Arenga pinnata (PROSEA)
Arenga pinnata (Wurmb) Merrill
- Protologue: Int. Rumph. Herb. Amb.: 119 (1917).
- Family: Palmae
- Chromosome number: 2n= 32
Arenga saccharifera Labill. (1801).
- Sugar palm, areng palm (En). Palmier à sucre, palmier areng (Fr)
- Indonesia: aren, enau, kawung
- Malaysia: enau, kabong, berkat
- Philippines: kaong (Tagalog), bagobat (Bisaya), hidiok (Bisaya)
- Burma (Myanmar): taungong
- Cambodia: chuëk', chraè
- Laos: ta:w ta:d (northern)
- Thailand: chok (southern), tao (northern)
- Vietnam: búng báng, doác, doát.
Origin and geographic distribution
Sugar palm is thought to be indigenous where it is encountered at present, except for the Pacific Islands and a few places in Africa where it has been introduced. This implies that its origin lies in an area covering SouthEast Asia up to Irian Jaya in the east, extending northeastwards to the Ryukyu Islands (Japan) and northwest to Annam (Vietnam) and the eastern Himalayas. It is mostly found near villages. It is found growing wild in primary or secondary forest.
All parts of the palm are used, and for a multitude of products. The main products are derived from tapping the inflorescence stalks: a sweet aromatic juice, both fresh ("nira") and fermented ("toddy"), vinegar resulting from continued fermentation, and yeast made from the residue deposited during fermentation; above all the darkred palm sugar, obtained from the juice, and widely used in all kinds of dishes, sweets, drinks and preserves. The juice is flavoured by adding leaves of Garcinia L. and bark of Xylocarpus Koen. and several Ulmaceae, which make the juice more bitter and improve storability; alcohol can be distilled from the palm wine.
Other food products are starch, extracted from the pith of the trunk, which may be used to prepare speciality foods such as "bakso" (Indonesia), and the grubs of the palm beetle (Rynchophorus ferrugineus) which are reared on fallen stems and eaten raw, fried or cooked. Young leaves, still white, are eaten in the same way as palm cabbage. Bees collect excellent honey from the flowers, and sweetmeat ("kolang kaling", Indonesia) is made by boiling the white endosperm of immature seeds with sugar. The production of these secondary foodstuffs is limited where tapping takes precedence.
Products from fibrous material take second place after those derived from tapping. Fibres are recovered from the roots, the pith of the trunk and leaf stalks, but most important are the long blackgrey fibres ("ijuk", Indonesia) surrounding the trunk. The latter, although coarse, are extremely durable even in seawater and have frequently been used for cordage on ships, and as a cover to protect wooden poles in soil and seawater against worms and insects. "Ijuk" mattresses are laid on roadbeds along the coast to stop burrowing prawns from surfacing with their mud piles. The fibre is also used to make sieves, to construct roofs, to reinforce concrete; heavyduty brushes and brooms the world over bristle with "ijuk" fibres. "Ijuk" paint brushes clean and paint ship hulls in a single operation.
The fine pulp occurring between the leaf sheaths used to be used for tinder and to caulk boats. Short ropes can be used as a portable fire lighter; the rope remains glowing in all weathers. Stout bristles between the thin fibres near the leaf bases have been used as pens and arrows. A bundle of these bristles makes a veritable torch, burning brighter when swung and producing a beam of light as the core burns deeper and glows more intensely. The bristles are sometimes used for burning as scent material. Fibres from the leaf stalks and roots are used for fishing-lines, snares and fine matting. The roots growing from the base of the stem are sometimes cut into fibrous boards for orchid cultivation.
The impressive leaves are also put to many uses. They serve to construct temporary shelters and the unfurling, still white leaves are used as hanging decorations for festivities. The leaflets are used in basketry, their stalks for brooms and sate sticks. Young leaflets may be eaten, but at a somewhat later stage they substitute for cigarette paper and serve as fastening ribbon. The large leaf stalks are used as firewood and to make walking sticks and musical instruments. Walking sticks can also be made out of the inflorescence stalks.
The green peel of the unripe fruit is poisonous and causes serious skin reactions on contact because of the calcium oxalate crystals; it is sometimes used to protect fish ponds from barefoot intruders. The pulped fruit in water brings fish to the surface. The seeds are favoured by pigs and used to bait wild pigs; many toys in the villages used to be carved from the seeds.
