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19: Essential-oil plants

Citrus bergamia Risso & Poiteau

Hist. nat. orangers 1: 111, t. 53—56 (1819).

RUTACEAE

2n = 18

Citrus aurantium L. var. bergamia (Risso & Poiteau) Wight & Arnott (1834), Citrus aurantium L. subsp. bergamia (Risso & Poiteau) Engler (1896).

Bergamot, bergamot orange (En). Bergamote, bergamotier (Fr). Indonesia: bergamet.

The origin of bergamot is unknown. It is said that Columbus brought it from the West Indies or Canary Islands to the Spanish town of Berga from where it was taken to Calabria in Italy, then a Spanish dependency. Others, however, have suggested that the origin must be sought in China or Turkey. Bergamot is almost exclusively grown in the narrow coastal plain of the south-western tip of Calabria in southern Italy between the towns of Villa San Giovanni and Brancaleone. Small but well established industries also exist in Ivory Coast near Sassandra and Soubré and in Guinea on the Foutah Djallon plateau. In northern Africa and Turkey bergamot is grown on a very minor scale. In South-East Asia its cultivation is not common.

Bergamot is mainly grown for the essential oil present in the peel of its fruit (bergamot oil). Bergamot oil is an important component of toilet water 'eau-de-Cologne', which was first developed around 1675 in Cologne (Germany) by the Italian immigrant Gian Paolo Feminis. His relatives further developed the industry and brought it to several other cities. As a result several formulae of eau-de-Cologne were developed, all characterized by bergamot oil. Later, bergamot oil became a constituent of high quality perfumes and of men's perfumes, such as aftershaves. Bergamot oil is also a characteristic additive of Earl Grey tea and of tobacco flavourings. In the Castelli area south of Rome it is customary to put a bergamot fruit in a cask of Frascati wine to impart its characteristic aroma. The oil is further used in skin care products (bronzers), soaps, lotions and creams. A different oil is obtained from the leaves (bergamot petitgrain oil), but is only produced to order. The juice of the fruit was formerly used to prepare calcium citrate and citric acid, while nowadays it is a component of citrus soft drinks. The pulp is used as animal feed or for the extraction of pectins.

The production of bergamot oil is almost entirely confined to Calabria (about 3000 ha in the early 1990s); only about 5% of the total production comes from Ivory Coast and Guinea. World production was 18 000 t fruit in the 1991/1992 season yielding about 100 t oil. Production of bergamot oil has been gradually declining since the early 1980s from 100—150 t annually. Bergamot oil fetches a price of about US$ 125/kg (1997). Although the oil is highly appreciated in perfumery, production did not increase with the increasing production of perfumery goods. This may be due to the frequent reconstitution or adulteration of bergamot oil by compounding it with cheap natural or synthetic aroma chemicals.

Bergamot oil is a mid-green to olive-green oil becoming yellowish or paler with age and exposure to sunlight. Its odour has a fresh citrusy top note; the body is extremely rich, sweet oily-herbal, peppery fruity and somewhat balsamic; the dry-out is dry and woody. It is characterized by a higher content of oxygenated compounds than in any other commonly encountered citrus oil. The main oxygenated compounds are linalool and linalyl acetate. The linalool content is highest at the beginning of the harvest season, after which it decreases gradually, while the linalyl acetate content exhibits an opposite trend. The content of oxygenated compounds varies from (20—)30—45(—60)%, of which on average linalool accounts for about 8% (5—15%) and linalyl acetate for about 28%. Bergamot oils from Ivory Coast have a somewhat higher linalool and linalyl acetate content than Italian oils. Bergamot oil of the best olfactive characteristics does not always contain high levels of linalool and linalyl acetate. Other quantitatively important compounds are: limonene, 'BETA'-pinene and 'GAMMA'-terpinene. The limonene content is lower than in other citrus oils.
Bergamot oil is in the Food Chemical Codex. In the United States it is 'generally recognized as safe' (GRAS No 2153) and has been approved by the Food and Drug Administration (FDA) under paragraph 182.20. The maximum permitted use level in foods is 0.02% (gelatins and puddings).
Bergamot oil contains bergapten which may cause skin irritation and photosensitivity and is thus a danger in sunscreen products. Bergamot oil NS (non-sensitizing) has been developed to overcome this risk; terpeneless grades produced by vacuum distilling whole oil can also be used.
In assessing the oil quality of a crop, samples must be taken from several trees and from several positions in these trees, as there are significant differences in oil content and oil composition between fruits growing in different positions on a tree. The degree of maturity also affects the quality of oil. The marketed product is often obtained from a mixture of ripe and immature fruits, but unripe ones produce inferior quality oil. See also: Composition of essential-oil samples and the Table on standard physical properties.

Bergamot oil has been adulterated by cheap orange oil (d-limonene) and by the addition of synthetic linalool and its acetate. The presence of orange can be detected by the presence of small amounts (up to 0.5%) of 'DELTA'-carene, a characteristic constitutent of orange peel oil. The latter adulterations can be detected because the synthetic compounds are chirally inactive and may contain impurities such as dihydrolinalool, dehydrolinalool and phenols.

Erect, unarmed, much branched tree up to 12 m tall, with trunk up to 25 cm in diameter; in cultivation trees are pruned up to 4—5 m in height with crown diameter of about 5 m. Leaves alternate, simple, glandular, aromatic when bruised; petiole about 13 mm long, moderately winged, articulated near the blade; blade lanceolate, up to 12 cm x 6 cm, in upper third part weakly indented. Inflorescence terminal, racemose, many-flowered; pedicel up to 8 mm long; flowers bisexual, 4—5(—10)-merous, fragrant; calyx cup-shaped with short lobes, yellow-green; corolla 3.8 cm in diameter, most often with 5, narrow-elongate, pure-white petals without any purple tinge; stamens (13—)21(—28), in (2—)4(—6) groups, sometimes petaloid; disk nectariferous; pistil with subglobose ovary, short and thick style, distinct to indistinct stigma. Fruit a slightly flattened subglobose to pyriform berry (hesperidium), 6.5—7 cm x 6—7.5 cm, often with a small navel and a persistent style; peel 6—7 mm thick, with numerous glands, tough, smooth to rough, sometimes ridged, adherent, shiny green turning yellow when ripe; flesh yellowish, firm, very acid and bitter, divided into 8—14 segments. Seed (0—)3(—13) per fruit, flattened, 11 mm x 6 mm x 4.4 mm, pale yellow, usually monoembryonic.

Bergamot trees develop slowly; a small harvest may be expected 7 years after planting, increasing annually to 12—15 years. Trees remain productive for 50—60 years if well maintained.
Temperatures during the first stage of fruit development and during mid-summer can greatly influence the essential oil accumulation. In Italy flowering is in March—April, fruits are harvestable in December—February.

Citrus bergamia is most probably of hybrid origin. It has been suggested that it is a hybrid between sour orange (Citrus aurantium L.) and lemon (Citrus limon (L.) Burm.f.), or a mutation of the latter. Others hold it as a hybrid between sour orange and lime (Citrus aurantifolia (Christm. & Panzer) Swingle). Bergamot is only known from cultivation and consists of a limited and well defined number of cultivars.
Four cultivar groups are recognized in bergamot: Common Bergamot, Melarosa (fruit rather flattened), Torulosa (fruit ridged) and Piccola (dwarf cultivars). Only Common Bergamot is commercially cultivated for the essential oil and 3 cultivars are grown: 'Castagnaro', 'Femminello' and 'Inserto'. Formerly, 'Femminello' and 'Castagnaro' constituted virtually all commercial plantings in the world, but they have largely been replaced by 'Inserto' ('Fantastico'), a hybrid of 'Femminello' and 'Castagnaro'. 'Femminello' is somewhat less vigorous and smaller than 'Castagnaro', but is earlier and more regular in bearing. Its fruit is spherical or nearly so, the rind smooth and more aromatic and hence it is preferred. 'Castagnaro' is more upright and vigorous, attaining a larger size than 'Femminello', but is less fruitful. Its fruit is roundish but frequently exhibits a short neck and obovate outline and is sometimes slightly ribbed; the rind is usually rougher and the oil usually less aromatic than in 'Femminello'. 'Inserto' is a fairly vigorous tree, that yields well and has only a slight tendency to alternate-bearing; its fruit is medium in size, averaging about 130 g with a rough rind texture.

In Calabria bergamot is grown in coastal areas protected from cold northerly winds by the nearby mountain range. The area has the highest average annual temperature and the highest number of sunshine hours in Italy and is further characterized by mild winters, a small difference between day and night temperatures and the absence of frost. The monthly average daily temperature varies between 26°C in August to 12°C in January. The average number of sunshine hours reaches a maximum of 10 h per day in July and is lowest in December and January with 3 hours per day. Average annual rainfall is about 550 mm with a maximum in December—January and a minimum in July. The soils are mainly alluvial. Although it was long thought that bergamot was adapted to a narrowly defined climate, it has proven to grow well in tropical conditions too, such as in Ivory Coast.
Oils obtained from fruits harvested in the interior of Calabria, where bergamot is not traditionally grown, show a high linalyl acetate and linalool content, but poor olfactive qualities.

In Italy, bergamot is grown mainly from budded seedlings. Until the beginning of the 20th Century lime (Citrus aurantifolia) was used as rootstock because it combined a high yield of essential oil of high quality with a short juvenile period. Nowadays sour orange (Citrus aurantium) is the only rootstock used. Rootstocks are grafted at a height of 60—70 cm. Planting distances range from 4 m x 4 m in the oldest groves in Italy to 5 m x 5 m or 6 m x 4 m in younger ones. Plantations are generally intercropped initially, to reduce establishment costs.

Young trees of bergamot are trained from 90—120 cm above ground level to produce a vase with 3—4 branches. Pruning is usually done every 2—3 years, only occasionally annually. In Calabria it is done in February after harvesting. The operation requires skilled labour to maintain each tree in a good shape. Weed control is usually done manually, though the use of herbicides is becoming more frequent. In older plantations on sloping land a grass cover is often maintained to control erosion. Ring weeding is then often practised, to reduce damage by diseases and pests. Irrigation is mostly by overhead sprinklers, but in the oldest groves basin irrigation is still practised. The amount of irrigation water applied annually in Italy is about 600 mm/ha. The amount of fertilizer applied and its timing varies from place to place and from farmer to farmer.

Little is known about diseases and pests affecting bergamot. Apart from diseases and pests attacking citrus in general, it is particularly sensitive to stylar end rot, a physiological disease affecting the fruit.

In Italy, fruits of bergamot are picked during winter (November—February) when the peel has turned yellowish and the fruit is fully grown but still unripe. Manual harvesting is still common, but tree shakers are occasionally used where possible. After picking, fruits are kept in the shade and quickly transported to the factory for processing.

The average annual fruit yield of bergamot in Italy is less than 12 t/ha with an essential oil yield of about 0.55%. In well-managed plantations yield per tree may reach 200—300 kg of fruit, yielding on average 300—600 g oil.

In Italy, bergamot oil is almost exclusively obtained by the 'Pelatrice' method, only a small amount is still produced by the traditional 'Calabrese' process. In the 'Pelatrice' process the essential oil is obtained by rasping the rind of whole fruits to cause the oil glands to break and release the oil. To remove the oil, rasping is done under water sprayers. The oil is extracted from the water by centrifugation. The residual peel is subsequently distilled, yielding a 'distilled essence' of slightly lower quality. The 'Calabrese' method is a 'sponge process', which gives low yields and requires much labour.

No extensive germplasm collections and breeding programmes for bergamot are known to exist.

Selection of cultivars of bergamot with a lower content or free of the toxin bergapten is important to improve the quality of the essence. Replacing the oldest groves with newly planted ones could increase yields and reduce production costs. However, the lack of relevant information on cultural practices, e.g. propagation, training, nutrition, irrigation, pest control and harvesting holds down production, therefore research in these fields is urgently needed. The growing interest in natural essential oils and the success of bergamot in tropical Africa justifies research into its suitability for South-East Asia.

Barone, E., Bounous, G., Gioffre, D., Inglese, P. & Zappia, R., 1988. Survey and outlook of bergamot (Citrus aurantium subsp. bergamia Sw.) industry in Italy. In: Goren, R. & Mendel, K. (Editors): Citriculture. Proceedings of the 6th International Citrus Congress, Middle East, 1988. Margraf Publishers, Welkersheim, Germany. pp. 1603—1611.
Chapot, H., 1962. Le bergamotier [The bergamot tree]. Al Awamia 5: 1—27.
Hodgson, R.W., 1967. Horticultural varieties of citrus. In: Reuther, W., Webber, H.J. & Batchelor, L.D. (Editors): The citrus industry. Vol. 1. University of California, Division of Agricultural Sciences, Berkeley, United States. pp. 489—494.
Huang, Y.Z., Chen, S.Q., He, C.Y., Chen, Q.Y. & Wu, Y.L., 1990. A study of chemical components of essential oils from Citrus bergamia and its close relatives and its taxonomy. Acta Phytotaxonomica Sinica 30: 239—244.
Huet, R. & Dupuis, C., 1967. L'huile essentielle de bergamote en Afrique et en Corse [The essential oil of bergamot in Africa and Corsica]. In: Essential oil production in developing countries. Tropical Products Institute, London, United Kingdom. pp. 79—95.
Mosandl, A. & Juchelka, D., 1997. Advances in the authenticity assessment of citrus oils. Journal of Essential Oil Research 9: 5—12.
Swingle, W.T. & Reece, P.C., 1967. The botany of Citrus and its wild relatives. In: Reuther, W., Webber, H.J. & Batchelor, L.D. (Editors): The citrus industry. Vol. 1. University of California, Division of Agricultural Sciences, Berkeley, United States. pp. 190—430.
Tanaka, T., 1954. Species problem in Citrus: a critical study of wild and cultivated units of Citrus, based upon field studies in their native homes. Revisio aurantiacearum 9. Japanese Society for the Promotion of Science, UENO, Tokyo, Japan. p. 112.
Weiss, E.A., 1997. Essential oil crops. CAB International, Wallingford, United Kingdom. pp. 459—464.

Sumeru Ashari

Ashari, S., 1999. Citrus bergamia Risso & Poiteau. In: L.P.A. Oyen and Nguyen Xuan Dung (Editors): Plant Resources of South-East Asia No 19: Essential-oil plants. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea