JPH02200170A - Drying of food under reduced pressure - Google Patents

Abstract

PURPOSE:To provide the title drying process causing no mutual adhesion of the food to be dried, no crushing of the dried food, and no generation of dust in the drying, so designed that a food is rapidly depressurized to effect expansion and the water contained therein is frozen and the resulting food is then dried by heating. CONSTITUTION:A food such as cooked rice or fruit chips is brought to a rapidly depressurized state (at <=4.58Torr, pref. <=0.8Torr) at a rate enough to be expanded (pref. within one second), and put to vibration treatment when its central temperature reached 0-7 deg.C. After freezing the water contained therein and before the water content becomes <=90wt.% of that prior to the treatment, the vibration treatment is terminated, the resultant food is then dried by heating through e.g. infrared heating or microwave heating, followed by returning the ambient pressure to normal pressure.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for drying cooked rice, livestock meat, fish meat, fruits, etc. under reduced pressure.

[Conventional technology]

Conventionally, foods have been dried under reduced pressure using various methods. For example, Japanese Patent Publication No. 59-4981 discloses that food is subjected to rapid depressurization at a rate sufficient to cause the food to swell, and the water in the food freezes, and after the water in the food freezes, A vacuum swelling drying method for foods is disclosed, which is characterized by drying by heating and then returning to normal pressure.

However, with this method, viscous foods such as cooked rice, fruits,
When drying thin flakes such as vegetable chips under reduced pressure, especially in a relatively large quantity, when drying them in a stacked state, (i) Foods may be stuck to each other or the food and food storage container. (ii) When dried food sticks to each other during drying, and in the case of cooked rice, it becomes unsightly as dried food forms multiple clumps. ) Also, there are various problems such as the adhesion reduces the surface area of the food to be dried, and therefore the drying efficiency deteriorates.

On the other hand, in order to prevent the rice grains from adhering to each other during freezing and to obtain frozen cooked rice in the form of individual pieces, it was proposed in 1983 that the rice be frozen while vibrating using a vertical multi-stage device equipped with a plurality of vibrating conveyors. -46352, and Japanese Patent Publication No. 62-25024 discloses the use of a vibrating fluidized bed dryer to improve drying efficiency of raw fish and prevent deterioration of color and texture. has been done. in particular,
When vacuum freeze-drying raw miso using a vibrating fluidized bed dryer, the dried material is sieved and the granules at the top of the sieve are used as a product.
The fine miso powder at the bottom of the sieve is fed back and a vibrating fluidized bed is used to spray raw miso onto the fine miso powder using a two-fluid nozzle to form granules, which are quickly frozen and then continuously dried in a vibrating fluidized bed dryer. This method is characterized by repeating the cycles of vacuum freeze-drying, sieving, granulation, and drying.

However, even if the special vibration treatment described above is applied to the vacuum drying method disclosed in JP-A No. 59-4981, it is impossible to prevent foods from adhering to each other, especially when drying cooked rice, fruit, or vegetable chips. Although this can be achieved, there are problems in that the food to be dried is broken and the resulting fine powder is generated, resulting in poor appearance and low yield.

Furthermore, if weak vibration treatment is applied in consideration of food breakage, there is a problem in that mutual adhesion cannot be effectively prevented.

[Problem to be solved by the invention]

Therefore, an object of the present invention is to provide a method for drying foods under reduced pressure, in which there is no adhesion of objects to be dried, there is almost no breakage of dried foods, and there is almost no generation of fine powder during drying.

[Means to solve the problem]

The present invention provides for drying foods under reduced pressure using a specific method.
This was done based on the knowledge that the above-mentioned problems can be efficiently achieved if the vibration treatment is started when the temperature of the food reaches a certain temperature.

That is, in the present invention, a food is subjected to a rapid depressurization state at a rate sufficient to cause the food to swell, and water in the food freezes, and when the center temperature of the food reaches 0 to 7°C, vibration is applied. To provide a method for drying food products under reduced pressure, which comprises starting a process, freezing water in the food product, drying it by heating, and then returning the pressure to normal pressure.

Foods to be dried under reduced pressure in the present invention are not limited, but include, for example, vegetables, fruits, grains, beans, and processed products thereof; processed meat products such as meat, ham, bacon, and sausage;
Fish meat, shellfish, shrimp, seaweed, and processed products thereof; Other examples include processed egg products, processed milk products, and mushrooms.

It is particularly effective when applied to viscous materials such as cooked rice, and flaky materials such as fruit chips, vegetable chips, and potato chips. Also, solids having a water content of about 50% or more are preferred. still,
When obtaining dried products such as fruits, vegetables, chips, etc., adjusting the thickness of the raw fruit and vegetable pieces to about 3 to 8 mm will effectively prevent the foods from adhering to each other and allow efficient drying. desirable.

In the present invention, the following two conditions must be satisfied when the food is placed under reduced pressure.

The first condition is to subject the food product to a certain reduced pressure quickly enough for puffing to occur. When food is placed under reduced pressure, the boiling point of the water in the food naturally decreases, thereby reducing the water content in the food.
In particular, the phenomenon of transpiration of free water occurs. When such a transpiration phenomenon occurs rapidly in a short period of time, in other words, when the food is placed under reduced pressure for a short period of time, the food puffs up. Therefore, the speed sufficient to cause the food to puff up as used in the present invention refers to the speed sufficient to cause the moisture in the food to evaporate rapidly in a short period of time. In general, the swelling phenomenon of various foods due to water evaporation differs depending on the type of food. Therefore, the speed sufficient for the expansion of the food product varies depending on the type of food product, but it is preferably within about 1 second.

Next, the second condition is that the specific degree of reduced pressure mentioned in the first condition is set to a degree of reduced pressure sufficient to cause the moisture in the food to freeze. The degree of vacuum sufficient to freeze water in the food according to the present invention is about 4.5 B torr or less, preferably about Q.3 torr or less.

After satisfying the above two conditions and placing the food under reduced pressure,
The food is maintained until the moisture in the food freezes, preferably until the food temperature no longer decreases. By this treatment, the shape of the puffed food is maintained as it is, and the finally obtained dried food is brought into a puffed state, thereby making it possible to speed up 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 food and freezing the water in the food in one step, i.e., the food is placed under a reduced pressure that is sufficient to freeze the water in the food and at a rate sufficient to cause puffing. This is the way to keep it. According to this method, the puffing of the food and the freezing of the water in the food occur almost instantaneously.

The second method involves puffing the food and freezing the water in the food in two stages, i.e., the food is heated under reduced pressure quickly enough for puffing to occur (and at a reduced pressure sufficient to freeze the water in the food). This is a method in which the food is expanded under a reduced pressure that does not exceed 100 degrees, and then the reduced pressure is lowered to a degree that is sufficient to freeze the moisture in the food. According to this method, the swelling of the food and the freezing of the water in the food occur separately over time.

Any of the above methods may be used without any problem in achieving the purpose of the present invention. After the moisture in the food is frozen, the food is dried by heating using a conventional heating method such as infrared heating or microwave heating.

Among the heating means mentioned above, radiant heating using infrared rays or the like is a suitable heating means for evaporating frozen water in food without completely melting it and in a state other than sublimation, that is, in a state where it is semi-frozen. As a result, drying provides an excellent advantage of not causing food shrinkage or deterioration of food texture after reconstitution, which is preferable. Next, the dried food obtained by heating and drying is returned to normal pressure, and then transferred to the next process such as a packaging process.

In the present invention, in the vacuum drying, the temperature of the food is 7 to 7.
It is characterized in that the vibration treatment is started when the temperature reaches 0°C. In other words, if the vibration treatment start temperature is higher than 7° C., the frequency of contact between foods due to vibration increases, the foods tend to stick to each other, and the foods tend to crack or break. - On the other hand, if the starting temperature is lower than 0°C, the foods will freeze while adhering to each other, and even vibration will not separate them. In addition, the vibration start temperature should be set at 4 to 6 degrees Celsius especially for cooked rice, and 0 to 2 degrees Celsius for kiwi pieces.
It is preferable to set the temperature to 3 to 5°C for lotus root pieces and 4 to 6°C for melon pieces, since the foods can be dried without adhering to each other, without cracking or breaking the food, and without generating fine powder.

The above-mentioned vibration processing can be ended at any time. but,
If the vibration treatment is continued until the moisture content of the food is less than 90% by weight (addressable moisture), the food will tend to crack or break, and fine powder will be produced, and the yield of the finished product will be reduced. Before the moisture content of the food reaches 90%, the moisture content of the food should be removed before the moisture content of the food reaches 90%. It is preferable to end the vibration treatment at .

In particular, if the vibration treatment is finished when the moisture content is 91 to 95% for cooked rice, and for fruit and vegetable pieces such as kiwi pieces, lotus root pieces, and melon pieces, the moisture content is in the range of 93 to 97%, the food may crack or break. A dried food with good restorability and texture without the generation of fine powder or fine powder can be obtained.

The vibration treatment described above can be performed using various devices, and can be easily performed using the device shown in FIG. 1, for example.

Here, 1 indicates a drying chamber, and a lid 2 is provided at the upper and lower portions of the drying chamber 1, and a container 4 for swallowing the food A is provided inside the lid 2. The container 4 is connected to a vibrator 5. An infrared heater 6 is installed inside the lid 2 of the drying chamber 1. A vacuum chamber 7 is connected to the side wall of the drying chamber 1 by a vibrator 8, and the vibrator 8 is provided with a cock 9 for opening and closing it. A cold trap 10 and a vacuum pump 11 are connected to the vacuum chamber 7 through a vibrator 12 in order to reduce the pressure inside the vacuum chamber 7, and a vacuum gauge 13 is also provided in the vacuum chamber 7. A vacuum gauge 14 for measuring the degree of reduced pressure inside the drying chamber 1 is provided on one side wall of the drying chamber 1, and an air supply vibrator 15 is also connected to the air supply pipe 15. A cock 16 is provided to open and close the air supply pipe 15.

The vibration source of the above device may be motor type, electromagnetic type, decompression type, etc., and the vibration direction may be vertical or horizontal, but it can more effectively prevent food items from adhering to each other. Vertical vibration is preferred. Vibration in two directions, vertical and transverse, can also be used. In addition, the above vibration is 0.5 to 5 mm (vertical amplitude) and frequency 30 to 7.
It is preferable to carry out the test under the condition of 0 Hz.

Various foods can be dried using the above method, but especially when processing cooked rice (obtaining dried rice), it is necessary to cook the rice and then cool it (to obtain dry rice).
Add 20 to 30% (by weight of cooked rice) starch (rice starch, waxy cornstarch,
It is preferable to perform the vacuum drying treatment described above after adding and mixing the processed starch (preferably processed starch), as this can more effectively prevent the cooked rice from adhering to each other.

[Effects of the Invention] According to the present invention, when drying viscous materials such as cooked rice, fruits, and vegetable chips under reduced pressure, mutual adhesion of the foods and the accompanying clumping of the foods occur during freezing and drying. do not have. Furthermore, even when food products are layered and dried under reduced pressure in large quantities, they do not stick to each other, so there is no uneven drying, and the drying process can be carried out more efficiently. For some reason, the generation of fine powder is hardly observed, and dried products of good quality can be produced with a high yield.Furthermore, the dried food obtained by the method of the present invention has good restorability.

Therefore, according to the present invention, it is possible to obtain a dried food such as puffed dry rice that can be restored in a short period of time by adding hot water or the like to the point where the texture after restoration is almost the same as the texture before drying. In addition, snack foods such as fruit chips, vegetable chips, potato chips, etc., which have a pleasant crunchy texture (can be eaten as they are) can be obtained.

Next, the present invention will be explained with reference to Examples.

〔Example〕

Example 1 300 g of rice was immersed in water at 20° C. for 60 minutes and then drained. To this, 500 g of water was added and rice was cooked in an electric pot. 100g of rice cooked in this way is 300g of 5
% ethanol solution for 1 minute, and then drained. Next, the cooked rice was washed several times with water at 30°C, drained (the temperature of the cooked rice was slightly raised to increase the degree of swelling), and the resulting cooked rice was placed in the apparatus shown in Figure 1. Ta.

By reducing the pressure in the vacuum chamber to 3 Q torr or less within 1 minute, and then to 0.5 torr or less within 4 minutes, the temperature of the sample decreased from 30°C to -15°C.

When the product temperature reaches 5℃, the vertical half amplitude becomes 1.5.
Vibration with a frequency of 4 Qhz was applied to reduce the moisture content of rice to 70 mm.
% (93.8% of initial moisture content)
). Next, vacuum freeze-drying was performed for 6 hours at a heater temperature of 60°C. The dried cooked rice thus obtained had individual grains that were separated and could be easily rehydrated in hot water.

Example 2 Dry cooked rice was obtained under the same conditions as in Example 1, except that the vibration was stopped when the moisture content of the sample reached 65%. The dry cooked rice obtained in this way has vI on the surface, although it is not attached.
Some powder adhered to it, and recovery was a little difficult.

Comparative Example 1 Dry cooked rice was obtained under the same conditions as in Example 1, except that vibration was started when the sample temperature reached 25°C. The dried cooked rice thus obtained had rice grains that stuck to each other and did not return well.

Comparative Example 2 Dry cooked rice was obtained under the same conditions as in Example 1, except that vibration was started when the sample temperature reached -1°C. The dried cooked rice thus obtained had rice grains that stuck to each other and did not return well.

Example 3 The kiwi was peeled and sliced into 4 mm thick pieces.

100 g of kiwi was placed in the apparatus shown in FIG.

When the pressure was reduced under the same conditions as in Example 1, the temperature of the kiwi fruit decreased from 30°C to -15°C.

When the product temperature reached 0°C, vibration was applied under the same conditions as in Example 1, and the vibration was stopped when the moisture content of the kiwi reached 81.9% (initial 95.9%). Next, heater temperature 6
Vacuum freeze-drying was performed at 0°C for 6 hours.

The thus obtained kiwi chips had no adhesion between the chips and were unbreakable.

Example 4 Kiwi chips were obtained under the same conditions as Example 2 except that the vibration was stopped when the moisture content reached 85%. The kiwi chips obtained in this way were cracked, and although the chips did not stick to each other, there were some breaks.

Comparative Example 3 Kiwi chips were obtained under the same conditions as in Example 2, except that vibration was applied when the product temperature reached 25°C. The kiwi chips obtained in this way were often cracked and broken, and the chips often stuck to each other.

Comparative Example 4 Kiwi chips were obtained under the same conditions as in Example 2, except that vibration was applied when the product temperature reached 11°C. The kiwi chips thus obtained had many chips that adhered to each other.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an apparatus used in the reduced pressure drying method of the present invention. In the figure, 1: Drying chamber, 2: Lid, 4: Container, 5: Vibrator, 6: Infrared heater 7
...Vacuum chamber, 8°°°0 vibe, 9-°0
Cook, 10... Cold Trap, 11...
Vacuum pump, 12...Pipe, 13.14...
Vacuum gauge, 15...air supply pipe, 16...cock, A...food.

Claims (4)

[Claims] (1) Place the food under rapid depressurization at a rate sufficient to cause the food to swell and freeze the water in the food, and when the center temperature of the food reaches 0 to 7°C, perform vibration treatment. A method for drying food under reduced pressure, which comprises: starting the drying process, heating and drying the food after the water in the food has frozen, and then returning the pressure to normal pressure. (2) The vacuum drying according to claim (1), characterized in that the vibration treatment is completed after the moisture in the food freezes and before the moisture content in the food becomes 90% by weight or less of the moisture content before treatment. Method. (3) Claim (1) characterized in that the food is cooked rice
) described vacuum drying method. (4) The vacuum drying method according to claim (1), wherein the food is kiwi pieces.