondary metabolites, which may interfere with desired flavor profiles. At scale, subtle changes in metabolic flux can result in significant variations in product composition, potentially leading to off-flavors or inconsistent quality [37, 53]. Addressing these challenges requires integrated bioprocess strategies, including improved bioreactor design, optimization of aeration and agitation systems, implementation of antifoam or cell immobilization techniques, and real-time monitoring of key process parameters. Such approaches are essential to ensure consistent bioflavor production, maintain product quality, and achieve industrial scalability [28, 74].
7. Industrial applications and case studies: bioflavor production by microbial fermentation
Industrial bioflavor production relies on the inherent metabolic pathways of specific microorganisms, enabling targeted synthesis of key aroma compounds across food and fragrance sectors (Figure 2). In dairy fermentation, lactic acid bacteria (LAB) such as Lactococcus lactis and Leuconostoc mesenteroides generate diacetyl and acetoin by converting citrate to pyruvate and α-acetolactate, which are then oxidatively decarboxylated. These metabolites provide buttery notes in butter and cheese. Lactobacillus helveticus contributes cooked and milky aromas during cheese ripening by catabolizing methionine to methional and dimethyl sulphide [22, 25, 71].
In beverages, Saccharomyces cerevisiae produces higher alcohols and esters through the Ehrlich pathway. Transamination and decarboxylation of leucine yield isoamyl alcohol, which is esterified with acetyl-CoA to form isoamyl acetate, a prominent fruity note in beers and ciders. Engineered Yarrowia lipolytica can overproduce β-ionone via carotenoid cleavage dioxygenases (CCDs), enhancing floral aromas in wines and fermented beverages. For plant-based meat analogues, Corynebacterium glutamicum catabolizes branched-chain amino acids to 3-methylbutanal and 2-methylbutanal, key precursors for meaty flavor development [74].
In bakery systems, yeasts such as Saccharomyces cerevisiae and Pichia kudriavzevii produce ethanol and CO₂ via glycolysis while simultaneously forming esters (e.g., ethyl acetate, ethyl butyrate) via alcohol acetyltransferases, which contribute fruity and floral notes to sourdough bread. The confectionery industry relies on microbial bioconversion for natural vanilla production, Aspergillus niger and Amycolatopsis spp. convert plant-derived ferulic acid to vanillin through oxidative decarboxylation and reduction, offering a sustainable alternative to synthetic vanillin for chocolates, ice creams, and confectionery products [115, 118].
Beyond food, the flavor and fragrance sector utilize engineered Escherichia coli and Saccharomyces cerevisiae to produce terpenes such as limonene and linalool via the mevalonate (MVA) pathway, supporting fragrance, beverage, and citrus-flavor applications. In parallel, waste valorization approaches employ Aspergillus oryzae in solid-state fermentation to convert fatty acids from citrus peels into lactones and aldehydes, generating fruity and creamy notes while upcycling agro-industrial residues. These case studies highlight how integrating microbial metabolism, metabolic engineering, and precision fermentation enables tailored flavor profiles, higher yields, and sustainable production systems aligned with regulatory and consumer expectations [57, 61, 119].
Microbial fermentation offers a compelling alternative to conventional chemical synthesis and plant extraction for bioflavor production, particularly when evaluated across efficiency, sustainability, cost, safety, and functional quality [97, 98, 99]. Compared with plant extraction, fermentation is largely independent of seasonal variability, land use, and climatic constraints, enabling more consistent yields and scalable production. While extraction from natural sources often requires large biomass inputs to obtain low concentrations of target compounds, engineered microbial systems can achieve higher volumetric productivity through pathway optimization and controlled bioprocess conditions [100, 101, 120].
Microbial fermentation has been successfully translated into industrial applications to produce a wide range of bioflavor compounds, including esters, alcohols, organic acids, and terpenes [74, 115, 118]. To enhance industrial relevance, it is important to contextualize these processes with representative quantitative performance metrics reported in the literature. For ester production (e.g., ethyl acetate, isoamyl acetate) using yeasts such as Saccharomyces cerevisiae, typical product titers range from approximately 0.5 to 5 g/L, with volumetric productivities of 0.05–0.5 g/L·h, depending on strain engineering and fermentation conditions [74, 115]. In optimized systems, particularly those employing metabolic engineering or fed-batch strategies, titers can exceed 10 g/L [115, 118].
In the case of higher alcohols and aldehydes, microbial systems such as yeasts and lactic acid bacteria commonly achieve titers of 0.1–3 g/L, with productivity generally lower than that of esters due to pathway constraints and cofactor limitations [100, 101]. For terpene biosynthesis (e.g., limonene, linalool) in engineered microbial hosts such as Escherichia coli or Saccharomyces cerevisiae, reported titers typically range from mg/L to low g/L levels (0.01–2 g/L), with ongoing advances in pathway optimization enabling gradual improvements in both titer and yield [97, 99].
