Preparation method and application of anthocyanin UiO-66-NH2 nano gas-sensitive pigment double-signal indication film

Abstract

The invention belongs to the field of food detection, and particularly relates to a preparation method and application of an anthocyanin UiO-66-NH2 nano gas-sensitive pigment double-signal indication film. The preparation method comprises the following steps: step 1, preparing a UiO-66-NH2 nano material; 2, packaging the anthocyanin in the metal organic framework; step 3, preparing an indicating film; and 4, establishing a freshness model and carrying out nondestructive real-time evaluation. On the basis that the UiO-66-NH2 nano material has the characteristics of high adsorbability, high porosity and multiple functional sites, the anthocyanin-coated UiO-66-NH2 nano gas-sensitive pigment dye is constructed, and the problem of poor stability of anthocyanin can be effectively solved; meanwhile, a fluorescent substance luminol is added, the freshness indicating film prepared on the basis of secondary improvement can have visible light color and fluorescent color changes in the application stage, lossless visual detection can be achieved on chilled meat, and the sensitivity and stability of a traditional gas-sensitive pigment indicating film are effectively improved.

Description

Technical Field

[0001] The invention belongs to the technical field of food freshness detection, and specifically relates to a preparation method and application of an anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film. Background Art

[0002] Chilled meat is popular for its delicious taste and nutritional value, but improper handling during production, transportation and storage can lead to high spoilage rates and losses due to the growth of environmentally affected microorganisms. These microorganisms produce amines through decarboxylation of amino acids and amination of carbon-based organic matter, which can lead to food spoilage and the production of toxic substances that harm human health. Therefore, it is of great significance to design a smart packaging with monitoring, detection, recording, tracking and communication functions. In recent years, freshness-indicating films have attracted much attention in the field of food research because they can provide intuitive information about food quality and safety. In the food industry, anthocyanins are considered to be natural pigments that are promising to replace synthetic pigments. They are widely available and respond to different pH values. pH-sensitive color changes provide a simple and effective way to monitor food quality, because the production of amines is always accompanied by pH changes during food spoilage. These pH fluctuations change the pigment structure, resulting in color changes visible to the naked eye.

[0003] Anthocyanins, as a class of natural gas-sensitive materials, can react with corruption gases to produce obvious color changes and are widely available. A large number of studies have shown that anthocyanins from various sources can be used as bio-based pH-sensitive dyes to monitor food freshness. However, anthocyanins undergo structural degradation under light or high temperature, which results in large deviations in the indication results of food freshness indicator films, which greatly hinders their application in food testing. This is a major problem currently faced in the field of food freshness indication, so improving the stability of anthocyanins is of great significance for the practical application of food freshness indicator films. In addition, most of the current freshness indicator films can only judge the freshness of cold fresh meat by visible light color changes, but a single signal may have low sensitivity and high detection limit for freshness judgment. Therefore, it is necessary to develop a non-destructive, effective and rapid detection method to evaluate its freshness. Summary of the invention

[0004] In order to solve the technical problems in the above background, the present application provides a method for preparing anthocyanin @UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, and uses it for evaluating the freshness of chilled meat, solving the technical problems of poor stability of natural gas sensitive membranes, strong subjectivity of response results and low sensitivity in the prior art.

[0005] The present invention firstly designs a photothermally stable phytochemical dye using different plant anthocyanins and the metal organic framework UiO-66-NH2, which significantly improves the photostability and thermal stability of anthocyanins. At the same time, considering that a single signal may have some deficiencies in judging freshness, a fluorescent signal is introduced as an auxiliary to achieve dual-signal detection of freshness, thereby making the detection results more accurate and reliable. At the same time, the structure of the film is characterized, and its pH sensitivity and storage stability are investigated; finally, an indicator film with excellent performance is obtained, which has achieved remarkable technical effects in the application of meat freshness detection.

[0006] This application achieves the above-mentioned purpose through the following technical means.

[0007] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, comprising the following steps:

[0008] S1. Extraction of anthocyanins: washing anthocyanin raw materials with distilled water and then air-drying them to obtain air-dried raw materials; at the same time, mixing anhydrous ethanol with water to obtain an ethanol-water extraction solution; then adding the air-dried raw materials to the ethanol-water extraction solution, stirring evenly at room temperature, vacuum filtering and collecting the supernatant, and rotary evaporating the obtained supernatant to obtain concentrated anthocyanins; finally freeze-drying to obtain a solid powder, which is an anthocyanin extract; the anthocyanin raw materials include but are not limited to grape skin, mulberry, roselle, and black wolfberry;

[0009] S2. Synthesis of UiO-66-NH2 nanomaterials:

[0010] The ZrCl4 and 2-aminoterephthalic acid powders are dissolved in N, N-dimethylformamide respectively, and ultrasonic treatment is performed until they are completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution; the 2-aminoterephthalic acid solution is placed in an oil bath, and acetic acid is added dropwise to the 2-aminoterephthalic acid solution, and then the ZrCl4 solution is added dropwise and stirred evenly; then, the stirring is stopped, and the constant temperature oil bath is maintained overnight to obtain a reaction solution; after the reaction solution is cooled to room temperature, anhydrous ethanol is added, and a precipitate is collected by centrifugation at room temperature, and finally the precipitate is washed, centrifuged and vacuum dried to obtain a product, namely, UiO-66-NH2 nanomaterial;

[0011] S3. Encapsulation of anthocyanins in metal organic frameworks: The UiO-66-NH2 nanoparticles obtained in step S3 and the anthocyanin extract obtained in step S1 are added to anhydrous ethanol and uniformly mixed by ultrasonication to obtain a mixed solution; the mixed solution is centrifuged and the precipitate is collected and dispersed in distilled water for washing, and the precipitate is collected after washing and vacuum dried in the dark. The product obtained after drying is UiO-66-NH2 loaded with anthocyanins, which is recorded as anthocyanin@UiO-66-NH2;

[0012] S4. Preparation of dual-signal freshness indicator film:

[0013] Add corn starch and polyvinyl alcohol into distilled water, heat and stir evenly to form a mixed solution; then cool to room temperature, add glycerol into the mixed solution and stir evenly to obtain a starch / polyvinyl alcohol polymer solution;

[0014] The anthocyanin @UiO-66-NH2 and luminol obtained in step S3 are added to the starch / polyvinyl alcohol polymer under ultrasonic treatment, and after ultrasonic treatment, they are poured into a mold and dried to form a film to obtain a dual-signal indicator film.

[0015] Preferably, in step S1, the volume ratio of anhydrous ethanol to water is 3:1; the ratio of the air-dried raw material to the ethanol water extract solution is 1 g:10 mL; the stirring is magnetic stirring, and the time is 2 h; the temperature of rotary evaporation is 50° C., and the time is 40 min; the temperature of freeze drying is -40° C., and the time of freeze drying is 2 days.

[0016] Preferably, the dosage of ZrCl4, 2-aminoterephthalic acid, N,N-dimethylformamide, acetic acid and anhydrous ethanol in step S2 is 300-350 mg: 200-300 mg: 1-2 mL: 20 mL: 10 mL.

[0017] Preferably, the temperature of the constant temperature oil bath in step S2 is 80°C; the rate of adding acetic acid is 3s / drop; the speed of the magnetic stirring is 400r / min, and the stirring time is 30min; washing is performed using anhydrous ethanol; the centrifugal speed is 8000-10000r / min, and the centrifugal time is 20-25min; the temperature of the vacuum drying is 75°C, and the time is 10-12h.

[0018] Preferably, in step S3, the ratio of UiO-66-NH2, anthocyanin extract and anhydrous ethanol added is 45-50 mg: 20-30 mg: 10 mL; the ultrasonic mixing time is 20 min; the number of distilled water washings is 3-5 times; the centrifugal conditions are: speed 10000 r / min, time 10-15 min; the vacuum drying temperature is 40°C, and the drying time is 12 h.

[0019] Preferably, in step S4, the dosage of polyvinyl alcohol, corn starch, luminol and distilled water is in the relationship of 1-1.7 g: 3-4 g: 10-20 mg: 100 mL; the dosage of glycerol is 1wt% of the dosage of corn starch; the dosage of anthocyanin @UiO-66-NH2 is 1%-5% of the total mass of corn starch and polyvinyl alcohol; the conditions of the ultrasonic treatment are: ultrasonic power of 100 W, ultrasonic time of 20 min, and ultrasonic temperature of 30°C; the mold includes a disposable culture dish and a polytetrafluoroethylene plate; the drying temperature is 40°C and the drying time is 12 h.

