In 137, the Japanese Imperial Army sanctioned an automobile-kitchen equipped with a primitive rice cooker. The rice cooker was a rectangular wooden box with two electrodes attached to opposite ends. To cook the rice, the box was filled with washed rice and water, and then a current was applied. This caused the water to boil. When the rice was cooked, the reduction of the water caused an increase in resistanc9e and reduced the heat, then it automatically became a stay-warm state. This method, however, was not suitable for different water qualities or degrees of rice washing and as such varied the amount of heat produced and the end results. It also presented a high risk of electrocution, thus it was not suitable for home cooking.
In 1945, the Mitsubishi Electric Corporation of Japan was the first company to produce a commercially available electric rice cooker. The Mitsubishi product was an aluminum pot with a heating coil inside. It had no automatic turn-off facility, and it required constant monitoring during cooking.
In the early development phase, electric home rice cookers used the simple concept of simply heating the rice to cook and turning off the heater when the temperature rose to a certain point. This method, however, is influenced too greatly with seasonal changes in room temperature and often produced under-cooked rice. Many makers continued to experience failures in their ongoing trial-and-error approaches. At this stage, there was even a trial model which embedded the heating element in a traditional wooden rice container.
The first practical electric rice cooker was invented by Yoshitada Minami, who had an association with Toshiba Electric Corporation. It became possible to cook rice practically by employing the triple-chamber rice cooker (that provided heat insulation by air layers).
In December 1956, the Toshiba Corporation placed the first commercially successful automated electric rice cookers on the market. It used a double-chamber indirect rice cooking method. Rice was placed into the rice pot, and water into a surrounding container. When the water in the outer pot boiled off, the temperature of the pot rose rapidly. A bimetallic thermostat then activated, and automatically turned off the cooker to prevent burning of the cooked rice. Soon, Toshiba was producing 200,000 rice cookers per month for the Japanese market. Four years later, rice cookers could be found in half of Japanese homes.
The double-chamber indirect cooking model took more time to complete cooking and also consumed more electricity. This model was gradually phased out in the 1960s, but there are some companies, such as Tatung, that still manufacture them. Today, electric rice cookers utilize an insulated outer container and an inner removable bowl, often coated with a non-stick surface, and stamped with water-level graduations marked in cups of rice used. The rice cup measure is normally based on the traditional Japanese measurement system 1 gō (合?), which has its origin in China (gě合). One gō (合?) is 180 ml, or approximately 25% smaller than the American measuring cup of 8 (US) fluid ounces/240 ml, and is regarded as producing enough cooked rice for a single meal for one person.
Initial models did not have a keep-warm feature and the cooked rice cooled down too quickly, thus it was often necessary to move the cooked rice to heat-insulated serving containers. In 1965, Zojirushi Thermos company started selling electric rice cookers with a stay-warm function, using a semi-conductor heat regulator. The product sold 2,000,000 units per year. Other makers soon followed suit. The stay-warm function can typically keep rice warm for up to 24 hours at a temperature high enough to suppress growth of Bacillus cereus, a cause of food poisoning. Another notable improvement was the use of electric timers.
In simple models, a mechanical thermostat is used to turn off the cooker when the rice is...