Introduction

Zingiber officinale var. rubrum (red ginger) serves as an alternative to synthetic drugs in the treatment of various health problems. Studies demonstrated that the antioxidant and anti-inflammatory properties of ginger extracts are useful in reducing the onset and progression of coronary atherosclerosis1,2,3. Shogaol and gingerols are bioactive compounds identified in ginger4. The extraction of gingerols from red ginger is usually performed by traditional extraction techniques. Multilevel maceration extraction yielded the highest concentration of 6-gingerol in the ethyl acetate fraction at 19.02%, the highest concentration of 8-gingerol in the hexane fraction at 4.49%, and a 10-gingerol concentration in the hexane fraction at 4.49%5. Additionally, these compounds can be extracted using ethanol maceration, and it was found that using 96% ethanol produced more of these compounds than using 30% or 70% ethanol6.

Considering extraction time and yields, conventional extraction processes such as maceration may take a long period from several hours to several days, thus not efficient7. In contrast, ultrasonic-assisted extraction (UAE) provides a rapid extraction process with less use of solvent and without the need for high temperature. It is known that compounds like gingerols and shogaol are heat-sensitive and prone to degradation when temperatures rise above 60˚C8. A number of studies have been conducted on the extraction of various plants using the UAE approach9,10. In addition, the use of organic solvents for extraction such as ethanol, hexane and ethyl acetate pose negative impacts on the ecosystem, environment and even humans, due to high volatility and flammability. Therefore, researchers are increasingly exploring alternative extraction methods that minimize the use of harmful solvents and reduce environmental impact. Techniques such as water-based extraction and supercritical fluid extraction (SPE) are gaining popularity for their efficiency and safety. It is known that the use of organic solvents has been associated with kidney damage, nervous disorders, cancer and other diseases11.

Natural Deep Eutectic Solvents (NADES) have emerged as a viable green substitute for volatile organic solvents2,3,12–20. NADES consists of a hydrogen-bonded network of natural compounds, including amino acids, carboxylic acids, alcohols, sugars, and amines. NADES have good physicochemical qualities since they are not very volatile, stay liquid below 0˚C, have a wide range of polarity, and can dissolve various substances. But the network of hydrogen bonding also makes the liquid very thick, which makes it hard for mass to move, therefore it can’t be used as an extraction solvent. By changing the amount of hydrogen bond acceptors (HBA), hydrogen bond donors (HBD), and water in NADES, you can change its polarity to target certain molecules. NADES are less hazardous, better for the environment, and more flexible than standard solvents since they have specific qualities and come from nature21,22.

NADES have been employed for the extraction of gingerols from ginger. The optimal NADES for extracting 6, 8, and 10-gingerol is a mixture of L-carnitine:1,3-butandiol (1:4) with a 25% water ratio under ultrasonication, with a solvent/solid ratio of 1:30 at 50˚C for 30 min8. Numerous NADES combinations remain available to enhance biomarker content. To do this, suitable polarity for the compounds can be generated through the appropriate mix of hydrogen bond acceptors (HBA) and hydrogen bond donors (HBD), as demonstrated in the instance of betaine with organic acid23.

Solvent screening process and method optimization in the laboratory are time consuming, costly, and resulting in solvent residue. Therefore, a molecular simulation method was developed to strengthen experimental work and used as a tool for selecting the most optimal solvent for extracting compounds8. Computer models can also provide valuable information about the underlying mechanism of the extraction process to help design and improve the extraction process.

COSMO-RS is a software tool utilized for predicting the equilibrium of a solution and its interaction with a substance, achieving a dependable level of accuracy24,25,26,27,28. COSMO-RS can forecast diverse thermodynamic and transport properties of fluids, thereby simulating molecular behavior in solution and predicting attributes such as solubility, distribution, and viscosity. The variety of combinations of NADES components makes the application of COSMO-RS for predicting NADES features particularly advantageous for enhancing solvent extraction procedures. COSMO-RS facilitates the assessment of diverse solvents under varying conditions, eliminating the need for costly and time-intensive experimental evaluations.

Conductor-like Screening Model for Real Solvents (COSMO-RS) has already been employed to forecast the solubility of prominent phytocompounds, including rutin, curcumin, carnosic acid, and carnosol in rosemary, across multiple deep eutectic solvents (DES)29, thereby considerably diminishing the number of solvents tested in the laboratory. Curcumin exhibits superior solubility in the choline chloride: glycerol combination compared to other NADES formulations, as indicated by data analysis using the COSMOTherm program29,30. Computational methods utilizing COSMO-RS identified HBA and HBD for the extraction of carnosic acid and carnosol. The findings indicated that hydrophobic DES, composed of ammonium chloride as HBA and fatty acids, aromatic carboxylic acids, or alcohols as HBD, were the most effective solvents for extracting these compounds29,30.

This work aims to examine NADES with optimal extraction for 6-, 8-, and 10-gingerol, as well as 6-shogaol, from red ginger. Fifteen NADES were formulated utilizing choline chloride, betaine, and L-carnitine as hydrogen bond donors (HBD) and organic acids (citric, malic, lactic, tartaric, and oxalic acids) as hydrogen bond acceptors (HBA). The COSMO-RS model was employed to predict and analyze the properties of NADES for the isolated gingerol and shogaol components. The evaluation involved assessing the sigma profile, sigma potential, solubility, and activity coefficient. Experimental validation was performed through the measurement of gingerols and shogaol utilizing the High Performance Liquid Chromatography (HPLC) technique.

Materials and methods

Computational method by COSMO–RS predictions

To screen NADES with high extraction ability of 6-, 8-, 10- gingerol and 6- shogaol from red ginger, COSMO-RS was used to model the molecular interactions between the studied NADES and these compounds. All molecular and SMILES structures of both NADES constituents and active substances were downloaded from the PUBCHEM database. The structures of compounds are presented in Table 1. The SMILES constituents of NADES; 6-, 8-, 10- gingerols and 6- shogaol were then input into COSMO-RS to calculate the σ profile, σ potential solubility and activity coefficient. It is known that for any compound, its solubility in the solvent is inversely proportional to its activity coefficient in the system29. COSMO-RS was used to predict the solubility of gingerols and shogaol in NADES at a temperature of 50 °C, in accordance with the experimental conditions where extraction were carried out29. Table 2 lists different combinations of NADES which are based on three HBAs (choline chloride, betaine and L-carnitine) and five HBDs (citric, malic, lactic, tartaric, and oxalic acids).

Table 1 Molecular structure, SMILES, and physicochemical characteristics of the substances used in this study.
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Table 2 The combinations of the studied NADES and their abbreviations.
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Experimental method

Plant materials and chemicals

The red ginger (Zingiber officinale var. rubrum) rhizomes were acquired from the Bogor Institute of Spice and Medicinal Research (BALITRO), Bogor, West Java, Indonesia (−6.576893345305293, 106.78633149414519). Macroscopic identification of the plant and rhizomes was conducted by PT. Palapa Muda Perkasa in Depok, West Java, Indonesia. The plant material was identified by the researcher and authenticated by the Indonesian Spice and Medicinal Crops Research Institute, Indonesian Center for Estate Crops Research and Development, Indonesian Agency for Agricultural Research and Development, Ministry of Agriculture, under the letter number B/013/KB.020/H.4.3/11/2021. The compounds used for the formation of NADES were the following: choline chloride (> 99%), betaine (> 99%) and L-carnitine (> 99%) were purchased from Qin Health Industry (Shaanxi, China). Citric acid (> 99%), malic acid (> 99%), lactic acid (> 99%), tartaric acid (> 99%), oxalic acid (> 99%) were purchased from Merck (New Jersey, USA). The standard compounds as biomarker for red ginger i.e. 6-gingerol standards (> 98%), 8-gingerol standards (> 98%), 10-gingerol standards (> 98%), 6-shogaol standards (> 98%) were obtained from Qin Health Industry (Shaanxi, China). The physicochemical properties of the NADES components and standard compounds were presented in Table 1. Organic solvents such as ethanol, methanol, and acetonitrile were of pro analysis and HPLC grade and were acquired from Merck (New Jersey, US). Aqua pro injection was bought from Ika Pharmindo Putramas (Jakarta, Indonesia); distilled water was obtained from the Research Laboratory (Universitas Pakuan, Indonesia).

