JPH0437698B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention weakens the degree of adhesion of rice grains to each other after drying, making it possible to form a single rice grain with a small amount of external force, and the texture and flavor after restoration are dry by adding hot water etc. This invention relates to a method for producing instant dried rice that can be restored in a short time to almost the same texture and flavor as before. Many techniques have been developed for instant dry cooked rice that can be easily restored by adding hot water or the like. The present inventors have also developed a vacuum swelling drying method for foods as a technique for obtaining instant dried cooked rice with excellent restorability, especially texture after restoration (Japanese Patent Laid-Open Publication No. 40057/1983). The gist of the vacuum swelling and drying method for food is to place the food under a rapid vacuum state at a rate sufficient to cause the food to swell and to freeze the moisture in the food. The present invention relates to a vacuum swelling drying method for foods, which is characterized by post-heat drying and then returning to normal pressure. However, when drying rice using this method to make instant dried rice, if the rice grains stick to each other and form clumps, the clumps are broken down and the rice grains are broken down one by one. The drawback was that it was difficult to let go. The inventors of the present invention have conducted research on methods to eliminate such drawbacks. As a result, we obtained the knowledge that it is effective to pre-dry the cooked rice after washing it with water, and then apply the vacuum swelling drying method for the above-mentioned foods.
This method was previously filed as a method for producing instant dried rice (Japanese Patent Application Laid-Open No. 59-31659). However, according to this method, since the cooked rice is washed with water in advance, the flavor of the cooked rice is eluted, which slightly reduces the flavor of the final product. Therefore, after further intensive research, we came to the knowledge that the above-mentioned problems can be solved by drying under reduced pressure, especially by increasing the drying temperature, rather than pre-drying the cooked rice. Ivy. The gist of the present invention, which was completed based on these findings, is to cook cooked rice at a rate sufficient for the rice to puff up.
The rice is placed under reduced pressure to the extent that the water in the rice self-freezes, and heating and drying is started immediately or within 20 minutes after the water in the rice self-freezes, so that the core temperature of the rice is 100 to 150°C. After heating and drying the rice until it becomes dry, the moisture content is 10% by weight under conditions that do not burn the rice.
There is a method for producing instant dried cooked rice, which is characterized by holding the pressure until the pressure drops to below and then returning it to normal pressure. The method of the present invention will be explained in detail below. In the method of the present invention, rice cooked in a conventional manner is first placed under reduced pressure. However, when placing the cooked rice under reduced pressure, at least the following two conditions must be satisfied. The first condition is to subject the cooked rice to a certain reduced pressure quickly enough for puffing to occur. When cooked rice is placed under reduced pressure, the boiling point of the water in the cooked rice naturally decreases, thereby causing a phenomenon of transpiration of water, especially free water, in the cooked rice. When such a transpiration phenomenon occurs rapidly in a short period of time, the cooked rice puffs up. Therefore, the speed sufficient to cause the puffiness of cooked rice as used in the present invention refers to the speed sufficient to cause the moisture in the cooked rice to evaporate rapidly in a short period of time. Specifically, it is preferably within 1 second. The second condition is to set the specific degree of pressure reduction mentioned in the first condition to a degree of pressure reduction sufficient to cause the water in the cooked rice to self-freeze. The degree of vacuum sufficient for self-freezing of water in cooked rice as used in the present invention is generally about 609 Pascal or less, preferably about 106 Pascal or less. After the above two conditions are satisfied and the cooked rice is placed under reduced pressure, it is maintained until the water in the cooked rice self-freezes. By this process, the shape of the puffed cooked rice is maintained as it is, and the finally obtained dry cooked rice is brought into a puffed state, thereby making it possible to speed up the restoration using hot water or the like. There are the following two methods of reducing pressure to satisfy the above two conditions. The first method involves puffing the cooked rice and self-freezing of the water in the cooked rice in one step. In other words, the rice is heated under a reduced pressure sufficient to cause the water in the cooked rice to self-freeze. This is a fast method. According to this method, the swelling of the rice and the self-freezing of the water in the rice occur almost instantly. The second method involves puffing the rice and self-freezing of the water in the cooked rice in two stages, i.e., the rice is cooked under reduced pressure at a rate sufficient to cause puffing (but not enough to cause the water in the rice to self-freeze). (under reduced pressure that does not reach the degree of reduced pressure)
There is a method in which the cooked rice is expanded, and then the pressure is reduced to a level sufficient to cause the water in the cooked rice to self-freeze. According to this method, the swelling of the rice and the self-freezing of the water in the rice occur separately over time. From the viewpoint of achieving the object of the present invention, there is no problem in using any of the above methods, and both are included in the reduced pressure state of the present invention. After the water in the cooked rice is self-frozen, it is heated and dried using conventional heating methods such as heat transfer heating, infrared heating, and microwave heating. Immediately or within 20 minutes, preferably within 2 to 7 minutes, after self-freezing. Whether or not the water in the cooked rice has self-frozen can be easily confirmed by observing changes in the temperature of the center of the cooked rice. FIG. 1 schematically shows the change in core temperature of cooked rice over time during vacuum expansion treatment. As shown in FIG. 1, when cooked rice is treated under reduced pressure and expansion conditions, the temperature of the center of the cooked rice rapidly decreases to a supercooled state, but after that, the temperature of the center of the cooked rice rises slightly. The moment when the rise stops is when the water in the cooked rice freezes. Apart from this, it is also possible to judge from the appearance whether the cooked rice has become almost completely cloudy or not. That is, as the water in the cooked rice self-freezes, the cooked rice gradually becomes cloudy. The time when the water in the cooked rice self-freezes depends mainly on the degree of pressure reduction and the rate of pressure reduction, but the water in the cooked rice generally self-freezes about 3 minutes after the start of pressure reduction. Therefore, from another perspective, the timing of starting the heat drying is about 3 to 23 minutes, preferably about 5 to 10 minutes after the start of decompression. If heat drying is started too early, drying will occur even though self-freezing has not completely completed, resulting in noticeable dry thinning. On the other hand, if heating and drying is started too late, the adhesion of rice grains to each other due to self-freezing will become stronger and the surface will gradually dry, resulting in less adhesion between the dried rice grains. It becomes strong and does not break easily with a slight impact. This heating and drying removes frozen water from the cooked rice, and in this case, it is extremely important to increase the temperature of the core of the cooked rice as quickly as possible in order to achieve the object of the present invention. That is, by rapidly raising the temperature of the core of the cooked rice and rapidly evaporating the water in the cooked rice, many cavities are formed inside the cooked rice and the cooked rice is dried. Even if the dry cooked rice obtained in this manner is in the form of lumps, the degree of adhesion between the rice grains is weak and can be easily disintegrated into single grains by a slight external impact. Note that the core temperature of cooked rice in the present invention refers to the temperature detected by a temperature sensor inserted into the cooked rice. In order to obtain dried cooked rice having such an effect, it is preferable to carry out the heating drying at as high a temperature as possible without causing burnt rice.
Specifically, it is carried out until the temperature of the center of the cooked rice reaches 100 to 150°C, preferably 120 to 140°C. Center product temperature
If heating drying is terminated before the temperature reaches 100°C, dry thinning will appear. On the other hand, the center product temperature
When the temperature reaches 150 degrees Celsius or higher, the rice becomes noticeably burnt. When carrying out heat drying under the above conditions, it is better to increase the temperature of the core of the cooked rice as fast as possible, and specifically, it is preferably 5 to 7°C/min. After the core temperature of the cooked rice reaches 100 to 150°C, it is maintained under conditions that do not burn the rice until its water content becomes 10% by weight or less. This enables long-term storage of dried cooked rice. In this case, if heat conduction heating is used as the heating means, the considerably high temperature will continue even if the heat source is stopped, which may result in the rice becoming burnt. It is preferable to perform forced cooling by appropriate means. After completing heating and drying as described above, the pressure is returned to normal to obtain instant dried cooked rice. Next, an example of a specific apparatus that can be used to carry out the method of the present invention will be explained based on FIG. 2. Reference numeral 1 indicates a drying chamber, and inside the drying chamber 1, a shelf 2 for placing the processed material A is provided. Heaters 3 and 4 are installed below and above the shelf 2. A cold trap 6 is connected to the side wall of the drying chamber 1 via a pipe 5, and the cold trap 6 is connected to a vacuum pump 8 via a pipe 7. On the other hand, the pipes 5 and 7 are provided with valves 9 and 10, and a reservoir tank 1 is connected to the front and back of the pipe 5 via pipes 11 and 12.
3 is connected. And the pipe 11,1
2 are provided with valves 14 and 15, respectively. After the drying process, an air supply pipe 16 for returning the drying chamber 1 to normal pressure is installed on the top surface of the drying chamber 1, and the air supply pipe 16 is provided with a needle valve 17. The experiment was carried out as follows using the above apparatus. Example 1 200 g of glutinous rice was soaked in water for 30 minutes and cooked in an electric pot in the usual manner. 100g of cooked rice obtained in this way
was placed in a tray and placed on the shelf 2 of the drying chamber 1 with the needle valve 17 closed. Separately, the valves 9 and 14 are closed, and the vacuum pump 8 is operated with the valves 10 and 15 open to reduce the pressure in the reservoir tank 13 to about 30 Pascals. Thereafter, the valve 15 was closed and the valve 14 was opened to instantly reduce the pressure inside the drying chamber 1 to about 400 Pascals.
After that, valves 14 and 15 are closed, and valves 9 and 1 are closed.
0 was opened, and the degree of vacuum in the drying chamber 1 was brought to about 40 Pascal using the vacuum pump 8. Self-freezing was completed approximately 3 minutes after the start of decompression, and 2 minutes later, heaters 3 and 4 were turned on to heat and dry the self-frozen cooked rice in the tray. The heating and drying conditions at this time are as follows: The heating rate of the central part of the cooked rice is 6.5℃/
The conditions are such that the When the temperature of the center of the cooked rice reaches 120℃, turn off the heaters 3 and 4, and run cooling water from outside the drying chamber 1.
The heat drying was completed by cooling for 40 minutes at a cooling rate of °C/min. At this time, the degree of vacuum in the drying chamber 1 was approximately 80 Pascals. After that, close the valves 9 and 10,
After stopping the vacuum pump 8 and gradually opening the needle valve 17 on the top of the drying chamber 1 to supply air into the drying chamber 1 from the air supply pipe 16 to return the inside of the drying chamber 1 to normal pressure, the drying The tray was taken out from chamber 1 to obtain instant dried cooked rice with a water content of about 5% by weight.
In order to confirm the effects of the instant dry cooked rice obtained in this way, the following comparative examples were conducted. Comparative Example 1 The conditions for heating and drying cooked rice under reduced pressure are that the rate of increase in the temperature of the central part of the cooked rice is 1°C/min;
The test was carried out in the same manner and under the same conditions as in the above example except that the temperature of the center of the cooked rice at the end of heating and drying was 50°C. Comparative Example 2 All the methods and conditions were the same as in the above example except that the heat drying was started 27 minutes after self-freezing. Comparative Example 3 The heating start time is 27 minutes after self-freezing,
The conditions for heating and drying the cooked rice under reduced pressure were as follows: except that the rate of increase in the temperature of the center of the cooked rice was 1°C/min, and that the temperature of the center of the cooked rice at the end of the heating and drying was 50°C. It was carried out using the same method and conditions as in the example. The comparison results are shown in Table 1. The comparison items are the following four items. 1 Degree of rice grain adhesion in instant dried rice 2 Degree of swelling in instant dried rice 3 Water absorption rate of instant dried rice 4 Texture of instant dried rice after restoration 5 Appearance of instant dried rice

[Table] Moisture content was measured.
