SUPERCRITICAL CO₂ EXTRACTION

What is supercritical CO2 extraction?

According to the review by Capuzzo et al. (2013), above the critical point (p > pc, T > Tc) the system is a supercritical fluid.

Why is CO2 the fluid used?

Coelho et al. (2012) describe CO2 as the most used supercritical fluid, because it is safe, allows supercritical operation at relatively low pressures and near room temperature, and gives a solvent-free extract.

Capuzzo et al., in their conclusions, describe supercritical CO2 as suitable for many non-polar to moderately polar compounds, while more polar compounds can be extracted with subcritical water or with supercritical CO2 and co-solvents.

What does Directive 2009/32/EC say about extraction solvents and carbon dioxide?

Under Article 1(1), first subparagraph, of Directive 2009/32/EC, the directive applies to extraction solvents used or intended for use in the production of foodstuffs or food ingredients. Under the second subparagraph of Article 1(1), it does not apply to extraction solvents used in the production of food additives, vitamins and other nutritional additives, unless these are listed in Annex I. Article 1(2)(b) defines an “extraction solvent” as a solvent used in an extraction procedure during the processing of raw materials, of foodstuffs, or of components or ingredients of these products, which is removed but “may” result in the unintentional, but technically unavoidable, presence of residues or derivatives in the foodstuff or food ingredient. Article 2(1), first subparagraph, provides that Member States authorise the use as extraction solvents, in the manufacture of foodstuffs or food ingredients, of the substances and materials listed in Annex I, under the conditions of use and, where appropriate, within the maximum residue limits specified in that annex. Annex I lists the extraction solvents which “may” be used during the processing of raw materials, of foodstuffs, of food components or of food ingredients; carbon dioxide appears in its Part I, extraction solvents to be used in compliance with good manufacturing practice for all uses. Note (1) to Part I of Annex I states that an extraction solvent is considered as being used in compliance with good manufacturing practice if its use results only in the presence of residues or derivatives in technically unavoidable quantities presenting no danger to human health. According to recital 8 of the directive, although a specific limitation is in general useful, it need not be laid down for the substances listed in Part I of Annex I, which have been found acceptable from the point of view of consumer safety when used under conditions of good manufacturing practice.

What is the product called?

Coelho et al. (2012) write that the oil obtained by supercritical extraction is designated “volatile oil” to distinguish it from essential oil, which by definition is produced mainly by steam distillation and hydrodistillation.

According to the ISO catalogue entry, ISO 9235:2021 is titled “Aromatic natural raw materials — Vocabulary” and falls under ISO/TC 54. The same entry gives it the status “Published”, a publication date of 2021-06, stage 90.20 (“International Standard under systematic review”), edition 3 and 8 pages.

How does an extraction plant work?

What does one laboratory apparatus look like?

Coelho et al. (2012) describe a flow apparatus allowing supercritical extraction studies between 298 and 353 K and up to 30.0 MPa, with a 1 L extractor and two 0.27 L separators. In their runs, 70 to 120 g of plant material filled the extractor, with CO2 flow rates of 0.71, 1.32 and 1.64 kg/h, at 8.0 to 10.0 MPa and 313 to 323 K. The first separator, at 8.0 MPa and 273 K, collected only waxes; the second, at 2.0 MPa and 265 K, collected the volatile oil, weighed as a function of time. According to Coelho et al. (2012), the best extraction conditions in their experimental programme are usually selected between 8.0 and 10.0 MPa and between 313 and 323 K. Under these conditions all the compounds of the volatile oil are largely soluble in supercritical CO2, but cuticular waxes are also co-extracted; a fractionation step, “as suggested by Reverchon”, precipitates the waxes in the first separator and the pure oil fraction in the second. 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 two models applied by Coelho et al., based on mass balances in the fluid and solid phases, assume plug flow of the solvent and distinguish a quicker initial extraction of easily accessible extract from open cells near the particle surface, followed by a slow extraction from intact cells, controlled by diffusion through poorly permeable cell walls. Applied to the best SFE conditions for each plant in their Table 1, the two models show no significant overall differences in describing the supercritical extraction of the volatile oils of these plants.

