A method for preparing a water-resistant intelligent colorimetric label for indicating food freshness

Abstract

The application provides a preparation method of a water-resistant intelligent colorimetric label for indicating food freshness, and belongs to the technical field of food packaging. A mixed dye is prepared by taking methyl red and bromothymol blue as raw materials, then the aqueous phase solution is obtained by adding the mixed dye into a chitosan gelatin solution, and the aqueous phase solution is added into an oil phase containing an emulsifier to perform emulsification and crosslinking, so that pH color developing microspheres are formed, and finally, the pouring method is adopted to prepare the water-resistant intelligent colorimetric label for indicating food freshness. The colorimetric label prepared by the application embeds the methyl red and the bromothymol blue in the ethyl cellulose film by using the microsphere method, which not only ensures the response of the colorimetric label to the acidic gas, but also avoids the influence of the environmental humidity on the indication accuracy, and the methyl red and the bromothymol blue cannot be dissolved out under high humidity, so that the stability of the film in the high humidity environment is ensured.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of food packaging, in particular to a preparation method of a water-resistant intelligent colorimetric label for indicating the freshness of food. BACKGROUND

[0002] Fruits and vegetables are rich in essential vitamins and dietary fiber that are beneficial to human health. However, they are prone to mechanical, physical, chemical and microbial damage during post-harvest storage and transportation, leading to quality deterioration. Therefore, it is necessary to detect the quality of fruits and vegetables in real time.

[0003] Research has found that the content of volatile gases such as carbon dioxide, aldehydes, sulfur compounds, ethylene, volatile organic acids and ethanol fluctuates dramatically during storage. Based on this, as the most widely used form of intelligent label, it is usually composed of a high molecular polymer (usually polysaccharide or protein) accompanied by chemical / natural pigments.

[0004] Among them, polysaccharides such as PVA films have good gas (oxygen, nitrogen) barrier properties and tensile properties, and have certain mechanical strength, oil resistance and transparency. Therefore, it is widely used, but because it contains a large number of hydroxyl groups itself, it has strong hygroscopicity, which hinders its application as a food packaging film. And because fruits and vegetables are usually stored in a high-humidity environment, hydrophilic polymer-based colorimetric sensing films lack long-term stability under such conditions.

[0005] Currently, research mainly improves the water resistance of PVA films through chemical crosslinking method, physical crosslinking method (such as freeze-thaw treatment), inorganic nano-material composite method and polymer blending method, etc. Chitosan molecules are easy to crosslink to form a spatial network structure, and have good film-forming properties. Such films have certain tensile strength and solvent resistance. And chitosan is a biodegradable natural substance, which is green, environmentally friendly and less polluting. However, the strength of pure chitosan film is far behind that of synthetic polymer film, so it is difficult to be widely used in the food industry. Currently, the main research trend is to prepare blended films of chitosan with polysaccharides, proteins and synthetic polymers.

[0006] Ethyl cellulose, as a water-insoluble film-forming material, has strong moisture barrier properties, but due to its own hydrophobicity, it hinders the combination of water-soluble color developing agents with fruit and vegetable characteristic gas molecules when used as an indicator label, thereby limiting the application of ethyl cellulose in intelligent labels.

[0007] Biodegradable materials in the prior art, such as PLA, PHAs, thermoplastic starch (TPS), chitosan, cellulose, gelatin, etc., all have good environmental protection, safety and microbial degradation advantages, but their performance is difficult to compare with plastic film, and they have poor water resistance. SUMMARY

[0008] In view of this, the present invention provides a method for preparing a water-resistant smart colorimetric label for indicating food freshness. The colorimetric label prepared by this method has high water resistance and can be stored stably in a high humidity environment.

