COLD EXPRESSION

What is cold expression?

According to the review by González-Mas et al. (2019), essential oils from the peel of Citrus L. can also be obtained by cold press extraction. According to a 2021 opinion of the EFSA FEEDAP Panel on lemon oil (opinion 6548), extraction by mechanical pressing is the most commonly used method to obtain essential oils from the peel of citrus fruits, and since it does not require heat, it is often referred to as “expression” or “cold pressing”. In the mechanised process described in that opinion, the surface of the lemon peel (Citrus × limon (L.) Osbeck; Citrus limon in the opinion) is first scarified to break open the cells containing the oil; water is then sprayed over the fruit to collect the released oil, the aqueous suspension is filtered to remove cell debris, and centrifugation separates the oil/water mix and removes fine particles. A 2021 EFSA FEEDAP opinion on mandarin oil (opinion 6625) states that expressed mandarin oil is obtained by cold expression from the fruit peel of Citrus reticulata Blanco, and describes the same mechanised sequence: scarification of the fruit surface, water spray to collect the released oil, filtration of the aqueous suspension, then centrifugation to separate the oil/water mix and remove fine particles.

What happens to the expressed oil after pressing?

EFSA opinion 6548 states that, besides the direct use of cold-pressed lemon oil, the applicant distils the expressed oil to obtain two further preparations: a volatile fraction collected after condensation and a residual fraction. A 2021 EFSA FEEDAP opinion on orange oil (opinion 6891), from Citrus × aurantium f. aurantium (Citrus sinensis in the opinion), distinguishes three types of orange oil: expressed or cold-pressed oil, distilled oil and “folded” oils, the last two using the expressed oil as starting material. According to that opinion, the distilled oil is obtained by steam distillation of the expressed oil and differs from it in composition only by the removal of the non-volatile material extracted by cold pressing. Folded oils are obtained by fractional distillation of the expressed oil, to reduce the content of volatiles (terpenes) and consequently increase the concentration of the non-volatiles extracted by cold pressing. The same opinion states that the wax content of cold-pressed oils is commonly reduced by winterisation (de-waxing).

What does a cold-pressed oil contain besides volatile compounds?

Dosoky et al. (2022) write that, when the volatile composition of Citrus oils is studied, the non-volatile fractions are hard to detect under standard gas chromatography conditions, because of their limited volatility and their relatively polar or heat-labile nature. According to Dosoky et al., the non-volatile components are far more species-specific than the volatile components, which show comparable patterns in different Citrus oils. EFSA opinion 6625 reports that the applicant carried out a literature search to identify the relative percentage and the composition of the non-volatile fraction of expressed mandarin oil. According to the opinion, its non-volatile constituents include mainly polymethoxylated flavones, for example tangeretin (0.2–0.5 %), nobiletin (0.07–0.25 %) and heptamethoxyflavone (0.05–0.15 %). EFSA opinion 6891 reports a literature search by the applicant on the relative percentage and the composition of the non-volatile fraction of sweet orange peel oil. According to that opinion, its non-volatile constituents include predominantly polymethoxylated flavones, predominantly heptamethoxyflavone (0.3–2.0 g/L), nobiletin (0.25–1.10 g/L), sinensetin (0.04–0.3 g/L) and tangeretin (0.36–1.0 g/L).

How does a cold-pressed oil differ from a hydrodistilled oil of the same fruit?

Eureka lemon, Li et al. (2021)

In the study by Li et al. (2021), fresh, mature Eureka lemon fruits were harvested in an orchard of the National Citrus Germplasm Repository (106°43′ E, 29°83′ N; altitude 241 m) in Chongqing, China. The cold-pressed lemon essential oil (CPLEO) was obtained by a method described in an earlier study by the authors.

The hydrodistilled comparison oil (HDLEO) was obtained with an improved Clevenger apparatus fitted with a second condenser, “1,000 g” of lemon flavedo being distilled with 2 L of water until no more oil was obtained. The non-volatile components of the cold-pressed oil included mainly 18 coumarins and furanocoumarins, against only 10 compounds in the hydrodistilled oil. The few coumarins and furanocoumarins found in the hydrodistilled oil were at very low concentrations, their total content being in the range of a few tens of ppm, and the total amount of the target components in the cold-pressed oil was more than 200 times higher. In the cold-pressed oil, the most abundant compounds were 8-geranyloxypsoralen and bergamottin, followed by 5-geranyloxy-7-methoxycoumarin, citropten, oxypeucedanin and byakangelicol, all at concentrations above 1.0 g/L. Furanocoumarins accounted for about 2/3 of the total mass, and coumarins for the remaining third. According to the authors, xanthotoxol and isomeranzin were identified for the first time in the cold-pressed essential oil of Eureka lemon.

Bergamot from Kefalonia, Melliou et al. (2009)

Melliou et al. (2009) studied the essential oils of the fruit rind and of the leaves of bergamot from Greece, Citrus aurantium subsp. bergamia in the study, a name that WCVP records as Citrus × aurantium subsp. bergamia (Risso) Wight & Arn. ex Engl., a synonym of Citrus × limon (L.) Osbeck; the trees were grown on the island of Kefalonia. Yellow fruits were collected from December to March over two years (2007-2008) and processed within 24 h of collection. The rind (3 kg per sample) was separated by hand, and the oil was obtained either by cold pressing or by hydrodistillation for 3 hours in a Clevenger apparatus. The maximum yield, calculated on the wet weight of fresh rinds, was 1.8 %; the passage does not say which of the two processes this yield refers to. The main constituent of the cold-pressed oil of the fruit rind was linalyl acetate, as its (–) enantiomer, with an optical purity above 99.9 %, which the authors describe as responsible for the characteristic odour of bergamot. The linalool/linalyl acetate ratio, a quality index the authors say has been reported at about 0.3 for Calabrian bergamot oil, reached what they call its best value (0.38) for bergamots harvested in mid-January. At the same harvest date, the sum of linalool and linalyl acetate reached its maximum value (55.8 %), against about 50 % reported for Calabrian oil. According to Melliou et al., the fruit oil obtained by hydrodistillation is of lower quality, because of the decomposition of linalyl acetate and the isomerisation of linalool, as previously reported. On the same sample treated by cold pressing or by distillation, the linalyl acetate content fell from 40.5 % to 10.7 % in the distilled oil, and the heat treatment led to a partial isomerisation of linalool, confirmed by the presence of (+)-linalool.

Which coumarins and furocoumarins are reported in cold-pressed citrus oils?

What a review reports

According to the review by González-Mas et al. (2019), drawing on the references it cites, the peel oil of C. aurantium (Citrus × aurantium L.), of C. aurantifolia (the review’s spelling; WCVP: Citrus × aurantiifolia (Christm.) Swingle) and especially of C. bergamia (a name that WCVP records as Citrus × bergamia (Risso) Risso & Poit., a synonym of Citrus × limon (L.) Osbeck), obtained by cold pressing or by solvent extraction, contains phototoxic furocoumarins of the bergapten type. The review reports that several coumarin and psoralen (furocoumarin) compounds have been isolated in C. bergamia essential oils. Being more polar and less volatile than most compounds of Citrus oils, they have been reported mainly in oils obtained by cold pressing, and some studies consider them part of the non-volatile peel fraction. According to the same review, the furocoumarins bergapten and bergamottin are characteristic of the rind of C. bergamia without being exclusive to it: they have also been described in other species, including C. aurantifolia.

What one study of 374 commercial oils measured

In the study by Dosoky et al. (2022), Citrus volatile oils from suppliers the authors describe as trusted were taken from the collection of the Aromatic Plant Research Center (APRC, Lehi, Utah, USA); a total of 374 cold-pressed Citrus oil samples from this collection are listed in their Table 4. A distilled yuzu essential oil, from Citrus × junos Siebold ex Tanaka (C. junos Sieb. ex Tanaka in the study), served as the matrix for method validation (total coumarins < 0.001 ppm). In that study, the lowest amount of coumarins and furocoumarins was detected in calamansi oil, from Citrus × microcarpa Bunge (C. × microcarpa (Bunge) Wijnands in the study), at 0.15 ± 0.02 ppm. The highest amount was found in Italian bergamot oil (Citrus bergamia Risso & Poit in the study), at 171,453.11 ± 9,227.11 ppm, followed by Brazilian bergamot oil at 52,473.90 ± 1,775.63 ppm. The expressed oil of Italian bergamot showed the highest furocoumarin content (167,281.60 ± 1,017.74 ppm) and the highest level of any individual furocoumarin (109,730.67 ± 3,150.55 ppm of bergamottin). In the same study, oils from fruits of the mandarin taxa showed low total coumarin levels (2.44–149.45 ppm) and furocoumarin levels (2.44–149.45 ppm). These taxa were mandarin (Citrus reticulata Blanco), clementine (C. clementina Hort. ex Tanaka in the study; WCVP attaches Citrus × clementina to Citrus × aurantium f. deliciosa (Ten.) M.Hiroe) and tangerine. The authors note that this result does not align with a previous report according to which this taxon is nearly devoid of furocoumarins.