Which parameters change the result?

Particle size and preparation of the plant material

According to Capuzzo et al., the moisture content of the solid material influences extraction quality and yield, as well as the fluid dynamics of the solvent. The review states that water can act as a co-solvent by changing the overall polarity of the fluid, but that extracted water can increase the formation of ice blockages; it associates these effects with drying the raw material beforehand to a water content of around 4 to 14 %. In Coelho et al.’s work on pennyroyal, Mentha pulegium L., extracted at 10.0 MPa and 323 K with particles of 0.3, 0.5 and 0.75 mm, the extraction curves “seem to overlap”, as if particle size had no effect. The authors relate this to the essential oil being extracted from glands on the surface of the leaves, so that milling the leaves does not make the oil more accessible.

How do the products of CO2 extraction compare with distilled oils?

According to Coelho et al. (2012), hydrodistillation and steam distillation can affect the quality of essential oil, because thermolabile compounds can degrade and some compounds can undergo hydrolysis and hydrosolubilisation. According to the same authors, one of the major differences between SFE oils and hydrodistilled oils is the presence of small percentages of waxes, which do not seem to affect their quality, since the natural aroma was maintained.

What did Coelho et al. measure, plant by plant?

According to Table 1 of Coelho et al. (2012), the SFE conditions retained for the volatile oils were 10.0 MPa and 323 K for pennyroyal (Mentha pulegium L.), 8.0 MPa and 313 K for cotton lavender (Santolina chamaecyparissus L.), and 9.0 MPa and 313 K for fennel, coriander (Coriandrum sativum L.), winter savory (Satureja montana L.), savory and thyme (Thymus vulgaris L.).

Table 1 names the savory Satureja fruticosa Béguinot, a name that WCVP records, with a different author, as Satureja fruticosa (L.) Briq., a synonym of Clinopodium serpyllifolium subsp. fruticosum (L.) Bräuchler. For pennyroyal, Coelho et al. identified 21 components of the essential oil (98.1 % of the total), mainly menthone (9 %), menthol (1.3 %), isomenthone (1 %) and pulegone (80.6 %), and found no significant difference between the compositions of the volatile oil and the essential oil. For fennel seeds (Foeniculum vulgare Mill., the name in the study’s abstract; its Table 1 names the fennel otherwise), they identified seventy components in the essential oil (96.5 % of the total), mainly (E)-anethole (42.2 %), estragol (20.9 %) and fenchone (16.8 %). The composition of the SFE oil is very similar to that of the essential oil obtained by hydrodistillation, although some differences were detected. For coriander (Coriandrum sativum L.), they observed no major difference in yield or composition between hydrodistillation and SFE. The main components of coriander were linalool (75.9 % by SFE, 67.6 % by hydrodistillation), γ-terpinene (5 % and 6.8 %), camphor (3 % in both cases), geranyl acetate (3.5 % and 2.8 %), geraniol (2.9 % and 2.9), α-pinene (1.5 % and 2.5 %) and limonene (1.4 % and 3.1 %). For winter savory (Satureja montana L.), the SFE and hydrodistillation extracts had as main compounds carvacrol (53 % in both cases), thymol (11 % in both cases), p-cymene (12.8 % by hydrodistillation against 10.1 % by SFE), γ-terpinene (8.9 % against 4.3 %) and β-bisabolene (2 % against 2.5 %). The major difference was the relative amount of thymoquinone, whose content “can be 15-fold higher” in the volatile oil. For savory (Satureja fruticosa Béguinot), the SFE oil was richer in piperitenone (11.2 %) and piperitenone oxide (10.7 %) than the hydrodistilled oil (9 and 6.8 %). Pulegone (40.6 %) and isomenthone (20.7 %), by contrast, were present in a higher percentage in the essential oil than in the SFE oil (35.7 and 17.6 %). For thyme (Thymus vulgaris L.), “52 components could be identified” in the essential and volatile oils, mainly p-cymene (24.4 % by SFE, 28.9 % by hydrodistillation), γ-terpinene (2.5 % and 5.1 %), linalool (4 % and 3.1 %), thymol (36.3 % and 41.6 %) and carvacrol (2.6 and 3.1 %). The main difference concerned thymoquinone, not detected in the essential oil, and carvacryl methyl ether (1.2 % by hydrodistillation, traces by SFE).