Preparations of NADES

Amounts of HBA and HBD were calculated and weighed in certain molar ratios (1:1 or 1:2). To form eutectic mixtures, NADES components were heated in a water bath at 80˚C and constantly stirred at 200 rpm until a clear solution formed, which should take around 15 min. Into the eutectic mixture, 25% water was then added to reduce the viscosity of the NADES and stirring continued for 30 min. To ensure solution stability, the NADES liquid was allowed to stand at room temperature for 12 h8,30,31. Table 2 displays the combinations of the examined NADES along with their acronyms.

Extraction of red ginger with NADES

Red ginger rhizomes were segmented into small fragments and ground for 5 min into a powder using a grinder. Unprocessed plant material was preserved in sealed containers at ambient temperature until subsequent utilization. Extraction was carried out using ultrasonic assisted extraction (UAE) method using different NADES (See Table 2). Red ginger powder (500 mg) was weighed carefully, and 15 ml of solvent was added (ratio 1: 30). Sonication was carried out at 50˚C for 30 min. The mixture was then centrifuged for 25 min at 2500 rpm. The filtrate was separated from the macerate and prepared for analysis using HPLC. Three replications were conducted for each solvent extraction8.

Analysis using HPLC

HPLC instrumentation and conditions

An HPLC system of a Prominence-I LC-2050 C 3D (Shimadzu, Tokyo, Japan), equipped with a photodiode array UV-Vis detector was used in this study. Chromatographic separation of 6-, 8-, 10- gingerol and 6-shogaol was performed using an Eclipse plus C18 column (150 mm x 4,6 mm; 5 μm). The mobile phase consisted of acetonitrile and water, with a gradient elution as described in Table 3. Before use, solvents were filtrated by a 0.45 μm Whatman filter paper in a Buchner funnel, and then subjected to sonication for 30 min. For analysis, samples of Z. officinale extracts (20 µL) were loaded and run at a flow rate of 1 mL/minute, and analytes were quantified at 280 nm.

Table 3 The gradient elution composition of the mobile phases for separation of 6-, 8-, 10- Gingerol and 6-shogaol.
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Preparation of standard and sample solution.

Each marker compound (6-gingerol, 8-gingerol, 6-shogaol, and 10-gingerol), 1 mg, was diluted in methanol (1 mL) to obtain respective primary stock solutions (1000 µg/mL). Mixture of standard solution at concentration of 20 µg/mL was prepared by pipetting 100 µL of each standard stock solution into a 5 mL volumetric flask and diluting with methanol. The solution was sonicated for 30 min, left for 15 min, and further sonicated for another 30 min. Afterwards, a 0.45 μm Whatman filter paper was used to filter the mixture. Similarly, working solutions were prepared by mixing and diluting appropriate amount of each primary stock solution to obtain six concentration points of 200, 150, 100, 50, 5, and 2.5 µg/mL.

Samples of each NADES extracts of red ginger were prepared by diluting 0.5 mL of extract solution into 10 mL methanol. The solution was sonicated for 30 min, left for 15 min, and further sonicated for 30 min. Each sample was filtrated with 0.45 μm Whatman filter paper before use.

Method validation.

The HPLC method was validated prior to marker analysis (6-, 8-, 10-gingerol and 6-shogaol) in red ginger extracts. The validation procedure included system suitability, linearity, precision, accuracy, limit of quantification (LOQ), and limit of detection (LOD) tests32.

