Shari (rice / 酢飯) - understanding aji, tsuya and nebari

Rice is everything in sushi. It is the foundation, and the most important ingredient.  When rice is prepared for sushi, it is called sushi-meshi. Once formed into a small piece (kan) to create nigiri, it is then called shari. Chefs use komai; rice that has been aged six months to a year or more, to prepare sushi-meshi. This is because new rice from a fresh crop contains significantly more moisture, making the rice too sticky for sushi. Additionally, older, dry rice grains develop small cracks that absorb vinegar, making it easier to season.

Delicious sushi rice should meet the following criteria:

• the taste (aji) should be flavorful and complex, with elements of sourness (suppai) from the vinegar, but also some amai (sweetness) - either from using a little bit of sugar, or from the natural sweetness of akasu (red sake lees vinegar).
• The appearance should be pleasing to the eye, with a visible luster and sheen (tsuya); it should not be dull in appearance. 
• The rice should have a sticky consistency and texture (nebari), and should be al dente, or "toothy" (hagotai). Each individual grain of rice should retain its shape and structure. 

photo credit: Bentenyama Miyako Sushi