Description

RICE COOKING AND VACUUM-PRESERVING METHOD AND ELECTRONIC RICE COOKER USING THE SAME

Technical Field

[1] The present invention relates to a rice cooking and vacuum-preserving method and an electronic rice cooker using the same. More particularly, the present invention relates to a rice cooking and vacuum-preserving method capable of improving quality of cooked rice by maximizing preservation efficiency of cooked rice by combining two steps, that is, cooling the cooked rice in a non-heating manner and decompressing the cooled cooked rice by high vacuum, as well as reducing time for water-swelling uncooked rice and minimizing damage of rice caused by the water-swelling, and an electronic rice cooker using the same. Background Art

[2] Generally, an electronic rice cooker is used for cooking rice and preserving the cooked rice therein. Since automatically controlling the cooking process using electricity without having to adjusting strength of fire in person, the electronic rice cooker is widespread in countries eating rice as main food.

[3] To explain the operation principle of the electronic rice cooker, a heat generation medium is mounted to a bottom or body of the rice cooker. As water with rice put in the rice cooker are heated, the water evaporates, thereby resulting in cooked rice.

[4] When first introduced, the electronic rice cooker is capable of only warmth-keeping the cooked rice. Nowadays, however, most electronic rice cookers are developed to directly cook rice as well as warmth-keeping of cooked rice.

[5] Such electronic rice cookers include a general type, a timer type, and a micom type.

The general-type electronic rice cooker cooks rice and then automatically converts to a mode for warmth-keeping the cooked rice. The timer-type electronic rice cooker is c onvenient especially when preparing early morning breakfast because a timer for setting a rice cooking time is further provided. The micom-type electronic rice cooker uses an integrated circuit (IC) chip of a semiconductor to control cooked states and tastes as well as setting the cooking time, so that taste of the cooked rice can be improved. According to a heating system or a cooking method, the electronic rice cookers may be divided into a heat-plate general rice cooker, a heat-plate pressure rice cooker, and an IH-type pressure rice cooker.

[6] In spite of the advantages as described above, the conventional electronic rice cookers have problems in preserving the remaining cooked rice after initial taking.

[7] More specifically, the conventional electronic rice cooker continuously heats the

cooked rice in an atmospheric pressure in order to preserve the remaining cooked rice. During this, the cooked rice is deteriorated in its texture and loses moisture. Furthermore, the cooked rice is discolored, generating its own stale smell and thereby losing the initial good taste.

[8] Moreover, the conventional electronic rice cooker still requires be water-swelling of rice for one or two hours before cooking. However, this process not only elongates the whole cooking time but also excessively softens the surface of rice, thereby deteriorating quality of the cooked rice. Disclosure of Invention Technical Problem

[9] Therefore, the present invention has been made in view of the above-mentioned problems, and it is an object of the present invention to provide a rice cooking and vacuum-preserving method capable of maximizing preservation efficiency of cooked rice by combining two steps, that is, cooling the cooked rice in a non-heating manner and decompressing the cooled cooked rice by high vacuum, and an electronic rice cooker using the same. Technical Solution

[10] According to an aspect of the present invention, there is provided a rice cooking and vacuum-preserving method for cooking rice by putting in an electronic rice cooker and boiling rice with water and preserving the cooked rice, the method generating a vacuum by vacuum-decompressing inside the electronic rice cooker holding the cooked rice, preserving the cooked rice maintaining the vacuum state in a non-heating manner, and essentially comprising a cooling step that cools the cooked rice in a non- heating manner before the vacuum-decompression.

[11] The cooling step drops inner temperature of the electronic rice cooker up to approximately 30~60°C before the vacuum-decompression of the inside of the electronic rice cooker.

[12] While vacuum-decompressing the inside of the electronic rice cooker after the inner temperature of the electronic rice cooker drops to approximately 30~60°C, the vacuum- decompression is performed when approximately 40-60 minutes have passed since starting the cooling step, regardless of the inner temperature of the electronic rice cooker, that is, although the inner temperature of the electronic rice cooker has not dropped to 30~60°C yet.

[13] While vacuum-decompressing the inside of the electronic rice cooker after the inner temperature of the electronic rice cooker drops to approximately 30~60°C, the inside of the electronic rice cooker is periodically heated so that the inner temperature is maintained.

[14] The inside of the electronic rice cooker is decompressed up to a vacuum pressure of approximately -300 to -600DHg.

[15] When inner pressure of the electronic rice cooker is measured to be beyond a preset range, the inside of the electronic rice cooker is vacuum-decompressed again to maintain the vacuum state.

[16] The inside of the electronic rice cooker is decompressed periodically in order to maintain the vacuum state.