The linearity test evaluates the linearity relationship between analyte signals and analyte concentration of the calibrators. A calibration graph for each standard was generated using six different concentrations (200, 150, 100, 50, 5, and 2.5 ppm). The linear regression equation was calculated based on the plot of the peak area against the concentration of the standard solution.

The LOD and LOQ of marker analysis were determined by the following equations:

$$LOD = \frac{{3XSD}}{b}$$
(1)
$$LOD = \frac{{10xSD}}{b}$$
(2)

In which SD is standard deviation, and b is the value in the regression equation.

The precision of the HPLC method was assessed by testing intra- and inter-day precision of six extract samples. Intra-day precision was accomplished by testing all six samples on the same day. Meanwhile, inter-day precision was determined by analysis of the levels of marker compounds the next day. The relative standard deviation (RSD) value yields precise statistics.

The accuracy of the HPLC method was assessed using a spike method to determine sample recovery. The procedure involved measurement of the levels of the biomarkers (6-, 8-, 10-gingerol and 6-shogaol) in unspiked samples, followed by addition or spiking the standard solution into the unspiked samples. This was then followed by measurement of the marker compounds once again. The unspiked samples were prepared and evaluated in the same way as outlined in the precision test. Three standard solutions were prepared at varying concentrations (80%, 100%, and 120%) and were added to the unspiked samples. Samples at 100% concentration were examined six times, whilst samples at 80% and 120% concentrations were examined three times each. The recovery percentage was calculated by the subtraction of marker compounds in spiked samples from the unspiked samples divided by the additional standard concentration.

The results of the method validation of 6-, 8-, 10-gingerols and 6-shogaol are shown in Table 4. All conditions were within the acceptance criteria. These results suggest that the HPLC method can be used for analysis of multiple marker compounds of 6-, 8-, 10-gingerols and 6-shogaol in the extracts with good separation and resolution.

Table 4 System suitability test results for red ginger standard.
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HPLC analysis of marker compounds of red ginger.

The developed HPLC method was employed for the determination of 6-, 8-, 10- gingerol and 6-shogaol in NADES extracts of red ginger. Samples were injected and the area was extrapolated in calibration curves to calculate the levels of biomarker compounds in the extracts.

Statistical analyses

Experimental data were summarized using standard deviations. These were expressed as mean ± standard deviation value and interpreted using Origin Pro (Origin Lab Corporation, Northampton, MA, United States). The linear regression equation was used for the plot of the peak area against the concentration of the standard solution.

Result and discussion

COSMO-RS predictions

σ-profiles analysis

The COSMO-RS theory analyzes solvent-solvent interactions using the charge densities σ and σ’. A histogram depicting the charge density distribution of a molecule is referred to as a σ profile. When solvent-solvent interactions occur at σ sites, solubility and extraction are enhanced. The robust interaction among these surface particles results in the positive chemical energy µ(σ). The σ profile is generally segmented into three sections representing the hydrogen-bonded, nonpolar, and hydrogen-bonded defect areas, rendering it an effective instrument for contrasting the polarities of various compounds. The molecular interactions, such as polar, electronic surface, and hydrogen bonding, of the mixture components can be anticipated by assessing the screening charge density distribution (σ) across all surface components. This probability distribution is referred to as the “σ profile”. The configuration, elevation, and breadth of the σ profile peaks fluctuate based on the molecular structure and the concentration of the active atoms33.

The σ profiles of two drugs can be utilized to ascertain their interactions. The intensity of molecular interactions between these two molecules escalates with the expansion of complementary information. In other words, a decrease in the presence of one molecule correlates with an increase in the presence of another molecule in the same vicinity34. A highly polar solvent exhibiting significant charge density in the HBA region (−0.0082 e/Ų) is projected to demonstrate a strong affinity for a solute with considerable charge density in the HBD region (+ 0.0082 e/Ų). This is also true for two atoms exhibiting substantial charge concentrations within the non-polar region of the sigma profile (± 0.0082 e/Å2)35,36. Figures 1, 2 and 3 show the sigma profiles of each compound and NADES. Commonly, the higher the σ-profile similarity between Z. officinale var. rubrum compound and the NADES, the better the solubility of the compound in NADES, in this research the compound is 6-, 8-, 10- gingerol and 6-shogaol37.

