A sponge-like gel phase-change cold storage agent for preserving fruits and vegetables and a preparation method thereof

Abstract

The present invention discloses a sponge-like gel phase-change refrigerant for preserving fruits and vegetables and a preparation method thereof, comprising the following steps: 1) adding sodium alginate powder, N,N'-methylenebisacrylamide aqueous solution, tetramethylethylenediamine, and ammonium persulfate aqueous solution to an acrylamide aqueous solution in sequence; 2) transferring the mixed solution to a polytetrafluoroethylene container and quickly placing it in a vacuum dryer containing color-changing silica gel, using a circulating water vacuum pump to evacuate and seal the container overnight for cross-linking to obtain a sponge-like gel; 3) soaking the sponge-like gel in a potassium chloride solution to prepare a sponge-like gel phase-change refrigerant. The sponge-like gel phase-change refrigerant obtained by the method of the present invention has both high phase-change latent heat and high flexibility; it can be freely cut to fit the logistics space, avoiding the problem of uneven internal environmental temperature of the logistics space caused by local cooling; using potassium chloride solution as an additive for adjusting the phase-change temperature of the gel refrigerant, the appropriate phase-change temperature can be adjusted for different fruits and vegetables.

Description

Technical Field

[0001] The invention belongs to the technical field of cold storage agents, and in particular relates to a sponge-like gel phase-change cold storage agent for preserving fruits and vegetables and a preparation method thereof. Background Art

[0002] With the rapid development of the Internet, refrigerants play an important role in the transportation of fruits, vegetables, fresh produce and other products. Traditional refrigerants require additional packaging bags or boxes, and the product shape is fixed. During transportation, they can usually only be placed in a corner of the packaging box. In addition, they are hard or have sharp edges, which can easily cause mechanical damage to the fruit and are not suitable for the retail needs of the fruit. In addition, during the use of existing phase-change refrigerants, if the outer packaging of the refrigerant breaks, causing the refrigerant to leak, it will cause contamination of fruits and vegetables. If ice cubes are used directly during transportation without packaging, a large amount of water will be generated during use, increasing the probability of microbial infection. In addition, the current phase-change refrigerants have fixed components and fixed latent heat of phase change. There are few refrigerants suitable for the storage and transportation system of fruits and vegetables, and it is difficult to adjust them according to the specific user during use.

[0003] Chinese patent CN115216276A discloses a method for preparing a guar gum-based jelly-like phase-change coolant. The specific preparation idea is: using guar gum, water, potassium chloride / urea / ammonium chloride as raw materials, and using any one or a combination of borax, organic chelated titanium, and organic chelated chromium as a cross-linking agent. The coolant prepared in this patent contains raw materials such as urea that are not suitable for contact with fruits and vegetables, and the selection of the phase change temperature is not optimized for fruits and vegetables.

[0004] Patent CN115386347A discloses a quick-matching gel-type phase-change coolant, and the specific preparation idea is: SAP polymer resin, urea and water are used to make a gel-type coolant. The preparation of the coolant in this patent requires additional packaging, so that flexibility cannot be guaranteed, and it does not optimize the fruit and vegetable system, so it is not suitable for transportation and preservation of fruits and vegetables.

[0005] Patent CN117659954A discloses a flexible composite coolant and its preparation method. The specific preparation idea is: it is made from sodium hydroxymethyl cellulose, polyvinyl alcohol, sodium silicate nonahydrate, propylene glycol, silicon dioxide, borax solution and water. The coolant box prepared by the patent has too small latent heat, and the mass of coolant required to achieve the same cool storage effect is too large, which will increase the transportation cost and is not suitable for the actual application of fruit and vegetable logistics after harvest. Summary of the invention

[0006] In view of the problems existing in the prior art, the purpose of the present invention is to provide a sponge-like gel phase change cool storage agent for preserving fruits and vegetables and a preparation method thereof, which is specifically achieved through the following technical solutions:

[0007] A method for preparing a sponge-like gel phase-change cool storage agent for preserving fruits and vegetables comprises the following steps:

[0008] 1) Add sodium alginate powder to the acrylamide aqueous solution in a water bath at 80°C, stir thoroughly for 10-30 minutes, and pre-cool in a refrigerator at 4°C for 1-3 hours for later use;

[0009] 2) The precooled mixed solution in step 1) is stirred for 5 minutes in an ice water bath, and after stirring, an aqueous solution of N,N'-methylenebisacrylamide is added to the mixed solution as a cross-linking agent, and the mixture is stirred for 5 minutes in an ice water bath to obtain a mixed solution containing a cross-linking agent;

[0010] 3) The mixed solution containing the crosslinking agent obtained in step 2) is continuously stirred in an ice water bath, and then tetramethylethylenediamine is added as a polymerization accelerator. After continuing to stir for 5 minutes, an aqueous ammonium persulfate solution is added as an initiator;

[0011] 4) The mixed solution of step 3) was continuously stirred in an ice-water bath for 10 minutes, and then transferred to a polytetrafluoroethylene container on ice. After being taken out of the ice, it was quickly placed in a vacuum desiccator containing color-changing silica gel, and vacuumed with a circulating water vacuum pump for 30 minutes, and then sealed overnight for cross-linking to obtain a sponge-like gel;

[0012] 5) Soaking the sponge-like gel obtained in step 4) in a solution with a volume of 10-20 times, wherein the solution is composed of a potassium chloride solution with a mass fraction of 0-10%, replacing the solution every 24 hours, and soaking for 14 days; then taking it out and absorbing the residual water to obtain a sponge-like gel phase change coolant.

[0013] Furthermore, the mass fraction of the acrylic acid aqueous solution was 16%, the concentration of the N,N'-methylenebisacrylamide solution was 0.1 M, and the concentration of the ammonium persulfate aqueous solution was 75 g / L.

[0014] Furthermore, in step 1), the amount of sodium alginate powder added is 2.2-3.2 g per 100 g of acrylamide aqueous solution.

[0015] Furthermore, in step 2), the amount of the N,N'-methylenebisacrylamide aqueous solution added is 450 μl per 100 g of the acrylamide aqueous solution.

[0016] Furthermore, in step 3), the amount of tetramethylethylenediamine added is 60 μl per 100 g of the acrylamide aqueous solution.

[0017] Furthermore, in step 4), the amount of the mixed solution added to the polytetrafluoroethylene container is such that the height of the mixed solution is between 1 / 3 and 1 / 2 of the height in the container.

[0018] Furthermore, when the sponge-like gel phase-change refrigerant prepared in step 5) is used, it can be cut into a suitable size according to packaging requirements, frozen in a -20°C refrigerator for 24 hours before use, and then taken out and placed directly in a fruit and vegetable packaging box.

[0019] The sponge-like gel phase-change cold storage agent prepared by the preparation method.

[0020] The sponge-like gel phase change coolant prepared by the method of the present invention has both high phase change latent heat (above 290 J / g) and high flexibility; it can be freely cut to fit the logistics space, avoiding the problem of uneven temperature of the internal environment of the logistics space caused by local cooling; potassium chloride solution is used as an additive to adjust the phase change temperature of the gel coolant, and the appropriate phase change temperature (0°C to -4.8°C) can be adjusted for different fruits and vegetables. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a DSC test result diagram of the gel coolant prepared in Example 1;

[0022] Figure 2 The figures are the impact resistance effect diagrams of the gel prepared in Example 1 and the gel prepared in Comparative Example 1;

[0023] Figure 3 This is a comparison chart of the refrigeration effects of the gel coolant obtained in Example 1 and the commercial coolant;

[0024] Figure 4 The fresh-keeping effect of the gel coolant prepared in Example 1 on the quality of winter jujube. DETAILED DESCRIPTION

[0025] The present invention is further described below in conjunction with specific embodiments to facilitate a better understanding of the present technical solution. Example 1

[0026] Step 1: prepare a 16% mass fraction acrylamide aqueous solution, a 0.1 M N,N'-methylenebisacrylamide aqueous solution, and a 75 g / L ammonium persulfate aqueous solution for later use;

[0027] Step 2: Take 200 g of acrylamide aqueous solution in a beaker, stir in an 80°C water bath, and gradually add 6.4 g of sodium alginate powder, stir thoroughly for 15 min to mix, and place in a 4°C refrigerator to precool for 3 h;

[0028] Step 3: Take the solution out of the refrigerator, stir it in an ice-water bath for 5 minutes, then add 900 μl of 0.1 M N,N'-methylenebisacrylamide solution, continue to stir it in an ice-water bath for 5 minutes, then add 120 μl of tetramethylethylenediamine, continue to stir it in an ice-water bath for 5 minutes, then add 8 mL of 75 g / L ammonium persulfate solution;

[0029] Step 4: After stirring for 10 min in an ice-water bath, transfer to a polytetrafluoroethylene container on ice with an inner height of 1.6 cm, add solution to a height of 1.0 cm, take it out of the ice and quickly place it in a vacuum desiccator containing color-changing silica gel, use a circulating water vacuum pump to evacuate for 30 min, seal it overnight for cross-linking, and obtain a sponge-like gel;

[0030] Step 5: Soak the obtained sponge-like gel in 4L of 5% potassium chloride solution by mass, continue swelling for 14 days, change the solution every day, take it out, and obtain a flexible fruit and vegetable coolant based on the sponge-like gel.

[0031] The phase transition temperature and latent heat of the gel refrigerant obtained in Example 1 were measured using a differential scanning calorimeter (DSC 3, Mettler Toledo, Swiss). The temperature was set to -70°C, kept stable for 5 minutes, and then raised to 30°C at a rate of 10 K / min. The carrier gas was nitrogen and the purge rate was 50 mL / min. The results are shown in Table 1. Figure 1 The phase change temperature was measured to be about -2.5 degrees Celsius and the latent heat of phase change was 307.9 J / g. Example 2

[0032] Step 1: Prepare a 16% acrylamide aqueous solution, a 0.1 M N,N'-methylenebisacrylamide solution, and a 75 g / L ammonium persulfate solution for later use;

[0033] Step 2: Take 50 g of acrylamide aqueous solution in a beaker, stir in an 80°C water bath, and gradually add 1.6 g of sodium alginate powder, stir thoroughly for 15 min to mix, and place in a 4°C refrigerator to precool for 2 h;

[0034] Step 3: Take the solution out of the refrigerator, stir it in an ice-water bath for 5 minutes, then add 225 μl of 0.1 M N,N'-methylenebisacrylamide solution, continue to stir it in an ice-water bath for 5 minutes, then add 30 μL of tetramethylethylenediamine, continue to stir it in an ice-water bath for 5 minutes, then add 2 mL of 75 g / L ammonium persulfate solution;

[0035] Step 4: After stirring for 10 min in an ice-water bath, transfer to a polytetrafluoroethylene container on ice with an inner height of 1.6 cm, add solution to a height of 1.0 cm, take it out of the ice and quickly place it in a vacuum desiccator containing color-changing silica gel, use a circulating water vacuum pump to evacuate for 30 min, seal it overnight for cross-linking, and obtain a sponge-like gel;

[0036] Step 5: Soak the obtained sponge-like gel in 1L of potassium chloride solution with mass fractions of 0%, 2.5%, 5%, 7.5%, and 10%, and continue to swell for 14 days. Change the solution every day and take it out to obtain a flexible fruit and vegetable coolant based on the sponge-like gel.

[0037] The phase transition temperature and latent heat of the gel refrigerant obtained in this embodiment were measured using a differential scanning calorimeter (DSC 3, Mettler Toledo, Swiss). The temperature was set to start from -70°C, kept stable for 5 min, and then raised to 30°C at a rate of 10 K / min. The carrier gas was nitrogen, and the purge rate was 50 mL / min. The measured phase transition temperature and latent heat are shown in Table 1.

[0038] Table 1: Relationship between gel coolant performance and KCl concentration

[0039]

[0040] Comparative Example 1

[0041] The steps of this comparative example are basically the same as those of Example 1, except that there is no vacuuming step in step (iv), that is, after being taken out from the ice, it is quickly placed in a vacuum desiccator containing color-changing silica gel, sealed and cross-linked overnight to obtain a comparative gel.

[0042] The gel prepared in Example 1 and the gel prepared in Comparative Example 1 were cut into 30 g of columns with a diameter of 5 cm, respectively, and placed on the pressure sensor. A polypropylene ball with a diameter of 3 cm and a density of 0.9 g / cm3 was released from 30 cm away from the gel surface and allowed to fall freely onto the gel surface. The impact force was recorded, and a blank control without the gel was performed at the same time. The results are shown in FIG. Figure 2 As shown by Figure 2 It can be seen that the sponge-like gel prepared in Example 1 has significant impact resistance, which is better than the gel prepared in Comparative Example 1 and the blank control.

[0043] Experimental Example 1

[0044] 100 g of the gel coolant obtained in Example 1 was cut into small pieces of about 10 g and frozen in a -20°C refrigerator for 48 h with 100 g of commercial coolant. The commercial coolant was a cold-proof deep water ice pack produced by Bingshuirun Industry.

[0045] Take two foam boxes, the internal space size of the foam box is 30 cm×20 cm×15 cm. Fix temperature probes on both sides of the foam box, the two probes are at the same height, one probe is closer to the coolant, and the other probe is farther away from the coolant. Place the gel coolant prepared in Example 1 and the commercial coolant in the foam box, cover and seal, place in a 25℃ incubator, and record and observe the temperature changes. The results are as follows: Figure 3 As shown, it can be found that the cooling effect is better when the gel coolant prepared in Example 1 of the present application is placed, and there is no uneven distribution of the cool storage effect.

[0046] Experimental Example 2

[0047] 100 g of the gel coolant obtained in Example 1 was cut into small pieces of about 10 g and frozen in a -20°C refrigerator for 48 h.

[0048] Take two foam boxes with an internal space size of 30 cm×20 cm×15 cm. Place 200 g of winter jujube in the foam box for the control group, and place 200 g of winter jujube and 100 g of gel coolant in the foam box for the gel group, cover and seal, place in a 25℃ incubator, take out after 24 hours, and continue to store at room temperature for 24 hours.

[0049] The hardness of winter jujube was tested during storage. The results are as follows: Figure 4 It can be found that the gel coolant can significantly delay the softening of winter jujube during storage.

Claims

1. A method for preparing a spongy gel phase change coolant for preserving fruits and vegetables, characterized in that The following steps are involved: 1) Add sodium alginate powder to the acrylamide aqueous solution while stirring it in a water bath at 80°C, stir thoroughly for 10-30 minutes, precool it in a refrigerator at 4°C for 1-3 hours, and set aside. The amount of sodium alginate powder added is 2.2-3.2 g per 100 g of acrylamide aqueous solution; 2) The mixed solution precooled in step 1) is stirred for 5 minutes in an ice water bath, and after stirring, an N,N'-methylenebisacrylamide aqueous solution is added to the mixed solution as a cross-linking agent, and the mixture is stirred for 5 minutes in an ice water bath to obtain a mixed solution containing a cross-linking agent, wherein the amount of the N,N'-methylenebisacrylamide aqueous solution added is 450 μl per 100 g of the acrylamide aqueous solution; 3) The mixed solution containing the crosslinking agent obtained in step 2) is continuously stirred in an ice water bath, and then tetramethylethylenediamine is added as a polymerization accelerator. After continuing to stir for 5 minutes, an aqueous solution of ammonium persulfate is added as an initiator. The amount of tetramethylethylenediamine added is 60 μl per 100 g of the aqueous solution of acrylamide; 4) The mixed solution of step 3) was continuously stirred in an ice-water bath for 10 minutes, and then transferred to a polytetrafluoroethylene container on ice. After being taken out of the ice, it was quickly placed in a vacuum desiccator containing color-changing silica gel, and vacuumed with a circulating water vacuum pump for 30 minutes, and then sealed overnight for cross-linking to obtain a sponge-like gel; 5) soaking the sponge-like gel obtained in step 4) in a solution with a volume of 10-20 times, wherein the solution is a solution of potassium chloride with a mass fraction of 0-10%, replacing the solution every 24 hours, and soaking for 14 days; then taking it out, absorbing the residual water, and obtaining a sponge-like gel phase change coolant; The mass fraction of the acrylamide aqueous solution is 16%, the concentration of the N,N'-methylenebisacrylamide solution is 0.1M, and the concentration of the ammonium persulfate aqueous solution is 75 g / L.

2. The method for preparing a spongy gel phase change coolant for preserving fruits and vegetables as claimed in claim 1, characterized in that In step 4), the amount of the mixed solution added to the polytetrafluoroethylene container is such that the height of the mixed solution is between 1 / 3 and 1 / 2 of the height in the container.

3. The method for preparing a spongy gel phase change cool storage agent for preserving fruits and vegetables as claimed in claim 1, characterized in that Step 5) When using the prepared sponge-like gel phase change coolant, it can be cut into a suitable size according to packaging requirements, frozen in a -20°C refrigerator for 24 hours before use, and then taken out and placed directly in a fruit and vegetable packaging box.

4. A spongy gel phase-change refrigerant prepared by the preparation method according to any one of claims 1 to 3.

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