Place the tuna meat in a cotton cloth bag made of fukin or muslin. Close the top of the bag tightly, secure with a rubber band, and put it under cold running tap water. Press the tuna in the bag several times until the water runs clear (this cleans the fish and removes excess fat and fishy flavor). Turn off the tap water and firmly press the fish in the cloth bag to remove excess water. Put the tuna in a bowl and add the soy sauce, mirin, sake, and sugar, stirring and mixing thoroughly with chopsticks. Transfer the tuna to a heated skillet (no oil is added) and cook over low heat, moving the fish around in the skillet, until all the moisture is cooked away and the fish becomes light and flaky, about 8 minutes. At the very end, add a small drop of red food coloring if you like and stir thoroughly. Transfer the sweet fish flakes to a bowl and let them cool. Store them in a sealable container in the refrigerator and use within two weeks or freeze.