Caesalpinia spinosa gum-chitosan-polyvinyl alcohol color-sensitive indicating film and preparation method thereof

Abstract

The invention relates to the technical field of food preservation, in particular to a caesalpinia spinosa gum-chitosan-polyvinyl alcohol color-sensitive indicating film and a preparation method thereof. The caesalpinia spinosa gum, chitosan and polyvinyl alcohol are used as film forming substrates, and anthocyanin and curcumin are added to prepare the color-sensitive indicating film which is green, safe and free of potential toxic and side effects. The raw materials of the film are non-toxic, harmless, degradable and renewable environment-friendly substances, so that the problem of environmental pollution caused by the existing color-sensitive indicating film can be effectively solved. In addition, the color-sensitive indicating film has reusability, good color stability and potential of serving as an intelligent pH response color indicator, and can be well applied to food fresh-keeping packaging.

Description

Technical Field

[0001] This invention relates to the field of food preservation technology, and more specifically, to a color-sensitive indicator film of prickly pear gum-chitosan-polyvinyl alcohol and its preparation method. Background Technology

[0002] With the rapid development of social science and technology and the improvement of people's living standards, people have higher requirements for quality of life. In the fast-paced modern life, people often store large amounts of food at home, but food is prone to bacterial and microbial growth when stored at room temperature or in the refrigerator, leading to spoilage and inedibility, resulting in food waste. Microbial growth and metabolism cause chemical changes in food, producing volatile alkaline nitrogenous substances, which cause changes in the pH environment inside the packaging. Color-sensitive indicator films can display corresponding colors according to the chemical changes during the food spoilage process, helping people to grasp the degree of food spoilage and whether it is still edible, and are used for food preservation and real-time quality monitoring. Anthocyanins and curcumin are natural pigments with good antioxidant, antibacterial, and pH-responsive properties. However, anthocyanins alone are unstable in indicator films, and curcumin alone has a small color change range, making it unsuitable as a freshness indicator for monitoring the entire shelf life of food.

[0003] Therefore, the current mainstream film-forming materials for food preservation films have problems such as limited functionality and difficulty in degradation, which can easily cause pollution. Finding natural, non-toxic, and biodegradable materials is urgently needed. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a color-sensitive indicator film made from *Ligustrum lucidum* guarana, chitosan, and polyvinyl alcohol, along with its preparation method. This invention uses *Ligustrum lucidum* guarana, chitosan, and polyvinyl alcohol as the film-forming matrix, and simultaneously adds anthocyanins and curcumin to create a green, safe, and non-toxic color-sensitive indicator film. The raw materials for this film are all non-toxic, harmless, biodegradable, and renewable environmentally friendly substances, effectively solving the environmental pollution problems caused by current indicator films.

[0005] The first objective of this invention is to provide a method for preparing a color-sensitive indicator film of *Hylocereus undatus* colloid-chitosan-polyvinyl alcohol. To achieve the above objective, this invention provides the following technical solution: A method for preparing a color-sensitive indicator film of *Hylocereus undatus* colloid-chitosan-polyvinyl alcohol includes the following steps: Step 1: Purify the prickly pear gum; Step 2: Dissolve polyvinyl alcohol and chitosan in pure water, and add purified prickly ash gum while stirring to prepare mixed solution I; Step 3: Add glacial acetic acid and glycerol to mixed solution I, stir well to obtain mixed solution II; Step 4: Prepare a mixed solution of anthocyanins and curcumin, add it to mixed solution II, stir evenly to obtain mixed solution III, cast mixed solution III into a film using the casting method, and dry it in an oven to obtain the final product.

[0006] Furthermore, the purification method in step 1 is as follows: under mechanical stirring, analytical grade glutenin (TG) is slowly added to pure water until it is fully dissolved into a gel-like liquid. After centrifugation to remove impurities, anhydrous ethanol is added to the gel solution under stirring to precipitate the precipitate. The precipitate is then washed with acetone and anhydrous diethyl ether, respectively, and then subjected to vacuum filtration and vacuum drying.

[0007] Preferably, the ratio of pure water to analytical grade argan oil is 1700 mL : (10.2~13.6) g.

[0008] Preferably, the mass ratio of the gel solution after centrifugation to anhydrous ethanol is 1:(1~3); the centrifugation parameters are: centrifugation at a speed of 3000r / min~4000r / min for 20min~30min; stirring time is 5h~8h; and vacuum drying time is 2~4 days.

