Prospects of Natural Chromogens - A Review

Dr. Madhu.C.Divakar

From time immemorial man had used colors; to decorate his household items, in dietary materials, in textiles, in cosmetics etc.

People used to extract dyes from natural resources in ancient times, but with the advent of synthetic dyes and colouring materials; from which one can make flamboyant shades of brilliant colours beyond imagination, had become common practice.

The use of natural dyes faded drastically for the last four to five decades and dyeing practice with the synthetic compounds enhanced manyfolds, without thinking much about the dreadful and silent killing effects of the synthetic colours on the environment and eco systems. The production and processing of the synthetic dyes and colourants were estimated to release many hundred tones of unfixed and non degradable wastes, which are hazardous to human health, also might cause major environmental pollution and ecological imbalance. Perhaps this may be the fact for the new awareness and resurgence of interest all over the world towards natural resources for getting biodegradable and environmental friendly dyes.

Many different types of synthetic dyes like azodyes, diphenylmethane derivatives, xanthene compounds, triphenylmethane compounds oxazine compounds etc having been used commonly for dyeing in textile industries, in dry colour powders, in the pharmaceuticals and also in nutraceuticals. The environment protection act 1986 section (6) subsection (2). rule 13, is obviously against the use of these synthetic dyes in food materials and in cosmetics. Already in developed countries like USA, Germany, France etc, the use of synthetic colours in food materials have restricted. The World Health Organization and USFDA specified and set defanite control limits for the use of these synthetic colours, because of their serious toxic reactions towards eyes and skin.

A dye, whether it is from a natural or synthetic origin, is used, not to just colour the surface of fibres, but it must become a part of the fibre. After dyeing the colour of the fabric should not be affected during washing process, dry cleaning with organic solvents etc and also the dye should give fastness to light, heat and bleaching. Mainly dyes are classified as follows.

a) Direct dyes :

These dyes usually contain acidic and basic functional groups and can combine with the polar groups in the fibre. These dyes are used to colour silk and wool. The free amino groups which are present in these fibres will attach with the acidic groups of the dyes.

b) Mordant dyes :

In this case, the fibre requires a pretreatment with the mordant material suitable for binding a dye. The mordant fixed to the fibre when combines with a dye to form an insoluble coloured complex, which is called as a lake. Mordant dyes are used to dye silk, cotton and wool fibres. For eg: Alizarin occurs in the root of Rubia tinctorium and in the heart wood of Morinda citrifolia gives a red colour with Aluminium and tin salts, brownish red colour with Chromium salts and black or violet color with Fe-salts. A mordant helps the dye to fix firmly with the fabric.

c ) Vat dyes :

These dyes which are insoluble in water, can convert into alkali soluble leucocompounds (colourless), when reduced with sodium hydrosulphite. After introducing into the fabrics, the dye will be oxidized on exposure to air and become insoluble in water again. Previously the reduction process of the dye was carried out in wooden vats, hence the name vat dyes. These dyes are used to colour cotton, fibres. Indigo (plant source : Indigofera tinctoria) is a good example for vat dye which is water insoluble and blue in colour when reduced, convert into indigo white which is colourless and water soluble.

d) Ingrain dyes :

Here in this case, the dye is synthesized inside the fabrics itself.

eg: Azodyes.

e) Disperse dyes :

These dyes are capable of dissolving some synthetic fibres. These compounds are used commercially as a finely divided dye, in a solution of soap and in presence of phenol or cresol at high temperature and pressure absorbed into the fibres like nylon, Dacron etc.

The major classes of synthetic dyes are the following.

a) Nitro dyes eg: Naplahol yellow

Maritus yellow

b) Azo-dyes (-N=N-) eg: Aniline yellow, Butter yellow, Methyl orange, orange II, Resorin yellow, Congo red, Bismark brown etc.

c) Diphenyl methane dyes

eg: Auramine ‘O’.

d) Triphenyl methane dyes (two benzene rings attached to p-quinoid group)(used for dyeing acrylic fibres). Eg: malachite green, crystal violet.

e) Xanthene dyes (made by condensing pthalic anhydride with phenols in presence of H₂SO₄ or ZnCl₂).

Eg: Fluorecein, Eosin, Rhodamine B.

f) Pthaleins eg: Phenolphthaleins.

g) Anthraquinoid dyes

Eg: Alizarin (used to dye cotton and wool).

Natural Chromogens

Normally in plants the coloured pigments are present in the leaves, flowers, heart wood, bark and in roots. These pigments can be extracted from the plant parts using suitable solvents depends upon the chemical nature of the coloured constituents. The major divisions of natural chromogens utilized or can be used in textiles food materials and pharmaceuticals are the following.

a) Indigoids (blue pigment)

eg: Indigofera tinctoira, Wrightia tinctoria, Murex brandaries (shell fish) etc.

b) Anthraquinones (yellow, pink and red pigments).

Eg: Morinda citrifolia (Morindone), Alkana tinctoria, Dactylopius coccus (cochineal) (carminic acid), Laccifer lacca (Lac insect) (Laccaic acid), Kermes ilicis (shield louse) (Kermisic acid), Alnus glutinosa, Rubia cordifolia, Rubia tinctoria etc (Alizarin) (Ruberethric acid).

c) Naphthaquinones (brown, pink or purple pigments)

Junglans nigra, J. regia (Juglone), Lawsonia innermis, (Lawsone), Woodfordia fruticosa, Lawsonia alba (Lawsone), Alkana tinctoria (Alkanin), Lithospermum erythorhizon (Shikonin)

d) Flavonoids or flavones (yellow to brown pigments)

Convallaria magalis, Butea monosperma, Nymphea alba, Madhuca indica, Ligustrum vulgare, Eupatorium gracile,

e) Phenalones (Yellow pigments)

eg: Curcuma longa, C. zeodora, C.aromatica (Curcumin).

f) Xanthophylls (yellow and red pigments)

eg: Tagetes erecta (marigold flowers), Tagetes patula, Capsicum annum (red pigment) (Capsanthin).

g) Tannis (Reddish brown, black, yellowish brown, bluish black, pigments)

Areca catechu, Alnus glutinosa, Acaciacatechu, Pterocarpousmarsupinm, Terminalea chetiula, Querus sativa (Galls) Prunus persica, Embellica officinalis annona reticulata, Amaranthus hypocondriacum.

h) Phloroglucinol derivatives (red, pigments)

eg: Mallotus philipinensis (kamala) (Rottlerin)

i) Nitrogen containing glucosides (red pigment) (oxyindol glycosides).

eg: Beta vulgaris

j) Anthocynanins (Blue, bluish red, mauve, brown, orange, reddish brown pigments).

eg: Grape fruits peel, corn flower, rose flower, apple, Prunus persica, Impatients balsamina, Papaver rhoeas Althearosea, red cabbage, Delphinium consolida, Plumbago Roseapetals, Clitoriaternata flowers, (Delphinidin), prinulahirsuta, Cantharella cinnabarium (edible mushroom), oral sunntanning agent.

Commiphoramukul flowers, Azolla, Adiantum ferns, Monarda didyma, Pisum sativum pods (Cyanidin).

Saliva splender (scarlet pigments), Lathyrus odoratus, Carthamus tinctorius (Carthamone), Butea monosperma flower (dye for wool fibres), Rubus idaeus (rasp berry), Raphanus sativus (Red raddish), Brassica oleracea (red cabbage) (cyanin),

k) Benzopyrones derivatives (yellow pigment)

Mangifera indica, Swertia chirata

l) Carotenoids (Orange, yellow, pink, brown colour pigments)

eg: Christisonia bicolor (Azafrin), Careca papaya fruits (anthera xanthin), Bixa orellina (Bixin), Ziziphus jujube, Crocus sativus (Crocetin), Rubus fruticosa,

Daccus carrota, Lillium hamsonii (b-carotene), Zeamays seeds, Helenium autumnale (cryptoxanthin), Ananas comosus, Lycopersicon esculatum, Rhodo pseudomonas bacteria.

m) Phenolic derivatives (red, pink, brown, pigments)

Caesalpinia echinata, Caesalpinina sappan, Haemotoxylon campechianum (Haemotoxylin), Chlorfora tinctoria (Maclurin).

n) Chlorophyll pigments (green pigments)

Chlorophyll – Cu⁺⁺ complex is a stable compound compared to the natural Mg⁺⁺ - chlorophyll combination.

Extraction pattern and analytical methods of some natural colour pigments

a) Plant pigments like chlorophyll can be extracted using acetone is presence of CaCo₃ or any mild alkali. The weak bases neutralise any acid, that may be liberated from the tissue and prevents the production of pheophytins during extraction process. Usually these extracts can be assayed at 654nm using UV/visible spectrophotometer.

b) Carotenoid pigmets (yellow to red colour) are exracted usually by acetone : Hexane (1:3) mixture. Acetone layer is removed by washing the extract continuously with water. The hexane extract is then treated with activated MgO₂ diatomaceous earth column, eluted with acetone and hexane. The acetone fraction usually contain polar xanthophylls and the hexane eluate contain non polar carotenoids. The carotenoid can be detected using a UV-spectrophotometer with a lmax value ranges from 430nm to 480nm (eg: b-carotene l max is 453nm in hexane).

Reverse phase HPLC methods using C-18 column are also popular for the analysis of carotenoid pigments in foods.     

c) Betalains constitute another group of colour pigments from plants. Betalains are present in two forms, one is b-cyanin which is purple red and found in high concentrations and the other one is b-xanthine which is yellow in colour and found only in low concentrations. Betalains are highly water soluble therefore it can be extracted easily with water from plant tissues. The water extract is then blended in ethyl alcohol : water (1:1) mixture. Ethanol in the mixture reduce the enzymatic action which is otherwise cause degradation of pigments. The extract can be screened for the presence of betanin and vulgaxanthin by the absorption maximum of there compounds at 535-540nm and 476-478 nm.

d) Anthocyanins : These colour pigments are present abundantly in plant kingdom, responsible for imparting purple, red and blue colour, of flowers, fruits and vegetables Anthocyanins used to show specific colours at a particular pH. These plant pigments are soluble in water and can be extracted easily from plant parts, by using slightly acidified water (0.05% HCl). Acidity in water prevent hydrolysis of anthocyanins in the extract. Usually extract containing anthocyanins with pH 1.0 show absorption maximum at wavelength 510 to 540 nm.

Conclusion

In this century, a global awareness is already set for the use of natural resources for saving the environment and earth from pollution and ecological imbalances. The present scenario is focused more towards the utilization of the vast diversity of natural resources of colour pigments for their use in food materials pharmaceuticals and textiles, instead of their synthetic counterparts, for safe guarding human health, as well as protecting and prolonging life on earth.

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About Authors

Dr. Madhu.C.Divakar

Pharmacy Department, Higher College of Technology,  Al-Khuwair,Muscat, Sultanate of Oman, madhu.divakar@gmail.com