Tuna is one of the most popular fishes and a true staple in any sushi bar menu. It is generally much easier to choose tuna for cooking than for sushi. Here I'll give you the exact information how to buy it, what tuna to buy and how to handle it in order to have a great safe sushi experience anytime. I used to run my own sushi bar and over the years this knowledge helped me a lot.
You have probably heard the term "sushi grade tuna". I personally believe that this marketing term is a little bit loose and only intended for people that don't have too much knowledge of this fish. Generally speaking, any fresh and sanitary tuna can be used for sushi if handled properly.
Fresh Yellowtail Tuna:
From my experience, I find fresh tuna to be the best choice to use for sushi. You can buy it in the fish section of most grocery stores. If you do so, ask for a piece to be cut for you from the loin they have in the walk-in fridge. Do not accept the tuna steaks that are on display, as they are usually discolored and not as fresh. Fresh tuna needs about 3 to 6 days aging to be at its best, but if you can't do that you should try to at least age it for 24 hours before using. If the flesh of the tuna is a little bit mushy and has a fairly strong flavor, then the best thing to do is to age it as you would do with steaks. After a few days in the refrigerator, carefully wiping the tuna with tissue paper to remove the excess moisture each day, it will develop its unique clean and smooth flavor, and firm texture that you can find in a good sushi bar.
I recommend always handling raw tuna with plastic gloves, not with bare hands, in order to avoid introducing bacteria on the surface of the fish which then grows.
Frozen Tuna (Saku):
Some specialty fish stores sell frozen tuna portions, usually rectangular-shaped vacuum-sealed steaks. Frozen tuna looks more red than the fresh tuna, may be because it has been treated with some gas before vacuum-sealing it, in order to preserve it. The frozen saku tuna usually comes from Philippines, Thailand or Vietnam. If the frozen tuna is not vacuum-sealed in its package or consists of several strips or chunks of tuna pasted together in a block, don't buy it. It will freezer-burn very fast and won't be safe to eat. If you do buy frozen tuna, it is very important to thaw it properly. I would usually cut a hole on its package to allow for the blood to run out, and leave it in the fridge in a container still in its original package for about 1 day to thaw.
After that I would wipe the tuna steak with paper towel to remove the excess moisture before using it. You can use the saku tuna for up to 2 days after thawing it, as it is not as fresh for serving it raw after that. I find the frozen tuna very convenient, but not as tasty as the fresh tuna. Do not refreeze the tuna once thawed.
Other kinds of tuna:
The albacore tuna is a very good substitute to the yellowtail and is not as endangered as a species. Albacore usually comes frozen and vacuum-sealed, and can be found in most grocery stores. Thaw it the same way as you'd do the saku tuna, although it only needs 12 hours to thaw. Due to its smaller size the albacore tuna is considered less exposed to environmental pollutants, therefore cleaner fish. Smaller fish normally don't have enough time to absorb many environmental pollutants during its life span.
Many sushi bars use escolar (also called oil fish) marketed as "white tuna". It is cheaper fish but is as tasty as the yellowtail. If you are worried about the tuna population you should definitely try the escolar. I have not heard so far that escolar is being threatened by extinction.
Substitutes of tuna:
When I used to run my sushi bar, I always pushed the salmon more then tuna for several reasons, not the least that salmon nigiri is my favorite sushi. Salmon is cheaper, more abundant and people are more familiar with it.
Kingfish (harimasa) is another tasty fish for sushi. I used to get it sometimes farm-raised, frozen and vacuum packaged from a sustainable and clean company in Australia.
The popularity of sushi is increasing throughout the world, so it is very important to know where your fish comes from and how it is being harvested, so we can enjoy the wonderful taste and health benefits of sushi for long, long time. Cheers.