Citrus paradisi Macf. (Grapefruit)

The genus Citrus (family Rutaceae), produces coumarins, flavanones, flavones, and flavonols which occur in the free form and/or as glycosides, in addition to an extremely complex group of compounds which are volatile and are responsible for determining the characteristic aroma of each Citrus variety.

Distribution and Cultivation Zones

Grapefruit (Citrus paradisi Macf.) is an important Citrus crop. Its origin is unknown, although recent studies point to a natural crossing between pummelo (Citrus grandis) and sweet orange (Citrus sinensis), in which the first parental species is the dominant genotype. The first written description is by Hughes, who in a journey to Barbados in 1750 refers to it as “the forbidden fruit” (Webber 1943).

Annual world production of grapefruit is 4.6 million metric tons, representing 7% of total citric produce, of which 89% (4.1 million metric tons) is produced in the Northern hemisphere, principally by the United States followed by Israel, Cuba, China, Cyprus, Mexico, Tunisia, Turkey, Spain, Morocco, Italy, and Algeria. Less than 5000001 (average of FAO figures for 1987 to 1990; FAO 1991) is produced in the Southern Hemisphere by Argentina and South Africa followed by Australia and Brazil.

Conventional Propagation Practices

For commercial growing Citrus paradisi plants are obtained by vegetative propagation, mainly by grafting. Among the most widely used rootstocks (propagated by seeds) for grapefruit grafting has been sour orange (Citrus aurantium L.). However, because of its susceptibility to Quick Decline, it is gradually being replaced by Troyer Citrange (a hybrid of Poncirus trifoliata L. Raf. and Citrus sinensis L. Osbeck), Carrizo Citrange (Poncirus trifoliata L. Raf.), and Cleopatra Mandarin (Citrus reshni Hort. ex Tan.).

Importance

Citrus paradisi accumulates naringin as the predominant flavanone glycoside in its fruits, leaves, and juice. To a lesser extent, it also produces narirutin, prunin, hesperidin, and neohesperidin.

Commercial interest in these compounds is great: pharmacologically, hesperidin has been shown to act in vascular permeability and naringin to behave as an antioxidant, protecting against lipid peroxidation, and antimutagenic activity.

Furthermore, naringin can be used as an alternative to caffeine or quinine in tonic beverages and other nonalcoholic drinks and produces a distinctly bitter taste at low concentration (usually 1 part naringin in 50 000 parts water). Naringin and neohesperidin also have another important industrial application in that they can be chemically converted into their corresponding intensely sweet dihydrochalcones, with values relative to sucrose of 1000 for neohesperidin dihydrochalcone and 300 for naringin dihydrochalcone.

In addition, although components such as the volatile aldehydes and several acetate esters are important to the flavor and aroma of grapefruit oils, sesquiterpene nootkatone has also been suggested as a major flavor-impact compound in this species, and is widely used in commercial flavors and fragrances.

Grapefruit seed oil, registered as DF-100, is also used commercially for its antimicrobial properties.

Demand in the World Market

The spot price for grapefruit oil in 1988 was $10/lb. Its primary use is in beverage flavoring, followed by perfumery, where it is used often as a base for artificial bergamot due to natural bergamot’s higher price and greater susceptibility to phototoxicity. The main sources of grapefruit oil are Israel and Belize.

Nootkatone is obtained from grapefruit oil by distillation. Its price in March 1988 was $4200/kg.

Apart from pectins, the flavonoid naringin is also obtained from grapefruit peel. Annual world consumption is approximately 100000 kg, its price varying from $30-40/kg depending on its purity. The principal producers are Israel, the United States, Spain, and Brazil.

Naringin, as well as being directly used in preparations, can be converted into the sweetener neohesperidin dihydrochalcone (which can also be obtained from neohesperidin) whose price (1993) is around $500/kg with an annual consumption of approximately 3000 kg.

The antimicrobial properties of grapefruit seed oil (DF-100) mean that it is widely used in canned meats, canned vegetables, jams, juices, lacteous products, etc. Its price (1993) is around $300/kg.

Conclusions and Prospects

Callus cultures of various Citrus species are capable of synthesizing nootkatone and valencene. Although the amounts of these metabolites produced by the callus cultures are < 2% of those found in the exocarps of the corresponding mature fruits, the fact that these plant cells express this secondary metabolism is considered important. In agreement with these results, other authors have described how callus cultures of various Citrus species accumulate small quantities of other terpenoid compounds.

The capacity of C. paradisi callus cultures to accumulate nootkatone is expressed only after a certain age and this might be associated with the development of morphological differentiation in the cells of these calli, as has been suggested for monoterpenes. Our results show clear differences between the cellular characteristics and the production capacity of the outer and inner zones of the callus. The greater capacity for nootkatone production of the cells of the inner zone of the callus can be clearly observed. In this zone, the cell wall is thinner and even disintegrates at some points, generating lysigenous intercellular spaces and a lesser cytoplasmic density when compared with the cells of the outer zone. These cellular characteristics bring to mind the differences detected by Bosabalidis and Tsekos in the peripheral cells of the oil gland and the inner secretory cells, respectively, in C. deliciosa fruits, suggesting that the cellular tissues present in our callus cultures have a similar integrated cellular function.

Finally, the results expressed here reveal the bioproduction of naringin, as the principal flavonoid, and narirutin in callus cultures of Citrus paradisi, similar to that which is found in developing fruit. In contrast to these results, other authors have suggested that Citrus callus culture has apparently an altered flavonoid pathway. We think that these discrepancies might be related to the morphological characteristics of the respective calli, since the degree of expression of these compounds is related to certain steps of cellular growth, and is generally associated with very young tissue such as that found in immature fruits. This is in accordance with the results obtained by us for different parts of Citrus aurantium () and in other Citrus sp..

Preliminary results suggest that the expression of both the sesquiterpenes and the flavonoids in the callus cultures can be modified by either accelerating the processes of cell differentiation or by prolonging the juvenile stages of cell growth.

Selections from the book: Medicinal and Aromatic Plants VII (1994).