Have you ever noticed how fruits and vegetables have a 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 order to assure the fruits fulfills their function of spreading around their seeds.
The colour of all this produce is all chemistry & physics. The way light is reflected, as we’ll discuss in more detail below, is highly influenced by the molecules onto which it bounces. These aren’t the simplest molecules out there. Their complex structures are such that they reflect light in a way to ‘produce’ colour!
What is colour?
Light, and thus colour, is just another type of electromagnetic radiation. All around us electromagnetic waves float around. These can be described by their wavelength and the resulting frequency. Most of these waves we can’t see, they are outside of the ‘visible spectrum’. Examples of these are waves with very long wavelength such as radio waves or microwaves as well as those with a very short wavelength such as X-rays. That visible spectrum that we can see sits in between these two extremities.
The wavelengths of visible light are about 380-750nm in length, which of course still is very small. Our eyes have developed in such a way that we can see these.
Wavelength determines colour
Once a wave is within the visible spectrum we can see it. What’s more, the specific length of that wave determines which colour it is that we see! For example, 600nm is orange, 450 nm is blue and 550 nm is what we perceive as green. For us to see one of these colours, a wave with this wavelength has to land onto our eye.
The sunlight around us is white in colour. It has this colour because it contains waves with wavelengths of all colours, as a result we see white. In order to see a colour, we only want a specific wavelength to fall on our eye. So how do fruits & vegetables manage that?
Light reflection & absorption
When sunlight shines on an object the molecules in the object (e.g. a pear skin) will absorb some of this light and reflect other parts. Molecules can absorb light because certain wavelengths cause them to vibrate and absorb the energy that sits within this wavelength. If a molecule absorbs certain wavelengths those waves cannot enter your eye anymore. As a result, only the ones that are reflected are the ones that we see. So, if a fruit or vegetable reflects green light but absorbs all the rest, we see it as being green. The same applies for all other colours.
Pigments are colour makers
Now that we know how we see colour, we can look at those fruits & vegetables themselves. Why do they reflect & absorb these specific colours? This is where we have to switch from physics to chemistry.
There is a special group of molecular structures that gives colour to fruits and vegetables, these are pigments. Pigments are not only present in foods, but are 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. They have this ability due 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):
- chlorophyll (green)
- carotenoids (yellow, red, orange)
- flavonoids: anthocyanins + anthoxantins (red, blue, purple)
- betalains (red, yellow, purple)
Within a family a variety of molecules exists, but they all have a similar basic structure. We’ll discuss each one of them briefly here and share more extensive further reading on each of the groups. When you look at the molecular structures of the different groups you might notice that there are a lot of double bonds within the structure. That is, two atoms are connected by two bonds, instead of just one. This is not a coincidence. These types of structures are especially good in absorbing light.
Chlorophyll – The green of this world
You can literally see chlorophyll in action all around you. All green plants contain chlorophyll, pistachios are just one example. Chlorophyll is actually one of the main engines in life. It absorbs sunlight and uses its energy to transform water and carbon dioxide into glucose. The glucose is then used by the plant as an energy source for growing and living. The process is called photosynthesis.
Chlorophyll of course absorbs most of the sun light since it uses this as an energy source. The more wavelengths it can absorb, the more energy it can use. There really is just one set of wavelengths that it cannot use. These are the green ones and those are thus reflected, which makes everything green around us!
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 thus create the green colour that we see. The role of the tail is not as relevant for the actual colour, but it does help the molecule to stay in place and do its job.
Once the ring structure breaks down, with that magnesium ion sitting in the middle, the green colour will be lost. This can happen during cooking for instance which is why overcooked green vegetables start turning brown instead of green (e.g. for broccoli). However, it can also happen in fresh produce during storage, causing them to turn yellow or brown.
Chlorophyll a and b
There are two main types of chlorophyll; A and B. They are actually very similar in structure, but have slightly different roles. The a type is required to actually do the photosynthesis reaction. However, it is quite picky in the type of wavelengths it can absorb. As a result, it cannot get as much energy out of the sun. That is why type b is there. Chlorophyll-b can absorb a wider range of wavelengths, but it cannot perform the photosynthesis reaction. Therefore, it passes its energy on to the a type. 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!
If carotenoids makes you think of carrots, that is not a coincidence. Carotenoids where first discovered in carrots, hence the name. However it’s a large group of different molecules that give fruits & vegetables it mostly red, orange and yellow colours.
As you can see below, carotenoids have long carbon chains with circular structures on the ends. They are quite stable molecules 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. It is also the reason why tomatoes only turn red over time, and why underripe oranges still have some green.
The two main flavonoids that colour fruits and vegetables are anthocyanins and anthoxantins. Anthocyanins and anthoxantins have several phenol rings in their structures which helps them absorb light. There are hundreds of molecules that belong to these classes.
The anthocyanins are known for their purple, blue and red colours. Their colour is very sensitive to the acidity of its environment. This is why a vegetable like red cabbage can by both bright pink or dark blue, depending on what type of dressing (acidic or not) is on it. Purple carrots are also a good example of anthocyanins in vegetables.
Anthoxantins on the other hand are white and can be found in cauliflower and onions for instance. They are not as sensitive to pH.
Betalains and betains both refer to the same group of molecules. 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 most common in beets as well as a few other types of produce. 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 colours as well which is why your urine may be red after you’ve eaten a good portion of betain rich beet root but not after you’ve eaten a lot of red cabbage.
Betalains dissolve in water and are sensitive to heat, light and again pH. They are one of many ways to colour your food red.
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.
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