Food freshness detection colorimetric film based on spirulina extract, preparation method and application

Abstract

The invention provides a food freshness detection colorimetric film based on a spirulina extract as well as a preparation method and application of the food freshness detection colorimetric film, the spirulina extract and gallic acid are added as natural extracts, and carboxymethyl cellulose and sodium alginate are used as matrixes to prepare the food freshness detection colorimetric film. The prepared food freshness detection colorimetric film can change color according to the change of volatile basic nitrogen (TVB-N) released in the shrimp decay process, and is used for indicating the decay degree of shrimps. In addition, the colorimetric film provided by the invention also has good mechanical properties, physical properties, oxidation resistance and antibacterial ability.

Description

Technical Field

[0001] The present invention belongs to the field of bioengineering, and particularly relates to a method for developing a colorimetric film for detecting food freshness based on spirulina extract. Background Art

[0002] Food packaging can maintain the quality and safety of food, facilitate transportation, ensure safe storage, prevent product spoilage and economic losses, contribute to product promotion, and safeguard the interests of consumers. Traditional food packaging materials are derived from petroleum-based sources, which effectively protect food from environmental and microbial hazards and extend the shelf life. However, they contain harmful chemicals such as bisphenol A and phthalates, which can pose risks to the environment and human health. Therefore, intelligent active food packaging has great research potential. Integrating natural antibacterial and antioxidant substances into packaging materials can combat the quality deterioration caused by chemical changes and microbial spoilage, and rapidly evaluate the freshness of food, completing inspections, detections, reports, sensory analyses, tracking, and transmissions to ensure food safety.

[0003] Carboxymethyl cellulose (CMC) is considered a potential edible film or coating material, with biocompatibility, biodegradability, non-toxicity, and high film-forming ability. Sodium alginate (SA) is derived from algae and is a linear polysaccharide formed by connecting different proportions of α-l-guluronate and β-d-mannuronate through 1-4 glycosidic bonds. It is non-toxic, has consistency, is biocompatible, and is soluble in water. However, their limited antibacterial and antioxidant capabilities pose difficulties for their wider applications.

[0004] Chinese patent document CN115808416A discloses an intelligent indicator film capable of real-time monitoring of shrimp freshness. The film is prepared by adding pericarp pigment of Daphniphyllum calycinum with anthocyanin content not less than 18 g / 100 g to an aqueous solution containing green aqueous polyurethane, sodium alginate, and nanofibrillated cellulose film matrix to obtain a film-forming solution; the film-forming solution is formed into a film and then cut. When in use, the intelligent indicator film is placed inside the lid of a fresh-keeping box and contacts the gas inside the fresh-keeping box; the indicator film can change with the changes of pH value and total volatile basic nitrogen (TVB-N) content during the storage process of fresh shrimp.

[0005] Spirulina, as a nutrient-rich substance, provides essential amino acids, fatty acids, phytohormones, and antioxidants. Its phycobiliproteins, especially phycocyanin and allophycocyanin, are widely used in food due to their bright colors, unique fluorescence, and various benefits, and have great commercial value. Gallic acid (3,4,5-trihydroxybenzoic acid) is a representative polyphenol with anti-cancer, antibacterial, antioxidant, food preservative, and cross-linking properties, and has potential uses as an antioxidant and antibacterial agent. However, the application of adding spirulina aqueous extract (UAAESP) and gallic acid in a film to create an intelligent color-changing sensor for detecting food freshness and enhancing the antioxidant and antibacterial activities of food packaging films has not been studied before. Summary of the Invention

[0006] Based on the prior art, the present invention uses spirulina aqueous extract and gallic acid as natural extracts, and carboxymethyl cellulose and sodium alginate as matrices to prepare a colorimetric film for detecting food freshness. The prepared colorimetric film for detecting food freshness has antioxidant and antibacterial potential and strong pH sensing and detection capabilities.

[0007] To achieve the above objectives, the present invention adopts the following solutions:

[0008] A method for preparing a colorimetric film for detecting food freshness based on spirulina extract, characterized by comprising the following steps:

[0009] (1) Ultrasonic-assisted extraction: Ultrasonically extract spirulina biomass in an ice bath; after the ultrasonic extraction is completed, centrifuge to collect the supernatant, then evaporate and concentrate the supernatant, and freeze-dry the obtained concentrated solution to obtain spirulina extract;

[0010] (2) Preparation of the colorimetric film: Add CMC and SA to deionized water to prepare a mixed solution, add agar and mix evenly, then add the spirulina extract obtained in step (1), stir in a magnetic stirrer until completely dissolved, add glycerol and mix evenly, and perform ultrasonic treatment in a water bath at 40°C - 50°C, and finally dry to form a film and store it in a desiccator.

[0011] Further, the ultrasonic extraction conditions in step (1) are: the amplitude is 20% - 100%, turn off for 2 - 4 seconds every 5 - 10 seconds of ultrasonic treatment, and cycle for 10 - 40 minutes.

[0012] Further, the centrifugation conditions in step (1) are: 13000 rpm, 4°C, 10 - 20 minutes; the evaporation temperature for concentration is 35 - 45°C, and the obtained concentrated solution is 1 / 5 - 1 / 7 of the volume of the supernatant; the freeze-drying conditions are: the temperature is -55°C - -15°C, and the vacuum degree is 1 - 5 Pa.

[0013] Further, the addition amounts of CMC, SA, spirulina extract, and agar in step (2) are as follows: 20 - 40 g, 20 - 40 g, 0.5 - 2 g, and 2 - 3 g are respectively added to every 100 ml of deionized water; the addition amount of glycerol is: 20 ml of a 2% glycerol solution.

[0014] Further, the conditions of the ultrasonic treatment in step (2) are: the amplitude is 80%; it is turned off for 2.5 seconds every 7.5 seconds of ultrasonic treatment, and this cycle is carried out for 5 - 10 minutes; preferably, the drying temperature in step (2) is 50 - 70 °C, and the duration is 5 - 7 hours; the temperature for dry storage is 23 ± 2 °C, and the humidity is 43 ± 2%.

[0015] Further, gallic acid is also added in step (2), and the addition amount is 0.1 - 0.3 g per 100 ml of deionized water.

[0016] Further, the spirulina biomass in step (1) is obtained by the following method: Spirulina is cultured using Zarrouk medium. After culturing to the stationary phase, spirulina is collected by filtration; the culture conditions are: the light intensity is 4000 - 5000 Lux, the temperature is 16 - 30 °C, and the time is 14 - 21 d; a filter screen or filter cloth with a pore size of 0.2 μm is used for filtration and collection.

[0017] A food freshness detection colorimetric film based on spirulina extract prepared according to the preparation method.

[0018] The application of the colorimetric film is characterized in that: it is used as a tool for detecting the freshness of fresh - keeping food. By placing or attaching it on a fresh - keeping film / bag / box / container, it is located in the food storage space.

[0019] The application of the colorimetric film is characterized in that: it is used to prepare food fresh - keeping film / bag / box / container for antibacterial and / or freshness indication; the antibacterial types include Escherichia coli and / or Staphylococcus aureus.

[0020] The beneficial effects of the present invention are as follows:

[0021] (1) The colorimetric film prepared by the present invention can change color according to the change of volatile basic nitrogen (TVB - N) released during the spoilage process of shrimp. The color can change from blue to dark blue or from blue to blue - gray and then to dark gray, corresponding to different spoilage degrees of shrimp, thereby providing consumers with intuitive information about the freshness of shrimp, helping consumers make purchase decisions and maintaining food safety and reducing food waste to a certain extent.

[0022] (2) The present invention has developed an economically viable and superior-performance intelligent packaging colorimetric film. The addition of UAAESP and GA not only improves the tensile strength of the film but also reduces the moisture content (39.53%), water solubility (23.99%), gas permeability (38.91%), and oxygen permeability (25.77%) of the film, making the packaging material more durable. At the same time, it enhances the antibacterial and antioxidant properties, extends the shelf life of food, reduces the risk of microbial contamination, and can better protect food from the external environment.

[0023] (3) The present invention promotes environmental protection and sustainable development: The packaging film made of the biopolymers CMC and SA as the substrate is biodegradable, and this colorimetric biofilm is edible; compared with traditional plastic packaging materials, it is more environmentally friendly and reduces environmental pollution.

[0024] The alginate and carboxymethyl cellulose biofilm loaded with UAAESP and gallic acid prepared by the present invention improves the mechanical strength, antioxidant capacity, anti-microbial growth capacity, pH sensing capacity, and spoilage monitoring capacity of the biofilm. It provides an effective way for the development of innovative algae-based intelligent packaging, which represents broad prospects for natural green extracts in packaging sustainability, rapid food detection, and food safety development. Description of the Drawings

[0025] Figure 1 In (a), it shows the color change of 1% aqueous extract of Spirulina UAAESP at different pH values from 3 to 10, and in (b), it shows the light absorption curve of UAAESP at different pH levels.

[0026] Figure 2 SEM photos of the surface (marked "S" in the figure) and cross-section (marked "CS" in the figure) of the colorimetric films prepared in the comparative example, Example 1, Example 2, and Example 3.

[0027] Figure 3 FT-IR analysis results of the colorimetric films prepared in the comparative example, Example 1, Example 2, and Example 3.

[0028] Figure 4 In (a), it shows the change trend of the TVB-N value of shrimp with the extension of storage time.

[0029] Figure 5 It shows the change trend of the color difference ΔE of the colorimetric film when using the colorimetric films prepared in the comparative example, Example 1, Example 2, and Example 3 to indicate the freshness of the preserved shrimp.

[0030] Figure 6 It shows the color change of the colorimetric films prepared in the comparative example, Example 1, Example 2, and Example 3 at different times when indicating the freshness of the preserved shrimp.

[0031] Figure 7(a) and (b) in the figure are the inhibition and antibacterial effect diagrams of BF1, BF2, BF3, and BF4 against Staphylococcus aureus and Escherichia coli respectively; (c) and (d) are the inhibition zones of BF1, BF2, BF3, and BF4 against Staphylococcus aureus and Escherichia coli respectively.

[0032] Figure 8 (a) and (b) in the figure are the antioxidant activities of BF1, BF2, BF3, and BF4 against DPPH and ABTS free radicals. Detailed implementation manners

[0033] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0034] Comparative example: Dissolve 20 g of CMC and SA in 100 mL of distilled water respectively, and stir with a magnetic stirrer for 6 hours to completely dissolve the solute. Then, weigh, mix these solutions, add 2% glycerol and vortex. The mixed solution is ultrasonically treated under water bath conditions, and the ultrasonic treatment conditions are 10 minutes of time, 80% amplitude, turn off for 2.5 seconds every 7.5 seconds of ultrasonic treatment, and cycle like this to remove bubbles. Finally, dry to form a film, and the prepared CMC / SA film is denoted as BF1.

[0035] Example 1:

[0036] Step 1: Spirulina cultivation: Cultivate spirulina using Zarrouk medium, with a light condition of 5000 Lux and a temperature of 25 °C. After culturing for 21 days until the stationary phase, collect spirulina using a filter cloth with a pore size of 0.2 μm. The spirulina strain used is purchased from the Freshwater Algae Species Bank of the Chinese Academy of Sciences.

[0037] The components and their concentrations of Zarrouk medium are:

[0038] NaHCO3: 16.80 g / L, K2HPO4: 0.50 g / L, NaNO3: 2.50 g / L, NaCL: 1.00 g / L, MgSO4·7H2O: 0.20 g / L, FeSO4·7H2O: 0.01 g / L, K2SO4: 1.00 g / L, CaCL2·2H2O: 0.04 g / L, EDTA: 0.08 g / L, A5: 1 ml / L, B6: 1 ml / L.

[0039] Among them, A5 is a mixed solution with the following components: H3BO3: 2.86 g / L, MnCl2·4H2O: 1.86 g / L, ZnSO4·7H2O: 0.222 g / L, CuSO4·5H2O: 0.079 g / L, Na2MoO4·2H2O: 0.390 g / L, Co(NO3)2·6H2O: 0.049 g / L; B6 is a mixed solution with the following components: NH4VO3: 22.9 g / L, NiSO3·7H2O: 47.8 g / L, NaWO4: 17.9 g / L, Ti(SO4)2: 40.0 g / L, Co(NO3)2·6H2O: 4.4 g / L.

[0040] Step 2: Ultrasonic assisted extraction: 4 g of Spirulina biomass was suspended in 200 mL of distilled water, mixed thoroughly, and then ultrasonically treated with an ultrasonic cell crusher (Ningbo Xinzhi Biological, model JY92-IIN) under ice bath conditions, with an ultrasonic time of 30 min and an amplitude of 80%; during the ultrasonic process, the ultrasound was turned on for 7.5 seconds and off for 2.5 seconds, and this cycle was performed for 10 to 40 minutes. After the ultrasonic treatment, the resulting solution was centrifuged 3 times at 13000 rpm, 4°C, and 15 minutes to collect the supernatant; and the supernatant was evaporated in a rotary vacuum evaporator at 40°C to obtain a concentrate, the volume of which was about 1 / 5 of the volume of the supernatant, and the resulting concentrate was freeze-dried to obtain a freeze-dried product, which was Spirulina extract, recorded as UAAESP.

[0041] The obtained spirulina extract was mixed with deionized water at 25° for 5 minutes to obtain a 1% spirulina water extract. Multiple portions of the spirulina water extract were taken and the pH of the mixed solution was adjusted to 3-10 using an acid-base buffer solution. The color of the mixed solution was as follows: Figure 1 (a) As shown. When the pH value is 3-5, it is dark blue-gray, when the pH value is 6-9, it is blue-gray, and when the pH value is 10, it is yellow-green, and all have high UV-visible absorption effects, such as Figure 1 (b) as shown.

[0042] Step 3: Preparation of the colorimetric film: Weigh 20 g of CMC and SA respectively, dissolve them in 100 ml of deionized water, stir for 6 hours, then add 2 g of agar to the mixed solution and mix well on a vortex mixer. Next, add 1 g of spirulina extract to the mixed solution and stir until completely dissolved on a magnetic stirrer. Then add 20 ml of a glycerol solution with a mass concentration of 2%, and mix well again on the vortex mixer. Then, perform ultrasonic treatment under the condition of a water bath at 40 °C - 50 °C. The ultrasonic treatment conditions are: amplitude 80%, turn off for 2.5 seconds every 7.5 seconds of ultrasonic treatment, and cycle like this for 10 minutes. Finally, dry to form a film, denoted as BF2, which is the CMC / SA / UAAESP film. And store it in a desiccator. The temperature for drying to form the film is 50 - 70 °C, and the duration is 5 - 7 hours; the temperature for dry storage is 23 ± 2 °C, and the humidity is 43 ± 2%.

[0043] Example 2: The same as Example 1, the difference is that 0.125 g of gallic acid is added in Step 3 to prepare the CMC / SA / UAAESP / 0.125GA film, which is BF3.

[0044] Example 3: The same as Example 1, the difference is that 0.25 g of gallic acid is added in Step 3 to prepare the CMC / SA / UAAESP / 0.25GA film, which is BF4.

[0045] Permeability of the colorimetric film: To evaluate the permeability of the prepared colorimetric film, the microstructures of the films of Examples 1 - 3 and the control example were observed by scanning electron microscopy (SEM), including the surface regularity and structural changes. As Figure 2 shown, the surface of the control film BF1 is smooth and dense, with a uniform structure, and the cross-section also shows a dense structure. Due to the high water solubility of UAAESP, UAAESP can be evenly distributed in the film-forming solution and can form a relatively dense film. The addition of GA increases the internal connection of the film, resulting in the formation of pores and cracks on the cross-sections of films BF3 and BF4; this characteristic is crucial for the high sensitivity of the thin film to total volatile basic nitrogen (TVB-N) released by shrimp meat during storage. The surface of BF3 is rough, and the introduction of UAAESP and GA causes partial decomposition of the film-forming biopolymer. The cross-sections of BF3 and BF4 with the addition of UAAESP and GA are similar to that of BF2 with only UAAESP used. This similarity indicates that the combined use of UAAESP and GA more effectively reduces the formation of molecular bonds between biopolymer chains, thereby enhancing the sensitivity of the thin film to guest molecules (such as TVB-N). In addition, the porous structure of the film provides channels for mass transfer, potentially increasing the gas adsorption capacity.

[0046] The FTIR spectrogram of the colorimetric film prepared by the present invention is as Figure 3As shown, the peak positions in the BF1 spectrum are consistent with those observed in BF2, BF3, and BF4. Since the addition amounts of UAAESP and gallic acid are small, no new peaks appear in the spectrum. However, there are slight changes in the width or intensity of the peaks near 3260, 2925, 2881, 1594, 1410, 1320, and 922 cm -1 The broad peak between 3200 and 3600 cm -1 corresponds to the NH2 and O-H bands within and between molecules. The peaks appearing at 2800 and 3000 cm -1 are attributed to C-H stretching vibrations. The smaller peak near 2929 cm -1 also corresponds to C-H stretching vibrations. The peaks at 1420, 1320, and 1026 cm -1 represent the symmetric stretching vibrations of symmetric carboxylic acid (C=O), alkyl, and the characteristic peak of the glucuronic acid ring, respectively. In addition, the peak near 1320 cm -1 is related to C-H bending, and the peak near 1026 cm -1 is related to C-OH stretching. At 1594 cm -1 , the width of the peak is slightly different, which is attributed to C=O (amide I) stretching.

