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Review

Supercritical Fluid Extraction—A Sustainable and Selective Alternative for Tannin Recovery from Biomass Resources

by
Patryk Słota
1,
Joanna Harasym
1,2,* and
Irena Jacukowicz-Sobala
3
1
Adaptive Food Systems Accelerator-Science Centre, Wroclaw University of Economics and Business, Komandorska 118/120, 53-345 Wroclaw, Poland
2
Department of Biotechnology and Food Analysis, Wroclaw University of Economics and Business, Komandorska 118/120, 53-345 Wroclaw, Poland
3
Department of Chemical Technology, Wroclaw University of Economics and Business, Komandorska 118/120, 53-345 Wroclaw, Poland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(11), 5914; https://doi.org/10.3390/app15115914 (registering DOI)
Submission received: 20 March 2025 / Revised: 15 May 2025 / Accepted: 21 May 2025 / Published: 24 May 2025
(This article belongs to the Special Issue Extraction of Functional Ingredients and Their Application)
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Abstract

:
Tannins are structurally diverse polyphenols prized for their antioxidant and protein-binding properties, making them valuable in leather tanning, adhesives, coatings, and water treatment. This article compares research on conventional versus supercritical fluid extraction (SFE), which is recognized as an eco-friendly and efficient technique. While SFE using supercritical CO2 is already widely studied and commercially implemented for various botanical compounds, its application to tannin extraction remains in an earlier stage of development, with limited industrial solutions currently available. Various solvents (CO2, water, ethanol, and methanol) and their mechanisms of action under supercritical conditions are discussed, demonstrating how adjusting parameters like pressure and temperature can selectively isolate specific tannin fractions. By reviewing multiple studies on yield, solvent choice, process efficiency, and product purity, the article highlights SFE’s advantages in preserving tannin quality while reducing energy consumption and contamination. The conclusions suggest SFE as a promising method for sustainable tannin production, offering tailored extracts that meet the growing demand for green processes in both industrial and biomedical applications.

1. Introduction

Tannins are polyphenolic compounds commonly found in the terrestrial and aquatic plant kingdom. Their various properties antioxidant and antibacterial activity and chemical reactivity—make them useful resources for food, feed, chemical production, and environmental protection solutions [1,2]. Their numerous applications result from chemical composition. The presence of phenolic rings acting as a radical scavenger grants them antioxidant properties. In turn, hydroxyl and carboxyl groups in their chemical structures confer the ability to bind proteins, alkaloids, and amino acids; they also form strong complexes with macromolecules such as starch and cellulose [3,4]. Thus, tannins are valuable components of various materials, reagents, and additives for many versatile purposes [1]. Tannin extraction is the subject of many studies [2,5,6,7]. Innovations aimed at improving the efficiency of tannin extraction include ultrasound, microwave, and enzyme-assisted processes. Some of these techniques are commercialized. The use of supercritical fluids in tannin extraction was only explored experimentally. Given the change in reactivity of solvents in a critical state, this technique may also be advantageous in obtaining tannins for various purposes. The aim of this paper is to present the current state of knowledge of supercritical fluid extraction of tannins. The principles, efficiency, limiting factors, and future outlooks for studies are also discussed.

2. Tannin Application

For centuries, tannins were used for the leather tanning process to improve resistance and enhance the durability and flexibility of the leather. Currently, 90% of leather production relies on the chromium tanning process, with an outstanding ease of processing and a high quality of products hardly achievable using other methods. However, the cancerogenic properties of chromium compounds, along with the challenges of recovering chromium from tanning wastewaters and solid wastes, and the risk of chromium contaminating groundwater and soils, make this process environmentally hazardous [5]. Therefore, the tanning industry is exploring more sustainable alternatives. In this context, turning back to tannins, which are non-hazardous, plant-based renewable resources, presents a promising alternative to chromium compounds (Table 1).
The polyphenolic nature resulting in reactivity with formaldehyde, other aldehydes, and non-aldehyde hardeners makes tannins a versatile reagent in plastics and adhesives production. Using tannin adhesives to produce plywood, particle board, and fiber biocomposites improved their mechanical, physical, and water resistance properties [2]. Moreover, the copolymerization of tannins with synthetic adhesives allows for decreased formaldehyde emission from synthetic resins. Other bio-sourced resins and adhesives are also obtained, such as epoxy resins, tannin–furanic and tannin–resorcinol–formaldehyde thermosets, and non-isocyanite and isocyanite polyurethanes. These materials found various applications in tire cords, grinding wheels, angle grinder disks, automotive brake-pad matrices, Teflon coatings, paper impregnation, and many others [34,35]. Additionally, tannin coagulants and tannin-based adsorbents have been applied in the removal of heavy metals from water and wastewater. Their chemically modified forms can also serve as promising materials for eliminating other challenging water contaminants [36,37]. Tannins are also widely explored in biobased packaging materials as additives to nanocellulose, starch, and chitosan, significantly improving their antibacterial and antioxidative properties [38]. Tannic acid has also proven an effective crosslinker for casein, gelatin, and other proteins in the production of edible foils for food packaging with an increased stability to oxidative degradation [36,39,40]. In feed production, applying tannins in animal food has positive effects on animal intestinal microbiota and gut health, combined with increased animal production. Due to their antibacterial and anti-inflammatory properties, they can also be a good alternative to in-feed antibiotics [41]. The biochemical activity of tannins makes them valuable components of nutraceuticals, pharmaceuticals, and medicinal preparations as antioxidants, radical scavengers, anti-mutagenic, anti-cancerogenic, hepatoprotective, and neuroprotective agents (Figure 1) [42].
The variety of tannin applications makes tannins the fourth most extracted group of biomass components, following cellulose, hemicellulose, and lignin. In 2022, the global tannin market was valued at nearly $2.5 billion, with an expected annual growth rate of 6.2% projected from 2023 to 2030. Most tannins, accounting for 62% of the market share, are used in the tanning industry. However, the increase in annual tannin production is primarily driven by the rising demand from the wine industry [43]. Tannins and their derivatives are renewable, sustainable reagents with various properties and reactivities, making them promising candidates for other market segments, such as plastics, packaging, wood, and environmental protection. It is important to note that tannins comprise a diverse group of polyphenolic compounds, and their chemical composition and structure significantly influence their applications. Moreover, the chemical composition of tannins is strongly related to their source and the extraction method used, including solvent and critical parameters selection.

