ORIGINAL ARTICLE
Extraction of citronella oil from lemongrass
(Cymbopogon winterianus) by sequential ultrasonic and microwave-assisted hydro-distillation
Maya Sarah
a,b,*, Dwiky Ardiansyah
a,b, Erni Misran
a,b, Isti Madinah
a,baChemical Engineering Department, Universitas Sumatera Utara, Indonesia
bJl. Almamater Kampus USU, Padang Bulan, Medan 20155, Indonesia
Received 28 January 2022; revised 2 July 2022; accepted 5 March 2023
KEYWORDS Extraction;
Essential oil;
Lemongrass;
Microwave;
Ultrasonic
Abstract The Ultrasonic and Microwave-Assisted Hydro-Distillation (US-MAHD) method is a process combination of ultrasonic-assisted extraction (UAE) and microwave-assisted hydro- distillation (MAHD) carried out sequentially. This method aims to improve the UAE and MAHD methods in extracting citronella oil. This study evaluates the performance of US-MAHD in the extraction of citronella oil from the citronella plant. Extraction is conducted for 90 min at various solvent-plant ratios (v/m) (10:1, 12:1, 14:1), ultrasonic bath temperature (30°C, 40°C, 50°C), and power of microwave oven (150 W, 300 W, 450 W). US-MAHD yields a maximum yield of citronella oil of 1.82 mg/g when extracting lemongrass plant with a combination of ultrasonic bath tempera- ture of 30°C, a solvent-plant ratio of 10:1, microwave power of 300 W, and time of 90 min. All quality parameters of the oil produced have met ISO 3848:1976 standard. Extraction of citronella oil using the US-MAHD method produces higher yields than the UAE and MAHD methods under the same operating conditions. The yields of citronella oil from the US-MAHD, UAE, and MAHD methods are 1.82, 0.92, and 1.48 mg/g, respectively. SEM analysis of residual lemongrass plant shows more cell wall damage which indicates more oil release from the plant matrix.
Ó2023 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Alexandria University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/
licenses/by-nc-nd/4.0/).
1. Introduction
Indonesia has many exotic aromatic plants containing complex volatile compounds such as lemongrass, lime orange, cinna- mon, Javanese frankincense, sandalwood, etc. Extraction of
roots, leaves, bark, stems, fruit, seeds, or flowers of aromatic plants results in an essential oil[1]. The essential oil is sec- ondary metabolites product of plants that has specific fragrant [2]. Essential oil, therefore, becomes a key ingredient to pro- duce perfume, soap, cosmetics, etc. Lemongrass plant with an essential oil content of 1–2% (dry basis)[3]is available from abundant sources in Sumatera Utara Province, Indonesia, but their market and utilization are low[4]. The essential oil of the lemongrass plant known as citronella oil is characterized by a strong lemon fragrance due to the citral compound in the oil
* Corresponding author.
E-mail address:[email protected](M. Sarah).
Peer review under responsibility of Faculty of Engineering, Alexandria University.
H O S T E D BY
Alexandria University
Alexandria Engineering Journal
www.elsevier.com/locate/aej www.sciencedirect.com
https://doi.org/10.1016/j.aej.2023.03.019
1110-0168Ó2023 THE AUTHORS. Published by Elsevier BV on behalf of Faculty of Engineering, Alexandria University.
This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
In industry, citronella oil is one of the ingredients used to produce ionone, vitamin A, and carotene[3]. The aroma of the oil is widely used in soaps, detergents, and perfumes. In addition, citronella oil contains several bioactive compounds that are medicinal[5], antioxidants, anti-microbial, and anti- fungal[3]. Citronella oil also functions as an insect repellent because of its high efficacy and low toxicity, this use has been registered with the US EPA (Environmental Protection Agency)[7]. Although citronella oil is widely used in the phar- maceutical, cosmetic, and food industries, its selling price is still low, so it is needed to increase its economic value by iso- lating its active components such as citronellal, citronellol, and geraniol[8].
Extraction of essential oils from exotic aromatic plants can be carried out by several methods such as conventional steam distillation and hydro-distillation solvent extraction[9], inno- vative methods such as sonoprocess-assisted solvent extraction [1011], ultrasonic-assisted extraction (UAE) and microwave- assisted extraction (MAE)[1112]. Comparisons between tradi- tional methods and innovative methods in the essential oil extraction process have been reported by several authors else- where. Hydro-distillation as a traditional method is a common method for extracting essential oils, resulting in low extraction yields with long processing times [13]. The hydro-distillation method has been improved by using a microwave to aid extrac- tion. Ranitha et al. [3] reported that at the same time the extraction process, microwave-assisted hydro-distillation (MAHD) resulted in a higher yield of citronella oil compared to hydro-distillation and the content of citronella oil obtained from both methods was almost the same. Hamdiet al. [14]
reported that the extraction process of Eucalyptus salubris essential oil with MAE resulted in a higher yield of UAE and hydro-distillation, while the UAE produced more monoterpene hydrocarbon compounds.
Hamzahet al.[15]extracted citronella using three methods, namely ohmic-heated hydro-distillation, hydro-distillation, and steam distillation. Ohmic-heated hydro-distillation was considered feasible as an alternative method of extracting citronella oil because the SEM test of citronella grass showed that ohmic heating caused the formation of transient pores in the cell membrane, although the cell wall was still rigid. Ghaz- anfariet al.[16]reported that essential oil from coriander seeds extracted with MAHD showed better antimicrobial activity, higher phenols yield, and antimicrobial activity than the hydro-distillation method. Nurulet al. [17]extracted emprit ginger oil using four methods, namely microwave ultrasonic steam diffusion (MUSDf), steam diffusion (SDf), microwave extraction (ME), and microwave steam diffusion (MSDf), the best results were obtained using the MUSDf method with a yield of 0.952%.
Dranca et al. [18] compared conventional extraction and microwave-assisted extraction to obtain pectin from Malus Domestica ’Falticeni’ pomace, both methods obtained the same pectin yield but microwave extraction required a shorter time, galacturonic acid content and the esterification rate of pectin obtained was similar in both methods. Microwave- extracted pectin is possible for the manufacture of films having low oxygen permeability. Alara,et al.[19]reported the extrac-
time of 120 min, microwave power of 500 W, a solvent volume of 200 mL, and a temperature of 100°C.
Silvaet al.[20]conducted a study on the extraction of pas- sion fruit seeds (Passiflora edulis Sims) to obtain residual seed cake from oil extraction containing piceatannol, the study was carried out using microwave-assisted extraction method and conventional Soxhlet extraction method. Microwave extrac- tion yielded a fine cocoa powder with 27.17 ± 0.9 g piceatan- nol per mg extract at 87°C, with 70% ethanol, for 30 min each cycle, whereas Soxhlet extraction yielded a dark extract con- taining 13.03 ± 0.4 g/mg for 120 min. This proved that micro- wave extraction is a promising alternative for extracting passion fruit seed and adding to the residual value of passion fruit by providing a faster extraction, more color friendly, and yielding higher piceatannol compared to a conventional method. Hu et al.[21]found that the essential oil by micro- wave extraction had better quality than Soxhlet extraction, especially with low acid and peroxide values.
