There are 24 such standard materials, and they each have their own shortcut keys.
Some materials, such as Rice, are uniquely identified by one letter, K, but are made of multiple Elements, in this case: Rice, Elastic, and Viscous (KEV). Some materials, such as Water, are made up of only one.
Although Fire has a shortcut key (@), it does not work in Mix Mode. To make a custom material containing fire, it has to be selected with the mouse after pressing Esc or simply by mixing in the constituent parts, Fuel and Hot.