3. Tannin Classification and Sources

The diverse group of tannin compounds can be classified in various ways. The most common classification considers their functional groups and chemical structures. According to this system, tannins are primarily divided into two groups—hydrolysable and condensed tannins [3]. Hydrolyzable tannins can be fractionated into their components through hydrolysis [44] since they are esters of monosaccharides (mainly D-glucose) and phenolic acids [3]. This group consists of two subgroups: gallotannins and ellagitannins (Figure 2), which differ in their hydrolysis products. Gallotannins break down into gallic acid and its derivatives, whereas ellagitannins decompose into ellagic acid and its derivatives [45]. The most common monomer in the latter group is hexahydroxydiphenoyl (HHDP) which can spontaneously convert to ellagic acid during hydrolysis. Ellagitannins are the most well-studied tannins, with over 500 naturally occurring compounds identified [45].
Condensed tannins (Figure 2), also known as proanthocyanidins, are the primary product of global tannin extraction, exceeding 90% of worldwide annual tannin production. They consist of at least three repeating units of flavonoid structures, mainly flavan-3-ols such as catechin and epicatechin. Depending on the number of repetition units, condensed tannins are classified as oligomeric or polymeric flavonoids with molecular weights ranging from 500 to over 20,000 Da [2,46]. Although tannins are water-soluble compounds, the increase in the molecular weight of the condensed tannins limits their solubility in water. Condensed tannins may also contain amino acids and imino acids [47]. Unlike hydrolyzable tannins, they do not contain any saccharide components [48] and do not readily undergo hydrolysis. During degradation, they may break down into anthocyanidins, a class of plant pigments responsible for the red, purple, and blue colors observed in many fruits, flowers, and vegetables [49].
Beyond these two main groups, other tannins, such as complex tannins and phlorotannins, are also reported (Figure 2). Complex tannins are hybrid structures combining hydrolyzable and condensed tannins, where flavan-3-ols link with gallotannins and ellagitannins via C–C bonds [4]. In turn, phlorotannins comprise phloroglucinol units, whose derivatives form oligomeric or polymeric structures with molecular weights ranging from 126 to even 625,000 Da [50]. These tannins are primarily found underwater in brown macroalgae, including species from the Fucaceae, Sargassaceae, and Alariaceae families [4,45,51]. In 2022, phlorotannins derived from brown algae comprised the largest revenue share of 78.4%, with the fastest forecasted annual growth of 6.8% from 2023–2030 [43].
Tannins are commercially extracted from different resources and various parts of many plants, depending on their application. Tannins used in the tanning industry are generally condensed tannins extracted from quebracho (Schinopsis balansae or Schinopsis lorentzii) and mimosa (Acacia mearnsii) trees. Proanthocyanidins, are also derived from the wood of acacia (Acacia mollissima, A. mearnsii), the bark of pine and oak, and sorghum seeds [48]. In turn, hydrolyzable tannins are obtained from gall-producing plants such as sumac (Rhus semialata), which yields tannic acid (Chinese gallotannin), and oak (Quercus infectoria), which produces Turkish gallotannin. Other sources include the fruits of chebulic myrobalan (Terminalia sp.), which contain chebulinic acid, as well as the wood of chestnut (Castanea sativa), oak, the pods of divi-divi (Caesalpinia coriaria), and sumac leaves [48]. Additionally, tannins are abundant in plants cultivated for food, such as coffee, tea, cocoa, nuts, fruits, vegetables, herbs, spices, and cereals [52]. The abundance of tannins varies between species, depending on the specific part of the plants. Higher concentrations of tannins have been found in barks, leaves, seeds, shells, roots, peels, etc. These components often are by-products of the agro-food industry, such as grape pomace, pomegranate peels, nut shells, tea, and coffee residuals. Their valorization into tannin-based reagents can contribute to a circular economy strategy and increase the sustainability of the obtained products.

