A slow-release water-absorbing fresh-keeping material, its preparation method and application

Abstract

The present invention belongs to the technical field of fresh-keeping materials, and particularly relates to a fresh-keeping material with slow-release water absorption, its preparation method and application. The raw material components of the fresh-keeping material with slow-release water absorption of the present invention include a matrix material, a cross-linking aid, a responsive aid, a porous material and a solvent; the cross-linking aid includes at least one of boric acid and borax; the preparation method of the responsive aid is: mixing a temperature-responsive monomer, an initiator and a solvent, and reacting to obtain; the temperature-responsive monomer includes N-isopropylacrylamide and methacrylic acid. The present invention uses N-isopropylacrylamide and methacrylic acid as temperature-responsive monomers to prepare a temperature-responsive responsive aid, which is cross-linked with a specific type of cross-linking aid to form a stable three-dimensional network structure, and combined with a reasonable amount of matrix material and porous material, so that the fresh-keeping material has good water absorption performance, slow-release performance, temperature responsiveness and ethylene adsorption property, and is green and environmentally friendly.

Description

Technical Field

[0001] The present invention belongs to the technical field of fresh-keeping materials, and particularly relates to a fresh-keeping material with slow-release water absorption, a preparation method thereof, and an application thereof. Background Art

[0002] In recent years, with the improvement of people's living standards, the demand for seven major categories of foods, namely vegetables, fruits, aquatic products, grains, meats, milks, and eggs, has been increasing. However, in the production and circulation links, most food packaging causes microbial growth due to excessive water vapor in the package and the lack of antibacterial function, resulting in food spoilage and causing great losses and waste.

[0003] Currently, most packaging materials applied to foods are prepared from traditional materials. These packaging materials have certain fresh-keeping functions and biodegradable functions, and can be gradually promoted for extending the shelf life of foods and improving the commodity value. Superhydrophobic materials have an adsorption function for the excess water vapor in the package, and can keep the appropriate humidity inside the package. The relatively dry environment and antibacterial raw materials can also have a certain inhibitory effect on bacteria, fungi, etc. Therefore, food safety can be effectively guaranteed.

[0004] Some studies have disclosed a slow-release intelligent fresh-keeping agent, which uses a solvent method to prepare a cellulose-based porous hydrogel. The porous hydrogel is combined with graphene oxide (GO) and citral to obtain a functional fresh-keeping agent. However, its preparation process is relatively complex, special equipment is required for preparation, and the cost is relatively high. Some studies have disclosed a 1-methylcyclopropene (1-MCP) slow-release fresh-keeping agent based on a hydrogel system, its preparation method and application. Using a polymer monomer and a cross-linking aid, in a solvent containing diphenylphosphine oxide, the 1-MCP powder is wrapped in a hydrogel system to obtain a product. However, diphenylphosphine oxide used in this method is a harmful substance, and it is difficult to meet the actual fresh-keeping application requirements.

[0005] Therefore, there is an urgent need to develop a fresh-keeping material with good water absorption performance and slow-release performance, which can extend the storage time of foods, extend the shelf life of foods, and is green and environmentally friendly. Summary of the Invention

[0006] The present invention aims to solve one or more of the above-mentioned technical problems existing in the prior art, and at least provides a beneficial option. Specifically, the present invention provides a fresh-keeping material with good water absorption performance and slow-release performance of slowly releasing water, which can extend the storage time of foods, extend the shelf life of foods, and is green and environmentally friendly.

[0007] Inventive concept of the present invention: The raw material components of the fresh-keeping material of the present invention include a matrix material, a cross-linking aid, a responsive aid, a porous material, and a solvent; the cross-linking aid includes at least one of boric acid and borax; the preparation method of the responsive aid is: mixing a temperature-responsive monomer, an initiator, and a solvent, and reacting to obtain it; the temperature-responsive monomer includes N-isopropylacrylamide and methacrylic acid. The responsive aid with temperature responsiveness cross-links with a specific type of cross-linking aid to form a stable three-dimensional network structure, enabling the fresh-keeping material to have a temperature-responsive three-dimensional network polymer structure. This structure is stable, can improve water absorption, and has the slow-release performance of slowly releasing moisture and the function of adsorbing ethylene. Good water absorption can prevent food from rotting due to excessive moisture, and the good performance of slowly releasing moisture can adjust the temperature and humidity after packaging, prevent foods such as fruits and vegetables from withering due to lack of water, and can also prevent accelerated deterioration due to high temperature. Adsorbing ethylene can slow down the ripening rate of foods such as fruits and vegetables and extend the storage time of food. At the same time, combining the matrix material and the porous material, and the joint action of each component enables the fresh-keeping material to have good water absorption performance, slow-release performance, temperature responsiveness, and ethylene adsorption performance.

[0008] Therefore, the first aspect of the present invention provides a fresh-keeping material with slow-release water absorption.

[0009] Specifically, the raw material components of the slow-release water-absorbing fresh-keeping material include a matrix material, a cross-linking aid, a responsive aid, a porous material, and a solvent;

[0010] The cross-linking aid includes at least one of boric acid and borax;

[0011] The preparation method of the responsive aid is: mixing a temperature-responsive monomer, an initiator, and a solvent, and reacting to obtain it;

[0012] The temperature-responsive monomer includes N-isopropylacrylamide and methacrylic acid.

[0013] Preferably, the matrix material includes at least one of polyvinyl alcohol (PVA), chitosan, sodium alginate, and konjac gum.

[0014] Specifically, the matrix material of the present invention is biodegradable, and when the matrix material is chitosan, it also has antibacterial properties.

[0015] Preferably, the molar ratio of N-isopropylacrylamide to methacrylic acid is (0.8 - 1.2):1.

[0016] More preferably, the molar ratio of N-isopropylacrylamide to methacrylic acid is (0.9 - 1.1):1.

[0017] Even more preferably, the molar ratio of N-isopropylacrylamide to methacrylic acid is 1:1.

[0018] Preferably, the porous material includes an inorganic porous material.

[0019] Preferably, the inorganic porous material includes at least one of activated carbon, silica white, and diatomaceous earth.

[0020] Preferably, the mesh number of the inorganic porous material is greater than 300 meshes; more preferably, the mesh number of the inorganic porous material is 350 - 600 meshes.

[0021] Specifically, the inorganic porous material has characteristics such as strong loading capacity and stable performance. Uniformly dispersed in the fresh - keeping material, it can greatly improve the water - absorption performance, temperature - response performance, and slow - release performance of the fresh - keeping material.

[0022] Preferably, the solvent includes water.

[0023] Preferably, the molar ratio of the initiator to the temperature - responsive monomer is 1:(80 - 120); more preferably, the molar ratio of the initiator to the temperature - responsive monomer is 1:(90 - 110); even more preferably, the molar ratio of the initiator to the temperature - responsive monomer is 1:100.

[0024] Preferably, the initiator includes persulfate.

[0025] Preferably, the persulfate includes at least one of potassium persulfate, ammonium persulfate, and sodium persulfate.

[0026] Preferably, the solvent includes water.

[0027] Preferably, the temperature of the reaction is 60 - 80 °C, and the time of the reaction is 50 - 70 min; more preferably, the temperature of the reaction is 65 - 75 °C, and the time of the reaction is 55 - 65 min; even more preferably, the temperature of the reaction is 70 °C, and the time of the reaction is 60 min.

[0028] Preferably, after the reaction, it further includes a process of separation and removal of the supernatant.

[0029] Preferably, by mass parts, the raw material components of the slow - release water - absorbent fresh - keeping material include 50 - 80 parts of matrix material, 5 - 15 parts of cross - linking aid, 1 - 10 parts of response aid, 5 - 15 parts of porous material, and 800 - 1200 parts of solvent.

[0030] More preferably, by mass parts, the raw material components of the slow - release water - absorbent fresh - keeping material include 60 - 70 parts of matrix material, 8 - 12 parts of cross - linking aid, 3 - 6 parts of response aid, 8 - 12 parts of porous material, and 900 - 1000 parts of solvent water.

[0031] The second aspect of the present invention provides a method for preparing the sustained-release water-absorbing fresh-keeping material described in the first aspect of the present invention.

[0032] Specifically, the method for preparing the sustained-release water-absorbing fresh-keeping material includes the following steps:

[0033] Mix the raw material components, heat, and prepare the sustained-release water-absorbing fresh-keeping material.

[0034] Preferably, the mixing is carried out by stirring, the rotation speed of the stirring is 160 - 240 r / min, and the stirring time is 4 - 6 min; more preferably, the rotation speed of the stirring is 180 - 220 r / min, and the stirring time is 4.5 - 5.5 min; even more preferably, the rotation speed of the stirring is 200 r / min, and the stirring time is 5 min.

[0035] Preferably, when heating to 90 - 110 °C, stop heating, cool to obtain a gel, and then dry to prepare the fresh-keeping material; more preferably, stop heating when heating to 95 - 105 °C; even more preferably, stop heating when heating to 100 °C.

[0036] The third aspect of the present invention provides an application of the sustained-release water-absorbing fresh-keeping material described in the first aspect of the present invention in food preservation.

[0037] Compared with the prior art, the beneficial effects of the technical solution provided by the present invention are as follows:

[0038] (1) The raw material components of the fresh-keeping material of the present invention include a matrix material, a cross-linking aid, a responsive aid, a porous material, and a solvent; the cross-linking aid includes at least one of boric acid and borax; the preparation method of the responsive aid is: mix a temperature-responsive monomer, an initiator, and a solvent, and react to obtain; the temperature-responsive monomer includes N-isopropylacrylamide and methacrylic acid. The responsive aid with temperature responsiveness cross-links with a specific type of cross-linking aid to form a stable three-dimensional network structure. This structure is stable, which can improve water absorption, and has the sustained-release performance of slowly releasing water and the function of adsorbing ethylene. Good water absorption can prevent food from rotting due to excessive water, and the slow release of water can adjust the temperature and humidity after packaging, prevent food such as fruits and vegetables from shriveling due to lack of water, and can also prevent accelerated deterioration due to high temperature. Adsorbing ethylene can slow down the ripening rate of food such as green grapes and extend the storage time of food. At the same time, combining the matrix material and the porous material and acting together make the fresh-keeping material have good water absorption performance, sustained-release performance, temperature responsiveness, and ethylene adsorption.

[0039] (2) The matrix material of the present invention is a degradable material, enabling the preservation material to be biodegradable. After use, the preservation material can be composted and degraded without white pollution problems. The inorganic porous material has strong loading capacity and stable performance, which can improve the water absorption performance and slow-release performance of the preservation material. In addition, by selecting the matrix material (such as chitosan) and controlling the addition amounts of the inorganic porous material and the matrix material, the antibacterial performance, water absorption performance, and slow-release performance of the preservation material can be regulated to solve the disadvantages of non-biodegradability and lack of slow-release function in current traditional packaging materials, and it can be widely applied in many fields.

[0040] (3) The preparation process of the present invention is simple, and the raw materials are green and environmentally friendly, facilitating large-scale production and application. Description of the Drawings

[0041] Figure 1 It is a schematic diagram of the preparation process flow of the preservation material in Example 1 of the present invention;

[0042] Figure 2 It is a diagram of the appearance state of green grapes after being packaged and stored with the preservation materials in Example 3, Example 5, Comparative Example 1, and Comparative Example 4 of the present invention. Detailed Embodiments

[0043] In order to make the technical solutions described in the present invention clearer and more understandable to those skilled in the art, the following examples are listed for illustration. It should be noted that the following examples do not limit the protection scope required by the present invention.

[0044] Unless otherwise specified, the raw materials, reagents, or devices used in the following examples can be obtained from conventional commercial channels or can be obtained by existing known methods.

[0045] Example 1

[0046] A slow-release water-absorbing preservation material is composed of the following raw material components: 50 g of PVA, 10 g of boric acid, 10 g of response assistant, 5 g of diatomite, and 1000 g of water.

[0047] The preparation method of the slow-release water-absorbing preservation material includes the following steps:

[0048] (1) Prepare the response assistant: Add an excessive amount of water to the reaction vessel, add the temperature-responsive monomer N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) into the reaction vessel in a molar ratio of 1:1, control the mass ratio of the temperature-responsive monomer to water at 5:100, add the initiator potassium persulfate to the reaction vessel and introduce nitrogen, the molar ratio of the initiator to the temperature-responsive monomer is 1:100, set the temperature at 70 °C, keep stirring for 60 min, after the reaction is completed, perform centrifugal separation, remove the supernatant, and repeat 3 times to obtain the response assistant generated by the water-phase grafting reaction;

[0049] (2)Raw material premixing: Add PVA, boric acid, diatomite, the response assistant obtained in step (1), and solvent water into a reactor for mixing, control the stirring rate at 200 r / min, and stir for 5 min to obtain a premixed material;

[0050] (3)Crosslinking and drying: Continuously stir the premixed material obtained in step (2), heat it to 100 °C and then stop heating, and form a gel after natural cooling. Cut the gel into pieces and dry it by freeze-drying method to obtain a slow-release water-absorbing fresh-keeping material.

[0051] The schematic process flow diagram for the preparation of the slow-release water-absorbing fresh-keeping material in Example 1 is as Figure 1 shown.

[0052] Example 2

[0053] A slow-release water-absorbing fresh-keeping material is composed of the following raw material components: 40 g of PVA, 10 g of konjac gum, 10 g of borax, 2 g of response assistant, 10 g of silica white, and 1000 g of water.

[0054] The preparation method of the slow-release water-absorbing fresh-keeping material in Example 2 is the same as that in Example 1.

[0055] Example 3

[0056] A slow-release water-absorbing fresh-keeping material is composed of the following raw material components: 60 g of PVA, 10 g of sodium alginate, 8 g of boric acid, 5 g of response assistant, 10 g of silica white, and 1000 g of water.

[0057] The preparation method of the slow-release water-absorbing fresh-keeping material in Example 3 is the same as that in Example 1.

[0058] Example 4

[0059] A slow-release water-absorbing fresh-keeping material is composed of the following raw material components: 60 g of PVA, 5 g of chitosan, 10 g of borax, 5 g of response assistant, 8 g of activated carbon, and 1000 g of water.

[0060] The preparation method of the slow-release water-absorbing fresh-keeping material in Example 4 is the same as that in Example 1.

[0061] Example 5

[0062] A slow-release water-absorbing fresh-keeping material is composed of the following raw material components: 60 g of PVA, 5 g of konjac gum, 10 g of borax, 5 g of response assistant, 10 g of activated carbon, and 1000 g of water.

[0063] The preparation method of the slow-release water-absorbing fresh-keeping material in Example 5 is the same as that in Example 1.

[0064] Example 6

[0065] A slow-release water-absorbing fresh-keeping material is composed of the following raw material components: 60 g of PVA, 20 g of chitosan, 10 g of boric acid, 1 g of response assistant, 12 g of silica white, and 1000 g of water.

[0066] The preparation method of the slow-release water-absorbing fresh-keeping material in Example 6 is the same as that in Example 1.

[0067] Comparative Example 1

[0068] The difference between Comparative Example 1 and Example 5 is only that Comparative Example 1 uses an equal amount of glycerol to replace the borax in Example 5, and the others are the same as Example 5.

[0069] Comparative Example 2

[0070] The difference between Comparative Example 2 and Example 5 is only that the dosage of the matrix material PVA in Comparative Example 2 is 40 g, which is insufficient, and the others are the same as Example 5.

[0071] Comparative Example 3

[0072] The difference between Comparative Example 3 and Example 5 is only that Comparative Example 3 does not add a response assistant. Correspondingly, in its preparation method, step (1) for preparing the response assistant is not carried out, and the others are the same as Example 5.

[0073] Comparative Example 4

[0074] The difference between Comparative Example 4 and Example 5 is only that Comparative Example 4 uses the metal oxide porous material alumina to replace activated carbon in an equal amount, and the others are the same as Example 5.

[0075] Comparative Example 5

[0076] The difference between Comparative Example 5 and Example 5 is only that the temperature-responsive monomer in preparing the response assistant in Comparative Example 5 is N-isopropylacrylamide. That is, in step (1) when preparing the response assistant, the temperature-responsive monomer methacrylic acid is not added, and only N-isopropylacrylamide is added, and the others are the same as Example 5.

[0077] Comparative Example 6

[0078] The difference between Comparative Example 6 and Example 5 is only that the temperature-responsive monomer in preparing the response assistant in Comparative Example 6 is methacrylic acid. That is, in step (1) when preparing the response assistant, the temperature-responsive monomer N-isopropylacrylamide is not added, and only methacrylic acid is added, and the others are the same as Example 5.

[0079] Performance Test

[0080] The water absorption performance test, temperature response performance test, and slow-release performance test are carried out on the fresh-keeping materials prepared in Examples 1-6 and Comparative Examples 1-6. The specific test methods are as follows:

[0081] Water absorption performance test: The liquid absorption ratio was used to characterize the water absorption performance. The tea bag method was used to measure the liquid absorption ratio. Specifically, 1.0 g of each of the fresh-keeping materials prepared in Examples 1-6 and Comparative Examples 1-6 was weighed and placed in a nylon bag, and then placed in a beaker containing 1000 mL of deionized water. After the fresh-keeping materials were fully swollen, they were taken out, dried and weighed. The liquid absorption ratio of the fresh-keeping materials was calculated according to the following formula:

[0082] Qeq=(m2−m1) / m1;

[0083] In the formula: Qeq is the liquid absorption ratio of the fresh-keeping material, g / g; m1 is the initial mass of each group of fresh-keeping materials, g; m2 is the mass of each group of fresh-keeping materials after water absorption and swelling, g.

[0084] Temperature response performance test: The fresh-keeping materials prepared in Examples 1-6 and Comparative Examples 1-6 were fully absorbed with water and weighed, denoted as m3. The fresh-keeping materials fully absorbed with water were placed in an oven at 45 °C, taken out after standing for 10 min, wiped dry the surface moisture and weighed, denoted as m4. Calculate the mass change (m3 - m4) of each group of fresh-keeping materials, repeat twice, take the average value, and judge the temperature response performance through the mass change. The greater the mass change, the better the temperature response performance.

[0085] Sustained release performance test: The sustained release performance of the fresh-keeping material was tested by the weighing method. First, a certain mass of the fresh-keeping material (1 g) was added to 100 mL of deionized water, and the shaker was fully oscillated at room temperature for 2 hours to reach saturation of water absorption. Then it was filtered, the fresh-keeping material was taken out and placed in a constant temperature and humidity chamber (25 °C, humidity 50%RH). Samples were taken for testing every 30 min. The initial weight was tested, and the dried weight was tested after drying in an 80 °C oven. The moisture content of the fresh-keeping material was calculated, and further the sustained release performance of each gram of the fresh-keeping material to water was obtained. Calculate the moisture content after 30 min, 60 min, and 90 min of being placed in the constant temperature and humidity chamber respectively. The slower the moisture content decreases, the stronger the sustained release performance.

[0086] The test results of the water absorption performance, temperature response performance and sustained release performance of the fresh-keeping materials in Examples 1-6 and Comparative Examples 1-6 of the present invention are shown in Table 1.

[0087] Table 1: Test results of water absorption performance, temperature response performance and sustained release performance of fresh-keeping materials in Examples 1-6 and Comparative Examples 1-6 of the present invention

[0088]

[0089] As can be seen from Table 1, the fresh-keeping material of the present invention has good water absorption performance, temperature response performance and sustained release performance, can fully exert the fresh-keeping function of the fresh-keeping material, extend the storage time of food, and extend the food shelf life.

[0090] In Comparative Example 1, an equal amount of glycerol was used to replace the borax in Example 5, resulting in a decrease in the water absorption, temperature responsiveness, and slow-release performance of Comparative Example 1. This shows that only by selecting a specific cross-linking aid can the water absorption, temperature responsiveness, and slow-release performance of the fresh-keeping material be better improved.

[0091] In Comparative Example 2, the amount of the matrix material PVA was 40 g, which was insufficient, resulting in a significant decrease in the water absorption, temperature responsiveness, and slow-release performance of Comparative Example 2.

[0092] In Comparative Example 3, no response aid was added, resulting in a relatively high water absorption of Comparative Example 3, but a significant decrease in both the temperature responsiveness and slow-release performance.

[0093] In Comparative Example 4, the metal oxide porous material alumina was used to replace the activated carbon in equal amounts, resulting in a decrease in the water absorption of Comparative Example 4, and a significant decrease in both the temperature responsiveness and slow-release performance.

[0094] In Comparative Example 5, only N-isopropylacrylamide was used to prepare the response aid, resulting in a significant decrease in the water absorption, temperature responsiveness, and slow-release performance of Comparative Example 5. In Comparative Example 6, only methacrylic acid was used to prepare the response aid, resulting in a significant decrease in the water absorption, temperature responsiveness, and slow-release performance of Comparative Example 6. This shows that both N-isopropylacrylamide and methacrylic acid are indispensable. Using only one temperature-responsive monomer to prepare the response aid cannot achieve the water absorption capacity, temperature responsiveness, and slow-release performance after compounding at all. Only by using both N-isopropylacrylamide and methacrylic acid as the temperature-responsive monomers to prepare the response aid can the fresh-keeping material have good water absorption and temperature responsiveness.

[0095] The fresh-keeping materials of Example 3, Example 5, Comparative Example 1, and Comparative Example 4 were used to package fresh and plump green grapes, and then stored at 10 °C for 50 days. The appearance state of the green grapes after being packaged and stored with different fresh-keeping materials was observed, and the results are as Figure 2 shown. In addition, a portable ethylene detector was used to measure the ethylene concentration in the bag.

[0096] From Figure 2It can be seen that after 50 days of storage, the green grapes packed with the fresh-keeping materials of Example 3 and Example 5 are in good appearance state and still plump. The ethylene concentrations are 0.11 mmol / L and 0.08 mmol / L respectively. However, the green grapes packed with the fresh-keeping materials of Comparative Example 1 and Comparative Example 4 become shriveled and some have deteriorated. The ethylene concentrations are 0.19 mmol / L and 0.23 mmol / L respectively. This shows that the fresh-keeping material prepared by the present invention has a good fresh-keeping effect. This is because the fresh-keeping material of the present invention has good water absorption performance, slow release performance and temperature response performance. When stored at a certain temperature, water will be slowly released to achieve the purpose of adjusting the temperature and humidity of the package, thus achieving a good fresh-keeping effect. Moreover, the fresh-keeping material of the present invention can adsorb ethylene and slow down the ripening rate of green grapes.

[0097] In summary, the present invention uses N-isopropylacrylamide and methacrylic acid as temperature-responsive monomers to prepare a temperature-responsive response aid, which is cross-linked with specific types of cross-linking aids to form a stable three-dimensional network structure, and combines a reasonable amount of matrix material and inorganic porous material, so that the fresh-keeping material has good water absorption performance, slow release performance, temperature responsiveness and ethylene adsorption performance.

[0098] The above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A fresh-keeping material, characterized in that: The raw material components of the fresh-keeping material are composed of 60-70 parts of base material, 8-12 parts of cross-linking auxiliary agent, 3-6 parts of response auxiliary agent, 8-12 parts of porous material and 900-1000 parts of solvent water in parts by weight; The cross-linking aid is selected from at least one of boric acid and borax; The preparation method of the response auxiliary agent is: mixing a temperature response monomer, an initiator, and a solvent, and reacting them to obtain the response auxiliary agent; The temperature-responsive monomer consists of N-isopropylacrylamide and methacrylic acid; The matrix material comprises at least one of polyvinyl alcohol, chitosan, sodium alginate and konjac gum; The porous material is an inorganic porous material; the inorganic porous material is selected from at least one of activated carbon, white carbon black and diatomaceous earth; The molar ratio of the N-isopropylacrylamide to methacrylic acid is (0.8-1.2):

1.

2. The fresh-keeping material according to claim 1, characterized in that: The molar ratio of the initiator to the temperature-responsive monomer is 1:(80-120); and / or the initiator comprises persulfate.

3. The fresh-keeping material according to claim 1, characterized in that: The reaction temperature is 60-80° C., and the reaction time is 50-70 min; and / or, the reaction further includes a separation process to remove the supernatant.

4. The method for preparing the fresh-keeping material according to any one of claims 1 to 3, characterized in that: The following steps are involved: The raw material components are mixed and heated to obtain the fresh-keeping material.

5. The preparation method according to claim 4, characterized in that: The mixing is carried out by stirring, the stirring speed is 160-240r / min, and the stirring time is 4-6min; and / or, when the heating reaches 90-110°C, the heating is stopped, the mixture is cooled to obtain a gel, and then dried to obtain the fresh-keeping material.

6. Use of the fresh-keeping material according to any one of claims 1 to 3 in food preservation.

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