Crab: I am cooked and red

Produced by: Science Popularization China

Produced by: XY Team (Dalian Institute of Chemical Physics, Chinese Academy of Sciences)

Producer: Computer Network Information Center, Chinese Academy of Sciences

It’s crab season again. Have you met crabs on the dinner table this year?

Generally speaking, live crabs are dark blue all over, and look like steel-like bones. After being steamed, they transform drastically, and the crab shell turns red. The bright color makes people salivate.

Having eaten so many crabs, do you know why the crab shell turns red during the steaming process?

Who was the first person to try it?

Crabs belong to the class Molluscs and order Decapoda. They are crustaceans protected by a hard shell.

Chinese people have been eating crabs since ancient times. Ancient scholars regarded eating crabs as a pleasure, and they enjoyed talking about tasting crabs. The poet Li Bai had a special liking for crabs, especially eating crabs with wine. He once said, "Crab claws are golden liquid, and the dregs are Penglai. You must drink good wine and get drunk on the high platform under the moon." Su Dongpo, a great man of his time, was also addicted to crabs and often exchanged poems for crabs. "It's laughable that the greedy prefect of Wuzhong exchanged a poem for two crabs."

Around the Double Ninth Festival, it is the season when chrysanthemums turn yellow and crabs become plump. Invite a few good friends to enjoy the chrysanthemums and crabs together. Isn’t it wonderful and enjoyable?

However, the appearance of crabs does not conform to traditional gourmet standards. Who was the first person who had the courage to eat crabs?

Image source: Veer Gallery

According to legend, thousands of years ago, there was a beetle with a pair of claws and eight legs in the lake. It not only dug holes to destroy rice fields, but also hurt people with its claws, so it was called the "pinching bug." This is also the original name of the crab.

Later, Dayu came here to control floods and sent a strong man named Bajie to supervise the construction. The existence of mangling insects slowed down the progress of the project, so Bajie came up with a clever plan and asked everyone to dig a ditch around the city and fill it with boiling water.

The mangling insects that crawled to this place fell into the ditch and were scalded to death. Their bodies turned red and exuded an attractive and delicious smell. Ba Jie broke open the shell and found that the fragrance was even stronger. He took a bite and found that it tasted delicious. Since then, the dreaded pest has become a household delicacy, praised by gourmets of all generations.

Later, people appreciated Ba Jie's wisdom in solving the problem of man-eating insects and his pioneering spirit, so they placed the character "解" (jie) above the character "虫" (insect) in Ba Jie's name and called the man-eating insects "蟹" (crabs).

Astaxanthin: The secret of crab shell color change

So, why does the crab shell turn from green to red during the steaming process?

It turns out that there is a layer of dermis under the crab's shell, which is dotted with many pigment cells, among which the most abundant is orange-red astaxanthin. This is the secret of the crab's shell color change.

Astaxanthin in live crabs will combine with different types of proteins, showing blue-purple or black-blue color, which is called astaxanthin. Once in a high temperature environment, such as a pot at about 100℃, the protein will be denatured by heat, and the binding force between it and astaxanthin will gradually weaken. Slowly, the astaxanthin bound to protein will turn back into astaxanthin not bound to protein.

At the same time, other types of pigment cells in the dermis are slowly decomposed because they are afraid of heat, only astaxanthin is not afraid of heat and is retained.

In addition, since astaxanthin is mostly concentrated in the claws, chest and feet, and very little in the abdomen, "cooked crab with a white belly" is also a common phenomenon we see.

The left side shows a common live crab, and the right side shows a cooked crab after steaming (Photo source: Veer Gallery)

Astaxanthin is not only found in aquatic crustaceans such as shrimp and crabs, but is also distributed in the feathers of some fish and birds.

The function of astaxanthin is not just as simple as coloring. It is also the strongest antioxidant discovered by humans in nature so far. It is known as "super vitamin E" and "super antioxidant".

Some foods rich in astaxanthin (Image source: Veer Gallery)

Three tips for choosing crabs worth eating

As the saying goes, "Round navel in September, pointed navel in October", it means that female crabs should be eaten in the ninth month of the lunar calendar, when the navel is round and the crabs are full of roe and thick in meat; male crabs should be eaten in the tenth month of the lunar calendar, when the navel is pointed and the crabs are full of roe and firm in meat.

How to choose a fresh and plump crab is indeed a technical job. In simple terms, it can be summarized into three skills: "one pinch, two tease, and three look".

"One pinch": pinch the crab claws with your fingers to check the crab meat quality. The claws of fully mature crabs that have molted are generally hard and firm, while the claws of immature crabs are softer.

"Two teases": tease the crab's eyes with your hands. If it reacts immediately, it means it has strong vitality; if the eyes are bulging and there is no reaction, it may be dead. Turn the crab over with its belly facing up. If it can turn right quickly and crawl, it means it has vitality; if it cannot turn right or has difficulty turning right, it means it has poor vitality.

"Three looks": look at the appearance of the crab. If the back of the shell is dark green and shiny, it means the meat is thick and strong; if the navel at the tail end is protruding and red, it is generally fat and juicy.

Crabs being sold in the market (Photo source: Veer Gallery)

The most common way to eat crabs is to steam them. After boiling water in a steamer, put the crabs in and steam them over high heat for 5 minutes, then switch to low heat and continue steaming for a while. The steaming time depends on the size of the crabs. Generally, a 150g crab needs to be steamed for about 10 minutes; for every 50g increase in weight, the steaming time increases by 2 minutes.

In addition, you can also look at the color of the crab shell with your naked eyes. Most crabs will appear orange-red when they are cooked. If the crab shell is blue, it needs to be cooked for a while longer.

As autumn winds blow, crab legs become fatter. Next time we meet our old friend the crab, we can show off that we have learned the secret of its color change.

References:

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[3] Ni Xiaofeng. Study on the antioxidant effect of astaxanthin and its biological mechanism of effect on prostate cancer[D]. Zhejiang University, 2017.

[4] Cao Yunrui, Yang Lu, Qiao Xing, Xue Changhu, Xu Jie. Dietary astaxanthin: an excellent carotenoid with multiple health benefits. [J]. Critical reviews in food science and nutrition, 2021