Fruit dehydration method

Abstract

To provide a fruit dehydration method suited for frozen storage, when frozen / preserved fruits are thawed, capable of maintaining the shape and texture of the fruits, reducing dripping, increasing sweetness and keeping the quality of the target fruits throughout one year at a same or better quality as in season.SOLUTION: A fruit dehydration method includes: a decompression step of storing fruits 11 in a vacuum chamber 12 and decompressing the inside of the vacuum chamber 12; and a dehydration step of applying microwave to the fruits 11 in the vacuum chamber 12 under the reduced pressure and dehydrating 10 to 35% of total moisture contained in the fruits 11 in an initial state.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for dehydrating fruit, which can increase the sweetness of the target fruit, and even if the fruit is thawed after being frozen (frozen) stored, the shape of the fruit is maintained, the texture is not impaired, and there is little dripping, making it possible to provide the target fruit with quality equal to or better than that during the season throughout the year. [Background technology]

[0002] Traditionally, various fruits have been cultivated in various parts of Japan and shipped as local specialties. However, depending on the type of fruit, even if it can be harvested in large quantities during the season, it does not last long, and the areas where it can be shipped are limited due to restrictions such as logistics, so there is a problem that consumption does not expand. Therefore, in order to effectively utilize (conserve) surplus fruit, dried fruit processing and freezing processing are performed. However, in dried fruit processing, the cell walls are denatured and hardened because the fruit is dried by applying hot air, so the processed fruit is harder than the unprocessed fruit, has a unique texture, and contains less water, making it a luxury product. In addition, in freezing processing, the water in the cells of the fruit becomes large ice crystals during freezing and destroys the cell walls, so that the umami components in the cells flow out of the fruit as drips when thawed, which impairs the deliciousness of the fruit and makes it lose its chewiness, so the processed fruit is only used for limited purposes such as ingredients for jams, smoothies, etc. Therefore, with the current dried fruit processing and freezing processing, it is not possible to expect an increase in fruit consumption, and the reality is that surplus fruit is discarded.In response to this, Patent Document 1 discloses a cryopreservation method for cells or tissues, which is characterized in that, when cryopreserving cells or tissues, the cells or tissues are first dried at room temperature to an extent that does not cause damage to the cells or tissues, and then frozen and preserved. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2007-289157 A Summary of the Invention [Problem to be solved by the invention]

[0004] Patent Document 1 was granted as a patent for its characteristic that cells or tissues are pre-dried at room temperature to 3-5% of the total mass using microwave vacuum drying. However, in Patent Document 1, sensory tests on taste, odor, texture, and color when pre-dried, frozen, and then thawed for each condition (moisture content) during pre-drying are only conducted for tuna and mackerel. In Patent Document 1, data is presented that dripping can be reduced by reducing the moisture content during pre-drying for raw oysters, strawberries, figs, chicken fillet, and boiled eggs in addition to tuna and mackerel, but pre-drying is only performed up to 15% of the total mass, and no sensory tests are conducted. Therefore, as mentioned above, tuna and mackerel are the only foods for which it has been proven that pre-drying to 3-5% of the total mass and then freezing is good, and the conditions (moisture content) of pre-drying suitable for frozen storage are unknown for foods other than tuna and mackerel. The present invention has been made in consideration of the above circumstances, and aims to provide a method for dehydrating fruit suitable for frozen storage, which, when fruit that has been frozen (frozen) and stored whole without being cut or coated is thawed, maintains the shape and texture of the fruit, reduces dripping, and increases the sweetness of the fruit, thereby enabling the quality of the target fruit to be maintained throughout the year at the same level or higher than that of the fruit in season. [Means for solving the problem]

[0005] The fruit dehydration method according to the present invention, which is in accordance with the above-mentioned object, is a fruit dehydration method for dehydrating a target fruit to a state suitable for frozen storage, comprising: The method includes a depressurization step in which the fruit is placed in a vacuum chamber and the vacuum chamber is depressurized, and a dehydration step in which microwaves are irradiated to the fruit in the vacuum chamber under reduced pressure, thereby dehydrating 5 to 35% of the total water contained in the fruit in the initial state.

[0006] In the fruit dehydration method according to the present invention, the inside of the vacuum chamber during the depressurization step is preferably in a low vacuum state of 2 to 20 kPa.

[0007] In the fruit dehydration method according to the present invention, the surface temperature of the fruit is preferably 20 to 40°C during the depressurization step and the dehydration step.

[0008] In the fruit dehydration method of the present invention, when the outer surface of the fruit is covered with a skin, it is preferable to have a hole-punching step in which a plurality of holes penetrating at least the skin of the fruit are drilled in the fruit before the decompression step.

[0009] A fruit dehydration device for use in the fruit dehydration method of the present invention is preferably one that includes a vacuum chamber for accommodating fruit, a vacuum pump connected to the vacuum chamber for reducing the pressure inside the vacuum chamber, and a microwave transmitter attached to the vacuum chamber for irradiating microwaves to the fruit inside the vacuum chamber. The fruit dehydration device also preferably includes a table disposed in the vacuum chamber facing the microwave transmitter on which the fruit is placed, and a rotary drive unit for rotating the table around a vertical axis of rotation. Effect of the Invention

[0010] The fruit dehydration method of the present invention irradiates fruit under reduced pressure with microwaves to remove 5 to 35% of the water contained in the fruit, thereby concentrating the sweetness of the fruit, reducing the dripping that occurs when thawing after frozen storage, preventing the fruit from losing its shape, and achieving a texture that is smoother than that of fresh fruit, making it possible to provide fruit of equal or better quality than that in season throughout the year.

[0011] In the fruit dehydration method of the present invention, when the inside of the vacuum chamber during the decompression process is in a low vacuum state of 2 to 20 kPa, moisture in the center of the fruit moves to the surface side of the fruit in a short period of time, resulting in efficient dehydration and improved energy saving.

[0012] In the fruit dehydration method of the present invention, when the surface temperature of the fruit during the decompression process and the dehydration process is 20 to 40°C, alteration and deterioration of the fruit is prevented, the original quality of the fruit is maintained even after frozen storage, and consumption of the fruit is increased.

[0013] In the fruit dehydration method of the present invention, when the outer surface of the fruit is covered with a skin, if a perforation step is performed before the decompression step in which a plurality of holes penetrating at least the skin of the fruit are drilled in the fruit, the dehydration of water from the fruit is not hindered by the skin, and the dehydration step can be carried out reliably and efficiently in a short period of time. [Brief description of the drawings]

[0014] [Figure 1] 1 is a schematic cross-sectional view showing a fruit dehydration device used in a fruit dehydration method according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Next, with reference to the attached drawings, an embodiment of the present invention will be described for understanding the present invention. A fruit dehydrating apparatus 10 shown in FIG. 1 is used in a fruit dehydrating method according to one embodiment of the present invention. Fruit dehydration device 10 comprises a vacuum chamber 12 that contains fruit 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 13 and irradiates microwaves to fruit 11 inside vacuum chamber 13. Fruit dehydration device 10 also comprises a table 15 that is disposed opposite microwave transmitter 14 inside vacuum chamber 12 and on which fruit 11 is placed, and a rotation drive unit 17 such as a motor that rotates table 15 around a vertical rotation axis 16.

[0016] The fruit dehydration method according to the present invention will be described below. The fruit dehydration method according to the present invention includes a depressurization step in which fruit 11 is placed in a vacuum chamber 12 and the inside of the vacuum chamber 12 is depressurized, and a dehydration step in which microwaves are irradiated to the fruit 11 in the vacuum chamber 12 under reduced pressure, so that 5 to 35% (preferably 15 to 25%) of the total water contained in the fruit 11 in the initial state is dehydrated. Here, the ratio of the amount of water dehydrated in the dehydration step to the total amount of water contained in the fruit 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 is appropriately selected depending on the type of fruit 11. It is preferable that the inside of the vacuum chamber 12 during the decompression process is in a low vacuum state of 2 to 20 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 fruit 11, the microwave intensity and irradiation time, the dehydration rate in the dehydration process (final dehydration rate after the dehydration process), etc. The surface temperature of the fruit 11 during the depressurization process and the dehydration process is preferably 20 to 40° C. The surface temperature of the fruit 11 during the depressurization process and the dehydration process is not limited to the above range, and is appropriately selected depending on the type of fruit 11, the pressure inside the vacuum chamber 12 during the depressurization process, the microwave intensity and irradiation time, the dehydration rate in the dehydration process (final dehydration rate after the dehydration process), etc.

[0017] When the fruit to be dehydrated is a fruit whose outer surface is not covered with a skin, such as strawberries, the fruit is stored as is in the vacuum chamber 12 (on the table 15) and dehydrated by the above method. However, in the case of a fruit whose outer surface is covered with a skin, such as kiwi fruit (particularly a fruit whose skin is thick enough to prevent the passage of moisture inside the fruit), it is preferable to open a plurality of holes penetrating at least the skin of the fruit in the hole-opening process before the depressurization process (before the fruit is stored in the vacuum chamber). This allows moisture inside the fruit to quickly pass through the skin during the dehydration process, and the dehydration process is carried out efficiently in a short time. The diameter of the holes opened in the hole-opening process is preferably about 200 μm, for example, but the diameter, number and arrangement (spacing) of the holes are appropriately selected. In addition, in the hole-opening process, a hole-opening jig such as a microneedle having a plurality of needles arranged vertically and horizontally on a base material, like a pin holder, is used. For example, microneedles having a tip diameter of 100 μm, a base diameter of 600 μm, and a needle pitch of 1.5 to 2 mm are preferably used, but the needle dimensions, pitch (arrangement) and number are appropriately selected.

[0018] In addition, even if the outer surface of a fruit is covered with a skin in its normal (initial) state, if the fruit can be handled with the skin removed, if the fruit can be handled with the inedible parts (discarded parts) such as the stem cut off to partially expose the inside, or if the fruit has a thin skin covering the outer surface that is not likely to hinder the passage of moisture (e.g., figs), the fruit may be placed whole in the vacuum chamber 12 and each process may be carried out. In order to dehydrate the fruit uniformly, it is preferable to rotate table 15 during the decompression process and / or dehydration process, but the rotation speed (number of rotations) of table 15, the timing of starting and stopping rotation, and rotation time, etc. are selected appropriately depending on the type of fruit and the dehydration rate, etc., and rotation of table 15 may be omitted. EXAMPLES

[0019] 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.

[0020] The fruits to be tested were the strawberry "Amaou", a specialty of Fukuoka Prefecture, the fig "Toyomitsuhime", and the green kiwi fruit grown in Fukuoka Prefecture. The effect of dehydration before freezing (difference in dehydration rate) on the quality of the fruit upon thawing was evaluated. Each fruit was subjected to a dehydration process by the fruit dehydration method of the present invention using the above-mentioned fruit dehydration device 10, with the pressure in the vacuum chamber during the depressurization process being 2 to 10 kPa, and the microwave irradiation time being 30 minutes to 3 hours, after which the dehydration rate was measured, and the fruit was subjected to a quick-freezing process and frozen storage. Note that the green kiwi fruit has a thick skin that is difficult to dehydrate, so as described in paragraph 0017, a plurality of holes were made through the skin by a perforation process before being placed in the vacuum chamber (before the depressurization process and the dehydration process were performed). For comparison, each fruit was subjected to a quick-freezing process without dehydration and frozen storage.

[0021] Comparative Example 1 Amaou strawberries were frozen without being dehydrated, and then thawed in the refrigerator after two days. Four in-house evaluators tasted the samples and rated the taste on an 11-point scale from 0 (bad) to 10 (delicious).

[0022] Example 1 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was 5% before freezing.

[0023] Example 2 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was 14% before freezing.

[0024] Example 3 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was 20% before freezing.

[0025] Example 4 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was 35% before freezing.

[0026] Comparative Example 2 The evaluation was carried out in the same manner as in Comparative Example 1, except that the dehydration rate was 50% before freezing.

[0027] The evaluation results of Comparative Example 1, Examples 1 to 4, and Comparative Example 2 are shown in Table 1. From Table 1, it was found that strawberries dehydrated to a dehydration rate of about 20 to 35% can be eaten deliciously even after being frozen and thawed. In addition, it was found that strawberries dehydrated to a dehydration rate of about 5 to 14% have a poorer sweetness concentration effect than strawberries dehydrated to a dehydration rate of about 20 to 35%, but have a hardness close to that of fresh strawberries (unfrozen strawberries) and a good texture. Therefore, for example, in cases where the hardness (texture) of strawberries is important when they are served, such as strawberry shortcake, strawberry parfait, or strawberry daifuku, a dehydration rate of about 5 to 14% is no problem, and it is thought that the market can be expanded by selecting the dehydration rate according to the purpose.

[0028] [Table 1]

[0029] Comparative Example 3 "Toyomitsuhime" figs were frozen without being dehydrated, and then thawed six days later. Four in-house evaluators tasted the figs and rated their taste on an 11-point scale from 0 (bad) to 10 (delicious).

[0030] Comparative Example 4 The evaluation was carried out in the same manner as in Comparative Example 3, except that the dehydration rate was 10% before freezing.

[0031] Example 5 The evaluation was carried out in the same manner as in Comparative Example 3, except that the dehydration rate was 30% before freezing.

[0032] Comparative Example 5 The evaluation was carried out in the same manner as in Comparative Example 3, except that the dehydration rate was 40% before freezing.

[0033] The evaluation results of Comparative Example 3, Comparative Example 4, Example 5, and Comparative Example 5 are shown in Table 2. From Table 2, it was found that figs dehydrated to a dehydration rate of about 30% were delicious even after being frozen and thawed.

[0034] [Table 2]

[0035] Comparative Example 6 Green kiwifruit from Fukuoka Prefecture was frozen without being dehydrated, and then thawed after three days. Four in-house evaluators tasted the fruit and rated its taste on an 11-point scale from 0 (bad) to 10 (delicious).

[0036] Example 6 The evaluation was carried out in the same manner as in Comparative Example 6, except that the dehydration rate was 15% before freezing.

[0037] Example 7 The evaluation was carried out in the same manner as in Comparative Example 6, except that the dehydration rate was 25% before freezing.

[0038] Comparative Example 7 The evaluation was carried out in the same manner as in Comparative Example 6, except that the dehydration rate was 40% before freezing.

[0039] The evaluation results of Comparative Example 6, Example 6, Example 7, and Comparative Example 7 are shown in Table 3. From Table 3, it was found that green kiwi dehydrated to a dehydration rate of about 15 to 20% can be eaten deliciously even after freezing and thawing. In addition, it was found that even green kiwi, which has a thick skin and is difficult to dehydrate, can be effectively dehydrated by performing a perforation process in which multiple holes are made through the skin before performing the decompression process and the dehydration process.

[0040] [Table 3]

[0041] From the above, it was found that the fruit dehydrated by the fruit dehydration method of the present invention has a concentrated sweetness, produces less drips when thawed after freezing, maintains its shape, has a good texture, and can be eaten as deliciously as raw fruit. Therefore, even if the fruit has a shelf life of less than one week at room temperature or in a refrigerated environment, the fruit dehydration method of the present invention can maintain a shelf life of more than two months in a frozen environment, eliminating restrictions such as logistics, and is expected to lead to market expansion. In addition, the fruit dehydration method of the present invention is considered to be applicable not only to fruits other than those mentioned above (e.g., persimmon, peach, lychee, plum, mango, etc.), but also to fruit tomatoes and bamboo shoots that are difficult to refrigerate whole without prior processing (removal of stems and thick skins that become inedible or discarded parts), and is considered to contribute to strengthening the brand power of specialty products in various regions both domestically and internationally and revitalizing the regions.

[0042] 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. [Explanation of symbols]

[0043] 10: fruit dehydration device, 11: fruit, 12: vacuum chamber, 13: vacuum pump, 14: microwave transmitter, 15: table, 16: vertical rotating shaft, 17: rotating drive unit

Claims

1. A method for dehydrating target fruit into a state suitable for frozen storage, comprising: This method for dehydrating fruit comprises a depressurizing step in which the fruit is placed in a vacuum chamber and the inside of the vacuum chamber is depressurized, and a dehydrating step in which microwaves are irradiated to the fruit in the vacuum chamber under reduced pressure, thereby dehydrating 5 to 35% of the total water contained in the fruit in an initial state.

2. 2. The fruit dehydration method according to claim 1, wherein the inside of the vacuum chamber during the depressurization step is in a low vacuum state of 2 to 20 kPa.

3. 2. The method for dehydrating fruit according to claim 1, wherein the surface temperature of the fruit during the depressurizing step and the dehydrating step is 20 to 40°C.

4. 2. A fruit dehydration method according to claim 1, further comprising a hole-making step of making a plurality of holes in the fruit penetrating at least the skin of the fruit before the pressure reduction step when the outer surface of the fruit is covered with a skin.

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