With the global population estimated to reach 9.7 billion by 2050 (World Population Prospects, 2017), the demands for food and fuel are escalating tremendously. In this regard, microorganisms, particularly yeasts, have been widely explored in the last couple of decades. Besides tons of other benefits, they have shown promise in addressing both the issues of food crisis by contributing as single cell protein (SCP) and fuel shortage by fermenting a wide variety of carbon sources to ethanol. Furthermore, yeasts are the preferred cell factories to produce high-value compounds having industrial applications owing to their rapid growth rate and flocculating ability, contributing to increased productivity and ease of harvest (Øverland and Skrede, 2017, Ritala et al., 2017). Due to the single-celled eukaryotic system, genetic manipulation in yeast is carried out easily for strain improvement.

Yeasts provide a good source of vitamins and amino acids; the presence of bioactives and immunostimulators further makes them ideal for human food as well as animal feed ingredients (Øverland et al., 2013, Ritala et al., 2017). They also offer an economic advantage by fermenting a wide variety of organic substrates (Wiebe, Koivuranta, Penttilä, & Ruohonen, 2012; Zhang et al., 2011), readily converting the low-cost organic byproducts from industries to proteins and lipid-rich compounds. Yeast systems are conventionally used for the sustainable production of industrial bioproducts such as pharmaceuticals, biodegradable polymers, food additives and supplements, cosmetics, and many more. However, most naturally occurring yeasts lack the enzymes to efficiently hydrolyze the polymers in lignocellulosic biomass (LCB) (Fig. 1), for their use in biorefineries. This raises the need for pretreating LCB to produce sugars that can then be assimilated by yeasts. Various pretreatment processes raise the production cost as well as pose several other concerns, which are discussed later in the following sections.

From the perspective of mitigating fuel needs, biofuel always stands out for environmental sustainability. Microorganisms can ferment sugar to produce ethanol, more commonly called bioethanol, which is the most widely used biofuel in the transportation sector today. Generally, biofuel is considered a cleaner alternative to conventional fuels as it significantly reduces greenhouse gas emissions. Liquid biofuels represent about 40 % of the total energy consumption in the world and mainly include ethanol and biodiesel (Tan, Lee, & Mohamed, 2008). Bioethanol can be used directly as fuel in internal combustion engines or blended with gasoline to produce gasohol. Bioethanol offers several advantages over the usual hydrocarbon fuel- gasoline or petroleum by possessing a higher octane number, increased heat of vaporization, less toxicity, high biodegradability, and reduced air pollution (Balat and Balat, 2009, John et al., 2011). Brazil started using bioethanol as an alternative fuel as early as in 1970s, and by 1978 they launched cars exclusively powered by ethanol (Grandis, Fortirer, Pagliuso, & Buckeridge, 2024; Rico et al., Rico, Mercedes, & Sauer,; 2010). Soon countries like Germany, France, USA started using ethanol in combustion engines (Demirbas & Karslioglu, 2007). Since then, global ethanol production has increased steadily, reaching 28 billion gallons in 2022 (U.S. Department of Energy, Alternative Fuels Data Center, 2024). The USA and Brazil are the largest ethanol producers, contributing to 85 % of global ethanol production (Celińska & Grajek, 2009).

Depending on the type of feedstocks used for its production, bioethanol has been classified into three categories. The first-generation (1 G) bioethanol is derived from sucrose and starch-based edible plant products, like corn, other cereal grains, sugarcane juice, sugar beet, potato, cassava, etc. The second-generation (2 G) biofuel comes from the non-edible residues present in LCB that are abundant in plant cell walls. Third-generation (3 G) bioethanol is derived from algal biomass (Nigam & Singh, 2011). Among these three, 2 G appears the most promising as it does not compete directly with the food industry and does not require additional area for plantation or cultivation.