Figures 1 and 2, and 3 illustrate that the σ profile of 6-, 8-, and 10-gingerol, as well as 6-shogaol, presents a symmetrical curve characterized by a narrow peak predominantly located in the non-polar area of −0.008 e/Ų < σ < 0.008 e/Ų, with a minor extension into the HBA region reaching σ = 0.012 e/Ų attributable to oxygen atoms. The σ profile for 6-, 8-, and 10-gingerol, as well as 6-shogaol, exhibits a wide shape, featuring peaks within the range of −0.003 e/Ų < σ < 0.003 e/Ų. The HBA region peak at 0.012 e/Ų is linked to oxygen and nitrogen, whereas the HBD peak at 0.013 e/Ų is attributed to hydrogen atoms. Simultaneously, it possesses a wide peak situated in the non-polar region centered at −0.004 e/Ų, which is ascribed to CH3 groups. Generally, 6-, 8-, and 10-gingerol, along with 6-shogaol, can be categorized as non-polar molecules exhibiting robust hydrogen bonding. Therefore, for enhanced solubility of 6-, 8-, and 10-gingerol, as well as 6-shogaol, it is preferable for NADES to consist of non-polar varieties.

The σ-profiles of ChCl: MaA (1:1) and Be: LaA (1:2) are natural deep eutectic solvents that exhibit significant similarities to the profiles of 6-, 8-, and 10-gingerol, as well as 6-shogaol. The two NADES exhibit a small peak predominantly ranging from − 0.008 e/Ų to 0.006 e/Ų, beside a large peak situated in the non-polar zone, centered at −0.004 e/Ų. Conversely, Figs. 1 and 2, and 3 illustrate that the σ-profiles of the remaining NADES, except from the two previously mentioned, exhibit a large peak in the non-polar region centered at −0.006 e/Ų, which is wholly dissimilar to the profiles of 6-, 8-, and 10-gingerol, as well as 6-shogaol.

σ-potentials analysis

COSMO-RS conducts statistical analysis on data derived from sigma points to ascertain the chemical potentials of the system’s components38. The sigma potentials provide additional insights into the hydrogen bond accepting and donating properties of NADES and their constituent components. The sigma potentials, denoted as µ(σ), signify the manner in which a molecule may engage inside a charged milieu. The potentials are classified into two categories: the middle region exhibiting nonpolar interactions (charge density < ± 0.0084 e/Ų) and the outer regions that can accept or donate hydrogen bonds (charge density > ± 0.0084 e/Ų)35. Figures 4 , 5 and 6 illustrate profiles obtained from their molar and mass-based averages.

In all visualizations, the y-axis denotes the sigma potential experienced by the molecule in an environment with a specific charge, while the x-axis illustrates the charge density, encompassing both hydrogen bonding and nonpolar interaction regions. The sigma potential profiles of 6-, 8-, and 10-gingerol, together with 6-shogaol, are illustrated in each picture, alongside the sigma potentials of the respective NADES. A key distinction between sigma profiles and sigma potential profiles is that the former relates to the charge density within the molecule, while the latter applies to the charge density of an exterior surface. A peak in the positive electron density region, along with a likely beneficial negative potential in the negative electron density segment of the sigma potential profile, would suggest heightened electron density on a sigma profile.

