Freezing

To keep fish in edible condition for more than a few days, it’s necessary to lower its temperature below the freezing point. This effectively stops spoilage by bacteria, but it doesn’t stop chemical changes in the fish tissues that produce stale flavors. And the proteins in fish muscle (especially cod and its relatives) turn out to be unusually susceptible to “freeze denaturation,” in which the loss of their normal environment of liquid water breaks some of the bonds holding the proteins in their intricately folded structure. The unfolded proteins are then free to bond to each other. The result is tough, spongy network that can’t hold onto its moisture when it’s cooked, and in the mouth becomes a dry, fibrous wad of protein.