Yingngamet al.[22]reported for the first time the extrac- tion and identification of volatile compounds in essential oils from Shorea roxburghii flowers. Essential oil isolation using solvent-free microwave extraction method, the extracted essen- tial oil showed better overall parameters than traditional hydro-distillation methods, including shorter isolation time, higher odor quality, and environmental compatibility. Leema et al.[23]conducted a study on microwave-assisted extraction to extract lutein fromChlorella sorokiniana (NIOT-2). X-ray diffraction (XRD) analysis of the microwave-treated biomass (83.85%) showed a much higher crystallinity index than the untreated sample (17.28%). Microwave pretreatment is con- sidered suitable for the extraction of lutein from marine microalgae in consideration of the speed of coalescence, homo- geneous heating, less energy intensity, and high extraction yield.
Both UAE and MAE have been shown to improve over- all extraction performance in terms of yield, oil quality, and extraction time [24]. In the extraction process using UAE, ultrasonic waves will produce cavitation bubbles which can cause cell wall damage so that compounds in cells can be extracted [25]. While MAE uses microwave energy which causes polar molecules in the plant to reorient fol- lowing the direction of the magnetic field movement contin- uously, causing heat from friction between molecules and heat coming out of the plant sample accompanied by evap- oration of essential oils from plant cells. This process increases the effectiveness of the extraction process includ- ing a shorter extraction time [26]. In addition to a shorter time, the microwave method offers an important advantage over traditional methods, namely producing a more valu- able essential oil with a high amount of oxygenated com- pounds [27].
Rodsamran and Sothornvit[28]reported extraction of phe- nolic compounds from lime peel waste using ultrasonic- assisted and microwave-assisted extraction showed that the UAE method was more effective for extracting total phenolics (54.4 mg GAE/g) with high antioxidant activity and 33% time saving compared to MAE. One of the other advantages of applying ultrasound in the extraction of essential oils from
medicinal plants is the use of less solvent compared to conven- tional methods such as mechanical and Soxhlet extraction[29].
Sharifzadehet al.[30]studied the essential oil ofLavandula coronopifolia Poir using a sequential ultrasonic-microwave technique. The highest yield was obtained at 1.15%, the value was 82% higher than the yield of the basic method. This method produced a much higher number of active ingredient compounds than the microwave method, and more antioxi- dant properties. The active ingredients obtained from this extraction method had the highest antimicrobial properties.
In addition, it is more environmentally friendly than the microwave-assisted extraction method.
Ultrasonic also shows tremendous potential in synthesizing nanomaterials, particularly nano biomaterials for biomedical applications. The ultrasonic synthesis produces nano biomate- rials with better properties and performance than conventional synthesis methods and eliminates the need for harsh and expensive chemicals [31]. Ngamkhae et al. [32] extracted the Kleeb Bua Daeng formula with ultrasonic and microwave extraction methods separately. Extracts from the microwave method contained higher phenols, flavonoids, carotenoids, and anthocyanins than extracts from the ultrasonic method.
The microwave method is considered more efficient in extract- ing the formula. A comparative study of ultrasonic and micro- wave extraction was also reported by Sharma and Dash[33]to extract phytocompounds from black Jamun pulp. This method will make it possible to extract polyphenols and anthocyanins from black Jamun pulp for nutraceuticals, pharmaceuticals, and functional food-based industries.
Yang et al. [34] have successfully developed microwave- induced hydro-distillation and simultaneous extraction in a rotary state to produce essential oil, rosmarinic acid, and polysaccharides from Perilla frutescens. This technique can be an alternative to improve the extraction of essential oils, polysaccharides, and other target analytes fromP. frutescens or other plant materials. Costa et al. [35]have developed a greener microwave-assisted extraction method by modifying it using a natural deep eutectic solvent that is applied for the preparation of medicinal plant samples. The optimization of the combined microwave method and the use of environmen- tally friendly methods resulted in excellent analytical parameters.
Wan et al. [36]simultaneously extracted essential oil and flavonoids from Baeckea frutescensby enzyme pre-treatment method combined with ultrasonic microwave-assisted surfac- tant. The concentration of flavonoid extract obtained was 9.7 times higher than the original extract. The essential oil obtained was confirmed to have potential bioactivity. These results indicated this method can be used as a promising, and environmentally friendly alternative to extracting the active constituents.
However, in the extraction process, UAE and MAE meth- ods have disadvantages[3738]. UAE has the problem of atten- uation effect[39]while in the extraction process using MAE, inhomogeneous heating can occur[40]. Because of these short- comings, Yuet al.[37]evaluated the extraction process of total flavonoids using sequencing of UAE and MAE (US-MAE) fromOsmanthus fragransLour. flowers. Yuet al.[37]reported that US-MAE resulted in a higher yield of total flavonoids than UAE and MAE.
To date, no study reported extraction of essential oil using a sequential combination of UAE and MAHD (US-MAHD),
especially for the extraction of citronella oil. Therefore, this study aims to extract the essential oil of lemongrass leaves (Cymbopogon winterianus) using US-MAHD. The effects of solvent to plant ratio, ultrasonic temperature, and microwave power on yield are evaluated. To compare the effectiveness of the US-MAHD method, extraction of lemongrass leaves with UAE and MAHD is carried out using the temperature, fre- quency, and power of US-MAHD which results in the highest yield. The performance of US-MAHD will be compared with UAE and MAHD based on the yield and the constituents of citronella oil. To maximize the yield of citronella oil from the lemongrass plant, US-MAHD extraction of lemongrass stems is also investigated. Overall, this study is expected to increase the quality and quantity of citronella oil production in Indonesia, especially in Sumatera Utara Province which is still done traditionally using the steam distillation process that is considered ineffective and very expensive due to high energy requirements.
2. Materials and methods 2.1. Materials
Materials in this study comprise lemongrass (Cymbopogon win- terianus) taken from Deli Serdang, Sumatera Utara Province, Indonesia, distilled water as solvent from the local chemical supplier (PT. Rudang, Medan), and ethanol 80% for alcohol solubility analysis supplied by PT. Merck Tbk. The extraction process was carried out in an ultrasonic bath (Elmasonic S 300 (H)) for the UAE method and a microwave oven (Microwave Sharp R-21D0(S)IN) for the MAHD method. The ultrasonic bath is equipped with tank internal dimensions of 505 x300x 200 mm that operated at an ultrasonic frequency of 37 kHz.
The microwave oven was modified with the hydro-distillation apparatus. The dimensions of the microwave oven cavity were 485400292 mm with 800 W of power consumption and a 229–240 Volt power source. This experiment was carried out at the Laboratory of Organic Chemistry, Chemical Engineering Department, Universitas Sumatera Utara, Medan, Indonesia.
2.2. Methods
The scheme of citronella oil extraction from lemongrass plants by sequential US-MAHD method in this study is illustrated in Fig. 1. Before US-MAHD, pre-treatment is conducted to pre- pare the material to obtain the best result. The sequential US- MAHD begins with the extraction by ultrasonic wave followed by microwave irradiation assisted by hydro-distillation. The extracted oil was separated from the solvent and dried to remove all the water before a further investigation of oil qual- ity (color, constituents, density, refractive index, and solubility in ethanol).
2.2.1. Pretreatment of lemongrass
Fresh lemongrass plants collected from the plantation were cut for the leaves and the stem. The leaves were dried at room tem- perature without direct exposure to sunlight for a week. The drying aims to reduce water content in the plant to increase extraction yield[3]. The dried plants were cut with a dimension of 1x1 cm and then ground using a grinder (Miyako
BL-101PL) with a power of 300 W for one minute. Samples that have been reduced in size will facilitate the oil extraction process with US-MAHD to obtain a high yield[41].