For the flower heads of cotton lavender (Santolina chamaecyparissus L.), 1,8-cineole was the major compound (24.8 by hydrodistillation, 38.3 % by SFE), followed by camphor (7.4 and 10.7 %), borneol (8.3 and 3.8 %), terpinen-4-ol (7.4 and 1.9 %), terpinolene (1.4 and 2.1 %) and isobornyl acetate (1.5 and 1.2 %).

What do studies report, species by species?

Hops

A 2018 opinion of the EFSA FEEDAP Panel assessed a supercritical carbon dioxide extract of Humulus lupulus L. flos (hop strobiles), dissolved in propylene glycol (an authorised food additive in the EU, E 1520), as a sensory additive for animal feed. According to the opinion, the starting material is the dried flowers of H. lupulus (strobiles), pelleted and then extracted; the rest of the description is redacted in the published version. The opinion states that evidence was provided showing that prenylflavonoids are not extracted by carbon dioxide and remain in the spent medium. The opinion also states that the extent to which individual compounds or groups of secondary metabolites are represented in hop products is highly dependent on the method of preparation and extraction. The opinion’s conclusions on safety apply specifically to the extract assessed, obtained by supercritical carbon dioxide extraction, and cannot be safely extended to all extracts of H. lupulus (L.).

Sweet basil

According to Capuzzo et al. (2013), in the cited comparison on sweet basil (Ocimum basilicum L., a species described as having several chemotypes; the chemotype studied is not stated), supercritical CO2 extracts showed, relative to the hydrodistilled essential oil, a higher percentage of 1,8-cineole (4-fold), linalool (5.8-fold), eugenol (1.2-fold) and germacrene D (28-fold). The essential oil, for its part, had higher percentages of T-cadinol (3-fold) and of some other sesquiterpenes.

What did one study on the essential oil of Lippia origanoides measure?

Sousa et al. (2002) present experimental results for SFE extraction of the essential oil of Lippia origanoides Kunth (Lippia sidoides Cham. in the study) using “near critical” CO2. The solubility in CO2 of the essential oil present in the plant was measured by the dynamic method, in which the solvent is saturated by the solute as it flows through the bed of solids at a predetermined constant flow rate.

How does temperature change solubility?

According to the authors, the vapor pressure of the solute increases with temperature, which raises solubility, while the density of the solvent decreases, which lowers it; the dominant effect will depend on the magnitude of each effect on the others, for each system. For essential oils the vapor pressure is low; however, small changes in temperature “can” cause significant changes in solubility. Higher solubility values were obtained at 66.7 bar between 288.15 and 293.15 K; the authors write that the increase in solubility over this range “should mainly be” the consequence of the increase in the vapor pressure of the solute, which “must have” overcome the effect of the lower solvent density. At 66.7 bar, a five-degree increase in temperature, from 288.15 K to 293.15 K, caused an increase in solubility that the text gives as “14%”. The same increase in temperature, from 293.15 K to 298.15 K (at 66.7 bar), resulted in a reduction in solubility that the text gives as “42%”. In the first case (288.15 to 293.15 K) the dominant effect was the vapor pressure of the solute, in the second (293.15 to 298.15 K) density.

What does Table 2 of the study record?

Table 2 of the study (solubility measured by the dynamic method) records, at 283.15 K and 66.7 bar, a flow rate noted 1.60 in the column “Q*×10-5, kg/s” and a solubility noted 13.4 in the column “Y*×10-3, kg/kg”. At 288.15 K and 66.7 bar, it records 1.50 in the column “Q*×10-5, kg/s” and 19.5 in the column “Y*×10-3, kg/kg”. At 288.15 K and 78.5 bar, it records 1.60 in the column “Q*×10-5, kg/s” and 17.8 in the column “Y*×10-3, kg/kg”. At 293.15 K and 66.7 bar, it records 1.53 in the column “Q*×10-5, kg/s” and 22.7 in the column “Y*×10-3, kg/kg”. At 293.15 K and 78.5 bar, it records 1.60 in the column “Q*×10-5, kg/s” and 20.1 in the column “Y*×10-3, kg/kg”. At 298.15 K and 66.7 bar, it records 1.57 in the column “Q*×10-5, kg/s” and 13.2 in the column “Y*×10-3, kg/kg”.

What did one study on bay laurel berries measure?

Marzouki et al. (2008) collected mature fruits of Laurus nobilis L. in November in El Agba, Tunisia, from cultivated plants; the berries were air dried in the shade for 20 days. Before use, the plant material was ground, and the particle sizes were in the range 250–425 μm.

How were the two oils extracted?

The volatile oil of the berries was obtained by supercritical CO2 extraction (SFE) at 90 bar and 40 °C, the CO2 density being 0.479 g cm-3. The yield of this step, expressed as the percentage by weight of the oil with respect to the weight of the material charged in the extractor, was 0.9 %. The extraction vessel worked at 90 bar and 40 °C, the first separator at 90 bar and –10 °C, and the second separator at 20 bar and 15 °C. In their conclusions, the authors write that the essential oil was isolated by supercritical CO2 extraction coupled to a fractional separation technique, the waxes being recovered in the first separator and the oil in the second. The authors compared this oil with the oil obtained by hydrodistillation of the same starting material, whose yield was 0.8 %. A further extraction at higher pressure, 250 bar and 40 °C (CO2 density 0.886 g cm-3), was performed on the exhausted matrix to extract the fixed oil, with a yield of 15 %. This step was run on the same samples previously treated at 90 bar, using only one separator, at 20 bar and 15 °C, to recover the extract.

What did the two fractions contain?

The essential oil extracted under SFE conditions had a high content of (E)-β-ocimene (20.9 %), 1,8-cineole (8.8 %), α-pinene (8.0 %), β-longipinene (7.1 %), β-pinene (4.2 %), α-terpinyl acetate (3.8 %) and α-bulnesene (3.5 %). According to the abstract, the last extraction step, at 250 bar, produced an odorless liquid fraction in which triacylglycerols were dominant. The most represented fatty acids of the fixed oil “from whole berry” were 12:0, 18:1 n-9, 18:2 n-6 and 16:0, the last at 17.1 %. In their conclusions, the authors write that it “has been confirmed” that essential oil is selectively extracted only at low supercritical CO2 density.

What did one study on camphor tree leaves measure?

In the study by Zhang et al. (2022), camphor tree material, Camphora officinarum Boerh. ex Fabr. (Cinnamomum camphora in the study), was picked at random on the campus of Jiangxi Agricultural University, cleaned and crushed. Supercritical CO2 extraction used a 1000 mL stainless steel extractor loaded with 60 g of leaves, held between 10 and 30 MPa and between 35 and 55 °C, the oil being recovered by expansion to about 5 MPa in the separator. The comparison steam distillation followed method A for the determination of essential oils in Appendix XD of the Chinese Pharmacopoeia (2015 edition), on 100 to 200 g of material per run, heated with steam for 6 h and stopped when the yield no longer increased, giving about 0.5 g of oil. Steam distillation yields differed by plant part: 0.480 % (flower), >0.326 % (leaf) and >0.212 % (stem). The mean yields of the three samples were below 0.5 %, which, according to the authors, “may be because” the camphor tree raw materials were fresh samples with an excessive water content.

Which conditions did the study compare?

In the single-factor series at 20 MPa for 2 h, at 35, 40, 45, 50 and 55 °C, the highest yield was about 5 % at 45 °C; below 45 °C it increased with temperature, and from 45 to 55 °C it decreased slightly, then tended to stabilise. In the single-factor series at 45 °C for 2 h, at 10, 15, 20, 25 and 30 MPa, the yield was highest at 20 MPa, close to 5 %; below 20 MPa it increased with pressure, rapidly from 15 to 20 MPa, and above 20 MPa it decreased significantly. In the orthogonal test of the same study, the factors acting on yield ranked extraction time > pressure > temperature, and the optimum combination was 25 MPa, 45 °C and 2.5 h. In the time series, the yield rose rapidly at first, slowed after 2.0 h and barely increased after 3.0 h; the pressure and temperature of this series are not given in the text.

