Chemical Composition of Essential Oils Obtained from Heteromorpha arborescens (Spreng.) Cham. and Schltdl Leaves Using Two Extraction Methods

This study was aimed at comparing the essential oils obtained from Heteromorpha arborescens leaves by Solvent-Free Microwave Extraction (SFME) and Hydrodistillation (HD) methods in terms of their chemical compositions, yield, CO2 emission, and energy consumption. The solvent-free microwave extraction method indicated a higher oil yield of 0.7 mL/200 g (0.35%) as compared to 0.59 mL/200 g (0.295%) obtained through hydrodistillation. GC-MS analysis of the oils revealed a total of 52 chemical components from both methods with the presence of 35 (96.52%) and 30 (71.15%) chemical constituents for HD and SFME, respectively. The major constituents observed in the essential oil extracted by SFME methods include α-pinene (6%), D-limonene (11.27%), β-ocimene (9.09%), β-phellandrene (6.33%), β-mycene (8.49%), caryophyllene (5.96%), and camphene (4.28%). However, in the hydrodistillation method, the oil was majorly composed of a-pinene (4.41%), β-pinene (10.68%), β-ocimene (6.30%), germacrene-D (5.09%), humulene (5.55%), and α-elemene (6.18%). The SFME method was better in terms of saving energy (0.25 kWh against 4.2 kWh of energy consumed), reduced CO2 emission (200 g against 3360 g of CO2), a higher yield, and better quality of essential oil due to the presence of higher valuable oxygenated compounds (8.52%) against that of the hydrodistillation method (2.96%). The SFME method is, therefore, a good alternative for extracting the oils of H. arborescens leaves since the essential oil yield is higher with more oxygenated compounds, considerable energy savings, lower cost, and reduced environmental burden at substantially reduced extraction time (30 min as opposed to 180 min).


Introduction
Plants have a long history of therapeutic use in the management of diseases since time immemorial. Essential oils are natural, complex, and aromatic volatile compounds produced by plants, and they are generally present at low concentrations. ese compounds possess antimicrobial and antioxidant activities, and they function in the treatment of various kinds of diseases [1]. ey are also beneficial in various food industries (for preserving food against oxidation), as alternatives to artificial chemicals in cosmetics and perfumery industries for the production of various cologne waters, bathing lotions, hair lotions, and shampoos, and as components of disinfectants and insecticides [2].
Among aromatic plant species, the genus Heteromorpha (Apiaceae) consists of seven species which are restricted to temperate and subtropical Africa and Southern Yemen [3].
ere is an increased interest for Apiaceae, especially in food science, because members of this family such as fennel, celery, dill, carrot, and caraway are commonly used. Apart from the biologically active essential oils found in this family, they contain coumarins, polyacetylenes, flavonoids, sesquiterpenes, and phthalides [4].
Heteromorpha arborescens (Spreng.) Cham. and Schltdl (Apiaceae) can be referred to as a large shrub, small or medium deciduous tree [5]. It is regarded as an important medicinal plant throughout its distribution area in tropical Africa and has been used for the treatment of many ailments including helminthiasis [6], abdominal pains, infertility, nervous disorders, and tuberculosis [5]. e plant is popularly known in parts of Africa for its therapeutic benefits and is therefore included in the monographic treatment of medicinal plants of South African medicine [7].
From previous studies, the volatile oil of H. arborescens leaves is known to contain sabinene, δ-3-carene, myrcene, germacrene-D, limonene, (Z)-β-ocimene, β-phellandrene, and α-pinene as major constituents, and it possesses both antibacterial and antifungal activities [5]. Furthermore, the essential oil of H. arborescensis is vital in the development of new pharmaceutical and health products in Southern Africa for headache, inhalant, and aromatherapy [8].
Various conventional methods such as hydrodistillation, steam distillation, and cold pressing have been used for essential oil extraction in aromatic plants. However, the use of these methods has been disputable for successive determination of essential oil composition because the long extraction times at high temperatures may cause changes in the essential oil composition or degradation of unsaturated or esterified compounds and the loss of highly volatile compounds [9,10]. ese limitations have led to the development of the solvent-free microwave extraction method in order to reduce the extraction time and obtain better quality of essential oils [9,10]. e solvent-free microwave extraction (SFME) set up is referred to as a "green technology" which involves a combination of heat from the microwave and dry distillation at atmospheric pressure [11]. It is a sustainable extraction method which relies on reduced energy consumption, ensuring a safe and high quality end product [12]. It involves heating the fresh plant material in a microwave reactor, devoid of water or any solvent at all for 30 min. e shortterm heating of the water in the plant material expands the plant cells causing the plant cells and oil producing glands to release the oil from the plant material [9]. SFME method has widely been employed in the extraction of oils from Melalaica leucadendra L. [13], Ocimum basilicum L. [14], Kananga odorata [15], Pogostemon cabin [16], Cuminumcyminum L. and Zanthoxylum bungeanum Maxim [17], and many others. However, until now, there has been no studies on the comparison of the quality and yield of essential oils obtained from H. arborescens leaves using hydrodistillation and SFME methods. is work is therefore aimed at making a comparative study in terms of extraction yields and chemical compositions of essential oils obtained from H. arborescens leaves using hydrodistillation and solvent-free microwave extraction methods. (1)

