HYDRODISTILLATION

What is hydrodistillation?

According to Wainer et al. (2022), hydrodistillation and steam distillation both use heat to vaporise the oil and allow it to escape the plant, and the separation of water and oil is made possible by the difference in density of the two. In hydrodistillation, as they describe it, the lavender is placed in boiling water, whereas in steam distillation only the steam passes through the lavender.

Where does it sit among extraction techniques?

According to the abstract of Elyemni et al. (2019), essential oils are generally extracted by two main techniques: azeotropic distillation (hydrodistillation, hydrodiffusion and steam distillation) and solvent extraction. Chen et al. (2021) describe hydrodistillation as “the simplest method” and one commonly used to achieve industrial production.

How is the course of hydrodistillation modelled?

Chen et al. model hydrodistillation with a first-order kinetic model taken from Zhang et al. (2020), in which Ye and Yt (%) are the oil yield at equilibrium and at any given time, t (min) is the distillation time, and the model includes a rate constant.

Can the process change the oil?

According to Coelho et al. (2012), these traditional extraction techniques, the distillation processes such as hydrodistillation and steam distillation named just before in their introduction, “can” affect the quality of essential oil, because thermolabile compounds can degrade and some compounds can undergo hydrolysis and hydrosolubilisation.

What is the product called?

Coelho et al. write that the oil obtained by supercritical extraction is designated “volatile oil” to distinguish it from essential oil, which by definition is mainly produced by steam distillation and hydrodistillation.

What did published laboratory hydrodistillations measure?

Rosemary aerial parts, Elyemni et al. (2019)

In the study by Elyemni et al., rosemary aerial parts (leaves and apical parts) were hydrodistilled in a Clevenger-type set-up. For each extraction, 100 g of rosemary were added to 800 ml of distilled water in a 2-litre flask, placed in a heating mantle attached to a condenser ensuring the condensation of the essential oils, for 3 hours. At the end of distillation, two phases were observed: an aqueous phase (aromatic water) and an organic phase (the essential oil), less dense than water.

The text of the article (§3.1) states that Clevenger hydrodistillation (CH) and microwave-assisted hydrodistillation (MAH) gave the same extraction yield, “of the order of” 1.35 % ± 0.04 % (95 % confidence interval). Table 1 of the same article, CH row, gives a yield ranging from 0.31 % to 0.37 %, with a mean of 0.347 %.

In this study, the extraction temperature of both techniques is the boiling point of water at atmospheric pressure (100 °C). Reaching it and obtaining the first droplet of oil took 45 min of heating with CH against 3 min with MAH, and 20 min of MAH gave a yield similar to that obtained after 180 min of CH. According to the authors, MAH uses three modes of heat transfer within the sample (irradiation, conduction and convection), whereas in HD heat transfer can occur only by conduction and convection. Sixteen compounds were identified, accounting for 99.80 % of the oil obtained by CH and 99.75 % of that obtained by MAH. Cineole was the major constituent of the CH oil (31.20 %, against 32.18 % by MAH); the CH oil contained 16.54 % camphor and 15.82 % α-pinene (16.20 % and 15.40 % by MAH). The authors write that the larger proportion of oxygenated compounds in the MAH oils is “probably” due to the low water content of the system and to the speed of heating compared with conventional hydrodistillation.

Rosemary leaves and branches, Chen et al. (2021)

Chen et al. (who write Rosmarinus officinalis L. in their introduction and “Rosmarinus officinales L.” in their abstract) studied the hydrodistillation of rosemary. In their exploratory experiment E1, on mixed fresh leaves and twigs with a material-to-water ratio of 1:3 and 3 h of distillation, the yield reached a maximum of 0.61 % at 3 % NaCl and 0.60 % at 5 %, fell to 0.36 % at 5.5 %, and was only 0.02 % without NaCl. Chen et al. also report that the oil yield of fresh leaves and of one-year branches was markedly higher than that of perennial branches; that of one-year branches was slightly higher than that of fresh leaves, a difference the authors write is not significant (p > 0.05). In the same study, the yield from the different parts of the plant showed a linear relationship with distillation time (R² > 0.98), consistent with the first-order kinetic model. In the L9 (3^3) orthogonal design, on leafy current-year shoots, the maximum yield measured was 1.52 %. The influence of the factors decreased in the order B > C > A (water-to-material ratio, NaCl concentration, leaf integrity), the water-to-material ratio being the main determinant of yield according to the R values. The authors write that the maximum yield was obtained with the material cut into 2 cm pieces, a water-to-material ratio of 1:3 and 5.0 % NaCl (combination A2B3C3). In their significance test, none of the three factors of the orthogonal design (leaf integrity, water-to-material ratio, NaCl concentration) had a significant effect on oil yield (p > 0.05). Under the kinetic model fitted to the A2B3C3 conditions, the equilibrium yield “can” theoretically reach 2.67 %; the final actual yield was between 2.52 % and 2.73 %, a value the authors consider equivalent to the theoretical one.

