Because it is not their own home water hair even if the supermarket is not hygienic. Most unscrupulous merchants will use chemicals. Even if it is not formaldehyde will use industrial alkali. Because it can increase the water absorption and thus increase the shipping rate.
The identification method is: fresh squid skin tight, gray-black. Flesh white piggyback with a little gray. Not fresh surface is black purple, and dried aquatic is pink and a little yellow transparent. Fresh has a fishy smell of the seafood itself. The aquatic almost no flavor, carefully smell may have the taste of medicine.
PS; I am Shandong coastal, my family open restaurant.