4. Extraction Methods

The diversity of tannins in chemical composition and structures, along with their sources and technical requirements linked to their applications, drives a search for the optimal extraction method. The efficiency of tannin extraction is strongly influenced by solvent choice, temperature, time, the solid-to-solvent ratio, biomass raw material preparation, particle size, and moisture content [5,34]. Industrial biomass processing to obtain tannins uses aqueous solutions of sodium sulfite or bisulfite (1.5–2.0%) and sodium bicarbonate (0.5%). The process is usually conducted with hot water at 50 to 100 °C [53]. Before extraction, biomass is ground or milled to the desired particle size. The post-treatment of tannin extract is typically performed through spray- or vacuum-drying, resulting in a powdered or solid final product.
Water extraction stands out due to its simplicity, versatility, cost-efficiency, and sustainability, mainly when water is used instead of other organic solvents [34]. However, aqueous extraction leads to the co-extraction of other water-soluble compounds. Therefore, tannin extracts may contain non-phenolic impurities, including saccharides, hydrocolloids, and stilbenes, requiring post-treatment procedures to purify the final product based on its application restrictions [53,54,55,56]. The content of impurities can also be controlled by selecting the appropriate extraction parameters [5]. Alternative solvents such as ethanol, methanol, and acetone are utilized to enhance process efficiency and selectivity. These solvents are typically used as co-solvents with water in varying concentrations ranging from 50% to 85% [5,34,57]. In many other studies, other solvents were also employed: disopropyl ether, ethyl acetate ethyl ether, n-hexane, chloroform, ionic liquids, and deep eutectic solvents [5,6,46]. Furthermore, new techniques were used to enhance the efficiency of the process, such as microwave-assisted, ultrasound-assisted, and enzyme-assisted extraction [58,59] and extraction with supercritical fluids [5,34]. A general schematic of the extraction process is presented in Figure 3.
Ultrasound-assisted extraction (UAE) enhances the recovery of tannins and other polyphenols by employing acoustic cavitation, a phenomenon through which sound waves (typically 20–100 kHz) induce the formation and violent collapse of microbubbles in a liquid medium. This collapse generates localized high pressures (>1000 atm) and temperatures (estimated up to ~5000 K), disrupting plant cell walls and enhancing solvent penetration and mass transfer rates [60,61]. The UAE significantly reduces extraction time and allows the use of low temperatures (25–60 °C), protecting thermolabile compounds like hydrolyzable tannins from oxidative degradation [62]. The technique also lowers solvent consumption and supports the use of GRAS solvents (e.g., ethanol and water), aligning with green chemistry principles [63]. However, UAE is not without limitations. The cavitation process can generate reactive oxygen species (ROSs) such as hydroxyl radicals, potentially leading to the degradation of sensitive phenolics if parameters (amplitude and time) are not optimized [64]. Moreover, UAE often lacks selectivity, extracting both desired bioactives and unwanted compounds like sugars, chlorophyll, or proteins [62]. Industrial scaling is also challenging due to uneven energy distribution in larger volumes, which affects reproducibility and efficiency [65].
Enzyme-assisted extraction (EAE) is an environmentally friendly and efficient method for recovering tannins and other polyphenolic compounds from plant materials. The method uses specific enzymes, such as cellulases, hemicellulases, pectinases, and tannases, to hydrolyze structural polysaccharides and protein–phenolic complexes in the plant cell wall, thus enhancing the release of bioactive compounds into the solvent. The main mechanism behind EAE is the enzymatic degradation of the plant matrix, which improves mass transfer and reduces resistance to solvent penetration [66]. EAE offers several benefits. It significantly increases extraction yield: for example, Carrera et al. [62] showed that applying pectinase to grape skins improved polyphenol extraction by more than 60% compared to untreated samples. Moreover, the process is conducted under mild conditions—typically 30–55 °C and pH 4–6—which preserves thermolabile polyphenols such as gallic acid, catechins, or ellagic acid [67]. It also enables the use of green solvents such as water or ethanol, aligning with principles of sustainable processing [63]. Finally, the polyphenol-rich extracts obtained via EAE often demonstrate enhanced biological activity, including antioxidant and antimicrobial effects, due to increased release of free and bound phenolic forms [68]. However, EAE also faces limitations. The high cost of purified enzyme preparations can restrict its industrial scalability [66]. Moreover, the process must be carefully optimized in terms of enzyme concentration, incubation time, and matrix composition to prevent over-hydrolysis or the degradation of sensitive compounds [67]. EAE is most effective when tailored to a specific substrate and can be further enhanced in combination with physical techniques such as ultrasound or microwave-assisted extraction [69].
Extraction conducted with supercritical fluids provides additional functionality. Controlling the parameters of the process conditions enables tuning of solvent properties. This phenomenon may change the mechanism of tannin extraction involving secondary reactions such as hydrolysis or depolymerization. Therefore, solvent choice, temperature, and pressure may be crucial for the composition of tannin extract in terms of both polyphenolic compounds and impurities.

4.1. Extraction in Supercritical Fluids

The supercritical state is when a fluid has a temperature and pressure above its critical point, and the traditional gas and liquid phases no longer exist. Over this point, in a wide temperature and pressure range, the density of supercritical fluid is continuously changing without an abrupt change in phase transition. In turn, the subcritical state refers to conditions below the critical point, where the substance still exists as a distinct liquid or gas. As pressure and temperature increase but stay below the critical point, the solvent transitions through a dense gas or compressed liquid phase. When pressure is increased at a constant temperature, gas molecules become more compressed. If the critical point is exceeded, the distinction between gas and liquid disappears, forming a supercritical fluid. At this point, the density increases while compressibility decreases, making it behave differently from typical gases and liquids. In the supercritical state, solvents exhibit gas-like viscosity, diffusivity, and liquid-like density. They show tunable properties, including solvent polarity and reactivity, along with temperature and pressure changes. Adjusting these parameters makes it possible to determine optimal mass transfer conditions, thereby improving process efficiency and making supercritical fluids exceptional solvents with numerous applications [70,71,72].