Baker’s yeast (Saccharomyces cerevisiae) is the most widely used microorganism for industrial bioethanol production because of its low cost, fermentation compatibility, and availability of improved strains. Additionally, it also tolerates a wide range of pH. However, cultivating yeasts in large-scale alcohol fermentation still faces a lot of challenges. Various stress factors like high temperature, osmotic stress, high alcohol concentration, and bacterial contamination can all affect the growth rate and viability of the fermenting yeasts and as such inhibit production. Moreover, yeast species that are mostly used in ethanol production are usually unable to ferment pentoses. Very few species of yeasts from the genera Pichia, Candida, Scheffersomyces, and Pachysolen are capable of fermenting pentose to ethanol (Delgenes et al., 1986, Ferreira et al., 2011). Pentose sugars are present in the hemicellulose part of plant residues, which are relatively abundant in agricultural and forestry residues. They are more readily recovered by acid hydrolysis from hemicelluloses than D-glucose from cellulose. Hence, they are considered important feedstocks for ethanol and other chemical production. Moreover, using these sugars from hemicellulose can substantially reduce the current production cost of these processes.

The LCB is the most abundant feedstock in the world, contributing to ∼200 billion tons each year (Narisetty et al., 2022). It is essentially derived from plant cell walls, which comprise mainly the outermost protective layer of lignin (15–20 %), the inner secondary wall of cellulose (40–50 %), and the hemicellulose (25–30 %) microfibrils in between connecting the two (Hazeena, Sindhu, Pandey, & Binod, 2020). Pectin (a heteropolysaccharide) and various proteins are the minor components of the plant cell wall. The prerequisite for generating 2 G biofuel is breaking down these major polysaccharides into their monomers, predominantly glucose.

Cellulose makes up to 50 % the organic component of plants and is a linear polymer of D-glucose units linked by β-1,4-glycosidic bonds. These linear polymers are joined to form microfibrils which in turn are grouped together to form cellulose fibers that are covered with hemicellulose and lignin. Hemicellulose is a complex hetero-polysaccharide containing various sugars, viz., D-xylose, L-arabinose, D-glucose, L-galactose, D-mannose, and sugar acids viz., D-glucuronic acid and D-galacturonic acid (Fig. 1). Unlike cellulose, they do not form aggregates and can be hydrolyzed readily. Lignin is a three-dimensional complex heteropolymer of phenylpropane units. To utilize this LCB as the feedstock , the polymer needs to be broken down into simple fermentable sugars. This remains a great challenge as most of this conversion requires various enzymes to be employed, making the overall process very expensive. On the other hand, the application of less preferred thermal or chemical pretreatments for LCB often generates many toxic byproducts. Even after the pretreatment, the major limitation is that most of the microbes cannot metabolize the pentoses, resulting from the hydrolysis of the hemicellulosic fraction of LCB. Xylan is the major polysaccharide of this fraction, composed of β-1,4-linked xylose residues. The depolymerization of the hemicellulosic fraction generates a mixture of sugars that contains about 90 % xylose. Xylose constitutes the second largest proportion of sugar in LCB after glucose (Fig. 1) (Kasavi et al., 2012). However, most of the currently employed industrial microorganisms do not possess a native xylose-metabolizing pathway. Therefore, to release the fermentable sugars from LCBs, various pretreatment strategies are adopted, viz., heat treatment, enzymatic hydrolysis, etc. Some fungal strains are reported to naturally biodegrade these materials as they contain cellulolytic and hemicellulolytic properties. For example, strains of Aspergillus niger, A. sojae, and A. terreus could biodegrade about 16.6 % of rice straw for the production of biomethane (Noonari, Mahar, Sahito, & Brohi, 2020). Yeasts like Clostridium thermocellum have been reported to have cellulolytic ability together with glucanase and xylanase production, which help to effectively degrade plant cell walls (Furukawa et al., 2014, Mazzoli and Olson, 2020; Kiyoshi et al., 2015) . Such microbes offer an environment-friendly pretreatment method, providing fermentable sugars from LCBs. Co-cultivations of biodegrading bacteria/fungi/yeast together with xylose-fermenting yeasts have proven to be largely successful towards this endeavor (Brethauer & Studer, 2014; Chi, Li, Wang, Zhang, & Antwi, 2018; Kumar, Fox, & Takasuka, 2023; Llamas, Greses, Magdalena, González-Fernández, & Tomás-Pejó, 2023).

Xylose metabolism in microbes offers some advantages over conventional glucose, although the latter is the preferred carbon source for many microorganisms; (i) xylose induces genes involved in the respiratory phase of central carbon metabolism even under anaerobic conditions, (ii) the metabolic flux distribution of the TCA cycle, the pentose phosphate pathway (PPP), and acetyl-CoA biosynthesis is different on xylose, and generates more important intermediates than on glucose (Kwak et al., 2017), (iii) xylose induces a different metabolic flux distribution than glucose, and generates some key metabolic intermediates such as acetyl-CoA, malonyl-CoA, and erythrose-4-phosphate. These serve as precursors for production of several value-added chemicals such as terpenoids and aromatics. Due to several benefits of cultivating yeasts as discussed earlier, they are employed in LCB-based biorefineries. Certain species of yeasts can directly convert xylose into xylitol and other value-added chemicals that find applications in pharmaceutical, cosmetics, and many other industries as discussed below.

It is a high-value 5-carbon sugar alcohol, naturally present in many fruits and vegetables and has many potential health benefits. Along with its anti-inflammatory and anti-carcinogenic effects, it also protects enamels and provides resistance to osteoporosis. Most importantly, xylitol metabolism does not involve insulin, which is why it is widely recommended as an artificial sweetener for diabetic patients (He et al., 2021). Apart from its low glycemic properties, it also offers 40 % fewer calories than sucrose, even though it tastes similar to the latter (He et al., 2021). Most yeast strains that naturally ferment xylose can produce xylitol in a one-step reaction, as it is formed as an intermediate in the oxidoreductase pathway. Among the native xylose-utilizing yeasts, strains of Candida showed the highest yields of xylitol recorded i.e., 17 gL−1 (Peterson, 2013). Candida guilliermondii FTI-20037 and C. parapsilosis produced xylitol with yields around 0.74 gg−1 xylose (Barbosa et al., 1988, Nolleau et al., 1993). However, for large-scale xylitol production, S. cerevisiae is a better candidate, for many advantages as discussed earlier (Mouro et al., 2020). Several genetically engineered strains of this yeast are available that express the heterologous xylose isomerase (XI) or xylose reductase-xylitol dehydrogenase (XR/XDH) pathway. However, the XR activity of these strains needs to be improved for better xylose reduction. Secondly, the NADPH/NADH supply in the wild-type yeast is not sufficient, as in these reduction processes, glucose is routinely used for cell growth, and carbon catabolite repression also comes into play, affecting the overall productivity of the process (He et al., 2021).

Xylose improved the production of acetyl-CoA-derived chemicals such as terpenes and fatty alcohols in S. cerevisiae (Guo et al., 2016, Kwak et al., 2017, Montanti et al., 2011). Since xylose alleviates the glucose-mediated repression of cytosolic pyruvate dehydrogenase (PDH) bypass, the component of respiratory metabolism, it increases the availability of acetyl-CoA, the main precursor for terpenes as well as fatty acid-derived chemicals (Jin et al., 2004, Kwak et al., 2017).

They are a structurally diverse group of natural compounds consisting of 5-carbon isoprene units. Usually, they exist as mono-, di-, tri-, and sesqui-terpenes; and all of these classes have huge commercial applications in food/feed, fragrance, pharmaceutical, and biofuel industries (Yang et al., 2022). For example, carotenoids such as lycopene and β-carotenes find widespread use in the food/feed industry due to their antioxidant properties and potential health benefits. Squalene and patchoulene are important precursors of drugs and are used in the pharmaceutical industry. Some terpenes, on the other hand, like caryophyllene, limonene, farnesene, sabiene, and pinene, find use as biofuel feedstocks due to their high density and high combustion heat (Yang et al., 2022). The traditional method of terpene extraction from plants is nowadays replaced by more eco-friendly and cost-effective microbial conversion (Marienhagen & Bott, 2013). While both Escherichia coli and yeast have been used for terpene production, the latter has been the preferred host due to higher tolerance to the toxicity of most of the terpenes, high activity of the endogenous mevalonic acid (MVA) pathway, as well as less branched pathway and byproducts (Martin et al., 2003, Siddiqui et al., 2012). Different species of yeasts have been employed for the production of different classes of terpenes; however, S. cerevisiae, Yarrowia lipolytica, and Pichia pastoris have been metabolically engineered for terpene production. Recombinant strains of S. cerevisiae have been employed to produce lycopene (Shi et al., 2019) and squalene (Zhu et al., 2021). Oleaginous yeast Y. lipolytica has a high acetyl-CoA flux, which has been utilized to produce α-pinene and bisabolene (Wei et al., 2021, Zhao et al., 2021). P. pastoris achieved high-density fermentation on cheap fatty acids and methanol, which was exploited to produce α-farnesene, β-carotene, and lycopene (Liu, Chen, Xu, & Zhang, 2021). Yang et al. have effectively illustrated the entire metabolic pathway for the synthesis of different classes of terpenes starting from acetyl-CoA, highlighting the regulation of specific genes/enzymes undertaken to improve the yield of terpenes in yeasts (Fig. 2; Yang et al., 2022).

In Crabtree-positive yeasts like S. cerevisiae, xylose relieves the glucose-dependent repression on the PDH bypass. This improves the availability of cytosolic CoA, which serves as the precursor to the MVA pathway, improving the titer of terpene production (Jin et al., 2004, Kwak et al., 2017). Further, xylose does not interact with Snf1 enough to induce the downregulation of acetyl-CoA carboxylase and upregulation of β-oxidation. This minimizes the loss of fatty acyl-CoA and generates more cytosolic CoA under xylose compared to glucose (Brink et al., 2016, Feng et al., 2015). Xylose also highly induces the gene encoding ethanol reoxidizing alcohol dehydrogenase (ADH2) (Matsushika, Goshima, & Hoshino, 2014). S. cerevisiae has been an extensively-used chassis to produce aromatic compounds, particularly terpenoids. Recently, several metabolic engineering strategies have successfully led to the production of vitamin A, lycopene, carotenoid, isoprenoid, etc. when the yeast is grown on xylose (Kwak et al., 2017, Su et al., 2020). For instance, Su et al. introduced the lycopene biosynthetic pathway consisting of CrtE, CrtB, and CrtI into a recombinant strain of S. cerevisiae overexpressing native genes encoding xylulokinase (XK) and Scheffersomyces (Pichia) stipitis-derived xylose reductase (XYL1) and xylitol dehydrogenase (XYL2). Further, to directly convert xylulose-5-phosphate into acetyl-CoA, the phosphoketolase (PK) pathway consisting of xylulose-5-phosphate phosphoketolase (xPK) and phosphotransacetylase (PTA) genes was also introduced in the yeast. The engineered strain exhibited 1.6-fold more lycopene production on glucose and xylose, instead of glucose alone (Su et al., 2020). Another recombinant strain of the yeast produced 8-fold higher squalene on xylose than on glucose (Kwak et al., 2017). Meadows et al. further combined the acetaldehyde dehydrogenase (ADA) gene with xPK and PTA to further increase the cytosolic acetyl-CoA and reduce the ATP cost for farnesene production (Meadows et al., 2016). To compensate for the reduced NADPH and NADH, they also introduced a NADH-dependent HMG-CoA reductase (Meadows et al., 2016).

The advantage of microbial conversion of a wide variety of cheap substrates to oleochemicals gained attention in the last decade over conventional plant oil and animal fats for their economic sustainability. Until now, microbial lipids or single-cell oils are non-edible; hence, their utilization avoids the ethical conflict over food resources, and their production is also climate-friendly, without damaging the ecology. Moreover, their amenability to genetic engineering puts them under a highly important class of feedstock for oleochemical production (Ledesma-Amaro & Nicaud, 2016), especially in the context of fossil fuel-derived oleochemical production. The xylose-utilizing Y. lipolytica has been the most extensively studied oleaginous yeast for years because of its high lipid content recorded up to 90 % of biomass under certain conditions (Rodriguez et al., 2016). Despite the presence of endogenous XDH/XR pathway, the yeast cannot efficiently metabolize xylose because of the limited expression of the enzymes of the pathway (Rodriguez et al., 2016). Overexpressing these enzymes, or alternatively, heterologous expression of these enzymes improved the growth of this yeast and lipid production on xylose, though the glucose repression effect was not completely alleviated (Ledesma-Amaro and Nicaud, 2016, Li and Alper, 2016, Rodriguez et al., 2016). Several other Crabtree-negative xylose-utilizing yeasts from the genera Rhodosporidium, Cryptococcus, and Candida have also been reported as oleaginous (Chattopadhyay and Maiti, 2020, Chattopadhyay and Maiti, 2021, Koivuranta et al., 2018, Lopes et al., 2020). Oleaginous microorganisms are those that accumulate neutral lipids (predominantly triacylglycerol or TAG) greater than 20 % of their dry cell mass. Both genetic and metabolic engineering approaches have been used in some of these oleaginous yeast species to improve lipid and oleochemical production (Chattopadhyay et al., 2020, Polburee et al., 2018). However, limited knowledge of xylose metabolism as well as a lack of suitable engineering tools have prevented their use in oleochemical production from xylose (Chattopadhyay & Maiti, 2021). Another xylose-fermenting recombinant strain of Y. lipolytica heterologously expressing the XR/XDH pathway from S. stipitis and overexpressing the endogenous XK gene showed similar sugar consumption rate on both glucose and xylose (Ledesma-Amaro & Nicaud, 2016). Over-expressing genes from the TAG synthesis pathway and deleting genes from the β-oxidation pathway in a xylose-fermenting Y. lipolytica also achieved a very high titer of fatty acid (6.16 gL−1) when grown on xylose (Ledesma-Amaro & Nicaud, 2016). Spagnuolo et al. published a beautiful illustration demonstrating the synthesis of different classes of lipid-derived oleochemicals in yeast after assimilating a wide range of carbon substrates, starting from lignocellulose-derived sugars and aromatics to acetate, and oil mill wastes-derived glycerol and lipids (Fig. 3; Spagnuolo, Yaguchi, & Blenner, 2019). A few examples are as follows:

1-hexadecanol production from glucose in S. cerevisiae was achieved by introducing genes encoding fatty acyl-CoA reductase from Tyoto alba and ATP-citrate lyase from Y. lipolytica (Feng et al., 2015). When this strain was further used for overexpression of S. stipitis XR, XDH, and XK, followed by laboratory evolution, the resulting yield of 1-hexadecanol production (0.10 gg−1 xylose, 0.03 gg−1 glucose) from xylose culture was even higher than that of glucose (Feng et al., 2015, Guo et al., 2016). Due to its water-binding property, this compound is primarily used as an emollient in the cosmetic industry, apart from its general use as a fatty alcohol in detergents, and perfumeries.

Isobutanol is another fatty alcohol produced by yeast that has gained significant attention for its application as an advanced biofuel due to its high energy density, and octane number (Atsumi et al., 2010). S. cerevisiae is a natural producer of this fatty alcohol through the degradation of valine (Hazelwood, Daran, Van Maris, Pronk, & Dickinson, 2008). However, simple overexpression of the constituents of the pathway did not improve the yield in this yeast due to compartmentalization/differential distribution of these enzymes between mitochondria and cytosol (Chen et al., 2011, Kondo et al., 2012). Brat et al. could increase the production in S. cerevisiae by relocating the mitochondrial valine synthesis enzymes to the cytosol (Brat, Weber, Lorenzen, Bode, & Boles, 2012). Further, by overexpression of Clostridium phytofermentans XI and endogenous XK, the yeast could directly convert xylose to isobutanol (Brat & Boles, 2013).

2,3-Butanediol (2,3-BDO).

Most often this compound is regarded as a platform chemical as it is used for the synthesis of various high-value products through dehydrogenation, ketalization, esterification, and dehydration (Celińska and Grajek, 2009, Kim et al., 2017). These products find use in pharmaceuticals, cosmetics, food/feed as well as many chemical industries (Kim et al., 2017). Conversion of 2,3-BDO to 1,3-butadiene, a precursor for synthetic rubber (Köpke et al., 2011), synthesis of anti-freezing agents from enantioselective pure (R, R)−2,3-BDO (Häßler, Schieder, Pfaller, Faulstich, & Sieber, 2012) are some of the commercial applications that have increased the demand for production of 2,3-BDO over time. For efficient utilization of lignocellulose biomass, production of this compound from xylose has been attempted by metabolic engineering in S. cerevisiae by introducing both the xylose metabolic pathway and the 2,3-BDO synthesis pathway in an evolutionary engineered Pdc-deficient strain of S. cerevisiae (Kim, Seo, Park, Jin, & Seo, 2014). The yield of 2,3 BDO reported was 43.6 gL−1 2,3-BDO from xylose. However, later to improve xylose metabolism, TAL1 (transaldolase) and mutated XYL1 (xylose reductase) genes from S. stipitis were overexpressed, together with the expression of noxE (water-forming NADH oxidase) gene from Lactobacillus lactis and PDC1 (pyruvate decarboxylase) gene from Candida tropicalis, which finally resulted in a yield as high as 96.8 gL−1 2,3-BDO from xylose (Kim et al., 2017).

Several other value-added chemicals are generated from yeasts that find numerous applications in food/feed, pharmaceuticals/nutraceuticals, fragrance, as well as biofuels. These include lactic acid, and its isomer hydroxypropionic acid, which find use in the food and medical industries, respectively. Genetic engineering strategies have been employed in yeast to improve the synthesis and yield of these important commercial compounds (Kildegaard et al., 2015, Novy et al., 2018, Turner et al., 2015). Microbial production of polyhydroxybutyrate (PHB), the biodegradable polymer of plastic alternatives, has been attempted through genetic engineering from acetyl-CoA. Successful strategies targeted the constituent enzyme of PHB synthesis (Carlson and Srienc, 2006, Leaf et al., 1996) and the cytosolic acetyl-CoA biosynthesis pathway to improve the yield of PHB in S. cerevisiae (Kocharin, Chen, Siewers, & Nielsen, 2012).

From the standpoint of bioethanol production, yeasts are preferred over other microorganisms as they exhibit high yield (>90.0 % theoretical yield) and productivity (>1.0 gL−1h−1), high tolerance to ethanol (>40.0 gL−1) and resistance to inhibitors (Mussatto, Machado, Carneiro, & Teixeira, 2012). Despite S. cerevisiae being the most employed yeast species for ethanol production, the major limitation is its inability to ferment pentose sugars. To overcome the problem of xylose utilization, in the last couple of decades significant attention was given towards identifying non-conventional yeasts capable of fermenting pentoses naturally. Usually, these yeasts belong to the genera Scheffersomyces, Candida, Spathospora, Pachysolen, etc., as shown in Table 1. Their genetic profiles may not be known, but their use of different metabolic pathways and variability of fermentative profiles are advantageous for xylose fermentation and ethanol production. Differential transcriptomic and metabolomic patterns are reported for the native and engineered yeasts when grown on glucose and xylose (Kwak, Jo, Yun, Jin, & Seo, 2019).

Among the xylose-fermenting yeasts, S. stipitis which can also ferment other sugars like glucose, galactose, and cellobiose, is the most promising strain for industrial application because of its high ethanol yield (Agbogbo & Coward-Kelly, 2008). A major advantage of using this yeast for sugar fermentation is its ability to cope with the toxic inhibitors (like 5-hydroxymethylfurfural or HMF and furfural) produced due to the degradation of the sugars in the hydrolysate. These compounds were found to be completely metabolized by S. stipitis (Almeida et al., 2008, Biswas et al., 2013, Wan et al., 2012). This makes the process cost-effective as a high yield of ethanol is obtained without the need for further detoxification. Other species of this genus, like Scheffersomyces shehatae are also known to produce high amounts of ethanol from xylose (Antunes et al., 2013). In fact, the reports suggest that this species is capable of converting pentoses into ethanol and other value-added products as efficiently as they convert hexoses.

Two thermotolerant yeasts that are capable of ethanol production from xylose include Kluyveromuces marxianus (Murata et al., 2015) and Candida krusei (previously known as Pichia kudriavzevii) (Rahman, Ismail, & Najimudin, 2021). One strain of P. kudriavzevii UniMAP 3–1, a strain isolated from a wastewater pond, was found to ferment xylose at temperatures as high as 40 °C and exhibited multi-stress tolerance, including acid-, ethanol-, and salt-tolerance (Rahman et al., 2021).

Many species of Spathospora, like S. passalidarum, S. arborariae, S. gorwiae, and S. hagerdaliae, are capable of fermenting xylose to ethanol, among which, S. passalidarum produces the highest amount of ethanol from xylose under oxygen-limited conditions (Cadete & Rosa, 2018; Long et al., 2012). In fact, it is a very good candidate for fermenting glucose, xylose, and cellobiose simultaneously; hence can optimally utilize lignocellulosic materials (Cadete & Rosa, 2018).

Over the years, several species of Candida have emerged as good candidates for xylose consumption. Candida shehatae is one of the oldest yeast strains that was found to efficiently utilize a wide range of sugars and hydrolysates in all environmental conditions, xylose being one of the highly utilized sugars (Abbi, Kuhad, & Singh, 1996). The potential of a particular isolate C. shehatae NCL-3501 to produce ethanol from complex hemicellulose in the free state as well as in the form of calcium alginate immobilized state was explored by Abbi and group (Abbi, Kuhad, & Singh , 1996). The immobilized cells produced better results, probably due to protection from inhibitors present in the hydrolysates. Candida intermedia has been found to be capable of co-fermenting glucose and xylose along with an exhibition of tolerance to lignocellulosic-derived inhibitors (Moreno, Tomás-Pejó, Olsson, & Geijer, 2020). Moreover, this species has several industrially relevant traits, such as a high specific growth rate in xylose and well-characterized transporters (Gárdonyi et al., 2003, Geijer et al., 2020, Moreno et al., 2020). C. tropicalis is one of the best xylose utilizers, producing a high amount of xylitol, while ethanol is produced as a byproduct (Sánchez, Bravo, García, Cruz, & Cuevas, 2008). Both process optimization as well as metabolic engineering strategies have been approached in this yeast to increase efficiency of xylitol production from xylose (Tamburini et al., 2015, Zhang et al., 2021). Further an oleaginous strain of C. tropicalis was identified that showed promising rate of xylose utilization when fed with both glucose and xylose (Chattopadhyay and Maiti, 2020, Dey and Maiti, 2013). Apart from C. tropicalis, Candida (Pichia) guillermondii has also been used for xylitol production from xylose under semi-aerobic conditions (Roberto et al., 1995). This species has also been shown to ferment glucose and xylose simultaneously in synthetic media, but a better utilization of xylose was observed in complex hemicellulosic hydrolysates like sugarcane bagasse (Roberto et al., 1995). C. guillermondii and Candida oleophila have been reported to produce good titres of ethanol from xylose even in the presence of inhibitors like furfural, HMF, acetic acid, formic acid, ferulic acid, and vanillin.