Citrus Fruit

Citrus fruits are native to southeastern Asia and are among the oldest fruit crops to be domesticated by humans. They are widely grown in all suitable subtropical and tropical climates and are consumed worldwide. The most important of the citrus fruits commonly eaten include sweet oranges, mandarins, lemons, limes, grapefruits, and pummelos. These are eaten fresh, juiced, and in processed products. Citrus fruits have well-documented nutritional and health benefits as well as industrial uses. Their beauty and utility were well described by Georges Gallesio in 1811:

Of all the plants spread by nature upon the surface of the globe, there are none more beautiful than those we know under the names of citron, lemon, and orange trees which botanists have included under the technical and generic name Citrus. These charming trees are both useful and ornamental. No others equal them in beauty of leaf, delightful odor of flowers, or splendor and taste of fruit. No other plant supplies delicious confection, agreeable seasoning, perfume, essences, syrups, and the valuable aides so useful to colorers. In a word, these trees charm the eye, satisfy the smell, gratify the taste, serving both luxury and art and presenting to astonished man a union of all delights. These brilliant qualities have made the citrus a favorite in all countries.

Botany

Citrus fruits constitute several species of the genus Citrus of the subfamily Aurantiodeae of the plant family Rutaceae. The Aurantiodeae has a total of thirty-three mostly subtropical and tropical genera, a few of which have economic importance. Most genera originated in Southeast Asia: the Malaysian and Indonesian Archipelagos, the Indochinese Peninsula, India, and China. A few genera originated in Australia or Africa.

The citrus fruits proper are characterized by their distinctive fruit, the hesperidium, which is a berry with the internal fleshy parts divided into segments (typically 10 to 16) and surrounded by a separable skin. The name is derived from classical mythology, referring to the "golden apples" grown in the garden of the Hesperides (the daughters of Hesperus, the evening star), located in the far west, in Paradise. When grown naturally, citrus plants are generally small to large trees, with glossy alternate leaves having oil glands. The attractive and fragrant flowers have an annular disk and generally bloom in the early spring.

The genus Citrus is divided into two subgenera, Citrus and Papeda. The former contains "edible" citrus fruits (including some less than palatable varieties), while the latter consists of the papedas. These are a distinctive group, the fruits of which have high concentrations of droplets of acrid oil in the pulp vesicles, rendering them inedible due to the bitter, unpleasant flavor. The leaves are also distinctive as compared to those of the subgenus Citrus, having large, prominent petioles. The leaves of one species, Citrus hystrix, are used as a condiment in Southeast Asian cooking.

The taxonomy of Citrus, as a genus, is unclear. So, for that matter, is the taxonomy of the other thirty-two genera in the subfamily. Different authorities have recognized anywhere from 3 to 170 species of Citrus. Obviously, this large a difference is due to more than mere hair-splitting. The most commonly used systems, that of W. T. Swingle (see Reuther, Webber, and Batchelor, pp. 190–430) or its modifications, recognize about sixteen species.

Most of the difficulties in Citrus taxonomy arise due the free hybridization that can occur between different varieties and even between different species of Citrus. In fact, many of the other Aurantiodeae genera are capable of free hybridization with Citrus. The result is that many types of citrus arose from these hybridization events at some time in the past. A strict interpretation of the "species" concept would result in fewer types being awarded species rank, whereas a looser interpretation would result in a higher number of species. This problem is compounded by the numerous controlled hybridizations and selections made by humans with the goal of producing a more desirable fruit.

Currently, the generally accepted concept is that there are three primordial or fundamental Citrus species: Citrus medica (citrons), Citrus maxima (pummelos), and Citrus reticulata (mandarins). All other types of citrus currently existing arose from single or sequential hybridization events between these species or their offspring. This concept is supported by various types of studies: classical taxonomy, chemotaxonomy, and molecular analysis.

Edible citrus is generally divided into sweet oranges, sour oranges, mandarins, grapefruit, pummelos, lemons, limes, and citrons. Within each of these types there are various subtypes, as well as types that arose from free or controlled hybridization. Depending on the taxonomic system used, these subtypes and hybrids may or may not be accorded species status. Table 1 presents some basic information on these standard types of edible citrus. Some of this information is not strictly accurate in a scientific sense: Oroblanco is actually a hybrid of a pummelo and a grapefruit, but it is generally marketed as a grapefruit; Meyer is probably a low-acid natural hybrid of a lemon and a sweet orange, but it is usually marketed as a lemon; Mediterranean Sweet is probably more accurately referred to as a "limetta" rather than a sweet lemon. However, the idea is to present the reader with some general information on fruits that might be encountered and eaten. Some types (low-acid sweet oranges, sour oranges, citrons) are not of much importance commercially, while others (sweet lemons, sweet limes) are important in some regions of the world but not in others.

There are four types of sweet oranges. The navel oranges possess a small, secondary fruit in the stylar end of the main fruit. This is the navel. Since navel oranges were introduced to California from Brazil in 1873, they have assumed a primary importance throughout the world as a sweet orange for fresh consumption. Like the navel oranges, common sweet oranges mature during the winter. Because of the popularity of the navel as a fresh fruit, common oranges are generally grown mostly for processing, although they can be important locally as fresh fruits. Blood oranges have a pigment called anthocyanin in the rind and juice, producing a reddish blush that becomes more pronounced with cooler night temperatures in the fall. The blood oranges have a distinctive taste compared to other sweet oranges. Valencia oranges mature later than the other sweet oranges and are generally harvested in the late spring or summer. Low-acid oranges have about the same levels of sugars as regular sweet oranges, but much lower levels of acid, resulting in a rather bland flavor.

Mandarins are often referred to somewhat incorrectly as tangerines. The word "tangerine" was used in the nineteenth century to designate Mediterranean types of mandarins, and referred to the city of Tangier. This term later became associated with other types of mandarins. Mandarins are of ancient cultivation in China, their probable area of origin, and other parts of Asia. The common mandarins include such important varieties as Ponkan, which is widely grown in Asia under different names. The Satsumas are a distinctive, seedless, early maturing group apparently originating in Japan relatively recently as compared to the common mandarins. The Clementines are another distinctive group that apparently originated in Algeria as recently as the 1890s. There are now many different selections of Satsumas and Clementines. The hybrids of tangelo (crossed with pummelo or grapefruit) and tangor (crossed with sweet orange) are included here as types of mandarins since they are generally thought of by the public as being more like mandarins than the other parent.

Sour oranges are not often eaten as fresh fruit or used for processing. However, the fruit is used to produce marmalade, and the flowers of certain types are used in the production of perfume. The Bergamot, a sour orange hybrid, has a distinctively scented oil that is used in

teas as well as perfume. Sour oranges often make attractive ornamentals.

Pummelos are generally large fruit that originated in more tropical areas than most other types of citrus. They are commonly grown in southeastern Asia, where consumption is the highest. Pummelos are not eaten much outside of that area. The pummelos are a very diverse group, with large variations in size and shape, rind, flesh pigmentation, and acid level.

Grapefruit is another natural hybrid (probably pummelo crossed with sweet orange) arising relatively recently (in the eighteenth century). In the twentieth century, it became widely planted and was used for both fresh fruit and processing. White-and pink-fleshed varieties exist. The pink-fleshed varieties derive their color from the pigment lycopene and require high heat levels for good color development (in contrast to the blood oranges).

Lemons have not been identified as a wild species, and probably arose sometime in the remote past as a cross between a citron and a sour orange (itself probably a hybrid of pummelo and mandarin). Lemons are rather variable and it is sometimes difficult to distinguish between different varieties and types. Low-acid, sweet lemons also exist. The limettas are similar to and more common than sweet lemons and are often referred to in this manner.

Limes are somewhat similar to lemons in appearance and ancestry, and the distinction between the two groups is not always clear. Like lemons, low-acid, sweet types exist. The acid lemons are generally divided into the large, fruited types (generally seedless) and the small-fruited, "Key" types.

Citrons were probably the oldest citrus fruit to be cultivated in the West, but today they are not widely grown. Citrons are a highly variable group including acid and sweet varieties, but to the general public they often resemble large lemons. Citrons are sometimes used in the production of a candied peel and in Jewish religious ritual. The citrons are aromatic and are occasionally grown as ornamentals.

The kumquats are not, strictly speaking, citrus fruits. They are, however, in the genus Fortunella, which is closely related to Citrus in the Aurantioideae. Kumquats are distinctive in that they have small fruits with a sweet, edible peel. The trees are small and attractive and they are generally grown as backyard trees rather than commercially. The most important varieties of kumquats are Nagami, Meiwa, and Marumi.

Natural History and Spread

As well stated in Reuther, Webber, and Batchelor, "The history of the spread of citrus reads like a romance. Even in very early times the beautiful appearance of both tree and fruit attracted the attention of travelers and received mention in their written narratives" (p. 1).

Citrus is native to and has its center of diversity in northeastern India, southern China, the Indochinese peninsula, and nearby archipelagos. A theoretical dividing line (the Tanaka line) runs southeastwardly from the northwest border of India, above Burma, through the Yunnan province of China, to south of the island of Hainan. Citron, lemon, lime, sweet and sour oranges, and pummelo originated south of this line, while mandarins and kumquats originated north of the line. The mandarins apparently developed along a line northeast of the Tanaka line, along the east China coast, through Formosa, and to Japan, while kumquats are found in a line crossing south-central China in an east-west direction.

The cultivation of citrus began in ancient times in these areas. In fact, citrus was one of the earliest crops to be exploited and domesticated by man. Probably cultivation of citrus began independently in several locations within the area of origin and spread throughout the Southeast Asian region, and eventually into the Middle East, Europe, and America.

The oldest mention of citrus fruits known is from China, in the Yu Kung, a book of tributes to the Emperor Ta Yu, who lived from about 2205 to 2197 b.c.e. This book mentions the use of various types of citrus as tributes to the emperor. Later writings describe other types of mandarins, sweet oranges, pummelos, and kumquats. The monograph on citriculture written in 1178 C.E. by Han Yen Chih mentions twenty-seven varieties of citrus. The earliest mention of citrus in Indian writings is from about 800 b.c.e. in a collection of devotional texts, the Vajasaneyi samhita. This text mentions citrons and lemons. Sweet oranges are not mentioned in Indian writing until about 100 C.E.

The sweet orange probably arose in southern China where both mandarins and pummelos were planted together. From there, it spread through Burma and Assam into India. Much the same route was probably followed by the mandarins. Mandarins also spread into Japan. This probably occurred in the middle of the first millennium C.E., but the first mention of mandarins in Japanese literature dates from the thirteenth century.

Conversely, the citron probably originated in northern India and spread northward into China later. The citron also spread from India westward to Medea (Persia) by the first millennium b.c.e., and then into Palestine and the Near East. It is supposed that it was brought to this area by Alexander the Great. The citron became established in Italy during Roman times. The sweet and sour oranges, lemons, and pummelos followed this route at a later date.

The Arabs were instrumental in introducing most of the citrus types to Europe and northern Africa. The invasion of southern Europe by the Moors introduced citrons, sour oranges, lemons, and pummelos to the Iberian Peninsula, which is still an important area of citriculture. However, the sweet orange was apparently not established in Europe until the fifteenth century C.E. This was probably due to an entirely different route by Portuguese trade with southern Asia. The mandarins were apparently not introduced to Europe until early in the nineteenth century, when they arrived directly from China. Kumquats were introduced from China in the middle of that same century.

Citrus can be, and is, grown in southern Europe. That citrus represented a new and appealing type of fruit and had more exacting climatic requirements created a sort of cult of citrus in the more northern areas of Europe that persists to this day. Since citrus cannot be grown outdoors in such areas as the British Isles, northern France, and Germany, special houses (later known as orangeries) were in use by the fourteenth century for growing oranges and citrons. Some of these structures, which can be considered precursors to modern greenhouses, are still standing. In some cases, the citrus overwintered in the orangeries and were brought outdoors to enjoy the brief and mild summers and to enchant the public.

Citrus was carried to America by the Spanish and Portuguese colonizers beginning in the sixteenth century with the second voyage of Columbus in 1493. From its initial establishment in the Caribbean islands, it spread to the mainland (Mexico) and from there into the southern United States and Latin America. Citrus was introduced into Florida earlier than into California. Citrus was introduced separately into Brazil by the Portuguese, who were also responsible for the introduction of citrus into West Africa. It had apparently been introduced to the African continent earlier by Arab or Indian traders. Citrus was introduced in Australia from Brazil in 1788 by the colonists of the First Fleet.

The World Citrus Industry

In the New World, as in the Old, wherever citrus was introduced it became a popular fruit. If climatic conditions were appropriate, citrus was planted for commercial and for personal use. It remains the most widely planted fruit, except for grapes, in the world today. Most grape production is for winemaking, so citrus is undoubtedly the most widely planted fruit for direct human consumption in the world.

Citrus is grown throughout the world in the "Citrus Belt" between approximately 40°N and 40°S latitude. Within this belt there are tropical, semitropical, and subtropical climates, and it is possible to grow citrus in all three. Although there is some influence of scion and rootstock in cold susceptibility, frost is the main climatic limitation to citrus production. At the northern and southern margins of production (Corsica, Japan, New Zealand), the mildness and shortness of the summers is a secondary constraint. In areas that have a Mediterranean climate, which has a long, dry summer, supplemental irrigation is necessary.

The majority of commercial production is in the subtropical regions between 20^o and 40^o northern and southern latitudes. In the tropics, flowering is often erratic, and fruit may mature throughout the year. Although fruit size is generally large in the tropics, fruit quality is usually lower. Fruit color is generally less intense and acids may be too low for good eating quality. Yellow-fleshed and high-acid types (lemons and limes) are not as affected by these factors and are widely grown in the tropics, as are pummelos. Although there is less large-scale commercial production in the tropics, citrus is important locally and when grown for personal consumption.

In the subtropical areas, the yearly cycle of flowering and fruit development, as well as vegetative growth, is more tightly regulated by climatic conditions. This results in a crop that matures at the same time and has higher fruit quality. Semitropical conditions are intermediate between tropical and subtropical conditions. These areas, which include such major production areas as Brazil and Florida, produce high yields of citrus that is of acceptable quality. Fruit quality for fresh consumption is lower than in subtropical climates such as California and Spain, but most fruit produced in Brazil and Florida is grown for processing, which has slightly lower-quality standards.

Within these climatic types, there are some variations in types of citrus successfully produced. For example, varieties that are colored by lycopene, such as the pigmented grapefruits, do well in these semitropical climates, while those colored by anthocyanins (blood oranges) do better in areas with lower winter temperatures. In marginal areas such as Japan and New Zealand, early maturing varieties such as Satsumas are grown.

Brazil has been the largest producer of citrus for some time, followed by the United States. Other important producing countries include China, India, Spain, Morocco, Argentina, Italy, South Africa, Australia, Mexico, and Egypt. The relative ranking of these countries varies from year to year. Recently, there has been much interest in large-scale production in countries such as China and India, where the climate is suitable and labor and infrastructure inexpensive. There are many niches within the world citrus production. For instance, in the United States, Florida produces a large proportion of the sweet oranges, the majority of which are used in processing. California produces a higher quality sweet orange, with emphasis on navel varieties, which is eaten fresh and largely exported. Countries such as Spain and Morocco produce large quantities of mandarins for export to the United Kingdom and northern Europe. Some of the Southern Hemisphere countries export to major Northern Hemisphere producers during the off-season.

As with any industry, there have been changes over the years. In the last decades of the twentieth century, the trend has been toward increased global trade and yearlong availability in most major markets. Processed products have grown in importance. In fresh fruit, the trend has been toward easy-peeling, seedless mandarins and sweet oranges. In grapefruit, the pigmented varieties are becoming predominant.

Production

Citrus is produced in slightly different ways in different areas. Commercial production is more uniform throughout the world than is local or personal production, but there are some differences here as well. Many of the differences are in the nature of farming inputs rather than the production of trees. For instance, fertilization and irrigation are necessary in most areas. However, a more industrialized producer in an exporting country may utilize drip irrigation with inorganic fertilizers injected through the drip system, while a producer for the local market in a poor country or area may use manure and flood irrigation.

Citrus can be grown from seed; however, there are some disadvantages. In some cases, seedlings are not true-to-type with the mother tree; due to juvenility factors, seedling trees do not usually bear fruit until they are nearly a decade old; and they are vulnerable to unfavorable soil conditions, diseases, and so forth. For these reasons, most citrus produced throughout the world utilizes budded (grafted) trees.

A budded tree consists of two parts: the scion, which is the fruit variety, and the rootstock, which supports the scion in the soil environment. Rootstocks are chosen based on a number of factors, including compatibility with the scion, resistance to diseases or pests, adaptation to soil conditions, effect on fruit quality. Citrus root-stocks can be grown from seed, since the commonly used rootstocks are apomictic (and hence true-to-type), and there are no confirmed seed-transmitted systemic diseases of citrus. Production from seed is easier than from cuttings, the common method of production for root-stocks for most other tree crops.

The rootstock is usually of an appropriate size for budding about nine months to a year after germination, when it is about the diameter of a wood pencil. The scion variety is budded onto the rootstock by making an incision into the bark of the rootstock, inserting a bud removed from the scion variety, and wrapping it with tape. A callus should form between the rootstock and scion tissues in two to four weeks. With appropriate training, the young tree is ready for planting in the field in about another year.

Once planted, it is usually about two to three years before the tree begins to produce fruit. Full production is usually achieved at about ten years of age. Under appropriate conditions, citrus trees may live a long and productive life and achieve a fairly tall height. This was common in many older citrus-producing areas. Since about the 1970s, citrus production has become more cyclical, like that of other tree crops, and the life of an orchard may be no more than twenty to thirty years.

Citrus requires relatively little cultural manipulation compared to crops such as grapes and deciduous trees, which require pruning and extensive training. In some areas, however, such as the Mediterranean basin, mandarins and sweet orange may receive somewhat more manipulation than in areas such as California. Lemons grow vigorously upright and require more frequent topping. Irrigation and fertilization are necessary. Certain production problems or challenges in citrus have been successfully managed with the application of plant growth regulators. This is more established in citrus than in most other perennial crops.

In contrast to the relatively low cultural inputs for citrus, disease and pest management in this crop is more critical and challenging than for many others. Because citrus is grown in warm areas of the world, reproduction of insect pests is rapid and insect pressure can be great. The individual insect pests vary greatly with geographic area. Compared to other crops, citrus is also subject to a larger number of systemic, graft-transmissible diseases caused by virus and viruslike pathogens that can potentially devastate industries. The most important worldwide is the tristeza virus, which destroyed many thousands of hectares in California and South America starting in the 1930s. This has been managed in some areas by certification programs requiring the use of virus-tested propagative materials and in a few cases with eradication programs. Other diseases, such as greening and citrus variegated chlorosis, are equally deadly but less widespread throughout the world.

Citrus is harvested by hand. At this point, there have not been any widely accepted methods of mechanical harvest. The time of harvest is dictated by the market or in some cases by legal maturity standards. Citrus is more forgiving than some other crops in that harvesting can be delayed somewhat and fruit quality is not decreased too much by the extra time on the tree. This varies with variety. However, if fruit are left on the tree too long, quality deteriorates as acid levels decrease and the taste becomes insipid. Other fruit quality problems can also occur. After harvest, citrus can be stored at low (refrigerated) temperatures for several months. This has had important implications in the development of the industry since the beginning of the twentieth century.

After harvest, commercial citrus is transported to a packinghouse. There, the fruit is washed, sorted and graded, treated with fungicides and waxes, and packed. In some cases, ethylene gas treatment is used for degreening. Citrus packing today is highly automated in some ways, with various sensors and other devices routing and sorting the fruit through a complexly routed pathway of conveyor belts, and bins. However, there is still a substantial amount of hand labor necessary for sorting, grading, and movement. After citrus is packed, it is transported away from the packinghouse and enters wholesale and retail market channels.

In addition to commercial production, citrus is widely grown for personal use in "door yards," roadsides, small subsistence plots. Growing citrus for this use is extremely variable. Trees are grown from seed, are grafted by the grower, are purchased from commercial sources, and so forth. Varietal selection is based on personal preference rather than economic factors. Citrus is also prized for its ornamental value and often serves a decorative purpose as well. There are some cultivars that are grown strictly as ornamentals, such as some variegated types and the Buddha's Hand citron.

Use of Citrus Fruits

Citrus is consumed fresh, juiced, and processed. The most nutritious ways of serving citrus are as fresh fruit or fresh-squeezed juice. Citrus fruits are well known for their vitamin C content, but are also good sources of vitamin A, folic acid, and dietary fiber. Nutritional profiles of some citrus fruit and fresh juices are shown in Table 2.

Fresh citrus fruits can be stored for several days at room temperature or for several weeks in the refrigerator. Fresh-squeezed juice should be stored in the refrigerator and is stable at refrigerator temperatures for several weeks from a nutritional standpoint. However, there is often a loss of quality when fresh-squeezed juice is stored. This is especially true of navel orange juice.

Processing is an important part of the citrus market worldwide. Two of the major producers of citrus, Brazil and Florida, produce fruit predominantly for the processing trade. Low-grade and excess fruit from fresh market production may also be routed into processing. Where production is oriented toward producing citrus for processing, different varieties and to some extent different cultural practices are employed than when grown for fresh market. Criteria for harvest and quality standards are also different. Internal quality is paramount for processing citrus, whereas external appearance counts for more in fresh market fruit.

The most important use of citrus for processing is the production of frozen concentrated orange juice. The production of this is different from but equally as complex as packing fresh fruit. After fruit enter the plant, they are washed, juiced with a press or extractor, and strained to remove peel and rag. The juice then goes to the finisher, where excess pulp and essential oils are removed from the juice. It is then concentrated by an evaporator.

The bulk concentrate is stored in tank farms and transported in refrigerated trucks, train cars, or ships. The bulk concentrate is packaged into consumer-or industrial-sized containers. Frozen concentrate orange juice is sometimes reconstituted into "single strength orange juice." This is also produced directly without first being concentrated. Processing has little effect on the nutritional value of orange juice, but there is generally a loss of palatability. Grapefruit juice and to a lesser extent lemons, limes, and mandarins are processed similarly. These products are sometimes used to blend with other types of fruit juice or for the production of frozen confections. By-products of processing include essential oils and pulp. The latter is used for cattle feed. Other processed products of citrus include canned segments, segments in juice, pectin, jellies, and jams. Peel products are used for animal feed, marmalade, and shaved peel.

In addition to processing for juice and its associated by-products, there are minor industrial uses of citrus. Although essential oils are extracted as part of juice processing, in some instances trees are grown specifically for the production of these oils. The center of this industry is Calabria, Italy, and the main variety used is Bergamot, of which there are various selections. Citrus is also used to produce pesticides, cleaning products, and hair care products.

Health Benefits and Traditional Usage

In addition to the nutritional value and vitamin content of citrus, there are certain health benefits associated with some of the secondary products. For instance, various limonoid compounds, particularly D-limonene, have been shown to reduce tumorgenesis under experimental conditions. Carotenoids, such as lycopene, have been associated with decreased risks of heart attacks as well as general antioxidant activity. The high pectin content of some types of citrus contributes to soluble fiber consumption, which has been linked to increased cardiovascular health and reduced risk of certain types of cancer.

As might be expected with a crop utilized by humans for a number of millennia, these health benefits are reflected in the traditional use of citrus by indigenous people. Many of these uses are focused around the center of origin in China and India. However, health-related use of citrus has also been reported from traditional peoples in such areas as Fiji, Guatemala, and Chile. Citrus has been reported to be used for treatment of various illnesses, to reduce vomiting or diarrhea, and for regulating fertility. The sour orange has been reported to be used in voodoo ceremonies in Haiti.

Many of these uses are also associated with other plants in the subfamily Aurantiodeae. The kumquats have been mentioned already as being edible, but some other types of fruits are sometimes eaten by traditional peoples. Of particular note are the use of the leaves of Murraya koenigii as condiments and in the preparation of curry (the common name for this tree is curry leaf) and the use of Aegle marmelos ("Bael") for the preparation of teas. Other traditional uses reflect some of the properties suggested by the industrial use of citrus: insecticides and shampoos. As more insight into ethnopharmacology and secondary plant products is gained, it is possible that industrial use of citrus may increase, and probably some of these uses will reflect traditional uses of these plants.

See also China; Dietary Guidelines; Ethnopharmacology; India; Scurvy; Southeast Asia; Vitamin C .

BIBLIOGRAPHY

Davies, Frederick S., and L. Gene Albrigo. Citrus. Wallingford, Oxon, U.K.: CAB International, 1994.

Gallesio, Georges. Traité du Citrus. Paris: Fantin, 1811.

Gebhardt, Susan E., and Ruth H. Matthews. Nutritive Value of Foods. Home and Garden Bulletin No. 72, rev. ed. Washington, D.C.: United States Department of Agriculture, Human Nutrition Information Service, 1991.

Institute of Food Technologists Citrus Products Division. "Nutrition and Health Benefits of Citrus Fruit Products." Food Technology 48, no. 10 (November 1994): 103–139.

Kalt, Wilhelmina. "Health Functional Phytochemicals of Foods." Horticultural Reviews 27 (2001): 269–315.

Kimball, Dan A. Citrus Processing: A Complete Guide. 2d ed. Gaithersburg, Md.: Aspen Publications, 1999.

Reuther, Walter, Herbert John Webber, and Leon Dexter Batchelor, eds. The Citrus Industry. Vol. 1. History, World Distribution, Botany, and Varieties. Rev. ed. Berkeley: University of California, Division of Agricultural Sciences, 1967.

Rinzler, Carol A. The New Complete Book of Food: A Nutritional, Medical, and Culinary Guide. New York: Facts on File, 1999.

Saunt, James. Citrus Varieties of the World. 2d ed. Norwich, U.K.: Sinclair International, 2000.

Spiegel-Roy, Pinchas, and Eliezar E. Goldschmidt. Biology of Citrus. Cambridge and New York: Cambridge University Press, 1996.

Wardowski, Wilfred F., Steven Nagy, and William Grierson, eds. Fresh Citrus Fruits. Westport, Conn.: AVI, 1986.

Robert R. Krueger