Natural dyes

NATURAL

DYES Zain-ul-Abideen

M.Sc Applied Chemistry GC University, Fsd

NATURAL DYES

Natural dyes are dyes or colorants derived from plants, invertebrates, or minerals. The majority of natural dyes are vegetable dyes from plant sources—roots, berries, bark, leaves, and wood—and other organic sources such as fungi and lichens.

Substances with intense colours are often called colorants. Colorants may be natural or synthetic. Plants are a source of natural colorants.

Colorants used for � textile and food are often called dyes.

Colorants used for inks, paints and �cosmetics are often called pigments.

NATURAL DYES IN PLANTS

A plant usually contains a mixture of natural dyes. This mixture is often extracted and used to dye textiles. However, a particular dye may be extracted from the mixture to be used alone.

Natural dyes may be grouped according to their chemical structures. Here are some important groups

Carotenoids 1.Carotenes (orange or red-orange)

2. Xanthophylls (yellow) Flavonoids 1. Flavones and flavonols

2. Anthocyanidins and anthocyanins

Anthracenes 1. Anthraquinones 2. Napthoquinones

CAROTENOIDS

There are two major types of carotenoids

orange or red-orange yellow

Carotenes are found in, for example, carrots, red peppers, oranges and tomatoes. They are also responsible for the colour of most yellow and orange flowers. Xanthophylls are found in, for example, nettles, French marigolds, annatto and saffron.

Most carotenes are hydrocarbons though some also contain oxygen atoms. The most well-known is often simply called carotene It’s found in carrots and has the chemical structure:

All carotenoids are based on this carotene structure. The central carbon chain remains the same, but the groups at either end differ slightly. The central chain consists of 18 carbon atoms bonded alternatively with single and double covalent bonds with four methyl groups attached (always in the same positions). The sequence of alternative single and double covalent bonds is called a conjugated system.

Xanthophylls Some carotenoids have molecules

containing oxygen atoms. These are xanthophylls. Here is an example:

All carotenoids are fat-soluble. They dissolve in organic solvents that are not too polar. They do not dissolve in water.

FLAVONOIDS

Flavonoids are water-soluble compounds with molecules derived from 2-phenyl-1,4-benzopyrone

Flavones and Flavonols have yellowish colours. The colours are

sensitive to pH. The yellow becomes much deeper in solutions of high pH. The colours of flavones tend not to fade in strong light as flavonols do, but they are paler.

Example: Quercetin, Kaempferol and Myricetin

(found in many families, including onions, horse chestnuts and tea )

Anthocyanidins and Anthocyanins are the most highly coloured of the flavonoids.

They are responsible for the scarlets, reds, violets and blues in many flowers, fruits and vegetables.

The general structure for an anthocyanidin

The most common anthocyanidinsR 1= OH

R 2= OH

R 3 = H

Cyanidin (Red-Purple)

When anthocyanidins bond to sugar molecules they become anthocyanins.

This positive charge on this cation is balanced by an anion, e.g. chloride, Cl-.

The chromophore (functional group responsible for color) is:

Occurance

Pigmented leaves and flowers of many plants

Fruit such as blackcurrants, aubergines, oranges, blackberries, raspberries, cherries and redcurrants.

ANTHRACENES

Anthraquinones

Alizarin and Madder Plant (Red Shades)

Napthoquinones

Juglone found in Walnut (Brownish shades)

SOME OTHER DYES FROM PLANTS The colour of red beetroot is due to betanin, which

can be extracted easily from the vegetable. It makes up 95% of the red pigments in the extract. Betanin is a betacyanin – a group of red dyes once thought to be flavonoids. Like anthocyanins they are glycosides. However, their molecules contain nitrogen. Also, they do not change colour reversibly when pH changes as anthocyanins do.

Tannins are present in most plant tissues, being produced from the flavonoids, especially the anthocyanins, when tissues break down. They are found, for example, in the bark of oak and willow.

APPLICATIONS OF NATURAL DYES Natural dyes are known for their use in coloring of

food substrate , leather as well as natural protein fiber like wool and silk and cotton as a major areas of application from pre-historic times.

Non-toxic and non-allergic effects of natural dyes instead of synthetic dyes.

These are biodegradable

Natural dyes produce very soothing and soft shades as compared to synthetic dyes

However, natural dyes are expensive

THE END