MICROWAVE ASSISTED EXTRACTION

Elyemni et al. (2019) compared microwave-assisted hydrodistillation with Clevenger hydrodistillation on rosemary, and Guelifet et al. (2025) compared microwave-assisted distillation with hydrodistillation and steam distillation on the powdered aerial parts of rosemary. Chenni et al. (2016) on sweet basil leaves and Mohanty et al. (2023) on the rhizomes of three Curcuma species compared solvent-free microwave extraction with hydrodistillation. Villa et al. (2022) compared two microwave techniques with hydrodistillation and steam distillation on fresh rose petals of Rosa x damascena Mill. (Kazanlik type).

What limits the technique?

According to the review by Youchret-Zallez et al. (2026), MAHD and SFME are not automatically superior for all plant matrices. The review states that their performance depends on plant moisture, particle size, water-to-plant ratio, microwave power, irradiation time, sample loading, reactor geometry and condensation efficiency. In the same passage, excessive microwave power or prolonged irradiation “may” alter essential oil profiles or promote the degradation of thermolabile constituents. According to Hedayati et al. (2025), the industrial scale-up of microwave extractors is not an easy task, because of the low irradiation depth of microwaves, the irregular distribution of the electromagnetic field, the non-uniformity of irradiation and the formation of hot spots.

What did Elyemni et al. measure on rosemary?

The study by Elyemni et al. (2019) concerns two methods for the essential oils of Salvia rosmarinus Spenn. (Rosmarinus officinalis L. in the study): microwave-assisted hydrodistillation (MAH) and Clevenger hydrodistillation (CH).

How were the two extractions run?

The rosemary samples were harvested at the flowering stage in May 2018 in the region of Fez. Only the aerial part of the plant was used: the leaves and apical parts, dried in the shade for eight days at 25 °C. MAH used a domestic microwave oven (20 L, 1100 W) connected directly to a Clevenger-type extractor and to a cooling system condensing the distillate continuously, the excess condensed water being refluxed to the extraction flask to restore the water to the plant material. For CH, 100 g of rosemary was added to 800 ml of distilled water in a 2-litre flask, placed in a balloon heater attached to a condenser ensuring the condensation of the essential oils for 3 hours.

What did the comparison show?

The authors report that the same yield of essential oils is obtained in only 20 minutes with MAH, while it takes 180 minutes with CH.

The time required to extract the essential oils contained in 100 g of rosemary was 180 min for CH and 20 minutes for MAH, and the energy required 2.25 kWh for CH and 0.23 kWh for MAH. The study also gives 1800 g of CO2 rejected for CH against 184 g for MAH. Sixteen compounds were identified for the two methods, representing a total of 99.80 % in CH and 99.75 % in MAH. In MAH and CH respectively, monoterpene hydrocarbons accounted for 35.84 and 37.19 %, oxygenated monoterpenes for 63.03 and 61.76 %, total oxygenated compounds for 63.64 and 62.5 %, and total non-oxygenated compounds for 36.11 and 37.3 %. Among the compounds listed, cineole accounted for 32.18 and 31.2 %, camphor for 16.2 and 16.54 %, and α-pinene for 15.4 and 15.82 %. According to the authors, the quality of the essential oil is “improved” thanks to a 1.14 % increase in oxygenates. They write that the amounts of oxygenated compounds are substantially higher, and the amounts of monoterpene hydrocarbons lower, in the rosemary oil extracted by MAH than in that extracted by CH.

What did Guelifet et al. measure on rosemary?

Guelifet et al. (2025) subjected the powdered aerial parts of Salvia rosmarinus Spenn. (Rosmarinus officinalis L. in the study) to three extraction techniques: hydrodistillation (HD), steam distillation (SD) and microwave-assisted distillation (MD). For each seasonal batch, 100 g of dried plant material was placed in a 2 L round-bottom flask containing 1.5 L of distilled water. HD consisted of boiling the mixture at atmospheric pressure for 3 h, and SD of passing steam through the plant matrix for 3 h under identical conditions. For MD, the plant material was combined with 250 mL of distilled water (solid-to-liquid ratio 1:2.5 w/v) and subjected to microwave heating in an oven operating at 800 W for 30 min. The authors report that HD produced the highest yields, whereas MD consistently gave the lowest. The maximum yield (0.81 %) was achieved in spring with HD, and the minimum (0.25 %) in winter with MD. SD yielded consistently lower amounts than HD but higher than MD across all seasons. Table 1 of Guelifet et al. gives the following extraction yields (“Extraction yield (%)”) for HD, MD and SD respectively: autumn 0.76, 0.50 and 0.66; winter 0.45, 0.25 and 0.32; spring 0.81, 0.65 and 0.78; summer 0.63, 0.43 and 0.55.

