Tragacanth (FAO, NWFP 6)
- Extract from : NWFP 6. Coppen J.J.W., 1995. Gums, resins and latexes of plant origin. FAO, Rome. 142 p. (Non-Wood Forest Products, 6). on line
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Contents
DESCRIPTION AND USES
Tragacanth gum is the dried exudate produced by tapping the tap root and branches of certain shrubby species of Astragalus, particularly those which occur wild in Iran and Turkey. The gum is exported from the country of origin in ribbon or flake form, and has a rather horny texture. Chemically, it is a complex mixture of acidic polysaccharides, mostly present as calcium, magnesium and potassium salts.
Tragacanth swells rapidly in water to form highly viscous colloidal sols or semi-gels, which act as protective colloids and stabilizing agents. The high viscosity of tragacanth solutions results from the molecular characteristics of the gum, and these depend on the grade and physical form of the gum, and the manner in which it is taken up in water. For example, the same concentration of solution prepared from whole gum is more viscous than one prepared from powdered gum. Unlike many other gums, solutions of tragacanth have a very long shelf life without loss of viscosity.
The most important applications of tragacanth are now in foods and pharmaceuticals. Its use for other, industrial purposes has declined over the years as cheaper alternatives to tragacanth have been developed.
Food use
In Europe, tragacanth has the food additive number E413. Its use in foods is not nearly so extensive now as it was some years ago, when it was widely used in salad dressings and sauces, savoury spreads, milk shakes, ice creams, and confectionery and bakery products. It functions as a thickener, stabilizer or emulsifier, but for many of these applications its advantage over other gums is its stability under acid conditions. Despite this, however, its high price has meant that for some of these end uses it has now been replaced by guar or xanthan gums.
Pharmaceutical use
Tragacanth has long been an important gum for pharmaceutical use: as a binder, suspender or emulsifier in tablets, ointments, lubricating jellies and oral suspensions, and particularly in dermatological creams and lotions. It is also used in toothpastes, hair lotions and other personal care products.
WORLD SUPPLY AND DEMAND TRENDS
Markets
In the 1950s, Iranian exports of tragacanth exceeded 4 000 tonnes/year (90% of it in flake form, the rest in ribbon); the United States and the United Kingdom were the major importers. Political upheavals and military conflict in the late 1970s and 1980s led to shortages of gum from Iran and a sharp increase in prices. Severe competition from other, cheaper gums, particularly xanthan gum, has resulted in a greatly diminished market for tragacanth.
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ROBBINS (1988) estimated the world market for tragacanth to be no more than 500 tonnes/year; almost half of this was estimated to be consumed in Western Europe. Severe problems are encountered in estimating consumption of tragacanth: firstly, export data from the major producers (Iran and Turkey) are not easily accessed and, secondly, tragacanth is not listed separately in the trade statistics of many importing countries.
Japan does treat tragacanth separately, however, and Japanese imports during the period 1988-94 are shown in Table 14; they averaged just under 30 tonnes/year. This is not much different to the situation in the early 1980s, although in 1979 imports into Japan were over 100 tonnes.
In the United States, a 1987 trade embargo which prohibits the import of most goods from Iran has influenced direct imports of tragacanth, although the United States still imports the gum from European dealers.
In the absence of any reliable data, and in the light of news items in the trade literature which continue to speak of shrinking usage, it is estimated that world demand for tragacanth is probably in the region of 300 tonnes/year.
Supply sources
Iran and Turkey have been the only significant producers of tragacanth for some years, with Iran being the principal source. They are both listed as sources in Japanese import statistics (Table 14); tragacanth from the other countries represents re-exports (the only Indian shipment may be karaya, sometimes known as Indian tragacanth).
Trade sources in London report that production in Turkey has now virtually ceased, due to the poor financial returns to the collectors.
Quality and prices
Tragacanth is bought from origin as ribbons or flakes; loss of viscosity of gum which has been powdered and stored for long periods means that powdered tragacanth is always produced in the importing country. Iranian tragacanth, which is generally regarded as superior to Turkish, is sold in about 12 different grades: five ribbon (Ribbon no. 1, Ribbon no. 2, etc.) and the remainder flake.
Ribbon no. 1 is the top grade, being the palest and cleanest. Ribbon grades are usually used for pharmaceutical purposes; flake is used for food applications. The lower flake grades are appreciably darker and contain some foreign matter. When powdered for the end-user, tragacanth is sold and specified by viscosity.
An FAO specification exists for food grade tragacanth and includes limits on arsenic, lead and heavy metals, as well as some other parameters. Tragacanth is also specified in many pharmacopoeias for pharmaceutical use, including the British Pharmacopoeia.
Trade sources in London quote current (mid-1995) prices at around US$ 22/kg FOB for the top grade (Ribbon no. 1), US$ 16/kg for Ribbon no. 4 and falling to US$ 3-4/kg for the lowest grades. These prices are higher than they were a year earlier although, historically, tragacanth has always been one of the most highly priced gums, and has been considerably
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higher in some previous years. In the mid-1980s it fetched around US$ 20-70/kg, depending on grade.
PLANT SOURCES
Botanical names
Family Leguminosae (Papilionoideae): Astragalus spp.
Astragalus is a very large genus and includes many Asian species. Astragalus gummifer Labill. is usually cited as the source of tragacanth but there is surprisingly little evidence to support this, and it is likely that other species which occur in the gum producing areas contribute to the total amount which enters world trade; whether to a greater or lesser degree then Astragalus gummifer is not known. These other species include Astragalus adscendens Boiss., Astragalus echidnaeformis Sirjaev (Accepted name : Astragalus echidniformis), Astragalus gossypinus Fisch., Astragalus kurdicus Boiss. and Astragalus microcephalus Willd. Numerous other Astragalus species occur in the region.
Description and distribution
The better gum-yielding species are small, low, bushy perennials, frequently with a cushion-like form. However, they have relatively large tap roots and it is these which are the primary source of the gum. Astragalus gummifer is a low shrub, up to 1 m tall, and is thorny and branching. Astragalus microcephalus, which produces a high quality gum, is a low, spreading bush, 8-12 cm tall.
The Asiatic species of Astragalus, which are the sources of commercial gum, are native to countries of Asia Minor: Iran, Turkey, Iraq, Syria, Lebanon, Afghanistan and parts of Russia. They are usually found in the drier mountainous regions, although they require some water.
COLLECTION/PRIMARY PROCESSING
The most striking feature of the gum-producing Astragalus is a central gum cylinder in the tap root, which is contained by the woody cylinder and may be as much as half the total diameter of the root. The gum is contained in the cylinder at high pressure and, when cut, exudes rapidly and hardens into the characteristic ribbons of tragacanth.
Some gum is collected from spontaneous exudation but most is obtained by tapping. The process of tapping entails clearing away the earth surrounding the tap root and making one or two cuts into the upper part of the root. The cuts are usually made longitudinally or crossangled to the root, 2-5 cm long. Sometimes the branches are also cut but this usually yields an inferior gum. After a period of time which varies according to local custom or circumstances, but may be a few days or a week or more, the tapper returns to the plants he has cut to collect the gum. Further collections may be made thereafter but the quality of the gum soon deteriorates to a point when it is not worth while to continue. Flakes of gum, rather than ribbons, are usually produced later in the season.
Tapping is carried out in the dry summer months and continues until the autumn rains. The collector sells the gum to the local merchant who then sells it on to the main trader. He, in turn, takes it to the main sorting and grading centre where it is graded and packaged for export.
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Yields
GENTRY (1957) lists a number of factors which influence gum yields. Some species are intrinsically better yielders than others. Older plants, and those with a large gum cylinder in the root, produce greater quantities of gum, and good spring rains prior to tapping also favour gum production. Unlike exudate gums obtained from the trunks of trees, where warm sunlight shining on the tree increases gum flow, most exudation of tragacanth occurs at night, under conditions which minimise drying out of the gum and maintain the outward flow under high osmotic pressure.
Based on experimental fieldwork, Gentry estimated the average yield of gum from Astragalus microcephalus at 15 g per 100-day tapping season.
VALUE-ADDED PROCESSING
As has been noted earlier, further processing such as grinding the gum to a powder is only done in the importing country, usually immediately before onward shipment to an end-user, so as to minimize loss of viscosity. Careful grinding, classifying according to particle size and, if necessary, blending, is essential to produce tragacanth gum of the prescribed viscosity.
PRODUCTS OTHER THAN GUM
No other products of economic value are obtained from the bushes.
DEVELOPMENTAL POTENTIAL
The decline in consumption of tragacanth gum is largely due to the high price brought about by the shortage of supply. If it were available in greater quantities, and at a lower price, it would be the gum of choice in most of its traditional applications. On the other hand, once end-users have switched to cheaper alternatives it is expensive for them to return to previous formulations. Much depends on the end-user. If Astragalus could be cultivated and gum produced at a cost which would make it significantly cheaper to the end-user than at present, then it may be possible to regain some markets. In these circumstances, Astragalus would be a crop worth developing in those countries with the appropriate ecological conditions for it to grow well.
Research needs
It is odd that so little research appears to have been carried out on the cultivation of Astragalus, given the high value of the product obtained from it. Gentry made some theoretical estimates of gum yield from cultivated plots based on 25 000 plants/ha and 15 g/plant (= 375 kg/ha). The following aspects need to be researched:
- Basic biology, propagation and cultural techniques. More needs to be learned about the response of Astragalus to attempts to cultivate it.
- Differences between species in their adaptation to cultivation. Planting trials, coupled with determination of gum yields (and quality), need to be carried out on a number of different sites to identify the best species for exploitation.
- Frequency of tapping. How often can the plants be tapped and for how many years?
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- Economic assessment. The economics of production under optimum conditions of cultivation and harvesting need to be assessed.
- Market for the gum. Close contact needs to be made with importers and end-users to determine whether a secure supply of tragacanth from cultivated sources would encourage them to maintain or increase their consumption.
SELECTED BIBLIOGRAPHY
- BP (1993) Tragacanth. pp 679-681. In British Pharmacopoeia, Vol. 1. London: Her Majesty's Stationery Office.
- DUKE, J.A. (1981) Astragalus gummifer. pp 24-26. In Handbook of Legumes of World Economic Importance. 345 pp. New York: Plenum Press.
- FAO (1992) Tragacanth gum [published in FAO Food and Nutrition Paper 34, 1986]. pp 225-227. In Compendium of Food Additive Specifications. FAO Food and Nutrition Paper 52 (Joint FAO/WHO Expert Committee on Food Additives. Combined Specifications from 1st through the 37th Meetings, 1956-1990). Rome: Food and Agriculture Organization.
- GECGIL, A.S., YALABIK, H.S. and GROVES, M.J. (1975) A note on tragacanth of Turkish origin. Planta Medica, 27, 284-286.
- GENTRY, H.S. (1957) Gum tragacanth in Iran. Economic Botany, 11(1), 40-63.
- GENTRY, H., MITTLEMAN, M. and McCROHAN, P. (1992) Introduction of chia and gum tragacanth, new crops for the United States. Diversity, 8(1), 28-29.
- MEER, G., MEER, W.A. and GERARD, T. (1973) Gum tragacanth. pp 289-299. In Industrial Gums. Whistler, R.L. (ed.). 810 pp. New York: Academic Press.
- ROBBINS, S.R.J. (1988) Gum tragacanth. pp 52-60. In A Review of Recent Trends in Selected Markets for Water-Soluble Gums. ODNRI Bulletin No. 2. 108 pp. London: Overseas Development Natural Resources Institute [now Natural Resources Institute, Chatham].
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| 1988 | 1989 | 1990 | 1991 | 1992 | 1993 | 1994 | |
|---|---|---|---|---|---|---|---|
| Total | 20 | 37 | 31 | 32 | 23 | 33 | 20 |
| Of which from: | |||||||
| Iran | 12 | 13 | 13 | 9 | 5 | 13 | 5 |
| Turkey | 8 | 17 | 12 | 15 | 11 | 1 | 1 |
| UK | 5 | 4 | 8 | 6 | 9 | 11 | |
| Germany | - | 2 | - | - | 1 | 4 | - |
| India | - | - | - | - | - | 6 | - |
| USA | - | - | - | - | - | - | 2 |
Source: National statistics
