Fish for sushi needs to be raw. Japanese cuisine is founded on this conviction, stating that nature cannot and should not be improved or altered in any way. And even though there are recipes in which fish are boiled or baked, this is considered a huge compromise.
Rice, on the other hand, is the one thing that links all of the regions of the country. It is a fundamental part of almost every dish. Along with fish, it is the most popular food in Japan, where any other type of meat does not enjoy much popularity.
"Sushi" is the term used for rice used in the fermentation process of the fish itself. In literal translation it means "sour".
From a scientific standpoint, this is explained by the vinegar of the fermenting rice, which breaks down the fish into basic amino acids. That is how decay is avoided. It's also where the 5th taste comes from, which has been familiar to the Japanese for millennia - umami.
In its early days, this process was used to preserve fish in Southeast Asia. There, the catch was left to ferment in rice. When the fish were taken out for consumption months later, the rice would be thrown out.
As time passed, they began adding rice vinegar for better taste. This way, the taste was improved, while the time needed for fermentation was shortened. In the centuries to come, they began mashing the rice and fish in wooden molds.
Among the 1st recipes to reach our modern day, there has been data found indicating that layers of salt and rice were placed atop the freshly caught fish, with stones to press down on top and covered with a lid or towel.
This way it successfully fermented after a few months and the rice was discarded. Over the years however, people decided that the time needed for fermentation was way too long and the throwing out of the rice - a waste. So, during the 17th century, they began adding rice vinegar to the sushi, which shortened the time needed to make it.
It is mandatory for the fish for sushi to be freshly caught. The most frequently used fish are salmon and tuna, but they also use cuttlefish, octopus, shrimp, crabs and caviar. If the fish is even slightly old, it is not used to make sushi.
The fresh fish you have chosen needs to be cut into thin strips. Then place it horizontally on the rice. Do not process it in any other way and do not salt it. Also, it can be puréed and mixed with mayonnaise.