Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Colorant, Natural

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_164

Synonyms

Definition

The term natural colorant comprises all kind of materials available from natural sources which are able to impart color to matter.

The main principle of color development for natural colorants is the specific absorption of light in the wavelength region of 400–700 nm [1].

Other principles on color formation may be based upon physical effects, for example, refraction of light (rainbows), interference (feathers of peacocks), or electron excitation (electroluminescence) [2].

Sources for natural colorants include minerals (red ochre, α-Fe2O3), plant material (e.g., flavonoids from Canadian golden rod), and animal-based dyes (e.g., indigoid colorants from selected mollusc species).

Colored pigments are applied as finely divided solid particles which remain in an insoluble state during their application and use. Water-soluble or oil-soluble natural colorants can be found in food and beverage applications. A colorant that is used as dye must exhibit a specific substantivity towards that substrate. A dye is thus able to be sorbed at the surface of the material.

According to structure, organic natural colorants can be divided into several main groups, namely, flavonoid dyes (including anthocyanins), substances containing naphthoquinoid and anthraquinoid groups, indigoid dyes, tannins, carotenoids, and chlorophylls [3].

The colorant content in natural sources is relatively low, usually of the order of a few % of the dry material. Thus, considerable amounts of raw material have to be processed.

Natural colorants are usually obtained by means of aqueous extraction of plant material; the use of solvents is less common as substantial amounts of plant material have to be extracted. As a result of the nonselective extraction process, natural colorants usually contain a number of different color-providing chemical substances. The classification of a dye plant or animal source to a certain group of colorant usually is based on the properties of the most important species of dye present in the extract.

Major applications of natural colorants are as the coloring elements in food, as dyes for traditional textile dyeing eco-textiles, and as pigments for cosmetics [4].

Natural colorants can be applied in different ways. In many cases the extracted dye can be used directly (e.g., food and selected textile applications), but often mordants are used to increase dye adsorption (e.g., for textile dyeing and dye-lake formation). Important mordanting substances are tannins or metal salts.

Major Classes of Natural Colorants

As mentioned, as a result of the extraction of natural colorants from native sources, with few exceptions, a number of colored substances are present in the extract, and, therefore, natural colorants are classified on the basis of the main constituent, which is of relevance for a given application. Important dyes with known chemical structure have been assigned Colour Index (C.I.) Generic Names and/or C.I. Constitution Numbers. The major classes of colorant are discussed below.

Indigoid Dyes

The two most important natural dyes of indigoid structure are indigo (C.I. Natural Blue 1) obtained from plant sources and Tyrian purple (6,6′-dibromoindigo, C.I. 75800) from the Muricidae family, e.g., spiny dye murex (Bolinus brandaris) or banded dye murex (Hexaplex trunculus).

The structures of these two dyes are given in Fig. 1.
Colorant, Natural, Fig. 1

Structure of indigo (left) and Tyrian purple (right)

Tyrian purple is found in extracts from the hypobranchial glands of the gastropod mollusc. While the applicatory properties and fastness of the colorant were generally acceptable, the use of this dye ceased due to the low dye content in the mollusc.

Indigo is the most relevant blue natural dye and different plants have been cultivated for indigo production all over the world. As a general rule, the indigo is present in the plant in the form of different colorless indoxyl glycosides as precursors. Selected plants for indigo production are summarized in Table 1.
Colorant, Natural, Table 1

Indigo plants

Plant

Botanical name

Climate

Region

Precursor

Indigo plant

Indigofera tinctoria L.

Tropical

India, Africa, and North, Central, and South America

Indican indigo-β-d-glucoside

Woad

Isatis tinctoria

Isatis indigotica

Temperate

Europe China

Isatan B indoxyl-5-ketogluconat

Dyer’s knotweed, Ai

Polygonum tinctorium AIT

Subtropical and temperate

Europe, Japan

Indican indigo-β-d-glucoside

To obtain indigo the precursor is first hydrolyzed enzymatically to obtain the intermediate indoxyl, which then rapidly oxidizes in the presence of air oxygen to indigo. The precipitated solid indigo dye then can be collected. The oxidation of indoxyl to yield the yellow isatin is an unwanted side reaction, which can cause substantial losses in indigo yield. A general scheme is presented in Fig. 2.
Colorant, Natural, Fig. 2

Main steps in production of natural indigo (R = glycosidic bond carbohydrate)

Besides the use as insoluble indigo colorant, indigoid dyes usually are applied as vat dyes. In vat dyeing the insoluble dye (indigo) is firstly converted to the alkali soluble leuco form, which exhibits substantial substantivity towards protein fibers (wool, silk) and cellulosic fibers (cotton, flax, regenerated cellulose fibers). The adsorbed reduced indigo dye (yellow) is then oxidized to generate the blue-colored insoluble form of the colorant using air or chemical oxidants. For deep shades, the procedure of immersion into the leuco dyebath and subsequent oxidation in air is repeated several times.

Dye reduction can be achieved by use of reductants and alkali, such as sodium dithionite (Na2S2O4) and NaOH; alternatively, anaerobic microbial reduction can be employed. Microbiological dye reduction is practiced in traditional handicraft dyeing, but up to now is not applicable for dyeing with indigo in denim production (jeans).

Flavonoids and Anthocyanins

Flavonoids contribute to the color of many food products and thus are consumed by humans in the form of fruits and food. The color gamut provided by these colorants ranges from the yellow flavonoids (e.g., flavonols, chalcones, aurons) to orange–red–purple anthocyanins. In plants many flavonoids are present as glycosides; anthocyanins are also glycosides, while the respective aglycones are named anthocyanidins. Representative structures of flavonoids and anthocyanidins are given in Fig. 3.
Colorant, Natural, Fig. 3

Basic structures of quercetin (flavonoid, left) and cyanidin (anthocyanidin, right)

Yellow flavonoid dyes can be extracted from a high number of plant sources; selected representatives are given in Table 2 [5].
Colorant, Natural, Table 2

Plant sources for flavonoids

Plant

Botanical name

Main colorants

 

Part of plant

Weld

Reseda luteola

Luteolin

C.I. Natural Yellow 2

Plant except roots

Apigenin

Roman chamomile

Chamaemelum nobile

Apigenin

C.I. Natural Yellow 1

Flower

Onion

Allium cepa

Quercetin

 

Outer shell of fruit

Black oak

Quercus velutina

Quercetin

C.I. Natural Yellow 10

Bark

For textile dyeing, flavonoids can be used as direct dyes or as mordant dyes in combination with metallic mordants such as Fe or Al salts. When Fe, Sn, or Cu salts are employed as mordants, metal complexes are formed and a distinct change in color is observed, e.g., olive shades are obtained when an onion extract is applied in combination with Fe mordant.

The range of colors of anthocyanin dyes is of high coloristic interest for application as food colorants and for general purposes of coloration. The limited stability of the molecules can be improved by co-pigmentation, metal complexation, and the use of additives. Dependent on solution pH, anthocyanins rearrange to form different species with different absorption spectra. Some of these products are colorless which explains the propensity of the colorants to fade.

A general scheme describing the pH-dependent rearrangement of an anthocyanin structure is given in Fig. 4.
Colorant, Natural, Fig. 4

pH-dependent structural transformation of anthocyanins (malvidin 3,5,-diglycoside)

At pH 1.0, orange–red/violet oxonium/flavylium ions are present which are of greatest stability (Fig. 4, AH+). With increase in pH to between 4 and 6, the colored quinoid base appears (Fig. 4a). At the same time the formation of the colorless hemiketal (Fig. 4b) gains importance. The hemiketal B is sensitive to tautomerization and ring opening, thereby leading to colorless cis- and trans-chalcone forms (Fig. 4, Cc and Ct) [6].

Stabilization of the dye molecules can be achieved by co-pigmentation. By inter- and intramolecular complex formations, self-association, and metal complex formation, the stability of the dyes can be increased; in many cases the color becomes bluer (bathochromic shift) and absorbance is increased (hyperchromic shift).

Anthocyanins are found in many intensively colored fruits and berries. Examples are summarized in Table 3.
Colorant, Natural, Table 3

Plant sources for anthocyanins

Plant

Botanical name

Main colorants

 

Part of plant

Elder

Sambucus nigra

Cyanidin glycosides

 

Fruit

Vine

Vitis vinifera

Malvidin glycosides

 

Fruit

Privet

Ligustrum vulgare

Malvidin, cyanidin, delphinidin glycosides

C.I. Natural Black 5

Berries

Hollyhock

Alcea rosea

Malvidin, delphinidin glycosides

 

Flowers

Quinoid, Naphthoquinoid, and Anthraquinoid Dyes

Important yellow, orange, and red dyes belong to this group of colorants.

The red–orange quinoid dye carthamin (C.I. Natural Red 26) can be obtained by extraction of blooms of Safflower (Carthamus tinctorius) (Fig. 5).
Colorant, Natural, Fig. 5

Structure of carthamin

Two important representatives of naphthoquinone dyes are lawson (2-hydroxy-1,4,-napthoquinone) and juglon (5-hydroxy-1,4,-naphtoquinone) (Fig. 6).
Colorant, Natural, Fig. 6

Structures of lawson (left) and juglon (right)

Lawson (C.I. Natural Orange 6) is extracted from henna (Lawsonia inermis). The plant grows best in tropical savannah and arid zones. The leaves contain lawson in the form of its glycoside, which hydrolyses and releases the aglycone, which then is oxidized to the quinone form. Lawson is of significant interest in hair dyeing. As a small molecule, its diffusion rate is sufficiently high to impart acceptable levels of color under the gentle conditions employed for hair dyeing. When henna leaves are mixed with leaves of the indigo plant, the so-called black henna is obtained. In the application as hair dye, lawson and indigo are fixed on the hair and brown-black shades are obtained.

Anthraquinone dyes can be extracted from plant sources (madder, Rubia tinctorum, C.I. Natural Red 8; Hedge bedstraw, Gallium mollugo, Natural Red 14) [7] or from animals (Kermes, Kermes vermilio, C.I. Natural Red 3; cochineal, Dactylopius coccus, C.I. Natural Red 4). Anthraquinoid dyes also have been extracted from fungi and lichen [8].

Most plant-based anthraquinoid dyes are extracted from the roots of the plants. The colorants are present as glycosides which hydrolyze during storage or extraction to the corresponding anthraquinoid dye. Representative structures (Alizarin, 1,2-dihydroxyanthraquinone, pseudopurpurin 3-carboxy-1,2,4-trihydroxyanthraquinone) are shown in Fig. 7. Through the high number of hydroxyl groups that neighbor the quinoid system, these compounds are able to form stable metal complexes with, for example, aluminum or calcium. Thus, in traditional textile dyeing with madder extracts, mordanting with metal salts is used to improve dye fixation and fastness. The high fastness properties of such dyeings makes them valuable for textile applications, although care has to be exercised to ensure that harmful components such as lucidin (1,3-dihydroxy-2-hydroxymethyl-anthraquinone) have been removed, e.g., through an oxidation step.
Colorant, Natural, Fig. 7

Structures of alizarin (a), pseudopurpurin (b), and kermesic acid (c)

Kermes, cochenille, and lac are important examples of insect-based natural colorants. In the cases of kermes and cochenille, the parasitic insects live on host plants from where the female insects are collected and dried. Lac is obtained from the secretions of the lac insect. Due to the high quality (light fastness and wash fastness) of red textile dyeings, these dyes were of high importance in traditional dyeing. The dyes still are produced in considerable amounts for cosmetic applications and as food colorants (Table 4).
Colorant, Natural, Table 4

Insect dyes

Insect

Name

Main colorants

 

Host plant

Kermes

Kermes vermilio

Kermesic acid

C.I. Natural Red 3

Kermes oak

Cochenille

Dactylopius coccus

Carminic acid

C.I. Natural Red 4

Nopal cactus

Lac insect

Kerria lacca

Laccaic acids

C.I. Natural Red 25

Not specific

Tannin-Based Dyes

Tannins are a complex group of polyphenolic compounds. Gallotannins comprise the structural unit of gallic acid esterified with sugar molecules (hydrolysable tannins), while tannins contain flavan as general structural element, by condensation complex polyphenolic structures are formed (condensed tannins) (Fig. 8).
Colorant, Natural, Fig. 8

Structure of gallic acid and a condensed tannin ((+)-catechin-(+)-catechin, B3) flavan structure marked blue)

Tannins are found in all parts of plant, e.g., bark, wood, leaves, floral parts, and gallnut. In contrast to other natural colorants, the tannin content in plant material is much higher and, depending on source, can reach levels of 50 % of the plant mass (Table 5).
Colorant, Natural, Table 5

Plant sources for gallotannins and tannin agents (condensed tannins)

Plant

Name

 

Main colorants

Content/%

 

Aleppo gall on oak tree

Quercus infectoria

Gallnut

Turkey tannin

50–70

 

Sicilian sumac

Rhus coriaria

Leaves, twigs

Gallotannin

23–35

C.I. Natural Brown 6

Sticky alder tree

Alnus glutinosa

Bark

Gallotannin

20

 

Pomegranate

Punica granatum

Pomegranate fruit bark

Gallotannin

28

C.I. Natural Yellow 7

Scots pine

Pinus sylvestris

bark

Tannin agent

17

 

Cutch/catechu

Bark, leaves

Tannin agent

C.I. Natural Brown 3

Tea

Camellia sinensis

Leaves

Tannins agent

25

 

A gallnut is formed as response to the egg deposition by a gall wasp, e.g., on a leaves and twigs of an oak tree.

The term catechu is used for a number of plant extracts with high content in tanning agents, obtained from different plants, e.g., mangroves, conifers, cutch, and acacia.

Tannins and tannin agents form intensively colored complexes with metals such iron and copper; thus, color development usually is coupled to use of metal mordants.

Tannins also are used as mordants in place of metal mordants to increase the extent of adsorption and fastness of other natural colorants.

Carotenoid Dyes

A number of yellow, orange, or red pigments are found in many plants and animals. Animals are not able to synthesize carotenoids, which they have to obtain from their food.

Carotenoids are polyisoprenoids; hydrocarbon carotenoids are carotenes, while oxygen-containing molecules are named xanthophylls. Characteristically, carotenoids comprise conjugated double bonds. Typical examples are α-carotene (carrots, Daucus carota; red palm oil), lutein (green leafy vegetables, broccoli, corn), bixin (annatto seeds, Bixa orellana, C.I. Natural Orange 4), capsanthin (paprika seed, Capsicum annuum), lycopene (tomato, Lycopersicon esculentum), and crocetin (saffron, Crocus sativus, C.I. Natural Yellow 6) (Fig. 9).
Colorant, Natural, Fig. 9

Structure of bixin (a) and crocetin (b)

From annatto seeds and saffron flowers, orange dyes can be obtained by aqueous extraction. The water-soluble colorants can be used to dye wool, silk, and cellulose fibers. Both dyes also are used as food colorants.

A major group of other carotenoids exhibit poor solubility in water, and extracts are obtained using solvents of low polarity (hexane, oil). As a result, their use as food colorants is dependent on the oil content of the product as in many cases the oily phase will contain the dye.

Chlorophylls

While chlorophylls represent the most abundant pigments in nature, their use as colorants is limited due to low chemical instability and high production costs. Different natural chlorophylls have been identified (chlorophyll a, b, c, d, and e). While the extraction of the green dye from plant leaves, algae, appears of high interest for the coloration of food, cosmetics, and textiles, difficulties in obtaining pure products and the rapid modification via endogenous plant enzymes to brown-green products prevent the simple, direct use of chlorophyll as a colorant.

Chlorophyllines are semisynthetic, metal-chelated chlorophyll derivatives which exhibit higher stability and are water soluble in many cases. After hydrolytic treatments and replacement of the central magnesium by copper, sodium copper chlorophyllin can be obtained. In Europe the green complex can be used as colorant for sweets, ice cream, and cheese. In the United States the use of copper chlorophyllin is more restricted, a possible use being in dentifrice.

The use of copper chlorophyllin for the coloration of paper, textiles, and leather is under consideration, although the Cu content in the product has to be considered.

Applicatory Aspects

As many natural colorants are readily soluble in water, this is the main solvent used to extract the dye from a source. Due to the low content of colored material in the natural source, the amounts of extracts are limited. Use of solvent extraction such as ethanol or hydrocarbons is limited to special purposes as considerable amounts of treated material, e.g., plant residues have to be deposited.

Concentration by solvent evaporation has to be considered carefully for energy reasons. Another technique to obtain concentrated dye preparations is precipitation as a dye lake by formation of insoluble complexes, e.g., by addition of calcium, iron, or aluminum salts.

The dye lakes can be used directly as pigment dyes for paints and cosmetics.

For textile dyeing purposes, the extracted natural colorant can be used either directly without recourse to mordants. Attachment of the colorant to the substrate is then based on H-bonding, dipole interactions, and van der Waals’ forces.

Mordants are auxiliary chemicals used to increase dye adsorption and fixation on the substrate. The mordant can be applied in a separate bath (pre-mordanting), added directly to the dye containing bath (meta-mordanting), or used as an aftertreatment (post-mordanting).

While in traditional dyeing with natural colorants, many different heavy metal salts were used (e.g., copper, tin, chromium), environmental legislation has restricted the use of heavy metals in textile processes, and as a result, mainly iron-, aluminum-, and tannin-based mordants may be used nowadays [9].

Cross-References

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© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Research Institute for Textile Chemistry and Textile PhysicsLeopold Franzens University of InnsbruckDornbirnAustria