Food frozen preservation method

Abstract

To provide a food frozen preservation method by which ice crystallization is made free from coarsening inside target food during freezing, with no cells and organizations destroyed, by which food can be frozen preserved (cold storage) for an extended period and, when the frozen preserved food is thawed, the shape of the food, texture and taste are maintained, with generation of drip lessened, and by which the target food can be circulated through one year in the same quality as fresh food harvested in the seasonal period and as processed food shortly after manufacture.SOLUTION: A food frozen preservation method includes an over-refrigeration state maintenance process by which target food is maintained under over-refrigeration state of -10 to 0°C and a refrigeration process by which the food after the over-refrigeration state maintenance process is frozen by -100 to -10°C.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for freezing and preserving food, which enables the target food to maintain its shape, not lose its texture, and produce little dripping even when thawed after being frozen (frozen) preserved, and thus to provide the target food at high quality throughout the year. [Background technology]

[0002] Traditionally, fruits and vegetables, as well as seafood such as sea urchins, each had their own seasonal period, and the period during which they were available on the market was limited, leading to price fluctuations throughout the year. Processed foods with a water content of 70% or more, such as tofu, also have a short expiration date, and foods that have passed their expiration date are disposed of.

[0003] Freezing is a method for maintaining the freshness and expiration date of food for a long period of time, but when freezing food, the water inside the food freezes and the ice crystals become coarse. This destroys the cells of fresh foods and the tissues of processed foods such as tofu, causing drips to leak out after thawing, resulting in problems of deterioration in quality such as deterioration in texture, taste, color, and hardness.

[0004] For example, in fruits such as strawberries and figs, whose water content exceeds 80%, the water in the cells turns into large ice crystals when frozen, destroying the cell tissue, and when thawed, the delicious components in the cells drip out together with the water, causing the deliciousness of the raw fruit to be lost, and furthermore, the original hardness (crunchiness, etc.) and shape of the fruit to be damaged. Therefore, frozen strawberries and figs available on the market are only eaten in a semi-thawed state, or used as ingredients in smoothies and jams. In addition, sweets such as frozen strawberry shortcake and frozen fig tart are made by coating the surface of the fruit with sugar, etc., and are far from sweets made with fresh strawberries or figs.

[0005] In the case of sea urchins, whose ovaries are eaten (water content 74%), the ovary cells are destroyed by ice crystals during freezing, and when thawed, they become paste-like and inedible, so in reality, they are soaked in alum or sealed in salt water to extend their expiration date after harvest to about 1 to 2 weeks.

[0006] Tofu types with a smooth texture made by solidifying soy milk, such as firm tofu, silken tofu, and deep-fried tofu, generally have a short shelf life, and even if they are frozen to extend their shelf life, the water in the tofu tissue turns into large ice crystals when frozen, and when thawed, the water in these ice crystals melts and the tissue becomes spongy (Kouya tofu is intentionally frozen and preserved, and this is not a deterioration in quality), and as a result the smooth texture of the tofu before it was frozen cannot be obtained, frozen tofu is not in circulation.

[0007] To prevent such quality deterioration, a method is known in which the time that food remains in the maximum ice crystal formation temperature range (approximately -5 to 0°C) during freezing is reduced to within 30 minutes and the coarsening (growth) of ice crystals (ice crystals, ice crystals) is prevented, for example, by quick freezing, in which cold air of -100 to -30°C is blown onto the target food. However, while quick freezing rapidly freezes water near the surface of the food where the cold air hits, preventing the ice crystals from coarsening, water inside the food freezes more slowly than near the surface due to the delayed heat conduction, and therefore it has not been possible to sufficiently prevent the quality deterioration of food caused by cell and tissue destruction associated with the coarsening of ice crystals.

[0008] In response to this, Patent Document 1 proposes a food freezing and preserving method for various food ingredients such as vegetables, fruits, seafood, and eggs, which allows the food to retain the same texture as before freezing when thawed after freezing. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2007-53969 A Summary of the Invention [Problem to be solved by the invention]

[0010] However, the food freezing and preservation method of Patent Document 1 includes a step of soaking food materials in an aqueous solution containing salt, sugars, and calcium salts, and a step of freezing the food materials soaked in the aqueous solution through a temperature range of 0° C. to -5° C. within 15 minutes until the temperature reaches a range of -20° C. or lower. Therefore, there are problems in that it is time-consuming to store the aqueous solution in which the target food materials are soaked, making it unsuitable for mass production, and in that after thawing, the flavor caused by the aqueous solution remains, impairing the original flavor of the food materials.

[0011] The present invention has been made in consideration of the above circumstances, and aims to provide a method for freezing and preserving food, which enables food to be frozen and preserved for long periods of time without causing ice crystals to coarsen inside the food during freezing, and without destroying cells or tissues, and which enables the food to be distributed throughout the year with the same quality as fresh foods harvested in season or recently produced processed foods, with the shape, texture and taste of the food being maintained when the frozen food is thawed, with minimal dripping or the like. [Means for solving the problem]

[0012] The food freezing and preserving method of the present invention, which meets the above-mentioned objectives, comprises a supercooled state maintaining step in which a target food is maintained in a supercooled state at -10 to 0°C, and a freezing step in which the food after the supercooled state maintaining step is frozen at -100 to -10°C.

[0013] In the food freezing and preserving method of the present invention, the freezing process may include a quick freezing process in which the food is quickly frozen and cooled to -100 to -40°C, and a normal freezing process in which the food is kept at -25 to -10°C after the quick freezing process. Here, quick freezing is a freezing (freezing) method in which the food passes through the maximum ice crystal formation temperature range of -5 to 0°C, where the moisture in the food turns into ice crystals, within 30 minutes. For this purpose, for example, it is preferable to blow cold air (cold wind) of -100 to -30°C onto the target food, but this is not limited thereto, and the food may be frozen using low-temperature alcohol or nitrogen gas, etc. The temperature of the food after the quick freezing process is appropriately selected, but it is preferable to keep the food at -25 to -10°C (frozen storage) by the normal freezing process, and it is particularly preferable to keep the food at -18°C or lower, where bacterial activity stops. In addition, the lower the temperature in the normal freezing process, the longer the period during which the quality of the food can be maintained.

[0014] In the food freezing and preserving method according to the present invention, it is preferable that the supercooled state maintaining step is carried out for at least 30 minutes or more, and an electric field is formed in the preservation chamber in which the food is stored. Here, examples of the electric field generating means for generating an electric field within the storage chamber include a pulsed static electricity generating means of a specific frequency and a high-voltage electrostatic induction generating means.

[0015] In the food freezing and preserving method according to the present invention, it is further preferable that the quick-freezing step is carried out at a timing when the temperature of the food starts to rise during the supercooled state maintaining step. Here, the rise in temperature of the food during the supercooled state maintenance process is due to sensible heat generated when the supercooled water (liquid) in the food undergoes a phase transition to ice (solid). The temperature at which the food is maintained in a supercooled state is lower than the freezing point of the food, and depending on the food, it can be lower than -5°C, and may even reach -10°C or even lower.

[0016] In the food freezing and preserving method according to the present invention, it is preferable to have a slow cooling step, prior to the supercooled state maintaining step, in which the food is cooled from a room temperature state to the supercooled state over a period of 0.5 to 3 hours. Here, room temperature refers to the temperature range at which the cells and tissues of the food are not thermally denatured, and the maximum temperature varies depending on the food. For example, for strawberries, it is about 40°C.

[0017] In the food freezing and preserving method according to the present invention, an electric field may be formed in the preservation chamber in which the food is accommodated during the slow cooling process.

[0018] In the food freezing and preserving method of the present invention, it is further preferable that the food is a dehydrated food that has been previously irradiated with microwaves under reduced pressure so that 5 to 35% of the total water content contained in the food in the initial state has been dehydrated at room temperature.

[0019] In the food freezing and preserving method according to the present invention, it is preferable that the food is in a low vacuum state of 2 to 10 kPa under the reduced pressure. In the food freezing and preserving method according to the present invention, the food dehydrating device used to dehydrate the food in advance is preferably one that includes a vacuum chamber for accommodating the food, a vacuum pump connected to the vacuum chamber for reducing the pressure inside the vacuum chamber, a microwave generator attached to the vacuum chamber for irradiating the food inside the vacuum chamber with microwaves, and a structure for introducing a small amount of a gas that does not oxidize the food, such as outside air, nitrogen gas, or argon gas, into the vacuum chamber for the purpose of releasing the steam generated during dehydration to the outside, and discharging the steam together with the gas to the outside of the chamber. It is also preferable that the food dehydrating device includes a table disposed in the vacuum chamber opposite the microwave generator on which the food is placed, and a rotary drive unit for rotating the table about a vertical axis of rotation, so that the food is evenly irradiated with microwaves. Effect of the Invention

[0020] According to the food freezing and preservation method of the present invention, food maintained in a supercooled state at -10 to 0°C is then frozen at -100 to -10°C, accelerating the rate at which the food changes from a supercooled state to a frozen state and shortening the time the food remains in the maximum ice crystal formation temperature range. As a result, ice crystals in the food are prevented from becoming coarse, drips that occur when thawing after frozen storage are reduced, and the quality of the food (color, taste, shape, and texture) is maintained, making it possible to provide frozen foods of the same quality as in-season foods or foods that have just been produced throughout the year.

[0021] In the food freezing and preservation method of the present invention, when the freezing process includes a quick freezing process in which the food is quickly frozen and cooled to -100 to -40°C, and a normal freezing process in which the food is kept at -25 to -10°C after the quick freezing process, the quality of the food can be maintained at -25 to -10°C for a long period of time while shortening the time that the food is exposed to low temperatures of -30°C or below.

[0022] In the food freezing and preservation method of the present invention, when the supercooled state maintaining step is carried out for at least 30 minutes or more and an electric field is formed in the preservation chamber in which the food is stored, the water molecules throughout the food vibrate finely and become supercooled water in which the water molecules are broken down, shortening the time until moving to the subsequent quick-freezing step and causing the supercooled water to freeze rapidly and become fine ice crystals, thereby suppressing cell destruction due to the growth of ice crystals and preventing food deterioration and alteration, improving the quality preservation effect of frozen preservation and maintaining the original quality of the food even after frozen preservation.

[0023] In the food freezing and preservation method of the present invention, when a quick-freezing process is carried out at a time when the temperature of the food begins to rise during the supercooled state maintenance process, the supercooled water in the entire food rapidly passes through the maximum ice crystal formation zone and freezes all at once, and the ice crystallizes finely, suppressing the formation of coarse ice crystals, preventing damage to the cells of fresh foods and the tissues of processed foods that would be caused by the formation of coarse ice crystals, and maintaining the original quality of the food.

[0024] In the food freezing and preserving method of the present invention, if there is a slow cooling step in which the food is cooled from room temperature to a supercooled state over a period of 0.5 to 3 hours prior to the supercooled state maintaining step, the temperature of the food becomes approximately uniform from the outer surface to the inside, and then the supercooled state occurs and stabilizes throughout the entire food, so that the supercooled water throughout the entire food freezes all at once, preventing the formation of coarse ice crystals.

[0025] In the food freezing and preserving method of the present invention, when an electric field is formed in the storage chamber in which the food is contained during the slow cooling process, the electric field causes the water molecules in the food to vibrate finely, thereby breaking down the water molecule clusters, thereby preventing the ice crystals from coarsening in the subsequent quick freezing process, and significantly improving the quality maintenance effect in frozen food preservation.

[0026] In the food freezing and preservation method of the present invention, if the food is a dehydrated food in which 5 to 35% of the total moisture contained in the food at an initial state has been dehydrated by previously irradiating it with microwaves under reduced pressure, the moisture inside (inside) of the food will move toward the surface of the food due to microwave irradiation, making the moisture content inside less than the outside (surface). In the quick-freezing process, the outer surface (outside) of the food will come into contact with the cold air and cool first, while the inside will cool gradually due to thermal conduction, so that the time spent in the maximum ice crystal formation temperature range will be longer. Even if the ice crystals grow large and expand in volume, destruction of cell membranes and tissues will be prevented inside the food with reduced moisture content, and drips that occur when thawing after frozen storage will be significantly reduced, there will be no thermal denaturation associated with dehydration, and the quality (color, taste, shape and texture) of the food will be well maintained.

[0027] In the food freezing and preservation method of the present invention, when food is in a low vacuum state of 2 to 10 kPa under reduced pressure, water evaporates (vaporizes) and is dehydrated at room temperature of 40°C or less, so that water can be efficiently dehydrated from the food without causing thermal deformation as occurs with the hot air drying method. [Brief description of the drawings]

[0028] [Figure 1]1 is a schematic cross-sectional view showing a food dehydration device used to pre-dehydrate target foods in the food freezing and preserving method of the present invention. FIG. [Diagram 2] FIG. 2 is a schematic cross-sectional view showing a preservation device in the food freezing and preservation method. [Diagram 3] 2 is a graph showing changes in food temperature during the food freezing and preservation method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Next, with reference to the attached drawings, an embodiment of the present invention will be described for understanding the present invention. In the food freezing and preserving method according to the present invention, a food dehydrating apparatus 10 shown in FIG. 1 is used to dehydrate food 11 to be subjected to pretreatment at room temperature. Food dehydration apparatus 10 includes a vacuum chamber 12 that contains food 11, a vacuum pump 13 that is connected to vacuum chamber 12 and reduces the pressure inside vacuum chamber 12, and a microwave transmitter 14 that is attached to vacuum chamber 12 and irradiates food 11 inside vacuum chamber 12 with microwaves. Food dehydration apparatus 10 also includes a table 15 that is disposed in vacuum chamber 12 facing microwave transmitter 14 and on which food 11 is placed, and a rotation drive unit 17 such as a motor that rotates table 15 around vertical rotation axis 16. As a result, food 11 can be irradiated with microwaves under reduced pressure to dehydrate it.

[0030] In the food freezing and preserving method according to the present invention, first, a depressurization step is carried out in which food 11 to be processed is placed in vacuum chamber 12 and the inside of vacuum chamber 12 is depressurized. Then, a dehydration step is carried out in which food 11 in vacuum chamber 12 is irradiated with microwaves under reduced pressure to dehydrate 5 to 35% (preferably 5 to 25%) of the total water contained in food 11 in the initial state. Here, the ratio of the amount of water dehydrated in the dehydration step to the total amount of water contained in food 11 in the initial state is called the dehydration rate. Therefore, the dehydration rate in the dehydration step is 5 to 35%. Note that the dehydration rate in the dehydration step is not limited to the above range and may be appropriately selected depending on the type of food 11.

[0031] It is preferable that the inside of the vacuum chamber 12 during the decompression process is in a low vacuum state of 2 to 10 kPa, but the pressure inside the vacuum chamber 12 during the decompression process is not limited to the above range and is selected appropriately depending on the type and amount of food 11, the microwave strength and irradiation time, the dehydration rate in the dehydration process (final dehydration rate after the dehydration process), etc. Furthermore, in order to prevent the internal temperature of food 11 from exceeding room temperature and thermally denaturing the internal tissue, which would result in damage, the surface temperature of food 11 during the decompression process and dehydration process is preferably 20 to 40°C at which thermal denaturation does not occur. Note that the surface temperature of food 11 during the decompression process and dehydration process is not limited to the above range, and is appropriately selected depending on the type of food 11, the pressure inside vacuum chamber 12 during the decompression process, the microwave intensity and irradiation time, the dehydration rate in the dehydration process (final dehydration rate after the dehydration process), etc.

[0032] Next, in the food freezing and preserving method according to the present invention, a refrigerated freezing and preserving apparatus 20 shown in FIG. 2 is used to carry out a slow cooling step, a supercooled state maintaining step, and a freezing step (a quick freezing step and a normal freezing step). The refrigerated and frozen storage device 20 is capable of switching between refrigeration and freezing, and is provided with a storage chamber 21 for storing food that has been dehydrated in advance (dehydrated food) or food that does not need to be dehydrated (hereinafter collectively referred to as food). As shown in Fig. 3, the food freezing and preservation method according to the present invention includes a slow cooling process in which food is stored in the storage chamber 21 and cooled from room temperature to a supercooled state of -10 to 0°C, a supercooled state maintaining process in which the food after the slow cooling process is maintained in a supercooled state of -10 to 0°C, a quick freezing process in which the food after the supercooled state maintaining process is passed through the maximum ice crystal formation temperature range (-5 to 0°C) in as short a time as possible within approximately 30 minutes, and a normal freezing process in which the food after the quick freezing process is frozen and preserved. The quick freezing process is performed by strongly blowing cold air at a low temperature of, for example, -30°C or less onto the food. In this embodiment, the temperature of the food immediately after the quick freezing process is -60°C, but this can vary within a range of about -100 to -40°C depending on the temperature of the cold air during the quick freezing process and the duration of blowing. The temperature in the normal freezing process is selected appropriately within a range of -25 to -10°C, but since -18°C or lower, at which bacterial activity ceases, is particularly preferable, it is set to -25°C in this embodiment. Since the freezing process is a combination of the quick freezing process and the normal freezing process, the temperature range for the freezing process is -100 to -10°C. The slow cooling process includes a refrigeration process in which the food is maintained at 0 to 5°C.

[0033] The refrigerated / freezer preservation device 20 has a temperature control section (not shown) and can control (set) the temperature inside the preservation chamber 21 within a range of, for example, −100 to 20° C. The slow cooling step is preferably carried out within a range of 0.5 to 3 hours, and the refrigeration step and the supercooled state maintaining step are each preferably carried out within a range of at least 0.5 to 2 hours, but the times required for the slow cooling step, the refrigeration step and the supercooled state maintaining step are not limited to these ranges and are appropriately selected depending on the type and dehydration rate of the food 11 to be frozen and preserved.

[0034] Since the supercooled state is unstable, it is preferable to control the temperature during the supercooled state maintenance step, paying attention to the fact that the preservation chamber 21 is not vibrated, and to start the quick-freezing step once the entire moisture content of the entire food reaches the supercooled temperature, but regardless of the temperature control in the preservation chamber 21, when the moisture content of the entire food reaches the supercooled temperature, there is a risk that, for example, the interface with the cell membrane or tissue will become ice nuclei, ice crystallization will begin, the supercooled state will be eliminated, and the food will freeze into large ice crystals. When supercooled water (liquid) crystallizes (solidifies) (phase transition), the temperature of the supercooled water will rise due to sensible heat, so the temperature of the food in the preservation chamber 21 should be monitored, and the quick-freezing step should be started before point P at which the temperature of the supercooled food starts to rise (when the moisture content of the entire food has reached the supercooled temperature). In this way, by immediately flash-freezing the food in conjunction with eliminating the supercooled state, the food can pass through the maximum ice crystal formation temperature range (approximately -5 to 0°C) in a short period of time and be cooled to -100 to -40°C before the ice nuclei within the food become too large. This reduces the size of the ice crystals, inhibits their growth, limits damage to tissue, and maintains the quality of the food.

[0035] Therefore, if the "preferred supercooling temperature" and the "time (point) at which phase transition occurs" at that supercooling temperature are measured in advance for each food to be frozen and stored, and this data is incorporated into the temperature control unit of the freezer-refrigerator storage device 20, the user can simply select the target food (e.g., dehydrated strawberries (dehydration rate 5%), figs, firm tofu, sea urchin, etc.) from the operation panel, just as one selects a dish menu on a microwave oven, and the conditions (temperature and time) for the supercooling state maintenance process and freezing process (quick freezing process, normal freezing process) appropriate for each target food can be automatically controlled.

[0036] The refrigerated freezer storage device 20 also includes a space potential generator 23 having a static electricity discharge means 22 installed inside the storage chamber 21 and forming an electric field inside the storage chamber 21. This space potential generator 23 can form a high-voltage electric field throughout the storage chamber 21 by discharging static electricity from the static electricity discharge means 22. The refrigerated freezer storage device 20 may be a dedicated product in which the space potential generator 23 is built in advance for this food freezing and preservation method, or may be an existing freezer-refrigerator that can be switched between freezing and refrigerating operations and to which the space potential generator 23 is later attached. The specific configuration of the space potential generator 23 is preferably one that has been publicly known in the art (see, for example, Japanese Patent No. 5683032), but is not limited thereto, and may be any device that can form an electric field inside the storage chamber 21, such as a pulsed static electricity generating means of a specific frequency and a high-voltage electrostatic induction generating means.

[0037] In an electric field (pulse electric field), the binding of water molecules is suppressed by the effect of micro-vibrations of electrons, and the clusters of water molecules contained in the food are thought to be broken down and activated, resulting in finer and more homogenous ice crystals. Therefore, an electric field is formed in the preservation chamber 21 at a temperature lower than the freezing temperature of the target food, and vibrations are applied to the water molecules, so that the clusters of water molecules are broken down and the generation of ice crystals is suppressed, and a good supercooled state is maintained. Of the slow cooling process, the supercooled state maintaining process, and the freezing process, it is considered necessary to form an electric field in the preservation chamber 21 at least during the supercooled state maintaining process. However, since a stable supercooled state can be obtained by forming an electric field in the preservation chamber 21 before the temperature of the food reaches the supercooled temperature range, it is preferable to form an electric field in the preservation chamber 21 even in the slow cooling process.

[0038] In particular, because the clusters of water molecules are at their largest at 4°C, where the specific gravity of water is at its largest, the formation of an electric field in preservation chamber 21 begins when the food temperature reaches around 4°C and the refrigeration process begins, which is effective in stabilizing the supercooled state. Also, the formation of an electric field in preservation chamber 21 during the normal freezing process after the supercooled state maintenance process suppresses the destruction of cells and tissues due to the growth of ice crystals (recrystallization, refreezing), significantly improving the quality preservation effect of frozen preservation. In addition, as a substance that has the same action and effect as the subdivision of water molecule clusters by forming an electric field, a substance that applies a uniform magnetic flux and electromagnetic waves similar to the conventionally known proton freezing (freezing), a substance that generates a magnetic field similar to CAS freezing, or a substance that generates magnetic resonance (resonance due to high vibration frequency) similar to Nanobeat Water may be used. EXAMPLES

[0039] Examples performed to confirm the effects of the present invention will be described below, but these examples are not intended to limit the technical scope of the present invention in any way.

[0040] The target food was strawberries (moisture content: about 90%, freezing temperature: about -0.8°C), and the pressure in the vacuum chamber 12 during the decompression process was set to 5 kPa to reduce the surface temperature of the strawberries to 40°C or less, and the microwave irradiation time was set to 30 minutes to 3 hours depending on the dehydration rate to perform a dehydration process to dehydrate the whole strawberries, measuring the dehydration rate and storing them under various conditions. For comparison, non-dehydrated strawberries were also stored under various conditions.

[0041] Furthermore, when strawberries were kept in a supercooled state of -3°C in advance, it was confirmed that a point P appeared at which the temperature of the strawberries began to rise, approximately 80 minutes after they were placed in a supercooled state of -3°C.

[0042] Example 1 The strawberries were dehydrated whole to a dehydration rate of 7%, and then cooled from room temperature to a supercooled state of -3°C over a period of 3 hours in a storage chamber 21 where an electric field was formed (slow cooling process with electric field). They were then maintained in the supercooled state of -3°C for 80 minutes (supercooled state maintenance process with electric field), and then flash frozen. They were then stored frozen at -25°C (normal freezing process with electric field), and refrigerated and thawed two days later, and a sensory evaluation was performed. Example 2 The evaluation was carried out in the same manner as in Example 1, except that the dehydration rate was set to 11%.

[0043] Example 3 The evaluation was carried out in the same manner as in Example 1, except that the dehydration rate was set to 14%.

[0044] Example 4 The evaluation was carried out in the same manner as in Example 1, except that the dehydration rate was set to 17%.

[0045] Comparative Example 1 The evaluation was carried out in the same manner as in Example 1, except that dehydration was not performed, and the product was transferred from room temperature to a storage chamber 21 at -25°C where no electric field was formed, and frozen for storage (normal freezing process without electric field).

[0046] Comparative Example 2 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was set to 8%.

[0047] Comparative Example 3 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was set to 15%.

[0048] Example 5 The evaluation was performed in the same manner as in Example 1, except that the dehydration rate was 15%, and the product was cooled over a period of 3 hours from room temperature to a supercooled state of -3°C in a storage chamber 21 where no electric field was formed (slow cooling process without electric field), and then maintained in the supercooled state of -3°C for 80 minutes in a storage chamber 21 where an electric field was formed (supercooled state maintenance process with electric field), before being quickly frozen and stored frozen at -25°C (normal freezing process with electric field).

[0049] The evaluation results of Examples 1 to 4, Comparative Examples 1 to 3, and Example 5 are shown in Table 1. The results of Comparative Examples 1 to 3 show that when strawberries at room temperature are simply transferred into storage chamber 21 and frozen and stored (normal freezing process without an electric field), there is a significant decrease in taste (sweetness, texture, etc.) after thawing, regardless of whether they are dehydrated or not. In contrast, by comparing Comparative Example 3 with Example 5, it was found that even with the same dehydration rate of 15%, compared to when strawberries are quickly frozen from room temperature and then simply frozen and stored at -25°C without an electric field as in Comparative Example 3, the deterioration of taste after thawing is suppressed by cooling strawberries from room temperature to a supercooled state of -3°C over a period of 3 hours (slow cooling process without electric field) as in Example 5, and then maintaining them in a supercooled state of -3°C for 80 minutes in the storage chamber 21 where an electric field is formed (supercooled state maintenance process with electric field) before being quickly frozen and frozen and stored at -25°C (normal freezing process with electric field).

[0050] Furthermore, by comparing Example 3 and Example 5, it was found that even with the same dehydration rate, the hardness, color, drip, and taste of the food after thawing are all improved by cooling strawberries in a storage chamber 21 where an electric field is not formed from a room temperature state to a supercooled state of -3°C over a period of 3 hours (slow cooling process without electric field), then maintaining the supercooled state at -3°C for 80 minutes in a storage chamber 21 where an electric field is formed (supercooled state maintenance process with electric field), and then freezing and storing them at -25°C (normal freezing process with electric field) as in Example 5. This confirmed that carrying out the slow cooling step with an electric field before the supercooled state maintaining step with an electric field and the frozen storage step with an electric field is more effective in maintaining quality.

[0051] From the results of Comparative Examples 1 to 3 and Examples 1 to 4, it was found that compared to simply transferring strawberries at room temperature into storage chamber 21 at -25°C and storing them frozen as in Comparative Examples 1 to 3, by cooling strawberries from room temperature to a supercooled state of -3°C over a period of 3 hours in storage chamber 21 where an electric field is formed (slow cooling process with electric field), then maintaining them in the supercooled state at -3°C for 80 minutes (supercooled state maintenance process with electric field) before quickly freezing them and storing them frozen at -25°C (normal freezing process with electric field), the deterioration of flavor after thawing is significantly suppressed, the color of the halved surface (cross section) of the strawberries is maintained, and dripping is also greatly reduced. In particular, a comparison between Comparative Example 2 and Example 1, and a comparison between Comparative Example 3 and Examples 3-4 confirmed that even for strawberries (dehydrated foods) that had been dehydrated to the same degree of dehydration in advance, the slow cooling process with an electric field, the process of maintaining a supercooled state with an electric field, the quick freezing process, and the normal freezing process with an electric field at -25°C were effective in improving the taste, color, and dripping after thawing. Furthermore, the results of Examples 1-4 showed that there was a tendency for the taste (mainly sweetness) to improve with an increase in the dehydration rate.

[0052] [Table 1]

[0053] The foodstuff used in the study was raw sea urchin (moisture content: approximately 74%), which was frozen and stored under various conditions and then evaluated.

[0054] Example 6 Using the above-mentioned food dehydration apparatus 10, the pressure inside vacuum chamber 12 during the decompression process was set to 2 kPa, and the surface temperature of the sea urchin was reduced to below 30°C. Five raw sea urchin abdomens were dehydrated at room temperature until the dehydration rate reached approximately 10%, after which they were maintained in a supercooled state of approximately 0°C from room temperature for approximately four hours in storage chamber 21 where an electric field was formed (supercooled state maintenance process with electric field), and then flash frozen and stored frozen at -25°C (normal freezing process with electric field). After five days, chilled thawing was started, and the appearance of the sea urchins and the presence or absence of dripping were checked three hours after the start, and similar checks were made 19 hours later, along with a sensory evaluation.

[0055] Example 7 The sensory evaluation was carried out in the same manner as in Example 6, except that the dehydration treatment was not carried out.

[0056] As a result, 3 hours after the start of chilling and thawing, the sea urchin in both Examples 6 and 7 did not become paste-like, the shape of the sea urchin was maintained, and no dripping was observed. Furthermore, even after 19 hours had passed, no change in shape or dripping occurred.

[0057] When tasted, it was found that both Examples 6 and 7 maintained the taste of raw sea urchin, had a good texture, and were comparable to real raw sea urchin. Moreover, Example 6, which had been subjected to a dehydration treatment, was superior to Example 7 in terms of improved flavor intensity and grainy texture on the tongue.

[0058] The food under test was deep-fried tofu (moisture content: approximately 76%), which was frozen and stored under various conditions and evaluated.

[0059] Example 8 A block of commercially available deep-fried tofu was taken out of the refrigerator's refrigeration chamber (refrigeration temperature: about 5°C. Hereinafter, the refrigerator's refrigeration chamber will be referred to as the "refrigeration chamber") and cut vertically into four equal pieces. Two of the cut pieces were gradually cooled to a supercooled state of about -8°C (supercooled state maintenance process with electric field) in storage chamber 21 where an electric field was formed, and the supercooled state was maintained for about one hour. After that, the electric field was stopped and the tofu was flash-frozen and stored frozen at -30°C without an electric field (normal freezing process without an electric field). Thawing was started 24 hours later, and the deep-fried tofu that had reached about 0°C four hours later was evaluated.

[0060] Comparative Example 4 The remaining two slices of deep-fried tofu were stored in the refrigerator as they were and evaluated at the same time as in Example 8.

[0061] Visual inspection of the thawed deep-fried tofu revealed no cavities in either Example 8 or Comparative Example 4, and there was no difference in color between the two. Tasting revealed no difference in texture or taste between the two.

[0062] The foodstuff studied was firm tofu (moisture content approximately 87%), which was frozen and stored under various conditions and evaluated.

[0063] Example 9 A block of commercially available firm tofu was removed from the refrigerator (5°C) and cut lengthwise into approximately 6 equal pieces. It was then dehydrated at room temperature using a microwave vacuum dehydrator to a dehydration rate of 15%, wrapped in plastic wrap and transferred to storage chamber 21 at approximately 0°C to be slowly cooled. Two hours later, an electric field was formed in storage chamber 21 and the tofu was further gradually cooled to a supercooled state of approximately -8°C. This supercooled state was maintained for approximately 4 hours, after which the electric field was stopped and the tofu was flash-frozen and stored frozen at approximately -30°C for approximately 1 hour. After thawing, the firm tofu had no discoloration, had almost no voids, and was resistant to losing its shape. It also had a smooth texture, not dry, like regular firm tofu, and was comparable in taste to regular firm tofu.

[0064] Example 10 The evaluation was carried out in the same manner as in Example 9, except that the dehydration rate was set to 30%. After thawing, the firm tofu showed no discoloration, had a few small voids but these were not noticeable, and was resistant to losing its shape. The texture was slightly dry, but it was no different from regular firm tofu, and the taste was comparable to regular firm tofu.

[0065] Comparative Example 5 The firm tofu was taken out of the refrigerator (refrigeration temperature: about 5°C) and, without being dehydrated, gradually cooled to a supercooled state of about -8°C (supercooled state maintenance process with electric field) in the storage chamber 21 where an electric field was formed, and after maintaining the supercooled state for about 4 hours, the electric field was stopped and the tofu was flash-frozen and stored frozen at about -30°C for about 1 hour (normal freezing process without electric field).The tofu was then irradiated with a pulsed electric field for 48 hours in the storage chamber 21 to return to about 0°C, and then tasted. After thawing, firm tofu did not easily lose its shape, but it had gaps and felt dry. Although it tasted like tofu, it was found that its quality was too low for commercial sale.

[0066] Comparative Example 6 Evaluation was carried out in the same manner as in Comparative Example 5, except that the temperature in the supercooled state maintaining step was set to about -5°C. After thawing, the firm tofu had air gaps, lost its shape easily, and had no tofu taste, making it very unpalatable.

[0067] According to the food freezing and preservation method of the present invention, the target food is dehydrated in advance to a dehydration rate of 5 to 35%, cooled from room temperature to a supercooled state of -10 to 0°C in a storage chamber in which an electric field is formed over 0.5 to 3 hours, and then maintained in the supercooled state of -10 to 0°C for 0.5 to 2 hours before being normally frozen at -25 to -10°C, thereby reducing drips generated during thawing and maintaining the quality of the food (color, taste, shape, and texture), and making it possible to provide food of equal or higher quality than that during the season throughout the year. In particular, by performing quick freezing before normal freezing, the growth of ice crystals is effectively suppressed, and the quality of the food can be maintained even if normal freezing is then performed for a long period of time (for example, 4 months or more), and almost no drips are generated during thawing, realizing long-term frozen storage and improving the expiration date. Moreover, it is presumed that the food freezing and preservation method of the present invention will condense the flavor of fruits such as strawberries, figs, kiwis, and peaches by the amount of water content reduced by dehydration, making them sweeter than fresh fruits after thawing, with a smoother texture and making them very delicious. Furthermore, it has been found that the sugar content can be controlled by adjusting the dehydration rate of the fruit before freezing and preserving it using the food freezing and preservation method of the present invention, and it is expected that frozen fruits with a higher sugar content than conventional fresh fruits will become more widespread.

[0068] Although the embodiments of the present invention have been described above, the present invention is not limited to the configurations described in the above embodiments, and also includes other embodiments and variations that can be considered within the scope of the matters described in the claims. For example, we have explained that the food freezing and preservation method most effective in maintaining food quality is the application of a slow cooling process with an electric field, a process for maintaining a supercooled state with an electric field, and a normal freezing process with an electric field to dehydrated foods, but the foods in question are not limited to dehydrated foods; the method is also expected to be effective on non-dehydrated foods.

[0069] It is not necessary that all of the slow cooling process, the supercooled state maintaining process and the normal freezing process be carried out in a storage chamber in which an electric field is formed. Depending on the type of food, its purpose and the dehydration rate of the food, the formation of an electric field may be omitted.

[0070] The refrigerated freezing and storage device is not limited to an integrated device (one that performs all of the slow cooling process, supercooled state maintaining process, and freezing process) and may be separated into a dedicated refrigeration device that performs the slow cooling process and the supercooled state maintaining process, and a dedicated freezing device that performs the freezing process at -100 to -10°C, and after the supercooled state maintaining process, the food may be transferred manually or automatically from the dedicated refrigeration device to the dedicated freezing device.

[0071] Foods that can be subjected to the food freezing and preservation method of the present invention may be fruits, seafood, agricultural products such as tomatoes and bamboo shoots, and processed foods, whose taste and texture deteriorate when frozen. In particular, for foods with an initial moisture content (the ratio of the amount of moisture contained in the food to the total weight of the food) of 70% or more, by carrying out a dehydration process before freezing, the surface after thawing is smooth and not dry or gritty, and the quality of the food before freezing is maintained.

[0072] When storing food in the freezer, it is preferable to seal the food in an airtight special bag together with an oxygen absorber so that the food does not come into contact with oxygen, or to evacuate the air inside the special bag in which the food is sealed (air pressure inside the bag: 10 to 50 kPa) before storing the food, as this prevents discoloration of the skin of frozen fruits such as grapes and peaches and discoloration of the flesh exposed to dew. [Explanation of symbols]

[0073] 10: food dehydration device, 11: food, 12: vacuum chamber, 13: vacuum pump, 14: microwave transmitter, 15: table, 16: vertical rotating shaft, 17: rotation drive unit, 20: freezer / refrigerator storage device, 21: storage chamber, 22: static electricity discharge means, 23: space potential generator

Claims

1. A method for freezing and preserving food, comprising a supercooled state maintaining step in which a target food is maintained in a supercooled state at -10 to 0°C, and a freezing step in which the food after the supercooled state maintaining step is frozen at -100 to -10°C.

2. 2. The method for freezing and preserving food according to claim 1, wherein the freezing step comprises a quick freezing step in which the food is quickly frozen and cooled to −100 to −40° C., and a normal freezing step in which the food is kept at −25 to −10° C. after the quick freezing step.

3. 2. The method for freezing and preserving food according to claim 1, wherein the supercooled state maintaining step is carried out for at least 30 minutes, and an electric field is formed in the preservation chamber in which the food is stored.

4. 3. The method for freezing and preserving food according to claim 2, wherein the quick-freezing step is carried out at a timing when the temperature of the food starts to rise during the supercooled state maintaining step.

5. 2. The method for freezing and preserving food according to claim 1, further comprising a slow cooling step in which the food is cooled from a room temperature state to the supercooled state over a period of 0.5 to 3 hours prior to the supercooled state maintaining step.

6. 6. The method for freezing and preserving food according to claim 5, wherein an electric field is formed in the preservation chamber in which the food is contained during the slow cooling process.

7. The method for freezing and preserving food according to any one of claims 1 to 6, characterized in that the food is a dehydrated food in which 5 to 35% of the total water contained in the food in the initial state has been dehydrated at room temperature by previously irradiating the food with microwaves under reduced pressure.

8. 8. The method for freezing and preserving food according to claim 7, wherein the food is in a low vacuum state of 2 to 10 kPa under the reduced pressure.

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