citrus fruit

Why Is an Orange, Orange?

Just imagine how the world would look like if there weren’t any colors. Everything would be in black and white, light and dark. Life would be less interesting, nature less beautiful and, oranges wouldn’t be orange!

Fruits and vegetables contain a wide spectrum of colours and orange is just one of them. Besides oranges, pumpkins, papaya, carrots, bell peppers and peaches all have this vibrant colour which is all due to chemistry inside of them.

Physics of colour

Before we dive into food colours, we have to discuss some physics basics to understand what light and color are. Visible light is a form of electromagnetic radiation, which are waves. When this radiation has a specific wavelength (390 – 700 nm) it is part of the ‘visible light spectrum’. A lot of lights sources do not emit wave of a single wavelength, instead, they emit light with various wavelengths, making light white or yellowish.

But, each individual wavelength of light represents a different color. A good example of this is a laser, a laser emits only one wavelength and therefore has one specific color.

When light hits an object, in our case an orange, part of the light will be reflected, part will be absorbed and some might be transmitted. Which is depends on the wavelengths of the incoming light as well as the composition of the material.

If light of a certain wavelength is absorbed, it will not reach our eyes. However, if a specific wavelength is reflected it can reach our eyes and we will see it.

Would like a deeper dive into the topic? I would advise reading this website.

Chemistry of color

Next up is the link between the physics (the wavelengths, reflection, etc.) and the chemistry of colour. Chemistry influences which wavelengths are absorbed, reflected or trasnmitted by a product. Certain large complex molecular structures are especially good in absorbing light of specific wavelengths.

There are a lot of different groups of molecules which can colour foods. One of the most well known is probably chlorophyll, a group of molecules which gives plants their green colour. They absorb light of the correct wavelengths to use in the process of photosynthesis. The light they don’t need is reflected, which is green.

Other well-known examples are myoglobin molecules, which colour meat, or anthocyanidins which colour wine red. These last ones also play a role in the colour of the flesh of blood oranges, making them slightly red instead of orange.

Carotenoids colour oranges

The group of molecules that makes oranges orange are called carotenoids. Carotenoids are a large group of molecules, several hunderds of carotenoids have actually been identified.

I’ve had some trouble finding a chemical definition for carotenoids, but it seems that there are a few characteristics of carotenoids. Nearly all of them are made up of 40 carbon (C) atoms. These carbon atoms form a long chain, with some double bonds in between as well as some small side chains (often only 1 carbon atom long). The ends of the chain may be simply linear, or there might be a ring of carbon atoms at the end.

Below, one of the most well known carotenoids is shown: β-carotene. Here you can see that the molecule is built up of 40 carbon atoms (when drawing chemical structures the carbon atoms are often left out since there are so many, all intersections represent carbon atoms). You can also see a high number of double bonds between these carbon atoms (the double lines). Double bonds are very important when it comes to absorbing light.

beta carotene
The chemical structure of beta-carotene, a precursor of vitamin A and an example of a carotenoid.

There are a lot of different carotenoids but all absorb light of a similar wavelength. This results in a the yellow/orange sometimes slightly red colours that they show.

Carotenes, a sub-group of the carotenoids, can also be added to foods to give them a yellow/orange colour. When they are added in the production process these should be declared in Europe by using an E-number (E160a). This shows that E-numbers can be both artificial as well as natural components.

Stability of the colour orange

Carotenoids are long carbon chains with limited oxygen molecules. This makes them hydrophobic, which means they don’t like to sit in water, but prefer to sit in fat. They are pretty well fat soluble. When cooking or heating something this also makes them pretty stable, they won’t leak out in the cooking water easily.


Some of the sources I used for this research are: phototroph, Boundless (resource no longer online), Wikipedia. For those interested in the medical side of carotenoids, learn more here.

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