Organic acids and flavor precursors (e.g., lactic acid, acetic acid, diacetyl) often reach higher concentrations, with titers commonly exceeding 10–100 g/L in industrial fermentations [61, 119]. However, their contribution to flavor depends on controlled accumulation within sensory-relevant thresholds. From a downstream processing perspective, recovery yields vary widely depending on the physicochemical properties of the target compound and the extraction method employed. For volatile flavor compounds, recovery efficiencies typically range from 50% to 90%, with techniques such as solvent extraction, distillation, adsorption, or membrane-based separation being commonly applied [98, 99].
It is important to note that these values are indicative and can vary significantly depending on microbial strain, substrate, fermentation mode (batch, fed-batch, continuous), and process optimization strategies. Nonetheless, they provide a useful benchmark for assessing the industrial feasibility of microbial bioflavor production systems and highlight the ongoing need for integrated improvements in metabolic engineering, bioprocess design, and downstream recovery to achieve commercially competitive performance.
Compared with chemical synthesis, microbial fermentation typically operates at milder temperatures and pressures, reducing energy demand and minimizing the use of hazardous reagents or solvents [111, 112, 113]. This translates into a lower environmental footprint and improved sustainability metrics, particularly when renewable feedstocks such as agricultural by-products are used as substrates. Although chemical synthesis remains cost-effective for certain simple flavor molecules at large scale, it often yields racemic mixtures or unwanted by-products, requiring additional purification steps. In contrast, microbial biosynthesis often provides superior regio- and stereoselectivity, yielding flavor compounds with higher sensory fidelity to their natural counterparts [114, 115].
From a safety and regulatory perspective, fermentation-derived bioflavors produced by Generally Recognized as Safe (GRAS) microorganisms are increasingly favored by consumers and regulatory agencies. Chemical synthesis may raise concerns about residual solvents or synthetic intermediates, while plant extraction may pose contamination risks, pesticide residues, or variable phytochemical profiles. Fermentation systems allow tighter control over product purity and traceability, although downstream processing remains a cost-intensive bottleneck [39, 119].
In terms of functional quality, microbial fermentation enables the tailored production of complex flavor profiles through metabolic network modulation, co-cultivation strategies, or precursor feeding. Such flexibility is difficult to achieve with plant extraction and often impractical via chemical synthesis. Nevertheless, challenges remain for fermentation-based approaches, including substrate cost, product inhibition, and regulatory approval for genetically engineered strains. Overall, while no single approach is universally optimal, microbial fermentation uniquely balances efficiency, sustainability, and functional quality, positioning it as a key platform for next-generation bioflavor production in food, beverage, and fragrance applications.
Regulatory acceptance represents a decisive factor in the commercial deployment of microbial bioflavors, particularly in the context of emerging precision fermentation technologies. While non-GMO microbial processes are generally well aligned with current “natural flavor” regulations, genetically engineered strains face heterogeneous approval frameworks across regions, creating uncertainty for global market adoption. This regulatory asymmetry is further compounded by the clean-label paradox, in which advanced metabolic engineering enables sustainable, efficient production but risks consumer rejection due to perceived artificiality. Addressing these challenges will require not only technical innovation but also regulatory harmonization and transparent communication linking microbial biosynthesis to sustainability and product authenticity.
Regulatory compliance and safety assurance are pivotal factors governing the commercial adoption of fermentation-derived and microbially synthesized bioflavors. From a food safety perspective, production organisms must meet stringent criteria, particularly in jurisdictions where only specific microorganisms are permitted for use in food applications [94]. In the United States, many commonly used production hosts, including Lactobacillus spp. and Saccharomyces cerevisiae, benefit from Generally Recognized as Safe (GRAS) status, facilitating regulatory approval when used under defined conditions. However, the use of non-traditional or genetically engineered strains often requires comprehensive safety assessments, including toxicological evaluation, allergenicity analysis, and demonstration of the absence of viable cells or recombinant DNA in the final product [94, 95].
Labelling requirements add to regulatory complexity. Although fermentation-derived flavors may qualify as “natural” in some regulatory frameworks, definitions vary substantially across regions. For example, the classification of a bioflavor as “natural flavor” under U.S. Food and Drug Administration (FDA) guidelines may not align with European Union (EU) regulations, which impose stricter criteria for source materials and processing methods. As a result, identical products may require different labelling strategies depending on the target market, complicating global commercialization [87, 121].
International compliance challenges are further amplified when genetically modified microorganisms (GMMs) are involved in production. While the final bioflavor compound may be chemically identical to its naturally occurring counterpart, regulatory authorities may still impose additional oversight related to containment, traceability, and environmental risk management. The EU applies a precautionary regulatory approach to GMM-derived products, whereas other regions adopt more product-based evaluation frameworks. Navigating these divergent regulatory philosophies increases development timelines and compliance costs [31, 33, 121].
A clearer distinction between major regulatory frameworks would strengthen the global relevance of this review. In the United States, regulatory oversight generally follows a product-based approach, in which the safety and characteristics of the final product take precedence over the production method. For example, flavor compounds produced via microbial fermentation may be classified as Generally Recognized as Safe (GRAS) if sufficient evidence demonstrates their safety, regardless of whether genetically modified microorganisms (GMMs) were involved in their production, provided that no viable modified organisms or recombinant DNA remain in the final product [94, 95]. In contrast, the European Union adopts a more process-based, precautionary approach, in which the use of GMMs in production triggers stricter regulatory scrutiny under GMO legislation, even if the final product is chemically identical to naturally derived counterparts [87, 121]. This distinction is particularly important for the classification of “natural flavor” substances: while the U.S. allows broader interpretation based on source and processing, the EU imposes more stringent criteria requiring that flavoring substances be derived from natural raw materials through traditional or explicitly permitted processes. Consequently, microbial bioflavor production strategies must be carefully aligned with regional regulatory definitions and risk assessment frameworks. These differences underscore that regulatory acceptance is not solely determined by product safety, but also by the technological pathway employed, making it a decisive factor in the commercial deployment and international trade of microbial-derived bioflavors [31, 33, 121].
Safety considerations also extend to process-related contaminants, residual solvents, and by-products generated during fermentation or downstream processing. Compared with plant extraction, fermentation generally offers improved control over contaminant profiles; however, rigorous hazard analysis and critical control point (HACCP) systems are essential to ensure consistent product quality and consumer safety. For bioflavors intended for use in food and beverage applications, compliance with international standards such as Codex Alimentarius and ISO guidelines is increasingly important to support cross-border trade. Overall, while microbial fermentation provides clear advantages in sustainability and functional quality, regulatory heterogeneity and safety assessment requirements remain significant barriers to widespread adoption. Early integration of regulatory strategy into strain selection, process design, and product positioning is therefore essential to accelerate market entry and ensure long-term commercial success of bio-derived flavor compounds [76, 77, 121].
The growing demand for natural, sustainable, and clean-label ingredients positions microbial fermentation as a strategic platform for next-generation bioflavor production. Its key advantages include the use of renewable feedstocks, scalable bioprocessing, and the capacity to fine-tune sensory profiles through strain selection, metabolic engineering, and synthetic biology. Recent advances in genome-editing technologies, particularly CRISPR-based tools, have increased the precision with which metabolic pathways can be rewired to improve flavor yield, specificity, and molecular diversity. When integrated with systems biology and process optimization, these approaches enable tighter control of metabolic flux, reducing by-product formation and improving overall process efficiency [37, 52, 116].
Beyond strain-level innovation, future progress will increasingly depend on substrate and process integration. The valorization of agro-industrial side streams, such as fruit peels, molasses, and lignocellulosic biomass, offers cost-effective, sustainable feedstocks that align microbial bioflavor production with circular bioeconomy principles. However, the compositional heterogeneity of such substrates introduces metabolic variability, which directly affects flavor consistency. Addressing this challenge requires coupling robust strain design with adaptive fermentation control strategies. Advances in bioreactor design, continuous fermentation, and real-time monitoring technologies provide opportunities to stabilize production at an industrial scale while enabling the generation of novel, market-tailored flavor profiles for food, beverage, and fragrance applications [53, 57, 58, 62].
Despite these opportunities, significant bottlenecks remain in translating laboratory-scale successes into industrial reality. Scale-up is frequently constrained by microbial instability, contamination risks, and the high costs associated with downstream recovery of volatile compounds. Moreover, the inherent variability of biological systems contrasts sharply with the reproducibility expected by the flavor industry, necessitating advanced quality control frameworks that integrate analytical chemistry, sensory evaluation, and process analytics. Regulatory and consumer acceptance challenges further complicate commercialization, particularly for flavors produced using genetically modified microorganisms, as regulatory approval pathways and labelling requirements vary substantially across regions [20, 24, 118].
Sustainability considerations also warrant critical evaluation. Although microbial fermentation is often promoted as environmentally superior to chemical synthesis, its actual footprint depends strongly on process design. Energy-intensive steps such as aeration, temperature control, and product purification can offset environmental gains if not carefully optimized. Furthermore, microbial platforms must compete with synthetic flavor production, which often benefits from lower costs, established supply chains, and predictable output [59, 68]. As a result, the long-term competitiveness of microbial bioflavors will depend not only on technological feasibility but also on techno-economic performance and regulatory alignment.
Despite significant advances in microbial bioflavor production, the transition from laboratory-scale demonstrations to industrial-scale manufacturing remains a major bottleneck. One of the primary challenges is process standardization, as fermentation performance is highly sensitive to variations in strain physiology, medium composition, and operating parameters such as pH, temperature, aeration, and redox potential. Unlike chemical synthesis, which relies on well-defined reaction kinetics, biological systems exhibit inherent variability, complicating reproducibility across production batches and facilities [122, 121].
Starter culture consistency is a critical determinant of scalability, particularly for bioflavors derived from complex metabolic pathways. Genetic drift, plasmid instability, and phenotypic heterogeneity can cause fluctuations in yield and product profiles over the course of prolonged industrial operations. This issue is especially pronounced in engineered strains, where selective pressure may reduce pathway efficiency over time. Robust strain engineering, chromosomal integration of key genes, and standardized inoculum preparation protocols are therefore essential to maintain long-term production stability [121, 123].
Contamination control is another major industrial concern, as large-scale fermentations create nutrient-rich environments that favor the growth of competing microorganisms. Contamination can result in reduced yields, altered flavor profiles, or complete batch failure, leading to significant economic losses. While aseptic processing and closed bioreactor systems mitigate these risks, they increase capital and operational expenditures. The use of extremophilic or acid-tolerant production strains, along with selective substrates or process conditions, has emerged as a strategy to enhance microbial robustness and reduce contamination susceptibility [26, 118].
From an economic perspective, scale-up feasibility is strongly influenced by substrate cost, volumetric productivity, and downstream processing efficiency. Although fermentation can outperform plant extraction in yield per unit volume, downstream separation and purification of volatile or low-concentration flavor compounds remain cost-intensive. In comparison, chemical synthesis often benefits from mature, highly optimized purification pipelines. Integrating in situ product removal, solvent-free extraction techniques, or membrane-based separations may significantly improve the economic viability of large-scale fermentation processes [35, 36].
Bioreactor scale-up introduces additional complexity related to mass transfer, oxygen availability, and metabolic regulation. Parameters that are easily controlled at bench scale may not translate linearly to industrial volumes, potentially causing shifts in metabolic flux and unintended by-product formation. Advanced process monitoring, digital twins, and real-time control strategies are increasingly being explored to address these challenges and to support consistent, large-scale bioflavor production. Microbial fermentation offers strong advantages in sustainability and functional quality. Its industrial scalability depends on overcoming challenges related to standardization, cultural stability, contamination management, and cost-effective scale-up. Addressing these issues will be essential for translating laboratory innovations into commercially viable bioflavor production platforms [121, 122].
Overall, microbial fermentation holds substantial potential to reshape bioflavor production by combining sustainability-driven demand with rapid advances in biotechnology. Realizing this potential will require coordinated progress across multiple dimensions: robust, stress-tolerant strain engineering; fermentation systems designed as active modulators of flavor biosynthesis; efficient downstream processing; and harmonized regulatory frameworks. Addressing these interconnected challenges will determine whether microbial bioflavors evolve from niche applications into a dominant industrial paradigm.
Microbial fermentation offers a versatile and sustainable platform for natural bioflavor production, drawing on the metabolic diversity of lactic acid bacteria, yeasts, filamentous fungi, actinomycetes, and microalgae to generate a wide spectrum of volatile and non-volatile flavor compounds through pathways such as amino acid catabolism, fatty acid metabolism, and terpene biosynthesis. Advances in systems biology, metabolic engineering, and bioprocess optimization have transformed traditional fermentation into a precision-driven biotechnology, enabling improved yield, specificity, and sensory control. However, persistent challenges, including metabolic burden, product toxicity, downstream recovery inefficiencies, sensory variability at scale, and regulatory constraints, continue to limit industrial translation. Addressing these barriers will require system-level integration of omics-informed strain design, adaptive fermentation strategies, sustainable feedstock utilization, and aligned regulatory frameworks. Within this integrated paradigm, microbial bioflavor biosynthesis aligns strongly with clean-label and circular bioeconomy principles, positioning it as a credible and scalable route for next.
R. Haryo Bimo Setiarto: Conceptualization, literature review, Writing – original draft. Fifi Afiati: Writing –review & editing. Fitri Setiyoningrum: Literature review, Data curation, Writing – review & editing. Gunawan Priadi: Validation, Writing – review & editing. Suprapedi: Validation, Writing – review & editing. Senlie Octaviana: Conceptualization, Validation, Writing – review & editing.
There are no competing interests