4.1.1. Solvent Selection—Efficiency

Supercritical fluids provide advantages regarding mass transport between a solid and a solvent, which is crucial for extraction. High density increases the extraction yield, while low viscosity enhances penetration of the biomass matrix. However, along with temperature and pressure modulation, other properties of the solvent are changed, such as the dielectric constant and ionization coefficient. As a result, these changes affect the solvents’ ability to dissolve various compounds.
The SFE tannins extraction is mainly conducted with water, ethanol, methanol, and carbon dioxide, used as a co-solvent. As shown in Table 2, the dielectric constant determining the polar or non-polar nature of the solvating properties decreases when the conditions approach a critical point. The most widely used solvent for supercritical extraction of tannins, carbon dioxide (SC-CO2), is already non-polar under standard conditions. Its dielectric constant changes from 1.6 [73] to below 1.1 [74] under supercritical conditions. In the case of water, as the temperature increases, its dielectric constant decreases from a higher value of 78 [75] to only 5.35 [76]. This means that, at temperatures above 200 °C, the ability to extract hydrophobic compounds increases. Similar changes are observed for ethanol and methanol, the most frequently used solvents under standard pressure.
SC-CO2 is the most widely used method for extracting different biomass components in various studies [7,83]. However, its nonpolar characteristics limit its ability to extract tannins, which are hydrophylic. Co-solvents like ethanol, methanol, or water are often used to enhance extraction, modifying the polarity of the supercritical fluid and improving tannin solubility. The high diffusivity and tunable solvating power of SC-CO2 allow selective extraction of certain fractions of tannins. Depending on pressure and temperature settings, different molecular weight fractions of tannins may be extracted, leading to changes in composition. The extracted tannins often exhibit enhanced antioxidant activity due to the selective removal of lower-molecular-weight compounds while preserving bioactive polyphenols. SFE can alter the balance between hydrolyzable and condensed tannins in an extract, leading to different functional properties in food and pharmaceutical applications [84,85]. Despite the low efficiency of SC-CO2 in total tannin extraction, its application is justified by technical and economic factors. Due to its low critical parameters, the use of carbon dioxide as a co-solvent with polar solvents reduces the energy demand for the process [86].
As reported by Veggi et al., the extraction of jatoba bark (Hymenaea courbaril L.) yielded different results depending on the solvent system used. CO2 alone and CO2 with ethanol resulted in the lowest extraction yield (0.28% d.b. and 0.58% d.b.). Supercritical CO2 with water as a co-solvent significantly outperformed the previous two examples (24% d.b.) [47]. Studies by Markom and Hasan on Phyllanthus niruri showed similar effectiveness, depending on the solvents used. The yield was lowest with pure CO2 (1.09%) and increased when ethanol (1.57%) or methanol (1.73%) were used as co-solvents. Still, the highest yield (19.83%) was achieved using 50% ethanol–water with CO2 as an extraction mixture under supercritical conditions [87]. SFE of tannins from Acacia mearnsii by Pansera and Iob showed that supercritical CO2 alone was ineffective, but the yield increased to 9.5% with ethanol and methanol mixtures [23]. The most effective co-solvent, yielding 25% tannins at 60 °C and 20 MPa, was water. Some polyphenols, especially flavonoid aglycones and phenolic terpenes, are poorly soluble in water and require organic solvents [79,88]. Therefore, chloroform was also used for tannin extraction among the abovementioned solvents under supercritical conditions [75]. Chloroform is mainly used for the isolation of low-polarity lignans and glycosides.
Many studies, especially those with a practical or industrial focus, prioritize the evaluation of antioxidant activity (AA) over the extraction yield. The antioxidant properties of polyphenolic compounds, particularly tannins, are a key aspect of their industrial relevance. These compounds are often added to materials such as polymers not only to enhance mechanical performance but also to improve oxidative stability [89]. This activity is commonly assessed using standardized assays such as DPPH ([2,2-di(4-tert-octylphenyl)-1-picrylhydrazyl]), FRAP (Ferric Reducing Antioxidant Power), TRAP (Total Peroxyl Radical Trapping Antioxidant Parameter), and ABTS (2,2’-Azinobis-(3 3-ethylbenzothiazoline-6-sulfonic acid) [90,91]. Notably, the antioxidant potential of polyphenols is closely linked to their chemical structure—extracts rich in flavonoid subunits generally exhibit higher AA [92]. In the study on jatoba, Veggi et al. demonstrate an increase in antioxidant activity (AA), along with an increase in extraction yield [47]. However, this activity can vary significantly, depending on the plant species, the specific plant part used, the growing conditions, and even the harvest season [65], as well as the extraction parameters (temperature, time, and pressure).

4.1.2. Influence of Temperature

Polyphenolic compounds are sensitive to high temperatures and can undergo degradation. Therefore, extraction under supercritical conditions may lead to the breakdown of compounds into their components [93]. However, the key difference lies in the duration of the process and, consequently, the exposure time of the compounds to high temperatures. In conventional extraction methods, the process typically lasts around 3 to 10 h [23,56,71]. In the case of supercritical extraction, higher density and reduced viscosity resulting from increased pressure and temperature lead to increased diffusion coefficients, which can shorten the exposure time of compounds to high temperatures to a maximum of 3 h [71]. Therefore, limiting the process time has not only economic significance due to reduced processing duration but also reduces the exposure time of tannins to high temperatures. Studies have shown that longer processing times decrease both the extraction yield and the antioxidant activity of the products, likely due to secondary reactions such as degradation and hydrolysis [94]. Additionally, by using CO2 as a co-solvent, the critical conditions of the entire mixture are lowered (40–70 °C) [47,75,95], usually not exceeding the temperature of conventional solvent extraction 40–100 °C [5,71]. Therefore, supercritical fluid extraction can be viewed as a method that preserves the chemical composition of polyphenols. Simultaneously, the ability to control process parameters can be a promising advantage in increasing the selectivity of tannin extraction using supercritical fluids.

4.1.3. Selectivity

The selectivity of the supercritical fluid extraction process is influenced by pressure, solvent type, and co-solvent concentration. Higher pressures increase CO2 density, enhancing solvating power but reducing selectivity by co-extracting non-target compounds such as waxes and chlorophylls. Studies of tannins extraction from the herb Labisia pumila [96] showed that moderate pressures (around 28.3 MPa) optimize selectivity by balancing solubility and compound fractionation. Co-solvent addition, particularly ethanol (60–70%), improved selectivity for polar phenolic acids (gallic acid, methyl gallate, caffeic acid) by modifying the polarity and hydrogen bonding capacity of CO2. However, excessive ethanol (>80%) lowered extraction efficiency due to changes in phase behavior and solvent interactions. The extracted products primarily included phenolic acids (gallic acid, caffeic acid), flavonoids, and minor carotenoids. Higher temperatures (>50 °C) degraded antioxidants, while lower temperatures (<40 °C) reduced solubility, impacting both yields, while 16% co-solvent concentration ensured high phenolic content recovery.
In the study [47], the composition of the tannin extracts from jatoba Hyamenaea coubaril was analyzed after different solvents were used. Using pure CO2 allowed for the removal of small amounts of terpenoids and flavonoids, as well as minor quantities of compounds exhibiting oxidative activity. Carbon dioxide with ethanol retained more compounds, but the activity remained low. When water was used as a co-solvent, alkaloids were extracted, and the amount of captured oxidatively active compounds increased significantly. Variations in extracted compounds were also reported in another study. By studying the extraction of tannins from stem and aerial parts of Phyllanthus niruri Linn [87], scientists have shown that CO2 alone extracted a minimal amount of phenolics. Ethanol and methanol improved flavonoid solubility, and ethanol–water mixtures maximized ellagitannin content (gallic acid: 0.39–0.48%; corilagin: 2.42–3.00%; and ellagic acid: 5.94–6.48%) (Table 3).

4.1.4. Reactivity of Supercritical Solvents

The ionic product of water (Kw) makes it a reactive solvent. Additionally, variations in temperature and pressure significantly affect the ionic product of water, influencing its reactivity. At 25 °C and 0.1 MPa, Kw is approximately 1.0 × 10−14 [101]. As temperature increases, Kw initially decreases, reaching a minimum of around 250 °C, and then increases reaching some order of magnitude higher at higher pressure values until the critical point of water at 374 °C. In supercritical water (above 374 °C and 22.064 MPa), the dielectric constant decreases, affecting the extent of ionization. This leads to a decrease in Kw compared to subcritical conditions. In subcritical regions, water, due to the high concentration of H3O+ and OH−, may act as an acid or basic-like catalyst [72]. The catalytic activity of water in this region may accelerate secondary reactions such as hydrolysis, leading to the decomposition of complex structures of tannins [94]. The extracts may contain components of hydrolyzable tannins such as D-glucose, tannic acid, gallic acid, and ellagic acid. These extracts can be used to produce cross-linking additives (e.g., for films) [102,103,104]. In turn, the studies of flavonoid extraction from onion skin showed that an increase in the temperature caused quercetin decomposition to additional unidentified components of the extracts [105]. Similarly, Plaza et al. discussed the tendency of anthocyanins to degrade in subcritical water [106].
Unlike water, CO2, ethanol, and methanol are not classified as highly reactive solvents. However, in the case of the alcohols methanol and ethanol under supercritical conditions, the hydrogen bonding and length of chain clusters are reduced, resulting in the breakdown of alcoholic agglomerates to free monomers. The change in polarity of alcoholic molecules may influence their role as nucleophiles and hydrogen donors in side reactions with organic compounds [107,108]. Considering the increased reactivity of the most widely used solvents under sub- and supercritical conditions for tannin extraction, obtaining polyphenolic products may differ from native compounds due to secondary reactions occurring during extraction.
This issue, however, is not explored in the literature. The comparison of results obtained in different studies is unreliable because of the variability in biomass resources, even when the studies were conducted on the same species. Another challenge is the diversity of tannin structures. The determination of all components of the usually complex extracted mixtures requires in-depth studies to elucidate the possible reactions and mechanisms involved in the process of tannin extraction with supercritical fluids.

