Daily Science Factlet – crustacean colouration

Whilst chowing down on some prawns recently, someone asked me why it was that they were a blue-grey colour when raw, and a pink colour once cooked. Here’s why…

If you look closely at a raw prawn, you can see that the colouration isn’t even, but is distributed in small spots on their shell, and on the flesh underneath. The spots are made up of pigment-containing cells called chromatophores, that you can find in all kinds of organisms from bacteria to birds and plants to parrotfish. The combination of overlapping cells containing different coloured pigments gives the final colour of the organism (and making the chromatophores expand or contract is how octopus and cuttlefish are able to change their colour).

The key pigments involved in the blue-grey to red change in prawns (and lobsters) are the red astaxanthin (that also makes salmon and flamingos pink), and the blue crustacyanin. Crustacyanin is actually made up of several astaxanthin molecules tied up together with a protein molecule. In raw prawns and lobsters you have a combination of the red astaxanthin and the blue crustacyanin in the cells, giving a greyish blue.

But when you heat prawns up, the protein holding the crustacyanin together unravels, freeing up the astaxanthins. These are very heat tolerant, so just stay red on heating, making the cooked prawns look that appetising pink colour…

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