Carotenoids: Why Carrots Are Orange and Tomatoes Are Red

Colour is quantum physics

Before looking at specific carotenoids, it helps to understand why molecules have colour at all. The answer lies in the quantum mechanics of π electrons.

When a molecule has double bonds alternating with single bonds (a conjugated system), the π electrons of all those double bonds are delocalised along the whole chain. The longer the conjugated system, the smaller the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied one (LUMO). When that gap (ΔE) matches a photon in the visible range (400–700 nm), the molecule absorbs visible light and we see the complementary colour.

Carotenoids are long conjugated systems. β-carotene has 11 conjugated double bonds (nine in the central chain and one in each ring). It absorbs most strongly around 450 nm (blue-violet) and we see the complement: orange. Lycopene in tomatoes also has 11 conjugated double bonds, but all of them in an open chain without β-carotene's end rings; it absorbs a little further towards longer wavelengths, so tomatoes look red.

The carotenoid family

Carotenoids are a family of more than 700 natural compounds, all derived from phytoene, a 40-carbon precursor built by joining eight isoprene (C₅) units. They fall into two main groups:

Carotenes: carbon and hydrogen only (pure hydrocarbons):

• β-Carotene (carrot, pumpkin, sweet potato)
• α-Carotene (carrot, in smaller amounts)
• Lycopene (tomato, watermelon, pink guava)

Xanthophylls: carotenes with one or more oxygen-containing groups (–OH, epoxide, ketone):

• Lutein (egg yolk, spinach, maize)
• Zeaxanthin (maize, yellow pepper, egg yolk)
• Astaxanthin (salmon, prawns, krill), responsible for the salmon-pink colour
• Fucoxanthin (brown seaweeds)

This chemical difference determines where they are found in food: non-polar carotenes accumulate in fat-rich tissues (the oil droplets of chromoplasts), whereas the more polar xanthophylls also associate with membranes and can appear in more water-rich tissues.

β-carotene: provitamin A and antioxidant

β-carotene (C₄₀H₅₆) is the most studied carotenoid and the most abundant in the human diet. Its structure is symmetrical: two identical β-ionone rings joined by a polyene chain, with 11 conjugated double bonds in all.

As provitamin A. In the small intestine, the enzyme β-carotene 15,15'-dioxygenase splits β-carotene down the middle into two molecules of retinal (vitamin A aldehyde, C₂₀), which is reduced to retinol (vitamin A) or oxidised to retinoic acid as the cell needs. In theory, one mole of β-carotene can give two moles of vitamin A, though real conversion in the body is far less efficient (a factor of about 12:1 by weight for mixed Western diets).

The resulting vitamin A is essential for night vision (as part of the pigment rhodopsin), epithelial differentiation, immunity and embryonic development.

As an antioxidant. Its long conjugated system makes β-carotene an excellent quencher of singlet oxygen (¹O₂), a reactive oxygen species that is particularly damaging to cell membranes. β-carotene absorbs singlet oxygen's energy and releases it as heat without being destroyed (a physical mechanism), so it can neutralise many ¹O₂ molecules before breaking down. This antioxidant role is especially important in plants, where singlet oxygen is a by-product of photosynthesis.

Lycopene: the red of tomatoes

Lycopene (C₄₀H₅₆) has the same formula as β-carotene but no end rings: its chain is completely open. This structural difference has two consequences:

A redder colour. Without the rings, whose double bonds are twisted out of the plane of the chain and conjugate poorly, lycopene's geometry is more extended and its double bonds are all-trans, maximising effective conjugation. Its absorption maxima shift to 471–503 nm (blue-green), so the colour we perceive is deep red.

No provitamin A activity. Conversion to vitamin A requires a β-ionone ring with a double bond in the right place. Lycopene, with no rings, cannot become retinal and so has no provitamin A activity.

Lycopene is, however, an even better antioxidant than β-carotene at capturing lipid peroxyl radicals, and epidemiological studies have linked it to a lower risk of prostate cancer, although evidence from controlled clinical trials is still inconsistent.

Cooking boosts bioavailability. Lycopene in raw tomatoes is locked in the cell matrix in the all-trans form, which is poorly absorbed. Cooking does two things: it breaks down the cell matrix, releasing the lycopene, and it promotes isomerisation from trans to cis, a form that dissolves better in bile micelles and is better absorbed. That is why tomato sauce cooked with oil delivers three to four times more lycopene than raw tomatoes.

Lutein and zeaxanthin: the eye's carotenoids

Lutein and zeaxanthin (together with its isomer meso-zeaxanthin) are the carotenoids that the human retina accumulates, concentrating them selectively in the macula lutea (the area of sharpest vision). Their job is to filter high-energy blue light (400–470 nm) and capture free radicals produced during light detection, protecting the photoreceptors from oxidative damage.

The macula concentrates lutein and zeaxanthin at levels more than 1,000 times higher than in blood plasma, thanks to specific binding proteins (GSTP1 for zeaxanthin, StARD3 for lutein) and transporters such as SR-BI that pick them selectively out of the blood. The human eye is literally a carotenoid-concentrating organ.

In the AREDS2 study (Age-Related Eye Disease Study 2), adding lutein and zeaxanthin to the supplement formula for macular degeneration did not further reduce risk across all participants, but it did in people who ate little of these carotenoids. It also proved a safer replacement for β-carotene, which increases the risk of lung cancer in smokers.

Why flamingos are pink (and salmon too)

Astaxanthin (C₄₀H₅₂O₄) is a xanthophyll with two ketone groups and two hydroxyls on its end rings. It is made by microalgae (Haematococcus pluvialis) and certain fungi and bacteria, and it concentrates up the food chain: microalgae → zooplankton → small fish → salmon and trout.

Flamingos hatch white. Their intense pink comes from the carotenoids (mainly canthaxanthin and astaxanthin) in the algae and crustaceans they eat; captive flamingos without these sources lose their colour within weeks. Farmed salmon, which do not eat enough krill, are given synthetic astaxanthin in their feed; otherwise their flesh would be a pale grey.

Astaxanthin is one of the most effective carotenoids at neutralising singlet oxygen: in laboratory measurements, about 10 times more effective than β-carotene and up to 500 times more than vitamin E, thanks to its carbonyl groups conjugated with the central double-bond system.

The next article leaves food chemistry for fundamental biochemistry: ATP, the molecule that stores and transfers energy in practically every living thing, and why its hydrolysis releases so much energy.