Colours in fruits & vegetables

Fruits and vegetables have a great wide variety of colours. The spectrum goes from orange to purple to green to bright red. It’s amazing how nature has developed all these bright and beautiful colours.

In most cases these colours originate from a group of molecules inside the plant. The unique and special chemical structures of the molecules gives them their bright colours. Not all colours are completely unique though, most are related in some way or the other through their structure, there are a few ‘families’ of colour.

We’ve discussed several of them here on the blog already. However, it is especially useful to understand all these relationships and that’s exactly what we’ll be discussing in this post.

What gives fruits & vegetables colour?

All around us electromagnetic waves come by. Most of these electromagnetic waves we can’t see, they are outside of the ‘visible spectrum’. An example of these waves are microwave waves, radio waves or infrared. However, some of these waves we can see. Whether we can see them depends on their wavelengths. The waves that we can see are 400-700nm in length, which of course is very small. The reason we can ‘see’ these waves has to do with how our eye catches and registers light, which we won’t focus on any further.

The length of a wave within this spectrum determines the colour that we see. For example, 600nm is orange, whereas 450 nm is blue and 550 nm is what we perceive as green. For us to see these colours, a wave with this length will have to end on our eye. So how do fruits & vegetables manage that?

When light shines on an object the molecules in the object (e.g. a pear skin) will absorb some of this light and reflect other parts. This has to done with how molecules can vibrate. The wavelengths that are reflected are the ones that we see. So it’s the reflecting wavelengths that determine the colour of a fruit or vegetable.

Pigments are colour makers

A special group of molecular structures give colour to fruits and vegetables, these are so-called pigments. Pigments are not only present in foods, but also used to colour a lot of other products such as paint, clothes, etc. Pigments are good in absorbing a specific set of wavelengths and reflecting the remainder, thus making colour. Pigments are good in this thanks to their chemical structures. The atoms are organized in such a way that they are good in absorbing these specific wave lengths.

Four families of plant pigments

There are four main families of pigments in plants (though some sources will define only three groups): chlorophyll (green), carotenoids (yellow, red, orange), flavonoids: anthocyanins + anthoxantins (red, blue, purple), betalains (red, yellow, purple). Within a family a variety of molecules may exist, but they all have a similar basic structure, see below for some examples. All molecular structure have been taken from Wikipedia (1,2,3,4).

You may notice that all these structures have a lot of double bonds between carbon atoms close to one another. This is because these types of structures are especially good in absorbing light.

Chlorophyll – The green of this world

You can literally see chlorophyll all around you, all green plants contain chlorophyll. Chlorophyll is one of the main engines in life. Chlorophyll absorbs sunlight and uses it to transform water and carbon dioxide into glucose. This photosynthesis process is what makes the earth run.

Chlorophyll of course absorbs most of the sun light since it uses this as an energy source. However, some of it is reflected, these are the green wavelengths.

As you can see above chlorophyll molecules consist of a long tail with a large ring on top, the heme ring. It’s the ring that does the work of absorbing the light and gives the colour. The tail itself helps the molecule to stay in place and doesn’t have a colour.

The chlorophyll structure is quite dependent on the structure of the cell. So, upon cooking of green vegetables, the chlorophyll structure might break down quite easily, resulting in a loss of colour.

Chlorophyll a and b

There are two main types of chlorophyll; A and B. The a type is required to actually do the photosynthesis reaction. It only absorbs a quite limited range of wavelengths though. Chlorophyll-b on the other hand can absorb a wider range of wavelengths and passes it energy on to -a. If there is only limited sunlight (in the shade for instance) plants will tend to produce more chlorophyll-b to help it collect more energy.

Chlorophyll on FoodCrumbles.com

  • Chlorophyll is what makes pistachios green and hence makes pistachio ice cream green(ish).

Carotenoids

If carotenoids makes you think of carrots, that is not a coincidence. Carotenoids where first discovered in carrots, hence the name. As you could see above, carotenoids have long carbon chains with circular structures on the ends. They are quite stable which is why carrots stay orange, even when you’ve boiled them for quite a while! Exposure to oxygen can over time result in oxidation of the molecules, resulting in some loss of colour.

There are a lot of different carotenoids, one of the most well-known is beta-carotene, which can be transformed into vitamin A in the human body. There are a lot more though, for example: lycopene, xanthofyll, lutein and zeaxanthin.

Most plants contain a lot of carotenoids alongside the chlorophyll in the plant, it protects the chlorophyll and serves several other vital functions. However, normally, the chlorophyll will hide all these other colours. Only when the chlorophyll is broken down will these colours become visible, you might have seen this in broccoli that was in your fridge for a little too long).

Carotenoids on FoodCrumbles.com

Flavonoids

The two main flavonoids  that colour fruits and vegetables are anthocyanins and anthoxantins. There are hundreds of molecules that belong to these classes. The anthocyanins are known for their purple, blue and red colours. Anthoxantins on the other hand are white and can be found in cauliflower and onions for instance.

Anthocyanins and anthoxantins have several phenol rings in their structures which helps them absorb light.

Anthocyanins & Anthoxantins on FoodCrumbles.com

red cabbage salad - showing the bright purple cabbage colour of raw red cabbage
Red cabbage is full of anthocyanins!

Betalains

If you’re confused about betalains vs betains, don’t worry, they seem to be used quite interchangably, here we’ll stick with betalains. Betalains aren’t as common as the other three pigment families. They are structurally quite similar to anthocyanins but contain that nitrogen atom that anthocyanins don’t.

Common betalains are betains (these tend to be red) and betaxanthines (these tend to be yellow). We can’t really digest these as well which is why your urine may be red after you’ve eaten a good portion of betain rich beet root.

Betalains dissolve in water and are sensitive to heat, light and again pH.

Betalains on FoodCrumbles.com

Unwanted colours

Vegetables and fruits in particular have a tendency to change colour, for instance are they’ve been cut. This is often unwanted discolouration and commonly caused by enzymatic browning.

Sources

Physics classroom, To learn more about the basic physics of light, link

Physics classroom, Visible light and the eye’s response, link ; to learn more about how our eyes register colours

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