Why shrimp change colour and shape when cooked

For those who have cooked raw shrimp before it is a series of well known phenomenon: the shrimp changes colour from a dark blue/grey to pink when being cooked, it turns white instead of opaque. At the same time shape changes as well: whereas the shrimp was straight and flexible before cooking it becomes curved and firm after.

These are interesting phenomena from a scientific point of view but can also be used to determine whether your shrimps have been cooked properly! Today we’ll be discussing why and how.

From blue to pink – Colour formation


Since the change of the colour of shrimp during cooking is probably the most visible one, we will start with discussing that. This colour change is a typical example of applying our knowledge on food chemistry*. The nice thing is that during this colour change no new molecules are made (as is often the case, for example with browning of pesto), instead, molecules that used to be ‘hidden’ now become visible!

The hidden molecule

Ever thought that the pink colour of salmon, flamingos and cooked shrimp seemed similar? Well, that isn’t so strange at all since the colours of these three animals are all caused by the same molecule: astaxanthin. This molecule is part of the group of carotenoids. The orange pigment in carrots (carotene) belongs to this same group.

In shrimp the astaxanthin is bound to proteins. This causes the colour to be ‘hidden’, light cannot be reflected in the right way for it to show pink. When shrimp are cooked this protein denatures (it unfolds and changes shape) due to the high temperatures. Most proteins show this phenomenon when heated, but the temperature at which it occurs differs per protein.

Because of the so-called denaturation, the protein isn’t bound to the astaxanthin anymore. As a result the pinkish colour becomes visible!

Flamingos and shrimp

There is also a good reason the colour of flamingo’s is so similar to that of shrimp. Flamingo’s turn pink because of their diet of shrimp! In other words, they eat the pink colour. In the flamingo the protein/pink colour structure isn’t present as such though.

In other words: if flamingos don’t eat enough shrimp, flamingo’s will lose their pink colour!

Pink shrimp = Heated shrimp

Ever doubted whether shrimp are raw or have been cooked? If that shrimp is pink there is no doubt about it, is will have always had a heat treatment.

Using colour change for determining readiness of shrimp

The temperature at which the colour change happens doesn’t seem to be a well-known fact. Even after quite an intensive search I wasn’t able to find out at which temperature the protein has changed sufficiently for the shrimp to turn pink. Disappointing, since for for instance egg proteins it is quite well known at which temperatures these set (think of sous vide eggs).

On the other hand it makes sense. Denaturation of proteins is a very common process (think: frying eggs, baking a steak). However, each protein denatures at slightly different temperatures, moreover denaturation tends to depend on the acidity of the product as well as on the salt content. A long story short: there isn’t one temperature at which shrimp turn pink! This depends on the shrimp, as well as the sauce in which they are cooked and probably quite a lot of other factors.

Shrimp turning from translucent to white

Raw shrimp aren’t only blue, they are also translucent. You seem to be able to watch through 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 causes by the same mechanism: protein denaturation.

Proteins are heat sensitive. Proteins tend to be curled up, but when they are heated to high enough temperatures they can uncurl and extend. Even though I haven’t been able to find literature supporting this, I expect that the unfolding causes light to be reflected different, making it white.

Ceviche

Ceviche is a dish in which fish is put in an acid an ‘cooked’. Acids can also cause denaturation of proteins, so a fish in a ceviche ‘brine’ will also turn white slowly! The Food Lab wrote a nice post on the topic.

Curling shrimp

When cooking shrimp colour is one criterium for determining whether your shrimp are close to being ready. When the shrimp start getting close to being ready you will also see them curling up slightly. If you continue cooking them after they’re done they will continue curling up further, eventually forming pretty tight little balls. This is something cooks and producers want to avoid, because it also means the shrimp are becoming overcooked.

Unfortunately I haven’t (yet) found the exact reason as to why this happens. My reasoning is that the heat denatures proteins in the fish (as it does around the pink colour as well). These proteins curl up and bind less water causing the product to shrink. Due to the build of the shrimp the inside shrinks more than the outside, also thanks to the opening at the back of the shrimp where the veins had been removed. This causes it to have space to expand at the back and the inside to curl up!

Food safety & doneness

All methods mentioned above to help you determine whether your fish is cooked focus on the sensory perception, thus, how tender, soft and juice is your fish. Whether your fish or shrimps are safe to eat is influenced by cooking, but especially with fish the quality of the raw product can be just as important.

 

*New to food chemistry? Follow my introductory mini-course, but don’t worry, also for those new to the topic it should be fine.

Sources

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.

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