PULSED ELECTRIC FIELD EXTRACTION

What did Barros et al. (2022) measure on eucalyptus, rosemary and thyme?

Which material and which equipment?

In the study by Barros et al. (2022), adult eucalyptus and rosemary leaves harvested in the area of Proença-a-Nova, in the centre of Portugal, and dried thyme leaves from “Argelia” were used. The methods section states that “both plant materials were used within 15 days of harvesting”. In their discussion, the authors describe eucalyptus and rosemary as used a couple of weeks after harvesting (fresh material), and thyme as completely dry. The PEF treatment was applied in batch mode, in a treatment chamber made of a plastic container with two steel electrodes. The high-voltage pulses came from a commercial pulsed generator which can deliver positive pulses of 1 μs to 200 μs, up to 10 kV and 200 A, with repetition rates of 1 Hz to 200 Hz, at a maximum average output power of 3 kW. This solid-state Marx generator is based on a direct capacitive discharge, and the authors write that it can deliver almost rectangular pulses, independent of the load impedance, as long as the generator’s stored energy is much higher than the pulse energy.

How was the 50 g series run?

For the control, 50 g of plant material was placed in a 1 L volumetric flask with 500 mL of tap water, connected to a Clevenger apparatus, heated to the boiling point and distilled for 30 or 60 min. For the PEF experiments, 50 g of plant material was mixed with 250 mL of tap water in the batch treatment chamber, to obtain a uniform mixture. PEF was applied to achieve a specific energy (Ws) of approximately 10 kJ/kg, corresponding in the authors’ words to “less than 2.5 °C temperature increase expected”, with a voltage of 10 kV, a frequency of 5 Hz, a pulse width of 10 μs and a field strength of 2 kV/cm. The mixture was then transferred to a 1 L volumetric flask with an additional 250 mL of tap water, connected to the Clevenger apparatus and treated with the same procedure as the control. In this series, the experiments for each condition were done in duplicate. The authors’ Table 1 gives a calculation example for one eucalyptus experiment at f = 5 Hz: L = 10 cm, d = 5 cm, h = 6 cm, U = 10 kV, ton = 10 µs, 312 pulses, ρ = 0.8 mS/cm, I = 96 A, E = 2 kV/cm, Wp = 9.6 J, tPEF = 3120 µs, Wt = 3 kJ, Ws = 10 kJ/kg and ΔT = 2.4 °C. The number of pulses, n, was calculated so that the specific energy is approximately 10 kJ/kg. In their justification of the PEF protocol, Barros et al. write that, considering what precedes and the results of other authors (their references 19 and 21), an electric field of 2 kV/cm and a specific energy of 10 kJ/kg were chosen.

What yields did the 50 g series give?

In Table 4 of the study, whose sample codes C, P, E and R stand for control, PEF, eucalyptus and rosemary (Table 3), the yield column in % reads 1.20 for C.E_30, 1.41 for P.E_30, 1.67 for C.E_60 and 1.70 for P.E_60. The same column reads 0.57 for C.R_30, 0.63 for P.R_30, 0.68 for C.R_60 and 0.73 for P.R_60. For eucalyptus, the authors report evidence of a PEF effect with 30 min of HD, with an average increase in extraction yield of approximately 17 % compared with extraction without pre-treatment, while for 60 min of HD the average yield with PEF is similar to the control. These results are based on two repetitions per condition, and only the amplitude of the results was calculated for the dispersion analysis. For rosemary, the authors report an average increase of 11 % in extraction yield with PEF and 30 min of distillation and, for 60 min of distillation, still an average increase in yield compared with the traditional method, in the order of 7 %. The authors put forward a “possible explanation” for these yield variations. For eucalyptus, they observed a more substantial PEF effect with the shorter distillation time (30 min), and write that 60 min of HD is “probably sufficient” to extract almost all the essential oil available in 50 g of eucalyptus leaves, the yields with and without PEF being very similar. For rosemary, they obtained a similar effect with both distillation times (30 and 60 min), the yield not being enhanced substantially, and write that 60 min of distillation “might not be enough” to extract all the essential oil present in 50 g of rosemary leaves. According to the authors, the observed dispersions of the results, seen in the amplitude values, did not allow a more definitive conclusion. They write that “it seems” PEF induced a bigger cell disintegration in eucalyptus than in rosemary, in the cells where the essential oils are located. The authors present this as a hypothesis that remains to be validated by further work.