Fennel seeds, Raza et al. (2024)

In the study by Raza et al., the essential oil of seeds of Foeniculum vulgare Mill. (written “Foeniculum vulgare Miller” by the source; accepted name in WCVP) was obtained with a Clevenger-type apparatus comprising a flask, a heating mantle, a condenser and a separating funnel. Three hundred grams of seeds (mesh 80), mixed with 3 L of distilled water, were heated for 3 h with an electric heating mantle; the hydrosol and the oil were collected in the separating funnel, and the oil separated by decantation.

According to the abstract, superheated steam distillation gave the largest quantity of fennel seed essential oil (5.24 %), against 3.47 % for steam distillation and 2.47 % for hydrodistillation. The authors also write that superheated steam distillation at 150 °C gave 53 % more oil than hydrodistillation and 45 % more than steam distillation, over a 60-min period. The oils obtained by HD, SD and superheated steam distillation contained 23, 20 and 17 compounds respectively; in all three, the major compounds were trans-anethole (58.48 % to 66.46 %), followed by fenchone (8.4 % to 9.8 %), estragole (6.23 % to 7.13 %) and limonene (6.5 % to 7 %). According to Table 1 of the study, the hydrodistilled oil contained 58.48 ± 0.03 % trans-anethole, 8.40 ± 0.03 % fenchone, 6.50 ± 0.02 % limonene (written “Limenone”) and 6.23 ± 0.03 % estragole. The authors report the maximum concentration of the major compounds in the oil distilled with superheated steam, and the minimum in the oil obtained by hydrodistillation.

Seven species, Léva and Gál (2026)

In the study by Léva and Gál (2026), the hydrodistillation of the seven plant species was performed under laboratory conditions, with a Clevenger-type apparatus. In total, 70 ± 1 g of dried, finely ground plant material were submerged in 1 L of distilled water, and the distillations lasted 90 min, when the amount of oil obtained was constant. The collected essential oils were dried over anhydrous MgSO4 and filtered. The authors write that the hydrodistillation of the fresh plant materials was “not feasible” and “not recommended” in their study. According to Table 1 of the article, laboratory hydrodistillation of dried, ground needles and twigs of Picea abies (name used by the source; accepted in WCVP as Picea abies (L.) H.Karst.) gave 3.71 mL of essential oil per kg of dried material in 1.5 h. Under the same conditions, dried, ground flowers of Achillea millefolium (name used by the source; accepted in WCVP as Achillea millefolium L.) gave 4.29 mL per kg of dried material. Dried, ground aerial parts of Lavandula angustifolia (name used by the source; accepted in WCVP as Lavandula angustifolia Mill.) gave 15.71 mL of essential oil per kg of dried material in 1.5 h. Dried, ground aerial parts of Hyssopus officinalis (name used by the source; in WCVP, Hyssopus officinalis L. is a synonym of Dracocephalum officinale (L.) Y.P.Chen & B.T.Drew) gave 4.29 mL per kg of dried material in 1.5 h. Dried, ground aerial parts of Salvia officinalis (name used by the source; accepted in WCVP as Salvia officinalis L.) gave 11.14 mL per kg of dried material in 1.5 h. Dried, ground aerial parts of Mentha piperita (name used by the source; accepted in WCVP as Mentha × piperita L.) gave 21.43 mL per kg of dried material in 1.5 h. Dried, ground aerial parts of Mentha spicata (name used by the source; accepted in WCVP as Mentha spicata L.) gave 14.29 mL per kg of dried material in 1.5 h.

Lavender ‘Margaret Roberts’, Wainer et al. (2022)

Wainer et al. (2022) give two names for the lavender they studied, Lavandula × intermedia ‘Margaret Roberts’ and Lavandula × heterophylla, without choosing between them; in WCVP, Lavandula × intermedia Emeric ex Loisel. and Lavandula × heterophylla Viv. are two distinct accepted names. The lavender was air-dried in the shade for a few days, cut into pieces of an average size of 1 cm and weighed; the spike (flowers) and approximately 30 cm of the stem were used for extraction. For hydrodistillation, 80 g of this plant material and 1 L of water were placed in a round-bottom flask and heated, and the steam was directed into a Liebig condenser, then into a separating funnel. When the separator level reached 200 mL of hydrosol, the oil was separated, collected and stored. In this study, the extraction time is the total distillation time, from room temperature up to the moment the separator reached 200 mL, without the pre-treatment time. The average extraction times were 57 min for steam distillation, 51 min for hydrodistillation and 49 min for cellulase-assisted hydrodistillation.

