The reason is that crabs contain a lot of water. In the process of heating, water will slowly evaporate into steam, which is a gas and easy to spread around. The lower part of the crab has high water vapor content and high density, and the gas will automatically diffuse from high density to low density, so bubbles will appear on the crab shell.

Steamed crabs retain the essence of the original flavor, so they are very popular with foodies. Steamed crabs seem simple, but they are actually very particular. You need to master the heat and time, otherwise it will spoil the delicacy of crabs.

Extended data:

Selection method of crab

High-quality hairy crabs have four characteristics: green back, yellow hair, golden claws and white belly. The back is bright and shiny, with good hardness; Open the navel and you can see that it is full of red. The deeper the two red spots on the abdomen, the fatter the crab roe and crab paste.

Careful selection of crabs requires "five looks": a look at the crab shell. All shells are dark green with light, thick and strong; The back of the shell is yellow, mostly thin. Second, look at the navel. The navel protrudes, and a boat is fat; Most dents lack fat. Third, look at the claws. All hairy feet are old and healthy;

However, if the pliers have no fluff, the body will be soft. Fourth, look at vitality. If you turn the crab upside down, you can turn it back quickly with a stinger, which has strong vitality and can be preserved; Irreversible vitality is poor and cannot be preserved for a long time. Fifth, look at men and women. Female crabs are picked in August and September in the lunar calendar, and male crabs are selected after September, because the gonads of male and female crabs are mature and the taste and nutrition are the best.