1.1. Origins of the Genus Citrus

The first modern classification of the genus Citrus (in the Family Rutaceae) was formulated by Carl von Linné, who, in his “Species Plantarum”, clustered all Citrus fruits under two species: Citrus medica L. (Citron) along with its only variety Citrus medica var. limon (Lemon), and Citrus aurantium L. (Bitter Orange) along with its two varieties, Citrus aurantium var. grandis (Pomelo) and Citrus aurantium var. sinensis (Sweet Orange) ^[13]. All subsequent classifications in the following centuries were based on morphological and geographical data, and different authors recognized a diverse number of species.

A revolution in the taxonomy of Citrus has come with the development of laboratory techniques involving specific molecular markers, such as Random Amplification of Polymorphic DNA (RAPD), Sequence Characterized Amplified Region (SCAR), cpDNA Simple Sequence Repeats (SSR), Inter-Simple Sequence Repeat (ISSRs), Amplified Fragment Length Polymorphism (AFLP) and Restriction Fragment Length Polymorphisms (RFLPs) markers ^[3][14], and the triangulation with information from historical literature and contemporary data about the natural distribution of these species ^[5].

According to one study ^[15], there are at least 10 progenitor species. Of these, seven are original to Asia: Pomelo (Citrus maxima), the true Mandarin (C. reticulata), Citron (C. medica), Papeda (C. micrantha), Ichang’s Papeda (C. ichangensis), Mangshanyegan (C. mangshanensis), and Oval Kumquat (C. japonica). The remaining three species are Australian: Desert Lime (C. glauca), Round Lime (C. australis), and Lemon Caviar (C. australasica).

Recently, important genomic work ^[4][16] has provided the most up-to-date evolutionary tree for Citrus, thus challenging previous proposals. The paper by Wu and colleagues ^[4] that used single nucleotide polymorphism (SNP), proposes that at least three genera (Fortunella, Eremocitrus and Microcitrus) are nested in the Citrus clade, and that of the ten progenitor species, three are to be considered ancestral, namely Citrus maxima, Citrus reticulata and Citrus medica. Four main hybrids are derived from these ancestral species: Lemons, Limes, Oranges and Grapefruits ^[17][18].

Wu and colleagues ^[4] state that, according to their analysis of the accessions, “the centre of origin of Citrus species was the southeast foothills of the Himalayas, in a region that includes the eastern area of Assam, northern Myanmar and western Yunnan”, while inferences from modern distribution point towards the central-southern part of China as the primary biodiversity centre, with southeast Himalaya and southern China as two accessory centres ^[1]. Citrus probably moved from West to East Malaysia and Australia sometime in the Miocene/Pliocene ^[19].

Because of the intricacy of the taxonomy of the Citrus genus, from an agronomic perspective, when dealing with commercially important species, a pure Linnaean classification could be substituted by just using cultivar names grouped under general agronomic groups. At least eight groups have been recognized so far, the Sweet Orange Group (cultivar: navel, common, blood, acidless), the Mandarin Group (Satsuma, tangerine/clementine, Mediterranean Mandarin and Mandarin), the Lemon Group, the Grapefruit Group, the Lime Group, the Sour Orange Group (cultivar: Seville, Bouquet de fleur, Granita, Chinotto, Bergamot), the Pomelo Group, and the Citron group ^[13].

1.2. Taxonomy of Bergamot

The phylogeny and taxonomy of Bergamot (Figure 1) are particularly unclear, as it is its exact area of origin ^[20].

Figure 1. Bergamot plant and fruit, as illustrated in the traditional Köhler’s Medizinal-Pflanzen ^[21].

The plant is an interspecific hybrid, and this is testified by the high percentage of heterozygosity ^[22] and has been clustered with Bitter Orange ^[23]. The actual progenitor species of Bergamot have been claimed to be Citrus x aurantium (Bitter Orange) and C. x limon (Lemon), or C. x aurantium and C. x aurantifolia (Key Lime) ^[24], or C. x aurantium and C. medica ^[25][26].

Bergamot seems to have originated quite late in the phylogeny of the Citrus clade and was unknown to ancient authors. Some sources claim that the hybrid was originated directly in Calabria, or at least in the southern part of Italy ^[27], while others state that the hybrid first appearance was outside Italy (either in the Antilles or in Greece or the Canary Islands) and that it was brought to Calabria only later ^[22]. One source hypothesizes that the hybrid was brought to Italy by Columbus via the Spaniard city of Berga, which inspired the fruit name, Bergamot ^[22]. A more solid hypothesis for the name origin is that the term “Bergamot” may derive from the Turkish word “beg-a-mudi” which means “pears of the Prince”, due to its certain resemblance to Bergamot pears ^[27].

Some authors also identify four subgroups of Bergamot, namely the “Piccola” group, the “Torulosa” group, the “Melarosa” group, and the “Common” group. All Bergamot fruits that are interesting from an industrial point of view come from the “Common” group, which includes three cultivars:

“Castagnaro”, whose fruits give perhaps the least valuable essential oil (EO), but can be harvested for production for longer than any other cultivar.

“Femminiello”, less vigorous and strong than Castagnaro, with a low yield of a superior, excellent EO.

“Fantastico” (or “Inserto”), supposed to be a hybrid of the first two varieties, certainly introduced more recently (about 1940) but representing now a good part of all the trees in Calabria ^[22].

Concerning nomenclature, opinions of different authorities diverge, as explained in Table 3.

1.3. Definition of EO and Peculiarities of Citrus EOs

The International Organization for Standardization (ISO) definition (ISO 2020 document ISO 9235: 2013-2.11) states that an EO is a “product obtained from a natural raw material of plant origin, by steam distillation, by mechanical processes from the epicarp of Citrus fruits, or by dry distillation, after separation of the aqueous phase—if any—by physical processes”. Such includes three production methods, one of them exclusively reserved for Citrus EOs, which are therefore clearly separated from other EOs by production method and, therefore, by chemical composition. Distilled EOs contain only volatile molecules, while cold-pressed EOs may contain much heavier molecules and different chemical compounds from those normally found in distilled EOs. The second characteristic of Citrus fruit EOs is that, from an economic point of view, in many cases, the EO is a secondary product, while the fruit juice is the primary product, and other products can be obtained from the fruit (candied peels, “cells”, albedo, etc.). This is true for Lemons, Sweet Oranges, Grapefruits and Mandarins, but not for Bergamots and Bitter Oranges, whose fruit juice is generally produced in small quantities or even not at all. This fact partly explains the higher costs of both EOs ^[34].

1.4. Anatomy of Citrus fruits

The fruit of Citrus species is a modified berry called “hesperidium”, characterized by an approximately spherical shape, and by a very thick and robust outer rind or peel protecting the fluid-filled trichomes that make up the segments of the fruit juicy part (endocarp). The rind is divided in a superficial, thin, coloured exocarp called flavedo (from classical Latin flavus, or “blonde”) and a deeper and thicker spongy mesocarp called albedo (from the Latin albedo, derivative of albus, or “white”).

The flavedo is surrounded by a thick, waterproof cuticle and by various stomata in the superficial layer of the epidermis, it is made of small, dense collenchyma cells with chromoplasts, and contains many EO-bearing cavities called lumina or lacuna ^[22]. In the Bergamot fruit, these more or less spherical central cavities are surrounded by four to six layers of secretory cells and are formed in a schizolysigenous way ^[35], that is first by the separation of parenchymal cells (schizogenesis of the intercellular space), followed by further disintegration of cells in later stages of cavity development (lysogeny). These lacunae have no walls, are not connected, are found at different depths, and fill up with EO as the fruit ripens. They are of different sizes (the range is 0.4–0.6 mm in diameter) and they are conventionally divided in primary, secondary and tertiary cavities ^[34][36].

It is precisely this superficial distribution of the oil cavities that has made the method of cold-pressing or mechanical treatment the preferred one to obtain Citrus EOs ^[5][22].

1.5. Distribution and Cultivation of Bergamot

Around 90% of total BEO global production comes from Italy and specifically from a small area of fewer than 1400 hectares in Calabria ^[37]. Other regions of the world where Bergamot cultivation and BEO production have been tried are the African continent and the Americas, and lately China. Of all these producers, only the Ivory Coast has a commercially important share of the market, with 8–10% of global BEO production, although Brazil and China are emerging as global producers ^[22].

The introduction of Bergamot in Africa started only in the 1950s, and up until 1959, apart from a few trees in the Botanical Garden of Bingerville close to Abidjan, there was no record of the plants in this region. Other African countries such as Guinea, Liberia, Mali, Algeria, Morocco, and Tunisia only have small industrial productions with no indication of possible expansion ^[22].

One of the main problems for BEO production in Africa is that it appears to be up to 3–5 times lower per cultivated hectare than that of Calabria ^[38][39]. The reason for this has been sought in some differences in production technology, in the different climate, but above all in the characteristic composition of the soil. It can be added that a difference of three centuries of cultivation in Italy has allowed a better selection of plants ^[38][39].

The introduction of Bergamot in the Americas started in the US, specifically in Florida, Louisiana and California, where the tree was imported before 1815 but where it failed to thrive. The cultivations in South America were slightly more successful. Introduced since the 1960s there is only a modest industrial production of a good quality product in Argentina, Brazil and Uruguay ^[22].

The differences in the chemistry of BEO from various regions of the world are difficult to gauge because of their large variability, due to non-technological parameters according to Dugo and Bonaccorsi ^[22]. What can be stated is that most of the EOs produced in regions different from Calabria show values of β-pinene and γ-terpinene lower or close to the minima determined for Calabrian EOs and that in terms of enantiomeric composition it seems, from the scarce available chiral data, that EOs from the Ivory Coast and Uruguay agree with those of EOs from Calabria ^[22].

2.1. Manual Extraction

One method that was practised a long time ago, particularly in Sicily, was the “spugna” (sponge) method, whereby fruits were halved by expert operators, called “sfumatori”, the pulp was separated from the rind using a special, sharpened spoon-knife called “rastrello” ^[41], and the EO was extracted from the peel by pressing the peel against a hard object of baked clay (“concolina”) or by bending the peel, with the compression causing oils sacs to burst and eject EO under pressure. In both cases, a large natural sponge was used to absorb the EO emulsion, which, in a later stage, was removed by squeezing it into the concolina or into another container. An improvement of this method that was introduced in later years was to wash the rinds with lime water to buffer the acidity caused by organic acids such as oxalic and malic acid and, then, to let the rinds drain for 24 h before extraction. The system was very time-consuming and gave a poor yield (but apparently of very high quality).

An improvement over the spugna method was the scodella or eucelle method. In this case, the entire fruit was placed in a copper or brass bowl with a hollow central tube at the bottom. The bowl was covered with blunt spikes over which the whole fruit was delicately rolled to burst oil sacs. Released EO was then drained down the funnel stem and collected in a vessel, together with some water content of the rind, which was then separated from the rest by decantation. This method allowed an improved yield of EO and did not require the halving of the fruits, which was a time-consuming process that also possibly led to some EO loss. However, both methods are impractical and very time-consuming and are not used anymore.

2.2. Mechanical Methods

To improve these methods, under the Kingdom of Borbone, a competition was organized for automation of the extraction, which was won in 1844 by Nicola Barilla and Luigi Autieri, with the Calabrese Machine, further improved by Domenico Carbone in 1875, which partially automatized compressing and rasping operations ^[22][27]. In 1931 an improved version, called “modern Calabrese” machine, or “Polimeni” machine, appeared, that could compress and scrape up to eight fruits at a time, thus reducing processing time maintaining an excellent quality of the product ^[22][27]. Over the years many other machines were invented and used, all variations on the Calabrese or Pelatrice design (see below for a description), although most of them are not in use any more, and most of today’s production of BEO comes from “Pelatrice Speciale” and “Moscato tipo MP-C3”, plus some production from Brown extractors, “Garozzo” Machine and Food Machinery Corporation (FMC) in-line extractors ^[22][27] (see Table 4).

Since no Citrus EO extraction can be profitable without exploiting other parts of the fruit, the expression is nowadays part of an integrated system that focuses on zero waste to make the entire chain of production profitable. Hence, modern production methods depend on the use of machinery that can often operate the pressing of the juice and the separation of other parts of the fruit.

We can roughly divide modern, mechanized methods into:

(1)

EO extraction after pulp squeezing (“Sfumatrice”, “Torchiatura”).

(2)

EO production before juice (and other components) extraction (“Pelatrice”, Brown Extractor, “Garozzo”).

(3)

Methods in which the two extractions are performed at the same time (FMC).

It should be noted that, while all these methods are widespread, for BEO extraction the Sfumatrici are not normally used because the Bergamot juice industry is not economically relevant and the EO is of much higher value than other Citrus EOs.

2.3. EO Extracted after Juice Extraction

In the Sfumatrice process, the fruits are halved and the pulp is removed. The rinds are soaked in lime to neutralize acids and then passed through two horizontal ribbed rollers that press and bend the rinds to extract the EO. The EO is washed away from the rollers by fine sprays of water, then it is passed through a separator to remove any solid matter and finally sent to two serial centrifuges to purify the EO. After extraction, winterization can be used to freeze out waxes.

2.4. EO Extracted before Juice Extraction

With these machines, the EO is extracted before squeezing the juice, through a rasping or perforation of the peels (Pelatrice) or the breaking of individual bags (Brown machine, or Garozzo machine). The EO is washed with water spray, and the emulsion between water and EO is filtered and then separated by centrifugation.

In the Pelatrice process, Citrus fruits are put into a slow-moving cochlea surrounded by an abrasive shell. The abrasive, rasping action causes EO sacs to burst and release their EO-water emulsion. The cochlea then transports the fruit into a hopper, in which rollers with abrasive spikes burst the remaining oil sacs. As in the Sfumatrice process, the EO and water emulsion is washed away, the emulsion passes through a separator and then through the two centrifuges. Most BEO and some Lemon EOs are produced this way in Italy. Some methods or machines are today of only historical interest, while others are still used by producers.

In the Brown extractor and the Garozzo machine (a modified and improved version of the Brown extractor) (Figure 2), the fruits, after washing, are moved continuously into a long bed of rolling and shifting cylinders, each bearing thousands of spikes or square teeth, that prick the peel to release the EO, which is collected in a shallow layer of water. Fruits are then moved to a smooth roller section, where the remaining water and EO sticking to the rind are collected. After this passage, the whole fruits are sent to the juicing step. The emulsion is then filtered and sent to the centrifuge to recover the cold-pressed EO. These newer methods have been tested with Bergamot with very interesting results both in terms of quality and in terms of total yield.

Figure 2. The bed of rolling toothed cylinders in a Garozzo machine.

2.5. EO Extracted at the Same Time as the Juice

The only machine that can operate the two extractions at the same time is the FMC in-line extractor, a highly mechanized and automatized contraption in which there are two stainless steel cups, made of interlacing metal fingers, one holding the fruit and the other one descending towards it (Figure 3). The descending cup interlaces with the inferior one, thus squeezing the fruit and sending the juice downwards through a hole made automatically by a circular knife at the bottom of the lower cup. At the same time, the fingers also press the rind and burst oil sacs, thus freeing the EOs which are washed away by a spray. The two operations are physically separated and this ensures that the EO does not mix with the juice and vice-versa, although some contamination is almost inevitable, with some reduction in quality of produced EO.

Figure 3. A detail of an FMC in-line extractor showing the upper and lower cups.

2.6. Low-Quality EOs

In the past, those fruits which fell on the ground during the summer season, well before reaching perfect maturation, were still manipulated in a different, dedicated extraction line ^[41]. The derived EO was named “Nero di bergamotto” (Bergamot Black) because often these fallen fruits lay on the ground for a long time before being collected, and were scorched, blackened by the Sun. The product was of inferior quality, and this practice has been discontinued because of the increasing demand for superior products. However, while the “Nero di bergamotto” is not in production any more, in certain instances fruits that have fallen before maturation but later on in the season, and that have consequently spent less time on the ground before being picked, are extracted with specialised, small machines of the Calabrese Pelatrice type (see above). The EO that results from this process, called “Bergamottella”, has a lighter colour than “Nero di bergamotto” and has a quality that depends on the moment the fruit has fallen and it was processed, but in certain cases, the EO can reach a high percentage of esters, to a maximum of 30% ^[22][41].

There are also EOs derived from the residues of the normal extracting techniques: “Ricicli” (recycled) EOs are obtained by centrifugation of the waters left at the end of the daily process of extraction, which can amount to about 1% of the total EO; “Torchiati” (screw-pressed) EOs are recovered by pressing the solid residues in the secondary centrifuge, with a yield of about 3.5% of the total EO;

“Pulizia dischi” (disc cleaning) EOs are those obtained by decanting the liquid residues from the secondary centrifuge disk at the end of the daily process, with a yield of about 0.5% of the total EO ^[13][22].

2.7. Distillation

It should be noted that it is also possible to obtain all Citrus EOs by distillation, both of the juice and of the separated skin, but these distilled EOs are quite rare because they often have a lower organoleptic quality. Distillation is used as a customary method only for Lime and is never used for Bergamot. This is because any Citrus EO obtained through distillation has a reduced olfactory quality, and is more fragile because of the lack of non-volatile molecules which appear to provide some antioxidant activity ^[47]. In the Bergamot industry, distillation is only used for refining the cold-pressed BEO, or for second-grade products. For the refining step, distillation is adopted to modify the chemistry of cold-pressed EOs, either to reduce the percentage of monoterpenes (deterpenation), or to reduce the percentage of bergapten (bergapten-free BEO), or to reduce the percentage of furocoumarins (furocoumarin-free BEO).

Distillation is also used to produce second-grade derivatives. Distilled EO of “Bergamottella” is obtained by distilling unripe fruits that are too small for the Pelatrice process; “Feccia” EOs are derived from the distillation of the solid and semi-fluids residues of the Pelatrice machines; the “Peratoner EOs” are produced by vacuum distillation of the waters coming out of the centrifuges separating the EOs, or of generic residues of mechanical processing (Figure 4). Because of the lower temperature during distillation, it gives superior product compared to the other methods, and it is slowly supplanting them; and the “distilled juice notes” are EOs obtained from the distillation of the juice ^[22]. Innovative distillation techniques such as solvent- and water-free microwave extraction have been tested with good results, but they are still at a laboratory or pilot level ^[48].

Figure 4. Two Peratoner still units.

2.8. Terpeneless BEOs

Distillation under vacuum using a fractionating column is the classic technology to selectively eliminate monoterpene hydrocarbons such as limonene. Most cold-pressed EOs are dominated by monoterpenes (usually between 90% and 98%), mostly (+)-limonene, but also terpinene and β-pinene), with a very low olfactory impact. Selectively eliminating these monoterpenes allows increasing the weight of compounds with a high olfactory impact, such as esters and aldehydes. The deterpenation, therefore, results in two products, a deterpenated EO with a high olfactory impact, and the terpenic fraction (Orange terpenes, Lemon terpenes, Bergamot terpenes, etc.), often used in perfumery, for low-cost aromatisation, or to adulterate other EOs. Terpeneless BEO is produced in smaller volumes if compared to other Citrus EOs because Bergamot has already a high percentage of oxygenated compounds and the need to concentrate them by reducing the monoterpene content is less relevant. The yield of deterpened BEO (folded Bergamot EO) is 33–44% of the original BEO, while the yield for Orange and Lemon EOs is 1–3% and 3.5–6%, respectively. Usually, the EO is distilled under vacuum after a winterization passage. New technologies for deterpenization are available: some examples include alcohol extraction followed by a concentration step (under vacuum distillation of alcohol), counter-current extraction with two immiscible solvents, extraction with a binary mixture of a polar and an apolar solvent, chromatographic separation, SC-CO2 extraction, or membrane extraction. However, none of these methods has reached an industrial level.

2.9. Bergapten-Free and Furocoumarin-Free BEO

Another refining step sometimes used for Bergamot is the production of bergapten-free BEO for the fragrance industry. Bergaptene-free BEO is a reduced-bergapten content EO, with bergapten present at a content level below 30 ppm. In Italy, this step is done using a chemical passage, the mixture of the EO with an alkaline solution of sodium hydroxide (NaOH). This treatment has the effect of hydrolyzing the lactonic ring of bergapten and citropten but does not affect bergamottin and 5-geranyloxy-7-methoxycoumarin, because the lactonic function is sterically protected by the geranyloxy group. The resulting EO has a bergapten content of 0–91 ppm, a bergamottin content of 11,726–16,250 ppm, and a 5-geranyloxy-7-methyl coumarin content of 1539–1975 ppm. The passage is followed by centrifugation and separation, further washing with water and more centrifugation and separation ^[49].

A similar product is the furanocoumarins free FCF BEO, which is an EO of Bergamot deprived not only of bergapten but of all furocoumarins. This elimination is done by vacuum re-distillation over a fractionating column, and it usually results in the production of a very high-quality EO, with a bergapten content of 0-41ppm, a bergamottin content of 0–3017 ppm, and a 5-geranyloxy-7-methylcoumarin content of 0–349 ppm ^[49]. A graphical summary of BEO extraction processes is reported in Figure 5.

2.10. Innovative Methods

Although strictly they are not EO-obtaining production methods, because they do not comply with the ISO definition of EOs, some innovative techniques have been proposed to extract EOs from Citrus peels. In most cases, these extracts have different chemical compositions from cold-pressed EOs ^[50], and most of them are still at a laboratory or pilot level: ultrasound-accelerated solvent extraction, solid-phase microextraction, supercritical CO2 extraction ^[48], and pervaporation ^[51].

3.1. Volatile Fraction

The volatile fraction amounts to roughly 93–96% of the total content of BEO ^[53]. The main components of this fraction are limonene, linalyl acetate, and linalool ^[13], but there are more than 100 compounds detected and described in the scientific literature ^[54]. Although the content of hydrocarbon compounds of BEO is much lower than that of other Citrus EOs, the volatile fraction is still dominated by monoterpene hydrocarbons, specifically by limonene (25–55%), β-pinene (4–11%), and γ-terpinene (5–11%) ^[54]. Additionally, in the volatile fraction, there are sesquiterpene hydrocarbons, such as β-bisabolene, (E)-α-bergamotene, and β-caryophyllene ^[55]. In contrast with other Citrus EOs, however, oxygenated derivatives of mono and sesquiterpene hydrocarbons are present in a much higher percentage and they can be more than 50% of the entire volatile fraction, while in other Citrus EOs they can range from 1% to 6% ^[56]. Terpenic esters and alcohols (and secondarily aldehydes and oxides) are particularly abundant, although with very wide composition intervals, alongside with small amounts of aliphatic aldehydes, alcohols, and esters ^[53][54]. From a quantitative point of view, monoterpene esters are the most relevant esters in BEO, and the more abundant ones are linalyl acetate (15–40%), geranyl acetate, linalyl propionate, and neryl acetate. (Z)-Sabinene hydrate acetate and (E)-sabinene hydrate acetate, although not very important in terms of percentage, seem to be exclusive of C. bergamia EO ^[53][55]. An important non-terpenic ester is the aliphatic ester octyl acetate, which is more abundant in C. bergamia EO than in other Citrus species ^[55]. In the category of monoterpene alcohols, linalool is the most important and can be present in very high amounts (2–20%), while the sesquiterpene alcohol α-bisabolol, although not present in high amounts, seems to be quite specific to Bergamot ^[55].

3.2. Non-Volatile Fraction

This fraction amounts to 4–7% of the total content of the EO ^[53] and, apart from inert substances such as pigments and waxes, is principally made up of oxygen heterocyclic compounds, mainly psoralens and coumarins, plus minor amounts of polymethoxyflavones (sinensetin, tetra-O-methyl scutellarin) (Table 5). In particular, four compounds make up the majority of this fraction, namely the two psoralens bergapten (5-methoxypsoralen, 5-MOP. 0.11–0.32%) and bergamottin (5-geranyloxypsoralen 1.02–2.75%), and the two coumarins geranyloxy-7-methoxycoumarin (0.08–0.22%), and citropten (5-geranyloxy-7-methoxycoumarin. 0.14–0.35%), which are characteristic but not exclusive of C. bergamia ^[54][55]. The variability ranges of these four components are very large ^[22] (see Table 5). Other psoralens identified in genuine EOs are bergaptol, byakangelicin, biakangelicol, epoxybergamottin, isoimperatorin, 5-isopentenyloxy-8-methoxypsoralen, oxypeucedanin, and oxypeucedanin hydrate ^[49].

Sawamura and colleagues have analysed the volatile fraction composition of BEO using GC coupled with olfactometry, to identify the most important compounds in the character definition of Bergamot aroma, namely a few molecules present in minute amounts: (Z)-limonene oxide, decanal, linalyl acetate, and geraniol. Another molecule, (Z)-β-ocimene, although not possessing a typical Bergamot character, seems important for the formation of the overall aroma ^[54]. A graphical representation of BEO composition is displayed in Figure 6, Figure 7 and Figure 8 and chemical description of its most relevant compounds is reported in Table 6.

Figure 6. Total contents of BEO.

Figure 7. Volatile compounds found in BEO.

Figure 8. Non-volatile, oxygen heterocyclic compounds found in BEO.

Table 6. Chemical description of the most relevant compounds found in BEO.

 

Molecule	Molecular Formula	IUPAC Name	Structure	CAS	Range (%)
Chemical class	Monoterpene hydrocarbons
Limonene	C10H16	1-methyl-4-prop-1-en-2-ylcyclohexene		138-86-3	25–55
α-Pinene	C10H16	2,6,6-trimethylbicyclo[3.1.1]hept-2-ene		80-56-8	0.72–1.4
β-Pinene	C10H16	6,6-dimethyl-2-methylidenebicyclo[3.1.1]heptane		127-91-3	4–11
γ-Terpinene	C10H16	1-methyl-4-propan-2-ylcyclohexa-1,4-diene		99-85-4	5–11
cis-β-Ocimene	C10H16	(3Z)-3,7-dimethylocta-1,3,6-triene		338-55-4	0.17–0.27
Chemical class	Sesquiterpene hydrocarbons
β-Bisabolene	C15H24	(4S)-1-methyl-4-(6-methylhepta-1,5-dien-2-yl)cyclohexene		495-61-4	0.21–0.65
(E)-α-Bergamotene	C15H24	(1S,5S,6R)-2,6-dimethyl-6-(4-methylpent-3-enyl)bicyclo[3.1.1]hept-2-ene		13474-59-4	0.16–0.50
β-Caryophyllene	C15H24	(1R,4E,9S)-4,11,11-trimethyl-8-methylidenebicyclo[7.2.0]undec-4-ene		87-44-5	0.15–0.55
Chemical class	Monoterpene alcohols
Linalool	C10H18O	3,7-dimethylocta-1,6-dien-3-ol		78-70-6	2–20
Geraniol	C10H18O	(2E)-3,7-dimethylocta-2,6-dien-1-ol		106-24-1	0–0.01
Chemical class	Sesquiterpene alcohols
α-Bisabolol	C15H26O	6-methyl-2-(4-methylcyclohex-3-en-1-yl)hept-5-en-2-ol		72059-10-0	0.01–0.03
Chemical class	Monoterpene oxides
(Z)-Limonene oxide	C10H16O	Terpene oxide (1S,4S,6R)-1-methyl-4-prop-1-en-2-yl-7-oxabicyclo[4.1.0]heptane		32543-51-4	tr–0.02
Chemical class	Monoterpene esters
Linalyl acetate	C12H20O2	3,7-dimethylocta-1,6-dien-3-yl acetate		115-95-7	15–40
Neryl acetate	C12H20O2	[(2Z)-3,7-dimethylocta-2,6-dienyl] acetate		141-12-8	0.13–0.67
Geranyl acetate	C12H20O2	[(2E)-3,7-dimethylocta-2,6-dienyl] acetate		105-87-3	0.11–0.87
Linalyl propionate	C13H22O2	3,7-dimethylocta-1,6-dien-3-yl propanoate		144-39-8	0.01–0.07
(E)-Sabinene hydrate acetate	C12H20O2	[(2S,5S)-2-methyl-5-propan-2-yl-2-bicyclo[3.1.0]hexanyl] acetate		Not Available	0.015–0.13
Chemical class	Aliphatic esters
Octyl acetate	C10H20O2	octyl acetate		112-14-1	0.06–0.22
Chemical class	Aliphatic aldehyde
Octanal	C8H16O	Octanal		124-13-0	0.02–0.09
Decanal	C10H20O	Decanal		112-31-2	0.04–0.1
Chemical class	Oxygen heterocyclic compounds (psoralens)
Bergaptene	C12H8O4	4-methoxyfuro[3,2-g]chromen-7-one		484-20-8	0.11–0.32
Bergamottin	C21H22O4	4-[(2E)-3,7-dimethylocta-2,6-dienoxy]furo[3,2-g]chromen-7-one		7380-40-7	1.02–2.75
Chemical class	Oxygen heterocyclic compounds (coumarins)
Citropten	C11H10O4	5,7-dimethoxychromen-2-one		487-06-9	0.14–0.35
5-Geranyloxy-7-methoxycoumarin	C20H24O4	5-Geranyloxy-7-methoxycoumarin		7380-39-4	0.08–0.22