2.2.2. The sequential US-MAHD
In this study, the main ingredient was citronella leaves because the fragrance was more dominant, while the stem was used as a comparison. Experimental devices to perform sequential US-
MAHD are shown inFig. 2. UAE and MAHD used to com- pare US-MAHD performance are shown inFig. 2(a) and 2(b) respectively. About 100 g of the treated lemongrass leaves were placed into the flask. The distilled water was added at various ratios of solvent to plant material (v/m) (10:1; 12:1; 14:1 respectively). The flask was transferred into the ultrasonic bath to extract the sample using the UAE method. The process was carried out for 25 min at various ultrasonic bath temperatures Fig. 1 Scheme of citronella oil extraction from lemongrass plant by sequential US-MAHD method in this study.
Fig. 2 Experimental apparatus for US-MAHD in this study (a) UAE and (b) MAHD: (1) transducer, (2) water, (3) ultrasonic bath, (4) flask, (5) condenser, (6) microwave, (7) flask, (8) timer and power regulator, (9) condenser, (10) Liebig condenser, (11) separating funnel, and (12) distillate bottle.
(30 °C; 40°C; 50 °C respectively). After that, the flask was placed into a microwave oven that have been connected to hydro-distillation apparatus for further extraction process using the MAHD method for 65 min. The microwave power was varied at 150 W, 300 W, and 450 W. The procedure was repeated for various combinations of solvent to plant material ratio, the temperature of the ultrasonic bath, and microwave power, each experimental run was conducted in triplicate.
Oil was separated from the distillate by using a separatory fun- nel, followed by drying the oil over anhydrous sodium sulfate to ensure there is no moisture content within the citronella oil, and the yield obtained was calculated using Eq(1). As a com- parison, extraction of citronella oil from the lemongrass stems was then carried out using the best combination of tempera- ture, frequency, and power of US-MAHD yielded the highest oil from the lemongrass leaves.
Yield ðmg=gÞ ¼ammount of essential oil obtained ðmgÞ ammount of raw material used ðgÞ
ð1Þ
2.2.3. The UAE method
Distilled water as solvent was added into a flask containing 100 g of lemongrass leaves concerning solvent to plant ratio obtained from the highest yield in the extraction of similar material by US-MAHD. The sample was then extracted using an ultrasonic bath with a frequency of 37 kHz for 90 min at the ultrasonic temperature of US-MAHD resulting in the highest yield. The experimental run was conducted in triplicate. The separation of distillate was then carried out using a separatory funnel and oil was dried further over anhydrous sodium sul- fate. The experimental apparatus for the UAE method in this study is shown inFig. 2(a).
2.2.4. The MAHD method
The microwave oven modified with hydro-distillation appara- tus was used to extract 100 g of lemongrass leaves. The extrac- tion process was carried out for 90 min using the ratio of solvent to plant material resulting in the highest yield in the extraction of citronella oil from lemongrass leaves by US- MAHD. The experimental run was conducted in triplicate.
Microwave power was regulated in accordance power of US- MAHD that yielded the highest yield. The distillate was col- lected, and oil was separated using a separatory funnel and dried further over anhydrous sodium sulfate to remove all the solvent. The experimental apparatus for the MAHD method in this study is shown inFig. 2(b).
2.2.5. Determination of citronella oil properties
The Standard of density, refractive index, and oil solubility in ethanol in this study is referred to as ISO 3848:1976. Determi- nation of citronella oil’s density (20/20 °C), refractive index (20°C), and solubility in 80% (v/v) ethanol (20°C) were car- ried out following procedure of ISO/R 279, ISO/R 280, and ISO/R 875, respectively[42].
2.2.6. Determination of citronella oil constituents
The citronella oil constituents mainly consist of geraniol, citronellal, citronellol, citral, etc. Citronella oil constituents in this study were identified by gas chromatography/mass spec-
trometric (GCMS) (Shimadzu-QP2010 Plus). GC was equipped with a capillary column (Cross bond Carbowax, polyethylene glycol, 30 m long, 0.25 mm ID, 0.25 mm film thickness) that was used to identify the composition of extracted citronella oil. The oven temperature was pro- grammed at 60°C for 2 min which rise to 160°C at the rate of 4°C/min and holding time of 5 min. A solvent, n-hexane (1/10 v/v) was used to dilute the samples[3]and about 8lml diluted samples were injected into the GC by split mode with a ratio of 100/1. The injection temperature was set at 200°C. The rate of helium as a gas carrier was adjusted lin- early by 46.3 cm/sec. A calibration curve was developed using pure citronella oil and a database from the Wiley library was also used to compare the mass spectral fragmentation patterns in identifying the constituents of citronella oil. The quantifying of components was carried out concerning their compound’s retention time. The percentage of each component was obtained as a ratio of the area under the peak and the total area of the chromatogram[3]. Citronella oil from the extrac- tion of lemongrass leaves by the US-MAHD method would be compared with citronella oil resulting from each UAE and MAHD method.
2.2.7. Scanning Electron Microscopy analysis of lemongrass The effect of extraction methods (UAE, MAHD, and US-MAHD) on the structure of the lemongrass plant was investigated by Scanning Electron Microscopy (SEM). SEM analysis aims to investigate the ability of ultrasonic and micro- wave energy to destruct the cell wall that facilitates the release of oil to come out from the plant cell. The residues of lemon- grass after the extraction process in every method were col- lected and dried at room temperature for further SEM analysis[3840]. The microstructures of plants including shape and surface characters were observed and recorded by SEM (JSM 6510 LA, JOEL Ltd, Japan). This analysis was carried out at Laboratorium Terpadu, Universitas Diponegoro, Semarang.
3. Results and discussion
3.1. The extraction process of citronella oil from lemongrass leaves
The extraction of citronella oil from lemongrass leaves using US-MAHD was carried out at various bath temperatures of 30 °C, 40 °C, and 50 °C (UAE), and microwave powers (150 W, 300 W, and 450 W).Fig. 3shows the yield obtained from extraction by US-MAHD in this study. Based on obser- vations, the overall increase in ultrasonic bath temperature from 30°C to 50°C and the ratio of solvent to citronella sam- ples from 10:1 to 14:1 decreased the yield of citronella oil. For example, increasing the ultrasonic bath temperature at a solvent-plant ratio of 10:1 results in a decrease in yield in the range of 3.0 to 4.5 mg/g. As with temperature, increasing the ratio of solvent to material also decreases yield. Increasing the solvent-ingredient ratio from 10:1 to 14:1 decreased the yield in the range of 3.6 to 7.3 mg/g. Meanwhile, increasing the microwave power from 150 to 300 W increased the oil yield up to 35.8 mg/g. However, the use of microwave power of more than 300 W reduces the yield of citronella oil.
The highest yield of extraction obtained was 1.82 mg/g while the lowest yield was 1.17 mg/g. The highest yield was obtained from a combination of ratio solvent to plant of 10:1, the ultrasonic temperature of 30 °C, and microwave power of 300 W with an extraction time of 25 min in an ultra- sonic bath and 65 min in a microwave oven. The lowest yield of citronella oil was obtained by using a solvent to plant ratio of 14:1, an ultrasonic temperature of 50 °C, and microwave power of 150 W at a similar period.
In this study, the effect of some parameters to yield citro- nella oil such as solvent-plant ratio, the temperature of the ultrasonic bath, and the power of the microwave oven were investigated in the extraction process from lemongrass leaves.
