A modified anthocyanin freshness indicating film and a method for preparing the same

Abstract

The application discloses a modified anthocyanin freshness indicating film and a preparation method thereof. The indicating film comprises a dialdehyde polysaccharide-anthocyanin conjugate and a polymer. The dialdehyde polysaccharide-anthocyanin conjugate is obtained by condensation reaction of hydrogen atoms at C-6 or C-8 positions on the A ring of an anthocyanin molecule and aldehyde groups of a dialdehyde polysaccharide, and then forms a non-covalent bond with the polymer. The preparation method comprises the following steps: (1) condensation reaction of the dialdehyde polysaccharide and the anthocyanin under acidic conditions to obtain the dialdehyde polysaccharide-anthocyanin conjugate; (2) dissolving the dialdehyde polysaccharide-anthocyanin conjugate and the polymer in solvents respectively, then mixing, adding a plasticizer, and finally forming a film. The modified anthocyanin freshness indicating film of the application modifies the anthocyanin by condensation reaction of the anthocyanin and the dialdehyde polysaccharide, further combines the modified anthocyanin with the polymer, so that the release of the anthocyanin from the film is inhibited, and the stability of the film is improved.

Description

Technical Field

[0001] This invention relates to a membrane, and more particularly to a modified anthocyanin freshness indicator membrane and its preparation method. Background Technology

[0002] With the increasing awareness of food safety, smart food packaging materials that can provide information on food ingredients, processing, and freshness are gaining popularity among consumers. As a novel smart food packaging technology, freshness indicator films can sense changes in food quality through natural pigments embedded in the film (such as chlorophyll, carotenoids, curcumin, betaine, and anthocyanins), and visually reflect the freshness of the food through color changes in these natural pigments. Among the many natural pigments, anthocyanins have the advantages of wide availability and good color-changing properties; therefore, the preparation of freshness indicator films based on anthocyanins has become a research hotspot in the field of smart food packaging. Currently, researchers typically prepare anthocyanin freshness indicator films by directly mixing anthocyanins extracted from plants with a polymer matrix (such as polysaccharides and proteins) and then using a casting method to form the film. It is worth noting that the freshness indicator film prepared by directly mixing anthocyanins with polymer matrix using traditional methods has the following disadvantages: (1) Anthocyanins have poor stability and are more sensitive to external environmental conditions (such as light, heat, etc.), and are very easy to degrade, which will cause the film to change color and affect its color-changing function; (2) In the high humidity food packaging environment, because anthocyanins have high hydrophilicity and weak interaction with polymer matrix, anthocyanins are very easy to be released from the film, which will cause the film to fade and lose its color-changing function.

[0003] Currently, various technologies (such as multilayer film technology, nano-encapsulation technology, and electrospinning technology) have been used to improve the stability of anthocyanin freshness indicator films. Multilayer film technology involves adding protective layers with UV resistance and hydrophobic properties to both sides of the anthocyanin film to enhance its stability. Nano-encapsulation technology first embeds anthocyanins in nanoparticles, then adds these anthocyanin-encapsulated nanoparticles to a biopolymer matrix, thereby improving the film's stability. Electrospinning technology embeds anthocyanins into micro / nano-scale fibers to enhance the film's stability. It is worth noting that all of these technologies rely on providing a protective barrier for anthocyanins to improve film stability. These protective barriers can weaken the anthocyanins' ability to detect changes in food quality, thus limiting the film's color-changing properties. Therefore, there is an urgent need to research new methods to enhance the stability of anthocyanin films. Summary of the Invention

[0004] Objectives of the invention: The first objective of this invention is to improve the stability of anthocyanin membranes by providing a modified anthocyanin freshness indicator membrane that improves the stability of anthocyanin membranes; the second objective of this invention is to provide a method for preparing the modified anthocyanin freshness indicator membrane.

[0005] Technical solution: The modified anthocyanin freshness indicator film of the present invention comprises a dialdehyde polysaccharide-anthocyanin conjugate and a polymer. The dialdehyde polysaccharide-anthocyanin conjugate is obtained by condensation reaction between the hydrogen atom at the C-6 or C-8 position on the A ring of the anthocyanin molecule and the aldehyde group of the dialdehyde polysaccharide. The dialdehyde polysaccharide-anthocyanin conjugate and the polymer form a non-covalent bond.