Extraction by Hydrodistillation. 200 g of fresh
H. arborescens leaves was subjected to hydrodistillation with a Clevenger-type apparatus as described by the European Pharmacopoeia and extracted with 1L of water for 180 min at 100°C. e essential oil was collected and analyzed by GC-MS.

Solvent-Free
Microwave Extraction (SFME). 200 g of fresh H. arborescens leaves was placed into the reactor without addition of any solvent. Microwave extraction was performed using a Milestone MAO20-A apparatus; a multimode microwave reactor 2.45 GHz with a maximum delivered power of 500 W variable in 5 W increments. e essential oil was completely extracted at atmospheric pressure and 99.85°C (373 K) within 30 min; collected, and subjected to GC-MS analysis.

Energy Consumption and CO 2 Emission.
Power consumption was 500 W and 1400 W for SFME and hydrodistillation methods, respectively. e energy consumption and CO 2 emission were calculated according to previous literature as energy consumption (kWh) � Pt/1000 and to obtain 1 kWh from the combustion of coal or fossil fuel, and 800 g of CO 2 will be emitted into the atmosphere [18][19][20].

Extraction
Yields. e extraction yields of the essential oils obtained from both methods were calculated as 2.6. Gas Chromatography-Mass Spectroscopy Analysis. e essential oils extracted were separately analyzed by gas chromatography-mass spectroscopy (Agilent 6890 GC, coupled to an Agilent 5975 mass spectrometric detector). Gas 2 e Scientific World Journal chromatography-mass spectrometry was combined with a chromatography column HP-5.5% phenylmethylsiloxane, with 30 m length, 0.32 mm film thickness, 0.25 μm internal diameter. e injection port was held at 230°C, while the interface was at 280°C. e temperature was set from 50°C to 280°C at 10°C per minute, using helium as the carrier gas. e chemical components of the essential oils were identified by comparing their mass spectra and retention indices with those in PubChem, NIST, and Wiley libraries [21]. e spectrogram of each identified compound was determined by integration of the peak areas.

Results
e moisture content of the 200 g of H. arborescens leaves was 10.20 ± 0.95%, and a slightly higher oil yield of 0.7 mL/ 200 g (0.35%) was obtained through SFME after 30 min of extraction as compared to 0.59 mL/200 g (0.295%) obtained after 180 min by the hydrodistillation method. e oils were pale-yellow liquids with fine aroma. For the energy consumption and environmental footprint, the amount of energy consumed and the quantity of carbon dioxide emitted into the atmosphere were higher for the hydrodistillation method (4.2 kWh and 3360 g CO 2 /g of essential oil) as compared to the SFME method (0.25 kWh and 200 g CO 2 /g of essential oil).
A total of 52 compounds obtained by hydrodistillation and solvent-free microwave extraction methods are summarized in Table 1.
irteen (13) of the total components were common to oils extracted by both methods.