The trunk consists of a soft starchy core with many tough fibres and a woody cylinder. The attractive black and yellow wood of the trunk is used for flooring, furniture, tool handles, and as a fuel wood of high calorific value. The trunk base is easily hollowed to obtain a durable barrel or a water conduit.
Medicinal uses have been reported for the young roots (kidney stones), old roots (toothache), the fermented juice (the alcohol obtained through distillation and mixed with several herbs and roots of other plants is considered a generalpurpose medicine), the sugar (laxative) and the fine pulp occurring between the leaf sheaths (to speed up recovery from burn wounds). The trees are sometimes planted to mark boundaries or to prevent landslides. In certain regions, dowry is paid in the form of a few sugar palm trees.
Production and international trade
Almost all products of sugar palm are locally traded and used. Export data are scattered and probably incomplete. In 1986, 200 t palm sugar per month were exported from Indonesia to Australia; in 1994 the export price was about US$ 3.0 per kg. In 1985 one of five exporters of palm fibres shipped 400 t, representing a value of about US$ 550 000. Most of the exported fibre passes through Singapore. Starch is exported on a small scale. Canned "kolang kaling" production and export is steadily increasing. Sugar palm is still a smallholder crop, but in Indonesia commercial plantations are being established. Through a national sugar palm promotion project, Indonesia hopes to achieve selfsufficiency in sugar through sugar palm and nipa palm.
The palm juice contains 5-21% sugar. The sugar crystallizes beautifully into pale-yellow storable sugar of good quality under the right cooling and stirring conditions. The palm sugar contains 75% sucrose (saccharose) and a maximum of 6% reducing sugars, which is low and considered a desirable quality. Starch should be white and dry, obtained through repeated washing and settling. The fibres are graded into five length classes, the longest and thickest ones (up to 2 m long) being most expensive.
- A moderate to tall unbranched, hapaxanthic, solitary palm.
- Roots black, very strong, extending far (sometimes more than 10 m) from the stem and going as deep as 3 m.
- Trunk 10-20 m long and 30-65 cm in diameter, covered by bases of brokenoff leaves and long blackgrey fibres; crown dense, with 12-20(-28) erect to spreading leaves.
- Leaves 6-10(-12) m long, pinnate; petiole 1-1.5(-2.3) m long, with sheath at base; leaflets numerous (80-130(-155)), straplike, 140-180 cm × 8-11 cm, crowded along the rachis and held in several planes, with auricles at base, rounded or obtuse and toothed at apex, glabrous above, scabrous beneath.
- Inflorescence usually unisexual, pendulous, often more than 2 m long, arising from leaf axil, peduncle breaking up into a number of flowerbearing spikes; female inflorescences 3-7, formed at the top, male ones 7-15, appearing later and lower on the stem.
- Flowers with 3 coriaceous sepals and a 3-lobed corolla, tubular at the base; male flowers up to 11 500 per inflorescence, with many stamens, greenish to bronzy when still closed, yellowish when open; female flowers up to 15 000 per inflorescence, with a globose, trilocular ovary.
- Fruit a globose to ellipsoid drupe, 5-8 cm long, fleshy, first green, later turning yellow and black after falling, 2-3-seeded.
- Seed black.
Growth and development
Germination is very unpredictable, taking from one month to more than a year, and is remote-tubular. A tube emerges from the germpore near the apex of the seed and enters the soil. The radicle and plumule appear from the side of this tube, followed by 3-4 roots which grow straight up. Soon the radicle and these early roots are overtaken by the normal adventitious roots.
The rosette stage takes 3-5 years and the trunk growth phase 5-10 years, depending mainly on temperature, but also on competition for light. The rate of leaf production during trunk formation greatly depends on the growing conditions, but it is of the order of 3-6 leaves per year. Fifty leaves may be the maximum over a palm's life. The last two leaves emerge simultaneously, signalling the onset of flowering. These leaves expand fully, but the few remaining primordia and the growing point itself "petrify": they become woody without further growth. The first inflorescences emerge from the axil of the uppermost leaves and bear female flowers. Flowering gradually proceeds downwards: 3-7 female inflorescences are followed by 7-15 male inflorescences, although the latter may include a few which bear female flowers as well or which are completely female. Sometimes the formation of lower-positioned female inflorescences can be induced by removing the top female inflorescences. Buds lower on the trunk tend to be underdeveloped and in the natural state the palm dies before it is their turn to bloom. Sometimes completely male trees are encountered which are known to produce larger amounts of sugary juice (called "puso lolon" trees in North Sulawesi). The trunk serves as a store for starch that is accumulated after the rosette stage in the parenchyma of the pith. When flowering starts the starch is converted into sugar and dissolved in the stem fluid. As with Metroxylon sagu Rottboell, additional sugar is produced by photosynthesis in the functional leaves. The flowers are presumably crosspollinated since there is little overlap in flowering of female and male inflorescences of the same palm. Bees pollinate flowers, but small flies also swarm in large numbers around male inflorescences. Each female inflorescence carries thousands of fruits which take 12 months from flowering to maturity if the palm is not tapped. One palm may produce as many as 250 000 seeds.
In the natural state the palm dies after the fruit of the inflorescences near the top have matured, that is about 2 years after flowering starts. Skilful tapping can extend the tree's lifespan by 10 years or more. Since a load of growing fruit is essential to keep the palm alive, few or no female inflorescences are tapped. The art of tapping is to tap so little that fruits do not starve, but to tap sufficiently to delay their maturation indefinitely. Overtapping results in the lower green leaves breaking off, followed by massive premature fruit fall and death. The sagging of these leaves is the first sign that the palm is bleeding excessively. An optimal tapping intensity maintains an adequate sap stream to the fruit, ensuring a minimal rate of fruit growth.
A smallholder in North Sulawesi pointed out a tree which he had been tapping for 15 years! This implies a leaf age of more than 15 years. The extended life of the leaves under judicious tapping must contribute substantially to total sugar yield over such long periods. One documented tree produced more than 20 000 l of sugary juice within a period of three years. This represents some 12 t sugar from one tree.
Ultimately, tapped trees produce fruit too; if tapping is stopped in time the unripe fruits can be used for "kolang kaling" production. The remaining starch level in the trunk is too low to make extraction worthwhile (less than 20 kg for tapped trees).
Sugar palm grows best in warm conditions with a maximum amount of light and abundant water supply on very fertile soils. It can, however, grow under a wide variety of conditions, both in equatorial and seasonal climates, from sealevel up to 1400 m altitude, on all soil types from heavy loam to loamy sand and lateritic soils that are not regularly inundated. The growth rate drops substantially where growing conditions are less favourable. Wild in primary or secondary forests, it occurs especially on sites poor in nutrients and in marginal areas such as denuded hillsides. The age of first flowering depends strongly upon the altitude, being 5-7 years at sea-level and 12-15 years at 900 m altitude.
Propagation and planting
Normally, people drop the seed where they would like the palm to grow. Sometimes wild seedlings are collected and transplanted. In a nursery, the following procedure has proved successful. Ripe, black fruits under superior palm trees are collected. Seed left behind after the fruit has decayed may also be collected. They are cleaned in water and those that float are removed. Seed with fungal or bacterial growth near the pore is also discarded. Then they are scratched on a rough stone to scarify the thin black outer layer down to the underlying brown layer, close to the pore. Filing down to the white endosperm may result in more seed rotting. The seed is left to soak in water or wet sacks for 24 hours. Then it is sown in coarse sand kept moist, with the germpore pointing downwards. Air humidity should be high during germination. After 3 weeks, some 75% have germinated; these are transferred to plastic containers when the germination tube is 2-3 cm long and before the upwardgrowing roots have formed, since these break off easily. A heavy-gauge polythene is used since flimsy material is easily perforated by the roots. Seed should be planted so deep that the germination tube does not dry out. The seedlings stay alive in heavy shade but growth is minimal under such conditions. Direct sunlight stimulates an early appearance of the first leaf. The seedlings can be planted out when the second leaf has unfurled. The young seedlings should be hardened off in the nursery before transplanting.
To obtain a closed stand under fair growing conditions, the palms may be spaced 6 m × 7 m (about 250 trees/ha). Intercropping with fastgrowing woody legumes that can be coppiced may be advisable to provide shade for the young palms and firewood to boil the juice. Wind-breaks may be needed in exposed sites to prevent breakage of leaves.
Crop care is limited to occasional weeding. Manures and fertilizer are not applied. However, appreciable amounts of nutrients, notably K and N, are removed by tapping. Based on data obtained in 1933, this amounts to (%): N 0.041, P 0.001, K 0.12, Mg 0.0096 and Ca 0.016. Fertilization does strongly increase the size of the leaves. Shortly before the rosette stage ends, the palms should be provided full light.
Diseases and pests
No serious diseases and pests occur. Consequently, crop protection chemicals are not normally used. On Java, a locust species (Valanga nigricornis) has been reported to attack leaves. Locally, caterpillars of Artona catoxantha, Elymnias hypermnestanesaea and Hidar irava have been observed eating young leaves. Top death of young plants has been reported and is possibly caused by flies of Atherigona arenga. Sometimes, caterpillars of Batrachedra spp. are observed on the male inflorescences. The top of young trees is occasionally damaged by the common coconut top borer (Oryctes). In North Sulawesi people use one of the sharp bristles of the palm to pick in the hole and kill the larvae inside, normally resulting in continued survival and good growth of the palm. In a young plantation, damage was caused by mice eating the sweetish growing tip of the seedlings. Rats may eat the ripening fruit.
Tapping is preceded by pretreating the peduncle of the inflorescence by beating it and swinging it about. The peduncle of both male and female inflorescences can be tapped, but the latter are tougher and pretreatment less often results in a satisfactory flow of juice. When the first male inflorescence is almost fully expanded, the scales covering the peduncle are removed. Then the beating and swinging starts. This is done to cause some internal ruptures so that the juice will continue to flow later on after cutting. The frequency and duration of the pretreatment differ widely in the various regions. Some reports mention beatings over only a 3-7 day period. In North Sulawesi, at about 900 m altitude, the peduncle is treated five times during five weeks, and the stalk is tapped for 6-12 months. In general pretreatment and tapping periods are longer with increasing altitude. The first beating is light, otherwise the peduncle may wilt. Subsequent beatings are steadily intensified. The peduncle is beaten from the base upwards, stripwise until the entire circumference has been treated. The beating is moderated during the rainy season and for lightcoloured peduncles. The peduncle is also swung about to rupture vessel connections close to the trunk where the mallet cannot reach.
The pretreatment and the subsequent cutting of the peduncle are timed in accordance with the condition of the male flowers. The inner perianth with the stamens of a flower bud is removed, and the interior colour and striping of the outer perianth indicate the development stage. The peduncle is almost ready to be cut when a watery sap oozes from the scar. Gradually the sap becomes more viscous until it turns hyaline yellow. This is also the time that bees are attracted by a dry yellow exudate on the inflorescence. Shortly after this stage the flowers open. The villagers first cut a single spike to see whether it bleeds for more than a day. If it does, the peduncle is cut with a single stroke aimed at making a clean cut close to the point where it bifurcates into the spray of flowering spikes. The length of the remaining peduncle determines the maximum duration of the tapping period.
For unknown reasons, it is customary in Indonesia not to collect the juice during the first day; in Peninsular Malaysia people used to wait for several days. To produce "toddy" a bamboo vessel is fixed under the dripping peduncle end. It is inoculated with some old toddy to introduce the yeast (Saccharomyces spp.) that converts the sugar into ethanol. Sometimes additives are used to obtain a specific taste or strength. When sugar is to be produced from the sap, the bamboo vessels need to be cleaned each time they are replaced. This is done by drying them in the smoke of a fire or by rinsing with boiling juice.
For sugar production, a very thin slice is cut from the peduncle twice a day, as this results in less but sweeter sap. For "toddy" the slices are thicker, also depending on the desired alcohol content. If rather thick slices are cut, a large palm may yield some 60 l of fairly sweet juice per day for several months, but this is often associated with signs of overtapping: sagging leaves which eventually fall down, and massive drop of unripe fruit. After each cut, the peduncle end is covered with some material - usually a large leaf - to protect it from the sun; this promotes the sap flow.
The starch is harvested in the same way as that of the sago palm. The tough fibres in the pith make it harder to obtain the starch. Normally the villagers choose trees that have failed to respond to tapping treatments. These palms yield the highest quantities of starch (100-125 kg/tree). The thick, very hard wooden zone surrounding the pith makes it very laborious to open the stem.
The "kolang kaling" is produced from immature fruits, which are burned or cooked and then peeled; finally the endosperm of the young seeds is boiled with sugar. The "ijuk" is cut with a machete for the first time at an age of 4-6 years; scissors have also proved practical. Farmers in three different regions in Indonesia claim to have better growth of the trees when the "ijuk" is removed regularly.
In North Sulawesi, village stands of sugar palm of all ages currently produce about 70 kg sugar per ha per day. This adds up to an annual yield of 25 t sugar per ha. The sugar yield of such stands of unselected material is limited, because about half the palms do not bleed. When the first three treated inflorescences of a palm fail to bleed satisfactorily, people give up and it is left for the fruit and sometimes for starch extraction. Sometimes such trees are felled with the top downhill by partly cutting through the stem so that it stays connected to part of the root system. Then the top is cleared of leaves, and a large amount of sugary juice can be collected by slicing ever deeper in the heart cabbage. This destructive method yielding large quantities of sugary juice for some 10 days is mainly used for the trees that could not be tapped successfully, and during special festivities for the production of toddy.
One tree may yield enough sweetmeat to fill about 2000 onelitre cans. Moreover, the fruit of tapped trees can still be used for making "kolang kaling". A 15 m high palm with a diameter at breast height of 40 cm contains 100-150 kg starch; in a mechanized largescale operation even extraction of the remaining starch might be worthwhile. About 15 kg of "ijuk" fibre can be gathered from a tree, 3 kg of which are valuable long fibres. The annual yield of these superior fibres is about 150 kg/ha. All these components add up to an appreciable yield, and it is not surprising that a family is considered well-off as long as it is tapping 3-4 sugar palm trees.
Handling after harvest
The juice is evaporated in open pans. To prevent the liquid from boiling over, some fatty or oily material (castor beans, coconut endosperm, tung seeds) is added. As soon as the liquid turns dark red and begins to set, the sugar is poured into moulds. If the sugar is to crystallize at this moment, vigorous stirring should be applied while stopping the heating altogether. For the stirring normally large wooden forks are used. Because of impurities, palm sugar does not store well. Keeping quality may be improved by adding sodium bisulphate or lime (CaO) during evaporation.
No substantial germplasm collections are being maintained. A survey including many different regions in Indonesia revealed large differences in productivity between trees not related to tapping techniques and soil types, indicating potential for selection.
In North Sulawesi, selection of superior trees to obtain seed for propagation is carried out in several sugar palm stands. It appears that selection may raise the percentage of palms that bleed well from 50 to 85. In Java, large heavybearing trees are often used to produce "kolang kaling". Since these trees do not produce offspring for the next generation, the practice may amount to a negative selection.
The future for sugar palm seems bright, even though the maritime world no longer depends on "ijuk" fibres. Limited, largely local demand has so far hampered largescale production of palm sugar and its by-products. However, calculations based on the sap flow of a 15 m high tree of 40 cm girth being tapped during the last 10 years of its 20year lifespan show the scope for improving yields. The average daily yield over this period was about 30 l juice or about 5 kg sugar. Because intervals of several months sometimes occur between the tapping of 2 successive inflorescences and there are occasional mishaps caused by failure of the pretreatment of an inflorescence, a palm may be in production for only half that time. Based on these figures, impressive annual yields per ha have been calculated. For instance, a stand of 200 palms per ha with 85% of the tappable palms bleeding well, with an average of about 34 palms per year, may produce annually well over 25 t sugar per ha. However, tapping extracts considerable amounts of K, N, Ca and Mg from the system. Therefore, adequate cropping techniques, including fertilization, should be designed and tested to create optimal conditions for sugar-palm cultivation.
Meanwhile, large sugar-palm plantations are being established in Indonesia; several so-called transmigration timber estate schemes have included sugar palm planting. High rates of return are anticipated provided large (export) markets can be supplied. The estates will generate a great deal of employment and in certain areas will protect unstable sites. Especially with the reduced sugar production from sugar cane due to urbanization, the increased demand by a growing population, and the relatively easy refining of the palm sugar, there should be a very large local market within Indonesia.
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