PROSEA, Introduction to Dyes and tannins

Introduction

Choice of species described in this volume

This volume deals with plants producing substances used as dye or tannin in South East Asia. Dyes and tannins are closely related. Certain plants (e.g. Phyllanthus emblica L.) contain substances that can be used for dyeing and others that can be used for tanning. As well as being used for tanning, tannins are often also used for dyeing or staining; for example, the dark coloured extract obtained by boiling chips of the heartwood of Acacia catechu (L.f.) Willd. known as "catechu"" is employed both for dyeing and for tanning. The basic components of many vegetable dyes are chemically comparable with those of tannins. For these reasons vegetable dyes and tannins have been dealt with together in one volume.

The species (genera) described are primarily used as dye or tannin, and have some economic importance (at least locally). Mangrove species are perhaps primarily used locally for firewood, charcoal and timber, but because they play a major role in the tanning industry, they have also been included. Similarly, Phyllanthus emblica L. and Terminalia catappa L. have been included here, even though they are also important for their edible fruits. Some dye and tannin producing plants are poorly known. Their use is occasional and very local, and they have been classified as "dye and tannin producing plants"" because they have no other uses; most of these species are presented in this volume in Chapter 3 "Minor dye and tannin producing plants"".

In some cases, plants that do not occur naturally or in cultivation in the Prosea region are described because they may have interesting prospects, or their products are used in the region, or, in one case (Crocus sativus L.) because the product is commonly confused with products of indigenous plants.

Many vegetable dyes and tannins are non timber products of the forest, just like resins, gums, and latex. The latter products are often used in paints and inks. Examples include "gum damars"" from species of Dipterocarpaceae and "karaya gum"" from the Indian Sterculia urens Roxb. They do not have colouring properties, but are sometimes used as a thickening agent in solutions of dyes. These products are not dealt with in this volume, but will be treated in the volume "Plants producing exudates"". Some exudates, however, have dyeing properties, e.g. the gum resin from Garcinia hanburyi Hook.f.; hence this species is treated in this volume.

Chapter 4 lists species that have another primary use and have been or will be dealt with in other volumes of the Prosea handbook. However, because of their importance as dyes or tannins in South East Asia, concise information is given in Table 1 (dyes) and Table 2 (tannins).

Table 1. Alphabetical list of important dye producing plants with another primary use. Scientific name, family name, vernacular name, part of the plant used for dyeing, colouring substance and colour, and material suited for dyeing are given, where known. See Chapter 4 for commodity group and synonyms.

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• Anacardium occidentale (Anacardiaceae), cashew: bark and fruit used for the manufacture of marking ink and for colouring hair black.
• Arcangelisia flava (Menispermaceae): stem, containing berberine, used for colouring cloth yellow.
• Archidendron pauciflorum (Leguminosae): bark used for dyeing matting black and fruit for dyeing silk purple.
• Areca catechu (Palmae), betel nut palm: nuts used for dyeing cotton and wool red brown or black.
• Artocarpus heterophyllus (Moraceae), jackfruit: heartwood, containing morin, used for dyeing cotton and silk cloth yellow.
• Artocarpus integer (Moraceae), chempedak: heartwood used in the same way as for A. heterophyllus.
• Baccaurea javanica (Euphorbiaceae), menteng: bark used for dyeing cotton and linen cloth and cigar boxes yellow red or violet; also used as a mordant.
• Baccaurea motleyana (Euphorbiaceae), rambai: bark used in the same way as for B. javanica.
• Baccaurea racemosa (Euphorbiaceae), kapundung: bark used in the same way as for B. javanica.
• Bischofia javanica (Euphorbiaceae), red cedar: bark used for dyeing matting red or black.
• Carthamus tinctorius (Compositae), safflower: flowers, containing carthamin, used for dyeing silk, cotton and linen cloth yellow red.
• Clitoria ternatea (Leguminosae), cordofan pea: flowers used for dyeing food, matting and cloth blue green.
• Cocos nucifera (Palmae), coconut palm: nuts used for dyeing silk green, and fruit stalk used for colouring teeth black.
• Curcuma longa (Zingiberaceae), turmeric: rhizome, containing curcumin, used for dyeing food, cotton and silk cloth, matting and parts of the skin yellow brown.
• Dracaena angustifolia (Liliaceae), suji: leaves used for colouring food green.
• Flemingia grahamiana (Leguminosae): glands on the pod, containing flemingin, used for dyeing silk orange.
• Flemingia macrophylla (Leguminosae): glands on the pod used in the same way as for F. grahamiana.
• Garcinia atroviridis (Guttiferae): fruit used as a mordant in dyeing silk.
• Garcinia dulcis (Guttiferae): bark used for dyeing cloth and matting brown.
• Garcinia mangostana (Guttiferae), mangosteen: fruit rind used for dyeing cloth brown black, bark for dyeing cloth yellow.
• Hibiscus rosa-sinensis (Malvaceae), shoe flower: flowers used for colouring food red, and shoes and eyebrows black.
• Mangifera indica (Anacardiaceae), mango: bark used for dyeing cloth and matting yellow.
• Melastoma malabathricum (Melastomataceae), senduduk: roots used in mixtures for dyeing red, leaves in mixtures for purple; fruits used for dyeing cloth black; ashes used as a mordant.
• Melastoma sanguineum (Melastomataceae): plant used in the same way as for M. malabathricum.
• Psidium guajava (Myrtaceae), guava: leaves used in mixtures for dyeing silk and cotton cloth and matting black.
• Pterocarpus indicus (Leguminosae), red sandalwood: wood, containing santalin, used for dyeing cotton cloth, wool, leather, bamboo and other woods red.
• Pterocarpus santalinus (Leguminosae): wood used in the same way as for P. indicus.
• Syzygium spp. (Myrtaceae): bark used for dyeing cotton cloth brown black.
• Tagetes erecta (Compositae), marigold: flowers, containing quercetagetin, used for dyeing silk and chicken food yellow.
• Tagetes patula (Compositae), marigold: flowers used in the same way as for T. erecta.
• Tamarindus indica (Leguminosae), tamarind: fruit pulp used as a mordant.
• Tectona grandis (Verbenacea), teak: root bark and leaves used for dyeing matting yellow-brown.

Table 2. Alphabetical list of important tannin producing plants with another primary use. Scientific name, family name, vernacular name, part of the plant used for tanning, percentage tannin on dry weight basis, and material suited for tanning are given, where known. See Chapter 4 for commodity group and synonyms.

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• Acacia farnesiana (Leguminosae), fragrant acacia: fruit, containing ca. 23% tannin, used for tanning leather.
• Adenanthera pavonina (Leguminosae), condori wood: bark, containing 25- ¬30% tannin, used for tanning leather.
• Albizia procera (Leguminosae), safed siris: bark, containing 12 17% tannin, used for tanning leather.
• Archidendron clypearia (Leguminosae): bark, containing 20 22% tannin, used for toughening fishing nets.
• Areca catechu (Palmae), betel nut palm: nut, containing 13 27% tannin, used for tanning leather and fishing nets.
• Bischofia javanica (Euphorbiaceae), red cedar: bark, containing ca. 16% tannin, used for toughening nets and ropes.
• Calophyllum inophyllum (Guttiferae), Alexandrian laurel: bark, containing 12 19% tannin, used for tanning leather and fishing nets.
• Cassia fistula (Leguminosae), Indian laburnum: bark, containing 12 18% tannin, used for tanning leather.
• Casuarina equisetifolia (Casuarinaceae), redwood tree: bark, containing 6 18% tannin, used for toughening fishing nets.
• Heritiera littoralis (Sterculiaceae), looking glass tree: bark, wood and fruit, containing 12 14% tannin, used for toughening fishing nets and sometimes for tanning leather.
• Hopea odorata (Dipterocarpaceae), rock damar: bark, containing ca. 15% tannin, may be used for tanning sole leather.
• Kandelia candel (Rhizophoraceae): bark, containing 17 20% tannin, used for tanning leather.
• Lithocarpus pseudomoluccus (Fagaceae), pasang kajang: cupules of fruit, containing 20 25% tannin, useful for tanning leather.
• Manilkara zapota (Sapotaceae), sapodilla: bark, containing ca. 20% tannin, used for toughening fishing tackle and sails.
• Pithecellobium dulce (Leguminosae), Manila tamarind: bark, containing 12 37% tannin, well suited for tanning leather and toughening nets.
• Punica granatum (Punicaceae), pomegranate: fruit rind and bark, containing 26 28% tannin, used for tanning leather.
• Shorea negrosensis (Dipterocarpaceae), red lauan: bark, containing 9 10% tannin, may be used for tanning sole leather.
• Sonneratia caseolaris (Sonneratiaceae): bark, containing 9 15% tannin, used for tanning leather and nets.
• Symingtonia populnea (Hamamelidaceae): bark, containing ca. 11% tannin, used for tanning leather.
• Syzygium spp. (Myrtaceae): bark, containing up to 28% tannin, used for tanning fishing nets.

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Definition and chemistry of dyes

What is a dye?

Vegetable dyes are colouring agents originating from plants. They are extracted by fermentation, boiling, or chemical treatment from small quantities of certain chemical substances present in plant tissues.

Sometimes the colour of the dye is already visible in the living plant (e.g. saffron is extracted from the orange coloured stigmas of Crocus sativus L.). However, some important vegetable dyes originate from plant components which are not coloured in their original state or which are hidden in the plant (e.g. indigo from Indigofera species).

Substances are coloured because they absorb light that humans can see between about 400 and 800 nm wavelength (Singleton, 1972). Most colours can be obtained from plant products: for instance, blue from Indigofera spp. and Haematoxylum campechianum L., yellow from Crocus sativus L., red from Rubia cordifolia L., brown from Peltophorum pterocarpum (DC.) Backer ex K. Heyne, and black from Macaranga tanarius (L.) Muell. Arg.; green is usually obtained by mixing blue and yellow vegetable dyes.

Chemistry

Plant pigments may differ substantially in their chemical structure. In general, major classes of plant pigments are chlorophylls, carotenoids, flavonoids, and quinones.

• Chlorophyll is a generic term for a number of closely related plant pigments responsible for the green colours that are so superabundant in vegetations. Chlorophylls are sometimes used for colouring foods and drinks.
• Carotenoids are characterized chemically by a long aliphatic polyene chain composed of isoprene units (Fig. 1). They have a bewildering variety of structures and intense characteristic colours: yellow, orange, red and purple. Examples of carotenoid pigments are bixin, obtained from Bixa orellana L. (annatto) and crocin found in Crocus sativus L. (saffron), Nyctanthes arbor-tristis L. and Gardenia jasminoides Ellis (Fig. 1).
• Flavonoids comprise all compounds with structures based on flavone or flavane (Fig. 2). The major subgroups of flavonoids are chalcones, flavanones, flavones, flavonols, anthocyanins, and isoflavonoids. Examples of flavonoid pigments are morin (found in several species of Moraceae) and rutin (for example present in the flowers of Sophora japonica L.) (Fig. 2). Derivatives of flavonoid tannins often give a particular colour to leather as a secondary effect of the tanning process.
• Quinones include various compounds containing a quinone structure (Fig. 3). The colour is usually yellow to red. Major subgroups are benzoquinones, naphthoquinones and anthraquinones. An example of a naphthaquinone pigment is lawsone from Lawsonia inermis L. (henna). Examples of anthraquinones are alizarin, morindin and purpurin found in species of Rubiaceae (Fig. 3).
• Other important vegetable dyes not included in the groups of pigments discussed above include the dark blue indigo, obtained by hydrolysis of the colourless indican present in plants such as Indigofera species and oxidation of the resulting indoxyl; the red crystalline dye brazilein, obtained by oxidation of the whitish phenolic compound brazilin which is present in the wood of Caesalpinia species; and curcumin, an orange yellow crystalline compound which is the colouring principle of turmeric (from Curcuma longa L.); see Fig. 4.

Fastness of colours

The quality and importance of a dye are especially determined by the fastness of the colour. Most plant pigments are not permanent, the colours fading rapidly when exposed to detergents and sunlight, especially in the tropics. The vegetable dyes used for dyeing textiles can be classified into 4 types, according to the properties that determine their use:

• "Direct dyes"" form hydrogen bonds with the hydroxyl groups of the fibre; the dye is not fast (an example is curcumin).
• "Acid and basic dyes"" combine with the acid and basic groups respectively of wool and silk; cotton is not permanently dyed; examples include flavonoid pigments.
• "Vat dyes"" are regenerated in the fibre by a redox process; these dyes often display excellent fastness to light and washing (an example is indigo).
• "Mordant dyes"" dye textiles that have been mordanted with compounds of polyvalent metals; the dye can be very fast; examples are alizarin and morindin.

Specific substances called mordants increase the adherence of various dyes to fabrics. In ancient times dung or urine were often added to the dyeing bath as mordants. Nowadays the mordants are usually the salts of metals, e.g. from aluminium, iron, tin, or chromium; they form a chemical bridge between the dye and the fibre molecules. Some plant products are also still used as mordant, e.g. leaves and bark of Symplocos species. Mordants can also affect the final colour of a dye. To make the colour more permanent, coloured textile is often treated with a mixture of products like lime and sugar, or with vegetable dyes (e.g. flower buds of Sophora japonica L.) in a final fixing bath.

In some dye processes using alizarin stain complexes, vegetable tannins are added to the solution together with proteins, in order to prevent white coloured parts of the textile from becoming coloured differently by binding the stain that is lost from the coloured parts.

Food colouring

Some pigments naturally occurring in plants are used in food colouring. They are grouped into the following 4 main groups:

• Anthocyanins. These are intensely coloured water soluble orange, red and blue pigments belonging to the larger group of flavonoid pigments. They are commonly found in fruits and vegetables, and usually consist of a combination of several pigments (often 4-6). These pigments are most stable under acid conditions and therefore must be kept in a rather acidic medium.
• Betanins. These form a small group of red and yellow pigments which are sensitive to pH, heat and light. The red beet (cultivar group of Beta vulgaris L.) is the most commonly used source of these pigments.
• Carotenoids. These yellow, red and orange pigments were discussed above. They are sensitive to oxidation; their stability in foods is maintained by limiting exposure to air.
• Chlorophylls. These green pigments were discussed above. They are sensitive to acid conditions and light.

Many plant pigments are still used to colour food, not only to make the food more attractive in appearance, but also to make it more palatable in combination with a certain flavour. However, vegetable dyes often give an undesired flavour or taste, they have poor stability, and they hardly produce a uniform colour. Moreover, they are often present in low concentrations in the plant tissues. In many cases, natural dyes are more expensive than synthetic dyes.

Definition and chemistry of tannins

What is a tannin?

Vegetable tannins are bitter and astringent substances in plants, often occurring as excretions in the bark and other parts (especially leaves, fruits, and galls). For tanning purposes, the excretions are either employed directly or used in a concentrated form by extraction of the tanning substance.

Tannins are able to react with proteins. After treatment with a tannin, a raw skin becomes stained and is protected against putrefaction. Vegetable tanning preserves the hide fibre from bacterial attack. It also builds into the fibre certain characteristics of fullness of feel and resilience which are not only characteristic to the type of skin, but also depend on the tanning materials and methods used. The result is leather with its multitude of uses. In addition to the production of leather, tannins also serve to tan nets, ropes and sails. After tanning, fishing tackle becomes more resistant to sea water. Tannins are also used in glues, stains and mordants. For instance, proanthocyanidin tannins can be employed in the production of chipboard as an alternative to synthetic phenols, the production of which greatly depends on the oil price. Several tannin producing plants are an ingredient of masticatories because of their astringent action, e.g. gambier from Uncaria gambir (Hunter) Roxb. Tannins are widely used for medicinal purposes. They are applied against diabetes, to regularize the balance of hormones excreted by the pancreas, as antiherpetic and as antibiotic. The tannins present in beverages like tea, coffee, wine and beer are essential for the flavour and aroma, and the concentration of tannins in several fruits is important for the fruit quality. Tannins are also used in ink manufacture, to remove boiler scale, and to reduce the viscosity of the drill mud when drilling deep oil wells.

According to specialists in leather technology like Thorstensen (1985) there is no such thing as a single tannin from a particular plant source. "The material extracted not only contains many different tannins, but also starches, gums, and other materials. The extract is not a true solution but will contain suspended insoluble materials. These non tannin materials also contribute to the leather producing properties of the extracts. The structure of the vegetable tannins and the estimation of the tannin content of extracts have been a major field of work by leather chemists. The practical application of vegetable tanning has been primarily an empirical skill".

Chemistry

The chemistry of tannins is complex. According to Buchanan (1952) tannins can be defined as complex polyhydric phenols with a molecular size and shape which permits suitable solubility in water.

Research on the chemistry of tanning started long ago. In 1803, Davy hypothesized about the forming of complexes between the tanning polyphenols and the animal skin. Research on the chemical formulas of tanning materials has been in progress since the mid 19th Century. This field of research still exists and knowledge about tannins and about the reaction of polyphenols with proteins is increasing every year (Spencer et al., 1988). Interest is now based on the use of tannins not only for the production of leather but also for many other products. Some important results about the chemistry of tannins have been published by Swain (1979) and Fengel & Wegener (1984).

The few basic chemical components found in tannins apart from sugars are gallic acid and its dimer ellagic acid, flavonoids (flavane related components), lignans, stilbenoids and quinones.

Gallic acid is formed by oxidation of shikimic acid, which is an elementary reaction product in the metabolism of plants (Fig. 5).

For the basic structure of flavonoids, see Fig. 2; of quinones, see Fig. 3; and of stilbenoids and lignans, see Fig. 6.

Flavane related components and compounds with gallic acid may be seen as the main constituents of tanning materials.

In the literature a separation has been made between hydrolysable tannins and condensed tannins. The criterion of the division was whether acids or enzymes could hydrolyse the components or whether they condensed the components to polymers. Although not absolutely correct, this separation corresponds merely to groups based on gallic acid and to groups based on flavane related components.

The monomers may be polymerized to oligomers to give tanning material. They may also be linked with sugars in the tannins. In that case, the component is called a glycoside. Therefore, a glycoside consists of a sugar and a polyphenolic part which is called aglycon. As will be discussed later, the polyphenols have tanning effects if their molecular weight varies more or less between 500 and 3000 (Haslam, 1979; Hillis, 1987). For the bulk of flavonoid tannins this implies degrees of polymerization (DP) of 3 to 8. A complete list of all the tannins that have so far been determined would fill many pages. Only the major tanning constituents of the most important groups of tannins are listed here, i.e. the group of gallotannins and ellagitannins, and the group of proanthocyanidins.

Gallotannins and ellagitannins are esters of gallic acid or its dimers digallic acid and ellagic acid (Fig. 7) with glucose and other polyols.

Proanthocyanidins are oligomers of 3 flavanols (catechins) and 3,4 flavandiols (leucoanthocyanidins). See Fig. 8.

Tannin and animal skin proteins

Tanning is, by definition, the complex of reactions of tannins with proteins in hides and skins. Tanning as a chemical reaction may be found in other processes such as dyeing and medicinal processes (Oliver Bever, 1986). Research on the tannin protein complex is still in progress.

It is clear that hydrogen bonds play an important role in the forming of the complex (Fig. 9), but the necessity of a certain molecular weight for an effective tannin cannot be explained by them (Spencer et al., 1986). If hydrogen bonds were the only driving force in tanning, synthetic tannins could easily be made. The molecular structure of the tannin plays an important role. To understand new theories about tanning it is necessary to explain certain aspects of animal skin proteins. The part of the skin that is used for leather is the inner part of 3 layers and consists almost entirely (98%) of collagen. In proteins 4 structures are defined. The primary structure is the sequence of the amino-acids. The secondary structure concerns the spatial orientation of the peptide chain (e.g. a helix). The tertiary structure is defined by the spatial form of the secondary structure (e.g. a bent helix). The quaternary structure is the intermolecular arrangement in the form of complexes. Because of the molecular and macroscopic structure of the proteins, it is clear that stereochemistry is important in relation to tanning. One of the characteristics of leather is its decreased hydrophilic property; another is its stability against rotting. These two may be linked. If the molecules of the tannin are too small (molecular weight M < 500) there is little or no reaction. The reasons for this are not clear. If the molecules of the tannin are too large (M > 3000) they physically prevent a complete reaction along the polyphenol molecule. Not all of the polyphenols with a molecular weight between 500 and 3000 have tanning properties because there must be a stereochemical resemblance between the tannin and the protein.

Tanning practice

In the laboratory, extractions of tannins and other polyphenols are made with methanol. In the leather industry, warm water is used for extraction. The plant extracts consist of tanning and non tanning components. The more the tannin content exceeds the non tanning material in the extracts, the more suitable the plant is as a tannin supplier. In general, gallotannins and ellagitannins are the most highly valued vegetable tanning materials. They often give leather of good quality with pale colours that do not fade in light. Examples are the myrobalans of Terminalia species and the tannins of Fagaceae species. Proanthocyanidins usually produce leather with a darker colour which is not fast to light. Plants containing proanthocyanidins are Acacia species (wattles), and mangrove species (Rhizophoraceae). However, the colour of the leather can also be improved by certain tanning techniques.

History and role of vegetable dyes

History and importance

From earliest times colours have played an important role in the life of man. Tens of thousands of years ago bodypainting was already part of the ritual connected with waging war and funerals. In southern Europe coloured drawings of hunting scenes have been found in caves used as shelter by men in the Stone Age, and coloured cloth and utensils have been found in ancient Egyptian and Indian tombs. In the beginning, minerals were often used to prepare dyes. Vegetable dyes and their uses were known in ancient times. More than 2000 years ago plants such as woad (Isatis tinctoria L.), Rubia species, and indigo (Indigofera species) were known to contain substances that could be used for colouring cloth, implements, and utensils. The first recorded use of indigo in China is over 6000 years old. Samples of leather that had been dyed red, yellow and green 4000 years ago have been found in Egypt. The rind of the pomegranate (Punica granatum L.) may have been the source of the yellow dye, and madder root (Rubia tinctorum L.) of the red dye. The red dye from henna (Lawsonia inermis L.) was used by the ancient Greeks and Romans as a cosmetic, especially for giving human hair a red sheen. Indigo (Indigofera tinctoria L.) was cultivated on a large scale in India and South East Asia in the 16th Century, but the plant and its product, the blue dye, is mentioned in Sanskrit records 4000 years old (Krochmal & Krochmal, 1974). Indigo and woad both produce a dark blue dye, and in Europe there was a competition between the dye from woad, which was cultivated mainly in France, Germany and Britain, and the indigo dye imported from India and South East Asia. Indigo finally won, but only after a turbulent period in which there was severe punishment for marketing it.

The culmination of the use of many vegetable dyes was in the 19th Century. At the end of that century the vegetable dyes were largely supplanted by synthetic dyes, starting in 1856 with the preparation of mauveine, a basic violet dye obtained as the first synthetic aniline dye and used chiefly for dyeing silk. Alizarin, the substance from Rubia tinctorum L. used for dyeing red, was synthetized for the first time in 1869 in Germany (von Wiesner, 1927), soon followed by indigo, which has been produced synthetically in large quantities since 1897. In a short time vegetable dyes had been almost completely superseded because the synthetic dyes proved to be purer and cheaper to produce. The rapid decline is illustrated by the fact that in 1896 only natural indigo, originating mainly from British India, was used; by 1914 only 4% of the indigo was of vegetable origin. Since the beginning of the 20th Century most vegetable dyes have had local importance only.

Vegetable dyes were, and partly still are, used for colouring cloth (e.g. indigo, logwood), parts of the body (e.g. henna), objects of art, utensils, wood and wickerwork (e.g. Caesalpinia sappan L., Phyllanthus emblica L.), and food (e.g. annatto).

Textile dyeing

Dyes obtained from several plant species are employed in the batik process. In this process, wax is used to construct a pattern in the textile by protecting certain patterns or designs from the watery solution of the dye. Batik is especially known from Indonesia, but comparable techniques are practised in other East Asian countries. In northern Thailand, northern Vietnam and southern China textiles are still coloured using wax. Techniques rather like batik are found in other parts of Asia, and also in Africa and Europe. The batik technique probably originated in China (Haake, 1984). In Indonesia the batik process seems to have been known for well over 1000 years; batik has certainly been practised for over 500 years. The batik process is best perfected in Java, where batik is closely linked with culture. There are hundreds of named patterns, each pattern symbolizing the status of the person wearing the batik cloth. The Javanese usually use cotton cloth which is mostly produced locally from domestically grown cotton (often the coarser qualities), or imported from the Americas (the finer qualities). Silk clothes are nowadays rarely used, but they were used more often before the Second World War. Different qualities of cotton require different types of wax. Beeswax is of excellent quality, but it is expensive; commonly used waxes are paraffin and coconut oil. The wax is heated to a certain temperature above the melting point. Then it is fixed to the textile in a particular pattern by writing ("batik tulis"") with a special device called a "canting"", or by stamping with a "cap"", a large copper stamp. The textile is then immersed in a cold solution of the dye because the wax would dissolve in a hot bath. This reduces the number of possible dyestuffs. The processes of waxing and dyeing are repeated many times so that several colours in different patterns can be given to the textile. The sequence of application of several dyes differs per method and determines the ultimate colours of the textile. Finally, the wax is removed by immersion in boiling water. The "soga batik"" of central Java is famous, producing batik cloth of very fine quality. Nowadays synthethic dyes are commonly used in the batik process. Only occasionally and very locally are vegetative dyes from Indigofera spp. (dark blue), Morinda citrifolia L. (red), Ceriops tagal (Perr.) C.B. Robinson, Peltophorum pterocarpum (DC.) Backer ex K. Heyne (brown), and Maclura cochinchinensis (Lour.) Corner (yellow) applied.

Food colouring

For centuries people of South East Asia have been using vegetable dyes to colour food. The rich flora of this region presents many sources of pigments to choose from. The use of plant pigments for colouring food, especially for traditional meals, is still widespread in South East Asia, although the number of plant pigments used is quite limited. It is common practice to extract pigments directly from fresh materials, for instance yellow from the tubers of Curcuma longa L., green from the leaves of Pandanus amaryllifolius Roxb., Dracaena angustifolia Roxb. and Sauropus androgynus (L.) Merr., red from the leaves of Iresine herbstii Hook.f. and the fruits of Capsicum annuum L., and brown from the sugar extracted from palms such as Arenga pinnata (Wurmb) Merr., Borassus sundaica Becc. and Cocos nucifera L. Besides the colour, people often also appreciate the typical flavour and taste given to the food by the plant product. For the choice of their food, people are often attracted by the colour first, then the flavour, followed by the structure, and at last by the nutritional value.

The medium and large scale food industries use almost exclusively synthetic food colourings. Synthetic dyes are not always harmless for human beings. To protect the safety and health of consumers most countries in the world have gazetted regulations on the use of food colourings. However, a food colouring permitted to be used in one country may be prohibited in other countries. In general, the use of vegetable dyes is considered safer than the use of synthetic dyes, although they are usually subjected to the same scrutiny prior to approval for use in foods. Whereas, for example, chlorophylls are permitted for colouring foods and drinks in the European Communities (EC), these pigments are not approved in the United States (Freund et al., 1988). Extracts from the fruit of cape jasmine (Gardenia jasminoides Ellis) are commonly used in Japan for colouring boiled beans, fish eggs, cakes, liquor, sweets, ices, noodles and candies, but they are not approved for food use in the United States. Annatto (Bixa orellana L.) and turmeric (Curcuma longa L.) are vegetable dyes commonly used in the large scale food industry.

Wood tar and hosts of lac insects

In many cultures it is a custom to blacken the teeth. For this purpose wood tar from species such as Cocos nucifera L., Eugenia tumida Duthie, Tamarindus indica L., Fagraea racemosa Jack ex Wallich, and many others is usually used. Sometimes the juice from the plant is used, e.g. from Rothmannia macrophylla (Hook.f.) Bremek.

Lac insects (e.g. Laccifer lacca), which are tiny scale lice that produce lac, are found from India to Thailand. Lac is the source of shellac, a purified lac resin that is used chiefly in varnishes, binding and stiffening agents and for electric insulators, but it is also the source of the scarlet lac dye. Some plant species are important as host for lac insects: these include the Leguminosae species such as Butea monosperma (Lamk) Taubert, Tamarindus indica L., Caesalpinia crista L., Acacia farnesiana (L.) Willd., and Pongamia pinnata (L.) Merr., as well as Ficus religiosa L. (Moraceae), Litchi chinensis Sonn. (Sapindaceae), Macaranga gigantea (Reichb.f. & Zoll.) Muell. Arg. (Euphorbiaceae), and Ziziphus jujuba Miller (Rhamnaceae). In this way these plant species play an important role in the production of a dye of animal origin.

The wood tar producing plants for blackening teeth and the hosts of the lac insects are not dealt with specifically in this volume.

History and role of vegetable tannins

History and importance

The history of tannins probably goes back to prehistoric times. Animal skins were used as warm clothes and as footwear, and made it possible to combat the cold in the temperate regions of the earth. Dry hides are not flexible and they rot when they get wet. To overcome these problems, the hides were probably initially treated with smoke from fires and later with oils, fats, and salts. How, when and where early man learned to make strong, flexible leather out of dry skins is not known (FAO, 1960). Probably it was a serendipity. Archaeological investigations of ancient civilizations in northern Germany dating back to 12 000 years ago have proved the existence of leather and of leather tanning at that time. In Egypt jars containing pods of Acacia nilotica (L.) Willd. ex Del. and pieces of leather have been found in the remains of a tannery dating about 7000 years ago (Howes, 1953). Almost 2000 years ago Plinius and Dioscorides reported the occurrence of astringent substances in some plants which could be used to tan hides and to heal certain diseases. Acacia species were mentioned for this purpose, together with oak (Quercus spp., "acorn cups""), pine (Pinus spp.), alder (Alnus spp.), sumach (Rhus spp.), and gallnuts (plant galls, especially from Quercus, Rhus, Tamarix and Pistacia spp.). The ancient Greek and Romans were competent tanners and produced large quantities of leather of good quality.

During the Middle Ages the Middle East was the centre of production of fine leathers. The Arabs took their tanning skills to India; however, although they ran a flourishing trade with Java during the 15th to 17th Centuries, they never introduced these skills there.

The use of mangrove bark for tanning purposes is known with certainty from the 13th Century in Persia (Wind, 1924). In the 19th Century, wattles (Acacia species) from Australia were introduced in British India and in South Africa, and later also in Java. The practical value for tanning of the South American quebracho trees (Schinopsis quebracho-colorado (Schldl.) F. Barkley & T. Meyer and other Schinopsis species) was not discovered until 1870. Quebracho is now one of the major sources of vegetable tannins, especially in America and Europe.

For a long time tanneries were run as one man industries, but from the second half of the 19th Century large tanneries were established in Europe and North America. In most South East Asian countries there was no real tanning tradition before the 16th Century. The South East Asian tanning agents often proved superior to those from Europe, where oak and chestnut barks were traditionally used. Only catechu and gambier were already known in the 16th Century in Europe, although the origin of these tanning materials was unknown to most tanners. Asia, Africa, and South America exported their raw material (i.e. the tannins and hides) to Europe and the United States where the leather was produced and sold. Efforts were made to develop a viable leather industry to export leather and leather goods. The modernization in the industrializing nations in South East Asia is progressing rapidly, and not step by step as in Europe and North America decades ago. Thailand is one country which is rapidly extending its leather industry. In 1988 no less than 126 tanneries produced more than 18 000 t of leather per year, and 40% of the processed hides were imported. That is still not enough to meet demand. Malaysian shoe manufacturers are also rapidly expanding their facilities to cope with the massive recent surge in export demand.

The most important vegetable tannins on a world scale besides quebracho (from Schinopsis species in South America) are mimosa (from Acacia mearnsii De Wild., especially from South Africa) and chestnut (Castanea sativa Miller from Europe). An overview of the extract shipments in the years 1950-1988 shows that the amounts of vegetable tannins shipped fell by 50% or more (Table 3).

In 1851 chrome tanning was discovered, and this rapidly took a major place in the commercial world. In chrome tannage the animal skin is impregnated with chromium salts. Later, with the development of the chemical industry and the knowledge of organic synthesis, it became possible to build molecules into synthetic tannins, which have a more specific activity and are more predictable and controllable in the tanning process. The use of synthetic tanning materials such as syntans, resin tannages and aldehyde tannages, has increased rapidly since 1950. The synthetic tannins allow the tanner to obtain special effects in processing or leather quality (Thorstensen, 1985). In the United States about 85% of all leather is tanned by mineral processes and about 15% by vegetable tanning processes (Seigler et al., 1986). In large tanneries vegetable tannins are especially used for "heavy leathers" such as soles, belts, straps and mechanical leathers. The processes involved are time consuming. It can take 2 months or more to tan sole leather (Thorstensen, 1985). The vegetable tannin imparts the property of mouldability to sole leather and gives more physical weight and better durability. The major vegetable tannins are available as powders. In large tanneries, they are usually mixed with syntans.

Chrome tanning is preferred for many types of leather, mainly because chrome tanned leather is more heat proof, stronger and more supple and elastic, is more water repellent, and is easier to dye (van Herwijnen, 1956). Chrome tannins are therefore used for shoe upper leather and light leathers. The chrome tanning for upper leather is a rapid process taking only a few days. However, vegetable re tannage of chrome tanned leather is often necessary to produce usable leathers, and numerous light leathers are subjected to vegetable tanning to develop special characteristics.

The production of leather

Leather is remarkable for possessing a combination of properties: it can be hard and tough, but also soft and flexible; it has a porous structure which enables it to "breathe"; it is easy to work and cut. Many attempts have been made in recent decades to produce substitutes for leather, but none of the products equals the particular properties of leather.

Leather has for millenia not only been used for clothing; it has also been made (and is still made) into all kinds of tackle and gear such as saddles and reins. It is used to make bags for carrying liquids, and for luggage, purses and wallets, and also as ornament or decoration. Leather covered furniture is currently fashionable in many prosperous countries. The properties of the hides as well as the tannins are important for the production of good quality leather. Many types of hides and skins can be used: cattle hides, goatskins, sheepskins, pigskins, skins of reptiles like snakes, lizards, crocodiles and alligators, and sometimes even the skins of sharks, kangaroos, camels, elephants, and ostriches. Each type has its own application. For instance, the heavy hides of bulls are used for sole leather, pigskins for suede shoe leather and gloves, sheepskins for garment suede leather, goatskins for durable types of shoes and gloves, and skins of reptiles for hand bags.

The properties of vegetable tannins differ and co influence the characteristics of the leather obtained. Gambier extract is very mellow and gives a buff coloured leather; bakau extract from mangrove trees and cutch or cachou are more astringent and produce red leather, whereas myrobalans from Terminalia chebula Retz. give a greenish tinge to the leather. Good tanning needs a skillful balance of pH, temperature, and concentration (Thorstensen, 1985).

Buyers estimate the value of leather largely by the colour: light coloured rather than dark coloured leather is preferred.

The tannins must be extracted from the vegetable tissues (often barks). In the past tannins were extracted in open tanks by allowing hot water to percolate through the bark. The resulting tan liquors were diluted. This made it unpractical to transport extracts over great distances, and consequently tanneries were often located near clusters of tannin producing plants. Nowadays the liquors can be concentrated and solidified, and shipping is much easier. For industrial production of sole leather the hides are first trimmed, soaked, and, if necessary, remnants of flesh are removed. Then they are placed in lime to remove the hair; this usually takes about a week. After treatment with deliming and detergent materials, the hides are ready for tanning. Usually a series of rockers is employed in which the concentration of the tanning materials starts out low and is gradually increased as the tannage proceeds. This takes about 3 weeks. The "butt" (i.e. the thick part of the hide corresponding to the animal's back and sides after trimming off shoulders and belly) is the most valuable part of the skin for sole leather, and is cut off and halved into "bends". The bends are tanned again for some weeks and then cleaned and bleached. Finally, the leather is treated with certain oils and chemicals, rolled with a heavy cylinder, and sponged with wax coating materials and dried.

The activities connected with the tanning processes can be environmentally destructive. Mangrove forests have been destroyed in several regions for the production of tannins and firewood, and wild populations of quebracho in South America have been locally overexploited. Tanneries produce large amounts of waste solids and chemicals; effluent treatment is now often one of the major considerations in the design and operation of a tannery.

Botany

Dyes in plants

Dyes can be found in many different parts of the plant: roots (e.g. the red dye from Rubia cordifolia L.), rhizomes (the orange yellow dye from Curcuma longa L.), bark (the black substance from Terminalia catappa L.), gum resin of the bark (the yellow dye from Garcinia hanburyi Hook.f.), wood (sappanwood, logwood), leaves (indigo), fruits (the purplish black dye from Terminalia bellirica (Gaertner) Roxb.), seeds (annatto), flowers (safflower), and stigmas (saffron).

The functions in plants of substances used as dye depend on their chemical structure and location in the plant. Chlorophylls are involved in the light conversion step in photosynthesis. The functional aspects of these substances are still not wholly understood. The most plausible proposal for the universal function of carotenoids is that they protect cells from photo oxidative damage caused by the incidental absorption of visible light (Burnett, 1976). Leaf flavonoids might have a protective role as a deterrent in plant animal interactions, and may have an even more important protective role as a light screen against damaging ultraviolet radiation (Harborne, 1976), comparable with carotenoids.

In general it is assumed that the presence of pigments in flowers subserves important roles in attracting insects, birds or bats for pollination. Coloured fruits and other parts of plants attract birds and other animals and favour dispersal of seeds and sometimes also vegetative fragments. The functions of plant components which are not coloured in their original state but can be converted into dyes are often obscure.

Tannins in plants

The role of tannins in plants is still not clarified. Sometimes tannins are considered waste products, but a deterrent effect on herbivores and a sterilizing effect on microbes have also been postulated. The latter opinions are supported by the fact that tannins are often found nearby essential and vulnerable parts such as the cambium in dicotyledonous plants. The deterrent effect on herbivores is also supported by the fact that plants with high tannin content are frequent in open vegetations in tropical and subtropical regions with heavy grazing pressure, as with Acacia species in the savanna. Although tannins may be an effective defence against herbivores, it is likely that their major role in evolution has been to protect plants against fungal and bacterial attack. To support this opinion, the high concentrations of tannins in nonliving cells of many trees (heartwood, bark), which would otherwise readily succumb to saprophytes, have been cited (Swain, 1979). It has also been suggested that the leaf tannins are active metabolites used in the growing tissues (Darnley Gibbs, 1974). However, tannins in different plant species probably have different functions. Tannins are absent or only found in small quantities in lower plants (algae, mosses, lichens, fungi, ferns). They are comparatively rare in monocotyledons (except in palms). Tannins are common in dicotyledons, and their occurrence is scattered over many families. However, in some families tannins do not occur or are very rare, e.g. in Cruciferae and Labiatae, but in others (e.g. in Rosaceae and Guttiferae) they are almost invariably present. In a few families many species contain tannins in large quantities, e.g. Rhizophoraceae and Combretaceae. On a world scale the most important species for tannin production belong to Leguminosae (e.g. black wattle, Acacia mearnsii De Wild.), Anacardiaceae (e.g. quebracho, Schinopsis spp.) and sumach (Rhus spp.), Rhizophoraceae (species of several genera), and Combretaceae (e.g. myrobalans from Terminalia spp.). Studies of the distribution of tannins in higher plants indicate that numerous families with a large number of tanniferous species are commonly considered primitive. It seems "... that the capacity to synthesize tannin is a primitive character that tends to become lost with increasing phylogenetic specialization." (Bate Smith & Metcalfe, 1957).

It has also been pointed out that there exists a remarkable relation in plants between the presence of types of flavonoids to which many tannins belong, and woody habit (Bate Smith, 1957, 1963). This relation will remain unexplained as long as the chemistry of the processes concerned in lignification is not well understood.

Prospects

The great importance of vegetable dyes such as indigo and madder in the 19th Century contrasts enormously with the application of vegetable dyes nowadays. Some are hardly used any more, others remain of local importance only. However, the demand for natural products is increasing slightly in the world. Some synthetic dyes have proven to be carcinogenic. Others used in foods have been associated with behavioural disturbances such as hyperactivity and learning disorders in children. Moreover, their waste products cause environmental pollution. These facts may improve the market for vegetable dyes, especially for those used in foods and drinks.

For centuries dyes of vegetable origin have been used in the manufacture of the brown coloured "soga" batiks, especially in central Java. The colours symbolize grandeur and have been greatly appreciated. Fine soga batiks have been worn by Javanese people in various traditional ceremonies, and they are still worn, but almost exclusively by middle class to rich, noble and older people. The use of vegetable dyes may only be revived if much effort is made to increase the demand for fine traditional batiks by stimulating the interest and developing the appreciation of young people to the well developed culture of their country. This will not be easy because of the still increasing influence of Western and Middle Eastern cultures in Indonesia.

Vegetable tannins are still important, but in the future, a gradual shift in favour of synthetic tanning materials is expected. Chrome tanning is most important at present, and is employed in many modern tanneries. The waste products of this type of tanning can be very detrimental to the environment. This has been demonstrated in some European countries, and as a consequence, dumping of the waste products is regulated by law. Similar pollution can be expected when aluminium and titanate tanning methods are employed. These problems should be taken into account when extension of tanning industry is considered. Vegetable tannins decompose easier and are thus less dangerous for the environment - provided their waste products are not dumped in too large quantities. Renewed phytochemical research on vegetable tanning might reveal tanning methods and possibilities that are competitive with synthetic methods, especially if the costs of dumping waste products are also considered.

The prospects for growing tannin producing plants in the tropics should not be neglected. Multipurpose crops should have priority, producing other products such as oils, dyes, timber, firewood and edible fruits, and providing protection against erosion at the same time. For example, in many places, artificial regeneration of mangroves - including selection of better tannin yielding species - has been shown to be possible.

R.H.M.J. Lemmens, N. Wulijarni Soetjipto, R.P. van der Zwan & M. Parren