The physical properties of citronella oil extracted from lemon- grass leaves by using the US-MAHD method are shown in Table 1. The physical properties indicated characteristics of citronella oil obtained from experiments at various combina-
tions of solvent to plant ratio, the temperature of the ultra- sonic bath, and the power of the microwave oven. Overall citronella oil resulting from the US-MAHD in Table 1meets the standard specification of ISO 3848:1976. Oil relative den- sity, refractive index, and solubility in 80% (v/v) ethanol were Fig. 3 Effect of solvent-plant ratio (v/m) on the yield of citronella oil using the US-MAHD method at various ultrasonic temperatures and microwave power: (a) solvent-plant ratio 10:1, (b) 12:1, and (c) 14:1.
Table 1 Physical properties of citronella oil.
Parameters Result of this study ISO 3848:1976
Colour Yellow-brownish
yellow
Pale yellow-brownish yellow
Density 0.880–0.917 0.880–0.922
Refractive Index 1.466–1.472 1.466–1.475 Solubility in Ethanol
80%
1:2 clear 1:2 clear
determined following ISO/R 279, ISO/R 280, and ISO/R 875 respectively.
The colors of citronella oil produced in this study were yellow-brownish yellow, this yellow color was from the yellow substance contained in the essential oil called carotene [43].
The refractive index is influenced by the length of the carbon chain and the number of double bonds. The higher the index value, the higher the carbon chain length and the number of double bonds. Essential oil with a large refractive index value has better quality than oil with a small refractive index[44].
Solubility in ethanol produced a clear mixture, which meant the oil was completely soluble in ethanol. This was following the theory because the essential oil must be 100% soluble in ethanol[43].
3.1.1. The effect of solvent-plant ratio of US-MAHD to yield of the citronella oil
To determine the effect of the plant-solvent ratio on the yield of citronella oil, an evaluation was carried out at (1) constant ultrasonic temperature (30 °C) and (2) constant microwave power (300 W) which gave maximum results. The solvent- plant ratio in the extraction can affect the level of interaction between solids and solvents which will affect the extraction results, the best results are achieved when the solution reaches a saturation concentration[45]. The effect of the plant-solvent ratio on the yield of citronella oil at a constant ultrasonic bath temperature of 30°C is shown inFig. 4, whileFig. 5shows the same effect at a constant microwave power of 300 W.Figs. 4 and 5 show the correlation between the solvent-plant ratio and the yield of citronella oil of extraction with US-MAHD is inversely related.Fig. 4shows the extraction yield decreased by 7.46 mg/g as the ratio increased from 10:1 to 14:1 at 150 W microwave power. Similar results were obtained at 300 W and 450 W microwave power. An increase in the plant-solvent ratio from 10: 1 to 14:1 reduces the extraction yield by 7.69 mg/g and 8.87 mg/g at 300 W and 450 W microwave power, respec- tively. In principle, electromagnetic waves cause the reorienta- tion of polar water molecules according to the direction of the magnetic field, causing friction and heat. The friction between the water molecules and the lemongrass raw material tears the cell walls and releases lemongrass oil. However, a substantial
increase in the amount of solvent and microwave power results in excess heat. If this happens in a long period (90 min), then there will be evaporation of the solvent and the resulting citro- nella oil. This resulted in a reduced oil yield as shown inFig. 4.
Similar results were also concluded from the extraction at a fixed microwave power of 300 W as shown inFig. 5. Extrac- tion of citronella oil using a microwave power of 300 W and a temperature of 30 °C reduced the yield by 3.3 mg/g and 4.5 mg/g as the plant-solvent ratio increased from 10:1 to 12:1 and 14:1, respectively. Increasing the temperature from 30 °C to 40 °C at a constant microwave power of 300 W decreased the yield by 9.7 mg/g (ratio of 12:1) and 2.5 mg/g (ratio of 14:1). Increasing the temperature from 30 °C to 50°C increases the water vapor pressure which weakens the cavitation bubbles thereby avoiding damage to the cell walls of the citronella plant samples.
Similar conclusions were reported by Wong et al. [46], Ranithaet al.[3], Hoet al.[47], and Hien Tranet al.[48]. Fun- damentally, the increment of solvent to plant ratio from 10:1 to 12:1 and 14:1 elevated the driving force of mass transfer due to the high gradient of concentration between the solvent and the phytochemical component. Overall increase the yield of extraction[46]. However, in the microwave-assisted extrac- tion process, an increase in the amount of excess polar solvent would cause excess thermal stress[3], so some of the extracted components will be lost. In addition, an excess amount of polar solvent could dissolve or emulsify oil compounds so that the extracted essential oil yield would be reduced[4748]. The amount of solvent used for the extraction of essential oils from plant material must be in sufficient quantities. The greater amount of solvent can limit the overheating of the plant mate- rial and there is no burning of the plant material. In addition, a higher amount of solvent can also cause more energy absorption, so the oil yield decreases[49].
3.1.2. Effect of ultrasonic temperature of US-MAHD on yield of the citronella oil
Fig. 6shows the effect of ultrasonic temperature on the extrac- tion yield at a fixed microwave power of 300 W. The yield of citronella oil decreases as the ultrasonic temperature increases.
Extraction using the US-MAHD method using a solvent to
Fig. 4 Effect of solvent-plant ratio (v/m) to the yield of citronella oil using the US-MAHD method at a fixed ultrasonic temperature of 30°C and various microwave powers of 150 W, 300 W, and 450 W.
plant ratio of 10:1 will produce a yield of 1.82 mg/g at an ultra- sonic temperature of 30°C. The yield decreased to 1.67 mg/g when the ultrasonic temperature was increased to 50 °C.
Extraction yield decreased when the solvent to plant ratio was increased to 12:1 and 14:1. When the ultrasonic tempera- ture was increased from 30 to 50°C, the yield decreased by 13.69 mg/g and 13.63 mg/g for the plant-solvent ratio of 12:1 and 14:1, respectively. In point 3.3.1 it has been explained that the temperature increment of the ultrasonic bath reduces the yield of citronella oil. An increase in temperature will increase the water vapor pressure thereby lowering the surface tension and reducing the effect of cavitation bubbles due to the move- ment of water vapor into the bubbles which weakens the bub- bles. As a result, the contact between the solvent and the plant sample was not optimal and this minimized damage to the sur- face of the citronella plant sample so that the oil released was reduced.
Chemat et al. [50] reported similar results elsewhere. An increase in ultrasonic temperature would elevate the vapor pressure of the solution that decreasing viscosity and surface tension. It will reduce the effect of cavitation in the extraction process because the solvent vapor would enter the cavitation
bubbles and caused the bubbles to collapse weakly. It would reduce the number of damaged cells due to the effect of cavi- tation causing a reduction of the extracted compound. Con- versely, selecting a low temperature such as 30 °C could increase the yield due to the increment of cavitation bubbles number and the solid-solvent contact area. Similar results were also obtained on ultrasound-assisted extraction of essential oils from Oliveria decumbens flowers, the yield of essential oils increased significantly with increasing temperature from 25 to 40°C. However, a further increase in ultrasonic temperature from 40 to 55°C caused a decrease in yield[51].