[0047] After incorporating UAAESP, there are slight changes in the absorption peak intensity, indicating that the physicochemical interactions between the aromatic ring of the natural pigment and gallic acid and the membrane matrix have changed. Among them, the intensity of the peak near 1594 cm -1 decreases with the addition of UAAESP. In addition, the peak near 2881 cm -1 indicates the existence of additional hydrogen bonds between the natural pigment and the biopolymer matrix used. In summary, the FTIR spectrum confirms that the bioactive components of UAAESP and gallic acid are successfully incorporated into the carboxymethyl cellulose and sodium alginate matrices, promoting hydrogen bond interactions within these systems.

[0048] Freshness detection effect: Stick the film on the lid of the packaging box, place it 6 cm above the shrimp body, and store it at 25 °C. Use a colorimeter to measure the color change ΔE of the colorimetric film every 3 hours; and record the state of the colorimetric film to monitor the response of the colorimetric film to the total volatile basic nitrogen (TVB-N) released by the shrimp meat.

[0049] During the storage of shrimp, the total volatile basic nitrogen (TVB-N) released by the shrimp meat during the storage time from 0 h to 24 h is as Figure 4 shown. The TVB-N content in the shrimp samples increases from 5.70 mg / 100 g to 33.91 mg / 100 g. The color changes of the films of Examples 1-3 and the comparative example of the present invention used for preserving shrimp samples are as Figure 6As shown, the color difference change ΔE of the colorimetric film is as Figure 5 shown, indicating a decrease in freshness. As the storage time of shrimp extends, due to the increase in pH value caused by the volatile amines released from spoiled shrimp, the colors of the colorimetric films in Example 1, Example 2, and Example 3 all change. BF2 changes from blue to dark blue, and the darker the blue, the worse the freshness. BF3 and BF4 transition from blue (very fresh) to dark blue (semi - fresh) and dark gray (spoiled). The addition of gallic acid enhances the response of the spirulina extract to total volatile basic nitrogen (TVB - N).

[0050] Antibacterial properties of the biofilm: The inhibitory activities of the biofilm against Staphylococcus aureus and Escherichia coli were quantitatively evaluated by the inhibition zone method, and the antibacterial experimental results are as Figure 7 shown in (a) and (b) in Figure 7 ; (c) and (d) in

[0051] are the sizes of the inhibition zones of BF1, BF2, BF3, and BF4 against Staphylococcus aureus and Escherichia coli. The results show that the control colorimetric film (BF1) does not show any inhibition zone. However, BF2 added with UAAESP, BF3 and BF4 added with UAAESP and different concentrations of gallic acid all show obvious inhibition zones against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). The inhibitory differences of the colorimetric films in Examples 1 - 3 against Escherichia coli are statistically significant (p < 0.05). Among them, BF3 and BF4 added with gallic acid have the inhibition zones against Staphylococcus aureus and Escherichia coli increased by 74.25% and 81.09% respectively compared with BF2 added with only UAAESP, indicating that the film prepared by the present invention has strong antibacterial effects against Escherichia coli and Staphylococcus aureus. Figure 8 shown, compared with BF1, BF2 added with UAAESP significantly (p < 0.05) enhances the antioxidant capacity of the polymer matrix. The scavenging abilities of the CMC / SA film BF1 prepared in the comparative example against DPPH free radicals and ABTS free radicals are 6.56 ± 1.51% and 8.89 ± 1.72% respectively, while for BF2 added with UAAESP, the scavenging abilities of DPPH free radicals and ABTS free radicals are 21.35 ± 0.43% and 23.06% respectively. In addition, the scavenging abilities of BF4 added with 0.25 g GA against DPPH free radicals and ABTS free radicals are significantly improved, being 92.76 ± 1.41% and 94.99 ± 1.53% respectively.

[0052] In summary, the film prepared by the method for preparing a food freshness - detecting colorimetric film based on microalgae extract proposed by the present invention can well indicate the freshness of food and has antibacterial and antioxidant abilities.