MD consistently produced the oils richest in oxygenated monoterpenes (54.27–59.65 %), particularly camphor, 1,8-cineole and borneol, with maximum concentrations observed in spring. According to the authors, the efficiency of this method “can be attributed” to rapid and uniform heating, which minimises thermal degradation and preserves oxygenated volatiles. Steam distillation yielded higher monoterpene hydrocarbons (45.02–50.14 %) and sesquiterpene hydrocarbons (8.95–12.14 %), while MD produced oils richer in oxygenated monoterpenes (54.27–59.65 %), camphor reaching 36.52 % against 28.47 % in the steam extracts.

What did Chenni et al. measure on sweet basil?

In the study by Chenni et al. (2016), the broken leaves of sweet basil, Ocimum basilicum L., with 9.4 % ± 0.4 % moisture, were purchased, and the source states that it was collected in Egypt in 2011 (number 28587).

How were the two extractions run?

SFME was performed in a multimode microwave reactor described by the source as “2.45 MHz”, with a maximum delivered power of 1000 W, the temperature being monitored by an external infrared sensor.

The run was conducted at atmospheric pressure with 150 g of O. basilicum immersed in water (600 g) for 30 min at 600 W. A cooling system outside the microwave cavity condensed the distillate continuously with a Clevenger-type apparatus; the condensed water was returned to the flask, and heating was continued at 100 °C until no more essential oil was obtained. For hydrodistillation, 150 g of O. basilicum immersed in 6 L of water were hydrodistilled in a Clevenger-type apparatus for 1 h, until no more essential oil was obtained.

What did the comparison show?

During the extraction process, the system temperature is equal to the boiling temperature of water at atmospheric pressure (100 °C). Reaching this temperature, and thus obtaining the distillation of the first essential oil droplet, required heating the sample for only 5 min with SFME, against 20 min with HD. An extraction time of 30 min with SFME gave yields similar (0.48 % ± 0.02 %) to those obtained after 60 min by HD. According to the authors, SFME is clearly advantageous with respect to conventional HD in terms of rapidity, efficiency and substantial energy savings. Among the oxygenated compounds, the monoterpene alcohol linalool is the most abundant component of the basil oil (43.5 % for SFME, 48.4 % for HD), followed by methyl chavicol (13.3 % and 14.3 %) and 1,8-cineole (6.8 % and 7.3 %). The text states that slightly lower amounts of oxygenated compounds (the text gives 89.8% and 89.6%, in that order) and higher amounts of hydrocarbons (9.7% and 9.2%, in that order) are present in the SFME oil than in the HD oil. In the composition table, total oxygenated compounds are 89.8 % in the HD oil and 89.6 % in the SFME oil, and total non-oxygenated compounds 9.2 % and 9.7 % respectively.

What did Mohanty et al. measure on three Curcuma species?

Mohanty et al. (2023) compared HD and SFME for three Curcuma species: Curcuma alismatifolia Gagnep., Curcuma aromatica Salisb. and Curcuma xanthorrhiza, named Curcuma zanthorrhiza Roxb. in WCVP.

According to Mohanty et al. (2023), the rhizomes of the three species were harvested in September 2022 from the greenhouse of the Centre for Biotechnology, Siksha O Anusandhan, Bhubaneswar (India), and the dried rhizomes were soaked in water for 30 min in the 2 L glass vessel of the SFME apparatus. SFME was run at atmospheric pressure on a Milestone ETHOS X, a 2.45 GHz multimode microwave reactor with an infrared temperature sensor, at 450 W for the first 30 min and 400 W thereafter, a glass Clevenger apparatus above the oven continuously condensing the volatile substances and returning the water to the reactor.

For each of the three species, the extraction time was 7 h by HD and 4 h by SFME, and the energy consumption 9.43 kWh by HD and 2.13 kWh by SFME. The water column of the comparison table reads 1000 mL/100 g for HD and 100 mL/100 g for SFME. The essential oil was pale yellow (HD) and deep yellow (SFME) for C. alismatifolia, deep brown (HD) and light brown (SFME) for C. aromatica, and pale yellow by both methods for C. xanthorrhiza.