According to Table 1 of the study, the three oils obtained by hydrodistillation (samples 4, 5 and 6) contained 13.96 %, 15.6 % and 27.91 % 1,8-cineole and 24.82 %, 36.74 % and 24.97 % camphor (wt%). In the same table, linalool is given as 16.96 % and 21.89 % in samples 4 and 5 and as a dash ("-") in sample 6, and terpinolene as a dash in samples 4 and 5 and as 14.86 % in sample 6. The authors write that the very similar retention times of linalool and terpinolene “suggest” that the linalool peak “could have been” erroneously interpreted as terpinolene on the chromatogram. They write that the oils from conventional hydrodistillation and from cellulase-assisted hydrodistillation contained more 1,8-cineole and linalool than the oils from steam distillation, which contained more camphor.

Comparison runs, Coelho et al. (2012)

In the work reported by Coelho et al., the comparison hydrodistillation ran for 4 h in a Clevenger-type apparatus, on 40 g of plant material of the same mean particle size as in supercritical extraction; the passage does not name the species.

How do hydrodistilled oils compare with supercritical CO2 extracts?

According to Coelho et al., for coriander (the passage says “coriander”; the source uses Coriandrum sativum elsewhere, accepted in WCVP as Coriandrum sativum L.), no major difference in yield or composition was observed between supercritical extraction (SFE) and hydrodistillation, the main component being linalool (75.9 % by SFE, 67.6 % by HD). According to the abstract of Coelho et al., the main differences between HD and SFE oils are the presence of a small percentage of cuticular waxes and the relative amount of thymoquinone, present in the oils of thyme and winter savory.

What does the European Pharmacopoeia list on essential oils in herbal drugs?

According to the knowledge database of the European Pharmacopoeia (EDQM), the text numbered 20812, titled in English “Essential oils in herbal drugs (2.8.12.)”, has the status “In use”. The same entry gives its French title as “Huiles essentielles dans les drogues végétales (2.8.12.)”. The text appeared in Pharmeuropa 29.4, and its last publication, in English as in French, is 9.8.

References

• 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
• Chen H., Gu Z., Yang L., Yang R., Ji Y. et al., « Optimization extraction of rosemary essential oils using hydrodistillation with extraction kinetics analysis », Food Science & Nutrition 2021;9(11):6069-6077, doi:10.1002/fsn3.2549 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8565215/fullTextXML
• Raza M.H., Ayub M.A., Zubair M., Hussain A., Saleem S. et al., « Comparative Study of Essential Oils Extracted From Foeniculum vulgare Miller Seeds Using Hydrodistillation, Steam Distillation, and Superheated Steam Distillation », Food Science & Nutrition 2024;12(12):10535, doi:10.1002/fsn3.4593 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC11667025/fullTextXML
• Léva N., Gál E., « Impact of Extraction Scale and Method on the Chemical Profile of Essential Oils: A Comparative Study Between Laboratory Hydrodistillation and Semi-Industrial Dry Steam Distillation », Molecules 2026;31(12):2105, doi:10.3390/molecules31122105 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC13305851/fullTextXML
• Wainer J., Thomas A., Chimhau T., Harding K.G., « Extraction of Essential Oils from Lavandula × intermedia ‘Margaret Roberts’ Using Steam Distillation, Hydrodistillation, and Cellulase-Assisted Hydrodistillation: Experimentation and Cost Analysis », Plants 2022;11(24):3479, doi:10.3390/plants11243479 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC9785002/fullTextXML
• Coelho J.P., Cristino A.F., Matos P.G., Rauter A.P., Nobre B.P. et al., « Extraction of Volatile Oil from Aromatic Plants with Supercritical Carbon Dioxide: Experiments and Modeling », Molecules 2012;17(9):10550-10573, doi:10.3390/molecules170910550 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6268743/fullTextXML
• EDQM, European Pharmacopoeia, monograph details 20812 « Essential oils in herbal drugs (2.8.12.) », consulted 02/10/2026 — https://extranet.edqm.eu/4DLink1/4DCGI/Web_View/mono/20812
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip