PROTA, Introduction to Dyes and tannins

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PROTA 3, 2005. Plant Resources of Tropical Africa. vol. 3. Dyes and tannins. ed. by P.C.M. Jansen & D. Cardon. Wageningen, PROTA Foundation - Backhuys - CTA. 238 p.


Choice of species

PROTA 3: ‘Dyes and tannins’ describes the cultivated and wild plant species of tropical Africa traditionally used for dyeing or tanning. Some of these are only used as a dye or a tannin, but most have several uses. PROTA normally assigns one primary use and if relevant, one or more secondary uses to all plant species used in Africa. PROTA 3: ‘Dyes and tannins’ comprises accounts of species whose primary use is dye or tannin. The primary use of Acacia nilotica (L.) Willd. ex Delile is as a tannin and dye, and thus it is treated in PROTA 3, but it has several secondary uses, e.g. the trees are widely planted as a fence, for shade and as ornamental, the trunk and branches yield a valuable gum, the wood is used for timber and fuel, the leaves and fruits are sources of fodder, the seeds are eaten, the bark is used for fibre, and from most plant parts traditional medicines are prepared. Also tamarind (Tamarindus indica L.) is quite important as a dye and tannin, but its primary use is undoubtedly its edible fleshy sweet-sour fruit, and consequently tamarind is described in PROTA 6: ‘Fruits’.

Species that are used as a dye or tannin but have another primary use are listed after the primary use dyes and tannins, and are fully described in other commodity groups. Some important species included in this list are: garden beet (Beta vulgaris L.), capsicum pepper (Capsicum annuum L.) and safflower (Carthamus tinctorius L.).

In PROTA 3, three species are treated which, in addition to their primary use as dye or tannin, also have another primary use and consequently will be described in 2 commodity groups. These species are Pterocarpus angolensis DC. (also in PROTA 7: ‘Timbers’), Pterocarpus soyauxii Taub. (also in PROTA 7: ‘Timbers’) and Sorghum bicolor (L.) Moench (also in PROTA 1: ‘Cereals and pulses’).

Dyes and tannins are closely related. The basic components of many vegetable dyes are chemically comparable to those of tannins and for this reason dye- and tannin-producing plants have been dealt with together in this volume. Some species are only used as a dye or as a tannin, but particularly the tannin producing species are often also used as a dye.

In PROTA 3: ‘Dyes and tannins’ comprehensive descriptions are given of 30 important dye and tannin species. These major dyes and tannins comprise most wild species, but also several cultivated or partly domesticated species. The accounts are presented in a detailed format and illustrated with a line drawing and a distribution map. In addition, accounts of 43 dyes and tannins of minor importance are given. Because information on these species is often scanty, these accounts are in a simplified format and do not include a drawing or map. For another 43 species the dye and tannin information was too scarce to justify an individual treatment and they have only been mentioned in the accounts of related species.

Theoretically numerous plant species can be used as a dye because most contain substances which can colour fibres, food or skin. Many plant species also contain tannins. For PROTA 3, however, only species have been chosen for which proof was found in the literature on African useful plants that they actually are or have been

used as a dye or tannin. Such literature, however, is scarce, often old and usually poor in details about the use. Only species for which at least a practical application is known have been considered for treatment. This means that species mentioned in the literature but only with remarks such as ‘the plant contains tannin’ or ‘the plant yields a black dye’ have not been included.

The appearance of synthetic dyes and tannins in the second half of the 19th century eventually caused an almost complete disappearance of the natural ones in commercial dyeing and tanning and in scientific attention. In tropical Africa small-scale dyeing and tanning with materials of plant origin persisted only very locally and often escapes attention in ethnobotanical reports. Therefore, PROTA 3 does not claim to be comprehensive and particularly the dyes and tannins of Madagascar, which are poorly known, may be underrepresented.

At present the interest in natural dyes and tannins is increasing worldwide, mainly because they are less polluting and less or non-toxic. Natural food colorants are becoming particularly commercially important. In PROTA 3, however, little attention has been paid to food colorants because of the insignificance of their use in Africa. Not treated are, for example, red headed cabbage (Brassica oleracea L.), tea (Camellia sinensis (L.) Kuntze), many Citrus species, oil palm (Elaeis guineensis Jacq.), tomato (Lycopersicon esculentum Mill.) and cacao (Theobroma cacao L.). These species feature prominently in other commodity groups.

Plant names

Family: Apart from the classic family name, the family name in accordance with the Angiosperm Phylogeny Group (APG) classification is also given where it differs from the classic name.

Synonyms: Only the most commonly used synonyms and those that may cause confusion are mentioned.

Vernacular names: Only names in official languages of regional importance in Africa are included: English, French, Portuguese and Swahili. It is beyond the scope of PROTA to give an extensive account of the names of a species in all languages spoken in its area of distribution. Checking names would require extensive fieldwork by specialists. Although regional forms of Arabic are spoken in several countries in Africa, the number of African plant species that have a name in written, classical Arabic is limited. Arabic names are therefore omitted. Names of plant products are mentioned under the heading ‘Uses’.

Origin and geographic distribution

To avoid long lists of countries in the text, a distribution map is added for major species. The map indicates in which countries a species has been recorded, either wild or planted. It should be realized that for many species these maps are incomplete because they are prepared on the basis of published information, the quantity and quality of which varies greatly from species to species. This is especially the case for wild species which are not or incompletely covered by the regional African floras, and for cultivated species which are only planted on a small scale (e.g. in home gardens). For some countries (e.g. Central African Republic, Chad, Sudan, Angola) there is comparatively little information in the literature. Sometimes they are not covered by recent regional or national floras and although species may be present there, this cannot be demonstrated or confirmed. For some major species, a distribution map has been omitted because there is too little information on distribution.

Properties

Although often poorly known, the chemistry of the colouring or tanning agents is given. In addition, compounds with possible or proven nutritional or medicinal value, toxins and other relevant chemical compounds are mentioned.

Description

A morphological characterization of the species is given. The description is in ‘telegram’ style and uses botanical terms. Providing a description for the general public is difficult as more generally understood terms often lack the accuracy required in a botanical description. A line drawing is added for all major and some lesser-known species to complement and visualize the description.

Management

Descriptions of husbandry methods including fertilizer application, irrigation, and pest and disease control measures are given under ‘Management’ and under ‘Diseases and pests’. These reflect actual practices or generalized recommendations, opting for a broad overview but without detailed recommendations adapted to the widely varying local conditions encountered by farmers. Recommendations on chemical control of pests and diseases are merely indicative and local regulations should be given precedence. PROTA will participate in the preparation of derived materials for extension and education, for which the texts in this volume provide a basis, but to which specific local information will be added.

Genetic resources

The genetic diversity of many plant species in Africa is being eroded, sometimes at an alarming rate, as a consequence of habitat destruction and overexploitation. The replacement of landraces of cultivated species by modern cultivars marketed by seed companies is another cause of genetic erosion. Reviews are given of possible threats for plant species and of the diversity within species. Information on ex-situ germplasm collections is mostly extracted from publications of the International Plant Genetic Resources Institute.

References

The main objective of the list of references given is to guide readers to additional information; it is not intended to be complete or exhaustive. Authors and editors have selected major and other references; major references are limited to 10 references (5 for minor species), the number of other references is limited to 20 (10 for minor species). The references listed include those used in writing the account. Where data available on the internet have been used, the website and date are also cited.

Editors

  • P.C.M. Jansen, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
  • D. Cardon, CNRS, CIHAM-UMR 5648, 18, quai Claude-Bernard, 69365 Lyon, Cedex 07, France

General editors

  • R.H.M.J. Lemmens, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands
  • L.P.A. Oyen, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands

Photo Editor

  • E. Boer, PROTA Network Office Europe, Wageningen University, P.O. Box 341, 6700 AH Wageningen, Netherlands