After the fish is purchased from the fish market there are some preparations that the chef must do before they can prepare it as sushi. Depending on the fish, some of them are bought whole, and the bigger ones like tuna, and yellow tail are bought in loins, or blocks.
For the smaller fishes that are sold as whole, they are first scaled beneath a container of ice water to keep it cold as possible, and to prevent the scales from flying everywhere. Then they are filleted and remaining bones are removed.
One medium sized fish yields four sashimi fillets. All of them are checked for sushi quality such as moisture, oiliness, color tone, muscle alignment, and the ones that don’t meet the requirements are usually used for cooking, but never used for sushi.
Some fillets can be aged for a day or two to concentrate the flavors, and some will be used immediately. Other types require a few hours of curing, so they are first salted and then washed with rice vinegar.
For the larger fishes, the loins are carefully examined and the best areas which are free from tendons, and bright in color, are cut into smaller rectangular sushi loins. The loins for the bigger fish are cut so the muscle grains run parallel along the length. This way each sushi will have horizontal grains along the length and have a better appearance.