Food freshness indicating film based on lithospermum red pigment as well as preparation method and application of food freshness indicating film

Abstract

The invention discloses a food freshness indicating film based on lithospermum red pigment as well as a preparation method and application of the food freshness indicating film, and belongs to the technical field of intelligent packaging. The invention discloses a food freshness indicating film based on lithospermum red pigment. The food freshness indicating film comprises the following raw materials in parts by mass: 3-10 parts of starch, 0.5-3 parts of sodium alginate, 0.5-2 parts of lithospermum red pigment, 100-300 parts of water and 1-5 parts of glycerol. Through interaction among the starch, the sodium alginate, the glycerol and the lithospermum red pigment, the stability of the lithospermum red pigment is improved, so that the lithospermum red pigment still has good pH response performance and relatively good color developing sensitivity after being subjected to a starch gelatinization process. The prepared food freshness indicating film based on the lithospermum red pigment has excellent mechanical property, water vapor barrier property and ultraviolet barrier property.

Description

Technical Field

[0001] This invention belongs to the field of intelligent packaging technology, specifically relating to a food freshness indicator film based on purpuric red pigment, its preparation method, and its application. Background Technology

[0002] Food is susceptible to microbial contamination and changes during storage and transportation, which can reduce its freshness and potentially generate harmful components. Therefore, developing rapid and convenient food freshness detection technologies is crucial. The rise of smart packaging materials has opened up new avenues for food freshness detection. With increasing awareness of food safety, consumers expect to grasp the quality of the food they purchase through faster and more intuitive methods. Consequently, freshness indicator packaging that provides consumers with food quality information during transportation and storage has emerged. Given the importance of food safety, more and more researchers are beginning to explore the use of natural pigments extracted from plants as freshness indicators, and to incorporate them into various film-forming matrices to prepare freshness indicator films for food packaging.

[0003] For natural pigments, existing technologies mainly focus on shikonin. CN114805875 A discloses a low-temperature molded intelligent indicator film for visualizing pork freshness, which prepares the indicator film by mixing shikonin with sodium alginate and gelatin. However, the shikonin solution appears red at pH 3-7 and blue at pH 8-12. Its color response range is narrow, the color transition is not obvious, and software analysis is required to determine whether the pork has spoiled. Furthermore, the indicator film composed of sodium alginate and gelatin has poor water vapor barrier properties and is costly.

[0004] Starch films are gaining popularity in food packaging due to their economic efficiency and renewability as film-forming matrices. However, starch films have certain limitations, such as poor mechanical properties, difficulty in film formation, and poor barrier properties. Furthermore, starch requires temperatures above 60°C to gelatinize and form a film. If starch is directly mixed with natural pigments, the structure of the natural pigments is destroyed under high temperatures, reducing their stability and leading to abnormal color development and decreased color sensitivity. Current technologies typically gelatinize the starch and cool it before adding the natural pigments. However, starch already forms a dense three-dimensional gel network, making it difficult for pigment molecules to penetrate the network. They can only adhere to the gel surface or local pores, resulting in uneven pigment dispersion. Pigment agglomeration areas may become stress concentration points, reducing the tensile strength and flexibility of the material. Moreover, the weak bonding between the pigment and the matrix makes pigment migration more likely. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the primary objective of the present invention is to provide a method for preparing a food freshness indicator film based on purpuric red pigment.

[0006] Another object of the present invention is to provide a food freshness indicator film based on purpuric red pigment.

[0007] Another object of the present invention is to provide the application of the above-mentioned food freshness indicator film based on comfrey red pigment in the preparation of food packaging.

[0008] To achieve the above objectives, the present invention provides the following technical solution: A food freshness indicator film based on comfrey red pigment, comprising the following raw materials in parts by weight: 3-10 parts starch, 0.5-3 parts sodium alginate, 0.2-2 parts comfrey red pigment, 100-300 parts water, and 1-5 parts glycerin; Its preparation method includes the following steps: Starch, glycerin, sodium alginate, and alkanet red pigment are mixed in water, heated to gelatinize the starch, allowed to stand to defoam, form a film, and dried to obtain the food freshness indicator film based on alkanet red pigment.

[0009] This invention uses natural biomass materials starch and sodium alginate as the base film for a food freshness indicator film based on lithospermum erythrorhizon pigment. Through the interaction between starch, sodium alginate, glycerol, and lithospermum erythrorhizon pigment, the stability of lithospermum erythrorhizon pigment and the mechanical properties, water vapor barrier properties, and UV barrier properties of the food freshness indicator film are improved. The main component of lithospermum erythrorhizon pigment is naphthoquinone compounds. Hydrogen bonds are formed between the polysaccharide hydroxyl groups and glycosidic bonds in starch and the phenolic hydroxyl groups in lithospermum erythrorhizon pigment. The carboxyl groups (COO-) in sodium alginate... - The phenolic hydroxyl groups in lithospermum erythrorhizon form weak electrostatic or hydrogen bonding interactions. These interactions allow lithospermum erythrorhizon to retain good pH response and colorimetric sensitivity even after starch gelatinization.

[0010] A food freshness indicator film based on erythromycin was prepared using a solution casting method. This film exhibits superior mechanical properties, water vapor barrier properties, stability, and biodegradability. Utilizing natural polymers such as starch and sodium alginate, the erythromycin-based food freshness indicator film not only provides an environmentally friendly solution for food freshness monitoring but also possesses a broad redox pH response colorimetric range, enabling the monitoring of the freshness of various foods. Furthermore, its natural safety helps prevent food contamination, addressing growing public concerns about food safety.

[0011] In this invention, natural biomass materials starch and sodium alginate are used as the base film for the lithospermum erythrorhizon-based food freshness indicator film; lithospermum erythrorhizon is used as the indicator functional material, giving the lithospermum erythrorhizon-based food freshness indicator film indicator properties. The plasticizer glycerin assists deionized water in improving the flexibility of the lithospermum erythrorhizon-based food freshness indicator film and promotes the dissolution of lithospermum erythrorhizon in the matrix.

[0012] Specifically, the main components of the lithospermum erythrorhizon include lithospermum erythrorhizon, acetyl lithospermum erythrorhizon, β,β-dimethylacryloyl lithospermum erythrorhizon, β-hydroxyisovaleroyl lithospermum erythrorhizon and 2,3-dimethylpentenoyl lithospermum erythror

[0013] As a natural naphthoquinone pigment, limonene red pigment can change color significantly with changes in redox potential and pH during food spoilage, thus providing a direct indication of food freshness.

[0014] Shikonin is a mixture of various structurally similar naphthoquinone derivatives. As a natural pigment, it exhibits distinct color changes within an acidic to weakly alkaline range (pH=3~9). At pH <5, it displays a deep red color (due to the stability of the quinone structure); at pH = 6~8, it displays a purplish-red to bluish-purple color (due to ionization of the phenolic hydroxyl group and expansion of the conjugated system); at pH = 9~11, it displays a dark blue color (due to ring opening of the quinone structure); and at pH = 12~14, it displays a yellowish-green color. Compared to shikonin, shikonin has a wider color response range, more pronounced color transitions, and is visible to the naked eye, making it suitable for monitoring acid-base changes during spoilage of fresh foods (such as fruits, vegetables, and dairy products).

[0015] Deionized water is used as the matrix solvent to ensure film uniformity. During starch gelatinization, hydrogen bonds between starch molecules break, while hydrogen bonds between starch molecules and water molecules form, causing the starch to absorb water and swell, forming a stable hydrophilic colloid.

[0016] By controlling the raw materials within the above-mentioned range, this invention can improve the stability of lithospermum erythrorhizon pigment while enabling the prepared lithospermum erythrorhizon pigment food freshness indicator film to have superior mechanical properties, water vapor barrier properties, and ultraviolet barrier properties.

[0017] Preferably, the heating temperature is 70~90℃.

[0018] By controlling the heating temperature at 70~90℃, the stability of the lithospermum erythrorhizon pigment is ensured, while the prepared lithospermum erythrorhizon pigment food freshness indicator film can have better mechanical properties, water vapor barrier properties, and ultraviolet barrier properties.

[0019] Preferably, the heating time is 30-50 minutes.

[0020] Preferably, the settling time is 10 to 180 minutes.

[0021] Preferably, the drying temperature is 40~70℃.

[0022] Preferably, the drying time is 4 to 12 hours.

[0023] Preferably, the food freshness indicator film based on comfrey red pigment comprises the following raw materials in parts by weight: 3-9 parts starch, 0.5-3 parts sodium alginate, 0.3-1 part comfrey red pigment, 100-300 parts water, and 1-5 parts glycerin.

[0024] More preferably, the food freshness indicator film based on comfrey red pigment comprises the following raw materials in parts by weight: 3-7 parts starch, 0.5-1.5 parts sodium alginate, 0.3-0.8 parts comfrey red pigment, 100-300 parts water, and 1-5 parts glycerin.

[0025] More preferably, the food freshness indicator film based on comfrey red pigment comprises the following raw materials in parts by weight: 3-7 parts starch, 0.5-1 parts sodium alginate, 0.3-0.5 parts comfrey red pigment, 100-300 parts water, and 1-5 parts glycerin.

[0026] Preferably, the starch is at least one of carboxymethyl starch, hydroxypropyl starch, thermoplastic starch, acetate starch, and cross-linked starch.

[0027] Preferably, the sodium alginate is at least one of sodium alginate from giant kelp, sodium alginate from brown algae, and sodium alginate from Fucus vesiculosus.

[0028] This invention also protects the food freshness indicator film based on purpuric red pigment prepared by the above preparation method.

[0029] Preferably, the tensile strength of the food freshness indicator film based on purpureus red pigment is 10.16~78.13 MPa.

[0030] Preferably, the water vapor transmission coefficient of the food freshness indicator film based on purpureus red pigment is 8.56 × 10⁻⁶. -6 ~9.87×10 -6 g / (m·s·Pa).

[0031] This invention also protects the application of the above-mentioned food freshness indicator film based on purpuric red pigment in the preparation of food packaging.

[0032] Preferably, the food is pork.

[0033] Compared with the prior art, the present invention has the following technical effects: This invention uses natural biomass materials starch and sodium alginate as the base membrane for a food freshness indicator film based on lithospermum erythrorhizon pigment. Through the interaction between starch, sodium alginate, glycerol and lithospermum erythrorhizon pigment, the stability of lithospermum erythrorhizon pigment is improved, so that it still has good pH response performance and good color development sensitivity after starch gelatinization. On the other hand, the prepared food freshness indicator film based on lithospermum erythrorhizon pigment has superior mechanical properties, water vapor barrier properties and ultraviolet barrier properties.

[0034] The food freshness indicator film based on purpureus red pigment prepared in this invention can effectively judge food quality through film color changes, realizing visualized freshness monitoring and effectively reducing the risk of health problems caused by accidentally consuming spoiled food. The food freshness indicator film prepared by this invention is based on natural biodegradable materials, which can reduce plastic packaging waste and contribute to the achievement of carbon emission reduction goals. Attached Figure Description

[0035] Figure 1 The color of the food freshness indicator film based on comfrey red pigment in Examples 1-5 and Comparative Example 1 is shown.

[0036] Figure 2 Fourier transform infrared spectra of food freshness indicator films based on purpuric red pigment in Examples 1-5 and Comparative Example 1.

[0037] Figure 3 The transmittance is the permeability of the food freshness indicator film based on comfrey red pigment in Examples 1-5 and Comparative Example 1.

[0038] Figure 4 The color changes of the food freshness indicator film based on purpuric red pigment in different pH solutions are shown in Examples 1-5 and Comparative Example 2.

[0039] Figure 5 Examples 1-5 illustrate the indicative effect of the food freshness indicator film based on purpureus red pigment on pork.

[0040] Figure 6 The TG graphs are for Examples 1-5 and Comparative Example 1, showing the food freshness indicator film based on comfrey red pigment. Detailed Implementation

[0041] The present invention is further illustrated below with reference to specific embodiments. These embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used, unless otherwise specified, are all commercially available from the conventional market. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention are within the scope of protection claimed by the present invention.

[0042] Example 1 This embodiment provides a food freshness indicator film based on comfrey red pigment, the preparation method of which includes the following steps: S1. Dissolve 7 parts hydroxypropyl starch, 1.5 parts sodium alginate and 5 parts glycerol in 300 parts deionized water and mix thoroughly. Add 0.2 parts comfrey red pigment to the mixture and heat and stir in a water bath at 90°C for 40 minutes to form a uniform film-forming solution.

[0043] S2. The film-forming solution obtained in S1 was allowed to stand for 120 minutes to defoam. The defoamed film-forming solution was poured onto a polytetrafluoroethylene plate and placed in an electric heating drying oven at 70°C for 8 hours. After removal, the film was peeled off to obtain a food freshness indicator film based on purpureus red pigment.

[0044] Example 2 This embodiment provides a food freshness indicator film based on comfrey red pigment, the preparation method of which includes the following steps: S1. Dissolve 5 parts cross-linked starch, 3 parts sodium alginate from Fucus vesiculosus, and 2 parts glycerol in 150 parts deionized water and mix thoroughly. Add 0.3 parts of comfrey red pigment to the mixture and heat and stir in a water bath at 75°C for 50 minutes to form a uniform film-forming solution.

[0045] S2. The film-forming solution obtained in S1 was allowed to stand for 60 minutes to defoam. The defoamed film-forming solution was poured onto a polytetrafluoroethylene plate and placed in an electric heating drying oven at 40°C for 8 hours. After drying, the film was peeled off to obtain a food freshness indicator film based on purple gromwell red pigment.

[0046] Example 3 This embodiment provides a food freshness indicator film based on comfrey red pigment, the preparation method of which includes the following steps: S1. Dissolve 5 parts hydroxypropyl starch, 0.5 parts sodium alginate from giant kelp, and 1.5 parts glycerol in 200 parts deionized water and mix thoroughly. Add 0.4 parts comfrey red pigment to the mixture and heat and stir in a water bath at 85°C for 40 minutes to form a uniform film-forming solution.

[0047] S2. Let the film-forming solution prepared in S1 stand for 30 minutes to defoam. Pour the defoamed film-forming solution onto a polytetrafluoroethylene plate and place it in an electric heating drying oven at 50°C for 6 hours. After removing it, peel off the film to obtain a food freshness indicator film based on purple gromwell red pigment.

[0048] Example 4 This embodiment provides a food freshness indicator film based on comfrey red pigment, the preparation method of which includes the following steps: S1. Dissolve 9 parts of acetate-esterified starch, 3 parts of sodium alginate from giant kelp, and 4 parts of glycerol in 300 parts of deionized water and mix thoroughly. Add 1 part of comfrey red pigment to the mixture and heat and stir in a water bath at 90°C for 50 minutes to form a uniform film-forming solution.

[0049] S2. The film-forming solution obtained in S1 was allowed to stand for 180 minutes to defoam. The defoamed film-forming solution was poured onto a polytetrafluoroethylene plate and placed in an electric heating drying oven at 60°C for 10 hours. After removal, the film was peeled off to obtain a food freshness indicator film based on purple gromwell red pigment.

[0050] Example 5 This embodiment provides a food freshness indicator film based on comfrey red pigment, the preparation method of which includes the following steps: S1. Dissolve 3 parts carboxymethyl starch, 1 part sodium alginate from giant kelp, and 0.5 parts glycerol in 200 parts deionized water and mix thoroughly. Add 0.4 parts comfrey red pigment to the mixture and heat and stir in a water bath at 70°C for 50 minutes to form a uniform film-forming solution.

[0051] S2. The film-forming solution obtained in S1 is allowed to stand for 60 minutes to defoam. The defoamed film-forming solution is poured onto a polytetrafluoroethylene plate and placed in an electric heating drying oven at 40°C for 10 hours. After removal, the film is peeled off to obtain a food freshness indicator film based on purple gromwell red pigment.

[0052] Comparative Example 1 This comparative example provides a membrane that differs from Example 1 in that, in step S1, no comfrey red pigment is added. The rest is the same as in Example 1.

[0053] Comparative Example 2 This comparative example provides a food freshness indicator film based on purpureus red pigment. The difference from Example 1 is that sodium alginate is not added in step S1. The rest is the same as Example 1.

[0054] Performance testing Tensile strength test method: The film sample was cut into strips (100mm × 15mm), and the thickness was measured at five random points on each strip using a thickness gauge, and the average value was taken. The mechanical properties were measured using a universal testing machine. The clamping length of the testing machine was set to 40mm, and the tensile speed was set to 10mm / min to control the rate of the tensile process and obtain accurate mechanical property data. After the sample fractured under tension, its tensile strength was recorded.

[0055] Color testing method for food freshness indicator film: The appearance of the prepared composite film was captured using a digital camera. The film was placed in a photographing box with a blank plate as a control to obtain a photograph of the film. The color parameters (L, a, b) of the film were measured using a colorimeter. The total color difference (ΔE) of the film was calculated according to the following formula:

[0056] In the formula: L*, a*, and b* are the color values ​​of the standard white board.

[0057] Test method for water vapor transmission coefficient: Select a 25mm × 25mm weighing bottle, fill it with 1 / 3 dry anhydrous CaCl2, cut a film into samples of similar size to seal the bottle opening, and then place the weighing bottle in a desiccator with sodium chloride at the bottom. Weigh the bottle every 2 hours for 2 days. The formula for calculating water vapor transmission rate (WVP) is as follows:

[0058] In the formula: WVP is the water vapor transmission coefficient, g / (m·s·Pa); q / t is the weight gain of the weighing bottle per unit time, g / h; d is the membrane thickness, mm; S is the test area of ​​the membrane, m². 2 ; △P is the vapor pressure difference across the sample membrane, in kPa.

[0059] Transmittance testing method: For transmittance measurement, an empty cuvette is used as a blank control. The membrane sample is cut to the size of the cuvette, attached to the inner wall, and measured using a UV spectrophotometer in the wavelength range of 300 to 800 nm. The opacity of the membrane is calculated using formula (2-5):

[0060] Where A600 is the absorbance value at a wavelength of 600nm, and T is the film thickness.

[0061] Thermogravimetric analysis of composite membranes The composite membrane was tested using a thermogravimetric analyzer at a nitrogen flow rate of 10 mL / min. The test was performed twice in parallel, with the temperature increased from 30 °C to 600 °C at a heating rate of 10 °C / min.

[0062] Experimental test of food freshness indicator film in different pH solutions: The color development of the film in buffer solutions with pH=2, 4, 6, 8, 10, 12 and 14 was measured. The film was cut into 2cm×2cm pieces and immersed in the buffer solution for 1 minute. The color of the film was recorded with a camera.

[0063] Pork Freshness Monitoring Experiment: Fresh pork and composite films of similar size and shape were selected for preservation experiments. 5 grams of pork (length × width × height = 3 cm × 1 cm × 1 cm) was placed on one side of a sealed bag, and the indicator film was placed on the other side. The bag was then stored at room temperature for 5 days. Photos were taken at 0, 1, 2, 3, 4, and 5 days during storage to record the color changes of the indicator film.

[0064] The appearance, thickness, and tensile strength test results of the food freshness indicator films of Examples 1-5 and Comparative Example 1 are shown in Table 1.

[0065] Table 1. Appearance, morphology, thickness, and tensile strength of food freshness indicator films

[0066] Table 1 shows that the tensile strength of the food freshness indicator film first increases and then decreases with the increase of the amount of lithospermum erythrorhizon pigment added, with the food freshness indicator film of Example 3 exhibiting the highest tensile strength. This indicates that when the amount of lithospermum erythrorhizon pigment added is 0.2~1 part, it improves the tensile strength of the food freshness indicator film to a certain extent, enabling the prepared food freshness indicator film to meet the usage requirements. Comparative Example 2, with its pure starch matrix food freshness indicator film, has a stiff and brittle indicator film due to the strong intermolecular forces, making it difficult to withstand external stretching or folding.

[0067] The colors of the food freshness indicator films of Examples 1-5 and Comparative Example 1 are shown in Table 2 and... Figure 1 As shown.

[0068] Table 2 Colors of Food Freshness Indicator Films

[0069] Table 2 shows that the color of the food freshness indicator film changed significantly after the addition of lithospermum erythrorhizon pigment. The L values ​​of the food freshness indicator films in Examples 1-5 were all lower than those of the base film. The L values ​​gradually decreased with the increase of lithospermum erythrorhizon pigment content, indicating that the transparency of the food freshness indicator film decreased with the increase of lithospermum erythrorhizon pigment addition. Parameter 'a' was positive, meaning the indicator film was reddish, and the positive value of 'a' gradually increased with the increase of lithospermum erythrorhizon pigment content, meaning the red color of the indicator film deepened with the increase of pigment addition. The color intensity of red and yellow was determined by the relative magnitudes of the 'a' and 'b' values.

[0070] Figure 2 The images show the infrared spectra of Examples 1-5 and Comparative Example 1. Figure 2 It can be known that 1647cm -1 The peak remained stable, indicating that the C=O of the lithospermum erythrorhizon pigment did not interact strongly with the carboxylate of the matrix. (1416 cm⁻¹) -1 ~1417cm -1 The slight change in the absorption peak at that point may be due to the COO of sodium alginate. - Symmetric stretching vibrations and the phenolic hydroxyl groups and COO in lithospermum erythrorhizon. - A weak electrostatic interaction is formed. The absorption peak in Comparative Example 1 is at 10¹⁵ cm⁻¹. -1 Towards 997cm -1 The migration is attributed to the formation of hydrogen bonds between the phenolic hydroxyl groups in the lithospermum erythrorhizon and the polysaccharide hydroxyl groups or glycosidic bonds of the matrix.

[0071] The water vapor permeability coefficient test results of Examples 1-5 and Comparative Example 1 are shown in Table 3.

[0072] Table 3. 24-hour water vapor transmission coefficient test results for Examples 1-5 and Comparative Example 1

[0073] The water vapor transmission coefficient test results for Examples 1-5 and Comparative Example 1 are shown in Table 3. As shown in Table 3, the water vapor transmission coefficient of the food freshness indicator film based on the purple gromwell red pigment after 24 hours is 8.56 × 10⁻⁶. -6 ~9.87×10 -6 g / (m·s·Pa). This indicates that the addition of lithospermum erythrorhizon pigment enhances the intermolecular interactions, thereby constructing a dense barrier layer. The indicator membrane prepared in Comparative Example 2, due to its high content of hydroxyl groups (-OH), exhibits strong hydrophilicity and cannot effectively block water vapor.

[0074] The transmittance test results of Examples 1-5 and Comparative Example 1 are as follows: Figure 3 As shown. By Figure 3 It is known that after adding lithospermum erythrorhizon pigment, the pigment molecules are dispersed in the matrix, forming light scattering centers and absorption sites. At low concentrations (2.4%), the pigment is more uniformly dispersed, and the transmittance in the ultraviolet region decreases but still has a certain degree of transmittance; at high concentrations (3.8%~10%), the light transmittance of the food freshness indicator film in the 300~350nm ultraviolet region is almost zero. This strong ultraviolet blocking ability may be attributed to the strong absorption of ultraviolet light by naphthoquinone compounds in lithospermum erythrorhizon pigment.

[0075] The color changes of Examples 1-5 and Comparative Examples 2 and 3 in solutions with different pH values ​​are as follows: Figure 4 As shown. By Figure 4 It can be seen that the food freshness indicator films of Examples 1-5 show significant color reactions at different pH values. At pH 2, the film is bright red; at pH 4, it is light red; at pH 6, it is light pink; at pH 8, it is light purple or light blue; at pH 10, it is blue or purple; at pH 12, it is green; and at pH 14, it is yellowish-green. This indicates that the food freshness indicator film prepared by this invention has a wide range of redox pH response colors, enabling the monitoring of the freshness of various foods. Comparative Example 2 shows bright red at pH 2. At pH 4 or 6, it shows light pink; at pH 8, a boundary transition color appears, with the pink gradually fading and blue gradually appearing; at pH 10, it shows blue; and at pH 12 or 14, it shows yellowish-green. Compared to Example 1, the color of Comparative Example 2 is paler, the color change is unstable, and the color response to pH is sluggish. This indicates that this invention requires the interaction between starch, sodium alginate, and lithospermum erythrorhizon to improve the stability of the lithospermum erythrorhizon.

