PROSEA, Introduction to Exudates
- 1 Definition and selection of species
- 2 Role of exudates
- 3 Properties
- 4 Botany
- 5 Management
- 6 Processing
- 7 Genetic resources and breeding
- 8 Research and development
- 9 Prospects
Definition and selection of species
When a tree is injured, the fluid that oozes out of the wound is referred to as an exudate. The injury may be deliberate ("tapping") or it may be natural or accidental (e.g. caused by insects, animals, pathogen attack, drought or storm damage). In most cases exudates are harvested from the stem or branches of the tree or shrub, but occasionally they are harvested from the roots. Exudation varies greatly amongst genera, and between species within a genus, and the function of the exudate in the plant is not fully understood. One probable role of resinous exudates is to serve as a defence mechanism: when the tree is wounded or attacked by insects or pathogens, it responds by "bleeding" and resin flows to the wound, sealing it off from further attack. Some terpenoids present in the resin may be fungitoxic so that in addition to physical protection there is some chemical protection in terms of inhibiting the spread of fungi. Depending on their physical and chemical characteristics, exudates are conveniently classified as resins, oleoresins, balsams, latexes or gums. Exudates are a fairly widely used and traded category of non-wood forest products.
Resins are solid or semi-solid amorphous materials, usually comprising a complex mixture of organic compounds called terpenes. They are insoluble in water but soluble in certain organic solvents. Oil-soluble resins are soluble in oils and hydrocarbon-type solvents, while spirit-soluble resins are soluble in alcohols and some other solvents. Resins can occur in almost any organ or tissue of the plant and a few, such as lac, are produced from insects. "Copal" and "damar" (from Agathis Salisb. and Dipterocarpaceae, respectively) are two important resins from South-East Asia and these are exported in large quantities, particularly from Indonesia, to many other parts of the world. Other resins, such as dragon's blood (from Daemonorops Blume ex Schult.f., also of South-East Asia) and mastic (from Pistacia lentiscus L. well-known from the island of Chios and other islands in Greece) are also articles of commerce, but are of much more restricted occurrence and are exported in very much smaller quantities.
Some resins, like copal, are very hard but others such as "Manila elemi" (from Canarium luzonicum (Blume) A.Gray in the Philippines) and pine resin (worldwide) are quite soft, particularly when fresh. Such resins are sometimes called "oleoresins" and they may be sufficiently fluid to run down the tree to a considerable distance from the point of exudation. Their softness is due to a high content of essential oil which, like the non-volatile part of the resin, is primarily a mixture of terpenes. When exposed to the air, the resin or oleoresin hardens as the volatile terpenes evaporate or polymerize. The essential oil is often a valuable product in its own right, and in the case of pine oleoresin it is distilled from the crude exudate as an integral part of the post-harvest processing (Espiloy, 1973). The volatile component of pine resin is called "turpentine" and the non-volatile part "rosin" (not to be confused with resin). Occasionally the term "turpentine" is used to describe the liquid resins from some other coniferous species (such as Abies Mill.). If the resin or oleoresin is characterized by a high content of benzoic or cinnamic acids and their esters, with a typical "balsamic" odour, then it is often called a balsam. Examples of balsams are "benzoin" (from South-East Asian Styrax spp.), "Tolu balsam" and "Peru balsam" (from Myroxylon balsamum (L.) Harms of Central and South America), and "copaiba" (from certain Amazonian Copaifera spp.).
Note that the term "oleoresin" is also used in another context to describe prepared extracts of spices or other plant materials. It is the soft extract that remains after the solvent used to extract the spice has evaporated. Most such oleoresins are used as food flavours. Another term applied solely in the context of a prepared extract is "resinoid". This is the viscous liquid, semi-solid or solid prepared from a natural resin by extraction with a hydrocarbon-type solvent; it contains any essential oils originally present in the resin and is often used for fragrance applications (Coppen, 1995b).
Latex is the colourless or milky sap of certain plants consisting of tiny droplets of organic matter suspended or dispersed in an aqueous medium. The best-known example is rubber latex, in which the solids content is over 50% of the weight of the latex. Usually, boiling the latex causes the solids to coagulate into a solid mass. The principal components of the coagulum are cis- or trans-polyisoprenes and resinous material. If the polyisoprene is mainly cis, it confers elasticity to the solid and makes it rubber-like; if it is mainly trans the solid is non-elastic and resembles gutta-percha from Palaquium spp. Several latexes are used as the natural ingredient of chewing-gums. Other useful latex-derived products are: "chicle", the coagulated latex obtained from Manilkara zapota (L.) P.Royen and produced on a commercial scale in Mexico and certain parts of Central America; "jelutong", the coagulated gutta-like material obtained from the latex of wild trees of Dyera spp. from Indonesia and Malaysia; and "sorva", the coagulated latex from certain Amazonian Couma spp. (Rehm & Espig, 1991). "Balata", the coagulated latex from trees of certain South American Manilkara spp. is an example of a latex coagulum with non-elastic properties.
The natural rubber obtained from the latex of the para rubber tree (Hevea brasiliensis (Willd. ex Juss.) Müll.Arg.) is of prime economic importance in South-East Asia and is by far the most important exudate-yielding plant worldwide. The term "rubber" includes both natural rubber obtained from plant latex and synthetic rubber, but in this book it refers solely to natural rubber.
Vegetable gums, i.e. those gums obtained from plants, are solids consisting of mixtures of polysaccharides (carbohydrates) that are water-soluble or that absorb water and swell to form a gel or jelly when placed in water. They are insoluble in oils or organic solvents such as hydrocarbons, ether and alcohol.
Exudate gums are usually obtained by tapping the stem of a tree but in a few cases roots are tapped. Seed gums are those isolated from the endosperm of some seeds. Other vegetable gums can be isolated from marine algae (seaweeds) or by microbial synthesis.
The term "gum resin" is occasionally found in literature, generally used to describe a resinous material which contains some gum. It has no precise meaning, however, and is best avoided. "Gum rosin" is used to describe the non-volatile product obtained by distilling crude pine resin or oleoresin but here, again, the word "gum" is used loosely and should not be taken to imply any similarity to gums as properly defined. The coagulum of some commercially important latexes such as "chicle" and "jelutong" is often referred to as non-elastic gum or masticatory (chewing) gum, but these are not true gums. The wide variety of gums with commercial importance include gum arabic, ghatti gum, karaya gum and tragacanth gum. In South-East Asia, however, gums are of little commercial value.
Foremost of the exudate gums is gum arabic. Although the term "gum arabic" is sometimes used to describe the gum from any Acacia species, it is strictly defined by regulatory bodies for food use as the dried exudate obtained from the stems and branches of Acacia senegal (L.) Willd. or A. seyal Delile. (Note that the explicit inclusion of A. seyal in the definition is fairly recent and was adopted by the Codex Alimentarius Commission in 1999). The principal producers of gum arabic are Sudan, Chad and Nigeria. Ghatti gum is obtained from Anogeissus latifolia (Roxb. ex DC.) Wall. ex Guill. & Perr. trees in India. Karaya gum is the dried exudate obtained from Sterculia trees, particularly S. urens Roxb. from India, and tragacanth gum is obtained by tapping the taproot and branches of certain shrubby Astragalus spp., particularly those which occur wild in Iran and Turkey (Coppen, 1995b).
In addition to the exudate or bark gums, there are also gums obtained from seeds, such as guar gum from Cyamopsis tetragonoloba (L.) Taub. and locust bean (or carob) gum from Ceratonia siliqua L., both obtained from the ground endosperm. Other natural gums are the mucilages produced by algae (e.g. alginates, agar, carrageenan) and by bacteria (dextran, xanthan) (Lawrence, 1976).
Selection of species
Though the prime use of the species discussed in this volume is to provide exudate, the group is diverse and not as clearly distinguishable as, for instance, "Timber trees" or "Medicinal plants". Recently, a global overview of "Gums, resins and latexes of plant origin" which includes the exudate-producing species of South-East Asia appeared (Coppen, 1995b). "Vegetable gums and resins" (Howes, 1949) is still much cited and although it gives a useful historical background, and discusses many of the very minor gums and resins, much of the information is now outdated. The same is true for an overview of gums and resins in the Philippines (West & Brown, 1921).
Whether a species is given an extensive entry in Chapter 2 or a brief entry in Chapter 3 depends on the present or past economic importance of the exudate. For example, Ficus elastica Roxb. was important for natural rubber production by the end of the 19th Century, so it has been given an extensive entry in this volume. There are brief entries for species whose exudates are or used to be used locally, or have not generally been commercialized, at least not in South-East Asia. Other plant resources with another primary use but also used for their exudate, are listed in Chapter 4, with a cross-reference to the Prosea volume in which they have been or will be treated in more detail. Some fairly important exudate-producing plants (e.g. Agathis spp., Palaquium spp., Pinus spp.) are also important timber-producing species. The timber-related aspects of these species were dealt with in detail in the Prosea Handbook volume 5: "Timber trees". They include many Dipterocarpaceae, the most important timber-yielding family of South-East Asia that also yield exudate ("damar"). However, two genera have been included in this volume; Dipterocarpus Gaertn.f. because of its current value as exudate producer, and Shorea Roxb. ex Gaertn.f. (mainly S. javanica Koord. & Valeton) because of its importance as an exudate producer in agroforestry.
Role of exudates
Many exudates have been used since time immemorial. Amber, a fossilized resin, has been prized since Neolithic times, around 10 000 years ago. The ancient Egyptians are believed to have employed natural resins as varnishes for coffins containing mummies and the Incas of South America used Tolu and Peru balsams as embalming resins. A 9th Century formula used to prepare an oil based varnish included galbanum, frankincense, myrrh, mastic, sandarac and several gum exudates among its ingredients. Gum arabic in North Africa has been an article of commerce from at least the 1st Century. Benzoin has a long history, being traded as long ago as the second half of the 8th Century; it is the oldest known export item from Indonesia. In his "Herbarium Amboinense" (written at the end of the 17th Century but not published until 1741-1747), Rumphius noted the traditional use of the exudates of Agathis, Canarium, Dipterocarpaceae and Sindora Miq. Rubber was used to make balls, bottles, crude footwear and for waterproofing fabric well before Columbus reported its use by the end of the 15th Century. Rubber was first brought to Europe in 1736 from South America. Gutta-percha was first taken to Europe in 1665 and again in 1843, when it aroused much more interest (van Romburgh, 1900). Generally, resins have been used for centuries for lighting, caulking boats, as incense in religious ceremonies and to repel mosquitoes as documented in the 15th Century. Some old terminology continues to be used today. The term "naval stores" to describe those products obtained from the resin of pine trees (especially rosin and turpentine) originates from the days when wooden sailing ships, including naval ships, were waterproofed with pitch and tar and other resinous products from Pinus L. Use and international trade in resins increased during the 19th Century as the technology for producing varnishes and other industrial products improved, as the chemistry of the natural products became better understood and as new applications were discovered. With some notable exceptions, the synthetic materials developed during the 20th Century replaced many natural resins (and latexes) and consumption declined.
Towards the end of the 19th Century various rubber-producing species were introduced into South-East Asia. The many exotic species trialled in the Botanic Gardens in Singapore in 1881-1900 included Landolphia heudelotii A.DC., L. kirkii Dyer, L. mannii Dyer, L. owariensis P.Beauv., L. watsoniana Vogtherr, Saba comorensis (Bojer) Pichon and S. senegalensis (A.DC.) Pichon, which are all African lianas from the Apocynaceae. Manihot caerulescens Pohl emend. D.J.Rogers & Appan ssp. caerulescens, M. dichotoma Ule, M. glaziovii Müll.Arg. and M. heptaphylla Ule (Euphorbiaceae) were imported, mainly from Brazil. M. glaziovii, which yields "Ceara rubber", was trialled fairly extensively in Central and East Java as it can grow in relatively dry areas. The other three Manihot species were trialled later, but without success. In 1899, Mascarenhasia arborescens A.DC. (Apocynaceae) was introduced from the Mascarene Islands and East Africa into Singapore and Bogor Botanic Gardens (Indonesia), but the trees grew slowly, latex production was low and the quality of the rubber was poor due to its high resin content. The rubber-yielding Sapium aubletianum (Müll.Arg.) Huber, S. jamaicense Swartz and S. jenmani Hemsl. (Euphorbiaceae) from tropical America were introduced into the Botanic Garden of Singapore early in the 20th Century. S. aubletianum produces poisonous latex, so has no potential for commercial rubber production. In Guyana, the surface of the tapping wounds of S. jenmani became too irregular to repeat tapping the same piece of bark; S. jenmani and S. jamaicense have never been tapped in Singapore (Burkill, 1966; Heyne, 1927).
Among the rubber-yielding species introduced into South-East Asia was Hevea brasiliensis which was taken from Sri Lanka to Malaysia in 1882, and then to Singapore and Indonesia. In 1899 it produced the first rubber, which was traded on the world market. Between 1908 and 1912 during the "rubber boom" rubber plantations expanded rapidly in South-East Asia (Hübner, 1934). It had become apparent that the quality, yield and harvesting of the rubber of this species were far superior to those all other rubber-yielding species. Therefore, the exploitation of South-East Asian plants for rubber that had become important during the second half of the 19th Century, declined rapidly after the rubber boom and the species introduced to assess their value for rubber production were also abandoned, though some were occasionally maintained as ornamentals.
Copernicia prunifera (Mill.) H.E.Moore, the "carnuaba" or "wax palm" from eastern Brazil, has also been tested in Malaysia, Singapore and Indonesia, but the leaves have never been exploited for their wax. The "Chinese lacquer tree" Rhus verniciflua Stokes (syn. Rhus vernicifera DC.) was introduced into Malaysia, Singapore and Indonesia, but it failed in experimental plantations. Sideroxylon foetidissimum Jacq. (Sapotaceae), a tree of the West Indies which yields a type of chewing-gum, was also introduced into Malaysia, but with only moderate success (Burkill, 1966; Heyne, 1927).
Two endemic lianas, Chilocarpus costatus Miq. and C. vernicosus Blume (Apocynaceae) have been assessed for their rubber-yielding properties. However, they can only be tapped when 10 years old and although latex production is abundant it yields only little rubber, of inferior quality. In C. vernicosus 74% of the solid material in the latex is resin. C. denudatus Blume also contains latex but has never been assessed for its rubber (Burkill, 1966).
Other exotic species that have had some impact as exudate-producing species are discussed in Chapter 3.
A major problem in the discussion of uses is that it is not always possible to know which ones are current and which ones have long since disappeared and are therefore only of historical interest. The existence of trade statistics for a particular exudate will give some measure of international demand and enable overall trends in usage to be quantified, but the extent (and nature) of local or domestic usage is usually much more difficult to determine.
Resinous products such as copal, damar and elemi are used extensively in the preparation of paints and varnishes. The benefit of incorporating "Manila copal", for instance in the manufacture of paints, is much appreciated by manufacturers in South-East Asia. The resin gives a brilliant sheen and a vivid colour to the paint, which is applied on roads and traffic signs. The term "Manila copal" arose from the time when Manila (the Philippines), was the main port exporting this resin.
Other resins are employed for medicinal purposes, the manufacture of soaps, printing inks, linoleum, plastic and waterproofing materials. Traditionally, rural people used resin as incense in religious ceremonies, against mosquitoes, as torches and for kindling fire. "Manila elemi" has a very agreeable resinous and balsamic odour and is used at present mainly by perfumeries or the fragrance industry after distillation of the essential oil. It still finds occasional use as an ingredient in lacquers and varnishes, where it gives toughness and elasticity to the dried film. It is used locally for caulking boats and for torches. It is obtained from the trunk of Canarium luzonicum and is exported from the Philippines in commercial quantities.
Important natural oleoresins are the crude resins from conifers. Almost one million tonnes of crude resin are produced annually from tapping Pinus spp., making it by far the most important natural resin of commerce and second only to rubber of all the exudates in volume terms. On distillation, the crude resin yields turpentine and rosin. Collectively, as noted earlier, turpentine and rosin (and their downstream products) are known as "naval stores". More specifically, those products obtained by tapping living pine trees are referred to as "gum naval stores" to distinguish them from turpentine and rosin obtained as by-products from sulphate pulping of pine chips ("sulphate naval stores") and the products from solvent extracting aged pine stumps ("wood naval stores"). Turpentine and rosin were originally used in unprocessed form in the soap, paper, paint and varnish industries, but today they are used mainly as precursors for the manufacture of a much wider range of products. Most rosin is modified and used in such diverse products as paper size, adhesives, printing inks, rubber compounds and surface coatings. Turpentine, like rosin, is a very versatile material and is used mainly as a feedstock by the world's chemical industries. The α- and β-pinene constituents of turpentine, in particular, are the starting materials for the synthesis of a wide range of fragrances, flavours, vitamins and polyterpene resins and form the basis of a substantial chemical industry. Synthetic pine oil, widely used in disinfectants with a pine odour, is made from turpentine. Nevertheless, despite the greater use of downstream products the simpler, more traditional uses to which rosin and turpentine can be put can still be of value to the domestic economies of countries which have standing resources of pines, including those in South-East Asia. Such local consumption may be additional to the opportunities offered by exporting the raw materials (Coppen & Hone, 1995).
Rubber is the most important product obtained from latex. Its elasticity makes it suitable for a wide range of applications, the most important being car tyre manufacture. Car components, engineering components (e.g. mountings, flooring) and consumer products (shoes, sport goods, toys, gloves) are also manufactured from rubber.
The characteristic property of gutta-percha, the coagulated latex of a number of Sapotaceae genera, is its thermoplasticity: when heated it can be moulded into any form, which is retained on cooling. Today, its use is restricted to dental surgery, but it has been used widely to insulate submarine electrical cables and in golf ball manufacture. The superior quality of the gutta-percha product obtained from the leaves led to the establishment of a large gutta-percha (Palaquium gutta (Hook.f.) Baill.) plantation in Cipetir (West Java, Indonesia) which today is the principal source of high-grade gutta-percha. The processing, which is done at the factory on the plantation, involves chemically or mechanically extracting gutta-percha from leaves.
Before Hevea plantations were developed in South-East Asia, "jelutong" (from Dyera spp.) was produced and exported for the manufacture of inferior rubber items, in which elasticity was not a prime consideration. With the advent of large-scale rubber production, exploitation of jelutong declined greatly but later regained importance as a basic ingredient of chewing-gum (Coppen, 1995b). Its properties also make it suitable for bubble gums. It is sometimes used in admixture with "chicle", the coagulated latex of Manilkara zapota, a Central American tree. "Sorva" is the coagulated latex of Couma spp. which serves as an ingredient in chewing-gums.
Common uses of gums are for cosmetics and soap, as stabilizer or emulsifier in food products (ice-creams, dairy products, sauces, jams and jellies), as paper size, and as anticoagulant compound in medicinal and pharmaceutical preparations and laxatives. Gum arabic is widely used in the manufacture of cola-type drinks and other soft drinks, for flavour encapsulation and in lithography. Gums have been extensively used as adhesives and found useful in increasing the strength of starch pastes. Emulsions of medicinal compounds, insecticides, kerosene, paraffin, neoprene and natural latex can be obtained by the addition of gums. Gum tragacanth is used in preparation of toothpastes, for coating soap chips and powders to prevent dusting and lumping, and also in hairwaving preparations (Mule, 1977). In the Philippines, Leucaena Benth. seed gum has been found suitable as paper size and as it is cheaper to obtain than the imported guar gum there is great potential for increased use (Pamplona et al., 1990).
The collection of non-timber forest products contributes much to local communities. In Palawan, the centre of almaciga resin or copal production in the Philippines, resin collection or tapping is a major income-generating activity of indigenous peoples, local traders and concessionaires. Resin gatherers, estimated to number 500-1000 earned monthly incomes ranging from 1470-2400 Philippine peso (PhP) (US$ 35-60) in 1996-1998. Gathering of almaciga resin is profitable (Garcia et al., 1999).
Tapping of elemi from Canarium on the other hand, is an additional source of income to the farmers of Quezon Province. Farmers earn at least 2000 PhP (US$ 50) as additional income per month from tapping elemi, while traders earn an average of 15 000 PhP (US$ 360) from a month's trading of elemi (1996-1998 figures) (Ella et al., 1996).
Tappers have direct links to the trees and thus hold the key to the sustainable management of the forest and the various other non-timber forest products that are collected. To ensure sustainability, tappers should use proper tapping techniques. This will not only prolong the life of trees but will also increase yield and thus the income of tappers.
In North Tapanuli (Sumatra, Indonesia) benzoin production and fruit production associated with the benzoin agroforests are major income-generating activities. Tapping of benzoin trees is a task for men, whereas the women are in charge of food crops and coffee. The same holds true for the damar forests of southern Sumatra, which bring in cash for the farmers, mainly from damar but also from other products of this agroforestry system such as fruits and timber. The exploitation of exudates in agroforestry systems implies that the farmers own the land and have complete control over it. They show responsibility over the resource, which they inherit, and they maintain it. However, if prices of the exudate are low, farmers abandon the tapping, resuming as soon as they judge prices are attractive again. Occasionally, there are disputes with the administration or timber concessionaires over borders of the land occupied by the farmers (Michon & Bompard, 1985; Watanabe et al., 1996).
It is interesting to note that cultivation and exploitation of rubber is mainly in the hands of rural smallholders: 95% in Thailand, 80% in Indonesia and 85% in Malaysia.
Trade and markets
An understanding of trade and markets is of prime importance to anyone contemplating setting up new industries or looking at the opportunities for improving or expanding existing ones. Plant exudates are no different to other commodities in this respect. Domestic markets are important but it is often very difficult to obtain information about them, either about the scale of domestic consumption or about the precise applications for which a particular exudate is used. If official statistics are available, international trade may be easier to assess and it may be possible to gain information on the volume and value of such trade and the destinations or origins of the shipments (according to whether export or import statistics are being examined). However, care needs to be taken in interpreting trade statistics.
Firstly, self evidently, unofficial trade - exudates which do not pass through customs points - does not appear in trade statistics or other documented sources. Secondly, some exudates are not separately specified in the statistics of the country concerned; they are simply included under an all embracing heading such as "Natural gums, resins, gum resins and balsams". If some major gums and resins are specified, then others may be included with "Natural gums and resins NES" ("not elsewhere specified"). Sometimes, unfortunately, classifications are changed and items that were previously separated are included under a general heading, so making it impossible to carry out any further analysis of the trade into or out of the country concerned. Thus, in 1996, Indonesia subsumed copal, one type of damar, dragon's blood and some other gums - all of which were previously separated - under a general heading "Gum", while other types of damar (e.g. mata kucing), frankincense and gaharu were subsumed under "Other resin"; gutta-percha and the various types of jelutong were also put under a general heading. Thirdly, the data are only as good as the customs' returns allow. That is to say, if the exporter chooses not to describe his shipment in precise terms - for whatever reason - then it clearly will not be recorded as such and the official returns will underestimate exports. Occasionally, items are misclassified, which can result in either inflated or deflated figures, or they may be missed by anyone examining the statistics. In pre-1996 Indonesian trade statistics, for example, benzoin is misleadingly described as "frankincense" and gum turpentine is erroneously recorded within the "Gums" section. Finally, for those cases where exudates are re exported from intermediate destinations, additional care must be taken in interpreting the statistics. Recorded exports of Indonesian origin benzoin from Singapore are far higher in volume terms than the export figures from Indonesia to Singapore would suggest, but this is due to the fact that re processing occurs in Singapore, which results in a substantial weight gain.
In all systems of customs classification, a numbering hierarchy groups commodities according to type and becomes increasingly more specific as the number of digits increases. If the commodity is a major item then it is usually specified, but otherwise it is included with similar commodities under a general heading. Most countries now use the Harmonized Commodity Description and Coding System (usually known simply as the Harmonized System (HS)) of the Customs Cooperation Council. A few countries still use the older Standard International Trade Classification (SITC) (Revision 3) of the United Nations or show the SITC number alongside the HS number. The classification numbers used by some countries for selected exudates are shown in Table 1.
The South-East Asian exudates all originate from trees. The majority of these trees also yield timber, which is often economically more important. Table 2 compares the trade in timber and exudates for a number of species.
Knowledge on the physical and chemical properties of selected plant exudates like resins, latexes and gums is important to optimize their utilization. Understanding the factors causing variation in the quality of exudates may lead to improved utilization and wider industrial application as in the case of resins such as copal, damar or elemi for the manufacture of paints and varnishes.
Resin is either a hard and brittle solid that breaks with a concoidal fracture or, if it contains more essential oils (oleoresin), it is sticky and highly viscous. Oleoresin eventually hardens through evaporation of the essential oils and/or the process of polymerization. If water from the sap of the tree has been mixed into the resin it is opaque, otherwise it is translucent or transparent. The colour of resin can vary from no tint at all to amber, red, yellow, green or black. Resins are generally soluble in alcohol, ether and essential oils, but usually insoluble in water; only damar is insoluble and Manila elemi only partially soluble in alcohol (Gianno, 1986).
Physico-chemical properties which are generally determined to assess the quality of resins are:
- solubility in 95% ethanol, not used for damar and Manila elemi. A high percentage of solubility indicates a high percentage of resin recovery. It also implies a higher grade or quality due to lesser amount of insolubles. The insolubles may include gummy and gelatinous substances inherent in the resin, and impurities or mixed particles of dirt such as soil and bark. For Manila copal this is the most important basis for commercial resin grading (Tavita & Palanginan, 1999).
- iodine number. This is a measure of the degree of unsaturation of the acids present in the resin. It indicates the presence of double or triple bonds in the acid structure to which oxygen is absorbed to effect drying. Unsaturated acids can absorb oxygen and iodine. The greater the degree of unsaturation, the greater is the amount of iodine absorbed.
- acid number. The acid number is a measure of the acidity of an acidic material. It is measured by the amount (in mg) of KOH required to neutralize one gram of resin. Typical acid numbers are 20-30 for damar mata kucing, about 20 for Manila elemi, 120-140 for copal, 160-170 for most types of rosin obtained from pine resin and 160-200 for Indonesian rosin. Copals are acidic in nature since they consist mostly of resin acids, rendering the resin insoluble in drying oils and incompatible with basic pigments. If copal resin is used for paint production, it is modified chemically or through thermal processing, which significantly reduces the acid number and converts the resin to an oil-soluble product. On the other hand, resins with high acid number are more suitable for the production of paper sizing agents.
- saponification number. The saponification number is a measure of the total amount of acids and the extent of esterification, a process which renders the resin more soluble in drying oils in the manufacture of paints and varnishes. It is determined by the amount (in mg) of KOH required to saponify one gram of resin. The saponification number is therefore important in testing the purity of the sample. Any unsaponifiable matter may be considered an impurity. The ester number is a measure of the neutral components of the resin and can be calculated by taking the difference between the saponification number and the acid number.
- solubility in organic solvents. Elemis like Manila elemi, and rosin are easily and completely soluble without heating, damar is soluble in aromatic hydrocarbon solvents like benzol and toluol. Sindora wood-oil and the essential oil of Dipterocarpus grandiflorus (Blanco) Blanco are soluble in common organic solvents except ethanol. Copal is only partly soluble in organic solvents (Tavita & Palanginan, 1999).
- melting/softening point. This is a less common descriptor of resin. When determined, it is usually given as a temperature range and not as a single value, as resin is a mixture of chemical compounds.
The essential oils distilled from resins are characterized by generally accepted physical properties such as relative density, refractive index, optical rotation and miscibility in ethanol (Oyen & Nguyen Xuan Dung, 1999).
The relevant physical properties of latex are specific gravity, acidity (pH). Also of importance is the rapid coagulation of the latex after it has exuded from the tree. In the case of Payena leerii (Teijsm. & Binnend.) Kurz it is the latex that is collected, not the coagulated latex as is the case for many other trees producing gutta-percha. Therefore, it contains much less debris and the colour of the coagulated product is whiter. The acidity influences the stability of the latex: under acid conditions the suspension becomes unstable and the latex starts to coagulate. Fresh Dyera Hook.f. latex has a pH of 7, but after standing for 48 hours the pH is 5.0, which triggers coagulation. This is caused by microorganisms and enzymes found in the latex, a phenomenon also encountered in rubber latex.
Solubility in water is the most important property. Related is the viscosity of the solution of the gum: for gum arabic, for instance, the solution does not become highly viscous even at high concentrations, whereas karaya gum forms a viscous mucilage at low concentrations. The quality of a gum depends on its purity, both in terms of absence of foreign material such as bark or soil particles, as well as the absence of gums of other species.
As indicated earlier, most resins are complex mixtures of terpenes. In the case of balsams, a significant proportion of non-terpenoids (benzoic and cinnamic acids) is present. For many resins, the terpene mixture consists of a volatile fraction (primarily containing monoterpenoids and sesquiterpenoids, i.e. C10 and C15 compounds) and a non-volatile fraction (containing mainly di- or triterpenoids, i.e. C20 or C30 compounds). For hard resins such as copal and benzoin the amounts of volatile compounds present are negligible or small and no attempt is made to separate them in any post-harvest processing from the much larger quantities of non-volatile compounds. For soft resins or oleoresins such as pine resin and Manila elemi, the volatile fraction (essential oil) forms a significant proportion of the total mass (up to 30% or more) and may be separated by steam distillation from the non-volatile part of the resin. Crude pine resin is always distilled to yield its essential oil (turpentine).
It is not only the proportions of volatile and non-volatile fractions that vary greatly amongst the different resins; the composition of the respective fractions is also very variable, both qualitatively and quantitatively. Within a genus the compositional differences between species may also be considerable. This is particularly true in Pinus, where the turpentine is much more variable than the non-volatile fraction. Even within a species, the composition of pine resin can vary markedly according to provenance; P. merkusii is a case in point: Indonesian and Thai populations have quite different turpentine compositions to Filipino ones and within the former provenances there are further significant differences. For all these reasons there is little point in trying to detail chemical compositions here, either actual, average or "typical"; where details are available they are given in Chapter 2.
Latex can be coagulated by destabilizing the colloidal dispersion which causes the polymeric phase to separate from the liquid phase. Coagulation can be provoked by a number of chemical compounds, called "coagulants". The coagulated latex is the "coagulum", the solid phase after coagulation.
The polymeric substance in rubber latex is predominantly cis-1,4-polyisoprene (C5H8)n, whereas that in the latex of species yielding gutta-percha is trans-1,4-polyisoprene. The cis-form of the polyisoprene renders the coagulated latex elastic and rubber-like, whereas coagulum mainly consisting of the trans-form is non-elastic and similar to gutta-percha. When gutta-percha is heated, however, it can be moulded into a shape that is retained upon cooling: it is "thermoplastic". In addition to containing rubber and gutta-percha the latexes may contain water, resin and foreign material. The amount of these substances determines the quality of the latex: a high resin content in the latex of Sapotaceae decreases the quality of the gutta-percha. Processing of the coagulum is aimed at increasing the rubber or gutta-percha content by removal of the other materials.
Ultracentrifugation of natural rubber latex gives the following fractions:
- light fraction, mainly rubber: 35% (= dry rubber content or DRC),
- a thin yellow layer containing the Frey-Wyssling particles: 2%,
- a watery and clear fraction, called "C serum" or simply "serum": 48%,
- a heavy fraction, the "bottom fraction": 15%.
The polyisoprenes in rubber have a very high molecular weight of 500 000-2 000 000. The particle size ranges from 15 nm to 5.6 μm.
The first main non-rubber particles are the Frey-Wyssling particles, less numerous than lutoids and mainly composed of lipid material. They are yellow or orange due to the presence of carotenoids, a by-product of polyisoprene biosynthesis.
The C serum is essentially aqueous and contains carbohydrates, inositol, nitrogenous compounds, proteins, nucleic acids, inorganic ions and metal ions.
The bottom fraction contains numerous vesicles or vacuoles, 0.5-3 μm in diameter, bounded by a semi-permeable membrane mainly consisting of lipids. The "lutoids" are the second main non-rubber particles in fresh rubber latex. The liquid content ("B serum") causes coagulation when released from lutoids ruptured by mechanical, osmotic, chemical or electrical activity during and after tapping. This process is still under intensive research and insufficiently understood.
Fresh latex is a dual colloidal system composed of: first, negatively charged particles (mainly rubber and lutoids) dispersed in the neutral C serum containing anionic proteins; and, second, a system within the lutoid membranes comprising the acidic B serum with metallic ions (especially Mg++ and Ca++) and some cationic proteins. The two antagonistic systems can only exist as long as they are separated by the intact lutoid membranes; release of the B serum results in interaction between its cationic contents and the anionic surfaces of the rubber particles, causing the formation of floc (Compagnon, 1986; Webster & Baulkwill, 1989).
Gums are complex mixtures of polysaccharides of high molecular weight, e.g. 240 000 for gum arabic. On hydrolysis, gums yield simple sugars such as arabinose, galactose, mannose and glucuronic acid. Gum quality varies with growing sites, individual trees and seasons, but this is still poorly understood.
Important resin-yielding families are: Burseraceae, Dipterocarpaceae, Leguminosae (mainly Caesalpinioideae), Styracaceae and two coniferous families each with one important resin-producing genus, viz. Araucariaceae (Agathis) and Pinaceae (Pinus). These families are presented in Table 3 with their estimated annual world production.
Other families of some importance for resin production are listed in Table 4.
Latex-producing plants are mainly restricted to a relatively small number of families, including Apocynaceae, Euphorbiaceae, and Sapotaceae (Table 5).
The Moraceae do not figure in Table 5 as the products obtained from their latexes are very limited. The latex from several Moraceae is used locally for glues (including birdlime) and to cover and cure wounds (Calophyllum spp. and Ficus spp.) and is used on arrow tips as it has a strong cardiovascular effect (Antiaris toxicaria Lesch. and Artocarpus spp.).
In South-East Asia, there are few plant species producing gums and their economic importance is insignificant. Worldwide, the Leguminosae is the most important family, yielding important gums of commerce such as gum arabic (Acacia Mill.), tragacanth gum (Astragalus L.), locust bean (Ceratonia siliqua), guar gum (Cyamopsis tetragonoloba, mesquite gum (Prosopis L.), tara gum (Caesalpinia spinosa (Molina) Kuntze), and tamarind seed gum from Tamarindus indica L. Ipil-ipil seed gum from Leucaena leucocephala (Lam.) de Wit is gaining importance in South-East Asia. In other families, only karaya gum from Sterculia L. (Sterculiaceae), ghatti gum from Anogeissus latifolia (Combretaceae) and kutira gum from Cochlospermum religiosum (L.) Alston (Cochlospermaceae) are important gums. Gum arabic, tragacanth gum, karaya gum and kutira gum are exudate gums, the others are seed gums. (Coppen, 1995b; Rehm & Espig, 1991)
Resin and gum ducts
Resin and gum ducts can be a normal feature in plant tissue, for instance the resin ducts in Pinus wood and in Canarium bark. Production from existing resin ducts is low, however, and exudates are usually formed by secondary resin and gum flow, i.e. by newly formed resin and gum ducts and associated tissues. Their formation is induced by microorganisms, insects, mechanical injury (e.g. tapping), and physiological disturbances. "Gummosis", as this process is known, is thought to be a result of metamorphosis of the organized cell-wall materials to unorganized amorphous substances such as resins and gums.
Secondary resin flow is generally produced in newly formed wood after injury of the cambium. Its resin acid content is different from resin in unwounded tissue (primary resin). Numerous ducts are formed in one, sometimes more, concentric circles in the wood. These are also known as "traumatic ducts" or "traumatic canals" (see Photo 1). After injury, the cambium forms special groups of parenchyma cells instead of normal wood elements. At first, these cells develop schizogenously: they grow intrusively between existing cells. Soon after, the central cells of these parenchymatous groups start to disintegrate and to produce resin and this process proceeds to the periphery. In each cell the disintegration of cell walls starts in the primary wall and then proceeds towards the innermost lamella of the secondary cell wall (lysigenous development). The resulting cavity is filled with resin. As both processes are closely interlinked, this type of resin duct formation is called "schizo-lysigene" (Tschirch & Stock, 1933).
The resin ducts of Coniferae have been extensively studied. In this taxon the traumatic ducts are located in the wood and are lined with epithelial cells. In some genera (e.g. Abies, Tsuga (Endl.) Carrière) epithelial cells die in the year of formation, whereas in others (e.g. Pinus) they remain active for several years. In the tangential plane the resin ducts anastomose extensively to form a network of ducts. They have an open connection to the wound and the resin exudes between the renewed bark and the wood exposed by the wound. More importantly, the wound stress reaches the tissues above the wound, where it induces resin duct formation in the wood for up to several centimetres above the wound. The number of resin ducts formed depends directly on the size of the wound (Hillis, 1987; Tschirch & Stock, 1933).
Normally-formed resin ducts in Pinus are elongated structures built of thin-walled epithelial cells surrounding an intercellular space (see Photo 2). The cross-section of this space is round, unlike that of the traumatic ducts, which are much more irregular in shape. On the outer side of the latter there is a sheath of cells of one or more layers with relatively thick walls, very rich in pectic substances. In secondary tissues, both vertical and horizontal ducts occur. The inner end of each radial duct is connected to a vertical duct of the secondary xylem, and the lumina of the two types of ducts are continuous. The vertical ducts have a larger diameter (100-200 μm) than the horizontal ones (30-50 μm) (Hillis, 1987). Radial ducts continue in the phloem, the outer end is enlarged into a cyst-like vesicle. There is no connection between the resin system of the different organs of the primary body (needles, shoots), between them and the duct systems of the xylem and between the separate systems of the xylem itself. This may explain why differences in composition of terpene fractions of resin could be detected at different heights of the trunk and between different organs and tissues. No vertical ducts occur in the phloem of Pinus (Fahn, 1979).
Epithelial cells produce the resin which passes into the lumen of the duct where it collects. In Pinus, the lipophilic droplets have been observed in the plastids, in the periplastidal and cytoplasmic endoplasmic reticulum, in the mitochondria and in Golgi vesicles of the secretory cells, suggesting that these are the locations of formation of the resin (Fahn, 1978). When the volume of resin in a duct increases it compresses the epithelial cells which, because of the reduction of their size, increases their osmotic potential. This encourages absorption of water and when this occurs cells grow larger and pressure is exerted on the resin in the duct. If the latter is ruptured, the resin is forced out, and the epithelial cells may then produce more resin to refill the duct. Obviously this procedure of emptying and refilling can best be performed in ducts lined by thin-walled cells, as found in pines (Hillis, 1987).
The amount of resin exuded when a duct is ruptured is also influenced by the length of the duct. In pines the ducts are much longer than in the other conifers and can average from about 10 cm in the centre to about 50(-100) cm in the outer sapwood layers. Pines are more sensitive to injuries than other conifers; moreover, this sensitiveness increases with the age of the tree. Many of the injuries made by insects result in greater development of ducts than do purely mechanical wounds of the same size. Wounding, pressure, and the auxins indoleacetic acid (IAA), naphthalene acetic acid (NAA), and 2,4-dichlorophenoxyacetic acid (2,4-D) induce the formation of vertical ducts, but radial resin ducts are formed after a layer of ductless wood has first been laid down (Hillis, 1987).
Gums are formed in this way in the bark of Acacia senegal and other Acacia spp. Occasionally, gums are formed in the wood, e.g. in the Prunoideae (Rosaceae), where vessels are frequently filled with gum. Here, the gum production most probably results from secretion from vessel-associated parenchyma cells.
A laticifer is a specialized cell or a row of cells containing latex. Two types of laticifers are distinguished based on their formation. Non-articulated laticifers arise from single initial cells in the embryo or seedling. Articulated laticifers consist of chains of cells whose adjoining walls may sometimes break down, forming tubes or vessels (Metcalfe, 1967). Both types can branch, giving rise to anastomosing laticifers, or to non-anastomosing laticifers.
Non-articulated laticifers are formed in the primary tissues and these cells extend by intrusive apical growth into intercellular air spaces. Successive mitoses which are not followed by the formation of cells make the laticifer a multinucleate cell. In the lower portion of the cell the nuclei soon disappear. Growth of the laticifer occurs only in tissues in which the cells have not lost their ability to divide. In lianas the non-articulated laticifers can become several metres long.
Articulated laticifers are formed by anastomosis of the cells in a chain which are of primary or secondary origin. The laticifers of rubber, for instance, occur in the bark, which is a secondary tissue. The end wall of such cells remains entire or becomes porous or disappears. In the latter case the final result is a large multinucleate structure which cannot be differentiated from the non-articulated laticifers. However, all laticifers of secondary origin formed by the cambium of stem or roots are articulated laticifers (Fahn, 1979).
Laticifers in wood (secondary xylem) usually extend radially in the rays (in Apocynaceae, Asclepiadaceae, Campanulaceae, Caricaceae, Euphorbiaceae and Moraceae) or axially (only known in Moraceae). Articulated laticifers are formed in concentric rings in the bark of the stem of Hevea brasiliensis and in the phloem of the root of Taraxacum kok-saghyz L.E.Rodin. Branching and subsequent anastomosis in these plants only occurs tangentially between laticifers of the same ring. Non-articulated laticifers are found in the Apocynaceae, Asclepiadaceae, Euphorbiaceae (excluding Hevea Aubl. and Manihot Mill.) and Moraceae, whereas articulated laticifers occur in Caricaceae, Compositae, Hevea and Manihot (Euphorbiaceae), Papaveraceae and Sapotaceae.
In Parthenium argentatum A.Gray ("guayule", Compositae), latex occurs in parenchymatous cells. This is, however, the only example of laticifers that are not morphologically differentiated. In a few species, notably Guajacum officinale ("lignum vitae", Zygophyllaceae) there is rubber in the xylem.
Exudate-producing plants are managed either in plantations, in agroforestry systems or in natural forests (see Table 6). Apart from rubber, the only exudate for which a plantation has been established is gutta-percha: a plantation of Palaquium gutta was established in Cipetir, West Java, in 1885 and is still functional despite the falling demand for the product. Due to the present low demand for gutta-percha, management of the plantation is at a low level, as the plantation is sufficiently large to supply the present annual demand. It is not economically viable to intensify harvesting, which would increase production. For the same reason, old and/or poorly productive trees are not being replaced and natural regeneration seems satisfactory in the present situation. Finally, note that in South-East Asia no indigenous exudate-producing plants are currently being managed intensively in plantations, primarily for their exudate.
There are plantations from which exudates are harvested, but which have another primary goal: the production of timber. This is the case of Pinus and Agathis, although pine resin is of particular economic importance in Indonesia. A century ago both genera were exclusively exploited in natural forest. Since then, exploitation of natural forest has gradually decreased because of dwindling resources and the advent of forest plantations for timber and raw material for pulp and paper manufacture. In these plantations, tapping increasingly proved to be a viable option, especially in the case of tapping Pinus trees a few years before felling. There are now fairly large plantations of both Pinus merkusii Jungh. & de Vriese and Agathis dammara (Lambert) Rich. in Java, where the bulk of pine resin and "Manila copal" is produced in South-East Asia.
It is interesting to draw a parallel with the developments in rubber cultivation. Here, plantations have always been primarily geared towards latex production. A fairly recent development is that in these plantations, increased attention is being paid to another product: wood. Rubberwood is now a well-known and a well-accepted product in international trade. The parallel between this development and trends in forest plantations of the other exudate-producing trees (Agathis and Pinus) is that the plantations are becoming more multifunctional, with greater interest being shown in the various products which these trees can yield. Whether this is a general trend in landuse in South-East Asia caused by an increasing pressure on land resources cannot be confirmed here.
The natural forest in South-East Asia is generally owned by the state. In most national legislations it is considered a resource from which non-timber products may be collected by all people living in or near the forest. In the case of the harvesting of exudates, it is customary for those persons who started tapping particular trees to exert use rights over these trees for the period of tapping. Only rarely are use rights exerted over individual exudate-yielding trees which are currently not tapped. The appropriation of the rights to individual trees guarantees the sustainable use of the trees. Towards the end of the 19th Century, when many exudates were rapidly becoming important as an economic commodity, exploitation of natural forest was the rule. At that time, some exudates (e.g. gutta-percha) were harvested by felling the tree whereas others (e.g. jelutong) were tapped in such a destructive way that the trees died. The financial gain from natural forest proved to be more important than the sustainability of exudate collection. Hence the exudate-yielding species in certain tracts of forest, especially those which were easily accessible, were rapidly depleted. This was the case for Palaquium trees at the end of the 19th Century. As a result national legislation was endorsed to counteract this depletion. In the Philippines, for instance, exploitation permits specifying the area and the amount of resin which may be collected are mandatory for the tapping of Agathis philippinensis Warb. (Callo, 1995).
The case of Styrax in the Tapanuli region (Sumatra, Indonesia) is an instance of natural forest that has always been maintained for the collection of benzoin. Instead of natural forest being depleted, there has been a well-managed transition from natural forest to a forest with a larger proportion of Styrax trees. During the transition, there was both natural regeneration and sowing or planting. Most of the production of benzoin from Sumatra currently comes from agroforests in which a number of annuals and perennials, mainly fruit trees such as durian (Durio zibethinus Murray), jengkol (Archidendron jiringa (Jack) Nielsen), petai (Parkia speciosa Hassk.) and kemiri (Aleurites moluccana (L.) Willd.) have been planted. The plantation and management of several (perennial) crops reduces the risk for farmers, as benzoin prices fluctuate (Achmad, 1998).
Another interesting case is the damar forests of Lampung Province (Sumatra, Indonesia), which are an agroforestry system producing the resin from Shorea javanica. In this region all the damar forests have been planted and there has been no gradual transition from natural forest to agroforest. As seed availability is irregular and seeds cannot be stored, the farmers "store" the seedlings instead, using them both in existing damar forests as well as for establishing new ones. The management of existing damar forests is aimed at maintaining a minimum level of different development stages of the damar trees, ensuring a more or less continuous production of these individually owned agroforests. Natural regeneration is probably too unpredictable to rely on when managing the damar forests (Michon & Bompard, 1985), so only the natural regeneration of a number of other tree species yielding timber is maintained. Here too, fruit trees, mainly durian and "duku" (Lansium domesticum Correa) are being introduced as a risk-minimizing strategy of the farmers (de Foresta & Michon, 1994).
Nowadays, resins like copal, damar and elemi which are intended for international markets, are obtained by tapping the tree, rather than collecting fossil resin (in the case of copals) from the ground. To obtain the copal and elemi from the living inner bark of the trunk, the bark of the trunk is incised. Fresh cuts are made at suitable intervals a few days or a week or more, gradually moving up the tree, and the exudate is collected. The size and shape of the cut and its frequency of application vary according to the country or region in which tapping is undertaken, or the tradition of the communities involved. The recommended way of tapping Agathis and Canarium trees is shown in Figure 1(1), in which a narrow horizontal strip of bark is removed above the panel without damaging the cambium (Ella & Tongacan, 1992; Ella et al., 1998a, 1998b).
Present practice in Indonesia, specifically in Java (the biggest producer and exporter of copal), is for the tapper to return to the tree to make fresh incisions every 3-4 days; up to 4 or more small tin cups may be in place at different points on the tree at any one time, depending on the size of the tree (Coppen, 1995b). In the Philippines, the second biggest producer of copal, research has been conducted on tapping methods very similar to those used in tapping pine trees (involving the use of sulphuric acid as chemical stimulant) but these methods are not yet being used commercially. Experiments have also been done on using Ethrel as a source of ethylene to stimulate resin flow, but these too have not yet been applied commercially (Ella, 2000).
Crude resin from Pinus trees is collected in a variety of ways worldwide. Some traditional methods, as used in Indonesia for example, entail making a shallow cut into the wood every 3-4 days. The panel opened is enlarged at every consecutive tapping by removing a narrow strip of bark and a little sapwood above the panel as indicated in Figure 1(2). In many other countries, "bark chipping" methods are used, which involve removing a small strip of bark from the stem and applying an acid stimulant, either as a liquid spray or as a paste. The stimulant does not increase the biosynthesis of resin but simply keeps the resin ducts open for longer. In this way the tapper only has to return to the tree to repeat the treatment every 1-2 weeks. In China an alternative method is used in which small channels are cut into the xylem in a V-shape or "herringbone" pattern, while in India a similar "rill" method of tapping is used, involving the use of an acid stimulant. In all cases, a small container is fixed to the tree, into which the soft resin runs and accumulates. If carried out correctly none of the tapping methods used, or the use of acid, are damaging to the tree.
Damar is obtained from the wood of Dipterocarpaceae (e.g. Dipterocarpus, Shorea) which are tapped by cutting holes in the stem ("boxing") in which the resin can collect (Figure 1(3)). Once again, the number of holes and the frequency of harvesting, scraping the holes and enlarging them vary with species and region (Torquebiau, 1984). Often, fire is used to stimulate the flow of the resin. Sindora wood oil is harvested in the same way.
Figure 1(4) shows the particular way in which Styrax is tapped: a small tongue of bark is loosened up to the cambium. The resin exudes from the rays in the wood and collects behind the tongue, which is cut off after the resin has solidified. The same tree can be tapped again by gradually moving upward along the stem.
To counter the detrimental effects of traditional tapping methods, the Forest Products Research and Development Institute (FPRDI) in the Philippines has produced guidelines for the proper tapping of "almaciga" resin (Agathis philippinensis) and elemi (Canarium spp.) (Ella et al., 1998a, 1998b). The FPRDI group has already conducted a number of training sessions for resin tappers all over the Philippine archipelago. The tappers learn improved tapping and harvesting techniques to prevent too deep tapping, overtapping by opening too many and/or too large panels, too frequent tapping, and resin contamination with e.g. bark and dirt. The training sessions have been welcomed by both almaciga and Canarium resin licensees, and the tappers are keen to learn the improved techniques that enable them to preserve the resin-producing trees which have for so many years been providing them with a source of livelihood.
The techniques for collecting latex differ according to the plant sources. Most latex-producing species, however, are tapped by making series of cuts around the trunk and removing the bark, as is the case for chicle, jelutong, sorva, and gutta-percha. This practice however, is indiscriminate and inflicts great damage to the trees, especially in the case of gutta-percha. The traditional method of harvesting the gutta-percha latex depicted in Figure 1(5) shows a felled tree with rings cut into the bark and with cups placed underneath to collect the exuding latex. With the threat of extinction of the gutta-percha trees and disappearance of the gutta-percha industry, attention has finally focused on more effective and less destructive methods, such as the one evolved in Singapore, Sumatra, Java, and Borneo which involves extracting gutta-percha from the leaves with a chemical or mechanical process. The superior quality of the product obtained from the leaves led to the establishment of a large gutta-percha plantation in Java, which is now the principal source of high-grade gutta-percha.
The latex of the para rubber tree is harvested by tapping the bark without damaging the cambium. Using a V-shaped tapping knife, grooves are cut at an angle of 30° with the horizontal. At each consecutive tapping a thin slice of bark is removed. The latex runs along the groove into a vertical groove which transports the latex via a metal spout to a cup (Figure 1(6)). After 5-6 years of tapping the panel is 150 cm high; in smallholdings, however, bark consumption is much greater. Stimulants are increasingly being applied in rubber plantations, to decrease the dependency on labour which is in short supply.
The tapping of Dyera trees for their latex is shown in Figures 1(7) and 1(8): in both cases only bark is removed, without damage to the cambium. The tapping panel in Figure 1(7) is enlarged horizontally until the edges meet up. The V-groove in Figure 1(8) is enlarged vertically in both directions and a cup is needed to collect the latex. Ficus and Madhuca, Palaquium and Payena trees are not retapped, because the laticifers are not anastomosing and the latex that is harvested is what drains from the immediate vicinity of the place where the bark has been removed. Ficus elastica used to be tapped similar to the techniques indicated in Figure 1(7) and 1(8), but without enlarging the tapping panel.
As can be clearly seen from Figures 1(6),(7),(8), techniques for tapping latex entail removing the bark, but never up to the cambium, thus pathogens do not get easy access to the wood. The undamaged cambium can form new bark tissue which may be tapped again.
The relationship between the tissues tapped and the type of exudate is given in Table 7 for the South-East Asian trees dealt with in this book.
Exudate gums are usually, but not always, obtained by tapping the trees. In Sudan, Acacia senegal (in natural stands or in "gum gardens") is tapped, but the gum arabic from A. seyal is collected from natural exudation. In Sudan, tapping methods have been developed which do not damage the trees and normally begin when the trees are just starting to shed their leaves. Traditional methods of making small incisions into the tree with an axe have largely been replaced by one which uses 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 subsequent years, the other branches on the opposite side of the previously treated branch are tapped (Coppen, 1995b).
Drops of gum accumulated on the exposed surfaces after tapping are left to dry and harden. It is recommended that hard pieces of gum be picked off by hand and not by knocking them off onto the ground, where they can pick up dirt. Whenever possible, gums should be placed in an open container; the use of plastic bags increases the risk of mould formation.
In the case of Astragalus, gum is produced in the taproot and when the root is cut, it exudes rapidly and the exudate hardens into the characteristic ribbons of tragacanth. The process of tapping entails clearing the earth away from the taproot and making one or two cuts into the upper part of the root.
Unlike exudate gums obtained from the trunk of trees, where the sunlight shining on the tree increases gum flow, most exudation of tragacanth occurs at night, under conditions which minimize the drying out of the gum and maintain the outward flow under high osmotic pressure.
The primary processing of collected resin entails a preliminary cleaning (picking over by hand and, less commonly, sieving to remove foreign matter such as pieces of bark). Sorting and grading is common practice and is carried out by hand and/or sieving. When grading benzoin, special care is taken not to break the whole "tears" of the resin, which are the most highly valued form of benzoin. Crude pine resin cannot be used in its original form and is rarely exported as such. The distillation of crude pine resin to obtain rosin and turpentine can be considered part of "primary processing". The oleoresin of Dipterocarpus is filtered locally in gunny sacks to clean it and to partially remove the non-volatile fraction which is more viscous and remains behind. Much of the Sumatra benzoin which enters international trade has undergone secondary processing. "Block benzoin" consists of pale pieces of damar embedded in a much darker matrix of low quality benzoin. Occasionally, pieces of high-quality benzoin ("almonds") are used instead of damar. The blocks are made according to a well-tried formula that involves cooking the mixture briefly in hot water. After cooling, the mass solidifies and is made into blocks that are easy to transport and handle. Damar acts as a binder, it improves the burning quality as does the presence of powdered bark, although the scent of damar is inferior (Coppen, 1995b; van de Koppel, 1950).
The value-added processing of resin may consist of purification by ethanol, or hydrocarbon solvent extraction to improve its quality for lacquer and varnish manufacture. Distillation of resin or oleoresin may yield valuable essential oil, as is the case for pine resin and elemi.
In the Philippines, the major application of almaciga resin is in the manufacture of varnish and paint by the cottage industry and small furniture makers. Value-added processing entails the pulverization of the raw almaciga resin, which is then dissolved in 95% ethanol. The solution is filtered to remove impurities and insolubles. The clean resin solution is heated at 60°C for 30 minutes, cooled, and is then ready for packing (Tavita & Palanginan, 1999). Modifying almaciga resin chemically or through thermal processing significantly reduces its acid number, usually by the preparation of copal esters, and renders it oil-soluble (Gonzales et al., 1991).
Value-added processing of "pagsahingin" (resin from Canarium asperum Benth.) for paint production involves heating the raw product with xylene. The solution is screened and filtered to remove impurities. The clean solution is distilled to separate the solvent and the "modified pagsahingin resin".
Primary processing of the latex like gutta-percha entails pressing the partially formed coagulum into blocks after first softening it in hot water and removing larger pieces of foreign matter. The blocks are then transported to the factory for further processing; if they need to be stored for any length of time before transportation they are best kept under water to avoid spoilage by aerial oxidation (van Gelder, 1950; Williams, 1963).
Preparation of purified gutta-percha involves chopping the blocks of crude material into small pieces, removing the resinous (non-gutta) fraction by dissolution in cold petroleum spirit, and then dissolving the remaining, separated gutta fraction in hot petroleum spirit. The hot extract is drained from any insoluble and foreign matter and then allowed to cool, whereupon the purified gutta-percha separates out. After separation and distillation of residual solvent the hot, plasticized gutta is rolled into sheets and stored, either in the dark in well sealed tins, or in water. Solvent extraction of gutta from harvested leaves follows the same principles as above, but involves pulverized leaf material instead of chopped crude gutta-percha. Bleaching earth is added to the hot mixture to remove unwanted leaf pigments.
An alternative method of processing the leaves involves digesting the leaf pulp in hot water, and collecting the coagulated latex which separates out and pressing it into blocks.
Rubber latex is filtered and bulked on arrival at the factory. Generally, it is coagulated with formic acid in tanks. To produce sheet rubber the coagulated latex is then milled through pairs of rollers, the last pair of which are ribbed. The milled sheets are dried in a smoke house for several days to produce ribbed smoked sheets ("RSS"). If crêpe rubber or air-dried sheets are required, the coagulated latex is milled using crêpers to produce a well-knitted thin crêpe. After milling, the crêpe can be dried in hot air rooms.
After being softened by grinding or by dissolving in a suitable solvent, the rubber is compounded with other ingredients. The compounds added are for instance carbon black, a filler to increase wearing resistance, whiting for stiffening, antioxidants, plasticizers (usually oils, waxes, or tars), accelerators and vulcanizing agents. After the compounded rubber has been sheeted, applied as coating or moulded, it is vulcanized.
Most commercial rubbers are diene polymers. These rubbers deform readily when the randomly coiled chains extend due to rotation about C-C bonds in the polymer backbone. However, slippage of chains gives unlimited extension, the sample gets thinner and eventually breaks. Vulcanization involves the formation of chemical crosslinks between neighbouring chains, thereby preventing chain slippage, limiting extension and ensuring that the original dimensions are recovered on removal of the load. Vulcanization is effected by heating the rubber up to 140-170°C with sulphur, one or more organic accelerators, a metal oxide (ZnO) and an organic acid (stearic acid). It comprises a complex sequence of parallel and consecutive reactions. Many different formulations are used, the properties of the product being a function of the particular recipe. Vulcanization can be applied in moulds (e.g. in car tyre manufacture) where the transfer of heat is more favourable than in the case of "boiler vulcanization" (e.g. with hot air). Non-vulcanized rubber is sticky and rapidly becomes hard and brittle (Compagnon, 1986).
Although the invention of the vulcanization process is ascribed to Charles Goodyear in the year 1839, in Ancient Mesoamerica (by 1600 B.C.) people were already making rubber articles like solid balls, important in religious ball games, and hollow human figures and bindings. They applied the juice of "morning glory" (Ipomoea alba L.) to the latex of Castilla elastica Sessé to make rubber articles elastic and tough, but still workable. Morning glory also contains sulphur compounds that are capable of cross-linking the latex polymers and introducing rigid segments into the polymer chains. Today, local communities in Chiapas, Mexico still process rubber using the same methods as recorded by the Spanish observers in the 16th Century, i.e. mixing the latex with the juice of Ipomoea alba (Hosler et al., 1999).
Gums such as gum arabic are often further processed into kibbled and powdered forms after they have been imported into the consumer countries. 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 dissolve more easily in water, and under more reproducible conditions, than the raw gum and so are preferred by the end-user.
Powdered gum may be produced kibbled, or by a process known as spray drying. This results in 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, after pasteurization to remove microbial contamination, the solution 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, puts it beyond the reach of most gum arabic producers. The difficulty of handling large volumes of aqueous solution of gum in a producer country - where ambient temperatures are high - without suffering unacceptable increases in the microbiological load aggravates the problem (Coppen, 1995b).
Genetic resources and breeding
In the past, the large-scale collection of lucrative exudates seriously endangered the tapped species as it often disrupted traditional management systems of the resource, which aimed at a reasonably steady supply. The destructive harvesting of the latex of Sapotaceae (Madhuca, Palaquium and Payena) by felling the tree is another example of genetic diversity being reduced by the simple fact that the number of individuals is reduced. Currently, regulations are such that valuable trees as those yielding exudates are protected and their felling is often prohibited. As many exudates are collected from trees in natural forest, their genetic diversity depends on this ecosystem. If forest is selectively logged or cleared for activities such as agriculture, industrial development or mining, the genetic resource base of the trees yielding exudates is considerably narrowed or completely eliminated.
Ex situ conservation is important for those species managed in plantations or in agroforestry systems. In the case of Styrax, because the trees are planted or deliberately maintained in a forest, there is some degree of selection of better-yielding individuals. However, the extent of selection is difficult to assess. The resin-yielding Agathis and Pinus species have been included in international provenance trials with the intention of improving timber production. Areas of particular interest to any of these species have been identified and the conservation of these sites has been recommended. In Indonesia seed orchards of Pinus merkusii have been established (Soerianegara & Lemmens, 1993).
Around 1910, at the time of the "rubber boom" in South-East Asia, breeding activities were started to improve rubber latex production. The first step was clonal propagation by budding, as it was realized that the yield per tree was very variable. Today, there are many clones that yield 6 times what the first plantation trees in South East Asia yielded. It is increasingly being recognized that rubber breeding should focus not only on yield but also on other desirable characteristics, such as vigour, quality of virgin and renewed bark, colour and stability of latex, resistance to leaf and bark diseases and to wind damage and timber volume produced (Webster & Baulkwill, 1989).
Breeding programmes have also been initiated for Agathis. The 3 main objectives of a programme in Indonesia are: improving wood quality and production, improving copal quality and production, and improving resistance to diseases and pests.
Research and development
The main organizations conducting research and development on exudate-producing plants are:
- Research and Development Centres, Forestry Research and Development Agency (FORDA), Ministry of Agriculture and Forestry, Bogor
- research on propagation, diseases and pests, ecology and post-harvest handling of Pinus merkusii
- propagation of Agathis dammara
- ecology of Shorea javanica
- post-harvest handling of Dyera
- Indonesian Rubber Research Institute, Medan
- agronomy, soil research, breeding, diseases and pests, processing and socio-economic aspects of Hevea brasiliensis
- latex properties of Ficus elastica
- Faculties of Agriculture and Forestry, Bogor Agricultural University, Bogor
- ecology, seed viability, management, diseases and pests, tapping of Pinus merkusii
- propagation and properties of Dyera
- diseases and pests and resin purification of Shorea javanica
- conservation of Sindora sumatrana Miq.
- SEAMEO Regional Centre for Tropical Biology, Bogor
- seed dormancy and natural regeneration of Agathis
- seed technology, diseases and pests, genetics and silviculture of Shorea javanica
- Research Unit for Plantation Crops Biotechnology, Agency for Agricultural Research and Development (AARD), Ministry of Agriculture and Forestry, Bogor
- biotechnology of Hevea brasiliensis
- Other universities and research institutes
- diseases and pests, ecology, progeny tests, tapping, resin technology of Pinus merkusii
- site requirements of Shorea javanica
- post-harvest technology, growth of Dyera
- seed viability of Agathis labillardieri Warb.
- Center for International Forestry Research (CIFOR), Bogor
- research projects on Shorea javanica and Styrax
- Perum Perhutani, Jakarta
- responsible for production of copal (from Agathis) and pine resin (from Pinus merkusii)
- tapping methods for Pinus merkusii
- Forest Research Institute Malaysia (FRIM), Kepong
- utilization and properties of Dipterocarpus wood-oil
- utilization of jelutong (Dyera)
- Malaysian Rubber Board, Kuala Lumpur
- all aspects of rubber cultivation, processing and utilization, especially new applications of rubber
- Universiti Putra Malaysia (UPM), Serdang, Selangor
- properties of wood-oil from Dipterocarpus
- rubber utilization
- new applications of pine resin, rosin and turpentine (Pinus)
- Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor
- rubber utilization
- Forest Products Research and Development Institute (FPRDI), Department of Science and Technology, College, Laguna
- improved tapping systems for Agathis philippinensis, Canarium luzonicum, Pinus and Dipterocarpus
- stimulant use in tapping, resin properties and application of Agathis philippinensis resin in end products
- application of Canarium luzonicum oleoresin in end products
- use of natural resin in paper size, and in rubber-based adhesives
- University of the Philippines at Los Baños, College of Forestry and Natural Resources, Department of Forest Products and Paper Science, College, Laguna
- establishment of a plant for semi-commercial production of Agathis philippinensis resin varnish
- physico-chemical properties of resin, rosin and essential oils use of Canarium luzonicum oleoresin in paints
- Philippine Industrial Crops Research Institute, Philippine Rubber Testing Centre, University of Southern Mindanao, North Cotabato the following aspects of Hevea brasiliensis:
- nursery techniques and management
- tapping technology and yield
- diseases and pest control
- latex processing and equipment
- germplasm collection, evaluation and breeding
- Regional Research Centres of the Department of Agriculture
- all agricultural aspects of Hevea brasiliensis cultivation
- propagation of Pinus, Shorea, and Canarium
- Rubber Research Institute Thailand
- Hevea brasiliensis
- Rubber Estate Organization, Nabon Station, Nakhon Si Thammarat
- rubber processing
- Office of the Rubber Replanting Aid Foundation, Bangkok
- introduction of high-yielding varieties of Hevea brasiliensis
- Songkla Rubber Research Center, Songkla
- rubber processing
- Faculty of Natural Resource, Department of Plant Science, Prince of Songkhla University, Songkhla
- genetic modification of Hevea brasiliensis
- Faculty of Forestry, Kasetsart University, Bangkok
- rubberwood production
- General Directorate of Rubber Plantation, Phnom Penh
- Hevea brasiliensis
- Rubber Research Institute of Vietnam
the following aspects of Hevea brasiliensis:
- soil research and classification; plantation establishment
- diseases and pests control
- post-harvest handling
- germplasm collection
- extension in agronomy and rubber quality
In the other countries, research is being done, mostly on Hevea brasiliensis. In Laos, provenance trials aimed at identifying superior resin-yielding trees of Styrax tonkinensis and tapping trials aimed at optimizing harvesting methods, established in 1997 with FAO assistance, are continuing to be monitored and evaluated with EU support.
It has already been pointed out that in the last 100 years the use of many of the species that were once important sources of resins, latexes and gums in South-East Asia has declined. Most of the decline is attributable to the availability of cheaper, technically superior synthetic alternatives. The production of synthetics is not constrained by the vagaries of the weather or the uncertain productivity of the natural resource. The quality, too, is more consistent and can be varied at will according to the requirements of the end-user. However, it would be wrong to be over-pessimistic about future prospects; there is some cause for optimism. The two most important exudates (rubber and pine resin) continue to be produced from natural sources and this will remain so for the foreseeable future. Other exudates such as copal, damar and benzoin, retain substantial markets and the fact that several major projects involving them have been undertaken in the region in recent years demonstrates the importance that is attached to them by governments and international organizations. For those exudates (such as gum arabic and the chewing-"gums") which have a food or pharmaceutical use, the "naturalness" of the natural material is a powerful selling point.
Rubber is a very large export commodity for Malaysia, Indonesia, Thailand and Vietnam and its prospects are promising: demand is expected to rise. Moreover, the added revenues from rubberwood will strengthen the economic viability of rubber cultivation. Demand for gum turpentine and rosin, the primary products of crude pine resin, is also buoyant, although it is influenced by factors other than those related to the use of synthetics, namely the availability of supplies from China (the leading producer of gum naval stores) and the supply of sulphate turpentine and tall oil rosin as by-products from pulping (sulphate naval stores). Demand and economic circumstances permitting, Indonesia has the potential to increase its production of turpentine and rosin if it chooses to tap larger numbers of pine trees.
With the exception of rubber, the exudates produced in agroforestry systems are of minor importance to the national economies in South-East Asia, but in some areas they are very important to the local communities. In recent decades the price fluctuations of exudates produced in agroforestry systems such as damar and benzoin gardens have had direct influence on the intensity of exudate collection. When exudates command low prices or are temporarily not traded at all due to periods of political unrest or war, farmers shift their attention to other components of their gardens, such as fruit trees. It is precisely this flexibility that sustains the production system from which exudates can be collected.