[17] The method further comprises a water- swelling step that vacuum-decompresses the inside of the electronic rice cooker and swells rice by water put in the electronic rice cooker, before actual heating for cooking.

[18] The method further comprises a step of heating the cooked rice for a predetermined time considering an eating time by heat having lower temperature than cooking temperature, wherein the steps including water-swelling, cooking, heating by the lower temperature, cooling, vacuum-decompressing again after cooling the cooked rice are all automatically performed by one-time operation of a user.

[19] The method further comprises a reheating step that reheats the cooked rice in preservation for user's eating.

[20] The reheating step is automatically performed at a preset time.

[21] The reheating step, the cooked rice is reheated periodically.

[22] In the reheating step, the cooked rice is reheated periodically.

[23] Water is supplemented to the cooked rice to compensate loss of moisture caused during the reheating.

[24] Another aspect of the present invention is to provide an electronic rice cooker comprising a cooker body including a receiving part for receiving rice and water therein, and a heat generator for heating the received rice and water; a cooking lid hermetically closing the receiving part of the cooker body; a vacuum pump mounted to the cooker body to vacuum-decompress the receiving part; and a controller controlling the heat generator and the vacuum pump to perform steps of cooking the rice in the receiving part, cooling the cooked rice in a non-heating manner, vacuum-decompressing the receiving part, and preserving the cooled cooked rice in a non-heating manner maintaining a vacuum state of the receiving part.

[25] While performing the vacuum-decompressing step after inner temperature of the receiving part drops to approximately 30~60°C in the cooling step, the controller performs the vacuum-decompressing step when approximately 40-60 minutes have passed since starting the cooling step, regardless of the inner temperature of the electronic rice cooker, that is, although the inner temperature of the electronic rice cooker has not dropped to 30~60°C yet.

[26] The vacuum-decompressing step decompresses the receiving part up to a vacuum

pressure of approximately -300 to -600DHg.

[27] Before actual heating for cooking, the receiving part is decompressed up to a vacuum pressure of approximately -300 to -600DHg and the rice is water-swollen.

[28] The electronic rice cooker further comprises a first solenoid valve mounted to the cooker lid to selectively open and interrupt communication between the receiving part and the outside of the cooker lid; a connection hose connecting the receiving part with the vacuum pump; and a second solenoid valve mounted on the connection hose to selectively open and interrupt communication between the receiving part and the vacuum pump.

[29] The electronic rice cooker further comprises a backflow prevention valve mounted on the connection hose to allow a flow of fluid only from the receiving part to the vacuum pump, while interrupting the flow in the opposite direction, wherein the connection hose is connected to the vacuum pump passing by the outside of the cooker body, and includes a disconnect part formed at a lowermost position thereof to drop m oisture drawn in from the receiving part by the vacuum pump as water drops, and a moisture receptacle hermetically enclosing the disconnect part and receiving the dropping moisture therein.

Advantageous Effects

[30] As described above, the rice cooking and vacuum-preserving method and an electronic rice cooker using the same according to the present invention preserve cooked rice in a high- vacuum state rather than by heating. Therefore, the cooked rice can be preserved for long with almost no change in initial texture and moisture thereof. Accordingly, initial taste of the cooked rice can be maintained without causing a stale smell and discoloration.

[31] Since the cooked rice is not continuously heated, electric power can be saved. Also, waste of food can be prevented.

[32] By using the electronic rice cooker according to the present invention, not only the cooked rice but also other various cooked foods such as sweet potatoes, potatoes and corns can be well preserved.

[33] In addition, damage of the rice surface can be minimized by reducing time for water- swelling the rice before cooking. As a result, more soft and glossy cooked rice can be obtained.

[34] Moreover, according to the present invention, the whole process including water- swelling of rice, preservation of cooked rice, and periodic eating of the cooked rice can be performed by a one-touch operation through a button or switch.

[35]

Brief Description of the Drawings

[36] The foregoing and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[37] FIG. 1 is an exterior perspective view of en electronic rice cooker according to an embodiment of the present invention;

[38] FIG. 2 is a longitudinal sectional view showing the structure of the electronic rice cooker according to the embodiment of the present invention;

[39] FIG. 3 is a block diagram schematically illustrating function of a controller of the electronic rice cooker according to the embodiment of the present invention;

[40] FIG. 4 is a flow chart illustrating a vacuum-preserving method for cooked rice according to an embodiment of the present invention; and

[41] FIG. 5 through FIG. 13 are a series of reference views illustrating states of the electronic rice cooker using the vacuum-preserving method according to the embodiment of the present invention. Best Mode for Carrying Out the Invention

[42] Reference will now be made in detail to an exemplary embodiment of the present invention.

[43] First, an electronic rice cooker adopting a rice cooking and vacuum-preserving method, according to an embodiment of the present invention, will be explained.

[44] The electronic rice cooker adopts a vacuum preservation method sequentially combining two steps, that is, cooling cooked rice in a non-heating manner and decompressing the cooled cooked rice by high vacuum, so that deterioration of texture and loss of moisture of the cooked rice can be prevented. Also, the electronic rice cooker uses a vacuum pump in order to maintain initial taste of the cooked rice without causing a stale smell and discoloration. In addition, when swelling the rice before actual cooking, the electronic rice cooker decompresses by high vacuum and leaves for a predetermined time the rice, or repeats vacuum-decompressing the rice. According to this, inconvenience and damage of the rice by a conventional water- swelling process can be minimized, while reducing the water- swelling time.

[45] Hereinafter, the structure of the electronic rice cooker will be described in detail.

[46] FIG. 1 is an exterior perspective view of en electronic rice cooker according to an embodiment of the present invention. FIG. 2 is a longitudinal sectional view showing the structure of the electronic rice cooker according to the embodiment of the present invention. FIG. 3 is a block diagram schematically illustrating function of a controller of the electronic rice cooker according to the embodiment of the present invention.

[47] Referring to the drawings, the exterior of the electronic rice cooker according to the embodiment of the present invention looks similar to general electronic rice cookers.

The electronic rice cooker according to the embodiment comprises a cooker body 111, a cooker lid 112, a vacuum pump 131, a connection hose 133, and a controller 140. The controller controls the overall operation of the other operational parts including the vacuum pump 131, thereby performing a series of processes such as water-swelling, cooking and vacuum-preserving.

[48] The cooker body 111 comprises a receiving part 121A for receiving rice and water therein, and a heat generator 113 generating heat upon application of electric power and thereby heating and cooking the rice and water. A cooking container 121 in which the receiving part 12 IA is formed, is separably connected to the cooker body 111. Similarly to other general electronic rice cookers, the electronic rice cooker according to the embodiment of the present invention includes the heat generator 113 of the cooker body 111, a lagging material 114, and an operation panel 11 IA having assorted buttons or switches.

[49] The cooker lid 112 is hinged upon the cooker body 111 to hermetically close the receiving part 12 IA of the cooker body 111. An inner lid 123 for covering the receiving part 12 IA of the cooking container 1211 is formed at a lower part of the cooker lid 112. A sealing O-ring 125 is mounted to a lower circumference of the inner lid 123 to seal up the receiving part 12 IA by compressed between the inner lid 123 and the cooking container 121. Any resilient material, for example, rubber, can be adopted for the sealing O-ring 125. The inner lid 123 may be omitted. In this case, the cooker lid 112 also functions as the inner lid 123. Therefore, the sealing O-ring 125 is mounted along a lower circumference of the cooker lid 112. The cooker lid 112 is equipped with a first solenoid valve 138 for selectively opening and interrupting communication of the receiving part 12 IA with the outside of the cooker lid 112. In other words, the first solenoid valve 138 selectively opens and closes the receiving part 121A.

[50] The vacuum pump 131, being built in a rear part of the cooker body 111, decompresses the receiving part 121 A of the cooking container 121 by vacuum. For this purpose, the vacuum pump 131 has capability for decompressing the receiving part 121 A up to a vacuum pressure of approximately -300 to -600DHg based on a pressure gauge. Such a vacuum pump 131 being compact enough for the cooker body 111 and capable of high- vacuum decompression is easily obtainable since being already used for a home vacuum packer. The vacuum pressure approximately from -300 to -600DHg is sufficient for drawing out the air from the receiving part 12 IA without affecting the component parts of the electronic rice cooker.

[51] If a higher vacuum pressure is applied, the receiving part 12 IA will be decompressed more perfectly. However, the component parts of the electronic rice cooker may be damaged, and more loss may be caused in consideration of the price

performance of the vacuum pump 131.

[52] If a lower vacuum pressure is applied, on the other hand, the air inside the receiving part 12 IA may not be perfectly drawn out, thereby deteriorating the preservation performance, especially when preserving the cooked rice for a long time. Therefore, the vacuum pressure needs to be at least -300DHg. In other words, a vacuum generation method generating a slight vacuum pressure by a manual compression cap or merely by temperature difference totally departs the intention of the present invention. More particularly, when the manual compression cap usually made of rubber in a dome shape is used, although a little air can be discharged from the receiving part 12 IA at first, a vacuum pressure higher than -100DHg can not be achieved in the receiving part 121A. Because such a minor vacuum pressure is insufficient to remove all the gases including oxygen from the receiving part 12 IA, oxidation and deterioration of the cooked rice are inevitable. Furthermore, the manual operation using the compression cap is contrary to the intention of the present invention which aims at user convenience. To summarize, the compression cap manually operated is incapable of generating the high vacuum required in the present invention.

[53] As aforementioned, the temperature difference can be used to generate vacuum. As an inner temperature of the receiving part 121A drops, elements of the cooked rice such as protein tissues, water molecules, and air particles being expanded at high temperature are contracted, thereby generating a vacuum pressure inside the receiving part 12 IA. According to an experiment applying this vacuum generation method, approximately lOODHg of pressure difference is caused when the temperature of the cooked rice in the receiving part 12 IA drops from 9O ⁰ C to 28 ⁰ C, which is a normal temperature, thereby naturally generating vacuum inside the receiving part 12 IA. However, thus -generated vacuum is never sufficient to prevent oxidation and deterioration of the cooked rice. Especially, although the inner pressure is decreased, oxygen is not discharged to the outside in this case.

[54] Thus, the vacuum pump 131 capable of generating the high vacuum is necessary to achieve the object of the present invention.

[55] Relations between a vacuum pressure in a sealed space and a preservation term of cooked rice are deduced experimentally as shown in [Table 1] below. Here, the vacuum pressure and the preservation term are used to determine the vacuum pressure for decompressing the receiving part 121 A. A maintaining term of permissible number of germs, a discolored state in 3 days, and moisture content in 5 days are measured according to each vacuum pressure in the sealed space like the receiving part 12 IA of the electronic rice cooker.

[56]

[57] Table 1

[58] [59] According to [Table 1], the maintaining term of permissible number of germs changes most abruptly when the vacuum pressure increases from -400DHg to -500DHg, and not much afterward. As a result of the experiment for 3 days, the discolored state is relatively good when the vacuum pressure is higher than -300DHg. The moisture content which is considered to be most influential in the taste is measured for 5 days. According to the measurement, 16% by weight is maintained almost constantly when the vacuum pressure is between -300DHg and -600DHg. In the experiment, 16% by weight is set as the initial moisture content for the best taste of the cooked rice.

[60] In conclusion, the receiving part 121A is decompressed by the vacuum pressure of approximately -400 to -500DHg. [61] The connection hose 133 is mounted to connect the vacuum pump 131 with the receiving part 12 IA through a rear wall of the cooker body 111 and the cooker lid 112. A second solenoid valve 139 is mounted on the connection hose 133. A backflow prevention valve 135 may be further mounted. The second solenoid valve 139 opens and interrupts fluid communication between the receiving part 12 IA and the vacuum pump 131 under the control of the controller 140, thereby properly controlling the vacuum pressure in the receiving part 121A. The backflow prevention valve 135 allows a flow of fluid only from the receiving part 12 IA to the vacuum pump 131, while interrupting the flow in the opposite direction. Although it is recommended that both the second solenoid valve 139 and the backflow prevention valve 135 be equipped, any one of them will do.

[62] Additionally, the connection hose 133 is connected to the vacuum pump 131 passing through the outside of the rear part of the cooker body 111. A lowermost position of the connection hose 133 is exposed out of the cooker body 111. A

disconnect part 133B is formed at the exposed part of the connection hose 133. Therefore, moisture drawn out from the receiving part 12 IA by the vacuum pump 131 naturally falls as water drops at the disconnect part 133B. A moisture receptacle 115 is formed to receive the water drops falling from the disconnect part 133B. More specifically, the moisture receptacle 115 is detachably mounted to the outside of the rear part of the cooker body 111, hermetically enclosing the disconnect part 133B of the connection hose 133, and receives the falling water drops. Here, importantly, the moisture receptacle 115 needs to hermetically enclose the disconnect part 133B in order to transmit a vacuum suction force of the vacuum pump 131 to the receiving part 121A without loss, in spite of existence of the disconnect part 133B.

[63] When the cooked rice in preservation in the receiving part 121A is cooled to near the normal temperature, quantity of the moisture drawn out through the connection hose 133 is actually minor in spite of the decompression operation by the vacuum pump 131. Therefore, the moisture receptacle 115 is dispensable if the electronic rice cooker does not have to cook rice in bulk for commercial use. In this case, the exposed part and the disconnect part 133B of the connection hose 133 are also unnecessary.

[64] Referring to FIG. 3, the controller 140 is in connection with the operation panel

11 IA, the heat generator 113, the vacuum pump 131, the first solenoid valve 138, the second solenoid valve 139, a pressure sensor 137, and a temperature sensor 117. Upon designation of operation modes by a user through buttons and switches formed at the operation panel 11 IA, the controller 140 controls the heat generator 113, the vacuum pump 131, the first solenoid valve 138, and the second solenoid valve 139 to perform a series of processes including water-swelling, cooking and preserving, manually or automatically.

[65] When the user puts rice in the receiving part 121 A, pours water, and selects a cooking mode through the operation panel 11 IA, the controller 140 drives the vacuum pump 131 to decompress the receiving part 12 IA by high vacuum and maintain the vacuum for a predetermined time, for example, 2-3 minutes, before actual heating for cooking the rice. Here, according to an exemplary embodiment, the vacuum pressure in the receiving part 121 A is -300 to -600DHg. Such a high vacuum pressure helps promptly draw out the air from pores of the rice so that water can infiltrate into the rice. When the vacuum pressure in the receiving part 121 A exceeds -600DHg, however, the parts of the electronic rice cooker may be damaged and the price for performance of the vacuum pump 131 may excessively increase. According to the embodiment of the present invention, infiltration of water into the rice can be promoted by using the vacuum pressure, so that the rice is swollen to contain proper moisture in a short time. For reference, the rice in a normal state contains moisture of approximately 15% by weight; however, the optimum moisture content for cooking the rice tastily is ap-

proximately 30% by weight. Conventionally, the rice has to be swollen by water for 1-2 hours until the moisture content becomes 30% by weight. However, when the rice is water-swollen for such a long time, the surface of rice is softened too much, thereby deteriorating quality of the cooked rice. Moreover, the dedicated work of water- swelling is inconvenient.

[66] In general, when food is salted down under a vacuum, the time for salt-maturing the food is reduced by more than half. This is being actively researched abroad with regard to meat, and established theories have already been published. According to the theories, commonly, protein pores of meat are opened widely under the vacuum, thereby soaking up moisture more easily.

[67] Although theories regarding rice are not fully established, it is generally known that rice has fine pores thereon. Therefore, it will be sure understood that rice can be swollen by water very rapidly under the high vacuum.

[68] After water- swelling of the rice under the vacuum in the cooking mode, the controller 140 operates the heat generator 113 to actually heat the rice with water. Here, the heat generator 113 is controlled to generate heat over 100 ⁰ C to boil the water.

[69] After the rice is cooked, the controller 140 controls the heat generator 113 to heat the cooked rice by lower temperature than cooking temperature, for a predetermined time considering the cooking time. For example, the cooked rice is heated for approximately 40-50 minutes.

[70] Next, the controller 140 stops the operation of the heat generator 113 and waits until the inner temperature of the receiving part 12 IA drops to 30~60°C. The vacuum pump 131 is operated by the controller 140 to decompress the receiving part 121 A by the high vacuum pressure of approximately -300 to -600DHg. When the receiving part 12 IA is thus deaerated, the cooked rice remaining after initial taking can be well preserved even at the normal temperature.

[71] Most importantly, the decompression should be performed after the cooked rice is cooled enough, that is, until the inner temperature of the receiving part 12 IA measured by the temperature sensor 117 drops to 30~60°C. Since vapor is not generated from the cooked rice at 30~60°C, decompression of the receiving part 12 IA can be performed more favorably. Therefore, oxygen and various microbes causing deterioration of the cooked rice can be discharged from the receiving part 12 IA almost completely. Additionally, nutrients and moisture of the cooked rice are protected from heat. Consequently, the cooked rice can be freshly preserved even for a long time.

[72] If the decompression is performed while the cooked rice is hot or being heated at high temperature, vapor generates continuously and therefore, inner pressure of the receiving part 121A is increased. Decompression is not favorably performed in this state. In order to perform decompression by high vacuum in this case, the vapor

generated in the receiving part 12 IA needs to be continuously discharged while operating the vacuum pump 131 intermittently by short periods during the decompression. However, this may even dry up the cooked rice by depriving the cooked rice of all the moisture. If the cooked rice did not lose moisture, texture of the cooked rice would be excessively softened due to the moisture and heat, consequently damaging the nutrients.

[73] As long as maintaining the cooked rice at 30~60°C, heating can be performed continuously or intermittently because such a problem regarding vapor is not caused.

[74] When a user wishes to perform the decompression at high temperature, while properly discharging the vapor, a low vacuum pressure may be applied instead of the high vacuum pressure. However, since the air in the receiving part 121A cannot be completely drawn out, oxidation and deterioration of the cooked rice can not be completely prevented.