How does the CO2 yield compare with steam distillation?

According to Zhang et al., for the leaves, the essential oil yield was below 0.5 % by steam distillation, compared with supercritical CO2 extraction at 25 MPa, 45 °C and 2.5 h.

How does the composition change?

According to Zhang et al., at 50 °C and 30 MPa the oil of camphor tree leaves extracted with supercritical CO2 is selective, with linalool in the majority and camphor and α-pinene in the minority. At 35 °C and 15 MPa its composition is complex, its main components being α-terpineol, cubebene and β-phellandrene. Twenty-six substances were extracted at low temperature and low pressure, against only 13 at high temperature and high pressure. More hydrocarbons were extracted at 35 °C and 15 MPa (50.03 % against 18.58 %), and more alcohols at 50 °C and 30 MPa (68.95 % against 33.56 %). Between 15 and 30 MPa, the percentages of linalool and camphor varied from 19.81 to 66.42 % and from 3.39 to 9.05 % respectively, while those of β-phellandrene, α-terpineol and α-pinene decreased significantly at higher pressures. The compositions of the oils obtained by steam distillation and by supercritical CO2 were very different: only four substances were common to both methods, with a content of about 30 %. Steam distillation extracted more oxides, and the supercritical method more alcohols. Compared with the supercritical method, the oxide content (for example caryophyllene oxide) is higher in the distilled oil, approaching 20 %, while substances of poor water solubility or thermal stability, such as aldehydes, phenols and acids, are lost in it.

References

• Capuzzo A., Maffei M.E., Occhipinti A., « Supercritical Fluid Extraction of Plant Flavors and Fragrances », Molecules 2013;18(6):7194, doi:10.3390/molecules18067194 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6270407/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
• Marzouki H., Piras A., Marongiu B., Rosa A., Dessì M.A., « Extraction and Separation of Volatile and Fixed Oils from Berries of Laurus nobilis L. by Supercritical CO2 », Molecules 2008;13(8):1702-1711, doi:10.3390/molecules13081702 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6245310/fullTextXML
• Sousa E.M.B.D., Chiavone-Filho O., Moreno M.T., Silva D.N., Marques M.O.M., Meireles M.A.A., « Experimental results for the extraction of essential oil from Lippia sidoides Cham. using pressurized carbon dioxide », Brazilian Journal of Chemical Engineering 2002;19(2):229-241, doi:10.1590/S0104-66322002000200003 — https://www.scielo.br/scielo.php?script=sci_pdf&pid=S0104-66322002000200003&lng=en&nrm=iso&tlng=en
• Zhang H., Huang T., Liao X., Zhou Y., Chen S. et al., « Extraction of Camphor Tree Essential Oil by Steam Distillation and Supercritical CO2 Extraction », Molecules 2022;27(17):5385, doi:10.3390/molecules27175385 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC9457539/fullTextXML
• International Organization for Standardization (ISO), catalogue record « ISO 9235:2021 — Aromatic natural raw materials — Vocabulary », edition 3, 2021-06, archived copy of 04/06/2026 (Internet Archive) — https://web.archive.org/web/20260604103220id_/https://www.iso.org/standard/78908.html
• Directive 2009/32/EC of the European Parliament and of the Council of 23 April 2009 on the approximation of the laws of the Member States on extraction solvents used in the production of foodstuffs and food ingredients (Recast) — https://publications.europa.eu/resource/celex/32009L0032
• EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), « Safety and efficacy of a super critical carbon dioxide extract of Humulus lupulus L. flos when used as a feed flavouring for all animal species », EFSA Journal 2018;16(10):e05462, doi:10.2903/j.efsa.2018.5462 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC7009665/fullTextXML
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip