Green technology is commonly defined as the development and use of processes that minimize the negative impact of human activity on the environment and society (Figure 1).

It encompasses a diverse range of technologies and practices that address environmental issues, paving the way for sustainable development. It creates solutions and strategies to mitigate the effects of climate change, reduce environmental degradation, and promote the efficient use of natural resources. Green technology leverages both scientific knowledge and innovation to conserve natural resources, mitigate greenhouse gas emissions, and promote a circular economy. A key aspect of green technology is also its positive contribution to human health, both through the development of processes that do not negatively impact the health of workers involved in production and through the nature of manufactured consumer goods that support consumer health (Al-Emran and Griffy-Brown, 2023).

Among the main principles of green technology, including renewable energy and energy efficiency, transportation, water and wastewater treatment, and carbon capture technologies, agro-industrial waste management also plays a crucial role. Waste management technologies include advanced recycling plants that convert waste into valuable resources. Sustainable agricultural practices, on the other hand, aim to reduce the environmental impact of food production and maintain food security (Domínguez et al., 2024; Mishra et al., 2023).

The idea of a “circular economy,” which refers to the use of organic waste from one industry as a raw material for another, is based on the sustainability principle known as the “5Rs” (reduction, recycling, reuse, recovery, and regeneration) and replaces the traditional linear model (production, use, disposal). In recent decades, the growth of the food and agro-industrial sectors has dramatically increased food waste production. The amount of waste generated by agro-based industries has more than tripled (Bibi et al., 2023).

The Food and Agriculture Organization of the United Nations (FAO) estimates that approximately 1.3 billion tons of food are wasted each year, representing one-third of global production (Food and Agriculture Organization of the United Nations, 2022). Wastes pollute the environment. However, they serve as beneficial biomass resources. In addition to food waste, various types of agro-industrial residues are generated annually worldwide (Figure 2). Such organic wastes, a rich source of carbohydrates, proteins, lipids, organic acids, and other essential compounds, can be used in bioconversion processes (Sharma et al., 2022). They can serve as an inexpensive substrate for the manufacturing of various goods, including biogas, biofuel, single-cell biomass, as well as probiotics, biocontrol agents, fertilizers, enzymes, vitamin supplements, or antioxidants (Bibi et al., 2023).

The production of high-quality products represents one possibility for exploiting rich and valuable sources of organic molecules derived from agricultural waste and beneficial microorganisms (Phiri et al., 2024).

Several types of yeasts, which are eukaryotic microorganisms, have been widely used in various industries due to their potential applications, both for fermentation and the production of specific metabolites. Metschnikowia spp. are non-conventional yeasts with great biotechnological potential, and knowledge about them is still relatively limited, in comparison to conventional Saccharomyces cerevisiae.

The Metschnikowia genus was first identified approximately 130 years ago, and currently, the number of described Metschnikowia species exceeds 80 (Vicente et al., 2020). Species of the genus Metschnikowia form a monophyletic group within the family Metschnikowiaceae, which also includes the several other genera, such as Australozyma, Candidiozyma, Clavispora, Danielia, Gabaldonia, Gaillardinia, Helenozyma, Hermanozyma, Isabelozyma, Osmozyma, Soucietia, Sungouiella, Tanozyma, or Wilhelminamyces (Liu et al., 2024; Index Fungorum, 2025; NCBI Taxonomy Browser, n.d.).

The principal habitats where Metschnikowia species are encountered regularly include various parts of plants, namely flowers, fruits, barks, and leaves, or also the digestive tract or frass of some insects, as well as aquatic organisms (Kurtzman et al., 2018; Agarbati et al., 2024a). It is worth noting that some species of the Metschnikowia genus are globally distributed, while others exhibit extreme endemism. The nature of the association between Metschnikowia species and insects remains unclear; however, the relationship seems to be species-specific (Lachance, 2016).