[0009] Furthermore, the mass concentration of polyvinyl alcohol in mixed solution I is 0.8%~1.2%, the mass concentration of chitosan is 0.2%~1%, and the mass concentration of argan oil is 0.2%~2.8%.

[0010] Furthermore, the mass concentrations of glacial acetic acid and glycerol in mixed solution II are 0.2%~1% and 0.4%~2%, respectively.

[0011] Preferably, in step 3, glacial acetic acid is added first and stirred for 2 to 5 hours, followed by the addition of glycerol and stirring for another 2 to 5 hours.

[0012] Furthermore, the preparation method of the anthocyanin and curcumin mixed solution in step 4 is as follows: prepare an anthocyanin solution with a mass concentration of 0.4% using pure water, prepare a curcumin solution with a mass concentration of 0.4% using anhydrous ethanol, and then mix the two in a volume ratio of (1~3):(1~3) and stir evenly.

[0013] Preferably, the stirring time is 1 hour to 3 hours.

[0014] Furthermore, in step 4, the volume ratio of the anthocyanin and curcumin mixed solution to mixed solution II is (0.2~5):(0.2~5), and the mixture is stirred evenly.

[0015] Furthermore, in step 4, before membrane deposition, the mixed solution III is centrifuged at 3000 r / min to 4000 r / min for 15 min to 20 min to remove air bubbles and impurities.

[0016] Furthermore, in step 4, the oven temperature is 40℃~60℃, and the drying time is 72h~84h.

[0017] The second objective of this invention is to provide a color-sensitive indicator film made of prickly pear jelly, chitosan, and polyvinyl alcohol, which is prepared by the above-described preparation method.

[0018] The third objective of this invention is to provide the application of a color-sensitive indicator film made from prickly pear jelly, chitosan, and polyvinyl alcohol in the field of food packaging technology.

[0019] The technical solution provided by this invention has at least the following advantages compared to the prior art: (1) This invention uses prickly pear gum, chitosan and polyvinyl alcohol as film-forming matrices, and adds anthocyanins and curcumin to make a color-sensitive indicator film that is green, safe and free of potential toxic side effects. The raw materials of this film are all non-toxic, harmless, biodegradable and renewable environmentally friendly substances, which can effectively solve the environmental pollution problem caused by current plastic wrap.

[0020] (2) The color-sensitive indicator film prepared by the present invention has good mechanical properties, low light transmittance that can effectively block ultraviolet radiation, good vapor permeability that can effectively block moisture evaporation, and strong antioxidant properties that can protect food that is prone to oxidation and deterioration and extend its shelf life. In addition, the color-sensitive indicator film of the present invention also has reusability, good color stability and potential as a smart pH-responsive color indicator, and can be well applied in food preservation packaging. Attached Figure Description

[0021] Figure 1 These are test graphs showing the tensile strength and elongation at break of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 1-3; Figure 2 The ultraviolet spectra of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 1-3 are shown below. Figure 3 These are water vapor permeability test diagrams for the color-sensitive indicator membranes of Examples 1-3 and Comparative Examples 1-3; Figure 4 Infrared spectra of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 1-3; Figure 5 The X-ray diffraction patterns of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 1-3 are shown below. Figure 6 These are test graphs showing the antioxidant properties of the color-sensitive indicator films in Examples 1-3 and Comparative Examples 2-3; Figure 7 The test results show the preservation performance of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 2-3 on strawberries. Figure 8To test the colorimetric reaction diagrams of mixed solutions of anthocyanins and curcumin at different pH values ​​(pH=1~14) according to the ratios of anthocyanins and curcumin in Examples 1~3 and Comparative Examples 2~3, i.e., the ratios of anthocyanins and curcumin were 3∶1, 2∶2, 1∶3, 4∶0, and 0∶4, respectively. Figure 9 These are ammonia-sensitive reaction test diagrams of the color-sensitive indicator films of Examples 1-3 and Comparative Examples 2-3; Figure 10 This is a test diagram showing the reusability of the color-sensitive indicator film; Figure 11 The values ​​of (a) L*, (b) a*, (c) b*, and (d) ∆E of the color-sensitive indicator film under repeated cycles are: Figure 12 The stability test diagrams are for the color-sensitive indicator films of Examples 1-3 and Comparative Examples 2-3. Detailed Implementation

[0022] To enable those skilled in the art to better understand this application, the present invention will be further described in detail below with reference to embodiments and accompanying drawings. However, it should be understood that the following embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims.

[0023] In the description of this invention, it should be noted that unless specific conditions are specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0024] Furthermore, to better illustrate the present invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that the present invention can be practiced without certain specific details. In some embodiments, materials, methods, and means well-known to those skilled in the art are not described in detail in order to highlight the spirit of the invention.

[0025] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprising" shall be understood to include the stated elements or components without excluding other elements or other components.

[0026] Example 1

[0027] This embodiment provides a method for preparing a color-sensitive indicator film of *Hylocereus undatus* colloid-chitosan-polyvinyl alcohol, including the following steps: Step 1: Purification of *Ligustrum lucidum* Gum: Under mechanical stirring, slowly add 12g of analytical grade *Ligustrum lucidum* gum (TG) to 1700mL of pure water until it is fully dissolved into a white gel-like liquid. Centrifuge at 3500r / min for 20min to remove impurities. Then, add anhydrous ethanol to the gel solution under stirring. The mass ratio of gel solution to anhydrous ethanol is 1:1. Stir for 6h to precipitate the precipitate. Wash the precipitate twice with acetone and anhydrous diethyl ether, respectively. Then, filter under reduced pressure and vacuum dry for 3 days. Step 2: Dissolve polyvinyl alcohol and chitosan in 280 mL of pure water, add purified strychnine gum while stirring, and stir for 8 hours to obtain mixed solution I; the mass concentration of polyvinyl alcohol in mixed solution I is 1.2%, the mass concentration of chitosan is 0.4%, and the mass concentration of strychnine gum is 0.8%; Step 3: Add 1.4 mL of glacial acetic acid and 2.8 mL of glycerol to mixed solution I, stir well to obtain mixed solution II; Step 4: Dissolve 0.04g of anthocyanin in 10mL of pure water to prepare a 0.4% anthocyanin solution, and stir magnetically for 1h; dissolve 0.4g of curcumin in 100mL of ethanol to prepare a 0.4% curcumin solution, and stir magnetically for 1h; mix 0.375mL of anthocyanin solution with 0.125mL of curcumin solution to obtain a mixed solution, add the mixed solution to mixed solution II in step 3, stir evenly to obtain mixed solution III, centrifuge mixed solution III at 3500r / min for 15min to remove impurities and air bubbles, cast film using the casting method, dry at 40℃ for 48h, cool to room temperature and peel off the film to obtain a color-sensitive indicator film with prickly pear gum / chitosan / polyvinyl alcohol as the matrix, named TPC-A3C1.

[0028] Example 2

[0029] This embodiment provides a method for preparing a color-sensitive indicator film of *Ligusticum striatum* colloid-chitosan-polyvinyl alcohol. The difference from Embodiment 1 is that in this embodiment, anthocyanin solution and curcumin solution are mixed at a volume ratio of 2:2. That is, in step 4, 0.25 mL of anthocyanin solution and 0.25 mL of curcumin solution are mixed to obtain a mixed solution of the two. The rest are the same. The resulting color-sensitive indicator film is named TPC-A2C2 (in this invention, 0.125 mL of curcumin solution or anthocyanin solution is limited to one part for naming. For example, in this embodiment, 0.25 mL of anthocyanin solution and 0.25 mL of curcumin solution are used, so each is two parts, hence the name TPC-A2C2. Other embodiments or comparative examples are named in the same way).

[0030] Example 3

[0031] This embodiment provides a method for preparing a color-sensitive indicator film of *Ligustrum lucidum* jelly-chitosan-polyvinyl alcohol. The difference from Embodiment 1 is that in this embodiment, anthocyanin solution and curcumin solution are mixed at a volume ratio of 1:3. That is, in step 4, 0.125 mL of anthocyanin solution and 0.375 mL of curcumin solution are mixed to obtain a mixed solution of the two. All other steps are the same. The resulting color-sensitive indicator film is named TPC-A1C3.

[0032] Comparative Example 1

[0033] This comparative example provides a method for preparing a color-sensitive indicator film of *Ligusticum striatum* colloid-chitosan-polyvinyl alcohol. The difference from Example 1 is that this comparative example does not add curcumin and anthocyanins. That is, this comparative example directly uses the mixed solution II obtained in step 3 to cast the film by casting. All other aspects are the same. The color-sensitive indicator film obtained is named TPC-A0CO.