4.1.5. Environmental Issues and Perspectives

Supercritical fluid extraction has numerous potential applications in tannin extraction, particularly in the food, nutraceutical, pharmaceutical, and polymer industries. For decades, SC-CO2 has been used for the extraction of various botanicals, including commercial processes such as oils and nutraceuticals extraction from marine and terrestrial biomass by Pharmalink Extracts Ltd. (Appleby, New Zealand) [94] and coffee bean decaffeination d by K. Zosel as described by Keglevich [109]. The main advantages of this process are the ease of separation of extracted compounds via simple depressurization and the closed-loop system in terms of solvent recovery (Figure 4). Moreover, carbon dioxide is considered a non-toxic, inexpensive, chemically inert, and nonflammable solvent.
The increase in the environmental impact of SFE processes is primarily associated with using co-solvents—methanol and ethanol. Although water yields better results for the efficiency of tannins extraction, the lower boiling point of these alcohols makes them more advantageous when considering solvent evaporation from extracted products, lowering the energy demand of the process. However, the life cycle analyses (LCAs) of phenolic compounds extracted from bark spruce show that using ethanol as a co-solvent is the dominating contributor, exceeding 70%, in all studied environmental categories except ionizing radiation. This is due to ethanol loss in the residual biomass, for which the recovery approximates only 82–83% [112]. Integrating SFE extraction with bioethanol production in a biorefinery system may reduce its environmental impact [113].
The use of SC-CO2 acting as a co-solvent with water or alcohols offers technical advantages. Its presence in the solvent mixture lowers the critical temperature and reduces energy consumption. Thus, supercritical fluid tannin extractions occur at temperatures similar to conventional extractions but in a significantly shorter time. Despite being an energy-demanding process due to solvent pressurization and heating, life-cycle analyses of various product extractions from different biomass resources have shown a reduced environmental burden in most categories compared to conventional solvent extraction [114,115]. However, energy demand remains the most important factor in the environmental performance of the extraction processes. Its consumption can be limited by internal heat recovery and switching to renewable energy.
The results of LCA studies, presented in the literature, were mostly obtained by analyzing processes performed on a laboratory scale. Improvements in process intensification and heat economy during scale-up can reduce the environmental impact of SFE processes. Therefore, a better understanding of the mechanisms, optimization, upscaling, and life cycle analyses is needed to increase the maturity of this environmentally friendly technology.

5. Conclusions

Tannins are bioactive compounds with a wide spectrum of applications, ranging from food and feed additives to pharmaceuticals, cosmetics, and traditional uses such as winemaking and leather tanning. As the global demand for tannin-rich products grows, interest in developing more efficient and selective extraction methods also increases. Currently, conventional aqueous extraction remains the primary industrial method. However, it often suffers from limitations such as poor selectivity, long processing times, and high energy requirements due to elevated temperatures and high amounts of water evaporation from the tannin extract.
As a response, alternative extraction technologies are being explored, among which is supercritical fluid extraction (SFE), which has emerged as a promising solution. SFE offers precise control over process parameters—including pressure, temperature, and co-solvent concentration—enabling the selective targeting of specific tannin types (e.g., hydrolyzable or condensed tannins), depending on the desired application.
Among the various supercritical fluids investigated, carbon dioxide (CO2) is most frequently used due to its moderate critical parameters, non-toxicity, and cost-effectiveness. However, its poor polarity limits its ability to extract polar compounds such as tannins. This limitation can be addressed through the addition of polar co-solvents (e.g., water, ethanol, or methanol), which improve solubility and extraction yield by modifying the overall polarity of the fluid mixture. Moreover, CO2 use reduces the critical conditions of the system, lowering energy consumption.
SFE systems offer further advantages, such as low viscosity, high diffusivity, and enhanced mass transfer rates, all of which contribute to improved process efficiency. The use of SC-CO2 in combination with water has been particularly effective, with reported tannin extraction yields reaching up to 35%. However, tannin extracts may contain their degradation products, resulting from high temperatures or the reactive nature of supercritical water or alcohols.
This reactivity can lead to unwanted by-products, but it also enables the creation of valuable compounds for industrial use. However, the mechanisms behind these transformations remain insufficiently explored, necessitating further investigation to recognize their potential fully.
Despite the proven versatility and commercial viability of SFE, its application to tannin extraction remains relatively new and under active development. Significant research gaps remain, particularly in process scale-up, optimization, and environmental impact assessment. While environmental concerns related to SFE typically focus on energy demand (for solvent heating and gas compression) and the use of organic co-solvents, these issues are context-dependent. SFE may offer a more sustainable alternative than conventional organic solvent-based methods, particularly when using food-grade or recoverable solvents like ethanol. Therefore, supercritical fluid extraction represents a promising, tunable, and efficient approach for tannin recovery. Continued research and process development are essential to realize its full potential and to address current technological and environmental challenges.

Author Contributions

Conceptualization, I.J.-S. and P.S.; methodology, P.S.; validation, J.H. and I.J.-S.; formal analysis, I.J.-S.; investigation, P.S.; data curation, J.H. and I.J.-S.; writing—original draft preparation, P.S. and I.J.-S.; writing—review and editing, J.H. and P.S.; visualization, P.S. and J.H.; supervision, J.H. and I.J.-S.; project administration, J.H.; funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 15 05914 g001
Figure 1. Applications of tannins (based on data from [1,2,3,4,5,6,7,8,9,10,11,12,13,14]).
Figure 1. Applications of tannins (based on data from [1,2,3,4,5,6,7,8,9,10,11,12,13,14]).
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Figure 2. Selected examples of tannin structures representing main tannin groups.
Figure 2. Selected examples of tannin structures representing main tannin groups.
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Figure 3. Scheme of conventional production of tannins [6,25].
Figure 3. Scheme of conventional production of tannins [6,25].
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Figure 4. Scheme of tannin extraction with SFE [110,111].
Figure 4. Scheme of tannin extraction with SFE [110,111].
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Table 1. Tannin content in various plant species obtained through different extraction methods, along with their applications.
Table 1. Tannin content in various plant species obtained through different extraction methods, along with their applications.
PlantExtraction
Method		Tannin Content
(Plant Part)		Type of
Tannins		ApplicationsRef.
QuebrachoC164.3 mg/g DMCondensedOil and ceramics industry, anticorrosive for metal, wood preservation, packaging animal food, leather processing[2,8,9]
OakC74.82 CTC mg TAE/g DM
863.07 HTC mg TAE/g DM		Hydrolysable
Condensed		Wine quality
improvement,
wood preservation,
antioxidants		[2,10,11]
MAE91 TTC mg/g DM (bark)
472 TTC mg/g DM (gall)
CoffeeC5.45 mg TAE/g (pulp)CondensedAntioxidants[12,13,14]
CFW12.2 mg GAE/g (husk)
13.2 mg GAE/g (skin)
CFS52.07 mg CE/g
TeaUAE30.63 TTC mg GAE/g DMCondensedAntioxidants,
antimicrobial		[14,15]
C110.96 TPC mg/g
CFS47.4 mg CE/g
OliveC3.28 mg EE/g (EOCs)CondensedCoagulants,
adhesives,
colorants,
feed		[16,17,18,19]
M41.34–67.99 mg CE/g DE
C11.1 mg/g DM
SFEFS14.01 TPC mg GAE/g
ChestnutSE299.63 mg/g DMHydrolysableLeather-soaking process, wood adhesives,
wine production,
anticorrosive primers 		[20,21]
CM257.59 mg/g DM
UAE297.24 mg/g DM
SFE132.59 mg/g DM
MimosaC108.2 mg/g DMCondensedLeather tanning,
wood adhesives,
feeding		[8,9,22,23]
SFE135 mg/g DM
TaraC647.5 mg EGE/g DMHydrolysableLeather tanning,
wood preservation,		[2,9]
GambierC169.5 mg C1E/g DMCondensed Leather tanning [9]
PineC35 mg/g (bark)CondensedFood and pharmaceutical industry,
nutrition,
agriculture,
food packaging		[24,25,26]
51.4 mg/g (bark)
70.4 mg/g
SpruceC70 mg/gCondensed
Hydrolysable		Antioxidants,
food,
medicinal,
adsorbents,
biocoagulants		[27,28,29,30]
94.82 TTC mg TAE/g DM (bark)
41.6 TEY mg/g DM (bark)
SFE253.64 TTC mg TAE/g DM (bark)
GrapeC9.93 mg EE/g (marc)CondensedPlastic and wood adhesive,[2,31,32,33]
MAE66.9 mg EE/g (marc)
C25.75 TTC mg GAE/g
UAE20.04 TTC mg GAE/g
C9.9 mg/g (skin)
106 mg/g (seed)
25 mg/g (stem)
C—conventional extraction; MAE—microwave-assisted extraction; CFW—conventional from waste products; CFS—conventional from spent products; UAE—ultrasound-assisted extraction; M—maceration; SFEFS—supercritical fluid extraction from spent products; SE—soxhlet extraction; CM—cold maceration; DM/DE—dry mass/dry extract; CTC—condensed tannin content; HTC—hydrolyzed tannin content; TTC—total tannin content; TPC—total phenolic content; TEY—tannin extraction yield; TAE—tannic acid equivalent; GAE—gallic acid equivalent; CE—catechin equivalent; EE—epicatechin equivalent; EOCs—exhausted olive cakes; EGE—epigallocatechin gallate equivalent; C1E—cyanidin 1 equivalent.
Table 2. Critical parameters of solvents used for the extraction of tannins.
Table 2. Critical parameters of solvents used for the extraction of tannins.
SolventCritical
Temperature		Critical PressureDensity
(20 °C) [77]		Critical
Density		Dielectric Constant
[75]		Critical Dielectric ConstantReference
Tc [°C]pc [MPa]ρc [g/cm3]ρc [g/cm3]Κ [-]Kc [-]
Carbon dioxide31.17.380.001980.4691.6<1.1[38,43,45]
Water374.322.120.9980.34878.365.35[78]
Methanol239.68.090.7910.27232.618.40
(10 Mpa, 218 °C)		[79]
Ethanol240.96.140.7890.27624.851.90
(244 °C)		[80]
Chloroform319.05.791.4790.4914.71n.a[81,82]
Table 3. Example results of supercritical tannin extraction.
Table 3. Example results of supercritical tannin extraction.
Raw MaterialSolvents; Conditions; EfficiencyAA; TPC; TTC; Specific Compounds ContentAnalysis MethodsSource
Jatoba bark
(Hymenaea courbaril L.
var stilbocarpa)		CO2; 50 °C, 35 MPa; 0.28%3.58 IC50 mg/cm3; n.a.; n.a.;
terpenoids, phenolic carboxylic acids		DPPH, ESI-MS, TLC,
Folin–Denis method		[47]
CO2:H2O(9:1, v/v); 50 °C, 35 MPa; 24%0.2 IC50 mg/cm3; 335.00 mg TAE/g extract;
335.00 mg TAE/g extract;terpenoids, alkaloids
CO2:EtOH(9:1, v/v); 50 °C, 35 MPa; 0.58%7.08 IC50 mg/cm3; n.a.; n.a.;
terpenoid, glycosylated flavonoid canferol, flavonoids
Black wattle bark
(Acacia mearnsii)		CO2:EtOH; 60 °C, 25 MPa; n.a.n.a.; n.a.; 95 mg/g raw material;
tannic acid		HPLC[23]
CO2:H2O; 60 °C, 25 MPa; n.a.n.a.; n.a.; 135 mg/g raw material;
tannic acid
Grapevine seed
(Vitis vitifera)		CO2:MeOH(7:3, v/v); 80 °C, 65.5 MPa; n.a.n.a.; n.a.; 600 mg/g raw material;
catechin, epicatechin		HPLC, MS,
SFC-UV		[97]
CO2:MeOH(65:35, v/v); 80 °C, 65.5 MPa; n.a.n.a.; n.a.; 720 mg/g raw material;
catechin, epicatechin
CO2:MeOH(60:40, v/v); 80 °C, 65.5 MPa; n.a.n.a.; n.a.; 770 mg/g raw material;
catechin, epicatechin
Chestnut shells industrial waste
(Castanea Sativa Mill.)		CO2:MeOH(8:2); 70 °C, 7 MPa, 1 h; 35.23%n.a.; 60.1%; n.a.;
no detailed content of the extract provided		Hide-powder method[20]
H2O:CO2; 80 °C, 10 MPa, 3 h; 25.65%n.a.; 55.35%; n.a.;
no detailed content of the extract provided
Barro fruit powder
(Terminalia bellirica)		CO2:EtOH; 60 °C, 20 MPa, 1 h; n.a.n.a.; 96.10% (chebulagic acid), 79.92% g/g sample;
chebulagic acid		NMR, HPLC, MS[71]
SFE dynamic CO2; 60 °C, 20 MPa, 1 h; n.a.n.a.; 15.79% (chebulagic acid); 48.40% g/g sample;
chebulagic acid
SE EtOH 95%; 100 °C, 0.1 MPa, 10 h; n.a.n.a.; 40% (chebulagic acid); 39.40% g/g sample;
chebulagic acid
Pomegranate leaves
(Punica granatum L.)		CO2; 40–50 °C, 10/15/20/30 MPa; 0.21–0.67%40.20–60.70%; n.a.; 257–389 mg/g;
eicosanol, squalene, tocoferol derivatives		MS[95]
Pine sapwood and knotwood (Pinus pinaster)CO2; 50 °C, 25 MPa; 1.60%n.a.; n.a.; 19.38 mg/g;
stilbenes, flavonoids and lignans		Folin-Ciocalteu method
TEAC, FID		[98]
CO2:EtOH(9:1); 50 °C, 25 MPa; 4.1%n.a.; 7.60%; 75.61 mg/g;
stilbenes, flavonoids and lignans
Pecan nut shell
(Carya illinoinensis)		CO2:EtOH(9:1); 50 °C, 10 MPa; n.a.4.95 µmol TEAC/g 1.91 mg TEAC/g; 0.34 mg GAE/g; 0.48 mg CE g;
gallic acid, epigallocatechin, epicatechin gallate		Folin–Ciocalteau method, HPLC, ABTS, DPPH[99]
CO2:EtOH(9:1); 50 °C, 20 MPa; n.a.100.00 µmol TEAC/g 79.20 mg TEAC/g; 9.30 mg GAE/g; 29.00 mg CE g;
gallic acid, epigallocatechin, epicatechin gallate
Tea leaves
(Camellia sinensis L.)		CO2:EtOH; 50 °C, 18,8 MPa; n.a.2.62 μMol TEAC/cm3;
1.31 mg GAE/cm3; 0.50 mg TAE/cm3;no detail content of extra provided		n.a.[100]
Spruce bark
(Picea abies)		CO2: 70% EtOHaq; 40 °C, 10 MPa; 0.65%n.a.; 690.94 mg/g dry extract;
111.47 mg/g dry extract;
ferulic acid, p-coumaric acid		HPLC[30]
CO2: 70% EtOHaq; 50 °C, 10 MPa; 0.80%n.a.; 477.16 mg/g dry extract; 71.71 mg/g dry extract; ferulic acid, p-coumaric acid
CO2: 70% EtOHaq; 60 °C, 10 MPa; 0.70%n.a.; 829.35 mg/g dry extract; 253.64 mg/g dry extract;
ferulic acid, p-coumaric acid
CO2: 70% EtOHaq; 50 °C, 15 MPa; 1.81%n.a.; 525.02 mg/g dry extract; 109.64 mg/g dry extract;
ferulic acid, p-coumaric acid
CO2: 70% EtOHaq; 50 °C, 20 MPa; 2.08%n.a.; 377.44 mg/g dry extract; 86.57 mg/g dry extract; ferulic acid, p-coumaric acid
n.a.—not available; DPPH—2,2-Diphenyl-1-picrylhydrazyl; MS—mass spectrometry; TLC—thin-layer chromatography; HPLC—high-performance liquid chromatography; TEAC—Trolox-equivalent antioxidant capacity; ABTS—2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid); IC50—coefficient derived from the DPPH method. The lower this parameter, the higher the antioxidant activity.
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Słota, P.; Harasym, J.; Jacukowicz-Sobala, I. Supercritical Fluid Extraction—A Sustainable and Selective Alternative for Tannin Recovery from Biomass Resources. Appl. Sci. 2025, 15, 5914. https://doi.org/10.3390/app15115914

AMA Style

Słota P, Harasym J, Jacukowicz-Sobala I. Supercritical Fluid Extraction—A Sustainable and Selective Alternative for Tannin Recovery from Biomass Resources. Applied Sciences. 2025; 15(11):5914. https://doi.org/10.3390/app15115914

Chicago/Turabian Style

Słota, Patryk, Joanna Harasym, and Irena Jacukowicz-Sobala. 2025. "Supercritical Fluid Extraction—A Sustainable and Selective Alternative for Tannin Recovery from Biomass Resources" Applied Sciences 15, no. 11: 5914. https://doi.org/10.3390/app15115914

APA Style

Słota, P., Harasym, J., & Jacukowicz-Sobala, I. (2025). Supercritical Fluid Extraction—A Sustainable and Selective Alternative for Tannin Recovery from Biomass Resources. Applied Sciences, 15(11), 5914. https://doi.org/10.3390/app15115914

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