If you enjoy seafood, or love watching cooking shows on TV, you must be familiar with the bright red color of a lobster shell or the slight orangish hue of shrimps. Surprisingly, if you look at these shelled animals when alive under the sea, they look quite different. Their shells come in different colors, ranging from blue to green to yellow, and even a muddy shade of brown, but rarely any red. However, as soon as lobsters or shrimps are cooked, the shell turns into a bright pink-orange color. What’s up with that?
Before we answer that question, let’s find out what gives these crusty creatures their blue hue.
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What do a carrot, a flamingo, and a lobster have in common?
Apart from the fact that they are exclusively found on earth, they are bound together by their looks. Or, we could say, the molecules responsible for their distinct looks.
The bright orange color of a carrot, the stunning pink of flamingo feathers, and the greenish hue of a lobster are all caused by carotenoids—a group of colored organic compounds.
Carotenoids are bright yellow, orange, and red-colored organic pigments that are naturally found in many plants and vegetables. They were first extracted from a natural source and isolated by German chemist Ferdinand Wackenroder in the year 1831. He was actually working to extract medicinal chemicals from plants that could be used to treat intestinal worms, but accidentally ended up extracting beta-carotene, a red-orange fat-soluble pigment that is abundant in carrots, tomatoes, and sweet potatoes.
Carotenoids—Nature’s bringer of light
Carotenoids are produced by almost all plants, as they are essential for their food-making process, photosynthesis. The green pigment chlorophyll and the carotenoids present in plants work together to absorb and collect sunlight to power the photosynthesis process. Carotenoids also have a second job, which is to protect the plants’ cells from damage caused by sunlight.
And not just in plants… carotenoids also play the role of a protector of human eyes. Carotenoids present in the retina act as light filters and protect eyes from sunlight damage. But unfortunately, the human body cannot produce carotenoids on its own. We depend on foods like fruits and vegetables as a source. So, don’t forget to snack on carrots and oranges for a healthy dose of carotenoids.
Even though carotenoid pigments are essential to plant life, we don’t get to see them during the summer, as they remain hidden under the green of chlorophyll. As autumn approaches, the green pigments slowly break down, revealing the beautiful orange and yellow colors of leaves in the fall.
So, what do carotenoids have to do with lobsters and shrimps? Do they carry out photosynthesis too? No, they don’t… but they like to chomp on things that do.
Crustaceans like lobsters and shrimp eat a lot of plant materials and algae that grow in the ocean. Astaxanthin, a reddish carotenoid commonly found in ocean algae, enters their bodies via their diet. It slowly builds up and gives them their green, blue, or brown hue.
The same red pigment also gives flamingos their bright pink-orange color. Flamingos are born with grey feathers and get their pinkish hue from feasting upon algae and brine shrimps, both of which are rich in astaxanthin.
But carotenoids are orange and shrimps are green… that doesn’t add up!
Trapped in a hug
Like any other colored organic molecule, astaxanthin has something known as a chromophore in its structure. A chromophore is the region of an organic compound that can absorb a certain section of visible light and reflect the rest, producing a color. For example, astaxanthin absorbs the violets, blues, and greens from the rainbow of visible light and reflects red and orange. This is why substances containing astaxanthin appear to be orange or red.
For a molecule to be a chromophore it must have a very specific structure of molecular bonds. Any changes in the structure can change the color absorbed and reflected by the chromophore. This is exactly what happens in crustaceans.
The hard outer shell of a lobster is made up of proteins. When astaxanthin molecules from the lobster’s skin reach a shell layer, the proteins hug onto them tightly and give them a slight twist. The rotation action changes the shape of the astaxanthin molecule and causes the color to shift from orange to blue. This also leads to the formation of a new blue pigment, called crustacyanin.
If the astaxanthin is rotated in a different direction, the color can change from orange to yellow. A lobster that has both blue and yellow shell layers appears green, and if it has some left-over red color in its skin from astaxanthin it appears brown. The same goes for shrimps.
Now that we are clear on shell chemistry we can move on to our main question…
What happens to lobster shells after they are cooked?
Most of what we eat is made of five different types of food molecules: fats, carbohydrates, vitamins, minerals, and proteins. When we apply heat from a stove or an oven during cooking, the extra energy provided by the heat causes the food molecules to vibrate. These vibrations can cause the food molecules to change their shape.
For example, a solid block of butter, which is fat, melts when heated. Table sugar, which is a carbohydrate, caramelizes when heated.
Similarly, protein molecules are usually curled up like a knot, held together by tiny bonds. When heat is applied the bonds start breaking, unable to hold their shape, and the protein molecules start to uncoil into straight chains. This process is called denaturing.
So, when we put a lobster into boiling water, the heat causes the proteins to unfold. The straight protein chains can no longer hold onto the astaxanthin. The blue crustacyanin pigment breaks down and astaxanthin becomes free from its protein shackles.
And what was the color of astaxanthin in its free and original state? You guessed it right: red!
Glossary
Astaxanthin: A red pigment with the chemical formula C40H52O4 that is naturally produced by marine algae and fungi. It is also a commonly used food dye.
Chromophore: Unsaturated organic molecules (molecules with carbon–carbon double bonds) can have a certain section in their molecular structure that has the ability to absorb energy from a light source and reflect it at an angle, thereby producing a color.
Crustaceans: A group of invertebrates, or animals without a spine, that have a hard shell protecting their soft bodies, including crabs, shrimps, lobsters, barnacles, and isopods.
Crustacyanin: Also known as β-Crustacyanin, this blue biological pigment is formed when two astaxanthin molecules stacked on top of each other are bound to proteins present in crustacean shells. This pigment absorbs light from the red–yellow–orange region of the visible light spectrum and reflects blue and green light.
Flesch Kincaid Grade Level: 9
Reading Ease: 60.9
References
Begum, S., Cianci, M., et al (2015). On the origin and variation of colors in lobster carapace. Physical Chemistry Chemical Physics, 17(26), 16723–16732. https://doi.org/10.1039/c4cp06124a Accessed on 8/2/2022
A Rainbow of Colorful Lobsters. (2018, May 22). Smithsonian Ocean. https://ocean.si.edu/ocean-life/invertebrates/rainbow-colorful-lobsters. Accessed on 8/2/2022
The discovery and early history of carotene. (2009). Bulletin for the history of chemistry, 32–38. http://acshist.scs.illinois.edu/bulletin_open_access/v34-1/v34-1%20p32-38.pdf . Accessed on 8/2/ 2022
Koren, M. (2013, April 24). For Some Species, You Really Are What You Eat. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/for-some-species-you-really-are-what-you-eat-40747423/ . Accessed on 8/2/2022
University of Hawai‘i at Mānoa Food Science. (2020, August 26). The Role of Proteins in Foods: Cooking and Denaturation – Demo NSC Book. Pressbooks. https://opentextbooks.library.arizona.edu/demonscbook/chapter/6-the-role-of-proteins-in-foods-cooking-and-denaturation/ Accessed on 8/2/2022