According to Mohanty et al. (2023), the essential oil yield of the three species, expressed as % (v/w) on a dry weight basis, ranged from 0.25 % to 0.68 %, with statistical differences at p < 0.05 (Tukey test). The maximum yield was recorded for C. aromatica (SFME 0.68 ± 0.05 %, HD 0.43 ± 0.04 %), followed by C. xanthorrhiza (SFME 0.52 ± 0.04 %, HD 0.35 ± 0.02 %) and C. alismatifolia (SFME 0.43 ± 0.03 %, HD 0.25 ± 0.02 %).

A total of 85 compounds were identified, constituting 90.76 % to 94.97 % of the total essential oils of the three species. In C. alismatifolia, 50 compounds were identified in the oils obtained by HD and SFME, accounting for 94.02 % and 94.84 % of the total oils respectively. In C. aromatica, 49 compounds “could be identified” by HD and SFME, constituting 93.12 % and 94.97 % of the total oil respectively. The essential oil of C. xanthorrhiza had 48 identified constituents, accounting for 90.76 % of the oil isolated by HD and 93.82 % via SFME. The oils of the three species were dominated by oxygenated sesquiterpenes (36.09 to 73.42 %), followed by oxygenated monoterpenes (4.43 to 31.13 %), both groups being present in substantially higher amounts in the SFME oils than in the HD oils. The amounts of sesquiterpene hydrocarbons (5.72 to 15.26 %) and monoterpene hydrocarbons (3.01 to 15.64 %) varied considerably between the two extraction methods. According to the authors, their Table 2 shows that HD is more efficient in isolating hydrocarbon molecules, whereas SFME is more effective in isolating most of the oxygenated compounds.

What did Villa et al. measure on Damask rose?

As a “preliminary” analytical screening approach, Villa et al. (2022) applied and compared two microwave techniques, Solvent-Free Microwave Extraction (SFME) and Microwave Hydrodiffusion and Gravity (MHG), and two conventional procedures, hydrodistillation (HD) and steam distillation (SD), to extract the volatile compounds of fresh rose petals grown in Italy.

How were the extractions run?

The fresh petals of Rosa x damascena Mill. (Kazanlik type), the name used by the source, came from Ronco Scrivia (Genoa, Italy); they were collected at the flowering stage, immediately refrigerated at +4 °C, then stored at −20 °C until extraction. The initial moisture content (“relative humidity”) of the fresh flowers was 78.62 % ± 2.2 % on a Sartorius moisture analyser, and triplicate samples of fresh petals, 30 to 50 g depending on the technique, received 30 mg of octyl octanoate as internal standard. For MHG, 30 g samples of fresh petals, with no solvent or water added, were irradiated in triplicate at different times and temperatures to optimise the conditions, the best results coming from a 5 min extraction (1 min up to 100 °C, then 4 min held) in a multimode microwave prototype with a maximum delivered power of 600 W, whose cavity holds a Pyrex reactor, a 2.45 GHz magnetron, two optical fibres for temperature measurement and a feedback control unit modulating emitted power, time and temperature. For SFME, 30 g samples of fresh petals, with no solvent or water added, were irradiated in triplicate at different times and temperatures to optimise the conditions, the best results coming from a 12 min extraction (2 min up to 100 °C, then 10 min at that temperature) in a single-mode scientific microwave reactor with a maximum power of 300 W in 10 W increments, the temperature being monitored by an optical fibre in the vessel and regulated by power feedback from 0 to 300 W, the vessel being fitted with a modified Clevenger apparatus and the oil separated from the aromatic water (hydrosol) by decantation. For SD, 50 g samples of fresh petals were steam-distilled in triplicate for 3 h with 2 L of distilled water (700 W heating isomantel), the temperature (100 °C) being measured by a digital thermocouple thermometer and the 3 h counted from the first condensed drop falling into the Florentine flask. For HD, 50 g of petals were hydrodistilled in a Clevenger-type apparatus according to the Italian Pharmacopoeia for 3 h with 2 L of distilled water (700 W heating isomantel), the temperature (100 °C) being measured by a digital thermocouple thermometer and the 3 h counted from the first condensed drop falling into the graduated burette. The essential oil was collected, dried over anhydrous sodium sulphate and stored in dark sealed vials at −20 °C until analysis. The extracts were analysed according to Robustelli della Cuna et al., the compounds being identified by retention indices and mass spectra according to Adams and by comparison with a NIST mass spectral library, the relative amount of each component being expressed as percent peak area relative to total peak area in GC/FID of the whole extracts, and the quantitative data obtained by GC/FID with the internal standard method, an equal response factor being assumed for all compounds.

