Preparation method and application of time and temperature indicating film taking amaranth leaf extract as raw material

Abstract

The invention provides a preparation method and application of a time and temperature indicating film taking an amaranth leaf extract as a raw material. The preparation method comprises the following steps: taking amaranth leaves as a raw material, and preparing the amaranth leaf extract by adopting an ethanol extraction freeze-drying method. The TTI film is prepared by taking ethylene glycol as an auxiliary agent, taking polyvinyl alcohol and citric acid as a bone framework and taking an amaranth leaf extract as a color developing material through a tape casting method, and the TTI film can play an indication role by directly observing the color change of the TTI film or measuring the change of the total color difference value of the TTI film. The prepared TTI film has irreversibility of color change and certain refrigeration stability. The apparent activation energy corresponding to the change of the total color difference value of the film can be changed by adjusting the addition amount of the amaranth leaf extract in the film, so that the film is suitable for predicting key storage indexes in various food systems. The mechanical strength, the light barrier property, the smoothness and the biodegradability of the film are improved by adding the amaranth leaf extract. The TTI thin film is simple in preparation process and relatively low in cost and has a wide application prospect.

Description

Technical Field

[0001] The invention relates to the field of food processing and preservation, and in particular to a preparation method and application of a time-temperature indicating film using an amaranth leaf extract as a raw material. Background Art

[0002] Indicators are used to convey information about food quality or the surrounding environment to suppliers, retailers and customers in real time through various signals. Temperature indicators are indirect indicators of food quality, which reveal information about the external environment. Instead of focusing on the change itself, they provide information about the external factors (temperature abuse) that cause the food to change. Consumers can detect potential microbial growth, protein denaturation or emulsion rupture by understanding the temperature abuse. Temperature fluctuations and extreme temperatures have a great impact on shelf life as they largely determine the microbial activity of refrigerated foods such as frozen foods, refrigerated foods, and chilled foods in the meat and fish supply chain.

[0003] Time-temperature indicators (TTIs) indicate irreversible changes due to the combined effects of temperature and time. In order to ensure food quality, food manufacturers are also used to closely monitor the temperature of their products at every link in the supply chain. TTI monitors the cold chain temperature through visible irreversible colorimetric changes to reduce waste. my country is in a transition period from "transporting livestock and poultry" to transporting meat. At present, the phenomenon of "broken chain" in the cold chain during the storage and transportation of fresh meat still exists, and it is difficult to monitor, which poses a potential threat to the safety of fresh meat.

[0004] The color response efficiency and chromaticity distinguishability of TTI, irreversibility of color change, storage stability, and apparent activation energy of total color difference change are important properties to ensure accurate TTI detection, while the structure, light barrier properties, mechanical properties and thickness, and biodegradability of TTI films are also important quality indicators of TTI films. Therefore, it is urgent to develop low-cost and practical TTI to monitor the "broken chain" phenomenon during the storage and transportation of fresh meat, and provide reliable guarantees for the high-quality development of the fresh meat industry. Summary of the invention

[0005] In view of the problem that the cold chain "break" problem in the existing fresh meat storage and transportation process is difficult to effectively monitor, the purpose of the present invention is to provide a method for preparing a time-temperature indicator film using amaranth leaf extract as a raw material and conduct an application test in beef storage.

[0006] The presence of betaine in amaranth gives its leaves different colors (red, orange or green). Among them, betalain is a water-soluble pigment containing nitrogen. It is converted into red betacyanin and yellow betacyanin after deposition in the vacuole. Temperature is an external factor that affects the stability of betalain. Compared with refrigeration temperature, the degradation rate constant of betacyanin in the extraction medium is higher at room temperature.

[0007] Therefore, the present invention uses amaranth leaf extract as an important color-changing element, together with biodegradable thermoplastic polyvinyl alcohol (PVA) and citric acid to prepare a TTI green degradable film, and uses it in beef storage. The film has the advantages of simple preparation, low cost, good mechanical strength, and green degradability, and is expected to be widely used in monitoring temperature fluctuations during the storage and transportation of fresh meat.

[0008] The technical solution of the present invention is as follows:

[0009] The first object of the present invention is to provide a method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material, the method comprising the following steps:

[0010] 1) Take amaranth leaves, homogenize the leaf tissue in ethanol, and soak overnight;

[0011] 2) filtering the extract after overnight to remove coarse particles, centrifuging, filtering to obtain the supernatant, removing ethanol by rotary evaporation, and freeze-drying the solution to obtain the amaranth leaf extract;

[0012] 3) Mix the hydrochloric acid aqueous solution and ethylene glycol and stir evenly;

[0013] 4) adding polyvinyl alcohol and citric acid to the mixed solution obtained in step 3), stirring until fully mixed, then heating and stirring to allow the polyvinyl alcohol and citric acid to fully react, and then keeping the temperature and standing until the bubbles on the top of the solution are removed; preferably, the stirring time is not less than 3 hours;

[0014] 5) cooling the mixed system obtained in step 4) to room temperature, adding the amaranth leaf extract to the solution, and stirring to uniformly disperse the amaranth leaf extract in the solution;

[0015] 6) storing the solution obtained in step 5) under refrigeration conditions below -15°C for not less than 3 hours;

[0016] 7) The refrigerated solution obtained in step 6) is freeze-dried to obtain the time temperature indicating membrane (TTI membrane).

[0017] The storage condition of the time-temperature indicating film of the present invention is refrigeration for future use.

[0018] Furthermore, in step 1), the concentration of the ethanol solution is 95%, and the soaking process after homogenization is carried out at 4°C.

[0019] Furthermore, the centrifugation process in step 2) is carried out at 4°C.

[0020] Furthermore, in step 3), the pH of the aqueous hydrochloric acid solution is 1.5, and the volume ratio of the hydrochloric acid solution to ethylene glycol is 4:1.

[0021] Furthermore, in step 4), the amount of polyvinyl alcohol added is 12 g / 100 mL, and the concentration of citric acid is 1 g / 100 mL; and the heating process in step 4) needs to be heated to 90° C., with a rotation speed of 250 rpm and a time of 2 hours.

[0022] Furthermore, the amount of the Amaranth leaf extract added in step 5) is 0.2-1 g / 100 mL, and the stirring process in step 5) is carried out at 4°C.

[0023] Furthermore, in step 6), the solution is stored at a temperature below -80°C.

[0024] Furthermore, in step 7), the freeze-drying condition is that the vacuum degree is controlled below 0.1 m bar after pre-cooling.

[0025] The second object of the present invention is to provide a time-temperature indicating film using amaranth leaf extract as a raw material, wherein the time-temperature indicating film is prepared by the aforementioned preparation method.

[0026] Preferably, the time-temperature indicating film is cross-linked with ethylene glycol and citric acid to form a skeleton, ethylene glycol serves as a film-forming aid, and amaranth leaf extract serves as a time-temperature indicator.

[0027] Further preferably, the mass ratio of amaranth leaf extract, citric acid and polyvinyl alcohol as raw materials is 1-2:2:24.

[0028] The third object of the present invention is to provide an application of the aforementioned time-temperature indicating film in the preparation of shelf life prediction of food. Preferably, the food is a physical object that is sensitive to temperature and / or time, and the time-temperature indicating film plays an indicating role by directly observing its color change or measuring its total color difference value change. When exposed to an environment of 4-15°C, the color of the time-temperature indicating film changes from purple-red to khaki within 0.5-8 days, and when exposed to an environment of 25-35°C, the color of the time-temperature indicating film changes from purple-red to khaki within 3-12 hours.

[0029] The beneficial effects of the present invention are

[0030] The present invention prepares a film body by cross-linking polyvinyl alcohol and citric acid, and prepares and adds amaranth leaf extract to give the film a time temperature indicator function. The total color difference value of the TTI film changes more sensitively under food storage temperature (4-35°C). The film has irreversible color change and certain cold storage stability. The apparent activation energy corresponding to the change in the total color difference value of the film can be changed by adjusting the amount of amaranth leaf extract added to the film, so that it can be applied to the prediction of key storage indicators of various food systems. At the same time, the addition of amaranth leaf extract improves the mechanical strength, light barrier, smoothness and biodegradability of the film. The TTI film preparation process is simple, the cost is low, and it has obvious social, economic and ecological benefits. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Color changes of the prepared TTI films at different temperatures. Figures AE are the storage results at -10°C, 4°C, 15°C, 2°C and 35°C, respectively. Figure F shows the color of the prepared film after it was stored at 4-35°C for 9 hours to 8 days and then returned to -10°C for 1 day to check the irreversibility of the film color change.

[0032] Figure 2 The relationship curve between ln K and 1 / T in the Arrhenius equation for the total color difference change of the prepared TTI film and the relationship curve between ln K and 1 / T in the Arrhenius equation for the total volatile basic nitrogen (TVB-N) value of beef;

[0033] Figure 3 SEM micrographs of the surface (A) and cross section (B) of the prepared TTI film;

[0034] Figure 4 The thickness, light barrier and mechanical properties of the prepared TTI films. Figure A is the thickness, Figure B is the opacity, Figure C is the tensile strength, and Figure D is the elongation at break.

[0035] Figure 5 To characterize the degradability of the prepared TTI film. DETAILED DESCRIPTION

[0036] The present invention is further described in detail below with reference to the examples. The reagents or instruments used without indicating the manufacturer are all regarded as conventional products that can be purchased on the market.

[0037] Example 1

[0038] 1) Amaranth leaf tissue was homogenized in 95% ethanol and soaked at 4°C overnight;

[0039] 2) After overnight, the extract was filtered with gauze to remove coarse particles, centrifuged at 4°C for 15 minutes, filtered with filter paper to obtain the supernatant, and the ethanol was removed by rotary evaporator. The solution was poured into a petri dish and freeze-dried to obtain amaranth leaf extract;

[0040] 3) Add 40 mL of aqueous hydrochloric acid solution (pH 1.5) and 10 mL of ethylene glycol into a 100 mL beaker, and stir the solution at 50°C and 600 rpm.

[0041] 4) Add 6g of polyvinyl alcohol and 0.5g of citric acid to the solution and stir for 30 minutes. Then raise the temperature to 90°C and keep stirring at 250rpm for 3 hours to allow the polyvinyl alcohol and citric acid to react fully, and then keep at 90°C for 2 hours without stirring to remove bubbles on the top of the solution.

[0042] 5) After that, cover the beaker with plastic wrap and leave at room temperature for 12 hours.

[0043] 0.250 g of Amaranth leaf extract was added to the above solution, and the mixture was stirred at 250 rpm at 4°C for 2 hours.

[0044] 6) Pour 5 g of the solution into a 95 mL Petri dish, cover the dish with plastic wrap and make small holes in the corners, and place the dish in a -80°C refrigerator for 3 hours.

[0045] 7) The culture dish is then placed in a freeze dryer and the vacuum is controlled below 0.1 m bar to freeze dry the PC-ALE-0.250 film. The film is stored at -20°C until use.

[0046] Example 2

[0047] 1) Amaranth leaf tissue was homogenized in 95% ethanol and soaked at 4°C overnight;

[0048] 2) After overnight, the extract was filtered with gauze to remove coarse particles, centrifuged at 4°C for 15 minutes, filtered with filter paper to obtain the supernatant, and the ethanol was removed by rotary evaporator. The solution was poured into a petri dish and freeze-dried to obtain amaranth leaf extract;

[0049] 3) Add 40 mL of aqueous hydrochloric acid solution (pH 1.5) and 10 mL of ethylene glycol into a 100 mL beaker, and stir the solution at 50°C and 600 rpm.

[0050] 4) Add 6g of polyvinyl alcohol and 0.5g of citric acid to the solution and stir for 30 minutes. Then raise the temperature to 90°C and keep stirring at 250rpm for 3 hours to allow the polyvinyl alcohol and citric acid to react fully, and then keep at 90°C for 2 hours without stirring. Remove the bubbles on the top of the solution.

[0051] 5) After that, the beaker was covered with plastic wrap and left at room temperature for 12 hours. 0.375 g of Amaranth leaf extract was added to the above solution and stirred at 250 rpm at 4°C for 2 hours.

[0052] 6) Pour 5 g of the solution into a 95 mL Petri dish, cover the dish with plastic wrap and make a small hole in the corner, and place the dish in a -80°C refrigerator for 3 hours.

[0053] 7) The culture dish is then placed in a freeze dryer and the vacuum is controlled below 0.1 m bar to freeze dry the PC-ALE-0.375 film, which is then stored at -20°C for later use.

[0054] Example 3

[0055] 1) Amaranth leaf tissue was homogenized in 95% ethanol and soaked at 4°C overnight;

[0056] 2) After overnight, the extract was filtered with gauze to remove coarse particles, centrifuged at 4°C for 15 minutes, filtered with filter paper to obtain the supernatant, and the ethanol was removed by rotary evaporator. The solution was poured into a petri dish and freeze-dried to obtain amaranth leaf extract;

[0057] 3) Add 40 mL of aqueous hydrochloric acid solution (pH 1.5) and 10 mL of ethylene glycol into a 100 mL beaker, and stir the solution at 50°C and 600 rpm.

[0058] 4) Add 6g of polyvinyl alcohol and 0.5g of citric acid to the solution and stir for 30 minutes. Then raise the temperature to 90°C and keep stirring at 250rpm for 3 hours to allow the polyvinyl alcohol and citric acid to react fully, and then keep at 90°C for 2 hours without stirring. Remove the bubbles on the top of the solution.

[0059] 5) After that, the beaker was covered with plastic wrap and left at room temperature for 12 hours. 0.500 g of Amaranth leaf extract was added to the above solution and stirred at 250 rpm at 4°C for 2 hours.

[0060] 6) Pour 5 g of the solution into a 95 mL Petri dish, cover the dish with plastic wrap and make a small hole in the corner, and place the dish in a -80°C refrigerator for 3 hours.

[0061] 7) The culture dish is then placed in a freeze dryer and the vacuum degree is controlled below 0.1 m bar to freeze dry the PC-ALE-0.500 film. The film is stored at -20°C until use.

[0062] Comparative Example 1

[0063] 1) Add 40 mL of hydrochloric acid aqueous solution (pH 1.5) and 10 mL of ethylene glycol into a 100 mL beaker and stir the solution at 50°C and 600 rpm.

[0064] 2) Add 6g of polyvinyl alcohol and 0.5g of citric acid to the solution and stir for 30 minutes. Then raise the temperature to 90°C and keep stirring for 3 hours, and then keep it at 90°C for 2 hours without stirring. Remove the bubbles on the top of the solution.

[0065] 3) After that, the beaker was covered with plastic wrap and left at room temperature for 12 hours. The above solution was stirred at 250 rpm at 4°C for 2 hours without adding the amaranth leaf extract.

[0066] 4) Pour 5 g of the solution into a 95 mL Petri dish, cover the dish with plastic wrap and make small holes in the corners, and place the dish in a -80°C refrigerator for 3 hours.

[0067] 5) The culture dish is then placed in a freeze dryer and the vacuum is controlled below 0.1 m bar to freeze dry the PC-ALE-0 film, which is then stored at -20°C for later use.