The measurement conditions were 105°C and 16 hours.
By comparing Example 1 and Comparative Example 1 in Table 1, it was found that the drying conditions for self-frozen cooked rice were such that the rate of temperature rise of the center part of the cooked rice was fast and the temperature of the center part at the end of drying of the cooked rice was It becomes clear that by increasing the height, it is possible to prevent the cooked rice from shrinking during drying. Next, Example 1 and Comparative Example 2
By comparing the above, it becomes clear that by accelerating the start of heating and drying of self-frozen cooked rice, the degree of adhesion of rice grains to each other can be weakened. Next, Example 1 and Comparative Example 3
By contrasting this with By doing so, it becomes clear that all of the requirements of rice grain adhesion, swelling degree, restorability, and texture after restoration can be satisfied. Next, FIGS. 3 to 6 are electron micrographs of cross-sectional structures of rice grains of instant dried cooked rice obtained in Example 1 and Comparative Example 3. FIG. 3 shows the entire cross-sectional structure of the rice grains of the instant dried cooked rice of Example 1, and FIG. 4 is a partially enlarged photograph thereof. FIG. 5 shows the entire cross-sectional structure of the rice grains of the instant dry cooked rice of Comparative Example 3, and FIG. 6 is a partially enlarged view of this. First, when comparing FIG. 3 and FIG. 5, the cavities inside the rice grains are larger in Example 1 than in Comparative Example 3. From this, it is clear that the present invention is superior to Comparative Example 3 in water absorption and water retention, and as a result, is superior in restorability.
Next, when comparing FIG. 4 and FIG. 6, Example 1
The surface of the rice grain is thinner and denser, and for this reason, the rice grain of the present invention has better water absorption and can impart elasticity to the texture after restoration.
That becomes clear. As described above, in the instant dried cooked rice obtained by the method of the present invention, the degree of adhesion of the rice grains to each other after drying is weak, and it is possible to separate the rice grains one by one with a small amount of external force. By adding water or the like, the texture and flavor after restoration can be restored in a short time to a state that is almost the same as the texture and flavor of cooked rice before drying.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing the change over time in the core temperature of cooked rice during vacuum expansion treatment. FIG. 2 shows an example of a specific apparatus that may be used in carrying out the present invention. Figures 3 and 4 are electron micrographs showing the shape of the fibers inside the instant dried cooked rice obtained in Example 1, and the magnification is 30x in Figure 3 and 30x in Figure 4.
It is 150 times more. FIGS. 5 and 6 are electron micrographs showing the shape of the fibers inside the instant dried cooked rice obtained in Comparative Example 3, and the magnifications are 30x in FIG. 5 and 150x in FIG. 6. 1... Drying room, 2... Shelf, 3, 4... Heater, 5, 7, 11, 12... Pipe, 6... Cold trap, 8... Vacuum pump, 9, 10, 1
4, 15...Valve, 16...Air supply pipe,
17...Needle valve.

Claims (1)

[Claims] 1. Cooking cooked rice at a rate sufficient to cause the cooked rice to swell.
The rice is placed under reduced pressure to the extent that the water in the rice self-freezes, and heating and drying is started immediately or within 20 minutes after the water in the rice self-freezes, so that the core temperature of the rice is 100 to 150°C. A method for producing instant dried cooked rice, which is characterized by heating and drying the rice until the rice becomes dry, then holding the rice under conditions that do not cause it to burn until the water content becomes 10% by weight or less, and then returning the pressure to normal pressure. 2 Heat drying conditions are such that the temperature of the main part of cooked rice rises between 5 and 7.
The method for producing instant dry cooked rice according to claim 1, characterized in that the conditions are such that the temperature is 0.degree. C./min.