[0006] Taking purple sweet potato anthocyanins and dialdehyde locust bean gum as an example, the structural formula of the dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate obtained by the condensation reaction is as follows:

[0007]

[0008] Preferably, the dialdehyde polysaccharide is a polysaccharide having two aldehyde groups in its molecular unit. Preferably, the dialdehyde polysaccharide is dialdehyde starch, dialdehyde dextran, dialdehyde agarose, dialdehyde galactan, dialdehyde locust bean gum, dialdehyde guar gum, or dialdehyde tara gum.

[0009] Preferably, the anthocyanins are plant-derived anthocyanins. More preferably, the anthocyanins are black goji berry anthocyanins, purple corn anthocyanins, purple cabbage anthocyanins, or purple sweet potato anthocyanins.

[0010] The polymer functions by forming a non-covalent interaction with the dialdehyde polysaccharide-anthocyanin conjugate, providing a film-forming matrix without affecting its color-changing properties. The polymer possesses advantages such as biodegradability, non-toxicity, low cost, and ease of film formation. Preferably, the polymer is a neutral polysaccharide. More preferably, the neutral polysaccharide is starch, dextran, agarose, galactan, locust bean gum, guar gum, or tara gum.

[0011] Preferably, the mass ratio of the dialdehyde polysaccharide-anthocyanin conjugate to the polymer is 1:10-15. The mass ratio of the dialdehyde polysaccharide-anthocyanin conjugate to the polymer mainly affects the color and color-changing ability of the membrane. At the same time, the dialdehyde polysaccharide-anthocyanin conjugate and the polymer can also affect the physicochemical properties of the membrane through non-covalent interactions.

[0012] The method for preparing the modified anthocyanin freshness indicator film of the present invention is characterized by comprising the following steps:

[0013] (1) Dissolve dialdehyde polysaccharide and anthocyanin in a solvent, adjust the pH to acidic, and carry out a condensation reaction to obtain dialdehyde polysaccharide-anthocyanin conjugate.

[0014] (2) The dialdehyde polysaccharide-anthocyanin conjugate is dissolved in a solvent to form solution 1, the polymer is dissolved in a solvent to form solution 2, solution 1 and solution 2 are mixed, a plasticizer is added, and the mixture is mixed to obtain a film-forming solution. Then, a film is formed to obtain the modified anthocyanin freshness indicator film.

[0015] Preferably, in step (1), the reaction temperature is 25–45°C. During the reaction, the reaction products at different time points are placed into dialysis bags, and the reaction endpoint is determined by measuring the absorbance value of the precipitate at different time points. That is, the reaction is considered complete when the absorbance value of the precipitate no longer decreases. Preferably, the reaction time is 24–48 hours.

[0016] Preferably, in step (1), the mass ratio of the reaction dialdehyde polysaccharide to anthocyanin is 1 to 5:1.

[0017] Preferably, in step (1), the solvent is water.

[0018] Preferably, in step (1), the pH is 2 to 4.

[0019] Preferably, in step (1), the reaction is completed and purification is also included. The purification is as follows: the reaction product is dialyzed using a dialysis bag with a molecular weight cutoff of 3000 to 12000 Da, and deionized water is used for dialysis for 24 to 72 hours, with the water changed every 6 to 12 hours; after dialysis, the product is freeze-dried under the following conditions: temperature -45 to -50°C, pressure 10 to 100 Pa, and drying time 24 to 48 hours.

[0020] Preferably, in step (2), the plasticizer is glycerin.

[0021] Preferably, in step (2), the mixing conditions are stirring at 30-50°C for 60-120 minutes.

[0022] Preferably, in step (2), the film-forming method is as follows: the film-forming solution is poured into an organic glass mold of 15-35cm × 15-35cm and dried in an environment of 20-35℃ and 30%-60% relative humidity for 36-72h.

[0023] Mechanism of Invention: This invention first involves a condensation reaction between the hydrogen atom at position C-6 or C-8 of the anthocyanin molecule's A ring and the aldehyde group of a dialdehyde polysaccharide to obtain a dialdehyde polysaccharide-anthocyanin conjugate (a modified anthocyanin). Since the dialdehyde polysaccharide-anthocyanin conjugate is a high-molecular-weight pigment, it exhibits higher stability compared to unmodified small-molecule anthocyanins. This invention then adds the dialdehyde polysaccharide-anthocyanin conjugate to a polymer matrix to prepare a polymer / dialdehyde polysaccharide-anthocyanin conjugate membrane. Through the multi-site non-covalent interactions formed between the dialdehyde polysaccharide-anthocyanin conjugate and the polymer matrix, the release of anthocyanins from the membrane is inhibited.

[0024] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The modified anthocyanin freshness indicator film modifies anthocyanins by condensation reaction between anthocyanins and dialdehyde polysaccharides, and further combines the modified anthocyanins with polymers, thereby inhibiting the release of anthocyanins from the film and improving the stability of the film; (2) Compared with the freshness indicator film prepared by directly mixing anthocyanins with polymer matrix using traditional methods, the modified anthocyanin freshness indicator film prepared by the present invention has a deeper color, higher mechanical strength, better color development performance and storage stability, and can effectively inhibit the release of anthocyanins from the film, and has application potential in the field of intelligent food packaging. Attached Figure Description

[0025] Figure 1 The infrared spectra of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention are shown below;

[0026] Figure 2 The X-ray diffraction patterns of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention are shown below.

[0027] Figure 3 These are scanning electron microscope (SEM) images of Embodiment 1 and Comparative Example 1, and Embodiment 2 and Comparative Example 2 of the present invention.

[0028] Figure 4 A is a membrane image of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention; Figure 4 B is the ultraviolet-visible light transmittance diagram of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention;

[0029] Figure 5 A is a graph showing the water vapor transmission rate (WVP) of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention; Figure 5 B is a graph showing the oxygen permeability (OP) of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention;

[0030] Figure 6 The mechanical properties of Embodiment 1 and Comparative Example 1, Embodiment 2 and Comparative Example 2 of the present invention are shown in the diagrams; wherein, 6A is the tensile strength (TS) diagram and 6B is the elongation at break (EAB) diagram.

[0031] Figure 7 The thermogravimetric analysis diagrams of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention are shown; wherein, 7A is the thermogravimetric (TG) diagram and 7B is the thermogravimetric differential (DTG) diagram.

[0032] Figure 8 These are color change performance diagrams of Example 1 and Comparative Example 1, Example 2 and Comparative Example 2 of the present invention; wherein, 8A is the color change diagram of the membrane in pH (3-12) buffer solution, and 8B is the color change diagram of the membrane in an ammonia atmosphere;

[0033] Figure 9 The images show the color changes and ultraviolet-visible transmittance changes of Examples 1, 2, and 37°C for 28 days after storage, respectively. 9A corresponds to Example 1, 9B to Example 1, 9C to Example 2, and 9D to Example 2.

[0034] Figure 10 The diagram shows the swelling and anthocyanin release in water for Examples 1, 1, 2 and 2 of the present invention; wherein, 10A is the membrane swelling diagram, 10B is the membrane swelling rate, and 10C is the cumulative anthocyanin release rate diagram.

[0035] Figure 11 These are application examples of the present invention, namely, Example 1 and Comparative Example 1, Example 2 and Comparative Example 2, for indicating shrimp freshness; wherein, 11A is a graph showing the color change of the membrane and the change of ΔE value, and 11B is a graph showing the change of total volatile basic nitrogen (TVB-N) value of shrimp during storage. Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the embodiments.

[0037] Example 1

[0038] The modified anthocyanin freshness indicator film of the present invention is prepared by the following steps:

[0039] (1) 0.2 g of dialdehyde locust bean gum and 0.2 g of purple sweet potato anthocyanin were dissolved together in 20 mL of deionized water and mixed thoroughly at 25 °C for 30 min to obtain a dialdehyde locust bean gum / purple sweet potato anthocyanin mixed solution. The pH of the dialdehyde locust bean gum / purple sweet potato anthocyanin mixed solution was adjusted to 3.5 and reacted at 25 °C for 24 h. The reaction product was placed in a 3500 Da dialysis bag and dialyzed with deionized water, with the water changed every 8 h for a total of 24 h. The dialyzed solution was freeze-dried at -45 °C and 20 Pa for 48 h to obtain the dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate, which was named PSPA-g-DALBG. The anthocyanin content in PSPA-g-DALBG was 1.3%.

