Strawberry freezing method

Abstract

This invention provides a method for freezing strawberries that can reduce the amount of drip that occurs when frozen strawberries are thawed. [Solution] As preparation, a solution containing an ingredient that solidifies into a jelly-like substance at room temperature is prepared. In step 1, strawberries are placed in a vacuum container and the pressure is reduced, causing moisture from inside the strawberries to seep out to the surface through the vascular bundles. In step 2, while maintaining the reduced pressure, the solution containing the ingredient that solidifies into a jelly-like substance at room temperature is brought into contact with the surface of the strawberries. In step 3, with the surface of the strawberries covered with the solution containing the ingredient that solidifies into a jelly-like substance at room temperature, the reduced pressure is released, filling the openings of the vascular bundles on the surface of the strawberries with the solution. In step 4, the strawberries after step 3 are rapidly frozen.

Description

Technical Field

[0006]

[0001] The present invention relates to a freezing method for strawberries, and particularly to a freezing method for strawberries with less drip occurring during thawing.

Background Art

[0002] The harvesting season of strawberries is from May to June in open field cultivation and from December to May in greenhouse cultivation, and the strawberries harvested during these periods are frozen and stored.

[0003] Patent Document 1 describes a method for producing food, which includes impregnating a food having intracellular or interstitial space into a gelling colloidal solution by a vacuum immersion method, then gelling the colloidal solution, and then freezing it (claims), and strawberries are exemplified as the target food (Example 2). In addition, it is described that the stability can be improved by including sodium alginate, low-methoxypectin, carrageenan, locust bean gum, etc. in the colloidal solution of the gelling agent.

[0004] Patent Document 2 describes a method for producing frozen fruits, which includes a step of pre-dehydrating fruit raw materials by ultrasonic treatment under reduced pressure, a step of impregnating fruit raw materials into a hydrogel solution under reduced pressure and ultrasonic treatment, and then a step of rapid freezing (claims), and it is suggested that it is applicable to strawberries ("0003").

[0005] Patent Document 3 describes a method for stabilizing frozen crops, which includes a step of impregnating agricultural products into an exudate solution containing polysaccharides such as pectin and divalent cations in a vacuum environment, a step of releasing the vacuum, and other post-treatment steps, and finally a step of freezing (claims), and strawberries are included in the agricultural products ("0028"), and it is described that pectin and calcium form a hard gel-like structure for fruits and plant tissues, reduce the gravy loss of food, and improve the texture of frozen food ("0034").

[0006] Patent Document 4 describes a method for improving the quality of frozen strawberries, which involves immersing strawberries in an aqueous solution containing sodium alginate under vacuum conditions, then removing the strawberries and rapidly freezing them (claims, examples, etc.). It can be said that sodium alginate and calcium ions react to form a gel.

[0007] Patent Document 5 discloses a method for producing frozen fruit products, which includes the steps of: immersing fruits such as strawberries in an aqueous solution containing pectin esterase with a sugar content 1 to 2 times that of the fruit, and then reducing the pressure in the reaction system containing the aqueous solution; removing the fruit from the aqueous solution and then immersing the fruit in an aqueous solution containing sugar to adjust the sugar content of the fruit to 25 to 50 degrees; and removing the fruit from the aqueous solution containing sugar and then freezing the fruit.

[0008] Non-patent document 1 discloses experiments aimed at creating "chewable" frozen strawberries through rapid freezing and dehydrofreezing. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 1-258173 [Patent Document 2] Chinese Patent Application Publication No. 116268266 Specification [Patent Document 3] U.S. Patent Application Publication No. 2021 / 0368833 [Patent Document 4] Chinese Patent Application Publication No. 105309600 Specification [Patent Document 5] Japanese Patent Publication No. 2010-124767 [Non-patent literature]

[0010] [Non-Patent Document 1] Yusuke Kanai, "Development of Freezing Technology for Locally Grown Strawberries Suitable for Frozen Desserts," Tochigi Prefectural Industrial Technology Center Report (2019) [Disclosure of the Invention] [Problems that the invention aims to solve]

[0011] Like other frozen foods, strawberries, when frozen, have their internal water turn into ice, which expands in volume and destroys the tissue (cells). Rapid freezing results in smaller ice crystals, which can minimize tissue (cell) damage. However, since the volume expands regardless of the size of the ice crystals, the internal pressure of the strawberry increases. When such frozen strawberries are thawed, the ice melts from the surface first, and the inside expands, maintaining a high internal pressure. As shown in Figure 9, the trapped water, along with soluble components such as vitamins, leaks out as drip, resulting in a deterioration of appearance and texture.

[0012] The method described in Patent Document 1 does not allow for "filling only the openings of vascular bundles on the surface of a strawberry with a solution containing an ingredient that solidifies into a jelly-like substance at room temperature." Patent Document 1 neither intends nor suggests "filling only the openings of vascular bundles on the surface of a strawberry with a solution containing an ingredient that solidifies into a jelly-like substance at room temperature." What Patent Document 1 intends to do, as described on page 3, bottom left, third line from the bottom, is to minimize ice crystal formation in intracellular and / or interstitial spaces and reduce dripping caused by ice crystals by filling the intracellular and / or interstitial spaces with a gelling colloidal solution, thereby binding the water in the intracellular and / or interstitial spaces with the colloidal solution and suppressing the formation of ice crystals. The method described in Patent Document 1 involves binding intracellular and / or interstitial water with a colloidal solution to minimize ice crystal formation, thereby reducing drip during thawing. However, even though drip is reduced, in the case of strawberries, foreign matter (gel) enters the fruit, resulting in a rubbery texture and preventing the crisp texture of fresh strawberries from being achieved.

[0013] Patent Document 2 describes a method for controlling (suppressing) discoloration of frozen fruit in acidic juice. The material is based on "fruit pieces" rather than whole fruit, and it involves forming a uniform hydrogel film on the fruit surface (inner layer) to prevent discoloration. Although it is suggested that this method may be applied to strawberries, it does not disclose or suggest anything about removing water through the vascular bundles of strawberries.

[0014] Patent documents 3 and 4 do not disclose or suggest anything about filling a solution containing an ingredient that solidifies into a jelly-like substance at room temperature only into a limited area, such as the openings of vascular bundles on the surface of a strawberry, and therefore cannot suppress the occurrence of drip during thawing.

[0015] Patent document 5 claims that a texture similar to fresh strawberries can be obtained when frozen, but when thawed, drip occurs and a texture similar to fresh strawberries cannot be obtained. Non-patent document 1 also assumes that the product will be consumed in a frozen state.

[0016] In particular, Patent Document 5 describes immersing the fruit in a pectin esterase-containing aqueous solution and then reducing the pressure within the reaction system containing the pectin esterase-containing aqueous solution to allow the pectin esterase-containing aqueous solution to permeate the fruit. However, even if the pressure is reduced within the reaction system while the fruit is immersed in the pectin esterase-containing aqueous solution, the reduced pressure does not effectively act on the inside of the fruit, and sufficient permeation is not possible.

[0017] As described above, in Patent Document 5 and Non-Patent Document 1, the purpose of freezing strawberries is for use as a frozen dessert or sherbet to be eaten in its frozen state, or for use as an ingredient in smoothies after thawing. Eating frozen strawberries fresh is not considered because thawing them would result in a lot of drip. [Means for solving the problem]

[0018] To solve the above problems, the strawberry freezing method according to the present invention includes: in step 1, putting strawberries into a decompression vessel and decompressing in the gas phase to allow the moisture inside the strawberries to exude to the strawberry surface through the vascular bundles of the strawberries; in step 2, while maintaining the decompressed state, bringing a solution containing a component that gels at room temperature into contact with the strawberry surface; in step 3, releasing the decompressed state with the strawberry surface covered with a solution containing a component that gels at room temperature (5 - 35°C), and filling the solution only into the openings of the vascular bundles on the strawberry surface; and in step 4, rapidly freezing the strawberries after step 3 is completed. In the above, steps 1 and 2 may be carried out simultaneously and in parallel.

[0019] As the component that gels at room temperature, an extract of carrageenan with little drip from the gel itself when thawed is preferable. Specifically, examples include Ina Agar F (registered trademark) manufactured by Ina Foods Industry Co., Ltd., Cool Agar Frozen manufactured by Nitta Gelatin Inc., and Pearl Agar 34 manufactured by Fuji Shoko Co., Ltd.

Effects of the Invention

[0020] According to the present invention, the amount of drip generated when thawing frozen strawberries can be suppressed, and the texture and appearance after thawing can be made close to those of freshly picked strawberries.

[0021] Also, according to the present invention, strawberries with a texture always close to freshly picked can be provided, so it becomes possible to provide strawberry sandwiches, strawberry mochi, etc. throughout the year.

Brief Description of the Drawings

[0022] [Figure 1] Cross-sectional view explaining the structure of a strawberry [Figure 2] Enlarged view of the main part of FIG. 1 [Figure 3] Block diagram explaining the freezing method of strawberries according to the present invention in the order of steps [Figure 4] Figure showing the state where internal moisture is exuding to the strawberry surface under reduced pressure [Figure 5] Figure showing the state where the vascular bundle openings of strawberries are filled with jelly [Figure 6] A diagram showing the state of strawberries after being frozen using the method of the present invention and then thawed. [Figure 7] A diagram showing the state of strawberries that have been frozen using the method of the present invention and then cut. [Figure 8] This diagram shows the state of strawberries that have been cut after being frozen using the rapid freezing method of the present invention, with varying conditions. [Figure 9] This diagram shows the state of frozen strawberries after thawing, using a conventional freezing method. [Modes for carrying out the invention]

[0023] First, as shown in Figures 1 and 2, the structure of a strawberry has a core extending from the stem in the center, a pith outside of the core, and flesh outside the pith. Vascular bundles extend from the pith to the surface (pericarp), and there are achenes, which resemble seeds, at the surface openings of the vascular bundles. Thus, strawberries differ significantly from other fruits in the vascular bundle structure. Traditionally, drip during thawing seeps out through the vascular bundles.

[0024] (Example 1) Next, the strawberry freezing method of the present invention will be described. First, as preparation, a solution containing an ingredient that solidifies into a jelly-like substance at room temperature (5-35°C) is prepared. Ingredients that solidify into a jelly-like substance at room temperature include gelatin and agar, but in this example, agar, an extract from seaweed (carrageenan), was used. Among agars, those that do not easily break down the three-dimensional network structure of the jelly when thawed are preferred, for example, Ina Agar F (registered trademark) is used.

[0025] To the 10g of agar mentioned above, heat 500ml of water and 50g of sugar, stirring gradually to prevent lumps, until it lightly boils (95°C), then cool to 45°C.

[0026] Once the above preparations are complete, in step 1, place the strawberries in a vacuum container and reduce the pressure to -300 mmHg or higher. A lower degree of pressure is preferable, but a pressure greater than -300 mmHg is sufficient.

[0027] As a result of the reduced pressure described above, internal moisture seeps out onto the surface of the strawberry through the vascular bundles, as shown in Figure 4.

[0028] Next, in step 2, the agar solution (a solution containing components that solidify into a jelly-like substance at room temperature) prepared in the preliminary step is sprayed in a mist form into a vacuum container in the gas phase, bringing the agar solution into contact with the surface of the strawberries. Upon contact, the moisture that had seeped out of the strawberry surface through the vascular bundles dissolves or is converted into the agar solution, so the surface of the strawberries is covered with the agar solution.

[0029] Although the above instructions state that step 2 should be performed after step 1, steps 1 and 2 may be performed simultaneously. For example, the agar solution may be placed shallowly in a vacuum container, the strawberries may be added, and the container may be depressurized while the strawberries are slowly stirred so that they come into even contact with the agar solution.

[0030] Next, in the third step, the reduced pressure inside the vacuum container is released. As shown in Figure 5, the agar solution that covered the surface of the strawberries then enters only into the openings of the vascular bundles and solidifies into a jelly-like substance within those openings.

[0031] In the fourth step, the strawberries, whose vascular bundle openings were blocked with a jelly-like substance, were removed from the vacuum container and rapidly frozen. The freezing conditions were -20°C for 6 hours. Rapid freezing is done to break down the ice crystals and prevent tissue damage. It is preferable to place the strawberries in a nylon bag or similar container, remove the air, and freeze them in an airtight state. When placing strawberries in a nylon bag, it is preferable to include a jelly-like substance with them. This prevents the strawberries from being damaged by impacts during transport or handling.

[0032] Figures 6 and 7 show the state of rapidly frozen strawberries after being slowly thawed in a refrigerator. As shown in Figure 6, very little drip occurs even after thawing, and as shown in Figure 7, the tissue remains intact. As a result, the texture and appearance are very close to that of fresh strawberries.

[0033] (Example 2) In Example 1, steps 1 and 2 were performed separately, but in Example 2, steps 1 and 2 were performed simultaneously. Performing steps 1 and 2 simultaneously allows for more efficient insertion of the agar solution into the openings of the vascular bundles in the next step, step 3. First, prepare an agar solution (a solution containing components that solidify into a jelly-like substance at room temperature) in the same manner as in Example 1, and place the agar solution and the prepared strawberries into a vacuum container. The amount of agar solution should be shallow enough so that the strawberries do not sink into it. In this state, the vacuum level inside the vacuum container is set to -300 mmHg or higher, and the vacuum container is moved to ensure that the agar solution comes into even contact with the surface of the strawberries inside the vacuum container.

[0034] Through the above process, moisture from inside the strawberry seeps out to the surface of the strawberry through the vascular bundles inside the vacuum container. This seeped water then mixes with or is replaced by the agar solution. When the vacuum is released, the agar solution enters the openings of the vascular bundles and solidifies into a jelly-like substance as shown in Figure 5.

[0035] Figure 8 shows a cross-sectional view of a strawberry that was rapidly frozen at -30°C for 1 hour using a blast chiller, and then thawed at room temperature for 3 hours. When rapidly frozen under the above freezing conditions, the ice crystals became smaller, the core was visible in the cross-section, and the texture after thawing was even closer to that of a fresh strawberry.

[0036] In conventional methods, the openings of the vascular bundles were not sealed, so when the ice on the surface of the strawberry melted during thawing, the internal pressure caused drip to leak out from the inside through the vascular bundles. However, in strawberries frozen using this method, the openings of the vascular bundles are sealed, so even if the ice on the surface melts, it does not seep out to the outside. Furthermore, as thawing progresses, the ice crystals inside also melt, the volume that had expanded due to freezing returns to its original size, the internal pressure decreases, the water stays inside the cells, and drip seepage becomes less likely.

Claims

1. A method for freezing strawberries, characterized by comprising the following steps. Step 1: Place the strawberries in a vacuum container and reduce the pressure, allowing moisture from inside the strawberries to seep out onto the surface through the vascular bundles. Step 2: While maintaining a reduced pressure state in the gas phase, a solution containing an ingredient that solidifies into a jelly-like substance at room temperature is sprayed, bringing the mist-like solution into contact with the surface of the strawberries. Step 3: With the surface of the strawberries covered with a solution containing an ingredient that solidifies into a jelly-like substance at room temperature, the reduced pressure is released, and the solution is filled into the openings of the vascular bundles on the surface of the strawberries. Step 4: The strawberries that have been processed in Step 3 are rapidly frozen.

2. A method for freezing strawberries according to claim 1, characterized in that step 1 and step 2 are performed simultaneously in a vacuum container.

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