3.1.3. The effect of microwave power of US-MAHD on yield of the citronella oil
The effect of microwave power on yield resulting from the extraction of citronella oil using US-MAHD at a fixed ultra- sonic temperature of 30°C is shown inFig. 7. Extraction using various solvent-plant ratios increases the yield of citronella oil as microwave power rises from 150 W to 300 W. The yield of citronella oil increases by 35.8 mg/g, 37.5 mg/g, and 35.5 mg/g at a solvent-plant ratio of 10:1, 12:1, and 14:1 respectively.
Increasing microwave power above 300 W decreases the yield Fig. 5 Effect of solvent-plant ratio (v/m) to the yield of citronella oil using the US-MAHD method at a fixed microwave power of 300 W and various ultrasonic bath temperatures of 30°C, 40°C, and 50°C.
Fig. 6 Effect of ultrasonic temperature on extraction yield at a microwave power of 300 W using the US-MAHD method.
by 7.14 mg/g, 6.81 mg/g, and 8.33 mg/g at a solvent-plant ratio of 10:1, 12:1, and 14:1 respectively.
Increasing the power of the microwave oven will increase the penetration of electromagnetic waves into samples of citro- nella plants and water solvents. This has implications for increasing energy conversion from microwave energy into heat energy. The mechanism of energy change from microwave energy to heat energy has been described in point 3.3.1.
Increasing the power of the microwave oven will increase the amount of heat energy so that excess heat occurs. Based on observations, it is known that the microwave power of 300 W is the maximum power that can be tolerated by the ingredients (water solvent and lemongrass plant samples) to prevent excessive heating. Heating a microwave oven with a power of less than 300 W produces a low yield of citronella oil because the amount of power density produced is low, and vice versa. The power density limits tolerated by the mate- rial in this study were 227.72 W/kg, 230.77 W/kg, and 200 W/
kg for extraction with US-MAHD at plant-solvent ratios of 10:1, 12:1, and 14:1 respectively.
According to Hien Tranet al.[48], the use of high micro- wave power will increase the yield of essential oils but only to a certain power level. The increase in extraction yield occurred because the use of high power would result in a higher temperature rise due to the more random reorientation of polar molecules[3]. This process could increase the solvent diffusion so that more of the essential oil would be extracted.
However, the use of microwave power could cause some of the extracted chemical components to be lost and decomposed [48]. Nour et al. [52] reported that during short extraction times, about 30 to 90 min, the yield increased with increasing microwave power. However, when the extraction was carried out long enough, about 120 to 150 min, the yields obtained were similar under different microwave power. The initial extraction rate increased with increasing microwave power.
This is due to the rapid heat generation in the extracted plant material by absorption of microwave energy and the formation of a higher pressure gradient in the plant material when applied to higher microwave power.
The increase in yield at the beginning of the extraction then decreased most likely due to the recruitment of balanced microwave forces which disrupt the plant cell wall thereby facilitating dissolution into the cell and mass transfer. On the
other hand, the target compound can be degraded by increas- ing the microwave power, resulting in a decrease in the extrac- tion yield. Therefore, suitable microwave power is very important for the extraction process[36].
3.2. The comparison of UAE, MAHD, and US-MAHD methods in the extraction process of citronella oil
A comparison of the extraction yields produced using the UAE, MAHD, and US-MAHD methods is shown inFig. 8.
The extraction process for each of the above methods was car- ried out for 90 min using lemongrass leaves.Fig. 8shows the US-MAHD method obtained a higher yield while the lowest yield was obtained by the UAE method. The yield of citronella obtained by the US-MAHD method was 49.45% higher than the UAE method, while compared to the MAHD method, the yield obtained by the US-MAHD method increased by 18.68%.
Yuet al.[37]reported combining the UAE and MAE meth- ods sequentially (US-MAE) results in a higher extraction yield of flavonoids as compared to UAE or MAE methods. In the process of extracting phytochemical compounds using the US-MAE method, it was reported that more cell walls were completely damaged than in the process with UAE and MAE so that the yield of the extracted compounds could be higher. The highest total flavonoid content was obtained in the sequential combination of microwave-and ultrasound- assisted extraction (SC-MUAE) method (7.85 mg/g) compared to other methods, MAE (4,789 mg/g), UAE (6,229 mg/g), and sequential combination of ultrasound- and microwave-assisted extraction (SC-UMAE) (6,548 mg/g). It is related to the SEM analysis in this study shown inFig. 9for extraction by UAE, MAHD, and US-MAHD methods at 30°C (UAE and US- MAHD), 300 W (MAHD and US-MAHD), and 90 min.
Fig. 9(a) shows the SEM results of the sample before the extraction process is carried out. The black color indicates that there is no damage to the outer surface of the citronella plant cell sample indicating that the cell wall is still intact.Fig. 9(b), 9(c), and 9(d) show the SEM of a sample after the extraction process by UAE, MAHD, and US-MAHD respectively.
Fig. 9(b) shows the SEM of a sample with a damaged cell wall after being treated with an ultrasonic wave to obtain citronella
Fig. 7 Effect of microwave power on extraction yield at a fixed ultrasonic temperature of 30°C using the US-MAHD method.
Fig. 8 Yield comparison of UAE, MAHD, and US-MAHD methods in this study at 30°C (UAE/US-MAHD), 300 W (MAHD/US- MAHD), and 90 min.
Fig. 9 SEM Analysis of lemongrass sample pre-extraction (a), post-UAE (b), post-MAHD (c), and post-US-MAHD (d) at 30°C (UAE/
US-MAHD), 300 W (MAHD/US-MAHD), and 90 min.
oil, as indicated by a red circle. A similar illustration is shown in Fig. 9(c) and 10(d) for extraction with MAHD and US- MAHD respectively. SEM results from extraction using the US-MAHD method (Fig. 9(d)) showed the highest level of cell wall damage. This can be explained as follows. Extraction by the US-MAHD method occurs because ultrasonic waves and microwave energy are gradually applied to the sample.
In the UAE stage, ultrasonic waves propagate through the solvent and cause the phenomenon of acoustic cavitation which on a micro-scale can increase the temperature and pres-
sure, thereby increasing the pressure difference and the sheer force of the solvent. The contact between the cavitated waves with the sample surface will break some of the sample cell walls and the citronella oil will come out. If further extraction is car- ried out with microwaves, the remaining solvent will be reori- ented following the movement of polar molecules so that there is an increase in temperature and the process of cell wall dam- age will increase. This has implications for obtaining high yields. This confirms why extraction with US-MAHD obtains the highest yield. In the case of extraction using the UAE and
Fig. 10 The chromatogram of citronella oil extracted from leaves using UAE (a), MAHD (b), and US-MAHD Method (c) at 30°C (UAE/US-MAHD), 300 W (MAHD/US-MAHD), and 90 min.
MAHD methods only, the cell wall destruction process is not as optimal as US-MAHD. The UAE and MAHD methods have the disadvantage of being carried out separately. In the UAE method, a wave attenuation effect can occur, namely a
inhomogeneous heating can occur due to uneven wave absor- bance in the sample which will reduce the extraction yield.
GC-MS analysis was performed to compare the com- pounds of citronella oil obtained by the UAE, MAHD, and US-MAHD methods. Chromatogram of GC-MS analysis for citronella oil from leaves is shown inFig. 10and information on the comparison of compounds produced by each of the UAE, MAHD, and US-MAHD methods are shown inTable 2.
The content of chemical compounds in citronella oil obtained from extraction by the UAE, MAHD, or US-MAHD method varies. The MAHD method produced citronella oil with the highest citronellal content (18.03%) while the highest citronel- lol (17.25%) was produced by the UAE method. The US-MAHD method produced the highest geraniol (46.69%), limonene (3.10%), and citral (3.30%). The difference in the content of citronella oil content from the 3 methods as shown inTable 2provides a distinct advantage because it can be used for various purposes such as perfume, a fragrance for house- hold appliances, anti-fungal, anti-bacterial and antioxidant.
Each process has its advantages concerning the content of citronella oil. This provides an opportunity for product diver- sification toward the use of lemongrass. US-MAHD produced oil with the highest content of geraniol and citral at 46.69%
and 3.30%, respectively. Geraniol and citral components are raw materials for perfumes and household fragrances. Mean- while, UAE produces an oil with the highest citronellol content (17.25%), while MAHD produces an oil with citronellal con- tent (18.03%). Citronellol is a compound that is widely used as an antifungal and anti-microbial agent, while citronellal functions as an antioxidant.
According to Wanyet al.[6], the most important chemical compounds of citronella oil were citronellal, citral, citronellol, and geraniol. Citronellal was a monoterpenoid that had anti- fungal activity, and antimicrobial activity, and is widely used as an insect repellent. Other compounds of citronella oil, citral is one of the raw materials for producing perfume and vitamin A while citronellol contains antioxidants [53]. Mazczka et al.
[54]reported geraniol is widely used for its application as a fra- grance in cosmetic and household products.
UAE MAHD US-MAHD
Limonen 1.65 2.05 3.10
Pinene 1.22 2.82 2.74
Carene – 1.21 0.96
Citronella 11.52 18.03 17.79
Linalool 3.58 2.51 1.83
Isopregol 3.14 1.66 1.98
Isopulegol – 2.81 –
Citronellol Acetate 2.57 1.85 1.11
Z-Citral – – 1.07
E-Citral 1.93 1.77 2.23
Caryophllene 1.42 – –
Geraniol Acetate 8.12 6.24 4.44
Citronellol 17.25 13.00 12.45
Isoeugenol – 3.65 –
Geraniol 42.60 38.78 46.69
Geranyl Butyrate 2.56 2.41 1.72
Alpha-terpinolene – – 1.16
Eugenol 1.46 1.20 0.73
Table 3 The compounds of citronella oil resulted from stems using US-MAHD method.
The compound of Citronella Oil Relative Content (%)
Limonene 1.05
Origanene 1.91
Citronella 17.19
Nerol Asetate 4.41
Citronellol 15.28
Geraniol 53.88
Geranyl Butyrate 1.68
Isopropyl Methyl Propylidene 2.07
Dimethyl Bicyclo Hexane 2.52
Fig. 11 The chromatogram of citronella oil extracted from stems using the US-MAHD Method at 30°C, 300 W, and 90 min.
3.3. The extraction process of citronella oil from lemongrass stems
Extraction of lemongrass stems was also conducted to com- pare the yield and oil constituents. The process of extracting citronella oil from lemongrass stems was carried out using the US-MAHD method under the same conditions of leaves extraction resulting in the highest yield. The yield resulting from leaves and stems under this condition was 1.82 mg/g and 0.78 mg/g respectively. The extraction yield obtained by the stem is lower than the yield from leaves. The same result was also reported by Iqbal et al. [55] that the essential oil extracted from the stem of Murraya koenighi was lower as compared to the leaves. The compounds of citronella oil from the stem were identified using GC-MS. The result can be seen inTable 3and its chromatogram of GC-MS analysis is shown inFig. 11.
Tables 2 and 3show the different compositions of citronella oil extracted from the leaves (Table 2) and the stems of the lemongrass plant (Table 3) using US-MAHD at 30 °C, 300 W, 90 min, and the ratio of the solvent to plant 1:10. It was known that geraniol was the highest compound of citro- nella oil (53.88%) extracted while the compound of citral was not found. Besides that, the relative content of citronella and citronellol extracted from stems respectively was 17.19
% and 15.28 %. The geraniol content of citronella oil extracted from the stem (53.88%) was more than that extracted from the leaves (46.69%). So that the stem is very potential to be used as raw material for making perfumes.
However, the absence of the citral component and the low lemon content (3.10%) from the citronella stems requires mix- ing the oil from the citronella stems with the oil from the citro- nella leaves to produce perfume and household fragrances.
Based on essential oil yield, Barbouchiet al.[56]reported that the yield of essential oil isolated from the leaves of threeRuta species was higher (1.86 ± 0.01%) than the yield of essential oil isolated from the stems (0.16 ± 0.00%) (Barbouchi, et al., 2021). Although the yield of lemongrass leaves was 2.34 times higher than the stems, lemongrass stems contain higher citronella oil content than citronellol and geraniol.
4. Conclusions
The sequential US-MAHD method in this study was carried out for 90 min at a combination of solvent-plant ratio of 10:1 (v/m), an ultrasonic temperature of 30°C, and a micro- wave power of 300 W obtained the highest yield of 1.82 mg/
g. The extracted oil has met the standard specified by ISO 3848:1976 with a characteristic odor of strong lemon scent, color pale yellow, a relative density of 0.917 mg/l, a refractive index of 1.472, and solubility in 80% (v/v) of ethanol was 1:2 (v/v). The oil constituents mainly consist of citronella (17.79%), geraniol (46.69%), citronellol (12.45%), limonene (3.10%), and citral (3.30%). Based on this study, it is con- cluded that extraction of citronella oil from lemongrass by US-MAHD has the following advantages: (1) Ultrasonic extraction acts as a preliminary extraction where ultrasonic waves damage cell walls so that some of the oil in the plant matrix can be released. The further extraction process through microwave heating expands the damage to the plant cell walls due to the re-orientation of polar molecules in the citronella
plant which increases the temperature (volumetric heating) and pushes more citronella oil out of the citronella plant cell-matrix, resulting in high yield of citronella oil. (2) It does not require too much solvent (water), so it does not require a large extraction vessel if it is developed on a commercial scale. (3) This extraction can be conducted with fairly low power (300 W) so that the electrical power requirements are not high. However, this extraction process also has several challenges in its implementation on a commercial scale, such as how to design inexpensive ultrasonic and microwave irradi- ation systems. Therefore, it is necessary to develop further research for this extraction using ultrasonic and microwave hybrid extractor.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors gratefully thank Universitas Sumatera Utara for supporting this research under TALENTA Scheme for the Fis- cal Year of 2020.
References
[1] E. Tajidin, Chemical composition and citral content in lemongrass (Cymbopogon citratus) essential oil at three maturity stages, Afr. J. Biotechnol. 11 (2012),https://doi.org/
10.5897/AJB11.2939.
[2]M.E. Ibrahim, S.S. Ahmed, A.K. Khalid, S.A. El-Sawi, Characterization of essential oil content isolated from Rhus flexicauli (Baker), Int. Food Res. J. 24 (2017) 897–899.
[3] R.M., A.H. Nour, Z.A. Sulaiman, A.H. Nour, T.R.S., A comparative study of lemongrass (Cymbopogon Citratus) essential oil extracted by microwave-assisted hydrodistillation (MAHD) and conventional hydrodistillation (HD) method, Int.
J. Chem. Eng. Appl. 5(2) (2014) 104–108, doi: 10.7763/
ijcea.2014.v5.360.