[0053] Mechanical strength and physical properties of the biofilm: As shown in Table 1, with the incorporation of UAAESP and GA, the tensile strength of the biofilm increased significantly, while the elongation at break decreased, indicating that the mechanical properties of the biofilm can be enhanced and it has higher crack resistance during storage and transportation. The incorporation of UAAESP and GA significantly reduced the water solubility of the film (23.99%), which has a beneficial effect on food packaging. The oxygen permeability decreased significantly with the addition of UAAESP and the increase in GA concentration, increasing the path length of gas molecules within the biofilm matrix, thereby reducing oxygen permeation, with a maximum reduction of 25.77%. UAAESP and GA interact with matrix molecules through hydrogen bonds, thus reducing the interaction between water molecules and the hydrophilic groups of CMC and SA. The water molecules become less accessible, resulting in a significant decrease in the moisture content and water permeability. The moisture content can be reduced by up to 39.53%; the water permeability can be reduced by up to 38.91%.

[0054] Table 1 Mechanical strength and physical properties of the color film

[0055]

[0056] In summary, the film prepared by the method for preparing a colorimetric film for detecting food freshness based on microalgae extract proposed in the present invention can well indicate the freshness of food and has antibacterial, antioxidant capabilities, mechanical properties and physical properties.

[0057] The above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention; therefore, although the present specification has described the present invention in detail with reference to the above embodiments, those of ordinary skill in the art should understand that the present invention can still be modified or equivalently replaced; and all technical solutions and their improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract, characterized in that: The following steps are involved: (1) Ultrasonic-assisted extraction: The Spirulina biomass is subjected to ultrasonic extraction in an ice bath; after the ultrasonic extraction is completed, the supernatant is collected by centrifugation, and then the supernatant is evaporated and concentrated, and the obtained concentrate is freeze-dried to obtain the Spirulina extract; (2) Preparation of colorimetric film: CMC and SA are added to deionized water to prepare a mixed solution, agar is added and mixed evenly, and then the Spirulina extract obtained in step (1) is added and stirred in a magnetic stirrer until completely dissolved, and then glycerol is added and mixed evenly, and then ultrasonically treated in a water bath at 40°C to 50°C, and finally dried to form a film, which is stored in a desiccator.

2. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: The ultrasonic extraction conditions in step (1) are: the amplitude is 20% to 100%, the ultrasonic wave is turned off for 2 to 4 seconds every 5 to 10 seconds, and this cycle is repeated for 10 to 40 minutes.

3. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: The centrifugation conditions in step (1) are: 13000 rpm, 4°C, 10-20 minutes; the concentration and evaporation temperature is 35-45°C, and the concentrated liquid obtained is 1 / 5-1 / 7 of the volume of the supernatant; the freeze-drying conditions are: temperature is -55°C to -15°C, and the vacuum degree is 1-5Pa.

4. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: The added amounts of CMC, SA, spirulina extract and agar in step (2) are: 20-40 g, 20-40 g, 0.5-2 g and 2-3 g respectively added to 100 ml of deionized water; the added amount of glycerol is: 20 ml of 2% glycerol solution.

5. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: The conditions of the ultrasonic treatment in step (2) are: the amplitude is 80%; every 7.5 seconds of ultrasonic treatment, the ultrasonic treatment is turned off for 2.5 seconds, and this cycle is repeated for 5-10 minutes; Preferably, in step (2), the drying temperature is 50-70° C. and the drying time is 5-7 hours; the drying storage temperature is 23±2° C. and the humidity is 43±2%.

6. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: Gallic acid is also added in the step (2) in an amount of 0.1 to 0.3 g per 100 ml of deionized water.

7. The method for preparing a colorimetric membrane for detecting food freshness based on spirulina extract according to claim 1, characterized in that: The spirulina biomass in step (1) is obtained by the following method: culturing spirulina using Zarrouk medium, and collecting the spirulina by filtration after culturing to a stable period; the culturing conditions are: light intensity of 4000-5000 Lux, temperature of 16-30° C., and time of 14-21 days; and using a filter screen or filter cloth with a pore size of 0.2 μm when filtration and collection.

8. A colorimetric membrane for detecting food freshness based on Spirulina extract prepared according to the preparation method according to any one of claims 1 to 6.

9. The use of the colorimetric film according to claim 8, characterized in that: A freshness detection tool for fresh-keeping food is placed or attached to a plastic wrap / bag / box / container so that it is located within the food storage space.

10. The use of the colorimetric film according to claim 8, characterized in that: Used for preparing food preservative film / bag / box / container for antibacterial and / or freshness indication; the antibacterial species include Escherichia coli and / or Staphylococcus aureus.

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