Short grained Japonica sushi rice is traditionally prepared as a lightly seasoned, vinegared and aerated sushi rice! This is the basis for all sushi, the substrate upon which ingredients are presented.
Rice has been cultivated at least since about 3500 BC, when farmers in what is today northern Thailand grew it along the Mekong river. Farmers there discovered that storing fish in salt preserved the nutrients for the dry season, when the waters receded from the rice fields. The result was the predecessor of asian fish sauces, still a staple in Thai cuisine.
But another technique, packing pieces of fish in cakes of cooked rice, did a much better job of preserving the fish. Cleaned and gutted fish were packed in rice and stored away in containers, where mold transformed it into a sour but edible food that could last for months.
The best sushi rice is short-grained japonica rice, which was first cultivated in Japan about 2400 years ago. Most of the rice grown commercially is jasmine rice, but the Japanese variety is a different species.
Gemmai is the unpolished Japanese rice that many health-conscious people eat, but most of the rice sold in Japan is the polished "hakumai" rice. Short grained rice is the best for making sushi, with some varieties special fetching much higher prices than others. Most notable perhaps is "koshihikari" which has a firm and flavorful outer wall. Japanese rice farmers are not unlike wine producers in that they try endless variations in an attempt to get slight improvements in the product.
Sushi rice is usually vinegared immediately after the rice has been cooked, although the amount of seasoning varies. Heavily vinegared rice is still the preferred sushi substrate in southern and western areas of Japan, especially in Kansai area where sushi first took hold in Japan.
The sourness has been a feature since the very origins of sushi. Ironically the rice was not suitable for eating, but the preservation of freshwater fishes that swam in the flooded rice fields was the earliest predecessor of modern sushi.
The mark of a great sushi chef is the rice, and preparation of great sushi rice comes from formal training. Great sushi rice has a firm texture, is plump and moist and sticky. It is not bruised or crushed by sloppy paddle work when the sushi chef quickly dries it after cooking. It has the right amount of seasoning for flavor, and has the stickiness that comes when cooked for the right length of time with just the right amount of water.
Good sushi rice should create good nigiri sushi. The stickiness allows the sushi chef to create nigiri with all the pieces of rice aligned instead of packed in like the machines produce. A low density rice rectangle makes nigiri that seems to melt in your mouth; otherwise you end up with a rice ball that requires chewing.