[0040] (2) Dissolve 0.5g of dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate (the anthocyanin content in the conjugate is 6.5mg) in 40mL of deionized water to obtain a dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate solution; at the same time, dissolve 5.1g of locust bean gum in 210mL of deionized water to obtain a locust bean gum solution; then mix the dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate solution and the locust bean gum solution at 40℃ for 60min to obtain a locust bean gum / dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate mixed solution; then add 1.02g of glycerol to the mixed solution and mix at 40℃ for 30min to obtain a film-forming solution; pour the film-forming solution into an organic glass mold with a specification of 24cm×24cm and dry it naturally in an environment of 25℃ and 50% relative humidity for 48h to obtain a locust bean gum / dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate membrane, which is named LBG / PSPA-g-DALBG.

[0041] Example 2

[0042] The modified anthocyanin freshness indicator film of the present invention is prepared by the following steps:

[0043] (1) In step (1) of Example 1, the dialdehyde locust bean gum was replaced with dialdehyde guar gum, and the purple sweet potato anthocyanin was replaced with purple cabbage anthocyanin. The other preparation conditions remained unchanged. The resulting dialdehyde guar gum-purple cabbage anthocyanin conjugate was named PCA-g-DAGG. The anthocyanin content in PCA-g-DAGG was 1.4%.

[0044] (2) Replace the locust bean gum in step (2) of Example 1 with guar gum, and replace the dialdehyde locust bean gum-purple sweet potato anthocyanin conjugate with the dialdehyde guar gum-purple cabbage anthocyanin conjugate (the anthocyanin content in the dialdehyde guar gum-purple cabbage anthocyanin conjugate is 7.0 mg). Keep the other preparation conditions unchanged, and name the prepared guar gum / dialdehyde guar gum-purple cabbage anthocyanin conjugate membrane GG / PCA-g-DAGG.

[0045] Comparative Example 1

[0046] The preparation method of locust bean gum / purple sweet potato anthocyanin membrane includes the following steps:

[0047] 6.5 mg of purple sweet potato anthocyanins were dissolved in 40 mL of deionized water to obtain a purple sweet potato anthocyanin solution; simultaneously, 5.1 g of locust bean gum was dissolved in 210 mL of deionized water to obtain a locust bean gum solution; the purple sweet potato anthocyanin solution and the locust bean gum solution were then mixed at 40 °C for 60 min to obtain a locust bean gum / purple sweet potato anthocyanin mixed solution; 1.02 g of glycerol was then added to the mixed solution and mixed at 40 °C for 30 min to obtain a film-forming solution; the film-forming solution was poured into a 24 cm × 24 cm plexiglass mold and naturally dried at 25 °C and 50% relative humidity for 48 h to obtain a locust bean gum / purple sweet potato anthocyanin film, which was named LBG / PSPA.

[0048] Comparative Example 2

[0049] The preparation method of guar gum / purple cabbage anthocyanin membrane includes the following steps:

[0050] In Comparative Example 1, the locust bean gum was replaced with guar gum, and the 6.5 mg purple sweet potato anthocyanin was replaced with 7.0 mg purple cabbage anthocyanin. The other preparation conditions remained unchanged, and the resulting guar gum / purple cabbage anthocyanin membrane was named GG / PCA.

[0051] Structural characterization

[0052] The membranes prepared in Examples 1, 2, 1, and 2 were tested using infrared spectroscopy, X-ray diffraction, and microstructure analysis.

[0053] (1) Using a Fourier transform infrared spectrometer with attenuated total reflection mode at 400–4000 cm⁻¹ -1 The infrared spectrum of the membrane sample was measured within the specified range, and the test results are as follows: Figure 1 As shown:

[0054] Depend on Figure 1 It was found that LBG / PSPA-g-DALBG and LBG / PSPA had similar infrared spectra, and GG / PCA-g-DAGG and GG / PCA had similar infrared spectra. This indicates that a non-covalent interaction was formed between the dialdehyde polysaccharide-anthocyanin conjugate and the polymer matrix.

[0055] (2) The X-ray diffraction pattern of the membrane sample was determined using a polycrystalline X-ray diffractometer. The membrane sample was scanned at a speed of 2° / min within the range of 5° to 75°. The test results are as follows: Figure 2 As shown:

[0056] Depend on Figure 2 The results showed that LBG / PSPA-g-DALBG and LBG / PSPA had similar crystallization peaks, as did GG / PCA-g-DAGG and GG / PCA. This indicates that the dialdehyde polysaccharide-anthocyanin conjugate does not significantly alter the crystallization properties of the membrane.

[0057] (3) The cross-sectional microstructure of the membrane sample was analyzed using a scanning electron microscope. The test results are as follows: Figure 3 As shown:

[0058] Depend on Figure 3 The results showed that LBG / PSPA and GG / PCA exhibited non-uniform cross-sections, while LBG / PSPA-g-DALBG and GG / PCA-g-DAGG showed more uniform cross-sections. This indicates that the dialdehyde polysaccharide-anthocyanin conjugate has a stronger interaction with the polymer matrix, and the more uniform internal structure contributes to improved physicochemical properties of the membrane.

[0059] Performance testing

[0060] 1. The ultraviolet-visible transmittance, WVP, OP, TS, EAB and thermogravimetric properties of the films prepared in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 were tested.

[0061] (1) Cover the membrane with printing paper, take a picture of the membrane sample with a smartphone, and then use a UV-Vis spectrophotometer to measure the UV-Vis transmittance of the membrane in the range of 200-800nm. The test results are as follows: Figure 4 As shown:

[0062] Depend on Figure 4 A shows that LBG / PSPA is light purple, LBG / PSPA-g-DALBG is purple, GG / PCA is light blue, and GG / PCA-g-DAGG is blue-gray; this indicates that when the membranes have the same anthocyanin equivalent, the membranes containing the dialdehyde polysaccharide-anthocyanin conjugate have a deeper color. Figure 4 B shows that the transmittance of all membranes in the 200-800 nm range is less than 60%, and the transmittance of LBG / PSPA-g-DALBG and GG / PCA-g-DAGG is lower than that of LBG / PSPA and GG / PCA, respectively. This indicates that the membranes containing dialdehyde polysaccharide-anthocyanin conjugates have better UV-Vis light blocking performance.

[0063] (2) The WVP of the membrane sample was determined by gravimetric method, and the OP of the membrane sample was determined by gas permeability meter. The results are as follows: Figure 5 Show:

[0064] like Figure 5 As shown in Figure A, the wVP of LBG / PSPA, GG / PCA, LBG / PSPA-g-DALBG, and GG / PCA-g-DAGG ranges from 16.05 to 16.90 × 10⁻⁶. -11 gm -1 s -1 Pa -1Within the specified range, this indicates that the membrane possesses certain water vapor barrier properties; however, LBG / PSPA-g-DALBG and LBG / PSPA have similar WVP values, as do GG / PCA-g-DAGG and GG / PCA. For example... Figure 5 As shown in B, the OP of LBG / PSPA, GG / PCA, LBG / PSPA-g-DALBG, and GG / PCA-g-DAGG ranges from 0.51 to 4.62 cm. 3 mmm -2 day -1 atm -1 Within the specified range, this indicates that the membrane possesses certain oxygen barrier properties. Furthermore, the membrane containing the dialdehyde polysaccharide-anthocyanin conjugate exhibits a lower OP value.

[0065] (3) The mechanical properties of the membrane sample were determined using a universal tensile testing machine. The test results are as follows: Figure 6 Show:

[0066] Depend on Figure 6 As shown in Figure A, LBG / PSPA-g-DALBG has a higher TS value than LBG / PSPA, and GG / PCA-g-DAGG has a higher TS value than GG / PCA, indicating that the membrane containing the dialdehyde polysaccharide-anthocyanin conjugate has higher rigidity and is more effective in resisting external damage; Figure 6 B shows that LBG / PSPA-g-DALBG and LBG / PSPA have similar EAB values, and GG / PCA-g-DAGG and GG / PCA have similar EAB values, indicating that these films all have good ductility.

[0067] (4) The thermogravimetric properties of the membrane samples were determined using a thermogravimetric analyzer. Under the protection of high-purity nitrogen gas at a flow rate of 20 mL / min, the TG and DTG curves of the membrane samples were recorded within a temperature range of 50–750 °C. The results are as follows: Figure 7 Show:

[0068] Depend on Figure 7 It can be seen that all membranes have three thermal decomposition stages, with the main thermal decomposition of the membrane occurring between 160 and 405 °C; and the membrane containing dialdehyde polysaccharide-anthocyanin conjugate has higher thermal stability.

[0069] 2. The color-changing properties, storage stability, swelling characteristics in water, and anthocyanin release characteristics of Examples 1, 2, Comparative Examples 1 and 2 were tested.

[0070] (1) After immersing the membrane sample in a buffer solution with pH 3–12 for 1 min, record the color change of the membrane. The results are as follows: Figure 8 As shown.

[0071] Depend on Figure 8 As shown in Figure A, all membranes exhibited significant color changes in different buffer solutions, with LBG / PSPA-g-DALBG showing a deeper color than LBG / PSPA, and GG / PCA-g-DAGG showing a deeper color than GG / PCA. This indicates that membranes containing dialdehyde polysaccharide-anthocyanin conjugates exhibited more readily apparent color changes. Membrane samples were exposed to a 100 mM ammonia atmosphere for 180 min, and color changes at different time points were recorded. Figure 8 As shown in Figure B, all membranes exhibited significant color changes in an ammonia atmosphere, with LBG / PSPA-g-DALBG showing a deeper color than LBG / PSPA, and GG / PCA-g-DAGG showing a deeper color than GG / PCA. This indicates that membranes containing dialdehyde polysaccharide-anthocyanin conjugates have better color-changing ability.

[0072] (2) The membrane sample was stored at 37°C and 50% relative humidity for 28 days. The color change of the membrane sample was recorded every 7 days, and the change in UV-Vis transmittance of the membrane sample was measured. The results are as follows: Figure 9 As shown:

[0073] Depend on Figure 9 It was observed that during storage, LBG / PSPA changed from light purple to bluish-purple, while LBG / PSPA-g-DALBG retained its original purple color. Simultaneously, the transmittance of LBG / PSPA decreased significantly, and its transmittance curve showed a marked shift. In contrast, the transmittance of LBG / PSPA-g-DALBG decreased only slowly, and its transmittance curve did not show a significant shift. Similarly, GG / PCA changed from light blue to gray, while GG / PCA-g-DAGG retained its original bluish-gray color. Furthermore, the transmittance of GG / PCA decreased significantly, and its transmittance curve showed a marked shift. In contrast, the transmittance of GG / PCA-g-DAGG decreased only slowly, and its transmittance curve did not show a significant shift. This indicates that the membrane containing the dialdehyde polysaccharide-anthocyanin conjugate exhibits better storage stability.

[0074] (3) The membrane sample was immersed in deionized water for 30 min. The shape changes of the membrane at different time points were recorded, and the swelling rate and cumulative anthocyanin release rate of the membrane sample were measured. The results are as follows: Figure 10 As shown:

[0075] Depend on Figure 10It was found that LBG / PSPA-g-DALBG had a lower swelling ratio and a lower cumulative anthocyanin release rate than LBG / PSPA, and GG / PCA-g-DAGG had a lower swelling ratio and a lower cumulative anthocyanin release rate than GG / PCA. This indicates that LBG / PSPA-g-DALBG and GG / PCA-g-DAGG can effectively inhibit the release of anthocyanins from the membrane.

[0076] 3. Examples 1, 2, Comparative Example 1, and Comparative Example 2 were used to indicate the freshness of shrimp, and the results are as follows.

[0077] First, fix the membrane sample inside the lid of a transparent polypropylene food storage container. Then, place 220g of fresh shrimp at the bottom of the container, close the lid, and store it in a refrigerator at 4℃ for 96 hours. Record the color change of the membrane sample every 16 hours, calculate the ΔE value, and determine the TVB-N value of the shrimp. The results are as follows: Figure 11 As shown:

[0078] Depend on Figure 11 It was found that all membranes exhibited significant color changes during shrimp storage; the TVB-N value of shrimp stored at 4℃ for 64 hours reached 20.58 mg / 100g, exceeding the freshness threshold. At 64 hours, LBG / PSPA and LBG / PSPA-g-DALBG turned blue, with LBG / PSPA-g-DALB showing a deeper blue; GG / PCA and GG / PCA-g-DAGG turned green, with GG / PCA-g-DAGG showing a deeper green. Compared to the initial membranes, the ΔE value of LBG / PSPA increased by 13.29 at 64 hours, LBG / PSPA-g-DALB by 17.83, GG / PCA by 15.37, and GG / PCA-g-DAGG by 16.69. The results show that membranes containing dialdehyde polysaccharide-anthocyanin conjugates exhibit more pronounced color changes, making them easier to identify visually. Furthermore, during shrimp storage, due to high humidity, LBG / PSPA and GG / PCA underwent significant swelling and deformation, while LBG / PSPA-g-DALBG and GG / PCA-g-DAGG only experienced minor morphological changes. This indicates that membranes containing dialdehyde polysaccharide-anthocyanin conjugates are morphologically more stable during shrimp freshness monitoring.

Claims

1. A modified anthocyanin freshness indicator film, characterized in that, It includes dialdehyde polysaccharide-anthocyanin conjugates and polymers. The dialdehyde polysaccharide-anthocyanin conjugates are obtained by condensation reaction between the hydrogen atom at the C-6 or C-8 position of the A ring of the anthocyanin molecule and the aldehyde group of the dialdehyde polysaccharide. Non-covalent bonds are formed between the dialdehyde polysaccharide-anthocyanin conjugates and the polymers.

2. The modified anthocyanin freshness indicator film according to claim 1, characterized in that, The dialdehyde polysaccharide is a polysaccharide with two aldehyde groups in its molecular unit.

3. The modified anthocyanin freshness indicator film according to claim 1, characterized in that, The anthocyanins are plant-derived anthocyanins.

4. The modified anthocyanin freshness indicator film according to claim 1, characterized in that, The polymer is a neutral polysaccharide.

5. The modified anthocyanin freshness indicator film according to claim 1, characterized in that, The mass ratio of the dialdehyde polysaccharide-anthocyanin conjugate to the polymer is 1:10-15.

6. A method for preparing the modified anthocyanin freshness indicator film according to any one of claims 1 to 5, characterized in that, Includes the following steps: (1) Dissolve dialdehyde polysaccharide and anthocyanin in a solvent, adjust the pH to acidic, and carry out a condensation reaction to obtain dialdehyde polysaccharide-anthocyanin conjugate. (2) The dialdehyde polysaccharide-anthocyanin conjugate is dissolved in a solvent to form solution 1, the polymer is dissolved in a solvent to form solution 2, solution 1 and solution 2 are mixed, a plasticizer is added, and the mixture is mixed to obtain a film-forming solution. Then, a film is formed to obtain the modified anthocyanin freshness indicator film.

7. The method for preparing the modified anthocyanin freshness indicator film according to claim 6, characterized in that, In step (1), the condensation reaction temperature is 25-45°C and the reaction time is 24-48 hours.

8. The method for preparing the modified anthocyanin freshness indicator film according to claim 6, characterized in that, In step (1), the mass ratio of the reaction dialdehyde polysaccharide to anthocyanin is 1 to 5:

1.

9. The method for preparing the modified anthocyanin freshness indicator film according to claim 6, characterized in that, In step (1), the reaction is completed and purification is also included. The purification is: the reaction product is dialyzed using a dialysis bag with a molecular weight cutoff of 3000 to 12000 Da.

10. The method for preparing the modified anthocyanin freshness indicator film according to claim 6, characterized in that, In step (2), the plasticizer is glycerin.

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