[75] Another proper method for decompressing the receiving part 121A is to operate the vacuum pump 131 by the controller 140 when approximately 40-60 minutes have passed since starting cooling the cooked rice regardless of the inner temperature of the receiving part 121A, that is, although the inner temperature of the receiving part 121A has not dropped to 30~60°C yet. Because ambient temperature differs according to residential areas, it may take so long to cool the receiving part 121A to 30~60°C when the ambient temperature is high. 40-60 minutes is long enough to cool the receiving part 12 IA even at abnormally high ambient temperature. Furthermore, as the temperature of the receiving part 12 IA drops more, further decompression can be achieved incidentally since the air existing in the cooked rice and the receiving part 12 IA is contracted.

[76] When the cooking container 121 is lifted up, it is preferable that the controller 140 temporarily stops the operations of the heat generator 113 and the vacuum pump 131.

[77] The controller 140 is related to most of the operations of the electronic rice cooker.

The functions of the controller 140 will be described in detail with reference to a rice cooking and vacuum-preserving method.

[78] FIG. 4 is a flowchart for explaining the rice cooking and vacuum-preserving method according to an embodiment of the present invention. FIG. 5 through FIG. 13 are a series of reference views showing states of the electric rice cooker according to the rice cooking and vacuum-preserving method of the present invention.

[79] As shown in FIG. 4, the rice cooking and vacuum-preserving method comprises a water- swelling step (SlOO), a rice cooking step (S200), a warmth-keeping and maintaining step (S300), a cooling step (S400), a vacuum-decompressing step (S500), a vacuum-preserving step (S600), a reheating step (S700), and a warmth-keeping and maintaining step (S800).

[80] As shown in FIG. 5, in the water- swelling step (SlOO), proper amounts of water W and rice R are put in the receiving part 12 IA of the cooking container 121 and the cooker lid 112 is closed. The inner lid 123 disposed under the cooker lid 112 covers an upper part of the cooking container 121. As the sealing O-ring 125 is compressed between the cooking container 121 and the inner lid 123, the receiving part 12 IA of the cooking container 121s is sealed. When the inner lid 123 is not dedicatedly formed, the cooker lid 112 serves as the inner lid 123.

[81] When the user selects the cooking mode by pressing the button of the operation panel 11 IA (FIG. 1) mounted to the cooker body 111, before the actual heating for cooking the rice R, the controller 140 operates the vacuum pump 131 with the first solenoid valve 138 shut and the second solenoid valve 139 opened. The vacuum pump 131 draws in the air from the receiving part 12 IA of the cooking container 121 through the connection hose 133 as shown by an arrow and discharges the air to the outside, thus forming the vacuum inside the receiving part 12 IA. Therefore, a vacuum is generated the receiving part 12 IA of the cooking container 121 until the vacuum pressure of the receiving part 121A, measured by the pressure sensor 137, reaches -300 to -600DHg. When the receiving part 121A is thus decompressed by high vacuum, the air A in the rice C is extracted through numerous pores AH so that enough water W can infiltrate into the rice R easily. In this state, 2-3 minutes is enough to sufficiently swell the rice R by water.

[82] Generally, 30% by weight is considered as the optimum moisture content of the rice

R. However, in this embodiment, when the moisture content approximates the optimum value in 2-3 minutes, it is regarded as a proper time to begin the cooking mode. As described above, the present invention accomplishes rapid water- swelling by using the vacuum pressure. More correctly, infiltration of water W into the rice C can be promoted by the vacuum pressure. Accordingly, differently from the conventional water- swelling method, the surface of rice R can avoid long-time contact with the water W and be prevented from being excessively softened. Consequently, more soft and glossy cooked rice can be obtained as well as saving time and inconvenience.

[83] When the water-swelling step (SlOO) is completed, the heat generator 113 actually begins heating under the control of the controller 140, thus entering the cooking step (S200). While the heat generator 113 progresses the cooking step, the vacuum state may be released or maintained. When the vacuum state is maintained, infiltration of the water W will be promoted. The cooking step (S200) can be automatically performed after completing the water- swelling step (SlOO) merely by selecting the cooking mode initially.

[84] When the cooking step (S200) is begun, as shown in FIG. 7, the controller 140 applies power to the heat generator 113 and the heat generator 113 generates high heat

H and heats the cooking container 121. As time goes, the water W put in the receiving part 12 IA boils and the rice R is cooked, thereby resulting in cooked rice BR. Vapor generated from the boiling water is discharged out through the first solenoid valve 138 being opened. In order for the effect of pressure-cooking, the first solenoid 138 is shut so that discharge of the vapor is artificially restrained.

[85] Besides the rice, objects of the cooking include other various foods such as rice cake, grains, vegetables, and meats which require preservation after cooking.

[86] The vacuum preservation according to the embodiment of the present invention is now actually performed. As shown in FIG. 8, warmth of the cooked rice is kept for a predetermined time (S300). FIG. 8 shows the state where the cooker lid 112 is once opened to take part of the cooked rice and then closed again. In this step, the controller 140 controls the heat generator 113 to generate low heat WH having lower temperature than the cooking temperature for a predetermined time, for example, approximately 40-60 minutes. This is to enable the user to still have warm cooked rice when additionally taking the cooked rice within the predetermined time after the initial taking.

[87] Referring to FIG. 9, a cooling step (S400) is performed. The controller 140 stops the operation of the heat generator 113 after the previous step (S300) is progressed for approximately 40-50 minutes. Therefore, the cooked rice BR remaining in the receiving part 12 IA of the cooking container 121 is naturally cooked. At this time, the controller 140 may keep the first and the second solenoid valves 138 and 139 shut or opened after the heat generator 113 is stopped. In case that the first solenoid 138 is kept opened, a little vapor generated during the cooling is discharged to the outside.

[88] When the cooked rice BR is cooled up to 30~60°C, the next step is performed.

Whether the cooked rice BR is sufficiently cooled can be determined by the temperature sensor 117 mounted in the electronic rice cooker. Although the temperature of the cooked rice BR has not dropped to 30~60°C, the next step is preferably performed in the predetermined time, that is, approximately 40-60 minutes. This is because considerable time can be required to drop the temperature of the receiving part 121A to 30~60°C according to areas. Especially, when the ambient temperature is so high, for example, in summer, the cooked rice may easily go bad during the cooling.

[89] As described above, the cooling step (S400) is an indispensable step preceding the vacuum-decompressing step (S500) in order to achieve optimum preservation effect of the cooked rice BR. Without the cooling step (S400), the vacuum-decompressing step (S500) would be meaningless and it would be hard to achieve the optimum effect.

[90] When the cooked rice BR is sufficiently cooled, the vacuum-decompressing step

(S500) is performed as shown in FIG. 10. In this step, the receiving part 121A of the cooking container 121 is decompressed by the high vacuum so that the receiving part

121A is under the high vacuum pressure of approximately -300 to -600DHg. For this purpose, the controller 140 operates the vacuum pump 131 with the first solenoid valve 138 shut and the second solenoid valve 139 opened. Therefore, the vacuum pump 131 draws in the air from the receiving part 12 IA of the cooking container 121 through the connection hose 133 as shown by arrows in the drawings and discharges the drawn air to the outside. The vacuum pump 131 generates the vacuum in the receiving part 121 A by repeating the above process. The pressure in receiving part 121A gradually decreased to be lower than the atmospheric pressure and reach a preset degree measured by the pressure sensor 137. The moisture drawn in along with the air from the receiving part 12 IA is not drawn into the vacuum pump 131 but dropped at the disconnect part 133B of the connection hose 133 in the form of water drops BW and collected in the moisture receptacle 115. Since the moisture receptacle 115 hermetically encloses the disconnect part 133B disposed at the lowermost position of the connection hose 133, the vacuum pressure is not affected by existence of the moisture receptacle 115. An end part 133A of the connection hose 133 after the disconnect part 133B is connected to an air suction part 22 of the vacuum pump 131. When the backflow prevention valve 135 is mounted on the connection hose 133 together with the second solenoid valve 139, the air is allowed to flow only from the receiving part 12 IA to the vacuum pump 131 but not in the opposite direction, accordingly enhancing the decompression effect of the vacuum pump 131. Here, it is important that decompression of the receiving part 121A should be performed after the cooked rice BR is sufficiently cooled in the cooling step (S400). If not, much vapor generated from the hot cooked rice BR hinders formation of the vacuum in the receiving part 121A. In this case, the decompression would be poorly achieved, thereby prompting deterioration of the cooked rice BR. Furthermore, generation of much vapor means much loss of the moisture contained in the cooked rice. Therefore, quality of the preserved cooked rice will be deteriorated. [91] Next, as shown in FIG. 11, the vacuum-preserving step (S600) is performed. This step just maintains the decompressed state of the receiving part 12 IA as sealed without a special further operation. However, the vacuum-preserving step (S600) prepares for an unexpected case where the vacuum is weakened or released. More specifically, the pressure sensor 137 continuously checks the inner pressure of the receiving part 121A and upon detection of decrease in the vacuum pressure, informs the controller 140 of the decrease. The controller 140 soon operates the vacuum pump 131 to restore the vacuum pressure in the receiving part 12 IA. If the vacuum pressure in the receiving part 12 IA holding the cooked rice BR is thus maintained, substances such as the air, oxygen, and microbes which deteriorate the cooked rice BR can be mostly prevented from existing in the receiving part 12 IA of the cooking container 121. Accordingly,