The ecological diversity of Metschnikowia yeasts is not the only characteristic of these unique yeasts. Their uniqueness also extends to their morphology and physiology. Metschnikowia sp. reproduces by multilateral budding. The cells are spherical or ellipsoidal and may be pear-shaped, cylindrical, or lunate in shape. Pseudohyphae are weakly developed but often absent. In sexual reproduction, the ascospores are needle-shaped, tapered at one or both ends, sometimes swollen along one half, and the asci are elongated, club-shaped, spheroid, or ellipsoid-stalked. Depending on the species, one or two spores are produced per ascus. In some cases, spores can reach enormous sizes, exceeding 200 μm in length. In some strains, ascospore formation is preceded by the development of chlamydospores. Metschnikowia species also differ in the efficiency of forming these thick-walled cells. The adaptive properties of these features remain to be elucidated (Lachance, 2011; Lachance, 2016). Some species of this genus can produce pulcherrimin, a red pigment constituting a chelate of pulcherrimic acid and iron ions (Sipiczki, 2020). M. pulcherrima species can express different extracellular hydrolytic enzymes, namely amylase, cellulase, glucanase, β-glucosidase, β-lyase, lipase, lichenase, pectinase, protease, sulfite reductase, and xylanase, which makes them very interesting microorganisms in bioconversion processes (Mateo, 2023; Chaudhary and Karita, 2017).

The diversity of Metschnikowia strains also extends to their molecular characteristics. The yeast identification is usually based on the assumption that differences in barcodes are more minor within a single species than between species. The most commonly used barcodes are chromosomal repeat segments encoding ribosomal RNA. Molecular analysis of such segments in strains of several species belonging to Metschnikowia, conducted by Sipiczki et al. (2024), showed that this is not possible for the species of this genus. In these studies, intragenomic diversity significantly exceeded the threshold gaps used to differentiate related yeast species. The genome structures of various Metschnikowia spp. isolates were compared using RAPD and RFLP of mitochondrial DNA, demonstrating their high heterogeneity. Also, the sequence analysis of the PUL4 gene (a component of the PUL cluster) involved in pulcherrimin production revealed substantial intragenomic differences, suggesting that the genomes may be chimerized. These features make Metschnikowia spp. unique among yeasts and indicate that these traits and features evolve in a non-standard manner. When the molecular differences were compared with the phenotypic differences, no clear correlation was observed between the examined genetic/genomic diversities and the phenotypic diversities. Thus, according to Sipiczki and co-workers, none of the molecular tests can be used for differentiating strains that exhibit different phenotypes (Sipiczki, 2022; Sipiczki et al., 2024).

Recent phylogenetic, genetic, and genomic studies at the molecular level have raised questions about the taxonomic classification of species within the Metschnikowia genus. These data, combined with the results obtained in many studies through comprehensive analysis of primary and secondary barcode sequences, physiological features, and hybridization experiments, prove that the species within Metschnikowia cannot be distinguished from each other based on any of the phenotypic, phylogenetic, or biological concepts. These taxonomic properties were further supported by Troiano et al. (2023), who conducted a comparative genomic analysis of seven strains belonging to the M. pulcherrima clade and revealed the absence of single-copy markers for species differentiation. Sipiczki proposed combining species belonging to the M. pulcherrima clade, characterized by the formation of pulcherrimin (M. andauensis, M. fructicola, M. leonuri, M. pulcherrima, M. rubicola, M shanxiensis, M. sinensis, and M. zizyphicola), into a single species under the oldest species name, M. pulcherrima (Sipiczki, 2022; Sipiczki and Czentye, 2024).

Molecular biology and advanced genetic techniques have become essential tools in various fields of interest, including taxonomy, identification, classification, and metabolite production, as well as in potential applications. However, the safety of yeast used in biotechnological processes is also crucial to ensure benefits for humans and the environment (Tullio, 2022).