Figures 4 , 5 and 6 illustrate the sigma potentials of choline chloride, betaine, and L-carnitine acid-based NADES, respectively, with each NADES potential referenced to 6-, 8-, and 10-gingerol and 6-shogaol. The sigma potentials of 6-, 8-, and 10-gingerol, as well as 6-shogaol, suggest that these compounds can absorb hydrogen bonds but are incapable of donating the hydrogens they lack, consistent with their sigma profiles. Positive sigma potentials exceed 0.0084 e/Ų, but negative sigma potentials fall below − 0.0084 e/Ų for 6-, 8-, and 10-gingerol and 6-shogaol. Each figure illustrates two distinct inflection points at the hydrogen bonding limits at roughly ± 0.012e/Ų. Comparable sigma potentials of 6-, 8-, and 10-gingerol, as well as 6-shogaol, were observed in NADES ChCl: MaA (1:1), Be: MaA (1:1), and LCa: MaA (1:1).

Solubility and activity coefficient prediction results

COSMO RS can predict the solubility of compounds in NADES. Figure 7 shows the effect of different NADES on the predicted activity coefficient of 6-, 8-, 10- gingerols, and 6-shogaol. The types of HBAs and HBDs influenced the predicted dissolving ability of the studied NADES. As shown in Fig. 7, NADES with MaA, LaA, and OxA, each combined with HBAs of ChCl, Be, and LCa were predicted to dissolve the marker compounds with good solubility based on activity coefficient. Meanwhile, TaA combined with each HBA were predicted to give low solubility to marker compounds, following the order of ChCl: TaA > Be: TaA > LCa: TaA. The solubility values of the five NADES combinations that have the greatest values were ChCl: MaA (1:1), ChCl: LaA (1:1), Be: MaA (1:1), Be: LaA (1:2) and Be: OxA (1: 1). The higher the activity coefficient of a compound, the more difficult it is for the compound to dissolve. Conversely, the lower the activity coefficient of the tested HBA and HBD, the higher the compound solubility29,39,40,41. As shown in Table 5, the activity coefficients of gingerols and shogaols in NADES can be predicted. The 6-gingerol compound tends to have a low activity coefficient compared to the 8-gingerol, 10-gingerol and 6-shogaol compounds in red ginger. This means that the 6-gingerol compound is more easily extracted as presented in the Fig. 7 . When viewed from the type of NADES, the HBD composition significantly affects the activity coefficient compared to its HBA composition. NADES with the lowest HBD is a mixture of Choline chloride: Malic Acid, Betaine: Malic Acid, and L-carnitine Malic Acid. Figure 7 illustrates the projected activity coefficients from COSMO-RS alongside the experimental yields of compounds in NADES for (a) 6-gingerol, (b) 8-gingerol, (c) 10-gingerol, and (d) 6-shogaol.

Table 5 Comparison between predicted activity coefficient by COSMO-RS and experimental yields of marker compounds in the studied NADES.
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Experimental verification

NADES preparation and extraction results

NADES were formed by mixing HBA and HBD, whereby after heating and stirring, obtaining clear liquid. Of the fifteen NADES synthesized in this study, two NADES i.e. L-carnitine: tartaric acid and L-carnitine: Oxalic acid, appeared unstable as shown by crystallization occurring after 12 h of storage. Meanwhile, the rest of NADES combinations remained stable after 12 h of storage, thus were further used for extraction.

When NADES starting components were mixed and heated, melting point depression occurred due to the eutectic conditions. Charge delocalization occurred as a result of hydrogen bonding interaction between HBA and HBD, giving rise to viscous liquid11. The highly viscous nature of NADES can hinder the mass transfer of bioactive compounds during extraction, thereby limiting their efficiency as extraction media. To overcome this limitation, 25% (v/v) water was incorporated into the system to reduce viscosity and enhance solvent diffusion. The addition of water decreases hydrogen-bond interactions between NADES components, leading to improved molecular mobility and extraction kinetics. However, excessive dilution may disrupt the eutectic structure and weaken solvent–solute interactions, thus reducing extraction efficiency. Therefore, optimization of water content is crucial to balance viscosity reduction and solvent stability. Future studies could further investigate different water ratios to better understand their influence on solubility, mass transfer, and the physicochemical behavior of NADES during extraction42. Ultrasonic waves were able to dissolve cell walls and liberate phytocompounds attached to the plant cell walls into the extraction medium. Application of UAE in NADES extraction promotes extraction efficiency because of less use of energy and solvent42. In addition, UAE was also good option for extraction of thermolabile materials42.