[0020] The anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film of the present invention is used for freshness detection of meat products; the meat products include chilled fresh meat, cooked meat products and semi-finished products;

[0021] Based on the use of dual signal indicator membrane in meat product freshness detection, the operation steps are as follows:

[0022] (1) Cut meat samples into the same mass, put them into a reaction container and store them under low temperature conditions, take out the samples at regular intervals, and determine the TVB-N content by Kjeldahl nitrogen determination according to the different contents of volatile amine substances in the meat products of different freshness levels, and divide the freshness of the meat products into three levels accordingly, namely, fresh: TVB-N content <15 mg / 100 g, sub-fresh: TVB-N content 15-25 mg / 100 g, and corrupt TVB-N content >25 mg / 100 g; the low temperature condition is 4° C., the storage time is 0-480 h; different freshness levels correspond to different storage days;

[0023] (2) Obtain the color difference of the dual-signal indicator membrane before the reaction, then store the dual-signal indicator membrane and meat products of different grades in the same closed reaction container, obtain the color difference of the indicator membrane again after the reaction, and calculate the color difference value based on the color difference before and after the reaction, recorded as ΔE, and then establish a correlation curve between ΔE and the corresponding TVB-N content, thereby quantitatively judging and realizing the freshness detection of meat products; at the same time, meat products at different grades will cause the dual-signal indicator membrane to change color, so the freshness of the meat products can be judged according to the color change of the dual-signal indicator membrane, realizing visual detection.

[0024] Preferably, the reaction time in step (2) is 1 to 16 minutes; the dual-signal indicator membrane is fixed on the top of the reaction container at a distance of 10 to 50 mm from the meat product.

[0025] Preferably, in step (2), the color difference extraction step is as follows: using a colorimeter to extract the color difference of the indicator film before and after the reaction, that is, the L, a, and b values; the values ​​before the reaction are recorded as L0, a0, and b0; the values ​​after the reaction are recorded as L1, a1, and b1; the values ​​of L, a, and b before and after the reaction are subtracted, that is, △L=L1-L0, △a=a1-a0, △b=b1-b0; and according to the formula Calculate the color difference value.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. The smart indicator area of ​​this application uses anthocyanins embedded in the metal organic framework UiO-66-NH2 as a high-stability pH indicator. UiO-66-NH2 has high porosity, large surface area and different functional sites, and specific guest molecules can be encapsulated in the pore size. The UiO-66-NH2 selected in this application is safer than other MOFs nanomaterials, and the formed complex has significantly improved light and heat stability compared to natural anthocyanins.

[0028] 2. The present application discloses a method for preparing a dual-signal indicator film for freshness of cold fresh meat based on anthocyanin @UiO-66-NH2, which comprises a metal organic framework material UiO-66-NH2 loaded with anthocyanin, a fluorescent substance luminol and a film-forming matrix solution, which are evenly mixed and then cast and dried to form a film after ultrasonic elimination of bubbles; wherein the polyvinyl alcohol and starch matrix block the transmittance of ultraviolet light, thereby minimizing the degradation of anthocyanin by light; at the same time, the matrix is ​​non-toxic, low in cost, highly stable and has good film-forming properties.

[0029] 3. The colorimetric / fluorescent dual-signal indicator film established in this application can react with the volatile components produced during the spoilage of fresh meat, resulting in changes in visible light and fluorescent colors. The changes in these two colors can effectively monitor the freshness of fresh meat products. Compared with a single visible light indicator, this indicator also has fluorescent indication characteristics, and its fluorescent signal changes sensitively, which can achieve accurate detection of the freshness of fresh meat, and has a low detection limit, ensuring the accuracy of detection.

[0030] 4. The method for visually detecting the freshness of meat established in this application can effectively judge the freshness of fresh meat according to the color changes of the indicator film under visible light and ultraviolet light during the processing, transportation and storage of fresh meat, and realize real-time and rapid monitoring. Natural anthocyanins as traditional color indicators are combined with fluorescent nanomaterials to make the color signal of the indicator label richer and the measurement results more accurate. The freshness of meat can be easily judged by the naked eye without expensive detection instruments, and the detection cost is low and the operation is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This is the preparation process of anthocyanin@UiO-66-NH2 and dual-signal indicator membrane.

[0032] Figure 2 The color change and ultraviolet absorption spectrum of anthocyanin @UiO-66-NH2 of the present application at different pH values.

[0033] Figure 3 Deprotonation of luminol in alkaline environment (A) of Example 1; luminescence principle after adding hydrogen peroxide (B).

[0034] Figure 4 This is the Fourier infrared image of the synthesized material UiO-66-NH2.

[0035] Figure 5 Schematic diagram of the color change of the smart film in Example 1 and the ammonia reaction time.

[0036] Figure 6 The indicator film color and TVB-N content (C) of pork stored at 4°C for different storage days under sunlight (A) and ultraviolet light (B). DETAILED DESCRIPTION

[0037] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0038] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those of ordinary skill in the art of the present invention generally understand. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe methods and / or materials related to the documents. In the event of conflict with any incorporated document, the content of this specification shall prevail.

[0039] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0040] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to. Figure 1This is the preparation process of anthocyanin@UiO-66-NH2 and dual-signal indicator membrane. The specific operation is detailed in the embodiment.

[0041] Embodiment 1:

[0042] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, comprising the following steps:

[0043] S1. Extraction of anthocyanins (the raw materials of anthocyanins are grape skin, mulberry, roselle, and black wolfberry);

[0044] The anthocyanin extract is prepared by the following steps: fresh grape skin, mulberry, roselle, and black wolfberry fruit are washed with distilled water and then naturally dried to obtain the corresponding dried raw materials; at the same time, anhydrous ethanol and water are mixed in a volume ratio of 3:1 to obtain an ethanol water extraction solution; then the dried raw materials are added to the ethanol water extraction solution, and the ratio of the dried raw materials to the ethanol water extraction solution is 1g:10mL; after magnetic stirring at room temperature for 2h, the supernatant is vacuum filtered to obtain anthocyanin extract; the anthocyanin extract is subjected to rotary evaporation at 50°C for 40min to concentrate anthocyanins and remove ethanol to obtain concentrated anthocyanins, and then freeze-dried at -40°C for 2 days to obtain a solid powder, which is anthocyanin extract;

[0045] Four anthocyanin extracts were obtained in this embodiment, namely, anthocyanin extracts from grape skin, anthocyanin extracts from mulberry, anthocyanin extracts from roselle, and anthocyanin extracts from black wolfberry;

[0046] S2. Synthesis of UiO-66-NH2 nanomaterials:

[0047] 320 mg of ZrCl4 and 300 mg of 2-aminoterephthalic acid powder were dissolved in 20 mL of N,N-dimethylformamide, and ultrasonicated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution;

[0048] The obtained 2-aminoterephthalic acid solution was added to a round-bottom flask and immersed in silicone oil for oil bath (the temperature of the oil bath was 80°C), 20 mL of acetic acid was added dropwise to the 2-aminoterephthalic acid solution at a rate of 3 s / drop, and then ZrCl4 solution was added dropwise at a rate of 3 s / drop, and magnetic stirring was performed at a speed of 400 r / min for 30 min, and the mixture was stirred evenly, and then the stirring was stopped, and the constant temperature oil bath at 80°C was maintained overnight to obtain a reaction solution; after the reaction solution was cooled to room temperature, 10 mL of anhydrous ethanol was added, and a precipitate was obtained after centrifugation at room temperature (8000 r / min, 20 min), the precipitate was collected, 10 mL of anhydrous ethanol was added, and centrifugation was performed twice (centrifugation conditions: 8000 r / min, 20 min), and finally the precipitate was taken and vacuum dried overnight, and the obtained product was UiO-66-NH2 nanoparticles; wherein Figure 4 This is the Fourier infrared image of the synthesized material UiO-66-NH2.

[0049] S3. Encapsulation of anthocyanins in metal-organic frameworks;

[0050] 50 mg of UiO-66-NH2 nanoparticles and 30 mg of anthocyanin extract were dissolved in 10 mL of anhydrous ethanol and ultrasonically dissolved for 20 minutes to obtain a mixed solution; the mixed solution was then centrifuged at a speed of 10000 r / min for 15 min, the precipitate was collected, and washed with distilled water for 3 times to remove the unencapsulated anthocyanin; finally, the mixture was vacuum dried at 40°C in the dark overnight to obtain anthocyanin-loaded UiO-66-NH2, recorded as anthocyanin@UiO-66-NH2; four anthocyanin extracts were obtained in this embodiment, namely, anthocyanin extract of grape skin, anthocyanin extract of mulberry, anthocyanin extract of roselle, and anthocyanin extract of black wolfberry;

[0051] S4. Preparation of dual-signal freshness indicator film;

[0052] (1) 1.7 g of polyvinyl alcohol and 3.4 g of corn starch were mixed and added into 100 mL of distilled water, and magnetically stirred at 400 r / min for 1 h at 100° C. After the mixture was cooled to room temperature, 1 wt % of glycerol based on the mass of corn starch was added to enhance the stability of the indicator solution, and stirred at room temperature for 20 min to obtain a starch / polyvinyl alcohol polymer solution;

[0053] (2) Add the anthocyanin @UiO-66-NH2 prepared in step S3 and 20 mg of luminol to 15 mL of corn starch / polyvinyl alcohol polymer solution under ultrasonic treatment, and dissolve to obtain a freshness indicator layer solution; wherein the amount of anthocyanin @UiO-66-NH2 is 5% of the total mass of corn starch and polyvinyl alcohol; then, cast the freshness indicator layer solution into a disposable culture medium with a size of 90×90 mm; and dry it at 40°C for 12 hours to form a dual-signal indicator film. Among them, the dual-signal indicator films prepared based on the four raw materials of grape skin, mulberry, roselle and black wolfberry are marked as A1@UiO-66-NH2, A2@UiO-66-NH2, A3@UiO-66-NH2, and A4@UiO-66-NH2.

[0054] Comparative Example 1: The operation is the same as that of Example 1, except that in step (2) of step S4, anthocyanin @UiO-66-NH2 is replaced by the anthocyanin extract in step S1; corresponding indicator films are obtained, wherein the indicator films prepared based on the four raw materials of grape skin, mulberry, roselle and black wolfberry are marked as A1, A2, A3, and A4 respectively.

[0055] In order to understand the possible response mechanism of the functional factors anthocyanin and luminol in the membrane matrix, the UV-visible absorption spectra of anthocyanin@UiO-66-NH2 under different pH (2-12) environments were studied in this example. Figure 2 As shown, (A) is anthocyanin extract from grape skin, (B) is anthocyanin extract from mulberry, (C) is anthocyanin extract from roselle, and (D) is anthocyanin extract from black wolfberry. From the UV spectrum, the maximum UV absorption peak of anthocyanin @UiO-66-NH2 is at 535nm; with the continuous increase of pH value, the maximum absorption peaks of the four anthocyanins @UiO-66-NH2 gradually decrease, and a red shift phenomenon occurs. The color of the corresponding solution also gradually changes, indicating that the structure of anthocyanin has changed. This shows that anthocyanin is a natural dye with good color response to pH value and can be used as an ideal freshness indicator in smart packaging.

[0056] Figure 3 The schematic diagram of luminol luminescence shows that the amino group in the luminol molecule is more sensitive to protonation and deprotonation, thus giving luminol pH-responsive characteristics. The blue fluorescence gradually weakens with the increase of pH, and fluorescence quenching occurs after pH ≥ 11. Under alkaline conditions and the presence of hydrogen peroxide molecules, luminol will produce excited 3-aminophthalic acid divalent anion (3APA*), and the excited 3-aminophthalic acid divalent anion (3-APA*) returns from the excited state to the ground state, producing blue fluorescence.

[0057] S5. Optimization of reaction conditions when the dual signal indicator membrane is used;

[0058] A dual-signal indicator film (20 mm × 20 mm) was suspended at 10 mm from the liquid surface and the color change of the indicator film was recorded using a smartphone every 2 min for a total of 16 min with 1% ammonia water. Figure 5 Figure (A) shows the color response of 4 different anthocyanin-based films in an ammonia atmosphere (8mmol / L) under sunlight. When exposed to an ammonia environment, anthocyanins from different sources can react with ammonia under sunlight to cause a significant color reaction. Based on the color difference change, it was decided to act in an ammonia atmosphere for 15 minutes to obtain the best reaction effect.

[0059] At a reaction time of 15 min, the color difference response of different anthocyanin-based films was further investigated. Figure 5 Figures (B) and (C) show that different anthocyanins have different responses to ammonia, which is related to their anthocyanin content. Among them, the membrane added with Roselle anthocyanin@UiO-66-NH2 has the best response to the simulated volatile biogenic amine model.

[0060] The dual-signal indicator membrane is used to monitor the freshness of chilled pork. Taking A3@UiO-66-NH2 as an example, the steps are as follows;

[0061] (1) Cut fresh pork into 20 ± 0.5 g pieces and place them in a sterile packaging box. Stick the dual-signal indicator film (A3@UiO-66-NH2, size 20 mm × 20 mm) on the top of the packaging box without contacting the meat and store it in a refrigerator at 4 ± 1 °C.

[0062] (2) Take out the dual-signal indicator film every 48 hours, take photos of the dual-signal indicator film under fluorescent light and ultraviolet light with a camera, and record the color change of the dual-signal indicator film.

[0063] (3) The freshness grade of pork is determined by the automatic Kjeldahl method in the national standard. Mix 10g of meat with 75mL of distilled water and emulsify it for 20 minutes. Use standard hydrochloric acid titration solution (0.100mol / L) and an automatic Kjeldahl nitrogen analyzer. Turn off the reagent addition and waste discharge functions. Set the volume of alkali and water to 0mL and the volume of boric acid receiving solution to 25mL. The distillation time is 3min. Use an automatic potentiometric titration to determine the endpoint of the nitrogen analyzer. Set the titration endpoint pH to 4.65. Add 1g of magnesium oxide to the distillation tube containing the treated sample, immediately connect it to the distiller, and start the measurement according to the above requirements. Measure the TVB-N content in pork every 48 hours.

[0064] The results are attached Figure 6As shown in the figure, as the number of days of storage increases, the content of TVB-N changes and the corresponding changes in the color of the indicator film under sunlight and ultraviolet light. Pork is divided into three freshness levels based on the change in TVB-N content. Pork is fresh meat from 0 to 72 hours, second-fresh meat from 96 to 144 hours, and spoiled meat from 168 to 240 hours. As the refrigeration time of pork increases, the color of the label changes significantly and is easier to identify with the naked eye.

[0065] Under sunlight, the color changes from red-pink (0-72h) to blue-pink (96-144h) to green (168-240h); in addition, under ultraviolet light, the color response changes from red-pink (0-72h) to orange-pink (96-144h) to yellow-green (168-240h). The color change characteristics are obvious, showing excellent sensitivity. The color change of the indicator film is consistent with the change of the TVB-N content indicating the freshness of pork, indicating that the prepared dual-signal indicator film can intuitively and effectively monitor the freshness of chilled pork in real time.

[0066] Embodiment 2:

[0067] A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, comprising the following steps:

[0068] S1. Extraction of anthocyanins;

[0069] The anthocyanin extract is prepared by the following steps: fresh grape skin, mulberry, roselle, and black wolfberry fruit are washed with distilled water and then naturally dried to obtain the corresponding dried raw materials; at the same time, anhydrous ethanol and water are mixed in a volume ratio of 3:1 to obtain an ethanol water extract solution; then the dried raw materials are added to the ethanol water extract solution, and the ratio of the dried raw materials to the ethanol water extract solution is 1:10 (g / mL); after magnetic stirring at room temperature for 2 hours, the supernatant is vacuum filtered to obtain anthocyanin extract; the anthocyanin extract is subjected to rotary evaporation at 50°C for 40 minutes to concentrate the anthocyanins and remove ethanol to obtain concentrated anthocyanins, and then freeze-dried at -40°C for 2 days to obtain a solid powder, which is anthocyanin extract; four anthocyanin extracts are obtained in this embodiment, namely, anthocyanin extract of grape skin, anthocyanin extract of mulberry, anthocyanin extract of roselle, and anthocyanin extract of black wolfberry.

[0070] S2. Synthesis of UiO-66-NH2 nanomaterials;

[0071] 320 mg of ZrCl4 and 300 mg of 2-aminoterephthalic acid powder were dissolved in 20 mL of N,N-dimethylformamide, and ultrasonicated until completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution;

[0072] The obtained 2-aminoterephthalic acid solution was added to a round-bottom flask and immersed in silicone oil for oil bath. 20 mL of acetic acid was added dropwise to the 2-aminoterephthalic acid solution at a rate of 3 s / drop, and then ZrCl4 solution was added dropwise at a rate of 3 s / drop. The mixture was magnetically stirred at a speed of 400 r / min for 30 min and stirred evenly. Then the stirring was stopped and the constant temperature oil bath at 80°C was maintained overnight to obtain a reaction solution. After the reaction solution was cooled to room temperature, 10 mL of anhydrous ethanol was added, and precipitate A was obtained after centrifugation at room temperature (8000 r / min, 20 min). The precipitate A was washed with 10 mL of anhydrous ethanol, centrifuged again twice (8000 r / min, 20 min), and the precipitate was taken and vacuum dried overnight to obtain the product, namely UiO-66-NH2 nanoparticles.

[0073] S3. Encapsulation of anthocyanins in metal-organic frameworks;

[0074] 45 mg of UiO-66-NH2 nanoparticles and 20 mg of anthocyanin were dissolved in 10 mL of anhydrous ethanol and fully dissolved by ultrasonication for 20 minutes to obtain a mixed solution; the mixed solution was then centrifuged at 10000 r / min for 15 minutes, the precipitate was collected, and washed with distilled water for 3 times to remove the unencapsulated anthocyanin, and finally, the anthocyanin-loaded UiO-66-NH2 was obtained by vacuum drying at 40 °C in the dark overnight, referred to as anthocyanin@UiO-66-NH2;

[0075] S4. Preparation of dual-signal freshness indicator film;

[0076] Step 1: Add corn starch and polyvinyl alcohol into 100 mL of distilled water and heat in a water bath at 100°C to obtain a film-forming matrix solution;

[0077] Step 2: Add glycerol, ethanol solution of anthocyanin@UiO-66-NH2 material and luminol to the film-forming matrix solution obtained in step 1, and mix them evenly with ultrasound to obtain a film-forming solution;

[0078] Step 3: The film-forming liquid obtained in step 2 is allowed to stand to remove bubbles hidden in the film-forming liquid, and then cast into a film; after drying, an anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film is obtained.

[0079] In the film-forming matrix solution, the content of corn starch is 0.04 g / mL, the content of polyvinyl alcohol is 0.01 g / mL, and the content of glycerol is 0.01 g / mL.

[0080] In the film-forming solution: the amount of anthocyanin @UiO-66-NH2 is 5% of the total mass of corn starch and polyvinyl alcohol; the amount of luminol added is 20 mg.

[0081] The preferred embodiment is that the constant temperature and humidity environment conditions for storing the anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film are: 25°C, 50% RH.

[0082] A preferred embodiment is: the preparation of anthocyanin @UiO-66-NH2 material is based on Roselle anthocyanin powder as raw material, and the steps include: adding Roselle anthocyanin powder and UiO-66-NH2 nanoparticles to ethanol in turn, ultrasonically mixing in a dark environment, centrifugally washing to remove impurities, and drying in the dark.

[0083] Among them: the ultrasonic power is 100W, the ultrasonic time is 20min, and the ultrasonic temperature is 30℃.

[0084] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention; therefore, although this specification has described the present invention in detail with reference to the above embodiments, a person skilled in the art should understand that the present invention can still be modified or replaced by equivalents; and all technical solutions and improvements thereof that do not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film, comprising the following steps: S1. Extraction of anthocyanins: The anthocyanin raw material is washed with distilled water and then naturally dried to obtain an air-dried raw material; at the same time, anhydrous ethanol and water are mixed to obtain an ethanol-water extraction solution; then the air-dried raw material is added to the ethanol-water extraction solution, and after being stirred evenly at room temperature, the supernatant is collected by vacuum filtration, and the obtained supernatant is subjected to rotary evaporation to obtain concentrated anthocyanin; finally, solid powder is obtained by freeze-drying, which is an anthocyanin extract; the anthocyanin raw material includes but is not limited to grape skin, mulberry, roselle, and black wolfberry; S2. Synthesis of UiO-66-NH2 nanomaterials: The ZrCl4 and 2-aminoterephthalic acid powders are dissolved in N, N-dimethylformamide respectively, and ultrasonic treatment is performed until they are completely dissolved to obtain a ZrCl4 solution and a 2-aminoterephthalic acid solution; the 2-aminoterephthalic acid solution is placed in an oil bath, and acetic acid is added dropwise to the 2-aminoterephthalic acid solution, and then the ZrCl4 solution is added dropwise and stirred evenly; then, the stirring is stopped, and the constant temperature oil bath is maintained overnight to obtain a reaction solution; after the reaction solution is cooled to room temperature, anhydrous ethanol is added, and a precipitate is collected by centrifugation at room temperature, and finally the precipitate is washed, centrifuged and vacuum dried to obtain a product, namely, UiO-66-NH2 nanomaterial; S3. Encapsulation of anthocyanins in metal organic frameworks: The UiO-66-NH2 nanoparticles obtained in step S3 and the anthocyanin extract obtained in step S1 are added to anhydrous ethanol and uniformly mixed by ultrasonication to obtain a mixed solution; the mixed solution is centrifuged and the precipitate is collected and dispersed in distilled water for washing, and the precipitate is collected after washing and vacuum dried in the dark. The product obtained after drying is UiO-66-NH2 loaded with anthocyanins, which is recorded as anthocyanin@UiO-66-NH2; S4. Preparation of dual-signal freshness indicator film: Add corn starch and polyvinyl alcohol into distilled water, heat and stir evenly to form a mixed solution; then cool to room temperature, add glycerol into the mixed solution and stir evenly to obtain a starch / polyvinyl alcohol polymer solution; The anthocyanin @UiO-66-NH2 and luminol obtained in step S3 are added to the starch / polyvinyl alcohol polymer under ultrasonic treatment, and after ultrasonic treatment, they are poured into a mold and dried to form a film to obtain a dual-signal indicator film.

2. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: In the step S1, the volume ratio of anhydrous ethanol to water is 3:1; the ratio of the air-dried raw material to the ethanol water extract solution is 1 g:10 mL; the stirring is magnetic stirring, and the stirring time is 2 h; the temperature of rotary evaporation is 50° C., and the time is 40 min; the temperature of freeze drying is -40° C., and the freeze drying time is 2 days.

3. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: The dosage of ZrCl4, 2-aminoterephthalic acid, N,N-dimethylformamide, acetic acid and anhydrous ethanol in step S2 is in the following relationship: 300-350 mg: 200-300 mg: 1-2 mL: 20 mL: 10 mL.

4. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: The temperature of the constant temperature oil bath in step S2 is 80°C; the rate of adding acetic acid is 3s / drop; the speed of the magnetic stirring is 400r / min, and the stirring time is 30min; washing is performed using anhydrous ethanol; The centrifugal speed is 8000-10000 r / min, and the centrifugal time is 20-25 min; the vacuum drying temperature is 75° C., and the time is 10-12 h.

5. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: In step S3, the ratio of UiO-66-NH2, anthocyanin extract and anhydrous ethanol added is 45-50 mg: 20-30 mg: 10 mL; the ultrasonic mixing time is 20 min; the number of distilled water washings is 3-5 times; the centrifugal conditions are: speed 10000 r / min, time is 10-15 min; the vacuum drying temperature is 40°C, and the drying time is 12 h.

6. The method for preparing anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 1, characterized in that: The dosage of polyvinyl alcohol, corn starch, luminol and distilled water in step S4 is 1-1.7g:3-4g:10-20mg:100mL; the dosage of glycerol is 1wt% of the dosage of corn starch; the dosage of anthocyanin @UiO-66-NH2 is 1%-5% of the total mass of corn starch and polyvinyl alcohol; the conditions of the ultrasonic treatment are: ultrasonic power of 100W, ultrasonic time of 20min, and ultrasonic temperature of 30°C; the mold includes a disposable culture dish and a polytetrafluoroethylene plate; the drying temperature is 40°C and the drying time is 12h.

7. Anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film prepared according to any one of claims 1 to 6.

8. Use of the anthocyanin@UiO-66-NH2 nano gas-sensitive pigment dual-signal indicator film according to claim 7 in detecting the freshness of meat products.

9. The use according to claim 8, characterized in that Here are the steps: (1) Cut meat samples into the same mass, put them into a reaction container and store them under low temperature conditions, take out the samples at regular intervals, and determine the TVB-N content by Kjeldahl nitrogen determination according to the different contents of volatile amine substances in the meat products of different freshness levels, and divide the freshness of the meat products into three levels accordingly, namely, fresh: TVB-N content <15 mg / 100 g, sub-fresh: TVB-N content 15-25 mg / 100 g, and corrupt TVB-N content >25 mg / 100 g; the low temperature condition is 4° C., the storage time is 0-480 h; different freshness levels correspond to different storage days; (2) Obtain the color difference of the dual-signal indicator membrane before the reaction, then store the dual-signal indicator membrane and meat products of different grades in the same closed reaction container, obtain the color difference of the indicator membrane again after the reaction, and calculate the color difference value based on the color difference before and after the reaction, recorded as ΔE, and then establish a correlation curve between ΔE and the corresponding TVB-N content, thereby quantitatively judging and realizing the freshness detection of meat products; at the same time, meat products at different grades will cause the dual-signal indicator membrane to change color, so the freshness of the meat products can be judged according to the color change of the dual-signal indicator membrane, realizing visual detection.

10. The use according to claim 9, characterized in that The reaction time in step (2) is 1 to 16 minutes; the dual-signal indicator membrane is fixed on the top of the reaction container, and the distance from the meat product is 10-50 mm; The steps of color difference extraction are as follows: use a colorimeter to extract the color difference of the indicator film before and after the reaction, that is, the L, a, and b values; the values ​​before the reaction are recorded as L0, a0, and b0; the values ​​after the reaction are recorded as L1, a1, and b1; the values ​​of L, a, and b before and after the reaction are subtracted, that is, △L=L1-L0, △a=a1-a0, △b=b1-b0; and according to the formula Calculate the color difference value.

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