597

13: Spices

Syzygium aromaticum (L.) Merrill & Perry

Mem. Am. Acad. Arts & Sc. 18: 196 (1939).

MYRTACEAE

2n = 22

Caryophyllus aromaticus L. (1753), Eugenia aromatica (L.) Baill. (1876), Eugenia caryophyllus (Sprengel) Bullock & Harrison (1958).

Clove (En). Clou de girofle (Fr). Indonesia: cengkeh. Malaysia: chengkeh, chingkeh. Philippines: klabong pako, clavo de comer. Burma (Myanmar): lay-hnyin. Cambodia: khan phluu, khlam puu. Laos: do:k chan, ka:nz ph'u:. Thailand: kanphlu. Vietnam: dinh h[uw][ow]ng.

The clove tree was first cultivated on some islands of the Moluccas, where it occurs wild, as well as in New Guinea. It is found in abundance as a second-storey forest tree on the lower mountain slopes. The crop and its trade has a long and fascinating history going back to the Han Dynasty in the 3rd Century BC. The story of the clove trade and the spread of the crop is full of intrigue and brutality. Apart from pepper (Piper nigrum L.), no other crop may have played a comparable role in world history.
Early in the 17th Century, when the Dutch ousted the Portuguese from the Moluccas, clove cultivation had spread to many islands. Under Dutch rule, the crop was forcibly eradicated everywhere and concentrated on Ambon (a southern island of the group) and 3 nearby small islands. This is the wettest part of the Moluccas. From the Moluccas the clove tree was taken to other parts of Asia: early in the 19th Century the British took plants to Pinang (Malaysia), Sumatra (Indonesia), India and Sri Lanka. In the 20th Century, much material spread throughout Indonesia. During expeditions in 1753, 1770 and 1772, the French appropriated some offspring from trees that must have escaped the Dutch axe, and took them from the North Moluccas to Mauritius. These plants gave rise to the clove populations outside Asia, in Zanzibar, Madagascar and recently also in Bahia in Brazil.

Since ancient times, the clove has been highly valued as a spice by the Chinese. In the early Middle Ages the spice became increasingly important in Europe and also in India, where to this day betel quid is fastened with a clove. In South-East Asia, however, the clove is not much used to flavour food; medicinal use of both the clove (the flower bud) and the mother-of-clove (the fruit) has always predominated. Cloves suppress toothache and halitosis; they are also a stimulant and carminative.
Now, more than 90% of the cloves are used along with tobacco to produce 'kretek' cigarettes, which are smoked mainly in Indonesia. It is not known where the habit originated. Rumphius reported it from the Moluccas in the 18th Century. He also described the curing of cloves over small fires. Perhaps it was not a great step from inhaling the smoke of the fires to the smoking of tobacco mixed with clove.
When used as a spice, the dried clove buds are added to the food as such or after grinding, or the oleoresin is extracted to standardize the flavour (for use, for instance, in manufactured foods).
Distillation of cloves yields an oil that is used in the flavouring and perfume industry. Lesser quality oils are distilled from the flower stalks ('clove stems'), a by-product of the clove harvest (Zanzibar), and from the leaves (Madagascar, Indonesia). The major component of the oil is eugenol, formerly used to produce vanillin. Because of its flavour and antiseptic properties, eugenol is used in soaps, detergents, toothpaste and pharmaceutical products. The oil is also a potent bactericide and nematicide. It is indicated for inflamed oral and pharyngeal mucosa and is used for topical anaesthesia in dentistry.
In the United States the regulatory status 'generally recognized as safe' has been accorded to cloves (GRAS 2327), clove bud oil (GRAS 2323), clove bud oleoresin (GRAS 2324), clove stem oil (GRAS 2328) and clove leaf oil (GRAS 2325).
The tree's timber is hard, heavy and durable, but, with its dull greyish colour, it is not decorative.

For over a hundred years, Zanzibar was the largest producer of cloves, with an average annual production of 11 000 t from 1960-1970, compared with 9000 t for Indonesia and 6000 t for Madagascar. During that decade, there was a remarkable expansion of the area under cloves in Indonesia, spurred by the rapidly rising demand for 'kretek' cigarettes. From 8200 ha in 1951, the area rose to 80 000 ha in 1971, jumping to more than 500 000 ha in 1981 and nearly 750 000 ha in 1987.
The clove tree caught the fancy of small farmers, who called it 'the 100 000 rupiah tree' and planted a few clove trees in the hope of striking rich one day. However, about half the trees never reached bearing age; production picked up slowly and did not peak until around 1996 when the declining area was reported to be 495 000 ha. In that year production was a phenomenal 90 000 t, but this was only just enough to meet the demands of the cigarette industry (which showed signs of levelling off).
Demand for all other uses worldwide has been stagnant for a long time at 4000-5000 t per year. In the rest of Asia annual production of cloves, including stems, averaged 2750 t over 1996-1998; in Africa during the same period, Madagascar produced 15 000 t, well above Zanzibar's 6000 t. The yield of clove stems is roughly one-fifth that of clove buds. To some extent stems are used as a cheap substitute for buds.
Clove prices have fluctuated wildly throughout the long history of the crop. It takes a long time before growers respond to high prices and even longer before the trees come into bearing, causing a glut. The increasing popularity of 'kretek' cigarettes kept prices high, but during the 1990s production caught up with detrimental results: in New York cloves fetched US$ 11 per kg in 1982 compared with US$ 1.25-1.45 per kg in 1996 and 1997 (for cloves from eastern Africa; top grade cloves from Sri Lanka fetched US$ 3.30 per kg). However, dwindling stocks and declining production in Indonesia may reverse the trend.

The quality of the spice is determined by its essential oil content and composition. The clove tree produces 3 different types of essential oils: from the flower buds (content 15-17%), from the flower stems (content 6%) and from the leaves (content 2-3%). The major components of those essential oils are eugenol (80-95%), eugenyl acetate (1-5%) and 'BETA'-caryophyllene (4-12%); the quality of the oil is determined by the varying proportions of those components and of minor and trace components, and is influenced by origin, season, maturity at harvest, post-harvest treatment and method of distilling of the original product. In general, the best quality oil originates from the flower bud, second best from the flower stem, third from the leaf.
Clove bud oil is a clear, colourless to yellow liquid (turning browner with age) with a strong characteristic sweet and spicy clove odour and a warm, almost burning and spicy flavour. Its main components are eugenol 70-90%, eugenyl acetate up to 17% and 'BETA'-caryophyllene 5-12%. It stores well in light-proof containers. Clove bud oil is used in seasonings, processed food, perfumery and to a lesser extent in pharmaceutical and dental preparations.
Clove bud oleoresin is obtained by solvent extraction of cloves. The yield is 18-22% oleoresin (90-92% volatile components) using benzene and 22-32% using alcohol. Supercritical fluid techniques are currently used at an industrial scale. The oleoresin is a viscous brown liquid which can deposit waxy particles on standing. The odour and flavour are regarded as superior to distilled oil and much closer to the natural spice. The main advantages of oleoresin over dry spice are little risk of bacterial contamination and standard strength and quality. It is increasingly being used in the food and perfume industries.
Clove stem oil is a colourless to light yellow liquid with a strong spicy somewhat woody odour similar to clove bud oil but less sweet and floral. Its eugenol content is 90-95%, but it has a low eugenyl acetate content. Clove stem oil is used mainly in flavouring and perfumery.
Clove leaf oil (crude) is dark brown, often with a purple or violet tinge, somewhat cloudy and with a harsh, woody, phenolic, slightly sweet odour, quite different to that of clove bud oil. Rectified (redistilled) oil is clear pale yellow with a sweeter, less harsh, odour, closer to that of eugenol. The oil is obtained by steam or water distillation of fresh or dried leaves, but also often including twiglets, undersized buds and opened flowers. The oil may vary considerably in composition but eugenol content is usually 80-88%, eugenyl acetate content low, caryophyllene content high. The leaf oil is mainly used for eugenol and caryophyllene production. Rectified oil is used in less expensive perfumes and soaps. The leaf oil is not well suited for food flavouring because its harsher note does not reproduce the genuine clove flavour.
Monographs on the physiological properties of clove bud oil, clove stem oil, and clove leaf oil have been published by the Research Institute for Fragrance Materials (RIFM).

The usual adulterants of clove bud oil are clove stem or leaf oil, or the clove terpenes remaining after eugenol extraction. Such adulterations are difficult to detect analytically; for use in flavourings or perfumery, organoleptic evaluation is necessary.
Synthetic clove oil sometimes replaces clove bud oil, especially when prices are high because the natural product is in short supply.
Eugenol, the main product obtained from clove leaf oil, can be isolated from a variety of starting materials (e.g. cinnamon leaf oil) and can also be manufactured synthetically.

Slender, evergreen tree, up to 20 m tall, conical when young, later becoming cylindrical, in cultivation usually smaller and branched from the base. Roots form an extensive dense mat close to the surface with some major laterals, from which occasional 'sinker' roots descend. Shoot growth determinate, appearing in flushes, forming a dense canopy of fine twigs. Leaves opposite, simple, glabrous; petiole 1-3 cm long, reddish, somewhat thickened at base; blade obovate-oblong to elliptical, 6-13 cm x 3-6 cm, base very acute, apex acuminate, coriaceous, shining, gland-dotted. Inflorescence terminal, paniculate, about 5 cm long, with 3-20(-40) bisexual flowers, usually borne in cymose groups of 3; flower buds 1-2 cm long, constituting the cloves just before opening; calyx tubular, tube subterete to subquadrangular, 1-1.5 cm long, yellowish-green with a red flush, slightly protruding beyond the ovary (hypanthium), with 4, ovate-triangular, fleshy lobes 2-4 mm long; petals 4, coherent, tinged red, rounded, 6 mm in diameter, shed as an hemispherical calyptra as the flower opens; stamens numerous, up to 7 mm long; pistil with 2-celled ovary, style 3-4 mm long, stigma 2-lobed. Fruit (called mother of cloves) an ellipsoidal-obovoid berry, 2-2.5 cm long, dark red, usually containing only 1 oblongoid seed 1.5 cm long.

Seedlings are raised immediately after harvest, because the seed loses its viability within a few weeks. The young plants grow slowly and are quite delicate. Losses are high until the young trees are firmly established. Under favourable conditions, the juvenile phase lasts about 4 years. Clove yields increase until the tree is about 20 years old, and good yields can be produced until a great age. However yield fluctuates wildly, a heavy crop usually being followed by 2 or 3 light and mediocre crops before another bumper crop is produced.
High or low yields occur simultaneously over an entire region and there is much evidence that a well-marked dry season triggers a heavy crop. Subsequent low yields cannot be due to exhaustion, since the tree is relieved of its natural task of bearing a crop of fruit to maturity; in fact, the dry matter in a heavy crop of inflorescences is only in the order of 10 kg per tree.
Flower initiation seems to take place only in mature shoots that have been quiescent for several months. Thus the shoot-growth pattern in the course of the year governs flowering, and it is surprising that the annual growth rhythm of the clove has hardly been studied. Even the timing of flower initiation and the process of floral differentiation are not clear.
The crop cycle starts with a major flush as soon as the rainy season has settled in. Well before this flush, there is a first indication of the coming crop: rather suddenly the plump, light-green, floral, terminal buds can be distinguished from the pointed reddish vegetative shoot tips which will leaf out during the flush. The inflorescences emerge from the green terminal buds a few weeks after the leaves of the flush have turned green. The inflorescences expand in a series of well-defined stages. First a trident is formed; thereafter the inflorescence branches further, largely in multiples of three, until the proliferation of the inflorescence is complete. The extent of branching varies and strongly affects the size of the crop. In the penultimate stage each flower primordium assumes the typical clove shape. Reversal of floral primordia into leaf primordia sometimes occurs during the early stages. It takes 6-8 months before the flower buds are ready for harvest, an extraordinarily long time in a tropical tree crop. If the tree is not harvested, the fruit matures 3 months later.
Minor flushes of leafy shoots occur at irregular intervals, but in bearing trees, flushing stops in the last few months before harvest. Hence the leaves senesce, and as more leaves fall, the ratio of tops to roots drops. This stimulates renewed flushing after harvest, which is further encouraged by the loss of branches during harvest, which is often severe. The post-harvest shoots are too young for flower induction; moreover, buds or shoots on twigs that have flowered do not as a rule produce flowers for the next crop, resulting in a form of biennial bearing at the shoot level which also occurs in some mango cultivars.
Thus the next crop has to be borne on shoots that emerged early in the cycle and not as laterals of flowering twigs. After maturing into twigs, these shoots bear the buds which are receptive during the dry season when floral induction occurs. If virtually all twigs bear cloves, the bumper crop is followed by crop failure, simply because there are hardly any receptive buds on the tree. The poor crop in the third year can be attributed to the disturbed shoot growth pattern in the second year. Shoot growth does not suffer competition from the developing crop during the off-year; hence flushing becomes more erratic and continues until late in the season. This may again result in a shortage of receptive buds at the crucial time for flower initiation and hence in a disappointing crop in the third year. This explains the cycles of 3 or 4 years.
So, in order to produce regular crops, only half the twigs should flower each year. Increases in yield should come from bigger inflorescences, which is a matter of genetic constitution, healthy foliage and timely induction of flowering. Regular bearing in clove is more difficult to achieve, since there is no fruit to assist in stabilizing the growth rhythm and because the tree is severely damaged during harvest. Note that biennial bearing has been observed in young trees, with their greater vitality and ease of harvesting, whereas from the 14th year after planting, the year with the first heavy crop, bearing becomes triennial. It might be possible to suppress late flushing during the off-year by root pruning or application of growth retardants.
This description of clove phenology, based on fragmentary published information, needs verification by field observations. In East Java, for instance, some trees in a plantation have been reported to exhibit profuse return bloom on twigs that had flowered the previous year. In this case small inflorescences emerged from mixed buds which, on breaking, first produced a pair of leaves. These mixed lateral buds were found just below the point of removal of the previous year's inflorescence. It remains to be shown whether these trees are indeed more regular in bearing and, if so, whether this is worthwhile given the expense of picking small inflorescences.

In the past, Syzygium Gaertn. has frequently been united with Eugenia L. Convincing differences in the structure of flowers and seeds have strengthened the arguments for two separate genera, which means that the clove is assigned to Syzygium.
The tree populations in Zanzibar and Madagascar are rather uniform, but in Indonesia three types are distinguished: 'Siputih', 'Sikotok' and 'Bunga Lawang Kiri'; the latter is thought to be identical with the Zanzibar type. The types differ in tree habit, leaf size, and clove size and colour, but few trees are true to type; transitional forms are common. 'Siputih' produces large cloves, valued in the spice trade, but is said to be less productive than the others. In Indonesia, young trees are mainly of the reintroduced Zanzibar type.

Notions about the ecological requirements of the clove vary, perhaps because of an underlying dilemma: a climate with a marked dry season promotes flowering, but the tree does not cope at all well with stress. There are two ways out of this dilemma. The first is to choose a climate with a pronounced dry season (Zanzibar, East Java), but to limit stress by going for deep fertile soils, providing water and shade during the early years. The other way is to choose a wet climate with a short dry season (Madagascar, Sumatra, Pinang).
The choice is linked with the use of the produce. Cloves from wet areas are less suitable for making cigarettes, since the smoke becomes pungent and there is no crackling ('kretek') sound during the smoking. In Indonesia, cloves for 'kretek' cigarettes are said to require 3 months in which the monthly rainfall is less than 60 mm, whereas for cloves to be used as spice, rainfall should not drop below 80 mm in any month. Annual rainfall should exceed 1500 mm; wet clove areas usually receive 3000-4000 mm. With mean temperatures of 21°C in July and August, Madagascar is the coolest clove country, reaching to the Tropic of Capricorn.
Cloves are almost exclusively grown on islands, but proximity of the sea may not be as necessary as it was once thought to be, nor is the crop restricted to the lowlands. In parts of Sumatra and Java, and in the Nilgiri Hills in south India, cloves are grown successfully far from the sea and at altitudes of 600-900 m. Sheltered sites are preferred, because wind causes additional stress, and strong winds are not tolerated. Shade is necessary for young trees until they are firmly established.
Growth can be sustained on poor and acid soils, but waterlogging is very harmful. Adequate depth of soil is essential and water-holding capacity should be in keeping with the severity of the dry season; if not, irrigation is needed.

Cloves are propagated by seed. Seed from selected mother trees is extracted from the fresh fruit and germination follows in 2-6 weeks. Seedlings are raised in shaded nursery beds and respond to care: controlled watering, excellent drainage, adequate spacing for sturdy growth, and timely hardening-off by reduced shading and watering. Plants should reach a height of more than 50 cm within one year and should be moved to the field before they get much older. During transplanting, speed, protection of the root system and trimming of the shoots greatly increase the chance of survival.
Propagation trials in the control programme for 'Sumatra disease' in Indonesia have shown that clove can be propagated by air layering (50% success) and approach grafting (more than 80% success), but the results of propagation by cuttings and less cumbersome grafting techniques are still too poor for general use. Approach grafts on rootstocks of, for instance, Syzygium pycnanthum Merr. & Perry and Psidium guajava L. have also been successful. Progress in tissue culturing is slow and has not yet resulted in in vitro propagation.
Trees are planted in the field under temporary shade. In the dry season, young trees may need extra water. The standard spacing is 8 m x 8 m, but smallholders often plant much closer. A range of spacings from 6 m x 8 m to 8 m x 11 m, to take account of differences in site quality, seems better; the rectangular pattern facilitates intercropping in the early years. Banana and cassava are common intercrops. Intercrops may also provide shade but, near the young clove tree, shade trees such as Erythrina, Gliricidia or Leucaena species are preferred, since these can be pruned to even out irradiance through the year.

When the intercrop is phased out, husbandry is often limited to weeding once or twice a year. Careful weeding limits root damage but a more positive approach is to improve the topsoil by mulching under the trees and by cover crops (e.g. Centrosema pubescens Benth., Vigna hosei (Craib) Backer). There is evidence that the equilibrium of top to root is quite delicate. The trees recover with great difficulty from undue loss of leaves. So, all efforts to keep the topsoil in good condition assist in preventing root stress and maintaining a healthy foliage.
Manure or fertilizers are applied to each tree according to age. Results of trials with nutrients were inconclusive, perhaps because nutrition was related directly to yield; it would be more logical to measure growth response and to interpret the yield response on the basis of the growth reaction. In Indonesia, trees respond to nitrogen and, on poor soils, to potassium; liming is recommended to raise the pH above 5.5.

In both Zanzibar and Indonesia, the clove is threatened by diseases that kill the tree. Identification of the causal agents has been difficult amidst tree decline through non-parasitic forms of stress.
'Sumatra disease' is the main problem in Indonesia, killing up to 10% of the mature trees each year in parts of Sumatra and West Java, with an estimated annual crop loss of US$ 25 million. A tenacious research effort identified Pseudomonas syzygii as the cause. The bacteria live in the xylem vessels and apparently spread upwards from the roots. The symptoms are dieback, starting in the crown, vascular discolouration and root decay. Injections of oxytetracycline, the most effective antibiotic treatment, delay the decline but cannot cure the tree. Since then, it has been found that Hindola striata and possibly Hindola fulva act as vectors. Nearby forest is an important source of these tiny insects (tube-building cercopids of the family Machaerotidae), but they can complete their life cycle on the clove tree. This opens up prospects for the control of the disease: in the 1990s researchers identified egg parasites of Hindola and tested insecticides for specific action on Hindola; plants infected with non-virulent strains of the bacterium have been found to be immune to the virulent strain. Resistant clove cultivars have not been found, but grafts on resistant related species are being tested.
Leaf blister blight, named 'cacar daun' in Indonesia, is second only to Sumatra disease in the loss of crop and trees it causes. The causal fungi are Phyllosticta syzygium and Guigordia hevea. Fungicides can be effective, but research into the biology of the fungi, occurrence of resistence, etc. is needed to control the disease efficiently. A number of fungi and parasitic algae cause leaf spots; other fungi are involved in root decay.
Insect pests recorded on clove include stem, branch and twig borers, root feeders, a few sucking insects and caterpillars. However, crop losses are largely incidental, recurrent damage being caused mainly by borers. Termites may cause havoc in a young plantation and the fiery red tree ants make life difficult for the clove pickers.

At harvest, the complete inflorescence is picked, just before the first buds are about to open. Earlier picking reduces yield, and undersized cloves spoil the appearance of the produce; late harvesting means a sharp drop in oil content and spice value. The right stage for harvesting lasts only a few days, and a tree is picked 3-8 times in a season. The timely harvest of a good crop demands skilled management; often a substantial portion of a bumper crop is not harvested at all. Pickers equipped with baskets, ropes and crooks to pull the branches towards them climb the trees, or they use ladders with props. An experienced picker harvests some 40 kg of green cloves from good trees in a day. Improvements in harvesting equipment based on work study are needed to reduce damage to the tree and to raise efficiency.
The harvest season shifts substantially from year to year, apparently in response to timing and severity of the dry season. There may also be freak off-season crops. In South-East Asia, the main season ranges from May-June in East Java to November-December in Ambon and Pinang. Migrant workers follow the maturing crop through some of the major areas of production.

Yield varies so much from tree to tree and year to year that it is practically impossible to give normal values. It is clear, however, that yields are low. Dividing the production in Indonesia in 1996 by the area gives a yield of 200 kg/ha: about 1 kg of dried cloves per tree! However, this figure includes non-bearing young and decrepit trees. More precise is the series of production data from a large trial in Cibinong, West Java, planted in 1956. Over the 10-year period 1968-1977, mean annual production was consecutively 5.7, 0.0, 9.7, 4.6, 0.7, 10.3, 0.0, 2.0, 6.3 and 1.3 kg per tree; the overall average amounted to 4.1 kg per tree. Top yields of 50 kg have been reported for individual trees in different parts of the world, and an average yield over a 5-year period of 16 kg/tree/year for an outstanding group of trees in Pemba (Zanzibar).

After harvest, the inflorescences are separated into buds and 'stems' (the flower stalks) and dried in the sun for several days. The dry weight of buds and stems is about a third of fresh weight. The dried product is sold in bags. An increasing portion of the clove production is being used for essential-oil and oleoresin production by hydrodistillation or steam distillation or by solvent extraction.
For distillation of the leaf oil, fallen leaves may be gathered every 2-3 weeks. The yield is about 1.5 kg of sun-dried leaves per tree each time. It is more common, at least in Madagascar, to cut and bunch small branches, which are taken to the still. Regular pruning of closely planted hedges is recommended for this manner of harvesting; the clove yield is then negligible. It takes about 60 kg of prunings to produce 1 kg of oil.

Germplasm of clove has been collected in Indonesia, but little information is available on the collections. Naturally set seed results mostly from cross-pollination, but the floral biology of the clove also favours self-pollination and fairly uniform populations developed in the areas where only a few trees were introduced initially. The eradication of the trees in nearly all places in the Moluccas may have decimated the germplasm in the cultivated clove and widened the gap from the wild cloves. Wild cloves are hardier and more vigorous, but they are hardly aromatic. Perhaps aromatic trees occur only sporadically in wild populations but they are easy to recognize; so their seed may have been collected for cultivation through the ages.

Hybrids of wild and cultivated cloves are similar to the wild parent. Hence the only direct way to widen the genetic basis is to trace clove populations descended from trees that escaped the eradication campaign (e.g. in New Guinea). Presumably the Zanzibar type is such a population. Hybrids between trees from Zanzibar and Indonesia are superior to both parents in both vigour and yield in the early years. Clonal propagation of selected mother trees may result in a breakthrough in productivity; early results in East Java show that rooted cuttings exhibit the same outstanding yield features as the mother trees.

The world powers no longer wage war for the control of the clove trade. The clove has become very much an Indonesian crop and product. The spectacular developments in that country during recent decades have subsided into a more stable situation. Much depends on the future demand for cloves for the cigarette industry, as world demand for the spice and the oil are unlikely to change much.
If a reasonable price level is restored, there is much scope for agronomic improvements:
- Further segregation (in respect of growing conditions and crop care) of production for the cigarette industry, for use as a spice and for distillation of leaf oil.
- Control of Sumatra disease and leaf-spot.
- Clonal propagation of superior trees, cutting out the juvenile phase.
- Manipulation of the growth rhythm to reduce yield fluctuations and to extend the harvest season.

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E.W.M. Verheij & C.H.A. Snijders

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