What did the phenolic measurement show?

A single-factor ANOVA gave statistical significance, showing that for the extraction of eucalyptus essential oil the TPC is higher with PEF pre-treatment than with the traditional method.

With 30 min of HD, where the average yield was +17 %, the TPC was +27 %; with 60 min of HD, where the yield difference was not significant (+2 %), the TPC was +12 %. According to the authors, this “suggests” that PEF application, on its own, allows a higher extraction of polyphenols.

How was the thyme series run?

For thyme, the experiments for each condition were done only once. The control used 10 g of plant material in a 1 L volumetric flask with 220 mL of tap water, distilled for 30 or 60 min. For the PEF experiments, 10 g of plant material was placed in a treatment chamber with 90 mL of tap water, treated according to the authors’ Table 2, then transferred to a 1 L volumetric flask, where 130 mL of tap water was added. Table 2 (f = 10 Hz, τ = 10 µs, n = 10 pulses) lists four conditions, with d in cm, U in kV, E in kV/cm and Ws in kJ/kg: condition 1, d = 2, U = 2, E = 1, Ws = 0.4; condition 2, d = 2.5, U = 5, E = 2, Ws = 2.3; condition 3, d = 2.5, U = 10, E = 4, Ws = 9.4; condition 4, d = 2, U = 15, E = 7.5, Ws = 21.1.

What did the thyme series give?

From the results of their Figure 5, the authors judge it “clear” that the application of PEF, “despite the energy supplied”, has a visible effect, with extraction yields higher than with traditional HD (control). They note that these results are based on only one repetition per condition test; comparing the eight experiments with PEF to the two without, with no significant difference between 30 and 60 min of extraction, the average values show an increase in essential oil extraction of approximately 40 % with PEF pre-treatment relative to the conventional method.

According to the authors, “one can assume” that even for the minimal energy delivery (condition one, 30 min of distillation, E = 1 kV/cm) it is “possible” to extract all the essential oil present in 10 g of dry thyme leaves. According to the authors, PEF application thus shows a beneficial effect on the energy efficiency of the process, lowering the extraction time to 50 %, from 60 min to 30 min, while obtaining a maximal extraction yield.

What energy balance do the authors give?

Barros et al. write that the heating mantle used has a 400 W power consumption, “corresponding to a 400 Wh energy consumption during the HD process”, which they restate as “400 W consumed in a HD with a 60 min duration”. In the same calculation, PEF application under the Table 1 protocol uses 96 W average power for a 62 s treatment time, given as 1.65 Wh. The authors write that PEF together with 30 min of HD “can represent an energy saving of near 200 W”, from 400 W for 60 min of HD to 201.65 W for 30 min of HD with PEF pre-treatment.

How do the authors read their results?

In their discussion, the authors write that the impact of PEF on the extraction of thyme essential oil was more evident than with eucalyptus and rosemary, “for the same PEF protocol and HD conditions”. Their methods section, for its part, describes for thyme the four conditions of Table 2, and for one eucalyptus experiment the parameters of Table 1. According to the authors, the difference “may” indicate a different cell disintegration mechanism depending on the plant material, but also reflect different material conditions, eucalyptus and rosemary having been used a couple of weeks after harvesting (fresh material) and thyme being completely dry. According to the authors, their results and those reported by other authors make it “possible to conclude” that applying PEF to plant materials before distillation delivers higher essential oil extraction yields, compared with traditional methods. They also note some dispersion in the results reported by other authors, which hinders the conclusions, and write that more investigation is needed.

In their conclusion, Barros et al. write that processing eucalyptus and rosemary leaves two weeks after harvesting, and dry thyme, by PEF before HD has a beneficial effect on essential oil yield, with an increase in a range of 7 % to 40 %. According to the authors, PEF application led to a “possible reduction” of HD time from 60 min to 30 min, “without sacrificing the EO quality”.

What did Miloudi et al. (2022) measure on spearmint?

Which material and which equipment?

In the study by Miloudi et al. (2022), samples of the aerial part (leaves and stems) of Mentha spicata L. (M. spicata in the source) were collected in June 2021 in Mascara, Algeria, then dried in the air and in the shade. The PEF treatment used a system designed and manufactured by the laboratory, delivering exponential decay pulses at a high voltage of 6 kV, with the pulse frequency kept constant at 1 Hz. The treatment chamber consisted of two flat, parallel stainless steel electrodes separated by 1.5 cm. A 20 g sample with water, at a water-to-material ratio of 3:1 (mL:g), placed between the electrodes, was subjected to an electric field of 1 kV/cm or 2 kV/cm with 100 or 200 exponential decay pulses. The temperature of the treatment medium, measured with a digital thermometer before and after PEF treatment, always varied by less than 2 °C. The study examined the effects of electric field intensity (1 and 2 kV/cm), number of pulses (100 and 200) and distillation time (30 and 60 min) on essential oil yield.

What yields did the study report?

The average essential oil yields, calculated on the dry plant matter of the aerial part, were 0.41 % (30 min), 0.70 % (60 min), 0.89 % (90 min) and 0.94 % (120 min); according to the authors, the yield reached its maximum value of 0.94 % after 120 min of distillation. The authors noted a non-significant increase of 5.62 % when the distillation time goes from 90 min to 120 min. With PEF at 1 kV/cm, yields were 0.46 % and 0.72 % for distillation times of 30 min and 60 min, the source writing “(100 pulses)” after each of the two values. At the same intensity with double the pulses (200n), the yield was 0.50 % after 30 min and 0.87 % after 60 min of distillation. According to the authors, at 1 kV/cm the yield increased compared with the untreated control from 12.20 % for 100 pulses to 21.65 % for 200 pulses with 30 min of distillation, and from 2.86 % for 100 pulses to 24.26 % for 200 pulses with 60 min. At 2 kV/cm, yields were 0.52 % (30 min) and 0.86 % (60 min) with 100 pulses, then 0.69 % (30 min) and 0.90 % (60 min) with 200 pulses. At 2 kV/cm, the authors report that the yield increased compared with the control from 26.83 % (100 pulses) up to 68.29 % (200 pulses) for 30 min of distillation, and, after 60 min, from 22.86 % (100 pulses) to 28.57 % (200 pulses). After 30 min of distillation with PEF at 2 kV/cm and 200 pulses, they observed a yield of 0.69 %, which they describe as approximately the same yield as the control samples after 60 min of distillation (0.7 %). From the results of their Figure 4, the authors conclude that the parameters of PEF treatment and extraction of the essential oil of Mentha spicata in their study are 2 kV/cm, 200 pulses and 60 min of distillation time. According to the authors, PEF treatment accelerates the kinetics of extraction from M. spicata leaf.

Did the composition change?

According to the abstract of Miloudi et al., GC/MS analysis of M. spicata showed that oil quality was not affected by PEF treatment. The major components were carvone (31.07 %; 33.09 % with PEF), limonene (13.84 %; 15.26 % with PEF), eucalyptol (5.17 %; 5.80 % with PEF), dihydrocarveol (1.74 %; 2.11 % with PEF), caryophyllene (1.46 %; 1.28 % with PEF), germacrene D (1.23 %; 1.39 % with PEF) and fenchone (1.14 %; 1.09 % with PEF).

In their conclusion, the authors write that GC-MS analysis indicated that the quality of the essential oil obtained after PEF treatment did not degrade, since the chemical composition was almost the same as that obtained with the conventional method.

What did Sukardi et al. (2022) measure on red galangal rhizome?

Which material and which equipment?

