5.2.1 Solid–Liquid Extraction (SLE)

SLE is one of the most common techniques used for extracting bioactive compounds (phenolic compounds), including CGA, from different plant materials (Alirezalu et al. 2020). SLE consists of placing a solid sample in direct contact with a solvent, enabling the release of phytochemicals into the solvent, which can then be isolated through pressing or filtering the solution (Sharma and Kaushik 2021). A large variety of solvents can be used, such as ethanol, methanol, acetone, or the aqueous phase of solvent blends. Although this method is simple and cost-effective, it enables the extraction of thermolabile constituents only (Lama-Muñoz and del Contreras 2022), which requires additional operations, such as solid-phase extraction or subsequent column chromatography, to remove unwanted residues (Abubakar and Haque 2020). Moreover, too many factors can impact the extraction effectiveness, including the prolonged extraction time, the number of extractions, temperature, the polarity and type of solvent, and the ratio of solvent to sample (Lama-Muñoz and del Contreras 2022). Despite solid–liquid extraction being a simple and widely used technique, it presents multiple limitations, including prolonged processing time, low extraction efficiency, and the requirement for significant volumes of hazardous organic solvents (Okur et al. 2021). Therefore, new methods and techniques should be employed to address these issues effectively.

5.2.2 Soxhlet Extraction (SOX)

SOX is a continuous solvent extraction technique developed in 1879, also known as hot continuous extraction (Sharma and Kaushik 2021). It requires a specific apparatus that recirculates an organic solvent (pure or mixed in water), at ambient pressure and boiling temperature, over a solid sample, gradually dissolving the target compound (Rocchetti et al. 2019; Yu et al. 2023). Expanding on that, in this process, finely ground plant material is placed in a porous thimble within the Soxhlet apparatus. The heated solvent evaporates, moves into the thimble chamber, condenses, and dissolves the desired bioactive compounds. Once the chamber fills, the enriched solvent flows back into the boiling flask. This cyclic process enables efficient extraction without the need for constant solvent replenishment (Solomakou et al. 2022). Various elements can influence the SOX extraction method, including the volume of solvent in relation to the sample, the temperature used, and whether the mixture is stirred or not (Lama-Muñoz and del Contreras 2022). The Soxhlet extraction method has several practical advantages that make it popular in research. It uses less toxic solvents and is faster than traditional methods (Yu et al. 2023). A key benefit is the ability to repeatedly rinse the waste matrix with fresh solvent, improving extraction efficiency (Frosi et al. 2021). It also requires less solvent to extract a larger quantity of compounds (Sharma and Kaushik 2021). This technique is suitable for heat-stable plant materials, does not require filtration, and can handle high temperatures (Abubakar and Haque 2020). Additionally, it is especially useful for materials that are partially soluble or contain insoluble impurities (Abubakar and Haque 2020).

However, Soxhlet extraction comes with some major drawbacks. One of the main issues is the need for high temperatures, which makes it unsuitable for thermolabile plant materials (Abubakar and Haque 2020; Yu et al. 2023). Another limitation is that the process does not allow continuous agitation or stirring, which can reduce the efficiency of extraction (Abubakar and Haque 2020). Additionally, this method is not environment-friendly, as it releases harmful gases and exposes users to hazardous chemicals, raising concerns about its environmental impact and safety (Okur et al. 2021). Although Soxhlet extraction yields high concentrations of CGA (up to 41 mg/g in some studies), it is solvent-intensive and time-consuming, making it less favorable for large-scale applications (Rocchetti et al. 2019).

These limitations highlight the necessity to develop and adopt innovative, more efficient non-conventional extraction techniques that are environmental-friendly and suitable for heat-sensitive compounds. These techniques should overcome the drawbacks of traditional methods while ensuring higher efficiency and sustainability.

5.3.1 Ultrasound-Assisted Extraction (UAE)

Ultrasound-assisted extraction (UAE) is a non-conventional technique that utilizes high-frequency sound (ultrasound) waves exceeding 20 kHz to induce cavitation within the solvent, leading to the rupture of the plant cell wall and enhancing the surface area for solvent absorption. Ultrasounds are able to enhance extraction yields by means of combined mechanisms (sonoporation, fragmentation, deterioration, erosion, sonocapillarity, etc.) (Žlabur et al. 2021). As a result, secondary metabolites will be liberated, and a mass transfer of target compounds into the solvent occurs (Beaudor et al. 2023).

In this approach, plant material is dried, ground into a fine powder, and properly sieved. The resulting sample is then mixed with a suitable extraction solvent and placed in the ultrasonic extractor (Žlabur et al. 2021). The application of high-frequency sound energy accelerates the extraction by lowering the heat requirements (Abubakar and Haque 2020). There are several factors that can affect the effectiveness of the UAE extraction, such as temperature, sonication time, frequency of the ultrasonic waves, the nature of the sample, and solvent properties (Alirezalu et al. 2020).

Several studies showed mass recovery of bioactive compounds from SCG using the UAE extraction technique (Al-Dhabi et al. 2017; Valdés et al. 2020; Zhang et al. 2021). Caballero-Galván et al. (2018) demonstrated that a chlorogenic acid concentration of 46.53 mg/L was obtained through the UAE technique using 60% (v/v) ethanol as solvent, a v/w ratio of 20:1, ultrasonic power and frequency set at 750 W and 20 kWh, respectively, and a constant temperature (50°C ± 2°C) during 60 min. Meanwhile, conventional solvent extraction and Soxhlet extraction produced lower values of chlorogenic acid, 24.87 and 25.22 mg/L, respectively. Recently, Bouhzam et al. (2023) demonstrated that SCG water extraction assisted by ultrasound at room temperature produced the highest quantity of chlorogenic acid and caffeine, yielding 1.15 mg of chlorogenic acid per gram and 0.972 mg of caffeine per gram, respectively. This approach was found to be more effective when compared to the supramolecular solvent and vortex techniques (Bouhzam et al. 2023).

UAE technique provides various advantages, including shorter extraction time, reduced solvent usage, and higher yield, making it suitable for small sample sizes (Al-Dhabi et al. 2017; Beaudor et al. 2023; Žlabur et al. 2021). However, this technique has some limitations, as it may be challenging to reproduce consistently, and the high energy input during the extraction process may degrade the sensitive phytochemicals by generating free radicals (Abubakar and Haque 2020).

5.3.2 Microwave-Assisted Extraction (MAE)

Microwave-assisted extraction (MAE) is one of the innovative green extraction techniques developed for phenolic compounds extraction from different plants. The MAE technique employs microwave radiation to target an object, which can absorb electromagnetic energy in frequencies between 300 MHz and 300 GHz and wavelengths between 1 cm and 1 m (Alvi et al. 2022). This energy is subsequently converted into heat, which facilitates the movement of solvent into the sample matrix and the breakdown of analytes from the sample into the solvent (Coelho et al. 2021). The use of polar solvent leads to dipole rotation and migration of ions, enhancing solvent penetration and assisting the extraction process (Alirezalu et al. 2020). The remarkable selectivity of this technique enables an increase in extraction yields while requiring less time and solvent, potentially tripling outputs in comparison to conventional extraction methods such as SLE (Abdul Mutalib et al. 2024). The common solvents used to extract phenolic compounds from various plant species using the MAE technique are: ethanol, water, methanol, and acetone, but ethanol is preferred for nutraceutical purposes (Frosi et al. 2021). Multiple factors can impact its effectiveness, including microwave power, dielectric properties, duration of extraction, solubility, and characteristics of the solvent (Alvi et al. 2022).

It has been reported that water-organic solvent mixtures are more efficient than pure solvents for the recovery of CGAs. Based on previous results, the optimal ethanol percentage for CGAs extraction using MAE could be considered in the range of 50%–70% (Abdul Mutalib et al. 2024; Coelho et al. 2021). In a study published by (Solomakou et al. 2022), the MAE technique demonstrated the highest recovery yield (32.2 mg GAE/g d.b.) compared to UAE (18.52 ± 0.52 mg GAE/g dry SCG) and SLE (22.02 ± 0.98 mg GAE/g dry SCG) in the shortest time (5 min). There are significant variations in polyphenol yields from spent coffee grounds (SCG) extracted by the MAE technique, ranging from 7.694 to 31.216 mg/g dry SCG (Ranic et al. 2014), 57.49 mg GAE/g dry SCG (Budaraju et al. 2017), and up to 398.95 mg GAE/g dry SCG (Pavlović et al. 2013). These differences are likely attributed to variations in coffee production and extraction conditions (Solomakou et al. 2022). Upadhyay et al. (2012) demonstrated that MAE could achieve a CGA yield of 84 ± 2.8 mg/g using water as a solvent and a 1:4 solid-to-liquid ratio. Microwave-assisted extraction offers several significant advantages. It allows faster extraction times due to the rapid dielectric heating, which enhances process speed, selectivity, and high extraction performance (Sharma and Kaushik 2021; da Silva et al. 2022). This technique requires less solvent than conventional methods and provides high extraction yields with good reproducibility (Abubakar and Haque 2020; Frosi et al. 2021).

Among the critical limitations of this technique is that it works well for smaller phenolic compounds like gallic acid, quercetin, isoflavin, and trans-resveratrol, which can endure heating at 100°C for about 20 min, while larger molecules like tannins and anthocyanins are prone to degradation and are not suitable for this extraction (Alvi et al. 2022; Budaraju et al. 2017). One more drawback associated with this method is the low efficiency for volatile substances and the high costs of equipment (Frosi et al. 2021). Additionally, this technique does not support the use of nonpolar solvents, as utilizing them leads to minimal heat generation from the microwave radiation (Alvi et al. 2022).

5.3.3 Supercritical Fluid Extraction (SFE)

A supercritical fluid is any substance that shows both liquid and gas properties at its critical point (Clifford 2018). It behaves like a gas but also retains liquid-like qualities, allowing it to solvate the sample (Sharma and Kaushik 2021). CO₂ is generally used as a supercritical liquid in the SFE method, among several other supercritical fluids, such as nitrous oxide, pentane, trifluoromethane, ammonia, ethane, butane, and water (Al Khawli et al. 2019). Carbon dioxide is non-flammable, non-toxic (GRAS), and “green” solvent; thus, it has no significant impact on the environment (Roselló-Soto et al. 2019; Týskiewicz et al. 2018).

Basically, in SFE method, the supercritical fluid is pumped into a cylinder containing the plant material, then the extract laden liquid is collected in a separation chamber (Sharma and Kaushik 2021). SFE produces solvent-free extracts, as CO₂ evaporates during depressurization (Týskiewicz et al. 2018). This technique also ensures high-quality products rich in phenolics and flavonoids with strong antioxidant activity (Alirezalu et al. 2020).

This selectivity can be adapted depending on the compound to be extracted with the use of a co-solvent, since CO₂ is not suitable for the extraction of polar phenolic compounds. Ethanol is the most used, because it meets the green technology requirements (Al Khawli et al. 2019). Moreover, carbon dioxide may be more selective when pressure and/or temperature are optimized (Clifford 2018).

There is no recent study for the extraction of phenolic compounds from spent coffee grounds, nor coffee beans, using the SFE method. However, in a previous study, the extraction of green coffee beans with pure CO₂ as well as with 5% (w/w) of isopropyl alcohol or 5% (w/w) of ethanol as modifiers demonstrated a higher yield of 17% obtained from the extraction with carbon dioxide and ethanol (De Azevedo et al. 2008). Andrade et al. (2012) reported a CGA yield of 41 mg/g using SFE with CO₂ and ethanol as co-solvents.

SFE is particularly effective for heat-sensitive compounds, as it operates at lower temperatures than traditional extraction methods (Sharma and Kaushik 2021). Furthermore, the most important advantage of the SFE technique is its ability to alter the supercritical solvent properties by changing temperature and pressure or by using co-solvents (ethanol, methanol…) for selective extraction (Clifford 2018; Týskiewicz et al. 2018). While SFE offers excellent selectivity and purity of the extracted compounds, the high cost of equipment and operational complexity limit its widespread industrial application (Týskiewicz et al. 2018).

5.3.4 Subcritical Water Extraction (SWE)

Subcritical water extraction (SWE) is a technique where water is heated to temperatures exceeding its boiling point (100°C–374°C) and is placed under sufficient pressure, up to 22.1 MPa, to remain in a liquid state (typically above 5 Mpa) (Lorenzo et al. 2020; Zhang et al. 2020) to extract bioactive compounds. This technique alters the polarity of the water, allowing it to function like an organic solvent and effectively extract a wide variety of polar and less-polar bioactive compounds (phenolics, flavonoids, pigments, peptides, and essential oils) without relying on harmful organic solvents (Yabalak et al. 2024). This method is considered a green technology due to its use of water as the primary solvent, reducing reliance on harmful organic solvents (Gbashi et al. 2017; Zhang et al. 2020). Xu et al. (2015) achieved a CGA yield of 72.5 mg/g from SCGs using SWE at 180°C and a solid-to-liquid ratio of 1:10. SWE is particularly attractive for industrial applications due to its scalability and environmental sustainability (Ballesteros et al. 2017), under SWE conditions (T = 180°C, t = 30 min, L/S = 15 mL/g), reported TPC of 36.88 mg GAE, FRAP of 1.0 mmol Fe(II)/g SCG, and DPPH values of 119.02 μmol TE/g SCG. Similarly, SWE extraction of SCGs gives values of 3.4 mg CGA/g SCG for CGA, 22.45 mg GAE/g SCG for TPC, 0.31 mmol Fe(II)/g SCG for FRAP, and 69.31 μmol TE/g SCG for DPPH, under the optimum conditions of 180°C, 30 min, 30 mL/g SCG (Massaya et al. 2023). Recently, the extraction of phenolic compounds from SCGs using the SWE technique produced extracts with higher TPC values (170–331 mg GAE/g DW), especially SWE 150°C, higher DPPH (217–394 mg TE/g DW), and ABTS⁺ (958–975 mg TE/g DW) scavenging activities (Fernandes et al. 2025).

Compared to traditional extraction techniques, SWE has the advantage of utilizing non-toxic water as a solvent instead of toxic organic solvents, which provides huge environmental benefits. SWE improves extraction efficiency on both polar and non-polar compounds and reduces extraction time and energy consumption, which considerably reduces the cost of extraction processes, as well as environmental pollution (Aminzai et al. 2024; Yabalak et al. 2024).

5.3.5 High Hydrostatic Pressure-Assisted Extraction (HHPE)

HHPE is a non-thermal extraction technique that applies high hydrostatic pressure, in the range of 100–900 MPa, to break down the cell matrix and release bioactive compounds (Okur et al. 2021). In this technique, the application of high hydrostatic pressure enhances solvent penetration into plant cells, increasing cell permeability and promoting the diffusion of bioactive compounds (Alirezalu et al. 2020). This method preserves heat-sensitive compounds and reduces the need for solvents. Okur et al. (2021) reported CGA yields of 68 mg/g using HHPE at pressures up to 600 MPa, demonstrating the technique's potential for high-efficiency extraction with minimal environmental impact. In a recent study, it was reported that HHPE extraction using 80% methanol for 15 min at 500 MPa yielded 81.2 mg/g of CGAs and a TPC of 9.42 mg GAE/g (Pyrzynska 2025). Furthermore, a previous study showed that using HHPE (370 MPa, 50°C, 5.7 min) for oil extraction from pepper seeds resulted in higher antioxidant activity, shorter extraction time, and superior oil quality compared to SLE and UAE (Ma et al. 2019). The expensive equipment is the main disadvantage of this technique.

5.3.6 Supramolecular Solvent Extraction (SUPRAS)

SUPRAS extraction involves the use of nanostructured solvents to selectively extract compounds based on polarity (Torres-Valenzuela et al. 2019). SUPRAS extraction relies on the self-assembly of amphiphiles (special molecules) to form nanoscale structures that efficiently capture and dissolve target bioactive molecules from complex matrices. Their ability to selectively extract specific compounds, versatility, and eco-friendliness make them an emerging tool in natural product extraction and food waste valorization (Kfoury et al. 2025). It demonstrates robust performance for polar and nonpolar compounds (Marcinekova et al. 2024). This method is gaining attention for its environmental benefits, as the solvents used are often biodegradable. Torres-Valenzuela et al. (2019) reported CGA yields of 55 mg/g using SUPRAS, demonstrating the potential for eco-friendly, selective extraction.

Although SUPRAS is a promising, efficient, affordable, and eco-friendly extraction technique with wide potential uses, it still has some limitations, like challenges with volatile compounds and lacking cleanup steps; more work is needed to improve the method (Marcinekova et al. 2024).

5.3.7 Natural Deep Eutectic Solvents (NADES)

NADES-based extraction is a green method that uses naturally derived solvents such as sugars, polyalcohols, organic acids, and amino acids (García-Roldán et al. 2023). NADES are created by combining hydrogen bond acceptors (HBAs) with hydrogen bond donors (HBDs) (da Silva et al. 2024). The strong hydrogen combination lowers the melting point of the mixture, resulting in a stable liquid solvent that remains at room temperature. The supramolecular structure gives NADES the ability to partially disrupt plant cell walls by the formation of hydrogen bonds between cell wall constituents and the eutectic mixture (Chen and Mu 2019; Liu et al. 2018; Zdanowicz et al. 2018). This technique is more suitable for thermolabile compound extraction (García-Roldán et al. 2023).

Recently, NADES have been successfully used for the extraction of bioactive compounds from (Ahmad et al. 2021; Fanali et al. 2020; Oomen et al. 2020; Yoo et al. 2018; Zdanowicz et al. 2018). These studies found that betaine:triethylene glycol-based NADES was the most effective in polyphenol extraction from SCG (Fanali et al. 2020). García-Roldán et al. (2023) achieved a CGA yield of 48 mg/g using NADES, highlighting the method's sustainability and potential for industrial-scale application. The combination of DES citric acid and mannitol showed the highest efficiency in extracting TPC from SCGs under the conditions of 10% water, 80°C, and a solid–liquid ratio of 1:15 (w/w), yielding 1620.71 ± 3.72 mg GAE/L (C. N. da Silva et al. 2024). This combination also demonstrated the greatest antioxidant activity in all three methods assessed: FRAP at 1.071 ± 0.006 mol ascorbic acid/L, DPPH radical scavenging activity at 0.234 ± 0.001 mol TE/L, and ABTS radical cation scavenging activity at 0.319 ± 0.002 mol TE/L (da Silva et al. 2024). NADES extraction of phenolic compounds from coffee husk (50% water, 30 min, and a (1:10) (w/v) ratio) produced a yield of 6.16 mg GAE/g and the concentration of polyphenols of 307.81 mg/L (Maimulyanti et al. 2023).

NADES advantages that make it an interesting alternative to organic solvents include that are inexpensive components, easy to synthesize, biodegradable, recyclable, and eco-friendly with lower toxicity than organic solvents (da Silva et al. 2024; García-Roldán et al. 2023). However, combining some HBAs and HBDs can lead to DES with high viscosity, making their use difficult.