Our Approach
2018
The Mitsubishi Electric Mitsubishi IH Jar Rice Cooker
‒Bringing back a food texture of oven rice‒
Hon Sumigama‒KAMADO
Adoption of DURAFIDE® PPS in inside pot bottom coil base
The method of cooking is important when it comes to delicious rice. While it is a well-known fact that rice that is cooked in the oven is delicious, nowadays you hardly ever see any Japanese kitchen that has an oven.
Mitsubishi Electric developed the Hon Sumigama IH rice cooker, which incorporated a pot inside the rice cooker that was finished with shaved charcoal, 10 years ago and in commemoration of its tenth anniversary, it has now released the Hon Sumigama KAMADO NJ-AW106 model.
The food texture of oven rice that was until then hard to replicate using a rice cooker became attainable. The rice cooker continues to be a best seller despite its considerably high price.
The coil base located under the inside pot of this Hon Sumigama KAMADO uses DURAFIDE® PPS.
We talked to Group Leaders Mr. Kazuyoshi Negishi and Mr. Yoshiyuki Fujita of the Cooking Equipment Technology Section, Home Appliance Technology Department as well as Mr. Satoshi Takeuchi of the company’s Machining Technology Section, Manufacturing Administration Department (currently at the Subcontracting Section of the Materials Department) at the manufacturer Mitsubishi Electric Home Appliance Co., Ltd. about the development history and features of the Hon Sumigama KAMADO.
How do you realize the food texture of oven (kamado) rice?
Until now, the food texture of rice was assessed by hardness and stickiness. While, needless to say, these factors are important, we also focused on the water content of the rice.
Rice becomes soft if there is a lot of water while conversely it becomes hard if there is not much water. However, while oven rice has a higher water content, hardness is retained.
The rice is comprised of firm grains yet at the same time it is moist. So, how is this achieved in the oven? Rice is cooked under high heat in short time in an oven. If you cook rice at low heat over an extended period, it will definitely be soft and the moisture-containing rice becomes sticky. Therefore, we need a lot of heat in order for rice to be cooked to a state that is close to that of oven cooking.
Realizing high heat
However, merely resorting to high heat alone is not enough.
Cooking at high heat results in the pot boiling over. To what extent one can raise the temperature reveals the limitations of a rice cooker and the challenge is how can we make a breakthrough in this respect?
■Hon Sumigama Manufacturing Process
99.9% purity charcoal fired at a maximum of approximately 3,000℃ is machined individually by a craftsman. It takes about 100 days to complete the inside pot.
The magnetic flux permeation depth (heat generation thickness) is calculated by Mitsubishi Electric using a formula. The charcoal’s thermal conductivity is that measured by Mitsubishi Electric for the Hon Sumigama material. The thermal conductivity of steel is from ‘The Elements of Stainless Steel (revised version)” published by the Japan Stainless Steel Association. The thermal conductivity of steel is cited from the JSME Mechanical Engineers’ Handbook (from Mitsubishi Electric Cooking Equipment Catalog).
●Carbon material inside pot …The entire inside pot generates the far infrared irradiation effect and thermal conduction are both high
The use of carbon material for the inside pot realizes high heat. Metals are often used for IH heating but the magnetic flux that permeates from IH heating only penetrates around 0.25 mm from the surface in the case of stainless steel, for example, and heat is thus only generated at the outer surface of the inside pot. However, charcoal is characterized by the depth of permeation of the magnetic flux and heat is generated up to a maximum of 10 mm depth in the inside pot.
Moreover, the thermal conductivity of charcoal is higher than that of metals and the heat that is generated is transmitted more readily.
The inside pot of the Hon Sumigama KAMADO is handmade by craftsmen from 99.9% purity carbon material that has been fired at a maximum temperature of around 3,000℃ and it apparently takes around 100 days to complete one.
●Broad-brimmed inside pot shape…continuous boiling that traps the flavor
The traditional oven pot features a supporting brim in the center with sections above and below. The section below the brim is entirely enclosed by the oven while the upper section is directly in contact with the outside air. The portion below the brim is directly heated by the oven and on this account, a high temperature is maintained, boiling is continuous, and the rice and water are heated at the maximum possible temperature in order to extract the flavor. The upper section completely catches the froth and the stickiness that results from the intense boiling and traps the flavor. The oven pot lid is thick and made of wood in order to trap the flavor. In addition, because the upper section is in direct contact with air, its temperature is somewhat lower and the intensity of the boil over is thereby weakened.
On account of such factors, the carbon material of the inside pot of the Hon Sumigama KAMADO adopts a broad brimmed form. Furthermore, the overall volume of the inside pot has been increased by more than 10% and as is the case with the broad brimmed pot, rice can be cooked at high heat without boil over.
●A “harvest form” that leverages a high thermal insulation structure
The shape of the body also retains high heat, and a structure was adopted whereby the lower part is somewhat swollen to prevent heat escaping to as great an extent as possible. The new design has been dubbed “harvest form” because it conveys a vision of fruit ripening in season and the deliciousness of a rice ball formed by hand.
Until now, thermal insulation material was used solely on the outside of the side wall heaters but in the Hon Sumigama KAMADO, an expanded “swollen” space is located in the lower part of the main body and 10-mmthick foamed melamine resin thermal insulation material has been added in order to achieve high thermal insulation. Through this, dispersion of the heat is prevented and temperature rise on the outside surfaces of the main body is also suppressed and with the multiplier effect of the broadbrimmed pot coming into play, high heat is achieved.
Furthermore, a silicone rubber “Heat Sealing Ring” eliminates gaps between the main body and the brim portion of the inside pot, meaning heat cannot escape and heating is very efficient. In addition, a charcoal coating is also used in the heat dissipation plate and the center area of the main body. The rice cooker is thus designed in order to realize efficient heating.
The “harvest form” shape featuring a somewhat swollen lower portion of the main body is difficult to mold from a single component and with a conventional tool design, the molded part cannot be released from the mold. For this reason, discussions were held numerous times with the toolmaker and through modifications to the tooling design, a structure molded in one piece from ABS resin was realized.
DURAFIDE® PPS used in coil base
Achieving a higher heat than previously brings about thermal issues for the material used until then. This was indeed the case with the PET resin that was used in the coil base. Thereupon, the decision was made to use DURAFIDE PPS.
The coil base is considered to be an electrically insulating material and for this reason, CMJ registration at the Japan Electrical Safety & Environment Technology Laboratories (JET) was necessary as part of the company’s internal standards.
Verification of the upper limit of the insulating material’s usage temperature resulted in 180-200°C for PPS versus 150-160°C for PET resin. The temperature of the coil base reaches close to 170°C because high heating is achieved but PPS resin is capable of enduring such a temperature.
A glass fiber and inorganic filler-reinforced grade was used. Besides its heat resistance of course, another factor behind the adoption of the grade was its good external appearance given that in some places the plastic is visible.
Conventional glass fiber-reinforced materials are often difficult to mold due to issues such as glass fiber orientation. Gate positioning is limited due to shape and design and for this reason, problems in terms of external appearance and impact resistance are feared if the resin is merely shot into the mold.
Repeated trial and error attempts were made through to selection of the molding method and final grade.
While the need for a higher temperature than PET resin was recognized, initially the required equipment was not in place. This meant tough challenges in the early stages. Once the equipment was in place and the molding technology established, by adopting a high flow grade in the end, the external appearance requirements could be satisfied.
Previous PET resin coil base
DURAFIDE® PPS coil base (back/front)
●Hon Sumigama KAMADO Effects
The Hon Sumigama KAMADO boasts unprecedented features through its high heat capability realized in this manner.
The entire inside pot is heated by using carbon as the material of construction while continuous boiling is achieved through adopting a broad-brimmed design for the inside pot. These features coupled with the “harvest form” design enable heating of rice grains to their cores and the cooking of rice with strikingly moist grains.
In addition, the rice cooker has various cooking settings built into its menu.
The “Meigara Houjun Taki” mode that can extract the character of all of Japan’s 23 classic rice types means that one can taste rice while enjoying its character. Furthermore, there are various other menus for varying the cooking style such as brown rice cooking and germinated rice cooking.
One can say that Hon Sumigama KAMADO is the ultimate rice cooker and in the words of Group Leader Negishi, “Needless to say we must continue to pursue taste while at the same time considering ease of use. There’s still a lot for us to do.”
● The attached rice scoop is universal design compliant.
A rice scoop comes together with the rice cooker. At first glance it looks like any other rice scoop but close inspection reveals a series of grooves in the handle that can accommodate a ring. This helps visually-impaired people to adjust the water level in the inside pot by standing the scoop inside and pouring water up to the desired level, which is easy to judge from the position of the ring. Furthermore, a Braille label can be adhered to the switch to assist in operation.
The ring and braille label are provided free of charge to anyone that requires them.
■DURAFIDE® is a registered trademark of Polyplastics in Japan and other countries.