Discussion
Some conventional methods used for the extraction of essential oils are hydrodistillation, steam distillation, Microwave-Assisted Extraction (MAE), and solvent extraction. Although these techniques have been used over time, they have some limitations such as longer time of extraction, low extraction effectiveness, reduced oil quality due to degradation of unsaturated or ester compounds through thermal or hydrolytic effects, and negative impacts on the environment [22]. SFME and Microwave Hydrodiffusion and Gravity (MHG) methods are two new green techniques employed in the extraction of quality essential oils at a lower cost, reduced time, and environmental footprints [23]. e chemical compositions of the essential oil of H. arborescens leaves obtained by the solvent-free microwave extraction method have been compared with those which were obtained by the hydrodistillation method. Our study indicates that the essential oil yield obtained by the SFME method was slightly higher with shorter extraction time (0.35% for 30 min) when compared with that by the hydrodistillation method (0.295% for 180 min). e lower yield obtained by the hydrodistillation method may be attributed to increasing extraction time [24]. e present study also revealed that the SFME method reduced energy consumption and CO 2 emission by about 16 times. Similar observations were made in previous studies [25,26]. e two oils were characterized with monoterpene hydrocarbons, sesquiterpenes, and oxygenated compounds. However, monoterpenes and sesquiterpenes were dominant in the SFME and hydrodistilled oils, respectively, but higher quantity of oxygenated compounds was present in the SFME-extracted oil. is is in agreement with the essential oil compositions of Angelica sylvestris L. var. sylvestris fruits [27], Ferula glauca, Ferula arrigonii, and Ferula communis [28], all members of the Apiaceae family. Lower quantity of monoterpenes with higher sesquiterpenes and oxygenated compounds was observed in the SFME-derived oil. Monoterpene hydrocarbons are less important than oxygenated compounds in terms of their impact on the pleasant aroma of essential oils, while the oxygenated compounds are highly fragrant and, hence, the most important [26]. Nevertheless, monoterpenes are generally useful for therapeutic purposes as antibacterial and antifungal agents [29]. Sesquiterpenes also possess anti-inflammatory and anticarcinogenic properties [1].
Higher oxygenated compounds observed in the solventfree microwave-extracted oil indicate the superiority of the method over hydrodistillation and could be attributed to reduced water content in the system which would have minimized the thermal and hydrolytic degradation of oxygenated compounds when compared with the hydrodistillation method [30,31]. e major constituents observed in the essential oil extracted by the SFME method include a-pinene (6%), D-limonene (11.27%), β-ocimene (9.09%), β-phellandrene (6.33%), β-mycene (8.49%), caryophyllene (5.96%), and camphene (4.28%). However, in the hydrodistillation method, the oil was majorly composed of α-pinene (4.41%), β-pinene (10.68%), β-ocimene (6.30%), germacrene-D (5.09%), humulene (5.55%), and a-elemene (6.18%). is is in correlation with the findings of Mwangi et al. [32] and Chagonda et al. [33] in their observations that sabinene, δ-3carene, myrcene, germacrene-D, limonene, (Z)-β-ocimene, β-phellandrene, and α-pinene are major constituents in H. arborescens leaves. e absence of sabinene and δ-3carene in this study could be as a result of the difference in their geographic areas or some biotic and biotic factors in the environment [34]. It is interesting to note that 22 compounds were unique to only the hydrodistillation method and 17 were distinctive to the SFME methods, while 13 were common to both methods. is buttresses the point that the chemical composition of essential oils is dependent on the extraction methods [35].

Conclusions
is study has shown that the chemical composition of the essential oils obtained from H.arborescens leaves is dependent on the extraction method. e SFME method resulted in a higher yield in terms of quantity and an essential oil of better quality due to the presence of higher valuable oxygenated compounds than that which was obtained by hydrodistillation. Furthermore, the use of the SFME method 4 e Scientific World Journal indicated lower energy consumption and CO 2 emission. It can therefore be concluded that the SFME method is considered as a good alternative for the extraction of essential oil from H. arborescens leaves at shorter extraction time, reduced energy consumption, lower cost, and environmental footprint when compared with the hydrodistillation method.

Data Availability
All underlying data supporting the results can be found in the manuscript.

Ethical Approval
Ethical approval was granted by the University of Fort Hare Animal and Plant Use Research Ethics Committee, South Africa, with protocol number AFO111SABI01.

Conflicts of Interest
e authors declare that they do not have any conflicts of interest regarding the publication of this paper.