A freshness indicating film based on carminic acid and its preparation method and application

Abstract

The application discloses a freshness indicating film based on cochineal carmine and a preparation method thereof, and raw materials include cochineal carmine, a pH value regulator and a polymer base film. The application composites the cochineal carmine and a biological base material with good loading performance to obtain a food freshness indicating film, utilizes the characteristics that the cochineal carmine has strong light and heat stability and can have more stable structure under a complex light and heat environment, further improves the stability of the freshness indicating film, and further improves the accuracy of color freshness monitoring and indicating effect of the freshness indicating film, and solves the problem of poor light and heat stability of the freshness indicating film of a natural dye food indicator using anthocyanin, curcumin and shikonin.

Description

Technical Field

[0001] This invention relates to the field of smart packaging technology, and more specifically, to a freshness indicator film based on cochineal, its preparation method, and its application. Background Technology

[0002] Traditional methods for testing the freshness of meat suffer from drawbacks such as complex operation, potential damage to food integrity, time-consuming nature, and high cost. Currently, freshness indicator labels that detect the release of ammonia compounds (such as ammonia, methylamine, and dimethylamine) during changes in food freshness are being used to indicate the freshness of meat and meat products. These freshness indicator labels offer advantages such as low cost, real-time accuracy, good biodegradability, and ease of use.

[0003] CN201910716973.8 discloses a method for preparing a pH-sensitive food freshness indicator label and its sensitivity control method. First, a pH-sensitive natural dye is extracted to obtain its concentrate. Then, a natural polymer material is dissolved in a solvent to obtain a film-forming reserve solution. Next, a plasticizer, a water-absorbing agent, a phase transfer catalyst, and the pH-sensitive natural dye concentrate are added sequentially to the film-forming reserve solution, and the mixture is stirred thoroughly until uniformly dispersed to obtain a film-forming solution. The film-forming solution is then degassed by ultrasonication, slowly poured into a petri dish, and the solvent is dried to obtain the freshness indicator label. Based on the relationship between the color change point of the freshness indicator label and the food spoilage point, the pH value of the film-forming solution during the label preparation process is adjusted to match the two time points, achieving accurate indication. Specifically, this patent uses natural dyes such as anthocyanins, curcumin, and shikonin as indicators, and agarose, chitosan, starch, cellulose, pectin, gelatin, corn protein, and alginate as polymer matrices to prepare the freshness indicator label. First, the prepared labels suffer from low strength and poor stability. For example, while agar has advantages such as good biodegradability, wide availability, and good gelation properties, agar films are brittle and prone to shrinkage, thus affecting the indication effect of freshness indicators. More importantly, natural dyes such as anthocyanins, curcumin, and shikonin have poor stability and are sensitive to light and heat. Their structure can be destroyed at temperatures exceeding 40°C and under ultraviolet light. In polymer materials with higher mechanical strength, the dissolution or molding temperatures are also high, easily compromising the stability of the dyes during the mixing process between natural dyes and the polymer matrix. Furthermore, prolonged exposure to light also reduces the sensitivity of food indicators. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the shortcomings of existing food indicator films using natural dyes (anthocyanins, curcumin and shikonin) in terms of poor light and heat stability and poor mechanical properties of indicator labels, and to provide a freshness indicator film based on cochineal.

[0005] Another technical problem to be solved by the present invention is to provide a method for preparing a freshness indicator film based on cochineal.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A freshness indicator film based on cochineal, the raw materials include cochineal, pH adjuster and polymer base film, by weight, 10-22 parts polymer base film, 1-3 parts pH adjuster, and the amount of cochineal added is 0.5%-6% of the dry weight of polymer base film.

[0008] Cochineal is an anthraquinone pigment extracted from female cochineal insects on cacti. It is chemically stable and highly safe, exhibiting good antioxidant properties, safety, and pH sensitivity. In acidic environments, cochineal's structure is primarily composed of α-hydroxyanthraquinones and β-hydroxyanthraquinones, with colors mainly orange and red. When the pH (8-12) of the environment changes, the carbonyl and hydroxyl groups in the cochineal molecule ionize, causing changes in its UV-Vis absorption spectrum and color. This invention combines cochineal with a bio-based material with good loading capacity to create a food freshness indicator film. During food spoilage, a large amount of alkaline compounds are released, causing changes in the pH of the environment surrounding the indicator film. The cochineal within the film can detect alkaline gases and change color, thus providing information on the food's freshness.

[0009] Furthermore, the raw materials of the polymer-based film include polyvinyl alcohol and agar, wherein, by mass, there are 8-16 parts of polyvinyl alcohol and 2-6 parts of agar.

[0010] Polyvinyl alcohol, agar, and cochineal all contain a large number of hydroxyl and carboxyl groups, which readily form hydrogen bonds, thus improving the compatibility of the mixed solution. Simultaneously, the hydrogen and ionic interactions between polyvinyl alcohol, agar, and cochineal better immobilize the pH-sensitive dye, further enhancing the stability of the freshness indicator film and consequently improving the accuracy and effectiveness of color freshness monitoring. Furthermore, agar's strong hygroscopicity means that higher humidity can increase the pH sensitivity of the freshness indicator film, thereby enhancing its indication effect.

[0011] Furthermore, the raw materials of the freshness indicator film also include 2-5 parts of plasticizer, wherein the plasticizer is one or more of ethylene glycol or glycerin.

[0012] Furthermore, the pH adjuster is one or more of acetic acid, citric acid, and lactic acid.

[0013] A method for preparing a freshness indicator film based on cochineal red pigment, comprising the following steps:

[0014] S1. Heat and stir agar and polyvinyl alcohol in water until completely dissolved, then add plasticizer and continue stirring to obtain a mixed solution;

[0015] S2. Dissolve cochineal in water to obtain a cochineal solution, mix it with the mixed solution in step S1 above, adjust the pH of the solution to acidic, and obtain a mixed film-forming solution;

[0016] S3. Cast the mixed film-forming liquid obtained in step S2 into a film and dry it to obtain the freshness indicator film.

[0017] Furthermore, in the mixed solution described in step S1, the concentration of agar is 0.5%-3% w / v, and the concentration of polyvinyl alcohol is 6%-10% w / v.

[0018] Furthermore, the pH value of the mixed film-forming solution in step S1 is 2-3.

[0019] Furthermore, the stirring temperature in step S2 is 70-80°C.

[0020] Furthermore, the drying method described in step S2 is constant temperature and humidity drying, with the drying conditions controlled as relative humidity of 20%-40%, temperature of 40-60℃, and protection from light.

[0021] The aforementioned freshness indicator membrane based on cochineal was applied to the detection of freshness in pork and shrimp.

[0022] Compared with existing technologies, the beneficial effects are:

[0023] This invention utilizes the strong light and heat stability of cochineal, which allows for greater structural stability under complex light and heat environments. Polyvinyl alcohol and agar are used as a composite matrix, with agar serving as a carrier for cochineal immobilization. Furthermore, polyvinyl alcohol, agar, and cochineal contain numerous hydroxyl and carboxyl groups, readily forming hydrogen bonds. This not only improves the compatibility of the mixed solution and enhances the physical properties of the individual membrane, but also facilitates better fixation of pH-sensitive dyes through hydrogen and ionic interactions, further improving the stability of the freshness indicator membrane and thus enhancing the accuracy and effectiveness of color freshness monitoring. The cochineal used in this invention possesses strong antioxidant and antibacterial properties, which can also endow the freshness indicator membrane with antioxidant and antibacterial functions, thereby extending the shelf life of food.

[0024] The materials used in this invention are all non-toxic, biodegradable, safe and reliable, and more convenient and accurate than existing freshness testing methods. Attached Figure Description

[0025] Figure 1 The freshness indicator film prepared in Example 1;

[0026] Figure 2 This is a graph showing the color change of the freshness indicator film in Example 1 at different pH levels.

[0027] Figure 3 The UV spectra of the freshness indicator membrane in Example 1 at different pH values ​​are shown below.

[0028] Figure 4 Sensitivity diagram of the freshness indicator membrane prepared in Example 1 under different concentrations of ammonia;

[0029] Figure 5 Antioxidant properties of the freshness indicator film prepared in Example 1;

[0030] Figure 6 The color stability of the freshness indicator film prepared in Example 1 under ultraviolet light (70°C) for 15 days; where A is the color stability of cochineal and B is the color stability of the freshness indicator film.

[0031] Figure 7 The effect of the freshness indicator film prepared in Example 1 on pork and shrimp at 4°C is shown in the figure.

[0032] Figure 8 The images show scanning electron microscope (SEM) images of the cross-sections of the freshness indicator films prepared in Example 3 and Comparative Example 1; where A is the SEM image of the cross-section of the polyvinyl alcohol / agar film without cochineal prepared in Comparative Example 1, and B is the SEM image of the cross-section of the freshness indicator film prepared in Example 3.

[0033] Figure 9 The mechanical properties of the freshness indicator films prepared in Example 3 and Comparative Example 1 are shown in the figure. Detailed Implementation

[0034] The following examples further explain and clarify the invention, but the specific examples do not limit the invention in any way. Unless otherwise specified, the methods and equipment used in the examples are conventional methods and equipment in the art, and the raw materials used are all commercially available.

[0035] Example 1

[0036] This embodiment provides a method for preparing a freshness indicator film based on cochineal, comprising the following steps:

[0037] S1. Weigh 1.0g of agar, add 100mL of deionized water, heat and stir evenly in a 95℃ water bath, then add 4.0g of polyvinyl alcohol, continue stirring evenly in a 95℃ water bath, add 0.5mL of polyethylene glycol, and continue stirring to obtain a uniform mixed solution.

[0038] S2. Take 0.05g of cochineal and add it to 10mL of deionized water. Stir at room temperature for 30min to obtain a cochineal solution.

[0039] S3. Mix the mixed solution obtained in step S1 with the carmine solution in step S2, add acetic acid to adjust the pH of the solution to 2, stir continuously until uniform, and let stand to remove air bubbles to obtain a mixed film-forming solution.

[0040] S4. After casting the solution obtained in step S3 into a film, dry it in an oven at 20% humidity and 40°C in the dark to obtain a cochineal freshness indicator film.

[0041] Example 2

[0042] This embodiment provides a method for preparing a freshness indicator film based on cochineal, comprising the following steps:

[0043] S1. Weigh 2.0g of agar, add 100mL of deionized water, heat and stir evenly in a 95℃ water bath, then add 6.0g of polyvinyl alcohol, continue stirring evenly in a 98℃ water bath, add 1mL of polyethylene glycol, and continue stirring to obtain a uniform mixed solution.

[0044] S2. Take 0.1g of cochineal and add it to 10mL of deionized water. Stir at room temperature for 30min to obtain a cochineal solution.

[0045] S3. Mix the mixed solution obtained in step S1 with the carmine solution in step S2, add acetic acid to adjust the pH of the solution to 3, stir continuously until uniform, and let stand to remove air bubbles to obtain the mixed film-forming solution.

[0046] S4. After casting the solution obtained in step S3 into a film, dry it in an oven at 30% humidity and 50°C in the dark to obtain a cochineal freshness indicator film.

[0047] Example 3

[0048] This embodiment provides a method for preparing a freshness indicator film based on cochineal, comprising the following steps:

[0049] S1. Weigh 3.0g of agar, add 100mL of deionized water, heat and stir evenly in a 97℃ water bath, then add 8.0g of polyvinyl alcohol, continue stirring evenly in a 97℃ water bath, add 1mL of polyethylene glycol, and continue stirring to obtain a uniform mixed solution.

[0050] S2. Take 0.5g of cochineal and add it to 10mL of deionized water. Stir at room temperature for 30min to obtain a cochineal solution.

[0051] S3. Mix the mixed solution obtained in step S1 with the carmine solution in step S2, add acetic acid to adjust the pH of the solution to 2, stir continuously until uniform, and let stand to remove air bubbles to obtain a mixed film-forming solution.

[0052] S4. After casting the solution obtained in step S3 into a film, dry it in an oven at 40% humidity and 60°C in the dark to obtain a cochineal freshness indicator film.

[0053] Comparative Example 1

[0054] This embodiment provides a method for preparing a freshness indicator film, the steps of which include:

[0055] S1. Weigh 3.0g of agar, add 100mL of deionized water, heat and stir evenly in a 97℃ water bath, then add 8.0g of polyvinyl alcohol, continue stirring evenly in a 97℃ water bath, add 1mL of polyethylene glycol, and continue stirring to obtain a uniform mixed solution.

[0056] S2. Allow the mixed solution obtained in step S1 to stand to remove air bubbles and obtain a mixed film-forming solution;

[0057] S4. After casting the solution obtained in step S3 into a film, dry it in an oven at 40% humidity and 60°C in the dark to obtain a carmine-free agar / polyvinyl alcohol film.

[0058] Comparative Example 2

[0059] This embodiment provides a method for preparing a freshness indicator film, the steps of which include:

[0060] S1. Weigh 8.0g of polyvinyl alcohol, continue stirring in a 97℃ water bath until homogeneous, add 1mL of polyethylene glycol, and continue stirring to obtain a homogeneous mixed solution;

[0061] S2. Take 0.5g of cochineal and add it to 10mL of deionized water. Stir at room temperature for 30min to obtain a cochineal solution.

[0062] S3. Mix the solution obtained in step S1 with the carmine solution in step S2, add acetic acid to adjust the pH of the solution to 2, stir continuously until uniform, and let stand to remove bubbles to obtain a mixed film-forming solution.

[0063] S4. After casting the solution obtained in step S3 into a film, dry it in an oven at 40% humidity and 60°C in the dark to obtain a freshness indicator film without agar.

[0064] The freshness indicator film based on cochineal prepared in Example 1 was subjected to indicator detection. The specific detection method is as follows:

[0065] 1. Fix the freshness indicator film (2×3cm) in a sealed test chamber with a relative humidity of 75%, inject ammonia water (2mL, 0.3mol / L) into the test chamber using a micro-injector, and calculate the NH3 concentration using the following formula (1).

[0066]

[0067] Among them, V μL To analyze the volume of the solution, D mg / L To analyze the density of the solution, W represents the mass fraction of the solution, and M... g / mol To analyze the molar mass of the solution, V L This refers to the volume of the testing chamber.

[0068] After maintaining the film for 3 hours, an image of the film was obtained using a camera, and color parameters (R, G, B values) were obtained using Adobe Photoshop 2021. Its sensitivity was then calculated using the following formula:

[0069]

[0070] R0, G0, and B0 are the initial color parameters of the freshness indicator film.

[0071] like Figure 2 As shown, with increasing pH value, the color of the freshness indicator membrane gradually changes from its original orange-red to purple, and the color intensity gradually deepens. Figure 3 It can be seen that the maximum absorption wavelength in its ultraviolet spectrum undergoes a redshift from 279 nm to 300 nm. From Figure 4 It is evident that the sensitivity of the indicator membrane increases with increasing ammonia concentration. Therefore, it can be determined that the freshness indicator membrane described in this invention possesses excellent sensitivity.

[0072] 2. Mix the freshness indicator membrane (10 mg) with 10 mL of ABTS solution (6 mmol / L) in the dark at room temperature for 30 min. Measure the absorbance of the mixture at 734 nm. The free radical scavenging rate of the sample can be calculated using the following formula:

[0073]

[0074] Where A c A is the initial absorbance of the ABTS solution at 734 nm. s It is the absorbance of the mixed solution at 734 nm.

[0075] like Figure 5As shown, the freshness indicator film exhibits significant free radical scavenging ability, i.e., antioxidant capacity. Furthermore, its antioxidant capacity increases with the increase of carmine content in the freshness indicator film, indicating that the carmine content can enhance the antioxidant capacity of the indicator film to a certain extent.

[0076] 3. Cochineal and a freshness indicator film based on cochineal were exposed to ultraviolet light (70℃) for 15 days. When cochineal was stored under ultraviolet light (70℃), the ΔE of cochineal increased with increasing storage time. * The value increases because ultraviolet light in the light decomposes or oxidizes cochineal pigment, causing it to fade; however, after coating with polyvinyl alcohol and agar, its ΔE value increases under the same storage conditions. * The smaller change in the value indicates that polyvinyl alcohol and agar have a good interaction with cochineal, which enhances its heat and light resistance.

[0077] 4. Place appropriate amounts of pork and shrimp in glass petri dishes, fix a 10mm×20mm freshness indicator film on the top of the petri dish, seal it, and place it in a 4℃ refrigerator.

[0078] like Figure 7 The freshness indicator film was used to monitor the freshness of pork and shrimp under refrigerated conditions. When the pork and shrimp spoiled, the freshness indicator film showed a clear color change from orange-red to light purple, indicating that the freshness indicator film can be used to monitor the freshness of pork and shrimp under refrigerated conditions.

[0079] like Figure 8 In Example 3 and Comparative Example 1, the SEM images of the cross-section of the freshness indicator film prepared with added cochineal showed a relatively smooth surface without obvious layering, indicating good compatibility of the composite film. This suggests that the addition of cochineal improved the compatibility between agar and polyvinyl alcohol. In contrast, the SEM images of the cross-section without added cochineal showed a brittle, rough, and cracked cross-section with poor film uniformity, indicating a certain degree of incompatibility between agar and polyvinyl alcohol.

[0080] like Figure 9 In Example 3 and Comparative Example 1, the tensile strength (TS) and elongation at break (EAB) of the freshness indicator film prepared by adding cochineal significantly increased, which is similar to the results of SEM. The addition of cochineal significantly improved the compatibility of agar and polyvinyl alcohol, thereby enhancing its mechanical properties.

[0081] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for producing a freshness indicating film based on carminic acid, characterized by, The raw materials include cochineal carmine, a pH adjuster, and a polymer-based film, the polymer-based film being 10-22 parts by mass, the pH adjuster being 1-3 parts by mass, and the amount of cochineal carmine added being 0.5 %-6 % of the dry weight of the polymer-based film; wherein the raw materials of the polymer-based film include polyvinyl alcohol and agar, the polyvinyl alcohol being 8-16 parts by mass and the agar being 2-6 parts by mass; and the pH adjuster being one or more of acetic acid, citric acid, and lactic acid; The preparation steps include: S1. Heating and stirring agar and polyvinyl alcohol in water until completely dissolved, and continuing to stir to obtain a mixed solution; S2. Dissolving cochineal carmine in water to obtain a cochineal carmine solution, mixing the cochineal carmine solution with the mixed solution obtained in step S1 above, adjusting the pH of the solution to 2-3, continuing to stir until uniform, and standing to remove air bubbles to obtain a mixed film-forming solution; S3. Casting the mixed film-forming solution obtained in step S2 to form a film, and drying to obtain the freshness indicator film.

2. The method for producing the freshness indicating film based on carmine according to claim 1, characterized by, The raw materials of the freshness indicator film further include a plasticizer, the plasticizer being 2-5 parts by mass and being one or more of ethylene glycol or glycerol.

3. The method for preparing the freshness indicating film based on carmine according to claim 1, characterized by, In the mixed solution in step S1, the concentration of agar is 0.5 %-3 % w / v, and the concentration of polyvinyl alcohol is 6 %-10 % w / v.

4. The method for preparing the freshness indicating film based on carmine according to claim 1, characterized by, In step S2, the temperature of the stirring is 70-80℃.

5. The method for preparing the freshness indicating film based on carmine according to claim 1, characterized by, In step S3, the drying method is constant-temperature and constant-humidity drying, and the drying conditions are controlled to be a relative humidity of 20 %-40 %, a temperature of 40-60℃, and light-avoiding.

6. The cochineal carmine-based freshness indicator film according to claim 1 is applied to the freshness detection of pork and shrimp.

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