[4] M. Sarah, D. Estherina, Essential oil extraction from citronella grass by microwave-assisted hydro-distillation technique: a preliminary study, IOP Conf. Ser. Mater. Sci. Eng. 801 (Jun.
2020) 12046,https://doi.org/10.1088/1757-899X/801/1/012046.
[5] S. Ranade, P. Thiagarajan, Lemon grass 35 (2015) 162–167.
[6]A. Wany, S. Jha, V. Nigam, D.M. Pandey, Chemical analysis and therapeutic uses of citronella oil from Cymbopogon winterianus: a short review, Int. J. Adv. Res. 1 (2013) 504–521.
[7] R. Sharma, R. Rao, S. Kumar, S. Mahant, S. Khatkar, Therapeutic Potential of Citronella Essential Oil: A Review, Curr. Drug Discov. Technol. 16 (4) (2019) 330–339,https://doi.
org/10.2174/1570163815666180718095041.
[8] Y. Anwar, V.S. Siringoringo, Fractionation of citronella oil and identification of compounds by gas chromatography-mass spectrometry, Pharm. Sci. Res. 7 (3) (2020) 138–144, https://
doi.org/10.7454/psr.v7i3.1101.
[9] T. Rihayat et al, Composition on essential oil extraction from lemongrass fragrant by microwave air hydro distillation method to perfume dermatitis production, IOP Conf. Ser. Mater. Sci.
Eng. 506 (1) (2019), https://doi.org/10.1088/1757-899X/506/1/
012053.
[10] S.R. Chia, K.W. Chew, H.Y. Leong, S. Manickam, P.L. Show, T.H.P. Nguyen, Sonoprocessing-assisted solvent extraction for
manipulation of nanobiomaterials and nanoformulations for drug delivery, Ultrason. Sonochem. 80 (2021), https://doi.org/
10.1016/j.ultsonch.2021.105805105805.
[12] L. Martinelli et al., Antimicrobial activity and chemical constituents of essential oils and oleoresins extracted from eight pepper species, Cieˆncia Rural, 47(5) (2017), doi: 10.1590/
0103-8478cr20160899.
[13] F. Bakkali, S. Averbeck, D. Averbeck, M. Idaomar, Biological effects of essential oils–a review, Food Chem. Toxicol. an Int. J.
Publ. Br. Ind. Biol. Res. Assoc., 46(2) (2008) 446–475, doi:
10.1016/j.fct.2007.09.106.
[14] S. Haouel Hamdi et al, A comparative study of Eucalyptus salubris essential oils efficacy extracted by innovative and conventional processes against Sitophilus oryzae, J. Plant Dis.
Prot. 127 (4) (2020) 495–506, https://doi.org/10.1007/s41348- 020-00341-x.
[15] M.H. Hamzah, H. Che Man, Z.Z. Abidin, H. Jamaludin, Comparison of citronella oil extraction methods from Cymbopogon nardus grass by ohmic-heated hydro-distillation, hydro-distillation, and steam distillation, BioResources 9(1) (2014) 256–272, doi: 10.15376/biores.9.1.256-272.
[16]N. Ghazanfari, S.A. Mortazavi, F.T. Yazdi, M. Mohammadi, Microwave-assisted hydrodistillation extraction of essential oil from coriander seeds and evaluation of their composition, antioxidant and antimicrobial activity, Heliyon 6 (9) (2020) e04893.
[17] I. Nurul, G.M. Alif, F. Rohmatul, Z. Andesra, Optimization of emprit ginger oil yield through operating temperature with microwave ultrasonic steam diffusion method, J. Appl. Sci.
Manag. Eng. Technol. 2 (2) (2022) 83,https://doi.org/10.31284/
j.jasmet.2021.v2i2.2315.
[18] F. Dranca, E. Talo´n, M. Vargas, M. Oroian, Microwave vs.
conventional extraction of pectin from Malus Domestica
’Fa˘lticeni’ pomace and its potential use in hydrocolloid-based films, Food Hydrocoll 121 (2021), doi: 10.1016/
j.foodhyd.2021.107026.
[19] O.R. Alara, N.H. Abdurahman, H.A. Ali, Dataset on oil recovery from Carica papaya leave as influenced by microwave- assisted extraction parameters, Chem. Data Collect. 33 (2021), https://doi.org/10.1016/j.cdc.2021.100724100724.
[20] G.C. Silva, R.A.F. Rodrigues, C.B.G. Bottoli, Passion fruit seed extract enriched in piceatannol obtained by microwave-assisted extraction, Sustain. Chem. Pharm. 22 (June) (2021) 3–9,https://
doi.org/10.1016/j.scp.2021.100472.
[21] B. Hu et al., A comparison of extraction yield, quality and thermal properties from Sapindus mukorossi seed oil between microwave-assisted extraction and Soxhlet extraction, Ind.
Crops Prod. 161 (2021) 113185, doi: 10.1016/j.
indcrop.2020.113185.
[22] B. Yingngam, A. Navabhatra, A. Brantner, Increasing the essential oil yield from Shorea roxburghii inflorescences using an eco-friendly solvent-free microwave extraction method for fragrance applications, J. Appl. Res. Med. Aromat. Plants 24 (2021),https://doi.org/10.1016/j.jarmap.2021.100332100332.
[23] J.T. Mary Leema, T. Persia Jothy, G. Dharani, Rapid green microwave-assisted extraction of lutein from Chlorella sorokiniana (NIOT-2) – Process optimization, Food Chem 372 (2021) 131151, 2022, doi: 10.1016/j.foodchem.2021.131151.
[24] A. El Asbahani et al, Essential oils: from extraction to encapsulation, Int. J. Pharm. 483 (1–2) (2015) 220–243, https://doi.org/10.1016/j.ijpharm.2014.12.069.
[25] M. Vinatoru, T.J. Mason, I. Calinescu, Ultrasonically assisted extraction (UAE) and microwave-assisted extraction (MAE) of functional compounds from plant materials, TrAC Trends Anal.
essential oils from Thymus vulgaris L, Food Chem. 109 (2008) 925–930,https://doi.org/10.1016/j.foodchem.2007.12.084.
[27] M.N. Boukhatem, M.A. Ferhat, M. Rajabi, S.A. Mousa, Solvent-free microwave extraction: an eco-friendly and rapid process for green isolation of essential oil from lemongrass, Nat.
Prod. Res. 36 (2) (2022) 664–667, https://doi.org/10.1080/
14786419.2020.1795852.
[28] P. Rodsamran, R. Sothornvit, Extraction of phenolic compounds from lime peel waste using ultrasonic-assisted and microwave-assisted extractions, Food Biosci. 28 (January) (2019) 66–73,https://doi.org/10.1016/j.fbio.2019.01.017.
[29]R. Richa, R. Kumar, R. Shukla, K. Khan, Ultrasound-assisted essential oil extraction technology: new boon in the food industry, SKUAST J. Res. 22 (2) (2020) 81–85.
[30] S. Sharifzadeh, S. Karimi, H. Abbasi, M. Assari, Sequential ultrasound-microwave technique as an efficient method for extraction of essential oil from Lavandula coronopifolia Poir, J.
Food Meas. Charact. 16 (1) (2022) 377–390, https://doi.org/
10.1007/s11694-021-01170-8.
[31] S.S. Low et al., Sonoproduction of nanobiomaterials – a critical review, Ultrason. Sonochem. 82 (2022) 105887, doi: 10.1016/j.
ultsonch.2021.105887.