Knowledge about microorganisms involved in biotechnological processes has increased over the past decades, and the rapid development of molecular biology techniques has allowed for a deeper understanding of the genetic basis and specific metabolic pathways of microorganisms involved in these processes. However, the genetic refinement of microbial strains involved in processing remains controversial. Genetically modified microorganisms (GMOs) still encounter disapproval and are subject to extensive regulatory requirements. The use of GMOs as cell factories in closed systems that prevent their release into the environment is the least problematic aspect. Still, in the presented idea of green technologies, it seems to be wholly excluded (Plavec and Berlec, 2020).

Genome sequences are keystone data to explore the applicability of Metschnikowia spp. bioresources. They are also crucial for a comprehensive safety assessment, which is the principal regulatory concern, as requested by the European Food Safety Authority (EFSA) (EFSA, 2018; Binati et al., 2021; Miguel et al., 2022).

The safety analysis of yeasts is still under consideration because protocols are less developed than for bacterial assessments, and standardized methods have not yet been developed. According to EFSA, genome sequences should be searched to identify the presence/absence of metabolic pathways involved in toxigenicity or antifungal drug resistance. If detected, appropriate analysis is required to validate in silico evidence (EFSA, 2018). Genome studies are necessary because drug-resistant fungal infections pose a growing global health threat. However, no databases have been developed specifically for detecting antifungal drug resistance genes. In the studies of Larini et al. (2025), none of the Metschnikowia strains were capable of producing biogenic amines. In a recent study conducted by Rahmat et al. (2024), the toxicity assessment of extracts of M. pulcherrima (formerly M. persimmonsis) strains showed that they had no harmful effects on the liver and mitochondria of zebrafish, and no potential risk of cardiotoxicity was observed. Furthermore, other strains of M. pulcherrima (formerly M. ziziphicola), selected as potential probiotics, did not show any hemolytic activity (Staniszewski and Kordowska-Wiater, 2023).

However, the safety data are not consistent with previous publications that described a single strain of M. pulcherrima causing disease in immunocompromised patients (Mohl et al., 1998). A case report later described a non-pigment-producing strain of Metschnikowia isolated from the skin of a patient with dermatitis (Kuan et al., 2016). Another case involved an atypical strain of Metschnikowia isolated from a patient with leukemia (Savini et al., 2013).

In this context, the issue of the significant genetic diversity of Metschnikowia strains, which may reflect certain features typical of pathogenic microorganisms, is still the subject of intensive research. For example, the whole genome sequence of the pathogenic M. bicuspidata LNES0119 strain, responsible for disease effects in the crab Eriocheir sinensis, was sequenced by Jiang and co-authors (Jiang et al., 2022). Within the 16.13 Mb genome of M. bicuspidata LNES0119, encoding 5,567 genes, 1,467 genes were identified with significant homology to genes from the pathogen-host interaction database. Comparative genomic analyses of three M. bicuspidata strains and one nonpathogenic M. pulcherrima strain revealed 331 unique genes in M. bicuspidata LNES0119, 30 of which were putatively associated with its pathogenicity. Genomic and comparative analyses showed that the genome of M. bicuspidata LNES0119 contains a variety of putative pathogenic genes, primarily involved in cell wall assembly and construction. These genes may play a crucial role in adapting to the host environment, acting as virulence factors in pathogenicity, or triggering a cell-mediated host immune response.

In addition, special attention was paid to the genome analysis of Candida lusitaniae, a pathogenic yeast responsible for candidemia in humans and most phylogenetically related to Metschnikowia in the so-called “GTC clade.” Phenotypic echinocandin resistance in C. lusitaniae results from a missense mutation (S645F) in the FKS1 gene, which was not found in Metschnikowia strains. Specifically, sequenced M. pulcherrima strains exhibited more than one copy of the FKS1 protein-encoding gene, which contained either serine at position 645 (similar to the nonpathogenic wild-type C. lusitaniae) or proline (Larini et al., 2025).

Formally, M. pulcherrima was included on the list of microorganisms approved for food use developed in a joint project by the International Dairy Federation (IDF) and the European Food and Feed Cultures Association (EFFCA) in 2002. Later additions suggested by the IDF National Committees and EFFCA members, as well as additions found through a scientific literature search, were also included on the list (Bourdichon et al., 2012).

In the United States, two Metschnikowia species, M. pulcherrima and M. fructicola, have been Generally Recognized As Safe (GRAS) by the Food and Drug Administration (FDA) (FDA, 2022). Under Sections 201 and 409 of the Federal Food, Drug, and Cosmetic Act, any substance that is intentionally added to food is subject to premarket review and approval by FDA, unless the substance has been generally recognized to be safe by qualified experts under the conditions of its intended use, or unless the use of the substance has been otherwise excepted from the definition of a food additive. Therefore, GRAS is not simply a special regulatory marker. It represents a comprehensive approach to ensuring that food and dietary products are safe for their intended purposes.

In the European Union, EFSA has published a peer review risk assessment of the use of the active substance M. fructicola NRRL Y-27328 as a pesticide (EFSA, 2017). A completely natural origin characterizes this strain - it has been isolated from grapes grown in central Israel. The positive opinion was reached based on the evaluation of the representative uses of M. fructicola NRRL Y-27328 as a fungicide on stone fruits, strawberries, and grapes. Regulation (EU) 2018/1915 of 6 December 2018 approved the active substance M. fructicola strain NRRL Y-27328 under Regulation (EC) No 1107/2009 of the European Parliament and of the Council, concerning the placing of plant protection products on the market (European Union, 2018).

The results of molecular studies may provide a novel resource of knowledge for further analysis of the pathogenic mechanisms in Metschnikowia, as well as for the identification of potential targets for further research and therapeutic intervention.

According to the last EFSA statement, microorganisms used in the food chain, either as active agents, biomasses, or as production organisms of substances of interest, should be subject to a premarket authorization process. This procedure includes a complete molecular characterization of the organism under assessment. Data analysis can provide information on the unambiguous taxonomic identification of the strains, on the presence of genes of concern (e.g., those encoding virulence factors, resistance to antimicrobials of clinical relevance for humans and animals, production of harmful metabolites, or of clinically relevant antimicrobials) and on the characterization of genetic modifications (EFSA, 2024).

Therefore, the application potential of the strain must be verified through extensive studies to exclude the presence of any potential pathogenic features.

The unique features of yeasts of the genus Metschnikowia provide potential opportunities for their use in many biotechnological processes as well as in enology, agriculture, and the food and cosmetics industries (Haniffadli et al., 2024). The potential use of these yeasts in biocatalysis is particularly interesting. Research results are available on the production, from precisely defined substrates, various metabolites such as ethanol (Chaudhary and Karita, 2017) and lipids (Santamauro et al., 2014). What is more, Metschnikowia spp. demonstrate the potential to produce biodegradable surfactants (Kumari et al., 2021), low-calorie sweeteners (Sasahara and Izumori, 2005), as well as analytical reagents (Meena et al., 2014). The list of selected metabolites of the yeast Metschnikowia spp. from precisely defined substrates is presented in Table 1. It is complemented by the possible uses of these yeasts in the cosmetics industry, winemaking, and brewing.

Currently, Metschnikowia spp. are being explored as promising agents for environmentally friendly and cost-effective green technologies. Biotransformation using waste is mainly used to produce lipids, proteins, enzymes, and biofuels. Table 3 shows many potential bioconversions of various by-products by Metschnikowia species.

Possible raw materials for bioconversion processes involving Metschnikowia spp. included fruit wastes (Chaudhary et al., 2021a, 2021b, 2022, 2024; Tatay-Núñez et al., 2024) and various lignocellulosic biomass (Vong and Liu, 2017; Zhou et al., 2017; Abomohra et al., 2021; Dygas et al., 2023a, 2023b). Potential substrates also involved whey (Chreptowicz et al., 2018; Zhang et al., 2022; Chua et al., 2018), and even post-production animal fats and glycerol (Němcová et al., 2021; Diamantopoulou et al., 2020).

In this context, lipid production seems to be particularly interesting. The interest of biotechnologists is closely linked to the rising prices of fossil fuels. Their negative impact on the environment is undeniable. The composition of microbial lipids is very similar to that of vegetable oils, which creates significant potential for their use in biodiesel production. This biotechnological strategy could also be interesting for the food industry (Abeln and Chuck, 2019; Abeln et al., 2019; Abeln et al., 2020).

Traditionally, economical and environmentally friendly production is achieved by metabolizing nutrients produced by algae. This method, which recycles solid waste such as macroalgae sugars and proteins, holds great potential for sustainable economic development. Li et al. (2021a) achieved a significant reduction in harmful substrate emissions from organic waste, alongside the production of Metschnikowia oil. It is worth noting that Metschnikowia spp. can utilize acetic acid from organic waste to enhance lipid accumulation (Li et al., 2021b).

Němcová et al. (2021) demonstrated the ability of yeast to produce significant amounts of unsaturated fatty acids from crude waste animal fat, with the accumulated lipids in yeast cells reaching 36% of cell dry weight. It was documented that controlling nitrogen content can increase lipid content compared to phosphorus-limited conditions.

Diamantopoulou et al. (2020) cultivated yeasts of the genus Metschnikowia, among others, under nitrogen-limited conditions using crude glycerol as a substrate. Lipids produced by yeasts contained mainly oleic and palmitic acids. However, Metschnikowia strain produced lower amounts of lipids than the best strain, Rhodosporidium toruloides, which was able to form oil from glycerol in an amount of 12.5 g/L.

In a study conducted by Tatay-Núñez et al. (2024), M. pulcherrima grew well in sugarcane and sugar beet molasses, as well as persimmon hydrolysate, and produced intracellular lipids. Their cells demonstrated increased tolerance to desiccation. However, the lipid yield was lower in comparison to other tested oleaginous yeasts.

Metschnikowia strains are also able to grow on lignocellulosic biomass for lipid production (Zhou et al., 2017; Abomohra et al., 2021). In the work of Zhou et al. (2017), rapid, microwave-assisted acidolysis of lignocellulosic biomass led to the production of fermentable saccharides. Under these conditions, 1.5 g/L of mono- and di saccharides were available for fermentation, and less than 0.5 g/L of acids and furfurals were produced, which prevented fermentation by S. cerevisiae and other ethanol-producing yeasts. M. pulcherrima could grow in this broth, producing small amounts of lipids with a composition similar to palm oil.

The valorization of food and lignocellulosic wastes into biodiesel using M. pulcherrima strain isolated from rotten food wastes was evaluated by Abomohra et al. (2021). Food waste hydrolysate was supplemented by rice straw and softwood sawdust as additional carbon sources to increase the C: N ratio. M. pulcherrima showed the ability to produce lipids with a maximum productivity of 2.49 g per liter per day, and good biodiesel characteristics.

Intensive research into fermentation technology for waste biomass revalorization resulted in a study of M. pulcherrima as a source of SCP production. In a study by Dygas et al. (2023b), M. pulcherrima was successfully grown on rapeseed meal. A slight increase in protein was obtained, measured by an increase in nitrogen content from 0.6 to 1.6%. However, simultaneous saccharification and fermentation led to the conversion of isoflavones into forms with fewer adverse effects and lower estrogenic activity. In other work, the same authors investigated the possible use of M. pulcherrima to enrich sugar beet pulp pretreated by enzymatic hydrolysis. In these conditions, Metschnikowia yeast can grow at the level 2 × 10⁷–1 × 10⁸ CFU/mL, and the protein increase measured by the increase in nitrogen content ranged from 0.9 to 1.7% (Dygas et al., 2023a). The results showed that sugar beet pulp provides a good matrix for SCP and feed production. However, when using lignocellulosic biomass, pre-treatment is necessary.

The production of bioethanol and other alcohols from agricultural byproducts offers a good solution for waste management. Metschnikowia strains were used for this purpose, even though they are moderate fermenting yeasts. Research on the use of Metschnikowia spp. in fermentation processes was conducted on various lignocellulosic wastes. Despite wide yeast enzymatic activity, lignocellulosic biomass pretreatment was necessary (Chaudhary et al., 2021a,b; Chaudhary et al., 2022; Chaudhary et al., 2024). However, the drastic thermal and chemical treatment of lignocellulosic biomass means these bioconversions cannot be classified as pure green technologies.

Chreptowicz et al. (2018) assessed the activity of M. chrysoperlae in the production of 2-phenylethanol from whey or molasses, but the results also were not satisfactory. Metschnikowia strain produced 2-PE at 1 g per liter in whey and sugar beet juice medium; however, the best producers were strains from S. cerevisiae, which produced about 3 g/L of this metabolite.

Mitri et al. (2022) described different weak points for 2-PE formation. Firstly, high concentrations of 2-PE inhibit the yeast cell growth. Of course, there are several ways to overcome the inhibiting restraint. For example, strain mutagenesis and culture medium composition, or the optimization of the fermentation conditions, are widely used strategies for the improvement of yield. Secondly, the nitrogen sources, including L-Phe, carbon sources, vitamins, and minerals, supplemented to the media affect the fermentation process. For this reason, these media are not cost-effective, and it is necessary to search not only for efficient strains but also for various alternative, cheap cultivation media. In addition, temperature, initial medium pH, and fermentation time are all factors that could affect the amount of product.

Dairy industry wastes can be explored as a cheap and attractive raw material also for producing various alcohols. Zhang et al. (2022) converted cheese whey powder into D-arabitol and L-galactitol in a two-step process. Firstly, the simultaneous lactose hydrolysis and isomerization of lactose-derived D-galactose was performed by an engineered E. coli strain. Subsequently, the mixture containing lactose-derived D-glucose and residual D-galactose was subjected to fermentation by M. pulcherrima E1 strain, which produced 60 g/L D-arabitol and 28 g/L galactitol, low-calorie sweeteners.

Metschnikowia spp. are a promising microorganism with huge biotechnological potential. However, so far, these biotechnological strategies are still not economically competitive with chemical synthesis. To sum up the examples of the use of waste materials by the Metschnikowia spp., it should be stated that the first ambitious goal for developing biotechnological production should be to identify highly productive yeast strains and substrates that can be revalorized. The task will also be to use appropriately selected mixed cultures, proper pre-treatment methods, and cultivation conditions.

The main standards of green technology focus on reducing environmental pollution and reusing waste to produce new products (Nandy et al., 2022). Green technologies offer numerous opportunities, but they also face significant challenges (Figure 3). Green innovations can leverage new ideas to develop new processes and products, as well as improve existing production processes through environmentally friendly practices. Therefore, implementing closed-loop green technologies is essential for a sustainable future on a global scale. Current research is focused on developing new industrial methods and technologies that not only reduce waste and dependence on raw materials but also promote ecosystem conservation.

The use of Metschnikowia spp. in green processes offers several advantages, such as utilizing their broad enzymatic capabilities, non-toxicity to humans and the environment, low cost, and long-lasting protection against pathogens. With the continuous advancement of green technologies, scientists are increasingly exploring the use of waste materials for culturing Metschnikowia cells, with broader applications in sectors such as food/feed, energy, cosmetics, and biocontrol. However, it is noteworthy that the long-term use of Metschnikowia in various natural environments can impact other beneficial microorganisms and disrupt microbial ecological systems (Nowak et al., 2025).

Several regulations govern the use of Metschnikowia spp. in the food/feed industry. According to these guidelines, particular attention should be paid to the safety of the yeast strain, human- and environmentally friendly process conditions, and the economic viability of such processes in large-scale industrial production. Therefore, to enable the full commercialization of Metschnikowia-based products, it is essential to establish quality standards and specifications that ensure safety, efficacy, and consistency. First, standards for raw materials should specify the required purity for yeast cell production. Additionally, process standards should be developed, covering fermentation conditions, hygiene protocols, and methods for detecting all potentially toxic substances. Both the stability of the innovative products and other key parameters must be clearly defined and implemented.