GUM ARABIC, GUM TALHA AND OTHER ACACIA GUMSDEFINITION
The term gum arabic is used with varying degrees of precision by different groups of people. In the context of its use as a food additive the most recent international specification, published by FAO (FAO, 1990), defines gum arabic as the "dried exudation obtained from the stems and branches of Acacia senegal (L) Willdenow or closely related species". The specification then proceeds to give limits for certain parameters which have been selected to try and ensure that only gum from A. senegal (and closely related species) satisfies the specification (see Quality and prices below). The need for such legislation arises from the need to assure the public on safety grounds that there are no hazards associated with ingestion of gum arabic; gum arabic which complies with the definition and specifications has been tested and shown to be safe to consume./
In Sudan, the term gum arabic is used in a wider context to include two types of gum which are produced and marketed, but which are, nevertheless, clearly separated in both national statistics and trade: "hashab" (from A. senegal) and "talha" (from A. seyal). In a still wider sense, gum arabic is often taken to mean the gum from any Acacia species (and is sometimes referred to as "Acacia gum"). "Gum arabic" from Zimbabwe, for example, is derived from A. karroo.
In practice, therefore, and although most internationally traded gum arabic comes from A. senegal, the term "gum arabic" cannot be taken as implying a particular botanical source. In a few cases, so-called gum arabic may not even have been collected from Acacia species, but may originate from Combretum, Albizia or some other genus. In the discussion which follows, the term "gum arabic" will generally be used in the generic sense as any Acacia gum unless it is qualified by some other statement or the botanical source is specified.
Statistical data originating in Sudan and shown in Tables 11a, 11b and 12 separate gum hashab and gum talha. Figures relating to Sudan in other statistical tables (Tables 6, 7, 9 and 10) are assumed to combine both types of gum arabic (hashab and talha).
DESCRIPTION AND USES
Gum arabic from A. senegal is a pale to orange-brown coloured solid, which breaks with a glassy fracture. The best grades are in the form of whole, round tears, orange-brown in colour and with a matt surface texture; in the broken, kibbled state the pieces are much paler and have a glassy appearance (see section on quality below). Inferior grades, and gum from species other than A. senegal, may not have the characteristic tear shape and are often darker in colour. Gum from A. seyal (gum talha) is more friable than the hard tears produced by A. senegal and is rarely found as whole lumps in export consignments.
The gum arabic-yielding Acacias grow in semi-arid areas and the vast majority of gum arabic which enters international trade originates in the so-called gum belt of Sub-Saharan Africa, extending from the northern parts of West Africa eastwards to Sudan and Ethiopia. A little gum is of Indian origin.
Gum arabic is a complex, slightly acidic polysaccharide. The precise chemical and molecular structure differs according to the botanical origin of the gum, and these differences are reflected in some of the analytical properties of the gum. As a result, the functional properties and uses to which gum arabic can be put (and its commercial value) are also very dependent on its origin.
Unlike some other gums such as tragacanth, locust bean and the seaweed gums, gum arabic is very soluble in water and forms solutions over a wide range of concentrations without becoming highly viscous. The combination of high solubility in water and low viscosity confers on gum arabic its highly valued emulsifying, stabilizing, thickening and suspending properties. Despite some substitution of gum arabic by cheaper alternatives, brought about in the past by shortages of supply and high prices, it has remained the most important of the exudate gums and in some applications it has technical advantages which make it difficult to substitute completely. Its uses fall into three main areas: food, pharmaceutical and technical.
Food useAs noted earlier, the FAO specification for gum arabic intended for food use stipulates that it should come from A. senegal or closely related species. Even apart from legislative requirements, the quality and technical performance of gum arabic from this source makes it the material of choice in most cases. In Europe, the food additive number of gum arabic is E414.
Confectionery remains a major use for gum arabic, although supply and price pressures have led to a marked reduction in the amount of gum arabic used in some traditional items such as "fruit gums" and pastilles. The role of gum arabic in confectionery products is usually either to prevent crystallization of sugar or to act as an emulsifier. In candy products it is also used as a glaze.
It finds wide application as a means of encapsulating flavours (for example, spray-dried flavours and citrus oils) and is also used in a range of dairy and bakery products (especially as a glaze or topping in the latter). It is used in soft and alcoholic drinks, either as a vehicle for flavouring or as a stabilizer or clouding agent.
Pharmaceutical useGum arabic's use in pharmaceuticals is much less than it once was, and it has been displaced in many of its applications by modified starches and celluloses. However, it still finds some use in tablet manufacture, where it functions as a binding agent or as a coating prior to sugar coating, and it is also used as a suspending and emulsifying agent, sometimes in combination with other gums.
Technical and miscellaneous usesAn important non-food/pharmaceutical application of gum arabic is in the printing industry, where it is used to treat offset lithographic plates: as a protective coating to prevent oxidation;
as a component of solutions to increase hydrophilicity and impart ink repellency to the plates; and as a base for photosensitive chemicals.
Other technical uses include ceramics, where gum arabic helps to strengthen the clay, certain types of inks, and pyrotechnics. Use in textiles, paints and adhesives (including the traditional office glue and postage stamps) has decreased to very low levels in recent years, at least in Western markets.
N. 1 Tapping Acacia senegal var. Senegal using "sunki", in Sudan. Plantation ca. 12 years old
( Photo: J.J.W. Coppen )
N.2 Exudated gum, formed into a pale to orange brown coloured solid, ready for collection.
( Photo: The Gum Arabic Co., Ltd., Khartoum)
N.3 Primary Cleaning and granding of gum arabic at the Gum Arabic Co. depot, El Obeid, Sudan.
( Photo: J.J.W. Coppen )
N.4 Final inspection of mechanically cleaned gum arabic at the Gum Arabic Co., export depot, Port Sudan. ( Photo: J.J.W Coppen )
WORLD SUPPLY AND DEMAND TRENDS
MarketsThe use of gum arabic has a very long history but in modern times production and trade has been dominated by Sudan. Levels of supply from Sudan are therefore a good indicator of consumption.
A more detailed discussion of production levels in Sudan is given below, but towards the end of the 1960s total gum arabic production (hashab + talha) was in excess of 60 000 tonnes/year; supplies of gum arabic from other countries meant that total world usage was around 70 000 tonnes. Events in the 1970s and 1980s led to fluctuations in both the supply and price of gum arabic and, as a consequence, to changes in demand. The severe Sahelian drought of 1973/74 resulted in a world shortage of gum arabic and high prices which, in turn, accelerated the replacement of gum arabic by substitutes such as modified starches. A low point of approximately 20 000 tonnes of Sudanese exports was reached in 1975, which recovered to around 40 000 tonnes during 1979. A further drought in 1982?84, combined with political and civil unrest, saw levels of exports fall to below 20 000 tonnes in some years in the mid?/late 1980s and early 1990s.
Demand for gum arabic has therefore been constrained at times by the supply, and under these circumstances end-users who switch to alternatives do not always revert to gum arabic when supply problems are eased. It is unlikely, therefore, that world markets for gum arabic will reach the heights that they once did, although the superior properties of gum arabic (especially good quality material from A. senegal) will ensure that it retains substantial markets if availability is assured and prices are favourable.
The European Community is by far the biggest regional market for gum arabic and imports into it, with sources, are given in Table 6 for the period 1988?93. Imports averaged almost 28 000 tonnes/year over the six years, with a peak of over 32 000 tonnes in 1991.
A breakdown into destinations of imports within the EC is given in Tables 7 and 8 for Sudanese and Nigerian gum arabic, respectively. France and the United Kingdom are the biggest markets (although they both re-export a large proportion of their imports) followed by Italy and Germany. The United Kingdom has been a consistent buyer of Nigerian gum, although France and Germany have imported large quantities in recent years. France is the main importer of gum arabic from the Francophone countries of West and Central Africa.
Outside the EC, the United States is the largest market for gum arabic. Imports for 1991?94, and their sources, are given in Table 9; they averaged 7 500 tonnes annually but exceeded 10 000 tonnes in 1994.
Japanese imports averaged 1 900 tonnes/year during 1988?94; year-by-year details are shown in Table 10.
Supply sourcesThe gum belt referred to earlier occurs as a broad band across Sub-Saharan Africa, from Mauritania, Senegal and Mali in the west, through Burkina Faso, Niger, northern parts of Nigeria and Chad to Sudan, Eritrea, Ethiopia and Somalia in the east, and northern parts of Uganda and Kenya. Most of these countries appear in the trade statistics as sources of gum arabic, although they differ greatly in terms of the quantities which are involved.
Sudan is the world's biggest producer of gum arabic, and since very little is consumed domestically it is also the main source of gum in international trade. Sudanese production data are given in Tables 11a and 11b: 5?year annual averages since 1960 are given in Table 11a and yearly figures for the crop years 1988?94 are shown in Table 11b. In both cases, gum hashab is distinguished from gum talha.
The data in Table 11a show a drop in production by more than half in the last decade compared to that in the 1960s (when it averaged about 48 500 tonnes/year). In the ten years 1950?59 (not shown) production averaged just under 41 000 tonnes/year. The more detailed data in Table 11b show an all-time low of 7 600 tonnes in 1992. Since then, production has increased and it is expected to be the highest for some years in 1995.
The proportion of gum talha in Sudanese production of gum arabic (Tables 11a and 11b) is usually around 5?15%. However, in recent years (Table 11b) it has varied from less than 200 tonnes (3%) in 1992 to over 11 000 tonnes (33%) in 1994.
Exports from Sudan averaged 20 300 tonnes/year in the period 1988?94 (Table 12). Comparison with production data is difficult because of the uncertainty in the level of carry-over of stocks from one year to the next.
Nigeria is the second biggest producer and exporter of gum arabic after Sudan. Direct imports into the European Community from Nigeria averaged 4 500 tonnes/year during 1988?93 (Table 6). Import data for the United States (Table 9) show that Nigeria was the second biggest primary source of gum arabic.
Of the other producers, Chad is the next most important after Sudan and Nigeria; direct imports into the EC for 1988?93 averaged 2 000 tonnes/year (Table 6). However, a significant proportion of the gum exported from Chad, as well as from the Central African Republic, is believed to originate in Sudan and enter the neighbouring countries through illegal cross-border trade. Likewise, some gum from Cameroon originates in Chad. The 1 000 tonnes of gum arabic imported into the EC from the former Soviet Union in 1988 represents re-exports of bartered gum from Sudan.
A few countries which have gum-yielding Acacias produce gum for the local market, but not in sufficient quantities to enable exports to be made. Two such examples are Zimbabwe and South Africa, which produce gum arabic from A. karroo.
Outside Africa, India produces small amounts of gum, similar in quality to gum talha, but a proportion of her exports of gum arabic consists either of re-exports of African gum or locally produced gum ghatti (from Anogeissus latifolia) misclassified as gum arabic.
Quality and pricesThe quality of gum arabic as received by the importer is very dependent on the source. Gum arabic (hashab) from Sudan is the highest quality and sets the standard by which other "gum arabics" are judged. Not only does Sudanese gum come from a species (A. senegal) which intrinsically produces a high quality exudate with superior technical performance, but the collection, cleaning, sorting and handling of it up to the point of export is well organized and highly efficient (see COLLECTION/PRIMARY PROCESSING). Within Sudan, gum arabic from the Kordofan region has the highest reputation, and traders and end-users in importing countries often refer to "Kordofan gum" when indicating their preferences.
Nigerian gum arabic, on the other hand, has a reputation for very variable quality. Some gum is comparable to the best Sudanese quality but much of it is poorer. A major problem for importers and end-users is the inconsistent, and often heterogeneous, nature of the consignments: gum of varying degrees of cleanliness and colour is present, which reflects the less rigorous methods of harvesting and post-harvest treatment practised in Nigeria compared with Sudan. One aspect, in particular, which adversely affects the quality is the mixing of different types of gum, i.e., gum collected from different species of Acacia.
Gum talha from Sudan (produced from A. seyal) is intrinsically a poorer quality gum than hashab ? it has inferior emulsifying properties and even light-coloured samples of whole gum sometimes form dark solutions in water due to the presence of tannins and other impurities. It is more friable than hashab.
Quality criteriaAs noted earlier, an FAO (JECFA) specification exists for gum arabic intended for use as a food additive; in the United States, a Food Chemicals Codex specification exists. For pharmaceutical use, gum arabic appears in many pharmacopoeias, including the British Pharmacopoeia.
The JECFA specification has undergone a number of revisions over the years. The present one (published in 1990) specifies limits on such things as loss on drying, ash, acid-insoluble matter, arsenic, lead and heavy metals. A departure of the present specification from earlier ones (other than a modified definition) is the inclusion of limits on optical rotation and nitrogen content. Their inclusion, and the numerical limits, are designed to ensure that as far as possible only gum from A. senegal or closely related species is able to satisfy the requirements (and that, for example, gum talha is excluded).
Although gum arabic intended for pharmaceutical use needs to be of high quality, the BP specification is not as demanding as the JECFA one. Neither optical rotation nor nitrogen content are specified.
Quality control measures in Sudan include a small laboratory at the cleaning and sorting warehouses in Port Sudan. Samples of gum are regularly checked and each export consignment receives a certificate giving analytical data such as moisture content, acid-insoluble matter and optical rotation.
Grades and pricesThere are four main grades of Sudanese gum arabic (hashab), although two of these (HPS and Cleaned) are the main ones to enter international trade. The names of the four grades arise from the way in which the gum is cleaned and sorted. Small amounts of "Natural" gum (i.e., gum which has not been cleaned or sorted) used to be available but there is very little demand for this. In addition, since 1994, a processed grade (kibbled) has been available (see COLLECTION/PRIMARY PROCESSING). The grades and their export prices for 1994/95 (FOB Port Sudan) are as follows:
|Kibbled||US$ 5 000/tonne|
|HPS (Hand Picked Selected)||US$ 4 850/tonne|
|Cleaned (or Clear Amber Sorts)||US$ 4 200/tonne|
|Dust||US$ 2 760/tonne|
Gum talha from Sudan has traditionally only been sold as one grade but from 1995 it is being cleaned and graded to form three grades:
|Standard Clean||US$ 850/tonne|
|Grade 1||US$ 3 500/tonne|
|Grade 2||US$ 600-700/tonne|
PLANT SOURCESBotanical names
Family Leguminosae (Mimosoideae):
Acacia spp., especially:
A. senegal (L.) Willd.
A. seyal Del.
Numerous Acacia species yield gum, either by natural exudation or after tapping, but almost all gum arabic of commerce originates either from A. senegal or A. seyal. There is disagreement over some aspects of Acacia taxonomy but A. senegalis generally regarded as occurring as four varieties:
A. senegal (L.) Willd. var. senegal
(syn. A. verek Guill. & Perr.)
A. senegal (L.) Willd. var. kerensis Schweinf.
A. senegal (L.) Willd. var. rostrata Brenan
A. senegal (L.) Willd. var. leiorhachis Brenan
(syn. A. circummarginata Chiov.)
A. seyal occurs as two varieties:
A. seyal Del. var. seyal
A. seyal Del. var. fistula (Schweinf.) Oliv.
Other species of Acacia from which gum is, or has been, collected for local use or as minor components of poorer quality shipments for export include:
A. karroo Hayne
A. paoli Chiov.
A. polyacantha Willd.
A. sieberana DC.
Description and distributionA. senegal var. senegal is the most widely distributed of the four varieties of A. senegal and the most important and best quality source of gum arabic. It is the only variety found in Sudan, where both natural stands and plantations are tapped. It is a small to medium sized thorny tree, with a stem which is irregular in form and often highly branched. In leaf, like many other Acacias, it has a dense, spreading crown. In common with other members of the A. senegal complex it has characteristic sets of prickles on the branches, usually in threes with the middle one hooked downward and the lateral ones curved upward. The bark is not papery or peeling. In Africa it occurs throughout the gum belt described earlier but is also found in the arid or semi?arid areas of Tanzania, Zambia, Zimbabwe and Mozambique. It has a limited occurrence in India and Pakistan.
The other varieties of A. senegal have a much more restricted distribution than var. senegal and provide only very tiny amounts of gum to the market. A. senegal var. kerensis has a slightly yellowish, sometimes peeling bark and smaller pods than var. senegal. It occurs in parts of Somalia, Uganda, Kenya and Tanzania. A. senegal var. leiorhachis is also found in parts of East Africa but it occurs also in Central and Southern Africa (Zambia, Zimbabwe, Botswana and South Africa). In Kenya it occurs in two growth forms: as a well-formed tree with spreading crown and as a "whippy" form in which three or four spindly branches extend upwards and away from the rest of the tree. A. senegal var. rostrata is also variable and occurs as a tree with flaking, papery bark or in a more shrubby form. It is mainly confined to parts of Central and Southern Africa.
A. seyal var. seyal is the source of gum talha and has a much more extended range than var. fistula. It has a single straight stem with a characteristic, pronounced colour, usually orange-red, to the powdery bark, and straight thorns rather than the curved prickles of A. senegal. It has a wide distribution in Africa and is found in most of the countries where A. senegaloccurs; in Sudan it occurs in greater numbers than A. senegal. A. seyal var. fistula is distinguished from var. seyal by its creamy white bark and the presence of ant galls. It is limited to the eastern half of Africa and is not known to be used as a source of gum.
Gum from A. karroo is produced in Zimbabwe and South Africa, although the species has a much wider distribution. It occurs over a wide range of altitudes and in many different habitats. In Ghana, A. polyacantha and A. sieberana occur in the hotter, drier, northern parts of the country and are occasional sources of gum.
COLLECTION/PRIMARY PROCESSINGIn Sudan and Nigeria, virtually all gum from A. senegal is obtained by tapping the trees; there is very little natural exudation. The reverse is true with A. seyal gum. In countries such as Kenya A. senegal does produce gum naturally and all of the gum which is collected comes from harvesting natural exudate.
The following account describes the collection, handling and primary processing (cleaning) of gum hashab - gum arabic from A. senegal - in Sudan. Tapping methods have been developed which do not damage the trees, and handling and cleaning practices have been optimized to produce a superior quality product.
Tapping begins when the trees are just starting to shed their leaves, usually about the end of October or beginning of November. After five weeks the first collections of gum are made, with further collections from the same trees at approximately 15-day intervals until the end of February, making five or six collections in total.
The older methods of making small incisions into the tree with an axe have largely been replaced by one which utilizes a specially designed tool, a "sunki". This has a metal head fixed to a long wooden handle. The pointed end of the head is pushed tangentially into the stem or branch so as to penetrate just below the bark, and then pulled up so as to strip a small length of bark longitudinally from the wood. Damage to the wood should be minimal. Several branches are treated in a similar manner at one tapping. In following years, other branches or the reverse side of the previously treated branch are tapped.
After this superficial injury, tears of gum form on the exposed surfaces and are left to dry and harden. As far as possible, the tears are picked by hand from the stems and branches where they have formed, and not by knocking to the ground where they can pick up dirt. They are placed in an open basket carried by the collector; the use of plastic sacks has been found to increase the risk of moisture retention and mould formation.
For trees which have been planted from seed, tapping starts at age 4-5 years; for those planted as seedlings, tapping can start in the third year.
In Sudan, the collector sells his gum at regular gum auctions, either to a trader who then sells it on to the Gum Arabic Company, or directly to the Company if they intervene because the price does not reach the guaranteed floor price. Any trader who buys gum then undertakes the process of cleaning and grading it. This is done by hand, usually by women, who sort it into piles of whole tears and smaller pieces, separating any dark gum and removing pieces of bark and other foreign matter.
The same principles of cleaning and sorting are followed in most other countries and the trader or trading organization then usually exports the graded gum. In Sudan, however, the cleaning process is repeated when the Gum Arabic Company receives consignments of gum from the regional centres at its export warehouses in Port Sudan. Since 1991 the cleaning operation has been mechanized using a system of conveyor belts and shaking and sieving machines. Final inspection of the cleaned gum and removal of any remaining foreign matter or dark coloured pieces is made manually as it moves on a belt to be bagged.
YieldsYields of gum arabic from individual trees are very variable and little reliable data are available on which to base sound estimates of "average" yields. A figure of 250 g of gum per tree per season is often cited as an average yield. Yields of several kg or more have been reported from individual trees.
In Sudan, yields from cultivated A. senegal are said to increase up to the age of 15 years, when they level out and then begin to decline after 20 years. At this stage, if desired, trees can be coppiced and after a suitable period of time (and pruning) tapping can recommence on the new stems. In Mali, the best yields from A. senegal are said to be produced between ages seven and 15 years.
VALUE-ADDED PROCESSINGWhen imported into the consumer countries most gum arabic is further processed into kibbled and powdered forms. Kibbling entails passing whole or large lumps of gum through a hammer mill and then screening it to produce smaller granules of more uniform size. These pieces are more easily dissolved in water, and under more reproducible conditions, than the raw gum and so are preferred by the end-user.
As an extension to its mechanized cleaning process, Sudan recently installed machinery to produce kibbled gum arabic. In so doing, it became the first producer country to gain added value in this way. Production began during the 1993/94 season and approximately 2 500 tonnes of kibbled gum was produced.
Powdered gum may be produced from kibbled gum but it may also be produced by a process known as spray drying. This furnishes a high-quality, free-flowing powder with even better solubility characteristics than kibbled gum. The gum is dissolved in water, filtered and/or centrifuged to remove impurities and the solution, after pasteurization to remove microbial contamination, is sprayed into a stream of hot air to promote evaporation of the water. By altering atomizing conditions, powder can be produced with varying particle sizes and bulk densities, according to the end-user's requirements. Spray drying is an energy-intensive process and this, together with the requirements for large quantities of pure water, makes it something that most gum arabic producers could not consider. The difficulty of handling large volumes of aqueous solutions of gum in a producer country ? where ambient temperatures are high ? without suffering unacceptable increases in the microbiological load adds further to the problem.
PRODUCTS OTHER THAN GUMNo other items of trade are produced from the gum-yielding Acacias, although they are used locally as sources of fuelwood. Many species of Acacia are important sources of browse for livestock.
DEVELOPMENTAL POTENTIALA. senegal has been widely planted in Sudan and some other countries as a means of combating the process of desertification; it has also been used more generally for afforestation of arid tracts and soil reclamation. As well as environmental benefits, A. senegal provides socio-economic benefits to many thousands of communities in the "gum belt" through the production of gum arabic. In Sudan, especially, tending the "gum gardens" remains an integral way of life for many people and a valuable source of cash income.
However, demand for gum arabic is such that importers in end-user countries are always keen to encourage new sources of supply to supplement traditional sources. Thus, in recent years, Kenya has emerged as a new supplier of gum arabic to the world market, albeit a tiny one in comparison to most of the established African producers. However, the Kenyan experience is one which could be followed in some other African countries. In the semi-arid areas where A. senegal is found, the local people are often pastoralists involved in herding activities. Climatic and ecological conditions are not favourable to agriculture and there are few opportunities for growing cash crops. In these circumstances production of gum arabic ? either from an existing, wild resource of a suitable Acacia species or from A. senegal planted as part of an agroforestry system ? can generate much-needed cash.
A further attraction of promoting gum arabic collection under the conditions described above is that the realization by the local people that an economic value can be placed on the trees is likely to encourage them to preserve the trees and not to cut them down so readily for use as fuelwood as happens at present.
There are therefore numerous benefits to be gained from the production of gum arabic, either through the utilization of natural stands of Acacia or from planted sources, providing it is carried out in a sustainable manner. If due attention is given to the production of high quality gum (in particular, that gum from different Acacia species is not mixed) then not only can a new producing country aim to meet domestic needs, but it should also be capable of entering the export market.
Research needsOf the gum-yielding Acacias, most research on agronomic aspects has concentrated (justifiably) on A. senegal, although further work remains to be done. Chemical analysis and quality assessment has been carried out on gum exudates from a large number of Acacia species (as well as gum arabic-like exudates from other genera), but relatively little detailed information is available on the intra-specific variation of A. senegal gum. Some areas requiring further research are therefore:
- Vegetative propagation. Successful development of vegetative methods of propagation of A. senegal would enhance selection and breeding programmes aimed at producing superior gum-yielding trees.
- Chemical screening. In-depth studies need to be carried out to learn more about site-to-site, tree-to-tree and seasonal variations in gum quality. This applies to all gum-yielding Acacias.
- Yield assessment. Trial plots need to be established (in both natural populations and plantations) to measure gum yields on a per tree basis, and to determine the variation between and within sites.
- Resource assessment. There is an urgent need to assess the size and suitability of wild, gum-yielding Acaciaresources in those countries where they exist but where there is no, or only minor, gum arabic production.
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ANDERSON, D.M.W., BRIDGEMAN, M.M.E., FARQUHAR, J.G.K. and McNAB, C.G.A. (1983) The chemical characterization of the test article used in toxicological studies of gum arabic (Acacia senegal (L.) Willd.). The International Tree Crops Journal, 2, 245-254.
ANDERSON, D.M.W., BROWN DOUGLAS, D.M., MORRISON, N.A. and WEIPING, W. (1990) Specifications for gum arabic (Acacia senegal); analytical data for samples collected between 1904 and 1989. Food Additives and Contaminants, 7(3), 303-321.
ANDERSON, D.M.W., MILLAR, J.R.A. and WEIPING, W. (1991) Gum arabic (Acacia senegal) from Niger - comparison with other sources and potential agroforestry development. Biochemical Systematics and Ecology, 19(6), 447-452.
ANDERSON, D.M.W. and PINTO, G. (1980) Variations in the composition and properties of the gum exuded by Acacia karroo Hayne in different African locations. Botanical Journal of the Linnean Society, 80(1), 85-89.
ANDERSON, D.M.W. and WEIPING, W. (1991) Acacia seyal and Acacia sieberana sources of commercial gum talha in Niger and Uganda. The International Tree Crops Journal, 7(1/2), 29-40.
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AWOUDA, El?H.M. (1990) Indicators for present and future supply of gum arabic. pp 45-54. In Gums and Stabilisers for the Food Industry, Vol. 5. Proceedings of 5th International Conference, Wrexham, July, 1989. Oxford: IRL Press.
BADI, K.H., AHMED, A.E.H. and BAYOUMI, A.A.M.S. (1989) The Forests of Sudan. 184 pp. Khartoum: Ministry of Agriculture, Department of Forestry.
BARBIER, E.B. (1992) Rehabilitating gum arabic systems in Sudan: economic and environmental implications. Environmental and Resource Economics, 2(4), 341-358.
BESHAI A.A. (1984) The economics of a primary commodity: gum arabic. Oxford Bulletin of Economics and Statistics, 46(4), 371-381.
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BOOTH, F.E.M. and WICKENS, G.E. (1988) Acacia senegal. pp 1?7; Acacia seyal. pp 8?12. In Non-Timber Uses of Selected Arid Zone Trees and Shrubs in Africa. 176 pp. FAO Conservation Guide 19. Rome: Food and Agriculture Organization.
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DIONE, M. and VASSAL, J. (1993) [Experimental study on the means of production of gum trees of Acacia senegal: looking back on the gum tree development programmes in the Senegalese Sahel] (in French). pp 22-41. In Proceedings of 5th Sahel Workshop: Natural Resources and Social Conflicts in the Sahel, 4?6 January, 1993. Aarhus, Denmark: Aarhus University Press.
DSS (1993) Gum Arabic. A Potential Source of Income for the Mukogodo Community- [in Kenya]. Report by Department of Social Services. 14 pp. Nairobi: Ministry of Culture and Social Services.
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FAO (1992) Gum arabic [published in FAO Food and Nutrition Paper 49, 1990]. pp 735?737. 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.
GIFFARD, P.L. (1975) [Gum trees for the reafforestation of Sahelian regions] (in French). Bois et Forêts des Tropiques, (161), 3?21.
GLICKSMAN, M. and SAND, R.E. (1973) Gum arabic. pp 197-263. In Industrial Gums. Whistler, R.L. (ed.). 810 pp. New York: Academic Press.
GUINKO, S. (1991) [Study of the role of the Acacias in rural development in Burkina Faso and Niger] (in French). Ouagadougou, Aug., 77 pp.
HALL, J.B. and McALLAN, A. (1993) Acacia Seyal: A Monograph. 88 pp. Bangor, UK: School of Agricultural and Forest Sciences, University of Wales.
HANSON, J.H. (1992) Extractive economies in a historical perspective: gum arabic in West Africa. Advances in Economic Botany, 9, 107-114.
JAMAL, A. (1994) Major insect pests of gum arabic trees Acacia senegal Willd. and Acacia seyal L. in Western Sudan. Journal of Applied Entomology, 117(1) 10?20.
JAMAL, A. AND HUNTSINGER, L. (1993) Deterioration of a sustainable agro-silvo-pastoral system in the Sudan: the gum gardens of Kordofan. Agroforestry Systems, 23(1), 23-38.
JAYASINGHE, S. (1981) Plant gum exudates - an unexploited forest resource of Sri Lanka [includes Acacia and Sterculiaspp.]. The Sri Lankan Forester, 15(1-2), 54-60.
JURASEK, P., KOSIK, M. and PHILLIPS, G.O. (1993) A chemometric study of the Acacia (gum arabic) and related natural gums. Food hydrocolloids, 7(1), 73-85.
JURASEK, P., KOSIK, M. and PHILLIPS, G.O. (1993) The classification of natural gums. III. Acacia senegal and related species (gum arabic). Food hydrocolloids, 7(3), 255-280.
LARSON, B.A. and BROMLEY, D.W.(1991) Natural resource prices, export policies and deforestation: the case of Sudan. World Development, 19(10), 1289?1297.
MISRA, C.M. and SINGH, S.L. (1985) Suitable tapping technique of gum from Acacia nilotica (Babool). Van Vigyan, 23(1/2), 10-12.
NAS (1981) Acacia. Gum arabic. p 7. In Food Chemicals Codex. Third edition, 735 pp. Washington, DC: National Academy Press.
OSMAN, M.E., MENZIES, A.R., WILLIAMS, P.A. and PHILLIPS, G.O. (1994) Fractionation and characterization of gum arabic samples from various African countries. Food Hydrocolloids, 8(3/4), 233-242.
OSMAN, M.E., WILLIAMS, P.A., MENZIES, A.R. and PHILLIPS, G.O. (1993) Characterization of commercial samples of gum arabic. Journal of Agricultural and Food Chemistry, 41(1), 71?77.
PEARCE, D.W., BARBIER, E.B. and MARKANDYA, A. (1990) Natural resources in the economy of the Sudan. pp 117-149. In Sustainable Development: Economics and Environment in the Third World. Aldershot, UK: Edward Elgar Publishing.
PHILIP, M.S. (1975) Gum Arabic and Acacia Senegal. Nigerian Forestry Information Bulletin No. 29. 12 pp.
ROBBINS, S.R.J. (1988) Gum arabic. pp 18-33. 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].
VAN DEN BERGH, J. (1986) [The Acacia gum tree, tree resource of the Sahel] (in French). Afrique Agriculture, (132), 10-17.
VASSAL, J. and MOURET, M. (1989) [Preliminary results of trials of some Australian Acacia species in southern Corsica [for gum production]] (in French). Forêt Méditerranéenne, 11(2), 113-120.
3/ These specifications are currently (mid-1995) under review by JECFA and it is planned to publish revised ones in 1996
Table 6. Gum arabic: imports into the European Community, and sources, 1988?93
|Of which from :|
|Central African Republic|
|Soviet Union, former|
Table 7. Gum arabic: imports into the European Community from Sudan,
and destinations, 1988-93
|Of which to :|
Table 8. Gum arabic: imports into the European Community from Nigeria,
and destinations, 1988-93
|Of which to :|
Table 9. Gum arabic: imports into the United States, and sources, 1991-94
|Of which to :|
Table 10. Gum arabic: imports into Japan, and sources, 1988-94
|Of which from :|
Table 11a. Gum arabic: production in Sudan (5-year annual averages), 1960-94
|Of which :|
Table 11b. Gum arabic: production in Sudan, 1988-94
|Of which :|
Table 12. Gum arabic: exports from Sudan, 1988-94
|Of which :|
DESCRIPTION AND USESKaraya gum is the dried exudate obtained from trees of Sterculia species. Most gum is of Indian origin, although increasing amounts come from Africa. The gum enters trade as irregular-shaped or broken tears, with colour ranging from whitish or tan in the better grades to dark brown in the lower grades. In the early years of large-scale, commercial use it was sometimes used as an inferior substitute for tragacanth, and this led to its alternative name of Indian Tragacanth.
Karaya is an acidic, partially acetylated polysaccharide. It absorbs water very rapidly to form viscous mucilages at low concentrations, although it is one of the least soluble of the gum exudates. Although it does have food use ? in Europe it has been assigned the food additive number E416 ? its usage is overwhelmingly in pharmaceutical, dental or other medical applications, particularly those which make use of karaya's strong adhesive properties. A very minor amount is used in miscellaneous industrial applications such as papermaking and textiles.
Pharmaceutical and related useThe three most important uses of karaya are as a dental adhesive for false teeth, in the manufacture of colostomy bag fixings, and as a bulk laxative. In the first two applications there has been some substitution of karaya by cheaper carboxymethylcellulose derivatives, although recent American trade reports have suggested that some of these substitutes are not as effective as karaya. An Indian market study (ANON., 1987) reported that in France and the United Kingdom, 95% of imported karaya is used in pharmaceutical products; in the United States and Japan, the proportion was about 85%.
Food useIn foods, karaya is used in small amounts as a texturizer and stabilizer in ice creams, and in ice sherbets to prevent the formation of ice crystals. Its stability in acid media makes it suitable for addition to salad dressings, sauces, cheese spreads and some other products.
WORLD SUPPLY AND DEMAND TRENDS
MarketsThroughout the late 1960s to mid-1980s, Indian exports of karaya were in the range 4 000-6 000 tonnes/year - more than that of all other Indian gums and resins combined ? and the United States, France and the United Kingdom (in that order) were the biggest importers. Average exports for the period 1977/78-1982/83 were approximately 5 700 tonnes/year (ROBBINS, 1988).
More recent data, for the years 1987/88-1993/94, are given in Table 13. The six-year annual average for 1987/88-1992/93 is less than 1 300 tonnes, a sharp decline on the same period a decade earlier. The United States, France and the United Kingdom remain the biggest markets for karaya, although demand in the United States has fallen to such a degree that France is now the main importing country. Approximate annual averages over the whole of the recent period are: France 400 tonnes, United States 360 tonnes and the United Kingdom 210 tonnes; Japan is the next biggest single market (110 tonnes). Germany, Italy, Belgium and the Netherlands have also imported directly from India in most years (averaging 130 tonnes/year between them) so Europe as a whole is about twice the size of the American market.
Indian government controls over pricing and exports of karaya in the late 1980s, which some trade sources feel contributed to the poor supply situation caused by restrictions on tapping and low productivity, have now been relaxed, although there are mixed views in the trade as to whether karaya can regain its former position in the international market.
India is traditionally the biggest producer and exporter of karaya but increasing amounts of gum enter international trade from Africa. The quantities involved are very uncertain but if the data for Senegal in Table 2 (imports into the EC of gums and resins excluding gum arabic) refer mainly to karaya, then they could amount to around 1 000 tonnes or more annually.
Domestic consumption of karaya in India was (and still is) considerable. No recent data are available but in 1972, for example, it was about twice the volume of exports.
Supply sourcesIndia remains the biggest producer of karaya and, apart from lac, karaya is still India's most important export item in the gums and resins category. However, the data in Table 13 indicate a sharp fall in exports in 1990/91 from the previous year, with an all-time low of 570 tonnes in 1991/92. It is not known to what extent supplies from Africa made up for the drop in Indian exports. Since then, Indian exports have recovered somewhat, although they are still below the level at the beginning of the period shown in Table 13, and considerably below the levels a decade earlier.
In Africa, Senegal is the biggest producer of karaya and significant quantities are exported to France and the United Kingdom. Sudan also exports small amounts although it has the potential to produce and export much more.
Quality and pricesThere are at least five Indian grades of karaya: HPS (Hand Picked Selected), Superior No.1 and No. 2, FAQ (Fair Average Quality) and Siftings. The first four grades are the main export grades. The main quality criteria at the sorting stage are colour and foreign matter, although even after grading the quality of consignments is often variable. The higher grades should be cleaner and paler than the lower ones, which may be dark brown in colour and have bits of bark present.
A BP specification exists for pharmaceutical grade karaya, and FAO and Indian specifications have been published for karaya intended for food use.
Indicative FOB prices quoted by importers in London for Indian karaya (mid-1995) are in the range US$ 2 250-6 000/tonne according to grade. FAQ gum is about US$ 3 000/tonne. Senegalese gum has two grades, hand-picked and standard, which are generally inferior to the Indian export grades, and this is reflected in lower prices.
PLANT SOURCESBotanical/common names
Sterculia urens Roxb.
S. villosa Roxb.
S. setigera Del.
Description and distributionS. urens is a deciduous tree, up to 15 m high. It has a smooth, greyish white or reddish bark, which peels off in papery flakes. In India it occurs wild in many places on the dry, rocky hills and plateaus of central and northern regions, but it is also grown in plantations as a timber crop. The greater proportion of recent production has come from Andhra Pradesh state.
S. villosa is a small to moderate sized, spreading tree, distributed in the sub-Himalayan tract of India from the Indus eastwards, as well as more southern regions.
Several species of Sterculia occur in Africa but S. setigera is the only species known to be exploited commercially for gum. It grows up to 15 m tall and has papery, peeling bark.
COLLECTION/PRIMARY PROCESSINGThere is some natural exudation of karaya but most gum is collected by tapping. Descriptions of the tapping vary somewhat according to the source of the information, but all entail removal of sections of bark from the trunk of the tree. Guidance rules have been laid down by the Forest Research Institute, Dehra Dun, in India, but in practice the rules are not adhered to and the dimensions of the "blaze" are often exceeded. Tapping which involves deep and wide wounds to the tree to maximize gum yields is damaging to the tree, and this led to a ban on tapping by one Indian Forestry Department in the 1980s.
In India, tapping should be confined to trees with a minimum girth of 90 cm and the initial size of the blaze should be limited to 15 cm tall, 10 cm wide and 0.5 cm deep. Sixteen successive visits should be made to the tree at two-week intervals, removing a further 2?cm high section of bark above the previous one at each visit, and leading to a maximum depth of the blaze of 2.5?3.0 cm. An additional blaze can be worked for every 50 cm girth increment above 90 cm, providing sufficient space is left between adjacent blazes. By staggering the position of each new season's blazes it is possible to leave a rest period of six years before returning to a previous one, by which time the scar should have healed. Tapping is best done during the hot season to maximize yields.
In India, the collected gum is usually sold by auction to government agencies in each of the producing states, who then undertake final cleaning, drying and grading of the gum.
YieldsNo reliable data are available but the yield of gum from mature trees has been variously estimated at 1-5 kg/tree during a season.
VALUE-ADDED PROCESSINGImported gum is purified by size reduction and removal of pieces of bark by air flotation methods. Other mechanical methods are used to remove sand, dirt and other types of foreign matter.
PRODUCTS OTHER THAN GUMThe wood finds some use although it is not a high class timber. It has been employed for making packing cases, match splints, pencils, picture frames and other miscellaneous items.
DEVELOPMENTAL POTENTIALThe market appears willing to take good quality gum if it is available, as evidenced by the recent upturn in Indian exports, and pharmaceutical usage of karaya seems to be firm. However, opportunities for exploiting market demand are likely to rest more with existing producers, especially those in Africa with underexploited stands of Sterculia, than with new ones. Sudan has very large areas of Sterculia and if attention is paid to harvesting and cleaning the gum to produce material of high quality - as it is for gum arabic - then it certainly has the potential to supply much larger quantities of gum than it does at present.
Research needsImprovement of harvesting, cleaning and handling practices, coupled with market studies, is required more than basic research. Trade evaluation should be undertaken of large, representative collections of gum, by those countries having the raw material resource, in order to ascertain the scope for increased production and to gain the confidence of end-users that they would be a reliable, consistent supplier of gum.
SELECTED BIBLIOGRAPHYANON. (1973) Karaya Gum from Sterculia Urens Roxb. Industrial Series No. 7. Dehra Dun, India: Forest Research Institute and Colleges.
ANON. (1976) Sterculia. pp 43?49. In The Wealth of India. Raw Materials, Vol. 10. New Delhi: Council for Scientific and Industrial Research.
ANON. (1987) Market Survey for Select Minor Forest Products in France, UK, USA and Japan. 157 pp. New Delhi: Indian Institute of Foreign Trade.
BABU, A.M. and MENON, A.R.S. (1989) Ethephon-induced gummosis in Bombax ceiba L. and Sterculia urens Roxb.Indian Forester, 115(1), 44?47.
BP (1993) Sterculia. p 631. In British Pharmacopoeia, Vol. 1. London: Her Majesty's Stationery Office.
BIS (1985, reaffirmed 1990) Specification for gum karaya. Indian Standard IS: 5025?1985. 6 pp. New Delhi: Bureau of Indian Standards.
BIS (1988) Specification for gum karaya, food grade. Indian Standard IS: 12408?1988. 6 pp. New Delhi: Bureau of Indian Standards.
FAO (1992) Karaya gum [published in FAO Food and Nutrition Paper 38, 1988]. pp 821?823. 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.
GAUTAMI, S. and BHAT, R.V. (1992) A Monograph on Gum Karaya. Hyderabad, India: National Institute of Nutrition, Indian Council of Medical Research.
GOLDSTEIN, A.M. and ALTER, E.N. (1973) Gum karaya. pp 273?287. In Industrial Gums. Whistler, R.L. (ed.). 810 pp. New York: Academic Press.
GUPTA, T. and GULERIA, A. (1982) Gums and resins. pp 73?84. In Non-Wood Forest Products in India: Economic Potentials. 147 pp. New Delhi: Oxford & IBH.
JAYASINGHE, S. (1981) Plant gum exudates ? an unexploited forest resource of Sri Lanka [includes Acacia and Sterculia spp.]. The Sri Lankan Forester, 15(1?2), 54?60.
ROBBINS, S.R.J. (1988) Gum karaya. pp 61?66. 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].
SHAH, J.J. (1983) Gum, resin and gum-resin secretion in plants. Acta Botanica Indica, 11(2), 91?96.
SHIVA, M.P., SINGH, N.P. and THAKUR, F.R. (1994) New designed improved gum tapping tools. MFP News (Centre of Minor Forest Products, Dehra Dun, India), 4(1), 8?11.
SINGH, M. (1981) Potentialities for gum collection in Maharashtra and Gujurat. Khadi Gramodyog, 27(5), 288?290.
VERMA, V.P.S. and KHARAKWAL, G.N. (1977) Experimental tapping of Sterculia villosa Roxb. for gum karaya. Indian Forester, 103(4), 269?272.
Table 13. Karaya: exports from India, and destinations, 1987/88-1993/94a
|Of which to :|
|United Arab Emirates|
Note: a Year runs April-March
DESCRIPTION AND USESTragacanth 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 useIn 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 useTragacanth 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
MarketsIn 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.
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 sourcesIran 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 pricesTragacanth 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 higher in some previous years. In the mid-1980s it fetched around US$ 20-70/kg, depending on grade.
Family Leguminosae (Papilionoideae): Astragalus spp.
Astragalus is a very large genus and includes many Asian species. A. 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 A. gummifer is not known. These other species include A. adscendens Boiss., A. echidnaeformis Sirjaev, A. gossypinusFisch., A. kurdicus Boiss. and A. microcephalus Willd. Numerous other Astragalus species occur in the region.
Description and distributionThe 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. A. gummifer is a low shrub, up to 1 m tall, and is thorny and branching. A. 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 PROCESSINGThe 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 cross-angled 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.
YieldsGENTRY (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 A. microcephalus at 15 g per 100-day tapping season.
VALUE-ADDED PROCESSINGAs 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 GUMNo other products of economic value are obtained from the bushes.
DEVELOPMENTAL POTENTIALThe 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 needsIt 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?
- 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 BIBLIOGRAPHYBP (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].
Table 14. Tragacanth: imports into Japan, and sources, 1988-94
|Of which from :|
Source: National statistics
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