What did the comparison show?

The petal extracts obtained by MHG, SFME, SD and HD, fragrant pale yellow oils, gave yields of 0.28 % ± 1.3 %, 0.40 % ± 1.8 %, 0.17 % ± 2.7 % and 0.22 % ± 3.4 % (w/w) respectively, on dry plant material. According to the authors, the two microwave extractions offer shorter extraction times (SFME 12 min and MHG 5 min, against 3 h for HD and SD) and a general better recovery in terms of essential oil yield. GC/FID and GC/MS analysis of the oils obtained by the different procedures revealed the presence of 61 constituents. In the composition table (mean of three determinations), benzyl alcohol accounts for 1.68 % (MHG), 1.39 % (SFME), 0.14 % (SD) and 0.10 % (HD) of the oil, and 2-phenyl ethanol for 59.61 %, 45.50 %, 25.58 % and 26.90 % respectively. In the same table, linalool accounts for 0.45 % (MHG), “-” (SFME), 4.62 % (SD) and 1.32 % (HD), citronellol for 14.41 %, 2.53 %, 40.58 % and 30.18 %, and geraniol for 16.70 %, 2.54 %, 8.48 % and 13.36 %, the source not stating whether “-” means not detected. The table gives trans-caryophyllene at 0.11 % (MHG) and 10.15 % (SFME), “-” in SD and HD, while trans-α-bergamotene (2.05 %), “Cariophyllene oxide” (11.66 %) and α-bisabolol (1.96 %) are quantified only in the SFME oil, “-” in the other three columns. Octane accounts for 0.40 % (SD) and 4.45 % (HD), “-” in MHG and SFME, nonadecane for 1.67 % (MHG), 2.23 % (SFME), 5.26 % (SD) and 6.42 % (HD), and heneicosane for 0.49 %, 1.04 %, 4.94 % and 7.69 % respectively. In the table of cumulative percentages by class of compounds, among non-terpenic compounds, saturated hydrocarbons account for 2.38 % (MHG), 3.52 % (SFME), 11.17 % (SD) and 18.99 % (HD), aldehydes for 0.49, 0.14, 1.81 and 1.56 %, ketones (0.28 %) and esters (0.25 %) appear in SFME only, “-” elsewhere, and total non-terpenic compounds come to 64.16, 51.08, 38.7 and 47.55 %. Among monoterpenes, oxygenated alcohols account for 32.47 % (MHG), 5.19 % (SFME), 55.03 % (SD) and 46.45 % (HD), ketones for 0.45, 0.19, 1.49 and 0.93 %, total oxygenated monoterpenes for 33.99, 6.13, 58.01 and 48.41 %, non-oxygenated monoterpenes for 0.03, 0.51, 0.52 and 0.81 %, and total monoterpenes for 34.02, 6.64, 58.53 and 49.22 %. Among sesquiterpenes, oxygenated alcohols account for 1.51 % (MHG), 5.61 % (SFME), 3.34 % (SD) and 2.97 % (HD), epoxides (“Epoxydes”) for 11.66 % in SFME, the other cells being empty, total oxygenated sesquiterpenes for 1.51, 17.27, 3.34 and 2.97 %, non-oxygenated sesquiterpenes for 0.17, 21.28 and 0.03 % and “-” (HD), and total sesquiterpenes for 1.68, 38.55, 3.37 and 2.97 %, non-oxygenated diterpenes appearing in SFME only (0.11 %, “-” elsewhere). Total oxygenated terpenes come to 35.5 % for MHG and 23.4 % for SFME, and total terpenes to 35.7 % (MHG), 45.3 % (SFME), 61.9 % (SD) and 52.19 % (HD). In their text, the authors give primary alcohols as the major constituents of the MHG oil, 2-phenyl ethanol (59.61 %) being the most abundant compound, oxygenated terpenes at 35.5 % led by geraniol (16.70 %) and citronellol (14.41 %), and saturated hydrocarbons (2.38 %) dominated by nonadecane (1.67 %). For the SFME oil, they give primary alcohols as the major constituents, 2-phenyl ethanol (45.50 %) being the most abundant compound, oxygenated terpenes at 23.4 % led by caryophyllene oxide (11.66 %), geraniol (2.54 %), citronellol (2.53 %) and α-bisabolol (1.96 %), non-oxygenated terpenes in high amount (21.9 %) with caryophyllene (10.15 %), trans-α-bergamotene (2.05 %), trans-β-farnesene (1.89 %), β-selinene (1.53 %) and α-selinene (1.20 %), and saturated hydrocarbons at 3.52 %, mainly nonadecane (2.23 %) and heneicosane (1.04 %). For the SD oil, they give oxygenated terpenes as the major constituents, citronellol (40.58 %) being the most abundant, followed by geraniol (8.48 %), linalool (4.62 %), isomenthone (1.49 %), α-terpineol (1.23 %), α-eudesmol (1.14 %) and β-eudesmol (1.80 %), primary alcohols at 25.72 %, mainly 2-phenyl ethanol (25.58 %) with a very low amount of benzyl alcohol (0.14 %), saturated hydrocarbons at 11.17 % led by nonadecane (5.26 %) and heneicosane (4.94 %), and terpenic aldehydes at 1.49 %, mainly neral (0.87 %) and geranial (0.62 %). For the HD oil, they give oxygenated terpenes as the major constituents, among them citronellol (30.18 %), geraniol (13.36 %), β-eudesmol (1.42 %), linalol (1.32 %) and α-terpineol (1.31 %), primary alcohols at 27.00 %, dominated by 2-phenyl ethanol (26.90 %), saturated hydrocarbons at 18.99 % led by heneicosane (7.69 %), nonadecane (6.42 %) and octane (4.45 %), and terpenic aldehydes at 1.03 %, with neral (0.56 %) and geranial (0.47 %). The authors attribute the different compositions of the volatile fraction, in particular for benzyl alcohol and 2-phenyl ethanol, to the speed of the microwave procedures, which in their reading prevents the partial loss of some constituents. According to the authors, the mildness of SFME can be inferred from the GC-FID and GC-MS data, which show in the microwave extracts many “aromatic” compounds not obtained by the conventional methods. On the dendrogram of oil composition, built with Minitab19 from data scaled in percentage and normalised to the standard deviation, SD and HD are correlated at 98 % similarity, MHG and SFME at 95 %, and the four techniques are about 75 % similar in composition.

