Stevia rebaudiana (PROSEA)
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Introduction |
Stevia rebaudiana (Bertoni) Bertoni
- Protologue: Anales Cient. Parag., ser. 1, 5: 3 (1905).
- Family: Compositae
- Chromosome number: 2n = 22
Synonyms
- Eupatorium rebaudianum Bertoni (1899),
- Stevia rebaudiana (Bertoni) Hemsley (1906)
Vernacular names
- Stevia, sweet herb of Paraguay, honey-yerba (En)
- Indonesia: stevia
- Thailand: ya-wan
- Vietnam: cỏ ngọt
Origin and geographic distribution
Stevia originated from the Sierra Amambay in north-eastern Paraguay and possibly from adjacent areas in Bolivia and Brazil. The sweet principle of stevia is believed to have been used by the Paraguayan Indians for centuries. Stevia is now cultivated in parts of South America, but most widely in the Far East (China, Korea and Japan in particular). It is grown on a smaller or experimental scale in Thailand, Israel, Canada, United States, Mexico and Europe. In the early 1970s stevia reached Indonesia through seed imported from Japan.
Uses
In Paraguay and Brazil, fresh, or more usually dried, pulverized leaves of stevia are used as a sweetener in tea and maté or as a herbal remedy for diabetes. Some intense natural sweeteners can be extracted from the leaves, the most important being the steviol glycosides stevioside and rebaudioside A. In Japan, stevioside is used to increase the sweetening intensity or to reduce the energy value of other sweeteners. It is added to sugarless chewing gum to enhance sweetness and flavour. Stevioside can be added to sugar cubes to increase their sweetness without adding to their energy value. After a slow start in the 1970s, stevia is now used in a wide range of applications in Japan, e.g. in soft drinks, pickles, dried seafood, flavourings and confectionery. It can also be used to modify and suppress flavour, for instance the pungent flavour of sodium chloride used in the preparation of soya sauce.
Stevia is now used as a food additive in the Far East, Malaysia, the Philippines, several countries in South America, Canada, the United States, Israel and the Ukraine and is being evaluated for approval as a sweetener in Europe and the United States. Experience of its use by humans and data from animal feeding trials indicate that it is probably safe for human consumption. Medicinally, replacing sugar in the diet with stevioside could alleviate some types of diabetes, obesity and could lower blood pressure. An aromatic resin in the plant has a tonic action on digestive organs. Crushed leaves are made into cosmetic products, such as facial masks used to invigorate the skin.
Production and international trade
No production statistics are available for Paraguay and Brazil. Japan has developed stevia production since the 1950s, overcoming the problems of eliminating undesirable flavours by refining. In 1981, 650-750 t of dried stevia leaves were used for stevioside extraction; 60% of the leaves originated from Japan, 40% were imported from other Asian countries, mainly from China. In 1981, about 50 t stevioside was produced in Japan, in 1996 over 200 t in Japan and 700 t in China. In 1982, the stevioside price was US$ 90 000-130 000 per t (90% purity). In the United States the regulatory status "generally recognized as safe” (GRAS) was accorded to stevia in the mid 1980s, but this was withdrawn in 1991 following reports that some chemical compounds from stevia or their derivatives might be harmful to humans. At the same time an aggressively formulated detention order and import ban was issued by the Food and Drug Administration. Rumours of pressure from the American sugar industry and manufacturers of chemical sweeteners such as aspartame, which incidentally was accorded GRAS status around 1991, persist. Following a change in general regulations in 1995, the import ban was lifted. Stevia is now permitted, but only for use as a dietary supplement. Its use as an industrial sweetener remains prohibited. Admission for use as a sweetener has also been applied for in Europe.
Properties
Stevia owes its sweetness to a number of diterpene glycosides consisting of a steviol unit to which 3 or 4 glucose (or rhamnose) units are attached. The most important ones and their typical concentrations (on a leaf dry-weight basis) are stevioside 9%, rebaudioside A 4%, rebaudioside C 0.8% and dulcoside 0.3%. While the dried and ground leaves are about 30 times sweeter than sugar, the steviol glycosides are even sweeter, stevioside being 110-270 times sweeter, rebaudioside A 150-300 times, rebaudioside C 40-60 and dulcoside A about 30 times. In organoleptic tests rebaudioside A is preferred to stevioside, the latter having a slightly bitter after-taste. The steviol glycosides have excellent properties as domestic and industrial sweeteners: stevioside is stable in the pH range 1-9, only when heated at pH 10 does it disintegrate; it is stable up to at least 200°C, is non-fermentable, blends well with other sweeteners such as aspartame and glycyrhizin and does not darken upon cooking or storage (rebaudioside A may cause slight discolouration). Dried leaves keep their sweetening ability for a very long time. Herbarium samples tasted intensely sweet even 62 years after collection. All tests conducted indicate that stevia and its products are safe for humans. Acute toxicity tests indicated a LD50 of 8.2 g/kg. Doubts remain about its carcinogenic properties, because steviol is produced in the ceca (enlarged appendix) of mice and rats fed with stevia, and in an activated form it is suspected of being carcinogenic. The presence of steviol has been demonstated in the colon of humans who have taken stevia, but a review on the carcinogenity of stevia concluded that stevia is safe for use.
Stevia leaves also contain about 8% tannins, while the leaves and inflorescence contain 0.1% and 0.4% essential oil, respectively. The composition of the essential oil varies. In a sample tested the main components were the sesquiterpenes β-caryophyllene, trans-β-farnesene, α-humulene, δ-cadinene and the monoterpenoids linalool, terpinen‑4-ol and α-terpineol. In other samples spathulenol, β-cubebene and γ-elemene were found as major components.
Stevia pollen can be highly allergenic.
The weight of 1000 achenes is 0.15-0.30 g.
Composition
Stevia oil (from Italy) (Source: Martelli et al., 1985.)
- 12.2% β-cubebene
- 11.7% γ-elemene
- 10.0% β-caryophyllene
- 8.1% sesquiterpene alcohols (unknown)
- 5.6% α-humulene
- 5.6% β-elemene
- 4.3% (E)-β-farnesene
- 3.7% α-terpineol
- 3.3% α-cadinol
- 3.1% torreyol
- 2.1% spathulenol
- 1.8% linalool
- 1.3% 1-octenol-3
- 1.1% 3-hexenyl acetate
- 0.1% α-pinene
- 0.1% β-pinene
- 0.1% δ-cadinene
- 0.1% γ-cadinene
- 0.1% eugenol
- 0.1% carvacrol
- 0.1% caryophyllene oxide
- 0.1% nerolidol (unknown isomer)
- 0.1% (Z)-3-hexenol
- 0.1% β-bourbonene
- 0.1% α-bergamotene
- 74.5% total
Description
- A slender, erect, perennial herb, 50(-100) cm tall in natural stands and up to 120 cm under cultivation. Rhizome vigorous; root system shallow, with conical, hardly branched roots. Stem semi-woody, pubescent.
- Leaves opposite, subsessile, glabrescent; blade spatulate-lanceolate, 3-6.5 cm × 0.8-1.9 cm, faintly 3-veined, the margin crenulate above the middle.
- Inflorescence a terminal head, aggregated in panicles or small corymbs, with 2-6 flowers; involucral bracts 5, thin and narrow.
- Flowers regular, 7-15 mm long, bisexual but self-incompatible; corolla tubular, 5-6-lobed, white; stamens 5, connective of anthers ending in an oblong appendix; pistil with 2-lobed style, lobes curved at top.
- Fruit an angular, glandulose achene, crowned by a pappus of about 20, filiform, scabrid setae that are shorter than the corolla tube.
- Seed without endosperm.
Growth and development
Light promotes germination of stevia seed. Germination takes place under warm (optimum temperature 20°C) and moist conditions 2-7 days after sowing. Seedlings are very sensitive to high temperature and water stress.
Growth is slow during the first 2 weeks, but speeds up during the next 2 months. The length of the period of vegetative growth depends on the planting material used and the photoperiod, stevia being an obligate short-day plant with a critical photoperiod of about 13 hours. Under the critical photoperiod, flowering occurs 50-60 days after sowing. In the case of a ratoon crop flowering may take place within 40 days after cutting. Long days promote leaf production. Older leaves have a higher content of steviosides. Flowers are self-incompatible. Cross-pollination is necessary for seed production.
The biosynthesis of steviosides is a complex process. Steviol synthesis starts in the mevalonic acid pathway and is closely related to the biosynthesis of gibberellins. It occurs in chloroplasts. Glycolysation of steviol occurs under the influence of operationally soluble enzymes and occurs outside chloroplasts. The glycosides are stored in the vacuole.
Other botanical information
The genus Stevia Cav., confined to tropical and subtropical America, is badly known; species estimates range from 150-300. It is not known if other Stevia species than S. rebaudiana also possess sweetening properties; an investigation of some 110 species was negative.
A number of genotypes with anomalous glycoside proportions have been reported in Korea and Japan. Two named cultivars with a high rebaudioside content have been developed in Korea: "Suweon 2” and "Suweon 11”. They are based on single plant selections and need to be propagated vegetatively. Named cultivars have also been developed in China, e.g. "Yuubin”, "Zhongpin 1” and "SM4”.
Diterpene glycosides similar to steviol glycosides have also been isolated from Rubus suavissimus S.K. Lee ("sweet tea”) from southern China.
Ecology
The climate of stevia's native habitat can be characterized as subhumid (sub)tropical with an average annual temperature of 24°C. Average annual precipitation amounts to 1400-1600 mm. Stagnant water and very heavy, prolonged rain are injurious. Stevia occurs naturally on the edges of marshes or in grassland communities up to 700 m altitude, which are permanently moist but not subjected to prolonged inundation. It occurs naturally on soils with a shallow water table, particularly on infertile acid sands or muck with a pH of 4-5. However, stevia grows well on less acid to neutral soils with a pH of 6.5-7.5. It is sensitive to water stress and salinity.
Under cultivation stevia grows best in areas with a long frost-free growing season. Growth will be retarded severely below 15°C. High light intensities and high temperatures promote stevioside production. In Java (Indonesia), stevia is generally grown in mountainous areas with steep slopes, up to 1500 m altitude. Under lowland conditions stevia starts flowering too early.
Propagation and planting
Stevia can be propagated by seed, stem cuttings and tissue culture. Since germination rates are poor and seedlings slow to establish, seeds are sown in a nursery. They are sown on a fine seedbed, shaded and covered with a transparent finely perforated polythene sheet. Germination starts within a week, after which the shade should be reduced gradually to harden off the seedlings. The plastic cover must be removed two weeks after sowing. At this time seedlings bear two pairs of leaves and can be transplanted individually into polybags and placed in the nursery. After 4 months in the nursery the young plants have grown to a height of 15 cm and can be planted out in the field.
Cuttings with 4-5 nodes, taken from the apical part of stem and branches, are used for vegetative propagation. Under constant high air humidity and with the use of growth regulators to stimulate root formation, 100% success can be reached after about 17 days. The stem cuttings are then planted in nursery beds and shaded with transparant polythene sheet which is secured on a bamboo framework. The plastic cover can be removed two weeks after planting. When cuttings are taken during the dry season the cover should be removed gradually. Planting out in the field can start at the end of the third week.
In West Java, rooted cuttings are used as planting material, and planting distance in the field is 25 cm × 25 cm, giving 160 000 plants/ha. In Japan, spacing is 50 cm × 60 cm, giving 33 000 plants/ha. Research in Paraña (Brazil) showed that plant densities of 50 000, 80 000 and 100 000 plants/ha did not influence leaf yield in the first year. The highest seed yield, however, was obtained with 80 000 plants/ha.
Recently, a simple and rapid method for vegetative multiplication has been developed by means of in vitro culture: nodal segments of stems are incubated on an artificial growing medium, giving rise to new plants.
In vitro production of active compounds
Stevioside can be synthesized in the laboratory.
Husbandry
There is little information about cultural practices to make commercial cultivation of stevia more efficient. Weeding is carried out either manually or chemically, using pre- or post-emergence herbicides. In West Java, weeding is preferably performed manually, to prevent possible negative effects of chemicals on the quality of the product; this makes maintenance very labour-intensive.
Irrigation, if necessary, must be regular and shallow, using good-quality water low in salts. Surface irrigation is very suitable, especially during the dry season. Sprinkler irrigation is also practised in Indonesia.
Stevia responds well to farmyard or liquid organic manure. Nutrient uptake per t dry matter is 20-25 kg N, 2-2.5 kg P and 25-30 kg K. Application of fertilizers benefits the leaf dry matter production but not the stevioside content of the leaves. Flower-heads that appear before harvest are removed.
Diseases and pests
No serious diseases and pests of stevia are known. In Indonesia 2 pathogenic fungi (Colletotrichum sp. and Sclerotium rolfsii) have been isolated from a stevia stand in West Java, and in Japan a black leaf spot on stevia was caused by Alternaria steviae. In Canada, the most important diseases are Sclerotia sclerotiorum rot and Septoria steviae leaf spot.
In Indonesia army worms (Heliothis spp.) occasionally destroy young leaves and flowers, especially in the highlands. In the lowlands, Aphis may destroy young shoots and leaves.
Harvesting
Stevia is harvested just prior to flowering when the steviol-glycosides content is highest. First harvest takes place 2 months after planting. The second and following harvests are carried out at intervals of 1 month. Stems and branches are cut at a height of at least 15 cm above the ground; cutting lower gives a high percentage of mortality. Harvesting may continue for a period of 5-6 years.
Yield
Dried leaf yields are estimated at 1.5-2 t/ha for Brazil. In Japan, yields in the first year (two harvests) are 400-500 kg/ha. Yields in the second and the third years vary from 1.5-2 t/ha of dried leaves, which is equivalent to 50-75 t/ha of sucrose sugar. In West Java annual yields of 3 t/ha of dried leaves are possible. Seed yields of 8 kg/ha are possible.
Handling after harvest
The leaves are separated from the stems by hand and kept in plastic bags at room temperature before drying. Alternatively, the whole plant is dried and leaves are separated by threshing. Stems have a very low glycoside content and are removed to reduce processing costs. The method of drying is of great importance for the quality of the product. Drying can be done in the sun or, during the rainy season, mechanically in a ventilated desiccator. To remove any undesirable taste notes and increase resistance to crumbling and agglomeration during packaging, the leaves are first steamed for 1-2 minutes at 125-130°C, partially dried to 60-63% moisture, rotated at 60-65 rpm, and dried again for 5-7 hours at 75-85°C.
For export, dried leaves should meet the following standards: maximum moisture content 10%, minimum stevioside content (including rebaudioside A) 11%, and no more than 11% impurities. Dried leaves can be stored and transported in plastic bags or in airtight drums.
Several commercial methods of extracting the glycosides from the leaves have been developed and patented in Japan. They mostly involve water or solvent extraction, decolouration and purification using ion-exchange resins, precipitation or coagulation, followed by filtration, crystallization and drying. Newer methods based on ultra-filtration have also been developed.
Genetic resources
Due to the removal of the natural vegetation, gene centres of stevia are threatened with destruction. There are germplasm collections of S. rebaudiana in Japan, Korea and Indonesia.
Breeding
Main objectives of breeding work on S. rebaudiana in China, Japan, Korea and Canada are to increase leaf yield and glycoside content, and to increase the ratio of rebaudioside A to stevioside. Heritability for these factors is high. A total sweet-glucoside content of 20% has been reported from China and a ratio of rebaudioside A to stevioside of 9:1 from Japan. Stevioside and rebaudioside A content appear to be inversely correlated.
Prospects
Stevioside has not yet been officially approved as a sweetener by the Food and Drug Administration (FDA) in the United States and by the West European countries. If approved, the demand for stevia may increase dramatically. In that case there are good prospects for Indonesia and other South-East Asian countries as suppliers of stevia leaves. Although there will be competition from temperate countries, higher glycoside contents can be obtained in the tropics. Research is needed on weed control, water management, fertilization, labour requirements and efficiency of harvest. Moreover, breeding and selection for higher content of steviosides and higher leaf-to-stem ratio are desirable.
Literature
- Brandle, J.E., Starrat, A.N. & Gijzen, M., 1998. Stevia rebaudiana: its agricultural, biological, and chemical properties. Canadian Journal of Plant Science 78: 527-536.
- Carneiro, J.W.P., Martins, E.N., Guedes, T.A. & da Silva, M.A.G.D., 1992. Desempenho de culturas de stevia transplantadas com differentes densidades de plantio e espacamentos duplos [The performance of stevia at different plant densities and double spacing]. Pesquisa Agropecuaria Brasileira (Brazil) 27: 1273-1282.
- Higginbotham, J.D., 1982. L'état présent des aspartame, stévioside et autres édulcorants [The current situation of aspartame, stevioside and other sweeteners]. La Sucrerie Belge 101: 235-241.
- Lewis, W.H., 1992. Early uses of Stevia rebaudiana (Asteraceae) leaves as a sweetener in Paraguay. Economic Botany 46: 336-337.
- Mochtar, M. & Hutasoit, G.F., 1986. Stevia rebaudiana Bertoni M.: prospects and problems. Gula Indonesia 12: 11-15.
- O'Brien Nabors, L. & Inglett, G.E., 1986. A review of various other alternative sweeteners. In: O'Brien Nabors, L. & Gelardi, R.C. (Editors): Alternative sweeteners. Marcel Dekker, New York, United States. pp. 309-323.
- Soejarto, D.D., Kinghorn, A.D. & Farnsworth, N.R., 1982. Potential sweetening agents of plant origin. Organoleptic evaluation of Stevia leaf herbarium samples for sweetness. Journal of Natural Products 45: 590-599.
- Suhendi, D., 1988. Seleksi massa tanaman Stevia rebaudiana Bertoni M. [Mass selection of Stevia rebaudiana Bertoni M.]. Menara Perkebunan 56(4): 93-95.
- Toyoda, K., Matsui, H., Shoda, T., Uneyama, C., Takada, K. & Takahashi, M., 1997. Assessment of the carcinogenicity of stevioside in F344 rats. Food and Chemical Toxicology 35: 597-603.
- Wardojo, S., 1984. Aspek penelitian budidaya Stevia rebaudiana Bertoni (Compositae) [Aspects of research on the cultivation of Stevia rebaudiana Bertoni (Compositae)]. Balai Penelitian Perkebunan Bogor, Indonesia. 12 pp.
Sources of illustrations
Stevia rebaudiana: Botting-Hemsley, 1906. In: Hooker, W.J. (Editor): Icones plantarum. Vol. 29. Fig. 2816 (flowering branch, flower, anthers); Westphal, E. & Jansen, P.C.M., 1989. Plant Resources of South-East Asia. A selection. Pudoc, Wageningen, the Netherlands. p. 253 (leafy twig). Redrawn and adapted by P. Verheij-Hayes.
Authors
- J. Mohede & R.T.M. van Son
