Prota 14: Vegetable oils/Oléagineux
Sp. pl. 2: 1007 (1753).
2n = 20
Castor, castor oil plant (En). Ricin (Fr). Rícino, carrapateiro, mamoneiro, bafureira (Po). Mbarika, mbono mdogo, mnyonyo (Sw).
Origin and geographic distribution
Ricinus communis is indigenous to north-eastern tropical Africa. It was already grown for its oil in Egypt some 6000 years ago and spread through the Mediterranean, the Middle East and India at an early date. It is now widely cultivated in most drier areas of the tropics and subtropics and in many temperate areas with a hot summer. It naturalizes easily and grows in many areas as a ruderal plant. Ricinus communis occurs across the African continent, from the Atlantic coast to the Red sea and from Tunisia to South Africa and in the Indian Ocean islands.
About 95% of castor seed is used for the expression of oil, which consists mainly of triglycerides of ricinoleic acid, is non-drying and non-edible. Traditionally, it is used for illumination and in medicine. As a lamp oil, it is believed to give a cooler and brighter light than other vegetable and mineral oils, burn more steadily and produce very little soot. It is now only used in rural areas and even there often mixed with or as a substitute for kerosene. Currently, castor oil is primarily used as a high-quality lubricant and a versatile raw material in the chemical industry. It has long been used as a lubricant in carts and Persian wheels. It is characterized by a high lubricity, high viscosity remaining constant over a wide range of temperatures, and insolubility in aliphatic petrochemical fuels and solvents, making it suitable for equipment operating under extreme conditions such as in arctic zones and in aviation. Another specialized use of castor oil is in crumb-rubber manufacturing, where it prevents rubber crumbs from coagulating. Highly purified, food-grade castor oil is used as an anti-stick agent for candy moulds and as a lubricant for machinery in industrial food processing. Castor oil is further employed as a plasticizer in the coating industry, as a disperser for dyes and as filler in cosmetics such as lipsticks, nail varnishes and shampoos. Saponification of castor oil yields a clear, transparent soap. Washing jute fibre with the soap gives it a shiny silky appearance. The soap has poor detergent qualities, but is easily water-soluble.
Partial oxidation of castor oil in air at about 100°C yields ‘blown oil’, which remains fluid at low temperatures and is a major component of hydraulic and brake fluids and is used as a plasticizer for inks, lacquers and leather. Dehydration of castor oil turns it into a very pale, odourless, quick-drying oil used in manufacturing alkyd resins, epoxy resins and acryl resins used in heavy-duty paints and varnishes e.g. for refrigerators and other kitchen equipment. Hydrogenated castor oil yields a hard and brittle, odourless wax, mainly applied to modify the qualities of other waxes. Its main component, hydroxystearic acid, is used in lubricants, insulators and surfactants and in the production of non-drip paints. Treating castor oil with sulphuric acid yields ‘Turkey red oil’ which is soluble in water. It is used as a wetting agent in dyeing cotton and linen fabrics, as a defoaming agent in the sugar industry, and in leather and fur manufacturing.
Cracking of ricinoleic acid yields a number of compounds, particularly suitable for the manufacture of high quality lubricants and synthetic polymers such as the polyamides nylon 11, nylon 6.10 and more recently developed polyurethanes. Other components derived from cracked ricinoleic acid include aroma chemicals, sebacic acid used in manufacturing jet-engine lubricants, synthetic detergents and additives for insecticides. Castor oil is so important in chemistry that the United States has declared it a ‘strategic material’ of which adequate stocks have to be maintained at all time.
In medicine, castor oil is used primarily as a purgative. It is commonly referred to in South Africa as ‘blue bottle’ because of the characteristic blue bottle in which it was traditionally packed and sold. It was much feared by children because of the unpleasant taste. The oil is now sometimes given as a sweetened aromatized emulsion or as capsules. It stimulates peristalsis by irritating the intestinal mucosa but causes little griping. It is also applied as an emollient in the treatment of sores and as a solvent for antibiotic eyedrops. Neutral sulphated castor oil can replace soap in certain cases of contact dermatitis. Castor oil has been used as an abortifacient and is given orally, alone or with quinine sulphate, to induce labour in pregnancy at term. Ricinoleic acid prepared from the oil is a component of contraceptive creams and jellies.
The presscake of castor seeds is poisonous and allergenic and is mainly used as fertilizer or as fuel. Methods to detoxify the presscake and make it suitable as an animal feed have been developed, but even after treatment some toxicity may remain; horses are particularly sensitive to it. Another product extracted from the presscake is a lipase used in the industrial processing of fats.
In China and tropical Asia, the leaves of castor are used to treat skin diseases. They are also fed to the eri silkworm (Philosamia ricini). Although they are somewhat toxic, mature leaves are occasionally used as a fodder, but care must be taken to avoid the more toxic young leaves. In Korea mature leaves are dried and stored until winter when they are eaten as a vegetable; in Bengal (India) the young fruits are eaten. Castor is commonly grown as an ornamental.
Production and international trade
Between 1985 and 2005 annual world production of castor seed gradually increased to 1.4 million tonnes, while the harvested area fluctuated around 1.4 million ha. In 2005 the most important producers of castor seed were India (870,000 t), China (268,000 t) and Brazil (177,000 t). In Africa important producers are Ethiopia (15,000 t), South Africa (4900 t), Angola (3500 t), Kenya (1000 t) and Tanzania (1000 t). Most of the castor seed is processed in the countries of production. Major importers are France, the United States, Germany and Japan.
Data on cultivated area and yield do not present a fair indication of the actual production in a country since much castor is collected from the wild and because sole cropping of castor by peasant farmers is the exception.
Per 100 g, castor seeds contain approximately: water 5 g, protein 15–30 g, fat 43–53 g, carbohydrate 7–10 g, crude fibre 15–25 g, ash 2–3.8 g. The seed and to a minor extent other plant parts, contain extremely toxic proteins, the toxic alkaloid ricinine and allergens. The oil is non-drying, viscous, nearly colourless, transparent and has a characteristic odour and taste. It has the highest viscosity of all vegetable oils; ricinoleic acid, which makes up about 90% of the fatty acids of the oil, renders the special properties to the oil. Other fatty acids include: palmitic acid (2%), stearic acid (1%), oleic acid (7%), linoleic acid (3%).
Ricinoleic acid (12-hydroxy- 9-octadecenoic acid) has a single double bond and is further characterized by a hydroxyl group. Dehydration of castor oil, in which part of the ricinoleic acid is converted to a polyunsaturated acid, yields a quick-drying oil with properties that compare favourably with those of tung oil and linseed oil. It is used in paints, varnishes, waxes and epoxy resins. Hydrogenation of castor oil in which the ricinoleic acid is partly or completely converted to 12-hydroxystearic acid yields a hard and brittle wax. Blown oil, i.e. oil that is oxidized and partially polymerized by bubbling finely dispersed air through it at 80–130°C is a major component of hydraulic fluids. In inks, it is used to reduce water pick up and improve drying characteristics.
When the oil is pressed or extracted from the seed, the poisonous proteins remain in the castor cake. The main toxic proteins are ‘ricin’, a potent cytotoxin, and ‘RCA’ (Ricinus communis agglutinin), a powerful haemagglutinin. Poisoning by ingestion of castor seed is due to ricin, as RCA does not penetrate the walls of the intestines. Ricin is extremely poisonous when injected into the bloodstream; as little as 1 mg can kill an adult. It irreversibly inhibits ribosome activity; a single molecule that has entered a cell can inactivate over 1500 ribosomes per minute. Because of its extreme toxicity, ricin is included in Schedule 1 of the Convention on Chemical Weapons (1994) imposing the most stringent restrictions and control on its production, transportation and use. The ricin molecule consists of 2 parts; one responsible for its transport through the cell wall, the other is the toxin proper. Pharmacological research is going on to combine the toxic part of ricin with monoclonal and polyclonal antibodies in the development of immunotoxins for treatment of cancer and Aids.
The pyridine-carbonitrile alkaloid ricinine is a convulsant agent; it causes respiratory depression. At low doses it improves memory retention. Castor seeds are allergenic. They may cause asthmatic reactions in sensitive persons, but others may work in castor processing facilities for years without developing any sensitivity. As a flavouring, castor oil has been accorded the ‘generally recognized as safe’ (GRAS) status, e.g. in the United States.
Castor is used extensively in physiological studies to elucidate mechanisms involved in phloem transport.
Adulterations and substitutes
Castor oil is sometimes adulterated with rosin oil (a product from pine trees), blown oils and other unheated oils from groundnut, coconut, sesame, rape and cotton seed. The addition of any of these lowers the acetyl value. Rosin oil is detected by the increase in the unsaponifiable matter content, while addition of fatty oils, that have not been blown, lowers the specific gravity and viscosity and increases the product’s solubility in petroleum ether.
Evergreen, glabrous, soft-woody shrub or small tree, often grown as an annual, up to 7 m tall; taproot strong and with prominent lateral roots; stem and branches with conspicuous nodes and ring-like scars, shoots usually glaucous, variously green or red; glands often present at nodes, petioles and main axes of inflorescences. Leaves arranged spirally, simple; stipules 1–3 cm long, clasping the stem, deciduous; petiole 3.5–50 cm long, terete; blade palmately 5– 12-lobed, up to 50(–70) cm in diameter, membranous, lobes acuminate, median one up to 8(–20) cm long, margins with glandular teeth. Inflorescence an erect terminal panicle, later appearing lateral by overtopping, up to 40 cm long, usually glaucous, consisting of cymes. Flowers unisexual, regular, with short pedicel, 1–1.5 cm in diameter; calyx lobes 3–5, acute; corolla absent; male flowers towards the base of the inflorescence, with many stamens in branched bundles; female flowers towards the top of the inflorescence, with early caducous sepals, ovary superior, 3-celled, usually soft-spiny, styles 3, red or green, 2-cleft. Fruit an ellipsoid to globose, slightly 3-lobed capsule, 1.5–2.5 cm long, brown, spiny or smooth, dehiscing in 3 cocci each opening by a valve and 1-seeded. Seeds ellipsoid, 9–17 mm long, compressed, with a brittle, mottled, shining seedcoat and with distinct caruncle at the base; endosperm copious, white; cotyledons thin. Seedling with epigeal germination; cotyledons petioled, broadly oblong, up to 7 cm long, flat, with entire margins; first leaves opposite.
Other botanical information
Ricinus comprises a single species. Ricinus communis exhibits considerable variation, especially in plant size and duration, shape and size of the fruit and of the seed and the pattern and colour of the seed coat. The numerous varieties are so thoroughly connected by intermediates and hybridize so freely when brought together, that it is untenable to consider them as separate taxa. Colour differences in leaves, stems and inflorescences have resulted in the selection of many horticultural variants that may be classified into cultivar groups. In many countries red and white types are distinguished based on the colour of young shoots. Within these groups, cultivars are recognized based on seed characteristics.
Growth and development
Seedlings of castor emerge 10–20 days after sowing. The development of the plant is in accordance with Leeuwenberg’s growth model in which the apical buds systematically die after one growth unit, so that the growth is sympodial. The successive formation of branches and inflorescences continues throughout the plant’s life. The node at which the first inflorescence originates is a cultivar characteristic. In annual cultivars, the first inflorescence is the largest one and may account for up to 80% of the seed yield. In perennial cultivars, common in peasant agriculture, flowering is more diffuse. Flowering starts early in the life of castor. The first flowers may open 40–70 days after sowing. Pollen is mainly shed in the morning and pollination is by wind. As growth is indeterminate, one plant may bear infructescences in different stages of development. Ripening of fruits within an infructescence is uneven, the lower fruits maturing before the upper ones. In wild types, the period of maturation between the first and the last fruits within a given infructescence may be several weeks. In cultivars grown as annuals, the period from emergence to maturation varies from 140–170 days.
Under favourable conditions, castor has a high rate of photosynthesis, which has been attributed to the high chlorophyll content in the leaves.
Ricinus communis is often found as a ruderal near villages and in urban regions; under natural conditions in north-eastern Africa it occurs commonly along seasonally dry rivers.
Castor is a long-day plant, but is adaptable to a fairly wide photoperiodic range. At a daylength of 9 hours, growth and development are reduced, while at 12–18 hours, development is normal. Castor grows throughout the warm-temperate and tropical regions. It has been commercially cultivated from 40°S to 52°N, from sea-level to 2000 m altitude at the equator, with an optimum at 300–1500 m, the limiting factor being frost. Suitable soil temperatures for germination are 10–18°C. Castor requires average day temperatures of 20–26°C with a minimum of 15°C and a maximum of 38°C. Temperatures of 40°C or higher are detrimental. It is susceptible to damage by frost. It prefers clear, sunny days with low humidity. Castor can withstand dry arid climates, but also heavy rains and short flooding. In regions with an average annual rainfall of 750 mm or less, sowing should be done on such a date that 400–500 mm rainfall up to the time of main flowering is assured for the crop. Castor can tolerate water stress because of its deep root system, but is sensitive to excess of water and humidity.
Castor will grow on almost any soil type as long as it is well drained and reasonably fertile. It prefers deep, sandy loams with pH 5–6.5. Plants with the best tolerance to salinity or alkalinity tend to be large bushy ones with little commercial value.
Propagation and planting
For mechanized cropping under rain-fed conditions, field preparation starts by ploughing deep enough to break any compact layers. Castor requires a moist topsoil for germination and early growth for a longer period than maize or cotton. In dry regions where total rainfall is low, ridging is recommended. Smallholders usually intercrop castor in annual crops or plant it along the edges of fields or as a shade crop e.g. for ginger, turmeric or sugarcane.
Castor is propagated by seed. In spot-sowing 2–3 seeds are planted per hole at a depth of 3–8 cm; alternatively castor is sown in rows. The weight of 1000 seeds is 100–1000 g. Short-cycle cultivars may be grown in sole cropping as a second crop. In intercropping, distances between rows of castor may be as much as 4–5 m, and it will receive the treatment of the main crop. With dwarf cultivars in sole cropping, planting may be at 1 m row distance. Closer spacing can result in considerable damage to branches during weeding. Recommendations for in-row spacing range from 25–35 cm for dwarf, to 30–40 cm for larger cultivars, or about 25,000–30,000 plants/ha for crops grown in locations with 750–900 mm rainfall. Under irrigation, row width may be determined by the system of water delivery and where water is not limiting 30, 000–40,000 plants/ha is feasible depending on the cultivar.
Castor is generally grown on sandy or clayey deep red loams and on light alluvial loams. It is one of the few crops which can be grown economically on gravelly and poor soils. Deep black-cotton soils are not usually used for castor nor are very fertile soils with high nitrogen content, as they produce excessive vegetative growth. Castor seedlings are poor competitors and weed control is essential. Two weeding rounds are normally sufficient. Where practical, application of a pre-emergence herbicide followed by hand weeding is probably most effective. The first weeding is about 6 weeks after sowing. It is often combined with thinning, earthing up and topping. Since the young crop is very susceptible to mechanical damage, weeding should be done carefully. Effective weed control often results in a relatively bare soil surface. As the root system of castor has a low soil-binding ability, fields are often susceptible to erosion. Conservation measures in the cropping system and care in selecting sites for large plantings of castor are necessary. Peasant farmers do not usually irrigate castor, although it is often beneficial for yield.
As castor takes 5–8 months to come to harvest, it is grown as a main season crop. In general, application of organic manures such as compost or farmyard manure, groundnut or castor cake and inorganic fertilizers is said to be beneficial, the organic manures having beneficial residual effects over a period of 2–3 years. It has been calculated that a crop yielding 3.3 t fruit (2 t seed and 1.3 t hulls) removes 80 kg N, 8 kg P, 26.5 kg K, 8.5 kg Ca and 6 kg Mg. Castor is often grown mixed with groundnut and an application of NPK 1:2:3 to the latter crop increases the yields of both crops.
Diseases and pests
Few diseases are of economic importance. Normally, serious attacks only occur in badly-growing crops under humid conditions. The most damaging diseases that attack seedlings are various rots caused by Fusarium, Phytophthora, Rhizoctonia and Sclerotinia spp. The most common foliar disease is rust caused by Melampsora ricini which is now probably of worldwide occurrence; symptoms are the presence of uredopustules on the lower surface of the leaves. In severe cases leaves may be covered completely and dry up. Widespread leaf spot diseases of castor in Africa are caused by Xanthomonas axonopodis and Cercospora ricinella. Among the capsule diseases, those caused by Alternaria and Botrytis are the most serious ones. Alternaria ricini causes damage worldwide. Symptoms are the appearance of brown lesions on the leaves surrounded by a yellow halo. Affected capsules may suddenly wilt and turn dark brown or purple; also sunken areas may develop which gradually enlarge to cover the whole capsule. Under very humid conditions inflorescences may become covered by black sooty spore deposits. Seed treatment with a fungicide may control the disease. In later stages foliar application of carbamates or copper-based fungicides may be effective.
The African cassava mosaic bigeminivirus (ACMV) transferred by the whitefly Bemisia tabaci affects castor throughout Africa.
Probably the most damaging pests are those attacking the inflorescence, such as the cacao bug Helopeltis schoutedeni occurring throughout Africa. Peach moth or castor shoot and capsule borer Dichocrocis punctiferalis is a most important pest in tropical Asia. Young caterpillars feed on the green capsules and bore their way inside at the apical or basal end. Throughout Africa the scale insects Pseudaulacaspis pentagona and Saissetia coffeae affect castor, as do Agrotis cutworms, the cotton leafworm Spodoptera littoralis and the false coddling moth Thaumatotibia leucotreta. In East Africa the hairy castor caterpillar Euproctis producta may cause damage. Many other pests have been observed, but damage is mostly minor and localized. Tall, perennial cultivars can often outgrow the effects of insect attack. However, because of their tall stature and long duration, they are more susceptible to damage caused by stem borers than short-term cultivars.
Castor is a host of head-bug (Eurystylus oldi), a pest in sorghum. In Mali it is becoming serious in newly introduced compact-panicled cultivars of sorghum, whereas traditional open-panicled cultivars are fairly resistant.
The duration of the crop in the annual types of castor varies from 4–9 months, but perennial types may continue to bear for 10–15 years. Improved types with non-shattering capsules are harvested soon after they are fully dry. In types with shattering capsules, the capsules are harvested before they dry up and while they are still green. Harvesting may be repeated every 2 weeks. For manual harvesting simple tools in the form of a tin with a notch have been developed. Where castor seeds are merely collected from wild or volunteer plants, their harvesting sometimes involves no more than collecting scattered seeds.
Under intensive cropping, harvesting and hulling are the most time-consuming operations. Suitable machines and cultivars which are adapted to large-scale cultivation have been developed. Mechanical harvesting consists basically of removing fruits from the standing plants. Important problems still to be solved are the uneven ripening and the varying thickness of the fruit wall, both producing a large proportion of unopened fruits or broken seeds.
Average seed yield of castor is about 1000 kg/ha, with a maximum of about 3000 kg/ha. Statistics on yields are very difficult to compile as castor is often intercropped or grown along field borders.
Handling after harvest
The fruits of traditional cultivars are mostly semi-shattering. After harvesting, the inflorescences are stacked in heaps till the capsules blacken; they are then spread out in the sun to dry. They lose most of their seeds in 4–6 days. Unopened fruits are threshed. After separation of the healthy seeds from the trash, the product is ready for storage or for sale. Fruits of modern cultivars are often non-shattering. Such cultivars should only be grown if mechanical hullers are available, because traditional threshing results in a large proportion of damaged seeds. Castor seed can only be stored in the open for short periods, as both heat and sunlight reduce its oil content and quality. Seed should be handled with care since the thin and often brittle seed-coat is easily damaged.
About 10% of the total production is estimated to be retained by producers for propagation and domestic requirements. Hardly any sorting or grading of the seed is done and the bulk of the crop is sold by the producers without cleaning. Castor seeds can be stored for 2–3 years in gunny bags or in other containers without any detriment to the content or the quality of the oil. They are, however, seldom stored for more than 6 months and are utilized for oil extraction soon after threshing. Storage trials have shown that cracked or damaged seeds deteriorate rapidly and that wetting further accelerates this deterioration. Damaged seeds yield oil with a higher acidity and a dark colour that is difficult to bleach.
As castor is distributed widely throughout the tropics, there seems to be no risk of genetic erosion, also because a great deal of genetic diversity is being maintained on farmers’ fields. Studies on the genetic variability are necessary to elucidate and categorize the wide morphological variability. A germplasm collection of more than 1000 accessions, partly from tropical Africa, is maintained at the N.I.Vavilov Institute of Plant Industry, St.Petersburg, Russian Federation. The Institute of Oil Crops Research (CAAS), Wuhan, China holds nearly 1700 accessions and the National Plant Germplasm System of the United States holds over 1000 accessions. In the Biodiversity Conservation and Research Institute, Addis Ababa, Ethiopia a collection of local castor is available.
All natural types of castor are diploid; they cross freely and are fully fertile. The frequency of natural out-crossing is commonly between 5–50%, but in some dwarf cultivars it may be as high as 90–100%. Male-sterile and female-sterile lines have been identified and are of great value in breeding. Selection has mostly focussed on problems associated with mechanisation such as annual life cycle, dwarf plant architecture and indehiscent thin hulled and sparsely spiny fruits, maturing synchronously. The main aim of modern castor breeding are high seed yield, high oil and ricinoleic acid contents, easy harvesting and resistance to diseases and pests.
Numerous cultivars exist; ‘Hale’ and ‘Lynn’ are dwarf cultivars in the United States, now mainly used as pollen parents in the production of hybrids. Other well-known cultivars include: ‘Conner’ and ‘Kansas’ in the United States, ‘Rica’ and ‘Venda’ in France, and ‘T-3’, ‘CS-9’ and ‘SKI-7’ and the GCH series of hybrids in India.
Castor is of great economic importance in the tropics and great steps have been made in the development of castor as an industrial oil crop for the temperate regions. It grows over a wide area, regenerates well, and is traditionally managed and protected by farmers. As a raw material for industry, castor oil has to compete with alternative raw materials. Demand depends on the price of the oil in relation to that of alternatives and the reliability of supply. Both supply and price have fluctuated considerably in the past. Currently, competition is strongest for dehydrated castor oil, as cheap alternatives prepared from soya bean oil are available. With increasing research efforts aiming at the development of new products based on the unique properties of ricinoleic acid, however, the demand for castor oil may increase in the future.
Castor is important because of its multipurpose functions and its adaptability to a wide range of ecological conditions, including degraded sites. Special consideration should be given to using castor for soil rehabilitation in local land-use systems.
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Sources of illustration
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Correct citation of this article:
Maroyi, A., 2007. Ricinus communis L. In: van der Vossen, H.A.M. & Mkamilo, G.S. (Editors). PROTA 14: Vegetable oils/Oléagineux. [CD-Rom]. PROTA, Wageningen, Netherlands.
wild and planted
1, branch with leaves; 2, inflorescence; 3, infructescence; 4, seed.