Biomarker content in red ginger extracts by HPLC analysis

The HPLC method was successfully applied to quantitatively analyze marker compounds in NADES extracts. The chromatograms of the standards and NADES extracts of red ginger are displayed in Figs. 8 and 9. Peaks in the chromatograms of 6-, 8-, 10-, gingerol and 6- shogaol in red ginger NADES extracts were identified in the same retention times as those observed for the standards. The compounds 6-gingerol appeared at retention times of 9.587 ± 0.0015 min, 8-gingerol at 19.963 ± 0.0176 min, 10-gingerol at 26.735 ± 0.0203 min for, and 6-shogaol at 21.484 ± 0.0170 min. Following COSMO-RS prediction of marker compounds extracted by NADES, the studied NADES were employed for experimental extraction of marker compounds from rhizomes of red ginger. Table 6 displays the levels of 6-, 8-, 10- gingerol and 6- shogaol in different NADES and ethanol extracts analyzed by the HPLC method. Results in Table 6 reveal that different NADES obtained varying quantities of marker compounds. In general, NADES based on betaine obtained higher yields of marker compounds, followed by ChCl-based NADES and LCa-based NADES. Betaine: lactic acid (1:2) extracted the maximum amount of marker compounds from red ginger (15.0882 mg/g), meanwhile ChCl: LaA obtained lower yields (9.6357 mg/g), whereas LCa: CiA obtained the least amount (1.764 mg/g). Extraction capacity of NADES is affected by several variables, including degree of polarity, surface tension, viscosity, and interactions between NADES and chemicals7. According to the “like dissolves like” theory, substances that have similar qualities—such as polarity—dissolve in solvents that share those properties. Polar compounds are drawn to polar solvents more readily, and vice versa. Solvent’s polarity can be determined by the number of hydroxyl groups in the chemical structure. Lactic acid differs from other acids (oxalic acid, malic acid, citric acid, and tartaric acid) in that it contains less hydroxyl groups in its structure. In addition to polarity, the viscous nature of NADES may have been the limiting factor of NADES extraction of marker compounds. High viscosity limits the ability of NADES to extract marker compounds since it prevents compounds for mass-transferring from the cell into the solvent.

Table 6 The active substances levels of 6-, 8-, 10- Gingerol and 6- Shogaol in different NADES extracts.
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Comparison between predicted and experimental yields of marker compounds in the studied NADES

Figure 10 shows comparisons of COSMO-RS predicted results and experimental results analyzed using HPLC on the total levels of 6-, 8-, 10-gingerols and 6-shogaol extracted from red ginger in NADES. By COSMO-RS prediction (Fig. 10-a), out of fifteen NADES, five NADES showed high yields of total gingerols and shogaol, in the following order Be: MaA (1:1) > Be: OxA (1:1) > ChCl: MaA (1:1) > Be: LaA (1:2) > ChCl: LaA (1:1). Meanwhile, based on experimental results (Fig. 10-b), five NADES obtained high yields of marker compounds, in the following order Be: LaA (1:2) > Be: MaA (1:1) > Be: CiA (1:2) > ChCl: LaA (1:1) > Be: TaA (1:1). The discrepancies between predicted and experimental yields of marker compounds are due to several factors. It should be noted that the number of prediction factors that can be input into COSMO-RS are still limited to temperature factor. On the other hand, when conducting NADES ultrasound extraction experiments, several parameters in ultrasonication that enhance extraction should be considered. Ultrasonic power, temperature, solvent-to-sample ratio, solvent concentration, pH, and extraction duration are the most significant parameters affecting the UAE process43,44. These parameters are not yet incorporated in the COSMO-RS model, leading to deviations between theoretical and experimental results. Parameters include ultrasonic power, solvent-to-sample ratio, and extraction duration are kinetic and procedural variables. These factors affect on the speed and efficacy with which the system attains this ideal equilibrium, however they do not alter the equilibrium state itself. Consequently, it is not conceptually viable to integrate parameters such as ultrasonic power or extraction time directly into the COSMO-RS thermodynamic framework. The observed inconsistencies are not indicative of a defect in the model’s thermodynamic computations, but these kinetic factors did not attain complete theoretical thermodynamic equilibrium.

Conclusion

The present study showed the potential of fifteen types of NADES combination for the extraction of 6-, 8-, 10-gingerol and 6-shogaol from red ginger. Theoretical and experimental evaluations were conducted to determine the extractability of the target compounds in NADES. By COSMO-RS, calculations of parameters such as sigma profile, sigma potential, solubility, and activity coefficient predicted that ChCl: MaA (1:1), ChCl: LaA (1:1), Be: MaA (1:1), Be: LaA (1:2) and Be: OxA (1:1) had affinity to 6-, 8-, 10-gingerol and 6-shogaol. Our study showed that Be: LaA (1:2) is a NADES for the extraction of gingerol and shogaol from red ginger with a total content of 15.0882 mg/g ± 0.062. The COSMO-RS results supported the experimentally obtained data, which showed that 6-, 8-, 10-gingerol and 6-shogaol had increased solubility in ChCl: LaA (1:1), Be: CiA (1:2), Be: MaA (1:1), Be: LaA (1:2) and Be: TaA (1:1). Results herein presented the possibility of replacing hazardous organic solvents with NADES. Additionally, the study demonstrated the use of COSMO-RS model to assist initial screening of NADES for extraction of target compounds, therefore contributing to the development of environmentally friendly extraction methods.

Fig. 1
figure 1

The sigma profiles of marker compounds of Zingiber officinale and of choline chloride: acid.

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Fig. 2
figure 2

The sigma profiles of marker compounds of Zingiber officinale for betaine: acid.

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Fig. 3
figure 3

The sigma profiles of marker compounds of Zingiber officinale for L-carnitine: acid.

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Fig. 4
figure 4

The sigma potential of marker compounds of Zingiber officinale and of choline chloride: acid.

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Fig. 5
figure 5

The sigma potential of marker compounds of Zingiber officinale for betaine: acid.

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Fig. 6
figure 6

The sigma potential of marker compounds of Zingiber officinale for L-carnitine: acid.

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Fig. 7
figure 7

Comparison of the predicted activity coefficients from COSMO-RS with the experimental yields of compounds in NADES for (a) 6-gingerol, (b) 8-gingerol, (c) 10-gingerol, and (d) 6-shogaol.

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Fig. 8
figure 8

The chromatograms of standard compounds and red ginger extracts in NADES (Choline Chloride: Acids) for (a) Chromatogram of the standards 6-, 8-, and 10-gingerol and 6-shogaol; (b-f) Chromatograms of Red Ginger extracted by (b) ChCl: CiA; (c) ChCl: MaA; (d) ChCl: LaA; (e) ChCl: TaA; and (f) ChCl:OxA.

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Fig. 9
figure 9

The chromatograms of red ginger extracted by NADES (Betaine: Acid and L-Carnitin:Acid) (a-e) Chromatograms of Red Ginger extracted by (a) Be: CiA; (b) Be: MaA; (c) Be: LaA; (d) Be: TaA; (e) Be:OxA. (f-h) Chromatograms of Red Ginger extracted by (f) LCa: CiA; (g) LCa: MaA, (h) LCa: LaA.

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Fig. 10
figure 10

Total gingerols and shogaol content of red ginger extracted by NADES (a) Prediction result with COSMO-RS, (b) experimental results.

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