Colours in vegetables, a work of chemistry

Vegetables are important in rational nutrition and shows favorable influence on the functions of the physiologic human organisms. The chemical composition of vegetables shows high water content, sugars, protein, starch, fat, energy value. Vegetables and fruits are strongly coloured because they contain a chemical compound named carotenoids.

These compounds have an area called choromophore, which absorbs and gives off particular wavelengths of light, generating the colour that we then perceive. Carotenoids are yellow, orange, and red pigments synthesized by plants.

Chromophore

The chromophore is formed by a sequence of linear carbon-carbon double bonds (C=C), so the atoms remain closer to each other. In general, it’s necessary at least seven linear conjugated double bonds for a carotenoid to produce a colour. Bigger the number of bonds conjugated, the bigger the wavelength of the light absorbed and more brightly the vegetable is coloured.

Carotenoid pigments, such as carotenes and xanthophylls, mix with light to provide colour to vegetables. Carotene pigment absorbs blue and indigo light to produce yellows and orange hues found in carrots, mangoes and yams. Lycopene pigments are similar to carotene and provide the red colours of tomatoes, guava, red grapefruit and papaya.

Flavonoids

Flavonoids are yellow plant pigments found in lemons, oranges and grapefruit. Flavonoids exist in many foods, such as berries, dark chocolate, nuts, eggplant citrus fruits and cabbage. Flavonoid pigments like anthocyanins, aurones, chalcones, flavonols and proanthocyanidins lower cholesterol and provide antioxidant properties. Anthoxyanins help ripen fruit and is responsible for the red and purple colour found in some olives.

Betalains

Betalain pigments consist of two subgroups: betacyanin and betaxanthin. Betaxyanin produces red-violet colour while betaxanthin produces yellow to orange. Betalains are responsible for the deep red colour of beetroot.Other pigments found in beets include - 

• Betanin
• Isobetanin
• Probetanin
• Neobetanin
• Indicaxanthin
• Vulgaxanthins
• Chlorophylls

Chlorophylls

Chlorophylls are green pigments containing a prophyrin ring. Prophyrin are ring-shaped molecules that move freely, allowing chlorophyll to capture energy from the sun. Several kinds of chlorophyll make photosynthesis possible. Vegetables contain mostly chlorophyll type A, while some vegetables also contain chlorophyll type B.

Colours of different vegetables and their functions - 

Tomato is red because of the carotenoid, lycopene. This compound is generated by the plant to protect itself from the air oxidation. So it is a good antioxidant.

Carrots are coloured because they contain betacarotene. The conjugate double bonds of betacarotene is less then lycopene, hence carrots are orange and not red. Beta-carotenes are the major source of vitamin A in our diet. These plant compounds are good for the eyes and the skin.

Green leafy vegetables are green as they contain chlorophyll, a pigment which enables the plant to carry on photosynthesis, transforming solar energy and carbon dioxide into chemical energy in the form of carbohydrates and oxygen. The chlorophyll masks the other colours in vegetables and as its amount decreases the rest of colours become evident. This explains why tomatoes are initially green and then become red when they ripen.

Carotenoids are beneficial for the reproductive system of women. Studies have shown that carotenoids could reduce the risk of ovarian, breast, prostate, colon and lung cancers. The antioxidant plant compounds might delay the aging process. A diet rich in carotenoid rich vegetables could lower the risk of cardiovascular diseases.

Thus, it is the work of chemistry which brings colour and health benefits for vegetables.

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