In the study by Sukardi et al. (2022), fresh red galangal rhizome was collected from a farmer’s garden in Tumpang-Malang Regency, Indonesia, with a water content of 80 ± 5 %. The rhizomes, harvested at 12 months, were cleaned of dirt and soil by washing with water, drained, and sliced 0.5 ± 0.01 cm thick. The PEF generator is a local product, calibrated in a high-voltage laboratory of the Faculty of Engineering, Universitas Brawijaya. It requires 300 W of electrical power and can operate at a voltage of 5 V to 15 kV, a frequency of 5 to 15 kHz and an exposure time of 5 s to 2 h, with a cathode–anode distance adjustable from 5 to 40 cm. In this study, DC current was used: 3000 g of sliced red galangal rhizome in a plastic bag container were treated at 0, 1000, 2000, 3000, 4000 and 5000 V, at a frequency of 5000 Hz, an exposure time of 30 min and a cathode/anode distance of 25 cm, each treatment being repeated three times. The material was then taken from the chamber and distilled to obtain the essential oil.

What specific energy did the authors calculate?

In Table 1 of the study (“The energy-specific input”), the energy, in “kJ/cm3”, is 0 at 0 V, 181.0086 at 2000 V, 271.5129 at 3000 V, 362.0172 at 4000 V and 452.5216 at 5000 V.

What did the micrographs show?

In the SEM micrographs, samples without PEF have dense cavities that tend to be closed. At 3000 V the cell cavity began to open, and it opened maximally at 4000 V; it began to be partially closed at 5000 V, which the authors attribute to the cell beginning to experience damage due to high-voltage application.

What yields and properties did the oil have?

Table 2 of the study gives, for red galangal essential oil, the yield in %, the density in g/ml and the refractive index (in a column headed “(%)”): control 0.300, 0.8682 and 1.4779; 1000 V 0.310, 0.8777 and 1.4783; 2000 V 0.317, 0.8873 and 1.4806; 3000 V 0.340, 0.8812 and 1.4753; 4000 V 0.520, 0.8879 and 1.4824; 5000 V 0.410, 0.8853 and 1.4788. According to the authors, there was no significant difference in refractive index and specific gravity between no PEF and PEF treatment, but a slight increase with PEF treatment (Table 2).

Did the composition change?

In Table 3 of the study, the columns for PEF 3000 V, 4000 V and 5000 V include the following components (% area): at 3000 V, eucalyptol 29.03, α-terpineol 14.18, terpinene-4-ol 11.12, chavicol 8.88 and β-pinene 7.89; at 4000 V, chavicol 9.04, eucalyptol 8.99, β-farnesene 8.67, α-cadinol 7.85 and β-copaene 6.52; at 5000 V, eucalyptol 12.21, chavicol 9.84, β-farnesene 6.71, α-cadinol 6.68 and caryophyllene 6.01. According to the authors, the investigated compound of the rhizome oil of Alpinia purpurata (Vieill.) K.Schum. (A. purpurata in the source) showed a significant difference (Table 3) at an applied energy above 90 “kJ/cm3”.

What did Gu et al. (2026) measure on Wurfbainia villosa leaves?

Which material and which procedure?

In the study by Gu et al. (2026), fresh leaves of Wurfbainia villosa (Lour.) Škorničk. & A.D.Poulsen (W. villosa in the source) were collected on 16 October 2024 from a cultivation base in Chenwuzhai, Chuncheng Subdistrict, Yangchun City, Guangdong Province, China, and immediately stored at −20 °C until use. The leaves were cut into segments approximately 3 cm long, and exactly 50.0 g of segments were mixed with a specified volume of distilled water. After soaking for 1 h, the mixture was treated in a PEF chamber, then the volatile oil was extracted by hydrodistillation according to General Rule 2204 of the Chinese Pharmacopoeia (2020 Edition). The equipment listed includes a five-millilitre volatile oil extractor and a PEF-SY-12 K high-voltage pulsed electric field system (Guangzhou Paihu Technology Co., Ltd., China). The yield equation defines Y, the extraction yield of volatile oil (g/100 g), from m, the mass of volatile oil, and M, the dry weight of the W. villosa leaves.

Which conditions were tested?

