Freezing Fish for Sushi with Dry Ice
Dry ice is used to prepare the "freshest" sushi by flash freezing freshly caught fish with dry ice. In fact, the Food and Drug Administration requires that fish must be frozen first (to kill parasites) before eaten raw. Tuna is the exception, but seafood experts believe that flash freezing tuna better preserves the flavor. Leading sushi chefs prefer fish frozen and kept at -70° F, rather than the industry standard of -10°F.
It takes about a day and a half to completely freeze a 500-pound tuna using dry ice and liquid nitrogen. The frozen fish can then be kept for as long as two years. The fish are cut into pieces and thawed in warm water just before serving.