[32] N. Ngamkhae et al, Optimization of extraction method for Kleeb Bua Daeng formula and comparison between ultrasound- assisted and microwave-assisted extraction, J. Appl. Res. Med.
Aromat. Plants 28 (2022), https://doi.org/10.1016/
j.jarmap.2022.100369100369.
[33] M. Sharma, K.K. Dash, Microwave and ultrasound-assisted extraction of phytocompounds from black Jamun pulp: Kinetic and thermodynamics characteristics, Innov. Food Sci. Emerg.
Technol. 75 (2022) 102913, doi: 10.1016/j.ifset.2021.102913.
[34] X. Yang et al, A modified microwave hydrodistillation and simultaneous extraction in a rotating state to obtain essential oil, rosmarinic acid, and polysaccharides with sucrose stearate as an additive from Perilla frutescens, Ind. Crops Prod. 181 (2022), https://doi.org/10.1016/j.indcrop.2022.114807114807.
[35] F.S. Costa et al, Natural deep eutectic solvent-based microwave- assisted extraction in the medicinal herb sample preparation and elemental determination by ICP OES, J. Food Compos. Anal.
109 (2022),https://doi.org/10.1016/j.jfca.2022.104510104510.
[36] N. Wan et al, Enzyme pretreatment combined with ultrasonic- microwave-assisted surfactant for the simultaneous extraction of essential oil and flavonoids from Baeckea frutescens, Ind. Crops Prod. 174 (2021),https://doi.org/10.1016/j.indcrop.2021.114173 114173.
[37] J. Yu, Q. Lou, X. Zheng, Z. Cui, J. Fu, Sequential combination of microwave- and ultrasound-assisted extraction of total flavonoids from osmanthus fragrans lour. flowers, Molecules 22 (12) (2017) pp,https://doi.org/10.3390/molecules22122216.
[38] Y. Wang et al, Green and solvent-free simultaneous ultrasonic- microwave assisted extraction of essential oil from white and black peppers, Ind. Crops Prod. 114 (2018) 164–172,https://doi.
org/10.1016/j.indcrop.2018.02.002.
[39] V. Gole, P. Gogate, Intensification of synthesis of biodiesel from non-edible oil using a sequential combination of microwave and ultrasound, Fuel Process. Technol. 106 (2013) 62–69,https://doi.
org/10.1016/j.fuproc.2012.06.021.
[40] Z. Lianfu, L. Zedong, Optimization and comparison of ultrasound/microwave-assisted extraction (UMAE) and ultrasonic-assisted extraction (UAE) of lycopene from tomatoes, Ultrason. Sonochem. 15 (5) (2008) 731–737,https://
doi.org/10.1016/j.ultsonch.2007.12.001.
[41] W. Guan, S. Li, R. Yan, S. Tang, C. Quan, Comparison of essential oils of clove buds extracted with supercritical carbon
dioxide and other three traditional extraction methods, Food Chem. 101 (4) (2007) 1558–1564, https://doi.org/10.1016/
j.foodchem.2006.04.009.
[42] International Organization for Standardization, ISO 3756:1976 Oil of cubeb, 1976.
[43] P. Kusumo, A. Shobib, R. Luthfi, Essential oil extraction from citronella stems (Cymbopogon winterianus) by vacuum distillation, IOP Conf. Ser. Mater. Sci. Eng. 1053 (1) (2021), https://doi.org/10.1088/1757-899x/1053/1/012119012119.
[44] S. Salamah, M. Sarah, D. Estherina, Hydro distillation of java citronella using microwaves technique: Effect of mass ratio and irradiation time on physical characteristics of java citronella oil [Online].Available:, Int. J. Psychosoc. Rehabil. 24 (2020) 193–
198. https://www.scopus.com/inward/record.uri?eid=2-s2.0- 85084346030&doi=10.37200%2FIJPR%2FV24SP2%2FPR20 1300&partnerID=40&md5=
e38816cce1613f97812bbdc55071bb3f.
[45] Z. Li, F. Yang, L. Yang, Y.-G. Zu, Ultrasonic extraction of oil fromCaesalpinia Spinosa (Tara) seeds, J. Chem. 2016 (2016) 1794123,https://doi.org/10.1155/2016/1794123.
[46]B.Y. Wong, C. Tan, H. Chun Wai, Effect of solid-to-solvent ratio on phenolic content and antioxidant capacities of Dukung Anak (Phyllanthus niruri), Int. Food Res. J. 20 (2013) 325–330.
[47] C.H.L. Ho, J.E. Cacace, G. Mazza, Mass transfer during pressurized low polarity water extraction of lignans from flaxseed meal, J. Food Eng. 89 (1) (2008) 64–71, https://doi.
org/10.1016/j.jfoodeng.2008.04.003.
[48] T. Hien Tran et al, Research on lemongrass oil extraction technology (Hydrodistillation, microwave-assisted hydrodistillation), Indonesia. J. Chem. 19 (4) (2019) 1000–
1007,https://doi.org/10.22146/ijc.40883.
[49] H. Jadhav et al, Combined impact of ultrasound pre-treatment and hydrodistillation on bioactive compounds and GC–MS analysis of cinnamomum cassia bark extract, Waste and
Biomass Valorization 12 (2) (2021) 807–821, https://doi.org/
10.1007/s12649-020-01031-3.
[50] F. Chemat, N. Rombaut, A.-G. Sicaire, A. Meullemiestre, A.-S.
Fabiano-Tixier, M. Abert-Vian, Ultrasound-assisted extraction of food and natural products. Mechanisms, techniques, combinations, protocols, and applications. A review, Ultrason.
Sonochem. 34 (2017) 540–560, https://doi.org/10.1016/j.
ultsonch.2016.06.035.
[51] S. Mollaei, Z. Mamizadeh, S. Hazrati, H. Hashempour, The effect of ultrasonic pre-treatment on the yield, chemical composition and biological activity of essential oil in Oliveria decumbens flowers, J. Appl. Res. Med. Aromat. Plants, vol. 24, no. November 2020, p. 100313, 2021, doi: 10.1016/
j.jarmap.2021.100313.
[52]H.A. Nour, R. Mathialagan, H.A. Nour, Extraction and characterization of essential oil from Ginger (Zingiber officinale roscoe) and Lemongrass (Cymbopogon citratus) by microwave-assisted hydrodistillation, Int. J. Chem. Environ.
Eng. 4 (2013,) 221–226.
[53]A.D. Jagdale, S.P. Kamble, M.L. Nalawade, A.U. Arvindekar, Citronellol: A potent antioxidant and aldose reductase inhibitor from Cymbopogon citratus, Int. J. Pharm. Pharm. Sci. 7 (3) (2015) 203–209.
[54] W. Maczka, K. Winska, M. Grabarczyk, One hundred faces of geraniol, Molecules 25 (14) (2020) 1–16, https://doi.org/
10.3390/molecules25143303.
[55] Z. Iqbal, Antioxidant activity of essential oil from the leaves and stems of Murraya Koenigii, World J. Pharm. Res. 6 (7) (2017) 267–273,https://doi.org/10.20959/wjpr20177-8566.
[56] M. Barbouchi, B. Benzidia, M. Choukrad, Chemical variability in essential oils isolated from roots, stems, leaves, and flowers of three Ruta species growing in Morocco, J. King Saud Univ. - Sci. 33 (8) (2021), https://doi.org/10.1016/j.jksus.2021.101634 101634.