[0034] Comparative Example 2

[0035] This comparative example provides a method for preparing a color-sensitive indicator film of *Ligustrum lucidum* guarana, chitosan, and polyvinyl alcohol. The difference from Example 1 is that only anthocyanins are added in this comparative example, and curcumin is not added. That is, step 4 of this comparative example is to mix 0.5 mL of anthocyanin solution with mixed solution II. All other steps are the same. The resulting color-sensitive indicator film is named TPC-A4C0.

[0036] Comparative Example 3

[0037] This comparative example provides a method for preparing a color-sensitive indicator film of *Ligusticum striatum* colloid-chitosan-polyvinyl alcohol. The difference from Example 1 is that only curcumin is added in this comparative example, and no anthocyanin is added. That is, step 4 of this comparative example is to mix 0.5 mL of curcumin solution with mixed solution II. All other steps are the same. The resulting color-sensitive indicator film is named TPC-A0C4.

[0038] Performance Characterization

[0039] I. Mechanical Properties Analysis of Color-Sensitive Indicator Films The tensile strength (TS) and elongation at break (EB) of the color-sensitive indicator films prepared in Examples 1-3 and Comparative Examples 1-3 were measured respectively to evaluate the mechanical properties of the color-sensitive indicator films. The test results are as follows: Figure 1As shown, the tensile strength of the membrane in Comparative Example 1 (TPC-A0CO) was 14.2 MPa, and the elongation at break was 32.6%, which was significantly lower than that of the pigmented membranes (Examples 1-3, Comparative Examples 2-3). The tensile strengths of the mixed-pigment membranes (Examples 1, 2, and 3) were 32.7 MPa, 29 MPa, and 33.4 MPa, respectively, which were significantly greater than those of the single-pigment membranes in Comparative Example 2 (26.7 MPa) and Comparative Example 3 (20 MPa). This may be due to the more stable interaction formed between anthocyanins and curcumin, arugula gum, chitosan, and other additives incorporated into the color-sensitive indicator membrane. In summary, the color-sensitive indicator membranes of Examples 1-3 exhibited the best mechanical properties.

[0040] II. Transmittance Analysis of Color-Sensitive Indicator Films The transmittance of the color-sensitive indicator films prepared in Examples 1-3 and Comparative Examples 1-3 was tested respectively (using an ultraviolet spectrometer), and the test results are as follows: Figure 2 As shown. By Figure 2 It can be seen that the color-sensitive indicator films prepared in Examples 1-3 (all containing anthocyanins and curcumin) and the color-sensitive indicator film of Comparative Example 3 (containing curcumin) have a transmittance range of 0%-40% in the 500-800 nm range, which is relatively low; and a transmittance range of 0%-7% in the 200-500 nm range, which is almost completely blocking. The transmittance of all films containing curcumin drops sharply at around 500 nm, indicating that curcumin has a significant blocking effect on ultraviolet radiation.

[0041] III. Water vapor permeability (WVP) analysis of color-sensitive indicator membranes Water vapor permeability (WVP) is the mechanism by which water is transferred through edible films. The WVP of a film can indirectly indicate its preservation ability; films with lower WVP help reduce or prevent moisture exchange between food and the external environment, thus preventing food spoilage. The water vapor permeability rates of the color-sensitive indicator films prepared in Examples 1-3 and Comparative Examples 1-3 were tested respectively, and the test results are as follows: Figure 3 As shown. By Figure 3 It can be seen that the water vapor permeability of the thin film in Comparative Example 1 is 7.67 × 10⁻⁶. -10 g·m(m 2 ·s·Pa) -1 The WVP was significantly higher than that of Examples 1-3 and Comparative Examples 2-3, indicating that the addition of pigment helps prevent moisture loss. The WVP of Comparative Example 3 was 6.19 × 10⁻⁶. -10 g·m(m 2 ·s·Pa) -1The water vapor permeability (WVP) is similar to that of Comparative Example 1, possibly because the addition of curcumin molecules weakens the polymer-polymer interactions in the TG and CS gel network structure, reduces membrane density, and increases the rate of water molecule diffusion through the membrane, thus improving the WVP value. In all embodiments, the water vapor permeability of the membrane in Example 2 is 2.17 × 10⁻⁶. -1 0g·m(m 2 ·s·Pa) -1 This demonstrates excellent water molecule barrier properties. Overall, the color-sensitive indicator film prepared in these embodiments effectively blocks moisture evaporation and holds promise for practical application in packaging films.

