Preparation method of intelligent indication film with double functions of preservation and visualization

Abstract

The invention belongs to the field of green and intelligent food packaging, and particularly relates to a preparation method of an intelligent indication film with double functions of preservation and visualization. Chitosan, curdlan and lysozyme are selected to prepare a hydrophilic single-layer antibacterial film, and zein and alkannin are selected to prepare a hydrophobic color indication film. The lysozyme is added into a mixed suspension prepared from chitosan and curdlan, so that the problems of thermal stability, pH stability, storage stability and the like of the lysozyme are solved. By adopting a coaxial electrostatic spinning technology, the alkannin is encapsulated in a shell layer formed by zein, so that lasting color development of the alkannin can be realized. Under the condition of no instrument monitoring, a consumer can directly observe the change of the color indicator with naked eyes, so that the aim of monitoring the freshness of the chilled fresh meat in real time is fulfilled.

Description

Technical Field

[0001] The invention belongs to the field of green and intelligent food packaging, and in particular relates to a method for preparing an intelligent indicator film with dual functions of freshness preservation and visualization. Background Art

[0002] With the rapid development of the economy and society, the income level of residents has increased year by year, and people's requirements for food quality have become higher and higher, which has promoted the rapid development of the chilled meat industry. In my country, most chilled meat is stored, transported and sold in the form of naked or simple packaging. In this process, it is easily affected by many factors such as the surrounding environment. Microorganisms can easily grow and reproduce in it, which accelerates the corruption and deterioration of chilled meat, the loss of juice, etc., resulting in a shortened shelf life and greatly reducing its commercial value.

[0003] In the market, the sales of fresh meat usually use non-renewable and non-biodegradable petroleum-based packaging materials, which cause great harm to the environment. In order to meet the requirements of green and sustainable development, natural polysaccharides and proteins have become important substitutes for food packaging materials due to their biodegradability and renewability, but they still cannot provide product quality information in an intuitive way. Therefore, in the process of preparing packaging films, reaction phenomena such as color changes can be used to qualitatively indicate the freshness of fresh meat. At the same time, in order to extend the shelf life of fresh meat, adding natural antibacterial ingredients to food packaging films can also better meet consumers' demand for natural, green and preservative-free food. Summary of the invention

[0004] The purpose of the present invention is to provide a method for preparing an intelligent indicator film with dual functions of preservation and visualization, so as to solve the problems of short shelf life and decreased sensory quality of fresh meat, and to monitor the freshness of fresh meat in real time.

[0005] The present invention is achieved through the following technical solutions: A method for preparing a smart indicator film with dual functions of freshness preservation and visualization comprises the following steps: S1. Preparation of chitosan suspension: dissolving chitosan powder in acetic acid solution and stirring with magnetic force to form chitosan suspension; S2. Preparation of curdlan suspension: dissolving curdlan in sodium hydroxide solution and stirring with magnetic force to prepare curdlan suspension; S3. Preparation of lysozyme solution: dissolving lysozyme in deionized water to prepare a lysozyme solution with a concentration ranging from 0.2% to 0.5%; S4, preparation of chitosan / curdlan / lysozyme film-forming liquid: mixing the obtained chitosan suspension, curdlan suspension and lysozyme solution, filtering to remove insoluble particles, adding glycerol, stirring with magnetic force, and subjecting the film-forming liquid to ultrasonic treatment to remove bubbles; S5, preparation of zein and shikonin solution: dissolving zein powder in ethanol aqueous solution to prepare zein solution, stirring and vacuum degassing to obtain a uniform solution; dissolving shikonin in ethanol to prepare shikonin solution; S6. Preparation of chitosan / curdlan / lysozyme hydrophilic monolayer antibacterial film by magnetic tape casting process; S7, preparing a zein / shikonin hydrophobic color indicator film: placing the monolayer antibacterial film prepared in step (6) on an electrospinning device, and preparing a zein / shikonin color indicator film on the film by a coaxial electrospinning process; S8, corona treatment of the film: using a corona treatment device to treat the chitosan / curdlan / lysozyme film with a color indicator film obtained in step (7); S9. Packaging of fresh meat: Cut appropriate film according to the size of the fresh meat, and wrap it tightly against the surface of the meat. Place the wrapped meat in a transparent packaging box with the color indicator film facing upwards to facilitate consumers to observe the freshness of the meat. After sealing, store and sell at 4°C.

[0006] Furthermore, the specific operation of step S1 is as follows: weigh a certain mass of chitosan powder and dissolve it in 1-2% w / w acetic acid solution to prepare a chitosan solution with a concentration range of 1.5-2.5%, and stir magnetically at room temperature for 4-6 hours to completely dissolve the chitosan to form a chitosan suspension.

[0007] Furthermore, the specific operation of step S2 is as follows: weigh a certain mass of curdlan and dissolve it in 0.25 mol / L sodium hydroxide solution, stir it with magnetic force for 3-5 hours to completely dissolve the curdlan, and prepare a curdlan suspension with a concentration range of 1.5-2.5%.

[0008] Furthermore, the specific operation of step S4 is as follows: preparation of chitosan / curdlan gum / lysozyme film-forming liquid: according to the ratio of chitosan:curdlan gum = (1-3): 1, slowly pour the chitosan suspension into the curdlan gum suspension, use the pH automatic control monitor to accurately adjust the pH of the suspension to 3-4, add the lysozyme solution obtained in step (3) to the suspension, mix the chitosan / curdlan gum suspension and the lysozyme solution in a ratio of 2:1, filter to remove insoluble particles, add glycerol, the glycerol addition amount is 25-30% of the dry weight of the chitosan / curdlan gum, mix for 3-5 hours under magnetic stirring, and ultrasonically treat the film-forming liquid to remove bubbles.

[0009] Furthermore, the specific operation of step S5 is as follows: dissolving zein powder in an ethanol aqueous solution with a mass fraction of 75-95% to prepare a zein solution, wherein the final concentration of the prepared zein solution is 10-15%, and the solution is continuously stirred at 60-80° C. for 30-60 min and then vacuum degassed to remove bubbles to obtain a uniform solution; dissolving 60-80 mg of shikonin in 5 mL of ethanol to prepare a shikonin solution.

[0010] Furthermore, the specific operation of step S6 is as follows: first, the prepared chitosan / curdlan / lysozyme film-forming liquid is spread on the acrylic plate of the surface support of the belt casting device; the film-forming liquid is spread using a scraper device with a scraper length of 30-40 cm, and dried on the bracket, and the temperature of the film-forming liquid is maintained by circulating hot water with a temperature of 50-70°C in the coil in the magnetic tape casting device; the scraper gap is changed using a micrometer screw to adjust the suspension thickness applied to the bracket, the gap is adjusted to 6-8 mm, the diffusion rate is selected to be 200-250 cm / min, and the drying is performed for 6-8 hours.

[0011] Furthermore, the specific operation of step S7 is as follows: placing the monolayer antibacterial film prepared in step (6) on an electrospinning device, and preparing a zein / shikonin color indicator film with a length of 4 to 6 cm and a width of 2 to 4 cm on the film by a coaxial electrospinning process; wherein the coaxial spinneret of the electrospinning is composed of an inner needle with an inner diameter of 0.41 mm and an outer diameter of 0.82 mm, which is coaxially installed inside a large needle with an inner diameter of 1.12 mm and an outer diameter of 1.48 mm; the solutions obtained in step (5) are respectively poured into two 20 mL syringes, the flow rate of the shikonin solution is between 0.1 and 0.5 mL / h, the flow rate of the zein solution is between 0.6 and 1.2 mL / h, the voltage is set at 10 to 20 kV, and the distance from the tip to the collector is 16 to 18 cm, control the temperature between 23-27°C, and the relative humidity between 18%-20%RH, and after drying, a chitosan / curdlan / lysozyme film with a color indicator film can be obtained.

[0012] Furthermore, the specific operation of the film corona treatment in step S8 is as follows: the chitosan / curdlan / lysozyme film with a color indicator film obtained in step (7) is treated with a corona treatment device, the voltage of the corona treatment device is 10000~20000 V, the frequency range is 4~6 MHz, the film moves at a fixed linear speed of 1~2 cm / s, and the interval between the electrode and the sample film is set to 3~5 mm; the film placed on the moving base is displaced from right to left and from back to front at a speed of 1~2 cm / s to ensure that the entire surface of the film is fully treated; the side of the film that is in continuous contact with air during drying is treated with the electrode for 3~5 s.