the cooked rice BR can be preserved for a long time in an almost perfectly germfree state even without heating. In the same manner as canned food being preserved for a long time almost without deterioration of initial quality at the normal temperature, the cooked rice BR can be preserved according to the method of the present invention. If the vacuum pump 131 is controlled to operate periodically, for example, at every 8 hours or 12 hours, the pressure sensor 137 may not have to continuously measure the pressure in the receiving part 121A. In this case, lifespan of the pressure sensor 137 can be improved.

[92] When the inner temperature of the receiving part 12 IA is dropped to the preset temperature, that is, approximately 30~60°C, the controller 140 re-operates the heat generator 113 to maintain the preset temperatures while preserving the cooked rice. At this time, the heat generator 113 generates lower heat than the low heat WH, for user s comfortable eating. By maintaining the preset temperature, time for reheating the cooked rice can be highly saved when further taking the cooked rice during preservation. The controller 140 may control the heat generator 113 to operate in a periodical pulse type rather than constantly. Since the cooked rice is preserved within a range of 30~60°C which is higher than the normal temperature, evaporation of the moisture, and deterioration and discoloration of the cooked rice can be prevented almost perfectly. Here, the controller 140 may operate the heat generator 113 for maintenance of the preset temperature in the vacuum-decompressing step (S500), but more preferably in the vacuum-preserving step (S600).

[93] The distinctive feature of the present invention is sequential combination of the cooling step (S400) and the vacuum-decompressing step (S500) for maximizing the preservation effect.

[94] As shown in FIG. 12, next, the reheating step (S700) is performed by directly pressing a reheating button of the electronic rice cooker when the user wants to eat the cooked rice again after some time. The controller 140 then applies power to the heat generator 113, opening the first solenoid valve 138 to release the vacuum state of the receiving part 12 IA immediately or in a specific time. The specific time may be a time point before steam pressure is generated in the receiving part 121A or a time point when the inner temperature of the receiving part 12 IA reaches approximately 50~60°C. Therefore, the cooked rice BR being preserved under the vacuum state at the normal temperature is applied again with the high heat H, thereby reaching a proper temperature for eating in a few minutes. Here, the controller 140 performs the periodical pulse-type operation rather than constantly generating heat, so that the cooked rice BR can be uniformly warmed but not burned. If the reheating step (S700) is set to be performed at a preset time automatically, user convenience would be much improved. For the time setting, the controller 140 needs to have a timer function. For

example, if the user sets the eating time to 7 a.m. and 6 p.m., the controller 140 starts the reheating step (S700) at 6:50 a.m. and 5:50 p.m., regarding 10 minutes as a proper time to reach the proper eating temperature.

[95] During this, loss of moisture may be compensated by spraying water into the receiving part 12 IA of the cooking container 121. This is enabled simply by adding a jet nozzle (not shown) for being supplied with water from the moisture receptacle 115 and spraying the water into the receiving part 12 IA. The supplemented water amount can be roughly determined based on experiences, and just a small amount of water is enough. Because the cooked rice is not continuously heated at high temperature, differently from the conventional preserving method, supplement of water for such a minor loss of moisture is not actually necessary.

[96] As shown in FIG. 13, the vacuum-preserving step (S800) is performed. Since the user may further take the cooked rice BR after the initial taking, the vacuum- preserving step (S 800) maintains the proper eating temperature of the cooked rice BR for a predetermined time, for example, approximately 40-60 minutes. During this, the heat generator 113 generates lower heat than in the reheating step.

[97] The cooked rice BR still remaining in the receiving part 121A is preserved by repeating the above steps, that is, the cooling step (S400), the vacuum-decompressing step (S500), and the vacuum-preserving step (S600).

[98] According to the embodiment of the present invention, the above serial steps are performed sequentially and automatically through one-touch operation of a button or switch by the controller 140. In other words, the user can perform all the steps from water- swelling the rice to periodically eating the cooked rice, safely preserving the cooked rice for a long time, by operating the button or switch just once. In addition, the user is able to have cooked rice with initial good quality any time. Industrial Applicability

[99] While the invention has been shown and described with reference to a heat-plate general rice cooker, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. That is, the present invention can be applied to electronic rice cookers of various types divided by the heating system and cooking system, such as a heat-plate pressure rice cooker and an IH-type pressure rice cooker.