You don't need to scald the jellyfish to make it cold.

Because if you blanch the jellyfish, they will shrink and lose their crispiness. If you want to blanch jellyfish, you can boil the water in a pot to about 60 degrees, then turn off the heat, put the jellyfish in and blanch them a little bit and immediately remove them and immerse them in cold water or ice water.

Expanded:

Jellyfish heads need to go through a cleaning and soaking step before they can be cooked and eaten because fresh jellyfish heads contain toxins that can be harmful to the human body, and this toxin has to go through a series of treatments in order to be completely removed.

Fresh jellyfish heads purchased at home should be soaked in water for two days and then in vinegar for five minutes to kill all the bacteria inside the jellyfish head. If the jellyfish heads are dried, soak them in salt water. Blanching jellyfish heads is very skillful, otherwise it will affect the taste of the jellyfish head.

Baidu Encyclopedia - Cold Jelly Fish Heads