[0042] IV. Structural Analysis of Color-Sensitive Indicator Films The thin film structures of Examples 1-3 and Comparative Examples 1-3 were tested using an infrared spectrometer, and the results are as follows: Figure 4 As shown. By Figure 4 It can be seen that the spectral profiles and peaks of the six thin films are roughly the same, with no significant differences, indicating that the addition of pigments did not change the basic structure of the films. Among them, the 3280 cm⁻¹... -1 Left and right, 2917cm -1 The peaks on the left and right are relatively broad and are attributed to the characteristic absorption of hydroxyl groups in polymers and the characteristic absorption of hydroxyl groups associated with intramolecular molecules; the peaks are sharp and have a high peak value of 1018 cm⁻¹. -1 Or it could be attributed to the bending vibration of a carbon-hydrogen bond; 1646 cm -1 1562cm -1 1414cm -1 The peak is attributed to the skeletal vibration of the benzene ring; 815 cm⁻¹ -1 The peaks around the left and right are characteristic absorptions of para-substituted benzene; 1707 cm⁻¹ -1 The absorption peaks around the left and right may be due to the stretching vibration of the carbonyl group, and the conjugation of the carbonyl group with the double bond increases the absorption intensity of the double bond, causing the carbonyl group position to shift towards lower frequencies.

[0043] V. XRD Analysis of Color-Sensitive Indicator Membranes The films of Examples 1-3 and Comparative Examples 1-3 were tested using XRD, and the results are as follows: Figure 5 As shown. By Figure 5 It can be seen that all six color-sensitive indicator films have obvious characteristic peaks at θ=19.3°, and the peak shapes are broad, indicating that the samples have an amorphous structure. The peak lines of all film samples are similar, indicating that the natural pigments (curcumin and anthocyanins) are well dispersed in the *Lysimachia christinae* gel-based film.

[0044] VI. Analysis of the antioxidant properties of color-sensitive indicator films The antioxidant activity of the films from Examples 1-3 and Comparative Examples 2-3 was tested using the DPPH method, and the test results are as follows: Figure 6 As shown. By Figure 6It is evident that the free radical scavenging rate of the membrane increases with increasing curcumin content. This may be attributed to the presence of both phenol and diketone structures in the curcumin molecule, with the phenolic hydroxyl group possessing the ability to capture and scavenge free radicals. Simultaneously, the flavonoid structure of anthocyanins can provide hydrogen atoms to hydrogen peroxide free radicals, generating stable hydroperoxides. Furthermore, the phenolic hydroxyl group in the molecule reacts with oxygen free radicals to form conjugated, stable semiquinone free radicals, interrupting the chain reaction of free radicals and exhibiting strong antioxidant properties. Considering both antioxidant properties and water vapor permeability, Examples 1-3 of this invention possess excellent performance, effectively preventing moisture evaporation and protecting easily oxidized and spoiled foods, thus extending shelf life.

[0045] VII. Preservation Performance Analysis of Color-Sensitive Indicator Films The preservation performance of the films in Examples 1-3 and Comparative Examples 1-3 was tested by wrapping beakers containing fresh strawberries with color-sensitive films containing different pigment ratios. The results are as follows: Figure 7 As shown (to avoid confusion during the experiment, the inventors labeled each beaker; the one labeled "no pigment" is the TPC-A0CO prepared in Comparative Example 1, without added anthocyanins and curcumin; the proportions labeled in the figure are the proportions of anthocyanins and curcumin in each example and comparative example). Figure 7 It can be seen that the color-sensitive indicator film of Comparative Example 1 showed a large number of bacterial strains growing on strawberries on day 4, and the strains completely covered the strawberries on day 7. In contrast, the films of Examples 1-3 and Comparative Examples 2-3 showed no bacterial growth on day 4, indicating that the addition of pigments increased the preservation performance of the films. The films of Comparative Examples 2 and 3 showed that the strawberries began to rot and spoil on day 7, while the films of Examples 1-3 maintained the freshness of the strawberries for the longest time. This may be because the chitosan used in the examples has certain antibacterial properties, and the β-diketone and phenolic hydroxyl groups in the curcumin structure can induce cell membrane depolarization and Ca2+ at certain concentrations. 2+ Inflow and DNA breakage; furthermore, curcumin not only causes phototoxicity under light irradiation, inhibiting bacterial growth, but may also bind to the cell wall, disrupting its integrity, thereby achieving an antibacterial effect. In particular, the strawberry film using Example 1 did not begin to mold and grow bacteria until the 12th day, exhibiting the strongest antibacterial performance. Therefore, the color-sensitive indicator film provided in this embodiment of the invention can inhibit bacterial growth in food, maintain food freshness, and extend the shelf life of food packaging.

[0046] VIII. Analysis of Solution Color Sensitive Reaction Following the anthocyanin to curcumin ratios in Examples 1-3 and Comparative Examples 2-3, i.e., anthocyanin to curcumin ratios of 3:1 (TPC-A3C1 in the figure), 2:2 (TPC-A2C2 in the figure), 1:3 (TPC-A1C3 in the figure), 4:0 (TPC-A4C0 in the figure), and 0:4 (TPC-A0C4 in the figure), the colorimetric reactions of mixed solutions of anthocyanin and curcumin in different ratios at different pH values ​​(pH=1~14) were tested. The test results are as follows. Figure 8 As shown. By Figure 8 It can be seen that solutions containing anthocyanins all turn red to varying degrees under strongly acidic conditions. As the curcumin content increases, the red color transitions to yellow, until the curcumin-only solution turns completely yellow under strongly acidic conditions. The color trends of the mixed solutions prepared according to the proportions of Examples 1-3 are roughly the same, all changing from red or orange (pH=1~pH=3) to brown or light yellow (pH=4~pH=6), then to dark brown or brown (pH=7~pH=10), then to blackish red (pH=11~pH=12), and finally to dark red (pH=13~pH=14). Solutions containing curcumin change color under alkaline conditions. In acidic and neutral environments, they exhibit the bright yellow color characteristic of curcumin itself. Starting at pH 8, the bright yellow color transitions to a deep red, fading as alkalinity increases. Solutions containing only anthocyanins show a better color trend, changing from red to pink to deep purple to light purple to dark brown to black to brown to light brown. This is because anthocyanins have a conjugated structure, whose structure changes with pH. Under strongly acidic conditions, they are in the form of red flavonoid cations, gradually transforming into pink methanolic pseudo-bases, purple quinoline anhydrous bases, and finally light yellow chalcones under strongly alkaline conditions. In summary, both anthocyanins and curcumin have the ability to show color under different acidic and alkaline conditions. Anthocyanins have a wider color range than curcumin, and mixed solutions of both show good color development. Therefore, the films prepared in Examples 1-3 of this invention should possess corresponding color development effects and can be further applied to food preservation films.

[0047] IX. Ammonia Sensitivity Analysis of Color-Sensitive Indicator Membranes To evaluate the color-changing ability of the color-sensitive indicator films, the color-sensitive indicator films prepared in Examples 1-3 and Comparative Examples 2-3 were exposed to an ammonia atmosphere for 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 40, 55, 85, 145, and 205 minutes. Sixteen color-changing phenomena were observed, and the results are as follows: Figure 9 As shown. By Figure 9 It was found that all color-sensitive indicator membranes exhibited sensitivity to ammonia exposure, and the color change trends of the color-sensitive indicator membranes in Examples 1-3 were roughly the same. These results indicate that the color-sensitive indicator membranes prepared in the embodiments of the present invention have the potential to be intelligent pH-responsive color indicators.

[0048] 10. Reusability Analysis of Color-Sensitive Indicator Membranes The color-sensitive indicator film of Example 3 was repeatedly exposed to ammonia atmosphere (alkaline environment) and acetic acid atmosphere (acidic environment) to stimulate its color-changing ability and application ability in different pH environments. The test results are as follows: Figures 10-11 As shown. Figure 10 This is a reusability test chart for the color-sensitive indicator membrane, where the numbers represent the number of cycles, J represents an alkaline environment, and S represents an acidic environment. Figure 10 It can be seen that the color-sensitive indicator membrane changes color in nine cycles.

[0049] Figure 11 The values ​​are (a) L*, (b) a*, (c) b*, and (d) ∆E of the color-sensitive indicator film under repeated cycles. Figure 11 It can be seen that a*, representing the red-green dimension, did not recover to its value in the ammonia atmosphere of the 7th cycle in the 8th cycle, and only recovered to its original value in the 9th cycle. DE, representing color difference, showed almost no color change in the acetic acid atmosphere of the final cycle, indicating that the reusable limit of the color-sensitive indicator film had been reached. In addition, L* fluctuated between 32.4 and 37.6 under alkaline conditions and between 49.2 and 52.8 under acidic conditions; a* fluctuated between 27.7 and 31.4 under alkaline conditions and between 21.4 and 30 under acidic conditions; b* fluctuated between 10.9 and 20.4 under alkaline conditions and between 39.3 and 46.3 under acidic conditions; DE fluctuated between 57.2 and 60.8 under alkaline conditions and between 54.6 and 60.7 under acidic conditions. In summary, the color-changing cycle of the color-sensitive indicator film is highly effective, with the curves fluctuating within the range of colors visible to the naked eye. This indicates that the color-sensitive indicator membrane not only has good pH response performance, but also that the reaction is reversible.

[0050] XI. Color Stability Analysis of Color-Sensitive Indicator Films The color-sensitive indicator films of Examples 1-3 and Comparative Examples 2-3 were sealed in plastic bags for 30 days, respectively. Then, the colorimetric value of each sample was tested and observed to compare it with the initial color. The test results are as follows: Figure 12 As shown. Figure 12 In the diagram, (a) is TPC-A4C0, (b) is TPC-A3C1, (c) is TPC-A2C2, (d) is TPC-A1C3, and (e) is TPC-A0C4. Figure 12 It can be seen that the color difference DE of the five color-sensitive indicator films is not much different after being sealed in a plastic bag for a month, and the color change is imperceptible to the naked eye.

[0051] Based on the aforementioned performance analyses, the color-sensitive indicator films of Examples 1-3 of this invention all exhibit excellent performance and can be applied to food preservation films.

Claims

1. A method for preparing a color-sensitive indicator film of *Pteris vittata* var. *spinosa*, chitosan, and polyvinyl alcohol, characterized in that, Includes the following steps: Step 1: Purify the prickly pear gum; Step 2: Dissolve polyvinyl alcohol and chitosan in water, and add purified strychnine gum while stirring to prepare mixed solution I; Step 3: Add glacial acetic acid and glycerol to mixed solution I, stir well to obtain mixed solution II; Step 4: Prepare a mixed solution of anthocyanins and curcumin, add it to mixed solution II, stir evenly to obtain mixed solution III, cast mixed solution III into a film using the casting method, and dry it in an oven to obtain the final product.

2. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 1, characterized in that, The purification method in step 1 is as follows: under mechanical stirring, slowly add the strychnine resin to the water until it is fully dissolved into a gel-like liquid. After centrifugation to remove impurities, add anhydrous ethanol to the gel solution under stirring to precipitate the precipitate. Then wash the precipitate with acetone and anhydrous diethyl ether respectively, and then perform vacuum filtration and vacuum drying.

3. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 2, characterized in that, The ratio of water to prickly ash gum is 1700mL: (10.2~13.6)g.

4. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 2, characterized in that, The mass ratio of the gel solution after centrifugation to anhydrous ethanol is 1:(1~3); the centrifugation parameters are 3000r / min~4000r / min for 20min~30min; the stirring time is 5h~8h; and the vacuum drying time is 2~4 days.

5. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 1, characterized in that, In mixed solution I, the mass concentration of polyvinyl alcohol is 0.8%~1.2%, the mass concentration of chitosan is 0.2%~1%, and the mass concentration of argan oil is 0.2%~2.8%.

6. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 1, characterized in that, The mass concentrations of glacial acetic acid and glycerol in mixed solution II are 0.2%~1% and 0.4%~2%, respectively.

7. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 1, characterized in that, The preparation method of the anthocyanin and curcumin mixed solution in step 4 is as follows: prepare an anthocyanin solution with a mass concentration of 0.4% using pure water and a curcumin solution with a mass concentration of 0.4% using anhydrous ethanol. Then, mix the two solutions in a volume ratio of (1~3):(1~3) and stir until homogeneous.

8. The method for preparing the color-sensitive indicator film of *Hylocereus undatus*-chitosan-polyvinyl alcohol according to claim 1, characterized in that, In step 4, the volume ratio of the anthocyanin and curcumin mixed solution to mixed solution II is (0.2~5):(0.2~5).

9. A color-sensitive indicator film of *Pyracantha fortuneana*-chitosan-polyvinyl alcohol, characterized in that, The color-sensitive indicator film was prepared using the method described in any one of claims 1 to 8, which is as follows.

10. The application of the color-sensitive indicator film of claim 9, namely, the chitosan-polyvinyl alcohol-based material, in the field of food packaging technology.

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