[0014] The beneficial effects of the present invention are embodied in: (1) The present invention uses chitosan and curdlan as film matrices, which have good water absorption and water retention properties but are not easily dissolved in water. Therefore, the film obtained expands but does not dissolve when exposed to water, so that the fresh meat maintains good sensory quality during sales, processing and transportation.

[0015] (2) The color indicator used in the present invention has a strong hydrophobic effect and will degrade under strong alkaline conditions. In order to maintain the stable state of shikonin in the color indicator film, the present invention lowers the pH value when preparing the chitosan / curdlan gum-based film-forming liquid. The volatile basic nitrogen (TVB-N) produced by the cold fresh meat dissolves in water to produce alkaline substances, which react with the acidic components in the monolayer antibacterial film to neutralize the alkaline effect reaching the color indicator film, thereby weakening the alkalinity. After the juice produced by the cold fresh meat is pH-controlled by the chitosan / curdlan gum-based membrane, it is transferred to the color indicator film to keep the alkalinity in a relatively mild state, so that shikonin gradually changes from red to blue under relatively stable alkaline conditions, so that the degree of deterioration of the cold fresh meat can be judged by observing the degree of color change with the naked eye.

[0016] (3) The present invention uses renewable and degradable natural film materials, which have less impact on the environment than traditional packaging materials. Chitosan, which is used as the film matrix, has antibacterial properties and can greatly reduce the consumption of the antibacterial agent lysozyme. After adding lysozyme, the freshness and quality of the packaged food can be greatly guaranteed.

[0017] (4) The present invention prepares a chitosan / curdlan / lysozyme monolayer film by lowering the pH of the film-forming solution, which can make shikonin develop color under relatively mild / weak alkaline conditions. In addition, there are not many films on the market that can adjust the pH of the film-forming solution to affect the color indicator.

[0018] (5) The present invention uses a coaxial electrospinning process to prepare a zein / shikonin color indicator film, and encapsulates shikonin into a shell layer formed by zein, which, on the one hand, protects shikonin, and on the other hand, reduces the rapid consumption of shikonin.

[0019] (6) The present invention uses a corona device for treatment, which increases the wettability and adhesion of the chitosan / curdlan gum-based film and allows the color indicator film to undergo electrostatic adsorption therewith, so that the two layers of film are fully combined and the color development effect is significantly improved.

[0020] (7) The freshness indicator film prepared by the present invention can change color according to the pH value fluctuation of the film matrix caused by the gas generated by the chilled meat. The color change difference under different pH conditions is obvious and the color development sensitivity is high, which can provide an accurate indication of the freshness of the chilled meat.

[0021] (8) The present invention uses chitosan, curdlan, and lysozyme to prepare a hydrophilic monolayer antibacterial film, and uses zein and shikonin to prepare a hydrophobic color indicator film. Lysozyme is added to a mixed suspension made of chitosan and curdlan, which solves the problems of lysozyme's thermal stability, pH stability, and storage stability. By using coaxial electrospinning technology, shikonin is encapsulated in the shell layer formed by zein, which can achieve long-lasting color development of shikonin. In the absence of instrument monitoring, consumers can directly observe the changes in the color indicator with their naked eyes, thereby achieving the purpose of real-time monitoring of the freshness of cold fresh meat. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is an optical microscope image of the prepared chitosan / curdlan / lysozyme film; Figure 2 is a graph of the mechanical properties of the prepared dual-functional smart indicator film; Figure 3 It is a color change diagram of shikonin solution at different pH values ​​and a color change diagram of the color indicator membrane in response to different pH buffer solutions; Figure 4 is a color response sensitivity diagram of zein / shikonin color indicator film under different ammonia concentrations. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is described in detail below in conjunction with specific embodiments to facilitate understanding of the present invention, but is not intended to limit the present invention.

[0024] Example 1: This example provides an intelligent indicator film containing a natural antibacterial agent and shikonin, which has both the functions of preservation and visualization and is used for monitoring the freshness of chilled pork.

[0025] (1) Preparation of chitosan suspension: Weigh a certain amount of chitosan powder and dissolve it in 1% w / w acetic acid solution. Stir it with a magnetic stirrer at room temperature for 6 h to prepare a chitosan suspension with a concentration of 1.5%.

[0026] (2) Preparation of curdlan suspension: Weigh a certain amount of curdlan and dissolve it in 0.25 mol / L sodium hydroxide solution. Stir it magnetically for 3 h to completely dissolve the curdlan and prepare a curdlan suspension with a concentration of 1.5%.

[0027] (3) Preparation of lysozyme solution: Prepare a 0.2% lysozyme solution in deionized water.

[0028] (4) Preparation of chitosan / curdlan gum / lysozyme film-forming liquid: Slowly pour the chitosan suspension into the curdlan gum suspension at a ratio of chitosan:curdlan gum = 1:1, use a pH automatic control monitor to accurately adjust the pH value of the suspension to 3, add the lysozyme solution obtained in step (3) to the suspension, mix the chitosan / curdlan gum suspension and lysozyme solution at a ratio of 2:1, filter to remove insoluble particles, add glycerol (25% based on the dry weight of chitosan / curdlan gum), mix for 3 hours under magnetic stirring, and ultrasonically treat the film-forming liquid to remove bubbles. The optical microscope image of the chitosan / curdlan gum / lysozyme film prepared in Example 1 is shown in Figure 2. Figure 1 shown.

[0029] (5) Preparation of zein and shikonin solution: Zein powder was dissolved in 75% ethanol aqueous solution to prepare a 10% zein solution. The solution was stirred continuously at 60 °C for 60 min and then vacuum degassed to remove bubbles to obtain a uniform solution. 60 mg of shikonin was dissolved in 5 mL of ethanol to prepare a shikonin solution.

[0030] (6) Preparation of chitosan / curdlan / lysozyme hydrophilic monolayer antibacterial film: The film was formed by tape casting process. First, the prepared chitosan / curdlan / lysozyme film-forming liquid was spread on the acrylic plate, the surface support of the tape casting device. The film-forming liquid was spread using a scraper device with a length of 30 cm. It was dried on the support and the temperature of the film-forming liquid was maintained by circulating hot water (50°C) in the coil in the tape casting device. The scraper gap was changed using a micrometer screw to adjust the thickness of the suspension applied to the support. The gap was adjusted to 6 mm. The diffusion rate was selected to be 200 cm / min and the drying was performed for 8 h.

[0031] (7) Preparation of zein / shikonin hydrophobic color indicator film: The monolayer antibacterial film prepared in step (6) was placed on an electrospinning device, and a 6×4 cm zein / shikonin color indicator film was prepared on the film by coaxial electrospinning. The solutions obtained in step (5) were respectively poured into two 20 mL syringes, with the flow rate of shikonin solution being 0.2 mL / h and the flow rate of zein solution being 0.8 mL / h. The voltage was set to 10 kV, the distance from the tip to the collector was 16 cm, the temperature was controlled between 23-27°C, and the relative humidity was controlled between 18%-20%RH. After drying, a chitosan / curdlan / lysozyme film with a color indicator film was obtained.

[0032] (8) Corona treatment: The chitosan / curdlan / lysozyme film with color indicator membrane obtained in step (7) was treated using a laboratory corona treatment device (10,000-20,000 V, frequency range 4-6 MHz). The film was moved at a fixed linear speed of about 1 cm / s, and the interval between the electrode and the sample film was set to 3 mm. The film placed on a moving base was displaced from right to left and from back to front at a speed of 1 cm / s to ensure that the entire surface of the film was fully treated. The side of the film that was in continuous contact with air during drying was treated with the electrode for 3 s.

[0033] (9) Packaging of fresh meat: In this example, a transparent packaging box with a lid of 14×8×5.8 cm was selected. The size of the fresh pork was approximately 12×6×3 cm, and the color indicator film was 6×4 cm. The prepared antibacterial film was cut into a rectangle of 31×17 cm to fully wrap the fresh meat. The wrapped pork was placed in a transparent packaging box with the color indicator film facing upwards to facilitate consumers to observe the freshness of the fresh meat. After sealing, the meat was stored and sold at 4°C.

[0034] Example 2: This example provides an intelligent indicator film containing natural antibacterial agent and shikonin, which has dual functions of preservation and visualization and is used for monitoring the freshness of chilled beef.

[0035] (1) Preparation of chitosan suspension: Weigh a certain amount of chitosan powder and dissolve it in 1.5% w / w acetic acid solution. Stir it with a magnetic stirrer at room temperature for 5 h to prepare a chitosan suspension with a concentration of 2%.

[0036] (2) Preparation of curdlan suspension: Weigh a certain amount of curdlan and dissolve it in 0.25 mol / L sodium hydroxide solution. Stir it magnetically for 4 h to completely dissolve the curdlan and prepare a curdlan suspension with a concentration of 2%.

[0037] (3) Preparation of lysozyme solution: Prepare a 0.4% lysozyme solution in deionized water.

[0038] (4) Preparation of chitosan / curdlan gum / lysozyme film-forming solution: Slowly pour the chitosan suspension into the curdlan gum suspension in a ratio of chitosan:curdlan gum = 2:1, and use a pH automatic control monitor to accurately adjust the pH value of the suspension to 3.5. Add the lysozyme solution obtained in step (3) to the suspension, and mix the chitosan / curdlan gum suspension and lysozyme solution in a ratio of 2:1. Filter to remove insoluble particles, add glycerol (30% based on the dry weight of chitosan / curdlan gum), mix under magnetic stirring for 4 h, and then perform ultrasonic treatment to remove bubbles.

[0039] (5) Preparation of zein and shikonin solution: Zein powder was dissolved in 75% ethanol aqueous solution to prepare a 12.5% ​​zein solution. The solution was stirred continuously at 70°C for 50 min and then vacuum degassed to remove bubbles to obtain a uniform solution. 70 mg of shikonin was dissolved in 5 mL of ethanol to prepare a shikonin solution.

[0040] (6) Preparation of chitosan / curdlan / lysozyme hydrophilic monolayer antibacterial film: The film was formed by tape casting process. First, the prepared chitosan / curdlan / lysozyme film-forming liquid was spread on the acrylic plate, the surface support of the tape casting device. The film-forming liquid was spread using a scraper device with a length of 35 cm. It was dried on the bracket and the temperature of the film-forming liquid was maintained by circulating hot water (60°C) in the coil in the tape casting device. The scraper gap was changed using a micrometer screw to adjust the thickness of the suspension applied to the bracket. The gap was adjusted to 7 mm. The diffusion rate was selected to be 225 cm / min and dried for 7 h.

[0041] (7) Preparation of zein / shikonin hydrophobic color indicator film: The monolayer antibacterial film prepared in step (6) was placed on an electrospinning device, and a zein / shikonin color indicator film of 5×3 cm was prepared on the film by coaxial electrospinning. The solution obtained in step (5) was poured into two 20 mL syringes respectively, with the flow rate of shikonin solution being 0.4 mL / h and the flow rate of zein solution being 0.9 mL / h. The voltage was set to 15 kV, the distance from the tip to the collector was 17 cm, the temperature was controlled between 23-27°C, and the relative humidity was controlled between 18%-20%RH. After drying, a chitosan / curdlan / lysozyme film with a color indicator film was obtained.

[0042] (8) Corona treatment: The chitosan / curdlan / lysozyme film with color indicator membrane obtained in step (7) was treated using a laboratory corona treatment device (10,000-20,000 V, frequency range 4-6 MHz). The film was moved at a fixed linear speed of about 1.5 cm / s, and the interval between the electrode and the sample film was set to 4 mm. The film placed on a moving base was displaced from right to left and from back to front at a speed of 1.5 cm / s to ensure that the entire surface of the film was fully treated. The side of the film that was in continuous contact with air during drying was treated with the electrode for 4 s.

[0043] (9) Packaging of chilled meat: In this example, a transparent packaging box with a lid of 15×8×5.8 cm was used. The size of the chilled meat was approximately 13×5×3 cm. The color indicator film was 5×3 cm. The prepared antibacterial film was cut into a rectangle of 33×18 cm to fully wrap the beef. The wrapped chilled meat was placed in a transparent packaging box with the color indicator film facing upwards to facilitate consumers to observe the freshness of the chilled meat. After sealing, the meat was stored and sold at 4°C.

[0044] Example 3: This example provides a smart indicator film containing natural antibacterial agents and shikonin, which has both freshness preservation and visualization dual functions and is used for monitoring the freshness of chilled chicken.

[0045] (1) Preparation of chitosan suspension: Weigh a certain amount of chitosan powder and dissolve it in 2% acetic acid solution. Stir it with a magnetic stirrer at room temperature for 4 h to prepare a chitosan suspension with a concentration of 2.5%.

[0046] (2) Preparation of curdlan suspension: Weigh a certain amount of curdlan and dissolve it in 0.25 mol / L sodium hydroxide solution. Stir it magnetically for 5 h to completely dissolve the curdlan and prepare a curdlan suspension with a concentration of 2.5%.

[0047] (3) Preparation of lysozyme solution: Prepare a 0.5% lysozyme solution in deionized water.

[0048] (4) Preparation of chitosan / curdlan gum / lysozyme film-forming solution: Slowly pour the chitosan suspension into the curdlan gum suspension in a ratio of chitosan:curdlan gum = 3:1, and use a pH automatic control monitor to accurately adjust the pH value of the suspension to 4. Add the lysozyme solution obtained in step (3) to the suspension, and mix the chitosan / curdlan gum suspension and lysozyme solution in a ratio of 2:1. Filter to remove insoluble particles, add glycerol (25% based on the dry weight of chitosan / curdlan gum), mix under magnetic stirring for 5 h, and then perform ultrasonic treatment to remove bubbles.

[0049] (5) Preparation of zein and shikonin solution: Zein powder was dissolved in 95% ethanol aqueous solution to prepare a 15% zein solution. The solution was stirred continuously at 80°C for 30 min and then vacuum degassed to remove bubbles to obtain a uniform solution. 80 mg of shikonin was dissolved in 5 mL of ethanol to prepare a shikonin solution.

[0050] (6) Preparation of chitosan / curdlan / lysozyme hydrophilic monolayer antibacterial film: The film was formed by tape casting process. First, the prepared chitosan / curdlan / lysozyme film-forming liquid was spread on the acrylic plate, the surface support of the tape casting device. The film-forming liquid was spread using a scraper device with a length of 40 cm. It was dried on the support and the temperature of the film-forming liquid was maintained by circulating hot water (70°C) in the coil in the tape casting device. The scraper gap was changed using a micrometer screw to adjust the thickness of the suspension applied to the support. The gap was adjusted to 8 mm. The diffusion rate was selected to be 250 cm / min and dried for 6 h.

[0051] (7) Preparation of zein / shikonin hydrophobic color indicator film: The monolayer antibacterial film prepared in step (6) was placed on an electrospinning device, and a 6×3 cm zein / shikonin color indicator film was prepared on the film by coaxial electrospinning. The solutions obtained in step (5) were respectively poured into two 20 mL syringes, with the flow rate of shikonin solution being 0.3 mL / h and the flow rate of zein solution being 0.8 mL / h. The voltage was set to 20 kV, the distance from the tip to the collector was 18 cm, the temperature was controlled between 23-27°C, and the relative humidity was controlled between 18%-20%RH. After drying, a chitosan / curdlan / lysozyme film with a color indicator film was obtained.

[0052] (8) Corona treatment: The chitosan / curdlan / lysozyme film with color indicator membrane obtained in step (7) was treated using a laboratory corona treatment device (10,000-20,000 V, frequency range 4-6 MHz). The film was moved at a fixed linear speed of about 2 cm / s, and the interval between the electrode and the sample film was set to 5 mm. The film placed on a moving base was displaced from right to left and from back to front at a speed of 2 cm / s to ensure that the entire surface of the film was fully treated. The side of the film that was in continuous contact with air during drying was treated with the electrode for 5 s.

[0053] (9) Packaging of chilled meat: In this example, a transparent packaging box with a lid of 15×11×6.8 cm was selected. The size of two chicken legs was approximately 12×10×5 cm. The color indicator film was 6×3 cm. The prepared antibacterial film was cut into a rectangle of 31×27 cm to fully wrap the chicken legs. The wrapped chilled meat was placed in a transparent packaging box with the color indicator film facing upwards to facilitate consumers to observe the freshness of the chilled meat. After sealing, the meat was stored and sold at 4°C.

[0054] Experimental data characterization (1) Determination of tensile strength (TS) and elongation at break (EB) The tensile strength (TS) and elongation at break (EB) of the prepared samples were measured using an automatic tensile testing machine. All samples were cut into the same size (60 mm × 20 mm) before use, fixed between the clamps, the original spacing was set to 40 mm, and stretched at a speed of 50 mm / min. The experiment was repeated three times. The measurement results are shown in Figure 2 As shown. Figure 2 It can be seen that the tensile strength of the bifunctional intelligent indicator film obtained by the present invention is significantly higher than that of the chitosan / curdlan film, the chitosan / curdlan / lysozyme film, the zein film and the zein / shikonin film; the elongation at break of the bifunctional intelligent indicator film obtained by the present invention is also higher than that of the chitosan / curdlan film, the chitosan / curdlan / lysozyme film, the zein film and the zein / shikonin film.

[0055] (2) Color change experiment of shikonin and color indicator membrane in buffer solutions of different pH values 3 mg of shikonin was dissolved in 25 mL of ethanol to obtain a shikonin ethanol solution (0.12 mg / mL), 2 mL of the shikonin ethanol solution was mixed with different buffer solutions (2 mL) with pH values ​​of 2-12 to prepare a series of shikonin solutions, and the colors of the obtained solutions were photographed.

[0056] The film (3 cm × 3 cm) was immersed in different buffer solutions with pH values ​​of 2-12 at room temperature for 10 min. The film was taken out, the surface moisture of the film was removed with blotting paper, and its color was monitored and recorded.

[0057] Results Figure 3 , the zein / shikonin color indicator film is red in the pH range of 2-7, gradually transitions from red to purple between pH 8-10, and finally turns blue between pH 11-13.

[0058] (3) The prepared zein / shikonin color indicator film was placed on the mouth of a conical flask containing different ammonia concentrations for 30 min to test the sample's reactivity to ammonia vapor. The results are shown in Figure 4 , as the concentration increases, the sensitivity of the response also increases.

[0059] (4) Water vapor transmission rate test The chitosan / curdlan / lysozyme film prepared in Example 1-3 after 24 hours of equilibration was cut into a 33 cm 2 The circle was placed in the instrument and measured at 38°C and 90% RH for 10 hours. The results are shown in Table 1.

[0060] Table 1 Water vapor transmission rate of chitosan / curdlan / lysozyme films prepared in Examples 1-3 CS: chitosan; CD: curdlan; LYS: lysozyme; CS / CD / LYS film: CS / CD / LYS: chitosan / curdlan / lysozyme film.

Claims

1. A method for preparing a smart indicator film with dual functions of freshness preservation and visualization, characterized in that: The steps include: S1. Preparation of chitosan suspension: dissolving chitosan powder in acetic acid solution and stirring with magnetic force to form chitosan suspension; S2. Preparation of curdlan suspension: dissolving curdlan in sodium hydroxide solution and stirring with magnetic force to prepare curdlan suspension; S3. Preparation of lysozyme solution: dissolving lysozyme in deionized water to prepare a lysozyme solution with a concentration ranging from 0.2% to 0.5%; S4, preparation of chitosan / curdlan / lysozyme film-forming liquid: mixing the obtained chitosan suspension, curdlan suspension and lysozyme solution, filtering to remove insoluble particles, adding glycerol, stirring with magnetic force, and subjecting the film-forming liquid to ultrasonic treatment to remove bubbles; S5, preparation of zein and shikonin solution: dissolving zein powder in ethanol aqueous solution to prepare zein solution, stirring and vacuum degassing to obtain a uniform solution; dissolving shikonin in ethanol to prepare shikonin solution; S6. Preparation of chitosan / curdlan / lysozyme hydrophilic monolayer antibacterial film by magnetic tape casting process; S7, preparing a zein / shikonin hydrophobic color indicator film: placing the monolayer antibacterial film prepared in step (6) on an electrospinning device, and preparing a zein / shikonin color indicator film on the film by a coaxial electrospinning process; S8, corona treatment of the film: using a corona treatment device to treat the chitosan / curdlan / lysozyme film with a color indicator film obtained in step (7); S9. Packaging of fresh meat: Cut appropriate film according to the size of the fresh meat, and wrap it tightly against the surface of the meat. Place the wrapped meat in a transparent packaging box with the color indicator film facing upwards to facilitate consumers to observe the freshness of the meat. After sealing, store and sell at 4°C.

2. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S1 is as follows: weigh a certain mass of chitosan powder and dissolve it in 1-2% w / w acetic acid solution to prepare a chitosan solution with a concentration range of 1.5-2.5%, and stir it magnetically at room temperature for 4-6 hours to completely dissolve the chitosan to form a chitosan suspension.

3. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S2 is as follows: weigh a certain mass of curdlan and dissolve it in 0.25 mol / L sodium hydroxide solution, stir it with magnetic force for 3-5 hours to completely dissolve the curdlan, and prepare a curdlan suspension with a concentration range of 1.5-2.5%.

4. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S4 is as follows: Preparation of chitosan / curdlan gum / lysozyme film-forming liquid: according to the ratio of chitosan:curdlan gum = (1-3): 1, slowly pour the chitosan suspension into the curdlan gum suspension, use the pH automatic control monitor to accurately adjust the pH of the suspension to 3-4, add the lysozyme solution obtained in step (3) to the suspension, mix the chitosan / curdlan gum suspension and the lysozyme solution in a ratio of 2:1, filter to remove insoluble particles, add glycerol, the glycerol addition amount is 25-30% of the dry weight of the chitosan / curdlan gum, mix for 3-5 hours under magnetic stirring, and ultrasonically treat the film-forming liquid to remove bubbles.

5. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S5 is as follows: dissolving zein powder in an ethanol aqueous solution with a mass fraction of 75-95% to prepare a zein solution, wherein the final concentration of the prepared zein solution is 10-15%, the solution is continuously stirred at 60-80° C. for 30-60 min, and then vacuum degassed to remove bubbles to obtain a uniform solution; 60-80 mg of shikonin is dissolved in 5 mL of ethanol to prepare a shikonin solution.

6. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S6 is as follows: first, the prepared chitosan / curdlan / lysozyme film-forming liquid is spread on the acrylic plate of the surface support of the tape casting device; the film-forming liquid is spread using a scraper device with a scraper length of 30-40 cm, and dried on the bracket, and the temperature of the film-forming liquid is maintained by circulating hot water with a temperature of 50-70°C in the coil in the tape casting device; the scraper gap is changed using a micrometer screw to adjust the thickness of the suspension applied to the bracket, the gap is adjusted to 6-8 mm, the diffusion rate is selected to be 200-250 cm / min, and the drying is performed for 6-8 hours.

7. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of step S7 is as follows: placing the monolayer antibacterial film prepared in step (6) on an electrospinning device, and preparing a zein / shikonin color indicator film with a length of 4 to 6 cm and a width of 2 to 4 cm on the film by a coaxial electrospinning process; wherein the coaxial spinneret of the electrospinning is composed of an inner needle with an inner diameter of 0.41 mm and an outer diameter of 0.82 mm, which is coaxially installed inside a large needle with an inner diameter of 1.12 mm and an outer diameter of 1.48 mm; the solutions obtained in step (5) are respectively poured into two 20 mL syringes, the flow rate of the shikonin solution is between 0.1 and 0.5 mL / h, the flow rate of the zein solution is between 0.6 and 1.2 mL / h, the voltage is set at 10 to 20 kV, and the distance from the tip to the collector is 16 to 18 cm, control the temperature between 23-27°C, and the relative humidity between 18%-20%RH, and after drying, a chitosan / curdlan / lysozyme film with a color indicator film can be obtained.

8. The method for preparing the intelligent indicator film with dual functions of freshness preservation and visualization according to claim 1, characterized in that: The specific operation of the film corona treatment in step S8 is as follows: the chitosan / curdlan / lysozyme film with the color indicator film obtained in step (7) is treated with a corona treatment device, the voltage of the corona treatment device is 10000~20000 V, the frequency range is 4~6 MHz, the film moves at a fixed linear speed of 1~2 cm / s, and the interval between the electrode and the sample film is set to 3~5 mm; the film placed on the moving base is displaced from right to left and from back to front at a speed of 1~2 cm / s to ensure that the entire surface of the film is fully treated; the side of the film that is in continuous contact with the air during drying is treated with the electrode for 3~5 s.

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