What three EFSA opinions report

EFSA opinion 6548 concerns the essential oil obtained by cold expression from the fruit peel of C. limon of multiple origins (Argentina, Brazil, South Africa, Spain, France, Italy). According to the opinion, HPLC-UV analysis of 10 batches showed detectable concentrations of 12 of the 14 linear derivatives sought, with high variability between batches of different origin (Mediterranean origin against other regions outside the EU). Table 4 of the opinion, which names the species Citrus limon (L.) Osbeck, gives the following contents in mg/kg of the peel essential oil (10 batches): 5-methoxypsoralen (bergapten), average 23.6, range 3.9–5.8 for the batches from outside the EU and 7.5–75.4 for the Mediterranean batches; 5-geranoxypsoralen (bergamottin), average 497.3, range 1.0–1.4 outside the EU and 250–1,272 Mediterranean (table notation). According to EFSA opinion 6625, in ten batches of expressed mandarin oil, 8-methoxypsoralen (xanthotoxin) was below the limit of detection (0.5 mg/kg) in all samples, by HPLC with UV detection. The same opinion reports that the applicant’s literature search showed that only the furocoumarins bergamottin (0.001 %) and 5-methoxypsoralen (bergapten, 0.0003 %) were reported to occur in very low amounts. According to the opinion, the phylogenetic analysis of Citrus species by Dugrand-Judek et al. (2015) confirmed that mandarins have only a low capacity to synthesise coumarins/furocoumarins, whereas the other ancestral taxa (pummelos, citrons and papedas) and their descendant species synthesise them in high amounts. According to EFSA opinion 6891, the analysis of five batches of expressed orange oil showed that eight furocoumarins (phellopterin, 8-geranoxypsoralen, psoralen, 8-methoxypsoralen, 5,8-dimethoxypsoralen, imperatorin, oxypeucedanin and bergamottin) were below the limit of detection of the HPLC method.

Which hazards are documented for cold-pressed citrus oils?

In its opinion 6548, the EFSA FEEDAP Panel concludes that the additives assessed, expressed lemon oil and its fractions, “should be considered” as irritants to the skin, the eyes and the respiratory tract, and as skin sensitisers. Since they contain furocoumarins, the opinion states that expressed lemon oil and its fractions “may” cause phototoxicity. In its opinion 6891, the Panel concludes that the expressed orange oil assessed “should be considered” as irritant to the skin and eyes, and as a skin and respiratory sensitiser.

References

• González-Mas M.C., Rambla J.L., López-Gresa M.P., Blázquez M.A., Granell A., « Volatile Compounds in Citrus Essential Oils: A Comprehensive Review », Frontiers in Plant Science 2019;10:12, doi:10.3389/fpls.2019.00012 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6370709/fullTextXML
• Dosoky N.S., Satyal P., Setzer W.N., « Authentication of Citrus spp. Cold-Pressed Essential Oils by Their Oxygenated Heterocyclic Components », Molecules 2022;27(19):6277, doi:10.3390/molecules27196277 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC9572141/fullTextXML
• Li G., Xiang S., Pan Y., Long X., Cheng Y. et al., « Effects of Cold-Pressing and Hydrodistillation on the Active Non-volatile Components in Lemon Essential Oil and the Effects of the Resulting Oils on Aging-Related Oxidative Stress in Mice », Frontiers in Nutrition 2021;8:689094, doi:10.3389/fnut.2021.689094 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8236505/fullTextXML
• Melliou E., Michaelakis A., Koliopoulos G., Skaltsounis A.-L., Magiatis P., « High Quality Bergamot Oil from Greece: Chemical Analysis Using Chiral Gas Chromatography and Larvicidal Activity against the West Nile Virus Vector », Molecules 2009;14(2):839, doi:10.3390/molecules14020839 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC6253838/fullTextXML
• EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), « Safety and efficacy of feed additives consisting of expressed lemon oil and its fractions from Citrus limon (L.) Osbeck and of lime oil from Citrus aurantiifolia (Christm.) Swingle for use in all animal species (FEFANA asbl) », EFSA Journal 2021;19(4):e06548, doi:10.2903/j.efsa.2021.6548 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8085978/fullTextXML
• EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), « Safety and efficacy of a feed additive consisting of expressed mandarin oil from the fruit peels of Citrus reticulata Blanco for use in all animal species (FEFANA asbl) », EFSA Journal 2021;19(6):e06625, doi:10.2903/j.efsa.2021.6625 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8190682/fullTextXML
• EFSA Panel on Additives and Products or Substances used in Animal Feed (FEEDAP), « Safety and efficacy of feed additives consisting of expressed sweet orange peel oil and its fractions from Citrus sinensis (L.) Osbeck for use in all animal species (FEFANA asbl) », EFSA Journal 2021;19(11):e06891, doi:10.2903/j.efsa.2021.6891 — https://www.ebi.ac.uk/europepmc/webservices/rest/PMC8603005/fullTextXML
• Govaerts R. (ed.), World Checklist of Vascular Plants (WCVP), Royal Botanic Gardens, Kew — https://sftp.kew.org/pub/data-repositories/WCVP/wcvp.zip