CITRUS FRUITS

Certain Citrus species, (including Fortunella) and especially hybrids, belonging to the family Rutaceae, are a most important group of tropical and subtropical fruits. Citrus species are evergreen shrubs or trees up to 10 m or more in height. They may bear spines or thorns and the leaf stalks may be winged or flattened. Most commercial production takes place in subtropical regions with a Mediterranean-type climate (latitudes 45°N–35°S). They do not grow well in the humid tropics. 

Most Citrus cultivars will only stand light frost for short periods. As regards commercial production, sweet orange (Citrus × aurantium Sweet Orange Group) (Citrus sinensis) is produced in greatest amount, then tangerines (mandarins) (C. reticulata), then lemons (Citrus × limon) (C. limon) and limes (C. aurantifolia), and finally grapefruits (Citrus × aurantium Grapefruit Group) (C. paradisi) and pummelos (Citrus maxima) (C. grandis). The fruit is known botanically as the ‘hesperidium’. The outer rind or peel consists of the coloured ‘flavedo’ and the inner white spongy ‘albedo’. In the immature state, the green pigment is chlorophyll; as the fruit matures this gives rise to the yellow or orange carotenes. Citrus fruits in the tropics oft en remain green, even in the mature state, but this can be changed with suitable gas treatment. Within the peel is the juicy pulp containing the seeds, although some cultivars are seedless because fruit development takes place without fertilization (‘parthenocarpy’). Individual Citrus seeds oft en contain more than one embryo (‘polyembryony’). As with other fruit types, Citrus fruits contain a very large amount of water (almost 90 per cent); the usual three sugars (sucrose, glucose, and fructose); a good deal of potassium but little sodium; some carotenes, B vitamins and vitamin E, but rich in vitamin C. The main acid is citric. Some dietary fibre is provided by these fruits.

Citrus fruits are used for dessert purposes, may be canned, and also provide juice (sometimes more important commercially than the fresh fruit). Essential oils (not to be confused with the triacylglycerol (triglyceride) oils described earlier in the book) are extracted from the peel (also the flowers and leaves) and used as food flavourings and in cosmetics. Pectin is also extracted from peel. Citrus fruit is used in marmalade production. Aft er juice extraction, the fruit residue is marketed as an animal feed. Citric acid is included in certain manufactured foods and was once extracted from citrus fruits, but is now produced by fermentation from maize (corn). The citrus group probably evolved in the Western Pacific region. Th e earliest records of cultivation are from China about 2200 bc. Many leading Asian cultivars were not imported into the western hemisphere until the nineteenth or twentieth centuries.

SWEET ORANGE  

Citrus × aurantium. Sweet Orange Group (C. sinensis). This is the most important of the Citrus fruits and is a tree growing to some 15 m in height. It perhaps originated in southern China as a hybrid between C. maxima and C. reticulata and was taken to Europe in the fifteenth century. Columbus carried orange seeds on his second voyage to America in 1493. Sweet orange is grown throughout the subtropics and tropics, but Brazil and the United States of America produce the greatest quantities of this fruit. In these two countries, the bulk of production is used to manufacture orange juice. Spain is the world’s largest exporter of fresh oranges. The species may be classified into:
1. Common orange — a well-known cultivar is ‘Valencia’.
2. Blood orange, the flesh having a blood-red tint. The pigment responsible for this tint is anthocyanin. A well-known cultivar is ‘Maltese’.
3. Navel orange, known because of a navel-like mark at its apex. A well-known cultivar is ‘Washington Navel’ (=‘Baia’).

Sweet oranges contain 6–9 per cent total sugars and 44–79 mg vitamin C/100 g. In addition to the fresh fruit, orange juice is also a good source of sugars, vitamin C, and potassium.

SEVILLE, BITTER, OR SOUR ORANGE 

Citrus × aurantium. Sour Orange Group (C. aurantium). The plant bears fruits with a bitter taste (the bitter compound is ‘neohesperidin’) which makes them unattractive for eating raw. The hybrids between C. maxima and C. reticulata originated perhaps in southern China and were introduced into Europe in the eleventh century, well before the sweet orange. Spain is the major producer of this fruit, although it has spread to many tropical and subtropical countries. The fruit is used to make the conserve, marmalade, and as a flavouring, and in liqueurs (Curaçao). Seville orange has been used as a rootstock for lemon, sweet orange, and grapefruit, but it is susceptible to the virus disease tristeza.

LEMON  

Citrus × limon (C. limon). This is a small tree 3–6 m in height, with fruit that is too acid to be consumed as a dessert but is of great importance in providing juice for culinary and confectionery purposes, also drinks (e.g. lemonade). The candied peel is a food constituent. The hybrids between oranges C. aurantium and C. medica probably arose in tropical Asia but the exact region is not known. Today, the United States and Italy produce most lemons, but other important producers are Argentina, Brazil, Spain, and Greece. The fruit has only about 3 per cent total sugars but a high content, nearly 5 per cent, of citric acid; there is 58 mg/100 g of vitamin C. Wellknown cultivars are ‘Eureka’, ‘Lisbon’, and ‘Villafranca’. Rough lemon is used as a rootstock for other Citrus species. Essential oils are prepared from the fruits (also the leaves and flowers); bergamot oil comes from C. x limon Bergamot Group C (C. bergamia).

GRAPEFRUIT

Citrus × aurantium. Grapefruit Group (Citrus paradisi). This is a tree up to 15 m in height. It arose in the West Indies as a hybrid (backcross) between pummelo (Citrus maxima), and an orange, or as a mutation from pummelo. Grapefruit was described in Barbados in 1750. Today, the major producing country is the United States of America, but it is also produced in other countries such as Israel, the West Indies, Cuba, Argentina, and South Africa. Grapefruit is generally a breakfast fruit, segments are canned and juice is a commercial item. It is unique among the citrus fruits in that it stores well on the tree and may be harvested all the year round. Cultivars are white, pink, or red-fleshed, also with seeds or seedless. Some cultivars are ‘Marsh’ (seedless and white flesh), ‘Duncan’ (seeded and white flesh), ‘Thompson’ (seedless and pink flesh), and ‘Ruby’ (seedless, deep flesh colour, reddish peel). The flesh pigment is a carotene—‘lycopene’. The fruit has a total sugar content of about 7 per cent and a citric acid content of about 1 per cent (similar to orange) but there is a bitter compound present—‘naringin’. Vitamin C is present at 36 mg/100 g—rather low in relation to other citrus fruit.

LIME  

Citrus aurantifolia. This is a small, much-branched tree up to 5 m in height. It probably originated as a hybrid between C x limon and C. hystrix in tropical Asia, but has now spread throughout the tropics, where it takes the place of lemon. The plant is the most frost sensitive of the commercial Citrus species. Its fruit is small, up to 6 cm in diameter, and is greenish yellow when ripe. Some countries involved in commercial production of the fruit are Brazil, Egypt, Mexico, and the West Indies. Like lemon, it is not a dessert fruit but it is used in marmalade, as a food flavouring (e.g. chutney, pickles, and sauces), and a source of juice for drinks; oil is prepared from the peel. Its total sugar content is only 0.8 per cent and the fruit is somewhat more acid than lemon. In earlier times, limes were carried on sailing ships to be consumed as a preventative measure against scurvy. Its vitamin C content is 46 mg/100 g.

MANDARIN OR TANGERINE  

Citrus reticulata. A small tree, up to 8 m in height, this is the hardiest of the Citrus species. It is probably wild in tropical China. Classifi cation of forms within the species is difficult but one view is that the species includes ‘mandarins’, ‘tangerines’, ‘satsumas’, ‘King mandarins’, and ‘willow-leaf mandarins’. One possible distinction between the common mandarin and tangerine is that the former is yellow-fruited, the latter deep orange. In southern Africa, the Afrikaans name haartje is oft en used. The species is cultivated in many countries but major producing areas include Japan, Brazil, the United States, and Mediterranean countries (it was introduced into Europe at the beginning of the nineteenth century). The fruit is consumed as a dessert and the segments are canned. Its peel or rind is easy to remove from the flesh. Total sugar content is 8 per cent and vitamin C is 30 mg/100 g. ‘Calamondin’ (Citrus × microcarpa) is a hybrid, originating in China, of mandarin and kumquat (C. japonica). Its fruit has a number of food uses and is cultivated commercially in the Philippines.

CITRON  

Citrus medica. This is a small tree or shrub up to 3 m in height. It probably originated in the sub-Himalayan region of northeastern India and Upper Burma, spreading through Persia to the western world and also eastwards to China. It was the first of the Citrus species to reach Europe, about 300 bc. Although it has been cultivated in many tropical countries, commercial planting is restricted to certain Mediterranean islands of Italy, Greece, and France, and in the mountainous coff ee regions of Puerto Rico. The fruit, 10–20 cm long, is elongated with a lumpy surface, thick peel, and mildly acid or acid flesh. Its most important use today is the production of candied peel for confectionery and cakes. For this purpose the green immature fruit is sliced into halves, fermented in brine, and soaked in strong sugar solution. The ‘Etrog’ cultivar is used in the Jewish Feast of Tabernacles; another cultivar is the ‘Fingered’ citron, cultivated in China and other eastern countries, with a fruit split into a number of finger-like sections and used as a perfume source and medicine.

KUMQUAT  

Citrus japonica. The two most important cultivars were formerly referred to as species (Fortunella margarita and F. japonica). The species was native in China but is now cultivated not only in that country, but also in Japan, Taiwan, and in a number of other countries, for example Argentina, Brazil, Cyprus, and the United States. Kumquat is a shrub or small tree (2–4 m tall) with small (1–4.5 cm in diameter) ovoid or round orange, or golden-yellow fruit. The fruit can be eaten fresh (the skin is edible), cooked, or made into chutneys, marmalades, jellies, or preserved in syrup or candied. Kumquats have quite a high total sugar (about 9 per cent) and the vitamin C content is 39 mg/100 g.

CLEMENTINE

Citrus reticulata ‘Clementine’. This is often regarded as a cultivar of tangerine, or possibly a hybrid between tangerine and sweet orange. Its peel is easily removed. It may have originated in North Africa but is also cultivated in other Mediterranean countries and South Africa. The total sugar is about 9 per cent with a predominance of sucrose. Its vitamin C content is quite high—54 mg/100 g.

UGLI OR HOOGLY

This rather misshapen Citrus fruit is a cross between tangerine and grapefruit. Its flesh is sweeter than most grapefruits. It is grown in and exported from Jamaica.

OTHER CITRUS FRUITS 

The ortanique (Citrus × aurantium) is a cultivar of a hybrid (backcross) between tangerine and sweet orange. It resembles the sweet orange in size and juice content but, like the tangerine, has a thin, easily peeled skin. It is exported from Jamaica. The fruit has a high percentage of total sugar (almost 12 per cent) and a high content of vitamin C (50 mg/100 g).

The pummelo, pomelo, or shaddock (Citrus maxima) (C. grandis) is quite a well-known Citrus crop of South-East Asia. Its yellowish fruit is the largest (10–30 cm in diameter) of the Citrus group. It was introduced into Barbados in the seventeenth century by a Captain Shaddock and was the ancestor of the grapefruit. The fruit is exported from Israel.

Papeda, wild kaffir, leech, makrut-lime (Citrus hystrix) is grown in Malaysia, Sri Lanka, and Burma. It is a small tree (3–5 m tall) bearing yellow, very wrinkled and rough fruit. The fruit is not eaten fresh but used as a food flavouring.

Many hybrids have been created between various Citrus species, also kumquat: tangelos (Citrus × aurantium Grapefruit Group backcrossed with C. reticulata), for example ugli; tangors (Citrus × aurantium Sweet Orange Group backcrossed with C. reticulata), for example ortanique; and limequats, orangequats, and citrangequats (hybrids between kumquat and various Citrus hybrids).

By J. G. Vaughan & C. A. Geissler in "The New Oxford Book of Food Plants", published in the United States by Oxford University Press Inc., New York, 1999, excerpts p. 94-99. Adapted and illustrated by Leopoldo Costa. 

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