There is a very easy way to distinguish a fresh uncooked shrimp from a cooked one: its colour. If you’ve ever cooked shrimp before, you will have noticed this colour transformation. The shrimp will convert from a blue, translucent colour into a white with pink!
The same happens to lobster as well as crab, a beautiful colour transformation. And behind all of this: fascinating food chemistry!
And what’s better, it’s not just that transformation going on. The colour changes, but the shape does as well, so we’re going to discuss three processes here: the colour change from blue to pink, the colour change from translucent to white and the curling up of the shrimps when cooked.
From blue to pink – How the colour transforms
Colour is a major part of most food, but very little foods change colour so drastically as shrimps do. Food turning brown is very common, thanks to the Maillard reaction, caramelization or enzymes. Those reactions are very different from a chemists perspective compared to the shrimp’s colour transformation.
Instead, what is going on in shrimps is probably most similar to the colour change that happens when fruit ripen. During ripening they lose most of their green (chlorophyll) colour, mostly because the chlorophyll disappears which otherwise hides a lot of the other colours (e.g. in oranges).
Pink = astaxanthin
Even an uncooked raw shrimp contains the molecule that makes it pink. This molecule is astaxanthin which actually belongs to the group of carotenoids. This is a large group of colour molecules, the molecule that turns a carrot (carotene) orange is also part of it.
However, in raw shrimp this astaxanhin is bound to proteins forming a complex called crustacyanin. This causes the two molecules bound together to show as blue! Light cannot be reflected in the right way for it to show the pink colour.
When shrimp are cooked the protein denatures (it unfolds and changes shape) due to the high temperatures. As a result, the astaxanthin is unbound now and now reflects the light differently, causing the shrimp to be pink! Colour therefore is a very good indicator for knowing whether a shrimp has been heated already, or is still raw: raw = blue, heated = pink.
Temperature at which the colour changes
The temperature at which the colour change happens isn’t known exactly. It probably is a combination of time and temperature that will determine whether enough of the protein has denatured for the shrimp to turn pink. This depends on the exact composition of the shrimp. Also, not all proteins will break down at the exact same time.
Shrimp turning from translucent to white
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Raw shrimp aren’t only blue when raw, they are also translucent. You seem to be able to watch into the shrimp slightly. During cooking this translucency changes in a white colour, as it does for many other (white) fish.
This change of colour is similar to that of an egg white turning white when boiled or cooked and is caused by the same mechanism: protein denaturation. Proteins are heat sensitive. Proteins tend to be folded up in all sorts of complex configurations (helices, curls). But when they are heated to high enough temperatures they can uncurl and extend.
Ceviche – protein denaturation by acid
It is not just heat that can cause this transformation from translucent to white. Acids can do the same by denaturing the proteins. It is a similar process going on but caused by acidity instead of heat. You use this phenomenon when making ceviche, which is essentially fish ‘cooked’ in acids such as lemon juice.
When you cook shrimp it is not just the colour that changes, the shape changes as well. Once the shrimps starts heating up considerably, they will start to curl up. If you overcook them they will continue to curl up into tight little balls. The texture will be off and not as juice and soft anymore. You’d want to avoid getting to this stage.
What happens here has not been described in detail, but most likely has to do with contracting muscles within the shrimp. Just like a piece of meat will shrink, so does a shrimp. However, since the muscles of a shrimp are structured very differently compared to those in for instance chicken or beef it will make itself curl up if allowed to. Cutting a shrimp in the right places can prevent curling up.
Again, shrimp flesh is mostly made up of proteins, so it’s again the proteins that play a vital role here. These proteins curl up and bind less water causing the product to shrink.
Food safety & doneness
All the chemical changes discussed here, the colour changes as well as the shape change, are ways to get some idea of the readiness of your shrimp. They are however no foolproof methods to determine when your shrimp is safe to east. For that, the shrimp needs to have reached a certain temperature to kill of any harmful micro organisms. Also, buying good quality, fresh shrimps will increase your chances of getting a high quality, safe product to eat!
Various books and articles have been used to write this post: Book – Atkin’s Molecules, Book – Colour additives for foods and beverages, Book – Food carotenoids, chemistry, biology and technology, The Reluctant Gourmet.
An article on feeding shrimp in captivity can be found here.
Shrimp aren’t the only seafood that curl up, lobsters show a similar phenomenon as do crawfish. Here’s a nice mythbusting article about the curling up of crawfish.
Exploratorium has also written a post on cooking fish which I used in my post.
Cosma D Dellisanti, Silvia Spinelli, Christian Cambillau, John B.C Findlay, Peter F Zagalsky, Stéphanie Finet, Véronique Receveur-Bréchot, Quaternary structure of alpha-crustacyanin from lobster as seen by small-angle X-ray scattering, FEBS Letters, Volume 544, Issues 1–3, 2003, Pages 189-193, ISSN 0014-5793, https://doi.org/10.1016/S0014-5793(03)00486-1 ; For the real chemistry diehards! In depth article on the structure of crustacyanin.