2.1. Soxhlet Extraction
In
Table 1, the extraction conditions and extraction yields using Soxhlet, in combination with different solvents, are presented. The Soxhlet extraction was performed to select the most suitable co-solvent in the supercritical extraction. The highest yield was observed using water as solvent, followed by ethanol (17.93% and 17.70%, respectively) (
Table 1,
Figure 1). Lower extraction yields were noticed with ethyl acetate and hexane (8.28% and 4.82%, respectively). This difference could be due to the different polarities of the used solvents (10.2 for water, 5.2 for ethanol, 4.3 for ethyl acetate, and 0.0 for hexane) [
22], and their ability to solubilize the oils from the biomass.
Similar to our results, Al-Areer et al. found that, [
24] when performing Soxhlet extractions at 90 °C for 9 h, the highest extraction yield was observed when ethanol was used as solvent, followed by ethyl acetate and hexane. A different trend was observed by Lemma and Egza, [
25] who noticed a higher yield in the hexane than the ethanolic extract when Soxhlet extractions were performed for about 4 h at the boiling point of each solvent.
Moreover, the antioxidant capacity, as well as the concentration of total phenols of the extracts, were evaluated. The results are shown in
Table 2. The highest antioxidant activities were observed in the ethyl acetate and ethanolic extracts. Regarding the total phenolic content, the ethanolic extract presented the highest concentration.
The differences in the extraction yields, total phenolic contents, and antioxidant activity may occur because different solvents can extract different active compounds [
26]. Water, even though it presented the higher extraction yield, showed the lowest total phenolic content and antioxidant activity. This can be due to the high temperature used in that case (100.5 °C), leading to the thermal decomposition of high bioactive compounds (
Table 2). It is well-known that the high temperatures used in the Soxhlet extraction process (up to the boiling point of each solvent), along with the long extraction time, can lead to the thermal degradation of some compounds [
27]. More specifically, the continuous evaporation and extraction of the target compounds caused by the solvents, the high extraction kinetics, and the prolonged extraction time can promote the decomposition of thermolabile target compounds [
28]. Thermolabile target compounds can consist of polyphenols that present high antioxidant capacity [
29].
Μoreover, it is reported in the literature that many factors can affect antioxidant activities. Besides the amount and strength of the antioxidant compounds, the ability of the antioxidants to transfer a hydrogen to a free radical such as DPPH· can also be affected by the environment where the reaction takes place; for example, in different types of solvents [
30].
Taking all of the above into consideration, along with the antioxidant capacity (
Table 2,
Figure S1) of each extract and their total phenolic content (
Table 2), ethanol was selected as the co-solvent in the supercritical extraction.
2.2. Sub- and Supercritical Extraction
In
Table 3, the extraction conditions for the sub- and supercritical extractions are presented. The primary biomass extraction resulted in high yield percentages (
Table 4,
Figure 2). The supercritical CO
2 extraction with ethanol as the co-solvent (scCO
2 + EtOH) gave an extraction yield of 6.06%, which was the highest of the three obtained, followed by subcritical propane (scPropane) and supercritical CO
2 (scCO
2), with yield percentages of 2.06% and 1.54%, respectively. It is worth noticing that scCO
2 + EtOH led to higher yields in less time (half the time in comparison with the other methods). Considering the increase in the yield percentage, and the decrease of the time needed, the addition of ethanol had a positive effect in the overall extraction.
Ethanol, when used as a co-solvent, solubilized the CO
2 and led to a decrease of the viscosity of the mixture of solvents (CO
2 + EtOH), and an increase of density [
31,
32]. The combination of the solvents accelerated the extraction process, led to less usage of CO
, and, and enhanced the extraction yield. Additionally, the polar mixture of solvents led to an increase of the extracted amount of polar and soluble compounds [
22]. In general, the usage of co-solvents may improve the extraction performance due to the enhanced transport of solute [
33].
Due to the enhanced performance of the scCO
2 + EtOH extraction, when a secondary biomass was used, high extraction yields were observed in all cases (
Table 4,
Figure 3). Specifically, in the case of the scCO
2 + EtOH extraction of the secondary biomass that was used once before in scCO
2 extraction, the extraction yield was significantly higher than that of the primary biomass. CO
2 behaves as a nonpolar solvent and presents with a low extraction performance for polar compounds [
34]. The co-solvent (EtOH) that was used in the secondary biomass extraction had a positive influence on the extraction procedure, since it lowered the viscosity of the solvents, increased the density, and altered the overall polarity of the mixture of solvents, which led to the enhancement of the extraction yield.
When the biomass, prior used in scCO2 + EtOH, was extracted in a multistep extraction twice with scCO2 + EtOH, the extraction yield was lower than the primary biomass, but still significant. Mainly, this was observed due to the high amounts of polar compounds that the mixture of solvents obtained through the first extraction, and when only CO2 was left in the chamber, the capability to extract polar compounds was minimized. In continuation, when the extraction procedure was repeated, and the mixture of solvents renewed, their capability to extract those compounds improved once again and the extraction yield remained significant.
High yield percentages for ginger have been reported by Mesomo et al., [
15] and Mesomo et al., [
11] using CO
2 and Propane as solvents, in various pressures and temperatures. For CO
2, the yield percentages ranged from 0.22% to 3.21%, and for Propane from 1.98% to 2.70%. For CO
2, the pressure led to a positive effect on the yield, and for propane, both pressure and temperature. Similarly, Salea et al., [
35] using scCO
2 in various conditions, calculated a range of yields from 1.55% to 2.95%. That range was attributed to the variety of pressure and temperature conditions used in each extraction.
Zancan et al. [
36] performed scCO
2 and scCO
2 + EtOH, and did not observe significant differences in the yield percentages between the two methods, but ethanol favored the extraction of gingerols and shogaols.
2.3. Total Phenolic Contents and Antioxidant Capacity
Table 4 presents the total phenolic contents acquired through using different extraction conditions and solvents, as well as both primary and secondary biomass. The total phenolic contents were expressed as mg of gallic acid equivalent per 100 g of sample. In both the primary and secondary biomass, similar total phenolic contents were observed, with the highest being in scCO
2 + EtOH using secondary biomass, which was formerly used in the same type of extraction.
Stoilova et al., [
37] found a total phenolic content of 871 mg/g dry extract acquired through high pressure CO
2 extraction.
A similar trend can be observed for the antioxidant capacity of the extracts (
Table 5,
Figures S2 and S3). Overall, according to the IC
50 values, the scCO
2 + EtOH extract presented the highest antioxidant activity, and the scCO
2 extract the lowest.
The antioxidant properties of ginger extracts obtained by different extraction methods have been widely studied. Its antioxidant properties are due to compounds such as gingerols, flavonoids, and phenolic acids [
38].
Mesomo et al., [
15] studying different conditions of supercritical extraction, observed the highest antioxidant capacity with scCO
2. Zancan et al. [
36] observed that, when no gingerols and shogaols were yet obtained by the extraction the antioxidant, activity was much lower. Stoilova et al. [
37] calculated the IC
50 for inhibition of DPPH to be 0.64 μg mL
−1.
2.4. GC-MS Analysis
Ginger includes both volatiles such as geraniol, borneol, terpineol, curcumene, zingiberol, α-farnesene, α-sesquiphellandrene, α- β-bisabolene, β-elemene etc., as well as non-volatile compounds such as gingerols, shogaols, zingerone, and paradols [
7]. The composition of ginger extracts and oils differ significantly. The bioactive compounds of ginger essential oils are mainly monoterpenes and sesquiterpene hydrocarbons, and their chemical composition depends on the nature (fresh/dry) and place of origin of the ginger rhizome, as well as the extraction method employed [
10]. They are also composed of oxygenated hydrocarbon compounds including aldehydes, phenols, esters, oxide ethers, alcohols, and ketones [
10].
The chemical composition of the extracts/oils was determined by GC-MS analysis, and all the compounds identified have been compiled and presented in
Table 6 along with their area percentage. The extracts obtained with all the tested systems were found to have quite similar chemical compositions, and the main substances were a-zingiberene, β-sesquiphellandrene, a-farnesene, β-bisabolene, zingerone, gingerol, a-curcumene, and γ-muurolene. Similar compounds have also been identified in the literature [
11,
38,
39,
40]. In the case of scPropane, some more compounds, i.e., monoterpenes and sesquiterpenes, were identified but only in traces. When a primary biomass was used, Soxhlet with ethanol extracted less compounds in comparison with scPropane, scCO
2, and scCO
2 + EtOH, signifying the increased efficiency of such advanced extraction techniques.
Our results regarding the main components are in agreement with the bibliography [
11]. Based on literature data, higher essential oil and β-zingiberene contents were obtained for the dried ginger rhizome than that of the fresh ones. Moreover, the drying method has been found to play a significant role in essential oil’s yield and its chemical composition. Temperatures lower than 70 °C can increase the yield of ginger oil without having any effect on the transformation of 6-gingerol to 6-shogaol [
41].
It is noteworthy that most of the above-mentioned compounds were identified after the reuse/recycling of the secondary biomass, highlighting the possibility to extract the maximum value from the used biomass. In
Table 7, the identified compounds of the secondary biomass extractions are presented with their area percentage. After the first extraction of the secondary biomass, the compounds identified were less in comparison with the primary biomass’s identified compounds, but their area percentage was higher.
The main identified compounds (a-zingiberene, β-sesquiphellandrene, a-farnesene, β-bisabolene, zingerone, gingerol, and a-curcumene) that can be found in almost all of the studied extracts have been previously reported to possess significant antioxidant properties. This is in agreement with our results [
42,
43,
44,
45,
46,
47]. Badrunanto et al., [
45] when studying the antioxidant components of Indonesian ginger essential oil, observed that, amongst others, a-zingiberene, β-sesquiphellandrene, a-farnesene, β-bisabolene, and a-curcumene, presented a high correlation with the antioxidant activity of the oil. Similarly, Misharina et al. [
46] highlighted the antioxidant activity of zingiberene, β-sesquiphellandrene and β-bisabolene. Gingerol analogues have been associated with ginger extracts’ antioxidant activity [
42,
43,
47]. Specifically, Danwilai et al. [
42] studied the antioxidant activity of ginger extract oral supplements in cancer patients who were receiving adjuvant chemotherapy. These supplements contained 20 mg day
−1 6-gingerol, and results found a significant increase regarding antioxidant activity, and a decrease of oxidative marker levels. Moreover, Wang et al. [
43] observed that 10-gingerol presented with about 34.2% DPPH radical scavenging activity, and 6-gingerol about 16.3%. Furthermore, they noticed that the antioxidant activity of those ginger compounds contributed to the antimicrobial activity against
Acinobacter baummannii infections. 6-gingerol, 8-gingerol, and 10-gingerol’s antioxidant activity was studied by Dugasani et al. [
47], who observed that the DPPH scavenging potential was in the order of 10-gingerol > 8-gingerol > 6-gingerol. Rajan et al. [
44] studied zingerone’s antioxidant activity using a DPPH free radical method, and observed a dose dependent increase of the compound’s antioxidant activity.
In general, it is well reported in the literature that the components of ginger extracts/essential oils have significant bioactivities and health-promoting properties, and thus can have applications in various sectors [
7,
8,
9,
10,
11]. Based on literature data, ginger extracts/oils containing most of the bioactive compounds found in the present have been reported to have significant pharmacological, medicinal, and cosmetic applications, as they have been found to possess antimicrobial and antiseptic activity, anti-carcinogenic potential, neuroprotective activity, anti-obese activity, anti-diabetic effect, and analgesic activity as well as provide cardiovascular protection [
8,
11]. Another significant application is in the food industry, as the bioactive compounds of ginger can provide oxidative and storage stability, sensorial properties, preservation, oxidative resistance, and anti-bacterial activity in consuming products [
7]. As well, another notable factor is its application in agriculture for the control of plant diseases which minimizes simultaneously the possible negative effects on the environment, animals, and human health [
9,
10].