[0009] To achieve the above effects, the present invention adopts the following technical solution:

[0010] A method for preparing a water-resistant smart colorimetric label for indicating food freshness includes the following steps:

[0011] (1) Preparation of mixed dyes: Methyl red and bromothymol blue are added to an aqueous ethanol solution and stirred to obtain a mixed dye;

[0012] (2) Preparation of chitosan gelatin solution: Chitosan and gelatin are added to an aqueous acetic acid solution and stirred to obtain a chitosan gelatin solution;

[0013] (3) Preparation of pH colorimetric microspheres: The mixed dye prepared in step (1) is added to the chitosan gelatin solution prepared in step (2) and stirred to obtain an aqueous solution. Then, the aqueous solution is added to an oil phase containing an emulsifier for emulsification and crosslinking to form pH colorimetric microspheres.

[0014] (4) Preparation of pH colorimetric microsphere composite membrane: The pH colorimetric microspheres prepared in step (3) were soaked in sodium hydroxide solution for 8-10 hours and then filtered. They were then added to ethyl cellulose ethanol solution and mixed. Castor oil was added and stirred until mixed. The mixture was then cast into a mold and vacuum dried at 0.1-0.2 Pa and 40-45 °C to obtain the pH colorimetric microsphere composite membrane, which is the water-resistant smart colorimetric label used to indicate the freshness of food.

[0015] Preferably, in step (1), the concentration of methyl red in the mixed dye is 0.1-0.2 g / 100 mL, and the concentration of bromothymol blue is 0.05-0.1 g / 100 mL; the volume concentration of ethanol in the ethanol aqueous solution is 30%-70%.

[0016] Preferably, in step (2), the concentration of gelatin in the chitosan gelatin solution is 1-2 g / 100 mL, and the concentration of chitosan is 1-2 g / 100 mL; the volume concentration of acetic acid in the acetic acid aqueous solution is 1-2%.

[0017] Preferably, the volume ratio of the mixed dye to the chitosan gelatin solution in step (3) is 5-10:90-110.

[0018] Preferably, the emulsifier in step (3) is Span 80, and the oil phase is liquid paraffin; the volume concentration of the emulsifier in the oil phase containing the emulsifier is 1% to 5%.

[0019] Preferably, the emulsification cross-linking method of step (3) is: slowly adding the aqueous phase solution into the oil phase containing emulsifier under stirring at a temperature of 50-60 DEG C, continuing to stir to form W / O emulsion, then cooling to 15-25 DEG C, adding cross-linking agent solution, stirring for 25-35 min, and then continuing to solidify at 4-8 DEG C for 20-25 h, centrifuging to collect the microspheres, washing with acetone or isopropyl alcohol, and forming the pH colorimetric microspheres.

[0020] More preferably, the cross-linking agent is glutaraldehyde, the mass concentration of glutaraldehyde in the cross-linking agent solution is 20-25%, and the volume ratio of the cross-linking agent solution to the oil phase is 1-5:50.

[0021] Preferably, the volume ratio of the aqueous phase solution to the oil phase in step (3) is 1:4-6.

[0022] Preferably, the mass concentration of ethyl cellulose in the ethyl cellulose ethanol solution in step (4) is 4%-7%.

[0023] Preferably, the volume ratio of the pH colorimetric microspheres to the ethyl cellulose ethanol solution in step (4) is 1-4 g:100 mL.

[0024] Compared with the prior art, the present application has the following beneficial effects:

[0025] 1. The methyl red and bromothymol blue are embedded in the water-insoluble ethyl cellulose film by the microsphere method to prepare a hydrophobic film, which not only ensures the response of the colorimetric label to the acidic gas, but also avoids the influence of environmental humidity on the indication accuracy, and can be used as a gas sensor in a high-humidity environment.

[0026] 2. The methyl red and bromothymol blue embedded in the hydrophobic polymer film will not be dissolved out under high humidity, which ensures the stability of the film in a high-humidity environment.

[0027] 3. The prepared film is prepared by casting method, which is suitable for industrial mass production, and the prepared film is safe and non-toxic, can be directly used in the field of food packaging, and has relatively economical cost.

[0028] 4. The prepared film can react with the gas volatilized in the food spoilage process to quickly produce a color visible to the naked eye, so that the freshness of the food can be detected in real time and non-destructively. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is a photo of the pH colorimetric microspheres in Example 1 under electron microscope.

[0030] Figure 2 It is a photo of the water-resistant intelligent colorimetric label for indicating the freshness of food in Example 1.

[0031] Figure 3 The electron microscope photo of the water-resistant intelligent colorimetric label for indicating the freshness of food prepared in Example 1.

[0032] Figure 4 The water content and water-solubility results of the colorimetric labels prepared in the examples and comparative examples;

[0033] Figure 5 The application results of the water-resistant film prepared in Example 1 in fresh-cut green peppers;

[0034] Figure 6 The response experiment of the colorimetric label prepared in Example 1 to acidic gas;

[0035] Figure 7 The stability experiment of the colorimetric label prepared in Example 1 in water;

[0036] Figure 8 The stability experiment of the pH color-developing microsphere composite film and the gelatin film in aqueous solution. DETAILED DESCRIPTION

[0037] The application will be further described below in conjunction with examples.

[0038] The test methods or test methods described in the following examples are all conventional methods unless otherwise specified; the raw materials and auxiliaries are all obtained from conventional commercial channels or prepared by conventional methods unless otherwise specified.

[0039] Example 1

[0040] A preparation method of a water-resistant intelligent colorimetric label for indicating the freshness of food, the steps are as follows:

[0041] (1) Preparation of mixed dye: 0.1 g of bromothymol blue and 0.15 g of methyl red powder are weighed and added to 100 mL of 50% volume concentration ethanol aqueous solution and stirred to mix, to obtain a mixed dye;

[0042] (2) Preparation of chitosan gelatin solution: 1 g of chitosan and 1 g of gelatin powder are taken and added to 100 mL of 1.5% volume concentration acetic acid aqueous solution and stirred at 50°C for 30 min, to obtain a chitosan gelatin solution;

[0043] (3) Preparation of pH color-developing microspheres: 7 mL of the mixed dye prepared in step (1) is added to the chitosan gelatin solution prepared in step (2) and stirred, to obtain an aqueous solution;

[0044] Then take 10 mL aqueous solution, under the condition of 55℃, 750r / min electromagnetic stirring, slowly add 50 mL temperature 55℃ liquid paraffin containing 2 mL Span80, stirring 30 min to form W / O emulsion, then cooling to 15℃, add 3 mL 25% glutaraldehyde solution, stirring solidification 30 min, continue to solidify at 4℃ for 24 h, centrifugal collection of microspheres, first with isopropanol washing, filtration, after freezer drying, forming pH color microspheres;

[0045] (4) Preparation of pH color microspheres composite film: take 2g pH color microspheres prepared in step (3) and soak in 0.05mol / L sodium hydroxide solution for 8h, then filter and add to 5% ethyl cellulose ethanol solution, mix evenly, add 0.7mL castor oil, stir and mix evenly, then pour into the mold, vacuum drying at 0.1Pa, 40℃, to obtain pH color microspheres composite film; it is the water-resistant intelligent colorimetric label for indicating food freshness (marked as ECMB).

[0046] The photo of pH color microspheres prepared in Example 1 under electron microscope is as follows Figure 1 ;

[0047] The photo of water-resistant intelligent colorimetric label for indicating food freshness is as follows Figure 2 ;

[0048] The electron microscope photo of water-resistant intelligent colorimetric label for indicating food freshness is as follows Figure 3 .

[0049] Comparative Example 1

[0050] A preparation method of a polysaccharide colorimetric label, the steps are as follows:

[0051] Step 1: take 1g chitosan and dissolve in 1% acetic acid solution, magnetically stir for 24h after swelling, add 0.75mg / g glycerol solution, to obtain 1%(w / v) chitosan solution;

[0052] Mix the gelatin solution and chitosan solution at 45℃ with a volume ratio of 1:2, and stir evenly, marked as CS solution;

[0053] Step 2: add 7ml BTBMR indicator solution to the CS solution respectively; pour into the mold after stirring at 45℃ for 1h, set the vacuum degree to 0.1Pa and the temperature to 40℃, vacuum drying to form a film (marked as CGMB).

[0054] Comparative Example 2

[0055] A preparation method of a polysaccharide colorimetric label, the steps are as follows:

[0056] Step 1: 1 g of gelatin was dissolved in 100 mL of distilled water, and 0.4 mg / g of glycerol was added after stirring in a 60 °C water bath for 30 min to obtain a 1% (w / v) gelatin solution;

[0057] The gelatin solution was mixed and stirred uniformly at 45 °C, and was recorded as the GE solution;

[0058] Step 2: 7 ml of the BTBMR indicator solution was added to the GE solution, respectively, and was cast into a mold after stirring at 45 °C for 1 h. The vacuum degree was set to 0.1 Pa and the temperature was set to 40 °C, and the film was dried to obtain a gelatin film (recorded as GEMB).

[0059] The colorimetric labels prepared in Example 1 and Comparative Examples 1 and 2 were tested for performance:

[0060] (1) Water resistance test: The water content (MC) was calculated according to formula (1): the mass of the label was m1, and the label was dried in an oven at a temperature of 105 °C until the mass of the label was constant. The mass of the label after removal was m2. Each time, 3 parallel samples of the label were tested, and the average value was taken as the water content of the label.

[0061] MC = m1-m2 / m1*100% (1)

[0062] In formula (1), m1 is the initial mass of the label, and m2 is the mass of the label after drying.

[0063] (2) Water solubility (WS) was calculated according to formula (2): 3 pieces of the prepared indicator label were taken, dried to a constant mass, and the mass was recorded as m3. The label was soaked in tertiary water at room temperature for 24 h, and then dried to a constant mass after removal, and the mass was m4.

[0064] WS = m3-m4 / m3*100% (2)

[0065] In formula (2), m3 is the value when the mass of the label is constant for the first time, and m4 is the value when the mass of the label is constant again after soaking.

[0066] The test results of water solubility and water content are shown in Figure 4 From Figure 4 It can be seen that the microspheres embedded in the ethyl cellulose film show excellent water resistance and are not easily dissolved in water.

[0067] (3) Application of pH color developing microsphere composite film in green pepper: 60 g of fresh-cut green pepper was placed in a transparent box, and the ECMB was pasted on the cover of the box. The color developing test results of the composite film at a temperature of 4 °C are shown in Figure 5 .The change from green to yellow represents the change from fresh to not fresh. Figure 5

[0068] (4) Coloration experiment of ECMB in acetic acid gas: 1 mL acetic acid was dropped on a petri dish, and ECMB was pasted on the cover. As the acetic acid solution volatilized, the response of ECMB to acidic gas was tested. The results are shown in Fig. 6, which indicates that ECMB turns yellow in acidic gas. Figure 6 , Figure 6

[0069] (5) Stability experiment of ECMB in aqueous solution: ECMB was taken in aqueous solution to test whether methyl red and bromothymol blue in the composite film would overflow. The results are shown in Fig. 7. Figure 7 .

[0070] (6) Stability experiment of ECMB and gelatin film in aqueous solution: gelatin film and ECMB were respectively taken in pH = 7 buffer solution, and then the supernatant of the soaking solution at 10 minutes, 30 minutes and 60 minutes was scanned for absorbance change. The results are shown in Fig. 8. Figure 8 , wherein CG is the absorbance change of gelatin film, and EC is the absorbance change of ECMB.

[0071] The above only describes the preferred embodiments of the present application, and it should be noted that those skilled in the art can make several improvements and refinements without departing from the principles of the present application, and these improvements and refinements should also be considered within the protection scope of the present application.​

Claims

1. A method for preparing a water-resistant intelligent colorimetric label for indicating the freshness of food, characterized in that, The method comprises the following steps: (1) Preparation of mixed dye: methyl red and bromothymol blue are added into an aqueous ethanol solution and stirred to obtain a mixed dye; (2) Preparation of chitosan gelatin solution: chitosan and gelatin are added into an aqueous acetic acid solution and stirred to obtain a chitosan gelatin solution; (3) Preparation of pH color developing microspheres: the mixed dye prepared in step (1) is added into the chitosan gelatin solution prepared in step (2) and stirred to obtain an aqueous phase solution, and then the aqueous phase solution is added into an oil phase containing an emulsifier to perform emulsification and crosslinking, so as to form pH color developing microspheres; (4) Preparation of pH color developing microsphere composite film: the pH color developing microspheres prepared in step (3) are soaked in a sodium hydroxide solution for 8-10 hours, filtered, then added into an ethyl cellulose ethanol solution, stirred, and then cast into a mold after adding castor oil, and vacuum dried under the conditions of 0.1-0.2 Pa and 40-45 DEG C, so as to obtain a pH color developing microsphere composite film, which is the water-resistant intelligent colorimetric label for indicating food freshness. In step (3), the emulsifier is Span 80, the oil phase is liquid paraffin, and the volume concentration of the emulsifier in the oil phase containing the emulsifier is 1%-5%. In step (3), the method of emulsification and crosslinking is as follows: the aqueous phase solution is slowly added into the oil phase containing the emulsifier under stirring at a temperature of 50-60 DEG C, W / O emulsion is formed by continuing to stir, then the temperature is lowered to 15-25 DEG C, a crosslinking agent solution is added, and stirring is continued for 25-35 min, then the temperature is lowered to 4-8 DEG C, and the crosslinking is continued for 20-25 h, the microspheres are collected by centrifugation, washed with acetone or isopropyl alcohol, and the pH color developing microspheres are formed. In the above method, the crosslinking agent is glutaraldehyde, the mass concentration of glutaraldehyde in the crosslinking agent solution is 20-25%, and the volume ratio of the crosslinking agent solution to the oil phase is 1-5:

50.

2. A process for the preparation of water-resistant intelligent colourimetric label for indicating freshness of food as claimed in claim 1 wherein, In step (1), the concentration of methyl red in the mixed dye is 0.1-0.2 g / 100 mL, and the concentration of bromothymol blue is 0.05-0.1 g / 100 mL; and the volume concentration of ethanol in the aqueous ethanol solution is 30%-70%.

3. The method of preparing water-resistant intelligent colorimetric label for indicating freshness of food as claimed in claim 1 wherein, In step (2), the concentration of gelatin in the chitosan gelatin solution is 1-2 g / 100 mL, and the concentration of chitosan is 1-2 g / 100 mL; and the volume concentration of acetic acid in the aqueous acetic acid solution is 1-2%.

4. The method of preparing water-resistant intelligent colorimetric label for indicating freshness of food as claimed in claim 1 wherein, In step (3), the volume ratio of the mixed dye to the chitosan gelatin solution is 5-10:90-110.

5. The method of preparing water-resistant intelligent colorimetric label for indicating freshness of food as claimed in claim 1 wherein, In step (3), the volume ratio of the aqueous phase solution to the oil phase is 1:4-6.

6. The method of preparing water-resistant intelligent comparative color label for indicating freshness of food according to claim 1, wherein, In step (4), the mass concentration of ethyl cellulose in the ethyl cellulose ethanol solution is 4%-7%.

7. The method of preparing water-resistant intelligent colorimetric label for indicating freshness of food as claimed in claim 1 wherein, In step (4), the ratio of the pH color developing microspheres to the ethyl cellulose ethanol solution is 1-4 g:100 mL.

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