References

• Youchret-Zallez O.B., Spigarelli R., Kouki A., Mbirki L., Ben-Attia M. et al., « Essential Oil Quality in Medicinal and Aromatic Plants: Linking Plant Biosynthesis, Matrix Selection and Extraction Strategy », Plants 2026;15(18):2771, doi:10.3390/plants15182771 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC13610020/fullTextXML
• Hedayati S., Tarahi M., Madani A., Mazloomi S.M., Hashempur M.H., « Towards a Greener Future: Sustainable Innovations in the Extraction of Lavender (Lavandula spp.) Essential Oil », Foods 2025;14(1):100, doi:10.3390/foods14010100 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC11720256/fullTextXML
• Mohanty S., Ray A., Naik P.K., Sahoo A., Jena S. et al., « Variation in Yield, Chemical Composition and Biological Activities of Essential Oil of Three Curcuma Species: A Comparative Evaluation of Hydrodistillation and Solvent-Free Microwave Extraction Methods », Molecules 2023;28(11):4434, doi:10.3390/molecules28114434 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC10254477/fullTextXML
• Elyemni M., Louaste B., Nechad I., Elkamli T., Bouia A. et al., « Extraction of Essential Oils of Rosmarinus officinalis L. by Two Different Methods: Hydrodistillation and Microwave Assisted Hydrodistillation », The Scientific World Journal 2019;2019:3659432, doi:10.1155/2019/3659432 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6463580/fullTextXML
• Chenni M., El Abed D., Rakotomanomana N., Fernandez X., Chemat F., « Comparative Study of Essential Oils Extracted from Egyptian Basil Leaves (Ocimum basilicum L.) Using Hydro-Distillation and Solvent-Free Microwave Extraction », Molecules 2016;21(1):113, doi:10.3390/molecules21010113 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6273689/fullTextXML
• Guelifet K., Kherraz K., Messaoudi M., Ferhat M.A., Khattabi L. et al., « Seasonal and Extraction-Dependent Variation in the Composition and Bioactivity of Essential Oils from Wild Rosmarinus officinalis L. », Molecules 2025;30(21):4258, doi:10.3390/molecules30214258 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC12610705/fullTextXML
• Villa C., Robustelli Della Cuna F.S., Russo E., Ibrahim M.F., Grignani E. et al., « Microwave-Assisted and Conventional Extractions of Volatile Compounds from Rosa x damascena Mill. Fresh Petals for Cosmetic Applications », Molecules 2022;27(12):3963, doi:10.3390/molecules27123963 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC9228154/fullTextXML
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip