JP7386454B2 - Underwater shortwave heating method - Google Patents

Description

The present invention relates to a method for heat-treating foods by applying alternating current at a frequency in a short wave band (for example, 3 MHz to 300 MHz) to the foods.

Among methods of heating and sterilizing food stored in packaging containers, heating using a heat source external to the packaging container, such as pressurized steam or hot water, does not generate heat from the food itself, so the temperature at the center of the food heat sterilization tends to be insufficient as the temperature does not rise.

Therefore, the present inventor has proposed a sterilization method in which the food itself generates heat in Patent Document 1. In this method, a pair of cylindrical electrodes are placed coaxially in a pressure container filled with water, the space between the pair of cylindrical electrodes is used as a food storage space, and a short wave band alternating current is applied between the opposing cylindrical electrodes. This is used to sterilize food by heating.

Furthermore, in Patent Document 2, a liquid food material is continuously flowed into a gap between narrow opposing electrodes, and a high alternating current voltage of 20 kHz or less is applied between the electrodes to generate an alternating current electric field between the electrodes. An alternating current high electric field sterilization method for continuous sterilization is disclosed.

In the method of Patent Document 2, the temperature of liquid foods and solid foods with high electrical conductivity increases more than necessary. Therefore, Patent Document 3 proposes applying an electric field in which a pulsed electric field is superimposed on an alternating current electric field to foods.

Japanese Patent Application Publication No. 2016-111928 Patent No. 2848591 Japanese Patent Application Publication No. 2007-229319

Patent Document 1 mentioned above has problems with the methods disclosed in Patent Documents 2 and 3, that is, the temperature of the food increases more than necessary, and if a frequency of 3 MHz or less is used, the food may be wrapped in a plastic film, or the food may be wrapped in a plastic film. This solves the problem that food contained in containers (pouched food) cannot be sterilized from the outside.

However, in Patent Document 1, an alternating current in a short wave range is applied between electrodes arranged coaxially. In the case of this configuration, not only the food (pouched food) placed in the water tank but also the water in the water tank is heated at the same time. Heating the water in the container requires extra energy and does not shorten the processing time.
Furthermore, the same problem as above occurs even when a pair of parallel plate electrodes are arranged along the inner wall of the container.

On the other hand, if the amount of water in the container (aquarium) is reduced, uniform heating cannot be achieved, and therefore more energy is required because more than a certain amount of water is put in and circulated.

In order to solve the above problems, the underwater shortwave heating method according to the present invention sets the food to be processed in a processing container filled with water, brings electrodes into close contact with both end surfaces of the food, and in this state, between the electrodes. The configuration is such that shortwave band alternating current is applied to the Here, a state in which water or microbubbles inevitably enter between the food and the electrode is also considered to be close contact.

In the above, when heat-treating multiple foods at the same time, the multiple foods are stacked, and the electrodes are closely attached to both end surfaces of the stacked foods, and a conductive plate (metal plate) is tightly attached between the stacked foods. Short wave band alternating current is applied between the electrodes in the arranged state.

It is preferable that the shape of the electrode and the conductive plate be the same as the shape of the end surface of the food to be processed, or a shape that can completely cover the end surface. If the electrodes (conducting plates) are extremely small compared to the shape of the end surface of the food, the temperature of the portion of the food sandwiched between the electrodes will be high, making it impossible to heat uniformly.

Since the food and the electrode are in close contact, there is no water between the food and the electrode, and the food is heated directly. Conventionally, the water between the food and the electrode was heated, but this method heats the food directly. Heating is not required and power consumption is reduced. Furthermore, since the food and the electrode are in direct contact, the temperature can be raised to the required temperature in a short time.

Water enters the gap where the food and electrode are not in perfect contact with each other, so the food can be heated evenly as if there were no gap.

Furthermore, according to the present invention, even when a plurality of foods are processed at the same time, uniform heat treatment can be performed without variations in the heating of each food.

In addition, the size or shape of the processing container should be selected so that the gap between the food stored in the processing container and the inner surface of the processing container is small, and the impedance of water around the food should be made larger than the impedance of the food. , the amount of alternating current flowing through the water existing in the gap is reduced, and power consumption can be reduced.

Further, by heating under pressure, it is possible to heat the material to a temperature of 100° C. or more in a short time. For example, in retort heat treatment, the center of the food is required to be maintained at 121° C. for 4 minutes or more. This condition can be met in a short time by underwater shortwave heating under pressure.
Specifically, since conventional retort heating takes about one hour to heat, the physical properties and flavor of fish and meat deteriorate significantly during that time. For this reason, foods such as retort curry that compensate for the deterioration of flavor with large amounts of spices are on the market, but bland foods with less spices heated in retorts are not commercially available. Also, when sausages and kamaboko fish are heated in a retort, the gel softens, so items heated at low temperatures have a shelf life of 2 to 3 weeks when chilled.
However, according to the present invention, the heating time above 100°C can be shortened, so it is possible to produce high-quality processed foods that can be stored at room temperature for a long period of time, like retort foods, and that are less susceptible to thermal deterioration. Sausages and kamaboko fish, which are sold in the chilled section of Japan and have a best-before date of two to three weeks, can now be sold on shelves at room temperature, and can be exported overseas.

(a) is a vertical cross-sectional view of a heating device in which a comparative example was implemented, and (b) is an equivalent circuit of the same heating device. Graph showing heating time and temperature changes of water and food when performing RF heating using the heating device in Figure 1 Graph showing heating time and temperature changes of water and food when hot water is heated using the heating device in Figure 1 (a) is a vertical cross-sectional view of a heating device that implements the underwater shortwave band heating method according to the present invention, and (b) is an equivalent circuit of the same heating device. Graph showing heating time and temperature changes of water and food when performing RF heating using the heating device in Figure 4 Longitudinal cross-sectional view of a pressurizing and heating device according to another embodiment Graph showing heating time and temperature changes of water and food when performing RF heating using the pressure heating device in Figure 6

Comparative examples and preferred embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1(a) is a longitudinal cross-sectional view of a heating device according to a comparative example, and FIG. 1(b) shows an equivalent circuit of the same heating device. However, in the case of a frequency of 27 MHz, the impedance of the plastic packaging material is smaller than the impedance of the food product, so it was ignored.
The heating device arranges parallel plate electrodes 3, 3 connected to a short wave band high frequency power source 2 in a processing container (water tank) 1, and allows food 4 to be suspended between these parallel plate electrodes 3, 3. In order to maintain the filled water at a constant temperature, the water in the container 1 is circulated via a pump 5 and a heat exchanger 6. Further, an optical fiber thermometer 7 is inserted into the food 4 to measure the internal temperature.

The dimensions of the device are as follows.
Inner dimensions of container (water tank): length 150cm x width 50cm x height 200cm
Food: A pouch containing 500 g of potato salad packed in a plastic packaging material with inner dimensions of 140 cm x 140 cm. Food Electrode: 150 cm x 200 cm copper plate Applied high frequency: 27 MHz x 2 kW
Water temperature: 80℃

FIG. 2 shows the results of heating food using the above heating device. Further, FIG. 3 shows the results of heating the food to 75° C. using the above-mentioned heating device and using only the temperature of the circulating water (external heating) without applying high frequency.

Comparing Figures 2 and 3, when heating food externally using hot water, it took 2000 seconds for the internal temperature of the food to reach 75°C, but when high frequency was applied, it took 2000 seconds to reach 75°C. The time it takes to reach this point was reduced to 180 seconds. The energy consumed during this period was 732W.

On the other hand, when high frequency was applied, the water temperature in the container (water tank) exceeded 80°C and rose to 97°C. This water temperature increase is the result of applying high frequency. In other words, in this comparative example, energy is wasted in order to raise the temperature of the water in the container.

FIG. 4(a) is a longitudinal cross-sectional view of a heating device that implements the method of the present invention, and FIG. 4(b) shows an equivalent circuit of the same heating device. The embodiment shown in FIG. 4 shows an example in which a plurality of foods 4 are heat-treated at the same time.

The dimensions of the heating device are as follows.
Inner dimensions of the container (water tank): Length 150cm x Width 150cm x Height 250cm
Food: 250g of potato salad packed in a plastic packaging material with inner dimensions of 140cm x 150cm Food Electrode: 130cm x 140cm copper plate Conductor plate: 130cm x 130cm copper plate Applied high frequency: 27MHz x 2kW
Water temperature: 80℃

In the method of the present invention, a plate-shaped electrode 3 is fixed to the bottom of the container 1, one food item 4 is placed on top of this electrode 3, a conductive plate 8 is placed on top of this food item 4, and this is repeated. A plurality of (four in the illustrated example) foods 4 are stacked one on top of the other with conductive plates 8 interposed therebetween, and electrodes 3 are placed on the upper end surfaces of the stacked foods.
The shape of the electrode 3 and the conductive plate 8 is preferably the same as the surface on which the food is stacked, or a shape close to this.

In order to bring the food 4 into close contact with the electrode 3 or the conductive plate 8, it is preferable to place a weight (approximately 200 g) on top of the electrode 3 at the upper end or press it down with an equivalent force.

FIG. 5 shows the results of heating food using the above heating device. The temperature of the second and fourth foods from the bottom was measured using an optical fiber thermometer.
From FIG. 5, results were obtained in which a plurality of foods follow substantially the same temperature increase process. This is because the four foods were connected in series, so an equal current flowed through each food. Incidentally, the temperature difference between the four foods was within 3°C. When connected in parallel, more current flows through the higher temperature food, increasing the temperature difference between each food.

In the case of the example, it took 250 seconds for the center temperature of the food (potato salad: 1000 g) to reach 75°C. If the specific heat of potato salad is 1.0, then 1172W of energy was used.

On the other hand, in the comparative example, it took 180 seconds for 500g of potato salad to reach 75°C.A simple calculation shows that in the case of the example, since it is 1000g, it should take 360 seconds (180 seconds x 2), but in reality Since four pouches of food are stacked and processed simultaneously, power is effectively consumed and the processing time per food is shortened.

Furthermore, it can be seen from FIG. 5 that the temperature of the food is higher than the temperature of the water. This reduces the gap between the inner surface of the container and the food, reducing the amount of water that can enter the gap, so the impedance of the water is larger than that of potato salad, and the current flowing through the water is smaller. It is thought that as a result, the water generated less heat and the temperature of the water did not exceed 80°C.

In the illustrated example, four pouched foods are stacked vertically and heated at the same time, but the number of foods that can be treated at the same time is arbitrary, and the present invention is also applicable to the case where one food is heated. can. Furthermore, although the illustrated example shows an example in which the food is stacked vertically, if the food is placed upright and stacked horizontally, a temperature difference will occur between the top and bottom of the food. For this reason, it is preferable to heat-process the foods by laying them out and stacking them vertically.

FIG. 6 is a longitudinal cross-sectional view of a pressure heating device according to another embodiment, and this embodiment has a structure in which RF heating is performed under pressure.
That is, a pressure vessel 11 with a lid is placed outside the processing vessel 1, and a pressure gauge 12 is attached to the lid of the pressure vessel 11. Further, water is stored in the processing container 1, and a drain pipe 13 for draining this water is connected to the bottom surface of the processing container 1.

A circulation pipe 14 is led out from the bottom of the processing container 1, passes through the outside of the pressure container 11, penetrates the lid, and has a tip located above the processing container 1. is installed.

A circulation pump 5 and a heat exchanger 6 are provided in the middle of the circulation pipe 13, and the heat exchanger 6 increases the water temperature by supplying temperature-controlled steam from the outside, and sends the water with this increased temperature to the shower head 14. The water is supplied into the processing container 1 from the inside. The water temperature in the processing container 1 is measured by an optical fiber thermometer 16.

A plate-shaped electrode 3 (non-grounded electrode) is arranged at a position insulated from the bottom of the processing container 1, and a plate-shaped electrode 3 (grounded electrode) is arranged near the water surface, and food is placed between these plate-shaped electrodes 3 through a copper plate 7. They are arranged in four tiers. Here, the plate-shaped electrode 3 (grounded electrode and non-grounded electrode) is connected to the short wave band high frequency power source 2 via a matching box.

A specific example using the above pressure heating device will be described below.
As food, minced pork, tapioca flour, salt, and pepper were kneaded and stuffed into sheep intestines to produce sausages weighing approximately 50 g each. Four sausages (approximately 200 g) were packaged in vacuum pouches and used as samples.

A stack of four pouch-packed samples with a copper plate sandwiched between each was sandwiched between the lower plate-shaped electrode 3 (non-grounded electrode) and the upper plate-shaped electrode 3 (grounded electrode) in the processing container 1, and a 8kW Short wave band alternating current was applied.

Figure 7 shows the results of applying 8kW shortwave AC. Figure 7 shows the temperature history of the surrounding water temperature and the center of the food (sausage) during heating.As a result of applying the short wave band for 500 seconds from 560 seconds to 1060, the temperature at the center of the sausage ranged from 16℃ to 130℃. The temperature rose to On the other hand, the water temperature around the pouch was heated only in the short wave band from 560 seconds to 960 seconds, and after 960 seconds, heat exchange water using 120° C. steam was also used to heat the water.

As is clear from FIG. 7, it can be seen that the temperature of the food becomes higher than the temperature of the water after about 700 seconds have passed from the start of the process. This is because, as mentioned above, the gap between the inner surface of the container and the food is made smaller to reduce the amount of water that enters the gap, so the impedance of the water is larger than the impedance of the food and the water flows. It is thought that as a result of the decrease in current, the water generated less heat, and more current flowed through the food, causing the temperature at the center of the food to exceed the temperature of the water.

The food heat treatment method according to the present invention is applicable not only to potato salad and sausage, but also to heat treatment of foods wrapped in plastic films such as miso and tofu, and foods packed in plastic containers. Since the same level of heat treatment as retort heating can be performed in a shorter time, it is expected that retort heating time will be shortened and retort food will be of higher quality.

DESCRIPTION OF SYMBOLS 1... Processing container, 2... Short wave band high frequency power supply, 3... Electrode, 4... Food, 5... Pump, 6... Heat exchanger, 7... Conductive plate (copper plate), 11... Pressure vessel, 12... Pressure gauge, 13... Drain pipe, 14...Circulation pipe, 15...Shower head, 16...Optical fiber thermometer.

Claims (3)

A packaging material containing food to be processed is set in a processing container filled with water, and air bubbles and water that inevitably enter the both ends of the packaging material are removed, and the electrodes are brought into close contact with the packaging material . An underwater short-wave band heating method characterized by applying short-wave band alternating current between electrodes so that the temperature of the food becomes higher than the water temperature in the processing container when the temperature of the food is heated to a maximum value. 2. The underwater shortwave heating method according to claim 1, wherein the object to be treated is a plurality of packaging materials wrapping a plurality of separated foods, and the plurality of packaging materials are stacked with conductive plates arranged in close contact with each other in the vertical direction, An underwater shortwave band heating method characterized by bringing the electrodes into close contact with both end faces of the stacked packaging materials , and applying shortwave band alternating current between the electrodes in this state. 2. The underwater shortwave heating method according to claim 1, wherein the processing vessel filled with water is installed in a pressure vessel.

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