The fish ingredients are marinated between sheets of kelp and then left overnight in the refrigerator. The kelp flavor is imparted to the light-tasting white fish, making it even tastier. This process removes excess moisture from the fish and thus it keeps longer and brings out the better texture. In the end, the white fish is the leading role here, so if the kelp flavor is too strong, it will counteract the delicate flavor of the fish.
The most important part of kobujime is the amount of salt in the kelp. Kelp that has been cleaned with diluted vinegar (in some cases freshwater is used instead) is lightly salted and then the white fish is sandwiched between the kelp without further preparations. The reason the fish is not salted directly is because chefs don’t want the flavor to be too salty. The aim of including the saltiness with the umami of the kelp is to give the white fish a mild taste. Kobujime can be thought of as the fruit of the efforts and knowledge used in Edo-style sushi to help toppings keep longer and also improve the taste.
To put this scientifically, combining the two umami components of ingredients with more glutamic acid and inosine acid, which is found in kelp and white fish, this umami gets dramatically stronger. This umami component Inosine acid, found in fish and meat, increases with time, so white fish is used after maturing a moderate amount.