Dry salt cod—bacalao—needs to be desalted before using. Soaking in water removes the salt, rehydrates the cod, and softens the flesh. The process takes at least 36 hours to 2 days, changing the water at least four to six times; fatter loin pieces can take as long as 4 days; the thickest loin pieces can take even longer.
Rinse the pieces under cool running water. Place skinside up (to keep the salt from concentrating here) in a bowl and cover with fresh water. Change the water immediately, rinse out the bowl, and refill with cool water. Arrange the pieces in the bowl skin-side up. Place the bowl in the refrigerator and let the cod soak for 12 hours. Drain, rinse out the bowl, change the water, and soak for another 12 hours. Change the water every 6 to 8 hours or so for the next 24 hours. Place the pieces skin-side down during the final 12 hours. Drain the cod, rinse, and gently squeeze out some of the excess water. Place in a strainer—with the skin side up again—and let drain for at least 30 minutes. (For certain recipes where you don’t want any salty moisture running into the sauce, I recommend draining for significantly longer.)
To check if it is ready, take a pinch of the fish and taste. It should be slightly salty but not disagreeably so. (If it is too salty, change the water and continue to soak.) Carefully debone. Use according to the recipe instructions.