Optimisation was performed using single-factor experiments and response surface methodology. For the single-factor experiments, the levels were 0.31, 0.64, 0.98, 1.31 and 1.60 kV/cm for electric field intensity, 120, 180, 240, 300 and 360 for pulse number, and 1:4, 1:6, 1:8, 1:10 and 1:12 g/mL for the solid-liquid ratio. The volatile oil yield increased with electric field intensity up to 0.64 kV/cm, peaking at 0.543 g/100 g, then declined significantly, by approximately 12.34 % at 1.6 kV/cm compared with the peak value. The authors write that near or above 0.64 kV/cm membrane permeabilization “likely saturates”. In this series, 0.64 kV/cm was identified as the optimal electric field intensity. The yield increased up to 300 pulses, reaching a maximum of 0.569 g/100 g, then decreased by approximately 11.42 % at 360 pulses. According to the authors, this biphasic response “may” be due to competing mechanisms. In this series, 300 pulses was determined to be optimal.

The yield showed a parabolic dependence on the solid-liquid ratio, with a maximum of 0.777 g/100 g at 1:6 g/mL and a sharp decrease to 0.337 g/100 g at 1:12 g/mL, a reduction of approximately 56.6 % from the maximum yield.

What was the composition of the oil?

According to the GC–MS analysis by Gu et al., the volatile oil of W. villosa leaves extracted by PEF-HD contained 59 identified compounds, in five major classes: terpenes (16 compounds, 66.74 %), oxygenated terpenoids (27 compounds, 25.82 %), aliphatic compounds (8 compounds, 3.47 %), aromatic/benzene-ring derivatives (4 compounds, 3.11 %) and others (4 compounds, 0.86 %). Terpenes and oxygenated terpenoids together accounted for 92.56 % of the total oil. In the same analysis, the five most abundant monoterpenes were β-phellandrene (29.95 %), D-limonene (9.02 %), γ-terpinene (8.46 %), β-pinene (7.14 %) and 4-carene (2.64 %). The most abundant alcohols were 4-terpineol (6.29 %) and α-terpineol (4.39 %), followed by linalool (2.58 %) and myrtenol (2.55 %). The major ketones were pinocarvone (1.24 %) and isopinocamphone (1.22 %), while myrtenal (2.20 %) was the predominant aldehyde.

What did one study on taxanes from Taxus cuspidata needles observe?

In the single-factor trials of Zhao et al. (2022) on the extraction of taxanes — compounds that are not an essential oil — from the needles of Taxus cuspidata Siebold & Zucc., the taxane contents gradually increased with electric field strength up to a maximum, and the yields of the target compounds decreased beyond the strength of that maximum. The authors write “We speculate” that this result “may” be associated with several factors, and that it is “reasonable to infer” that taxanes “may” be degraded by the Joule heating effect.

References

• Barros M., Redondo L.M., Rego D., Serra C., Miloudi K., « Extraction of Essential Oils from Plants by Hydrodistillation with Pulsed Electric Fields (PEF) Pre-Treatment », Applied Sciences 2022;12(16):8107, doi:10.3390/app12168107 — https://mdpi-res.com/d_attachment/applsci/applsci-12-08107/article_deploy/applsci-12-08107.pdf
• Miloudi K., Hamimed A., Bouhadda Y., Benmimoun Y., Belhouala K. et al., « Impact of pulsed electric field treatment for extracting essential oil from Mentha Spicata L. », International Journal of Electrochemical Science 2022;17:220829, doi:10.20964/2022.08.41 — http://electrochemsci.org/papers/vol17/220829.pdf
• Sukardi S., Pulungan M.H., Asmara S.N.L., « Pulsed electric field energy calculation to damage red galangal (Alpinia purpurata, K. Scumm) rhizome slices and its essential oil yield and quality with hydrodistillation », Česká a slovenská farmacie 2022;71:103-115 — http://csfarmacie.cz/doi/10.5817/CSF2022-3-103.pdf
• Gu Y., Lv B., Xie X., Nian X., Yang X., « Antibacterial mechanism of pulsed electric field-assisted extracted volatile oil from Wurfbainia villosa leaves against Escherichia coli and Staphylococcus aureus », Frontiers in Microbiology 2026;17:1828154, doi:10.3389/fmicb.2026.1828154 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC13226615/fullTextXML
• Zhao Z., Zhang Y., Meng H., Li W., Wang S., « Identification and Optimization of a Novel Taxanes Extraction Process from Taxus cuspidata Needles by High-Intensity Pulsed Electric Field », Molecules 2022;27(9):3010, doi:10.3390/molecules27093010 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC9104932/fullTextXML
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip