Life on earth is full of colour. This short blog aims to uncover why colour exists chemically and why shrimps turn pink!

Evolutionarily, it is just a question of natural selection and for some flora and fauna, colour is breathtaking. For others it’s just not.

Vibrant colours – not so much the bee though

Colour depends on many things-

  • Biology-how we see things and interpret them
  • Chemistry of why something is that colour (molecules and stuff)
  • The physics of how light reflects or is absorbed into something

This whole blog actually started because of the colour of some shrimps I was cooking. Crustacean muscle is grey in colour- but when you cook it, it turns pink. So I sat in my kitchen pondering this, then did a bit of digging.

In fact, if you start to look at other crustaceans- like some lobsters, this common colour change can also be seen. Blue in the sea they become red when they cook.

What is more, they don’t turn grey afterwards following cooking when they cool down. So what is happening is permanent.

Why is that?

Shrimp are little crustaceans that feed off of plankton and other things off of the bottom of the sea have something about the chemistry of their muscle that causes the change.

Before starting I needed to understand why and how things exhibit colour.

What is colour?

You know that if you pass a beam of white light through a prism, or water droplets it separates into its separate colours-(see rainbows as an example in nature) and white light is made up of a variety of many different frequencies of light-

Yosemite national park - waterfalls make the best rainbows
Yosemite national park – waterfalls make the best rainbows

So, if white light is all around us , why if white light is beamed off of an object like an orange, is that object not white?

Selective absorption:

White light contains all the different colour frequencies- colours of the rainbow in fact- and when it shines that light can be transmitted, reflected or absorbed

When the lightwaves strike the object (red, orange, yellow, blue)- some of them are absorbed (called resonance)

[Resonance: molecules in an object resonate at the same frequency as the lightwave, the light wave is absorbed. ]

An orange for example, looks orange in white light because most of the frequencies within the light is absorbed into an orange except for one: ORANGE

The orange light is reflected by the skin of the fruit.

With citrus on my mind I drew the infographic below; lemons this time.

In a nutshell:

An object is a specific colour within a beam of white light- because it is reflecting that colour . The remaining colours from the white light are absorbed.

The molecules in each pigment/colour found in any object absorb diverse light frequencies in white light, and that’s to do with their chemistry (resonance).

In animals, pigments occur, exist and are obtained from carotenoids or melanins that are consumed or you are genetically programmed to have.

And if you are devoid of colour- i.e: white feathers, albino, that means a lack of pigment prevents frequencies of light being absorbed and all light is reflected

Absorption of light is a useful adaptation in plants. Plants are green (mostly) so they absorb all light except the green light- which if you look at the volume of frequencies being absorbed is ALOT.

This enables plants to gain a lot of energy from sunlight which is their means of growing ; but that’s for another lesson (photosynthesis).

Passionflower: Original watercolour by franscienceart
Passionflower: Original watercolour by franscienceart

Summary:

The colour of a pigment is such because it is reflecting that colour light (and absorbing all the others. )

So why is absorbance known as being subtractive in nature?

Subtractive colours exists where the energy of the light is absorbed– by pigments. Absorption of the light by the pigment is technically it taking away light from the spectrum, which is why it is known as subtractive

Pigmentation absorbs different light frequencies

So if an object reflects off all wavelengths and absorbs none, my eye will see it as white.

If I make a mess in my paint palette and mix all the colour pigments together, I will be mixing all different molecules with a lot of different capacities of colour absorption.

As a consequence most light will be able to be absorbed and nothing will be reflected- the resulting absence of colour will be black

So back to my shrimp. Why do they turn from bluish/grey to pink?

Grey/blue colour is caused by a special pigment called β-crustacyanin (crusta- for crustacean and cyan is a blue colour), which is not red but along the blue spectrum

Lobsters are blue because that chemical is reflecting blue light and absorbing all other colours. Shrimp are grey and contain a similar pigment.

Citation: Ertl NG, Elizur A, Brooks P, Kuballa AV, Anderson TA, Knibb WR (2013) Molecular Characterisation of Colour Formation in the Prawn Fenneropenaeus merguiensis. PLoS ONE 8(2): e56920. doi:10.1371/journal.pone.0056920

However, the red colour found in cooking is down to another protein (a Carotenoid protein) called Astaxanthin.

Whatchamawhatcha?

In shrimp the red Astaxanthin protein that gives you the red colour is not working properly because its bound up by another protein. It is bound to β-crustacyanin.

As mentioned, normally shrimp are a blueish/grey colour. Upon boiling, the blue β-crustacyanin is altered by the heat so that the red/ pink astaxanthin is set free and released.

So now the pigment present in my cooked shrimp ( astaxanthin) is available to reflect red light and absorb the rest of the light spectrum. The blue β-crustacyanin no longer works and can’t bind the red pigment and itself cannot reflect light or anything else.

The blue-black to pink-orange color change on cooking of lobster, due to thermal denaturation of an astaxanthin–protein complex, α-crustacyanin, in the lobster carapace

On the origin and variation of colors in lobster carapace – now published in Physical Chemistry Chemical Physics https://pubs.rsc.org/en/content/articlelanding/2015/cp/c4cp06124a

Astaxanthin is more common than you think and the major carotenoid found in crustaceans and is seen in their blue, purple and yellow colours. In larger animals, astaxanthin is found in octopus, which they obtain by eating plankton. Many fish accumulate carotenoids, in fact salmon accumulate astaxanthin in muscle.

So now you know. Crustaceans have colour created by the plankton they eat and this colour is temperature regulated.

The differences in the colours between crustaceans is due to the chemical nature of the pigments in the carotenoids they consume and how the crustacean biology handles them.

Pigments themselves exist as different colours because their chemistry enables them to absorb light at different frequencies.

This can be nicely summarized by looking at flamingos.

Flamingos and their feathers are white naturally but eating shrimp and blue green algae provide them with the carotenoids that gives them their distinctive colour.

So, it appears to be true; you are what you eat……….

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