[0076] Figure 5This image shows the effect of different composite films on the freshness of pork. Figure 5 As can be seen, at room temperature, the indicator films of Example 1 and Example 2 were light pink for the first three days. On the fourth day, the film color changed to yellowish-green, indicating that the pork had spoiled. On the fifth day, the film turned light yellowish-green, indicating that the pork was completely rotten. The indicator films of Example 3, Example 4, and Example 5 were pink for the first two days. On the third day, the indicator film color began to turn purple. On the fourth day, the indicator films of Example 4 and Example 5 began to change to yellowish-green, while the indicator film of Example 3 remained dark purple. On the fifth day, all three films turned completely light yellowish-green. The indicator effect diagram illustrates that the food freshness indicator film prepared by this invention can cause the indicator film to change color by adsorbing amine gases (ammonia, trimethylamine, etc.) produced during the pork spoilage process. The above results fully demonstrate that as the content of lithospermum erythrorhizon gradually increases, it can provide a very effective indication of the freshness of pork in a more rapid and intuitive way. In other words, the higher the content of purpuric pigment in pork, the faster and more sensitively it can reflect this change in freshness, thus providing a convenient and reliable basis for judging whether pork is fresh.

[0077] Comparative Example 2 shows that the molecular structure of pure starch film is easily altered during pork storage, leading to a continuous decline in its performance.

[0078] Figure 6 The figures show the TG curves of the food freshness indicator films based on lithospermum erythrorhizon pigment in Examples 1-5 and Comparative Example 1. As can be seen from the figures, the curves of Examples 1-5 generally show a trend of "slow weight loss initially, then rapid weight loss, and finally slow weight loss followed by stabilization." In the low-temperature range (below 200°C), the main loss is adsorbed water, bound water, and a small amount of low-boiling-point volatile substances in the composite film. The weight loss rate of each curve decreases slowly and the differences are small, indicating that the initial stage of water and low-boiling-point volatile substance loss is similar regardless of the presence or absence of lithospermum erythrorhizon pigment and different amounts added. In the medium-temperature range (approximately 200-350°C), the weight loss rate decreases rapidly, which is the main stage of thermal degradation of starch and sodium alginate molecular chains and thermal destruction of the lithospermum erythrorhizon pigment structure. In the high-temperature range (above approximately 350°C), the weight loss rate gradually slows down until it stabilizes. At this point, most of the thermally decomposable organic components have decomposed, leaving only residues with relatively high thermal stability (such as inorganic components and carbonaceous matter). Compared to Comparative Example 1, Examples 1-5 (containing comfrey red pigment) exhibited higher weight loss onset temperatures; that is, when the weight loss rate of the materials was the same, Examples 1-5 corresponded to higher temperatures. This indicates that the composite film begins to decompose or undergo mass change at higher temperatures. This further demonstrates that Examples 1-5 possess superior thermal stability compared to Comparative Example 1.

[0079] The above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A food freshness indicator film based on purpureus red pigment, characterized in that, The food freshness indicator film comprises the following raw materials in parts by weight: 3-10 parts starch, 0.5-3 parts sodium alginate, 0.5-2 parts alkanet red pigment, 100-300 parts water, and 1-5 parts glycerin; Its preparation method includes the following steps: Starch, glycerin, sodium alginate, and alkanet red pigment are mixed in water, heated to gelatinize the starch, allowed to stand to defoam, cast into a film, and dried to obtain the food freshness indicator film.

2. The food freshness indicator film according to claim 1, characterized in that, The heating temperature is 70~90℃.

3. The food freshness indicator film according to claim 1, characterized in that, The heating time is 30-50 minutes.

4. The food freshness indicator film according to claim 1, characterized in that, The settling time is 10~180 minutes.

5. The food freshness indicator film according to claim 1, characterized in that, The drying temperature is 40~70℃.

6. The food freshness indicator film according to claim 1, characterized in that, The food freshness indicator film based on comfrey red pigment comprises the following raw materials in parts by weight: 3-9 parts starch, 0.5-3 parts sodium alginate, 0.3-1 part comfrey red pigment, 100-300 parts water, and 1-5 parts glycerin.

7. The food freshness indicator film according to claim 1, characterized in that, The starch is at least one of carboxymethyl starch, hydroxypropyl starch, thermoplastic starch, acetate-esterified starch, and cross-linked starch.

8. The food freshness indicator film according to claim 1, characterized in that, The sodium alginate is at least one of sodium alginate from giant kelp, sodium alginate from brown algae, and sodium alginate from Fucus vesiculosus.

9. The application of the food freshness indicator film based on purpureus red pigment as described in any one of claims 1 to 8 in the preparation of food packaging.

10. The application according to claim 9, characterized in that, The food in question is pork.

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