Shiojime is the process of sprinkling salt on raw fish and dehydrating it to concentrate its flavor. By removing the water, the fish's fishy smell is reduced, which in turn reduces the growth of bacteria and improves shelf life.

Most people who cook know what shiojime is.

Shiojime can be divided into three levels, Usu-shio, Furi-shio, and Gou-shio, depending on the amount of salt shaken. Usu-shio is so that you cannot tell whether or not the fish is covered with salt, and at Gou-shio, the fish is covered with salt.

Using shiojime for sushi items has the purpose of denaturing the proteins in the meat with salt. Myofibrillar protein, which accounts for 50% of the protein in fish flesh, is soluble in 2~6% salt water. When salt is shaken and left for a while, water near the surface of the flesh is brought to the surface by dehydration. The surface of the flesh becomes covered with a high concentration of salt water, and the protein on the surface of the flesh becomes soft like a jelly, which is called gelatinization.

However, if the fish is left longer than necessary after being shaken with salt, the concentration of brine covering the surface of the meat will increase. When the salt concentration is close to 15%, gelatinization does not occur, only dehydration continues, and the fish meat becomes hard and less palatable.

The same phenomenon occurs not only with salt, but also with sugar. shiojime can make the fish too salty or too tough, so sugar is sometimes used with mackerel, for example. Some sushi chefs use sugar first before shiojime and then shiojime. Either way, the aim is to change the texture of the sushi toppings.