Rice cookers may be divided into a variety of rice cooking sauce pans or pots (e.g. risotto pan, paella pan, porridge pot), rice steamers (e.g. 釜 (zhēng lóng), ceramic or plastic containers for microwave oven, and gas and electrical appliances. Most dedicated home rice cookers are of electric type. In commercial or industrial use, there are many varieties, such as large gas or electric rice cookers, a large-scale rice cooker that is called "rice boiler", and fully automatic versions which handle the whole process of rice cooking from washing rice to the end of the cooking cycle. Dedicated rice cookers date from long ago in human history. A ceramic rice steamer dated to 1250 BC is on display in the British Museum.
Cooking rice has traditionally been a process which required attention to ensure the rice was cooked properly. Electric rice cookers automate the process by mechanically or electronically controlling heat and timing, thus freeing up a heating element on the cooking range that had to be occupied, otherwise for rice cooking. Although the rice cooker does not necessarily speed up the cooking process, with an electric rice cooker the cook's involvement in cooking rice is reduced to simply measuring the rice, preparing the rice properly and using the correct amount of water. Once the rice cooker is set to cook, the rice will be cooked with no further attention. Many modern cookers have heat insulating casing and a warming mechanism. When the rice is determined to be fully cooked, the unit will automatically switch to the "keep warm" cycle, thus preventing the rice from being overcooked and keeping the rice warm until it is ready to be served. They can also be used to keep cold solids cold because they are made to sustain the same heat that is inside them.
It takes about 30 minutes to 1 hour for most of electric rice cookers to complete cooking. Some advanced models can back-calculate the cooking start time from given finish time. The time required for cooking rice depends on amount of rice, power of the heating elements, and atmospheric pressure, thus it is not constant. Pressure-cooker models are not influenced by atmospheric pressure. The special features distinguish high-end models from lower-cost, simpler models.
Some types of rice dishes require special ways of preparation and are not suited to the mode of cooking represented by electric/gas rice cookers. These recipes include risotto, paella, and stuffed peppers (Capsicums).
In the early days of electric home rice cookers in Japan, the products were manufactured, marketed and purchased only within Japan. With the trend of Japanese cuisine spreading abroad, they were exported to Western countries. They were also exported to rice-consuming Asian countries as the average family income has improved and reduced manual labor is desired. Many products are now manufactured in China, Korea, and Malaysia. Even some rice cookers intended only for Japanese domestic market are now manufactured outside Japan.
