Preparation method of water-resistant intelligent colorimetric label for indicating food freshness

Abstract

The invention provides a preparation method of a water-resistant intelligent colorimetric label for indicating food freshness, and belongs to the technical field of food packaging. Methyl red and bromothymol blue are used as raw materials to prepare a mixed dye, then the mixed dye is added into a chitosan gelatin solution to obtain a water-phase solution, the water-phase solution is added into an oil phase containing an emulsifier for emulsification and crosslinking to form pH developing microspheres, and finally, a pouring method is adopted to prepare the water-resistant intelligent colorimetric label for indicating the food freshness. According to the prepared colorimetric label, methyl red and bromothymol blue are embedded in the ethyl cellulose membrane through a microsphere method, the response of the colorimetric label to acid gas is guaranteed, the influence of environment humidity on indication accuracy is avoided, methyl red and bromothymol blue cannot be dissolved out under high humidity, and therefore the colorimetric label has the advantages of being simple in preparation process, low in cost and the like. And the stability of the membrane in a high-humidity environment is ensured.

Description

Technical Field

[0001] The invention relates to the technical field of food packaging, and in particular to a method for preparing a water-resistant intelligent colorimetric label for indicating the freshness of food. Background Art

[0002] Fruits and vegetables are rich in essential vitamins and dietary fiber that are beneficial to human health. However, they are easily damaged by mechanical, physical, chemical and microbial damage during storage and transportation after harvest, resulting in quality deterioration. Therefore, real-time detection of fruit and vegetable quality is very necessary.

[0003] Studies have found that during storage, the content of volatile gases such as carbon dioxide, aldehydes, sulfur compounds, ethylene, volatile organic acids, and ethanol fluctuates dramatically. Based on this, as the most widespread form of smart labels, they are usually composed of high molecular polymers (usually polysaccharides or proteins) 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, they are widely used, but because they contain a large number of hydroxyl groups and are highly hygroscopic, they are hindered from being used as food packaging films. And because fruits and vegetables are usually stored in high humidity environments, hydrophilic polymer-based colorimetric sensing films lack long-term stability under such conditions.

[0005] At present, the research mainly improves the water resistance of PVA film by chemical cross-linking, physical cross-linking (such as freeze-thaw treatment), inorganic nanomaterial composite method and polymer blending method. The polysaccharide chitosan molecules are easily cross-linked to form a spatial network structure, which has good film-forming properties. This film has certain tensile strength and solvent resistance. Chitosan is a biodegradable natural substance, which is green and environmentally friendly with little pollution. However, the strength of pure chitosan film lags far behind that of synthetic polymer film, so it is difficult to use on a large scale in the food industry. At present, the main research tends to prepare blended films of chitosan with polysaccharides, proteins and synthetic polymers.

[0006] As a water-insoluble film-forming material, ethyl cellulose has strong moisture barrier properties. However, due to its own hydrophobicity, it hinders the combination of water-soluble colorants and characteristic gas molecules of fruits and vegetables when used as an indicator label, thus limiting the application of ethyl cellulose in smart labels.

[0007] Biodegradable materials in the prior art, such as PLA, PHAs, thermoplastic starch (TPS), chitosan, cellulose, gelatin, etc., all have the advantages of good environmental protection, safety and microbial degradation, but they are difficult to compare with plastic films in performance and have poor waterproofness. Summary of the invention

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

[0009] In order to achieve the above effects, the present invention adopts the following technical solutions:

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

[0011] (1) Preparation of mixed dye: adding methyl red and bromothymol blue into an ethanol aqueous solution and stirring to obtain a mixed dye;

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

[0013] (3) Preparation of pH color-developing microspheres: adding the mixed dye prepared in step (1) to the chitosan gelatin solution prepared in step (2) and stirring to obtain an aqueous phase solution, and then adding the aqueous phase solution to an oil phase containing an emulsifier for emulsification and crosslinking to form pH color-developing microspheres;

[0014] (4) Preparation of pH color-developing microsphere composite film: Soak the pH color-developing microspheres prepared in step (3) in sodium hydroxide solution for 8 to 10 hours and then filter, then add to ethyl cellulose ethanol solution and mix evenly, add castor oil and stir to mix evenly, then cast into a mold, and vacuum dry under the conditions of 0.1 to 0.2 Pa and 40 to 45° C. to obtain a pH color-developing microsphere composite film, which is the water-resistant intelligent colorimetric label for indicating the freshness of food.

[0015] Preferably, the concentration of methyl red in the mixed dye in step (1) is 0.1-0.2 g / 100 mL, the concentration of bromothymol blue is 0.05-0.1 g / 100 mL; and 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, the concentration of chitosan is 1-2 g / 100 mL; and 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 in step (3) is: at a temperature of 50 to 60° C., slowly adding the aqueous phase solution to the oil phase containing the emulsifier while stirring, continuing to stir to form a W / O emulsion, then cooling to 15 to 25° C., adding the cross-linking agent solution, stirring and curing for 25 to 35 minutes, and then continuing to cure at 4 to 8° C. for 20 to 25 hours, collecting the microspheres by centrifugation, and washing with acetone or isopropanol to form the pH color-developing microspheres.

[0020] More preferably, the cross-linking agent is glutaraldehyde, and the mass concentration of glutaraldehyde in the cross-linking agent solution is 20-25%; 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% to 7%;

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

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

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

[0026] 2. Methyl red and bromothymol blue embedded in the hydrophobic polymer membrane will not dissolve under high humidity, thus ensuring the stability of the membrane in a high humidity environment.

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

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

[0029] Figure 1 This is a photo of the pH color-developing microspheres in Example 1 under an electron microscope.

[0030] Figure 2 This is a photo of the water-resistant smart colorimetric label used to indicate the freshness of food in Example 1;

[0031] Figure 3 This is an electron microscope photograph of the water-resistant intelligent colorimetric label used to indicate the freshness of food in Example 1.

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

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

[0034] Figure 6 The colorimetric label prepared in Example 1 was tested for its response to acidic gas;

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

[0036] Figure 8 This is a stability experiment of pH colorimetric microsphere composite membrane and gelatin membrane in aqueous solution. DETAILED DESCRIPTION

[0037] The present invention will be further described below in conjunction with the embodiments.

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

[0039] Example 1

[0040] A method for preparing a water-resistant intelligent colorimetric label for indicating the freshness of food, comprising the following steps:

[0041] (1) Preparation of mixed dye: Weigh 0.1 g of bromothymol blue and 0.15 g of methyl red powder, add them into 100 mL of 50% ethanol aqueous solution, stir and mix to obtain a mixed dye;

[0042] (2) Preparation of chitosan gelatin solution: 1 g of chitosan and 1 g of gelatin powder were added to 100 mL of 1.5% 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) was added to the chitosan gelatin solution prepared in step (2) and stirred to obtain an aqueous solution;

[0044] Then, 10 mL of the aqueous phase solution was slowly added to 50 mL of liquid paraffin containing 2 mL of Span80 at 55 ° C at 55 ° C and 750 r / min electromagnetic stirring conditions, and stirred for 30 minutes to form a W / O emulsion, and then cooled to 15 ° C, 3 mL of 25% glutaraldehyde solution was added, stirred and solidified for 30 minutes, and then continued to solidify at 4 ° C for 24 hours, and the microspheres were collected by centrifugation, washed with isopropanol, filtered, and then dried in a freezer to form pH color-developing microspheres;

[0045] (4) Preparation of pH color-developing microsphere composite film: Take 2 g of the pH color-developing microspheres prepared in step (3), soak them in 0.05 mol / L sodium hydroxide solution for 8 h, filter them, add them to 5% ethyl cellulose ethanol solution and mix them evenly, add 0.7 mL of castor oil, stir and mix them evenly, then cast them into a mold, and vacuum dry them under the conditions of 0.1 Pa and 40° C. to obtain a pH color-developing microsphere composite film; this is the water-resistant intelligent colorimetric label (denoted as ECMB) for indicating the freshness of food.

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

[0047] Photo of a water-resistant smart colorimetric label used to indicate food freshness Figure 2 ;

[0048] Electron microscope photo of water-resistant smart colorimetric label used to indicate food freshness Figure 3 .

[0049] Comparative Example 1

[0050] A method for preparing a polysaccharide colorimetric label, comprising the following steps:

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

[0052] The gelatin solution and the chitosan solution were mixed and stirred at a volume ratio of 1:2 at 45°C, and recorded as CS solution;

[0053] Step 2: Add 7 ml of BTBMR indicator solution to the CS solution; stir at 45°C for 1 hour and then cast into a mold, set the vacuum degree to 0.1 Pa and the temperature to 40°C, and vacuum dry to form a film (referred to as CGMB).

[0054] Comparative Example 2

[0055] A method for preparing a polysaccharide colorimetric label, comprising the following steps:

[0056] Step 1: Dissolve 1 g of gelatin in 100 mL of distilled water, stir in a 60°C water bath for 30 min, then add 0.4 mg / g of glycerol to obtain a 1% (w / v) gelatin solution;

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

[0058] Step 2: Add 7 ml of BTBMR indicator solution to the GE solution; stir at 45 °C for 1 hour and then cast into a mold, set the vacuum degree to 0.1 Pa and the temperature to 40 °C, and vacuum dry to form a film to obtain a gelatin film (referred to as GEMB).

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

[0060] (1) Water resistance test: The moisture content (MC) is calculated according to formula (1): a label with a mass of m1 is placed in an oven at a temperature of 105°C and dried until the mass of the label is constant. The mass of the label after being taken out is m2. Three parallel samples are tested each time, and the average value is taken as the moisture content of the label.

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

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

[0063] (2) Water solubility (WS) is calculated according to formula (2): Take 3 pieces of the prepared indicator labels, dry them until the mass remains unchanged, record the mass as m3, soak them in grade tertiary water at room temperature for 24 hours, take them out and dry them until the mass remains unchanged, record the mass as m4.

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

[0065] In formula (2), m3 is the value when the label mass remains unchanged for the first time; m4 is the value when the label mass remains unchanged again after being immersed.

[0066] The test results of water solubility and water content are as follows Figure 4 As shown, from Figure 4 It can be seen that the microspheres are embedded in the ethyl cellulose membrane, and the colorimetric membrane as a whole shows excellent water resistance and is not easily soluble in water.

[0067] (3) Application of pH color microsphere composite membrane in green pepper: Weigh 60g of fresh-cut green pepper and put it in a transparent box. Paste ECMB on the lid of the box. Test the color development of the composite membrane of fresh-cut green pepper at 4℃. Figure 5 . Figure 5 The medium green to yellow color changes represent the transition from fresh to not-so-fresh bell peppers.

[0068] (4) Color development experiment of pH color microsphere composite membrane under acetic acid gas: 1 mL of acetic acid was added to a culture dish and ECMB was pasted on the lid. As the acetic acid solution evaporated, the response of ECMB to acidic gas was tested. The results are shown in Figure 2. Figure 6 , Figure 6 It shows that ECMB turns yellow under acidic gas.

[0069] (5) Stability test of pH color microsphere composite membrane in aqueous solution: ECMB was placed in aqueous solution to test whether methyl red and bromothymol blue in the composite membrane would overflow. The results are as follows: Figure 7 .

[0070] (6) Stability test of pH color microsphere composite membrane and gelatin membrane in aqueous solution: The gelatin membrane and pH color microsphere composite membrane were soaked in pH=7 buffer solution respectively, and then the supernatant of the soaking solution was taken for 10 minutes, 30 minutes, and 60 minutes to scan its absorbance change. The results are shown in Figure 8 , where CG is the absorbance change of gelatin film, and EC is the absorbance change of pH colorimetric microsphere composite film.

[0071] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a water-resistant intelligent colorimetric label for indicating the freshness of food, characterized in that: The following steps are involved: (1) Preparation of mixed dye: adding methyl red and bromothymol blue into an ethanol aqueous solution and stirring to obtain a mixed dye; (2) Preparation of chitosan gelatin solution: chitosan and gelatin are added into an acetic acid aqueous solution and stirred to obtain a chitosan gelatin solution; (3) Preparation of pH color-developing microspheres: adding the mixed dye prepared in step (1) to the chitosan gelatin solution prepared in step (2) and stirring to obtain an aqueous phase solution, and then adding the aqueous phase solution to an oil phase containing an emulsifier for emulsification and crosslinking to form pH color-developing microspheres; (4) Preparation of pH color-developing microsphere composite film: Soak the pH color-developing microspheres prepared in step (3) in sodium hydroxide solution for 8 to 10 hours and then filter, then add to ethyl cellulose ethanol solution and mix evenly, add castor oil and stir to mix evenly, then cast into a mold, and vacuum dry under the conditions of 0.1 to 0.2 Pa and 40 to 45° C. to obtain a pH color-developing microsphere composite film, which is the water-resistant intelligent colorimetric label for indicating the freshness of food.

2. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: The concentration of methyl red in the mixed dye of step (1) 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%.

3. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: 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%.

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

5. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: In step (3), the emulsifier 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%.

6. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: The emulsification cross-linking method of step (3) is: at a temperature of 50 to 60° C., slowly adding the aqueous phase solution to the oil phase containing the emulsifier while stirring, continuing to stir to form a W / O emulsion, then cooling to 15 to 25° C., adding the cross-linking agent solution, stirring and curing for 25 to 35 minutes, and then continuing to cure at 4 to 8° C. for 20 to 25 hours, collecting the microspheres by centrifugation, and washing with acetone or isopropanol to form the pH color-developing microspheres.

7. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 6, characterized in that: The cross-linking agent is glutaraldehyde, and the mass concentration of glutaraldehyde in the cross-linking agent solution is 20-25%; the volume ratio of the cross-linking agent solution to the oil phase is 1-5:

50.

8. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: The volume ratio of the aqueous phase solution to the oil phase in step (3) is 1:4-6.

9. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: The mass concentration of ethyl cellulose in the ethyl cellulose ethanol solution in step (4) is 4% to 7%.

10. The method for preparing a water-resistant intelligent colorimetric label for indicating food freshness according to claim 1, characterized in that: In step (4), the pH color developing microspheres and the ethyl cellulose ethanol solution are used in a ratio of 1-4 g:100 mL.

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