[0068] Performance Testing

[0069] The prepared films were stored at different temperatures to investigate the response of their total color difference to temperature and time. The TVB-N value prediction feasibility, color change irreversibility, cold storage stability, light barrier properties, surface morphology, mechanical properties, thickness and biodegradability of the films obtained in Examples 1, 2, 3 and Comparative 1 were tested respectively.

[0070] The detection method of the indicator is as follows:

[0071] 1. Color response efficiency and chromaticity analysis of films

[0072] In order to study the colorimetric response effect of TTI film, the color change of the film (2×2 cm) was studied at different temperatures (-10, 4, 15, 25 and 35°C). The colorimetric values ​​of the TTI film (L*, a* and b*) based on the surface color were measured using a colorimeter, and the color change of the TTI film was photographed using a camera. Before the measurement, the colorimeter was calibrated using a white standard plate. The total color difference (ΔE) was calculated using the following formula:

[0073]

[0074] Among them, Li*, ai* and bi* are the chromaticity parameters of the standard white plate, while L, a and b are the chromaticity parameters of the sample film.

[0075] 2. Kinetic Modeling of Chromatographic Changes

[0076] To study the kinetic modeling of the TTI film, the total color difference (ΔE) values ​​over time at different temperatures (-10, 4, 15, 25, and 35°C) are shown. The colorimetric response of the time-temperature indicator F(X) is as follows:

[0077] F(X)=kdt

[0078] k is the reaction rate constant and t is the storage time of the reaction. By plotting the response value F(X) versus time, a straight line can be obtained; the slope of the line can be used to determine the value of k at different storage temperatures.

[0079] Temperature dependence of the reaction The temperature dependence of the reaction was calculated using the Arrhenius equation as follows:

[0080] In k=In(k0)-(E A / RT)

[0081] E A represents the activation energy of the reaction under study, and R is the universal gas constant (8.314 J mol -1 K -1 ).

[0082] 3. Determination of total volatile basic nitrogen (TVB-N)

[0083] To investigate the applicability of the indicator film on beef, beef samples were placed at the same temperature (-10, 4, 15, 25 and 35°C) as the TTI film for a certain period of time. In a digestive tube, 15 g of beef sample was homogenized with deionized water, and 1 g of magnesium oxide powder was added to the mixture. Finally, the TVB-N values ​​of different beef samples were determined using a nitrogen analyzer, expressed in mg / 100 g.

[0084] 4. Irreversibility test of TTI film color change

[0085] To examine the irreversibility of the TTI films, the films (2 × 2 cm) that had changed color at different temperatures (4, 15, 25, and 35 °C) were placed back at −10 °C for 1 day, and the chromaticity values ​​(L*, a*, and b*) were measured with a colorimeter (Osaka, Japan CR-400-Chroma meter), and photos were taken with a camera.

[0086] 5. Cold storage stability of TTI film

[0087] The stability of TTI films was determined by utilizing the prescribed method with minor modifications (Yar et al., 2024). Briefly, the stability of PC-ALE-based films was monitored based on color change. The films (2 × 2 cm) were stored at -15 °C for 20 days and the film color was monitored at 10-day intervals using a colorimeter.

[0088] 6. Scanning electron microscopy observation

[0089] The surface and cross section of the indicator film were scanned using SEM. The film sample was fixed on a bronze sample stage and a thin layer of gold was sprayed. The sprayed film was fixed on the SEM sample stage and the microstructure was observed at a voltage of 10 kV.

[0090] 7. Light transmittance test

[0091] The transmittance or opacity of the film sample (80×50 mm) was measured at 600 nm using a spectrophotometer. The data was calculated using the following formula:

[0092] Opacity = A / L

[0093] Where A represents the absorbance value at 600 nm, and L represents the sample thickness (nm).

[0094] 8. Mechanical properties and thickness test

[0095] The thickness of the indicator packaging film was measured using a digital handheld micrometer. Five measurements were taken at different locations on each film to obtain an average value. The tensile strength and elongation at break of the film were checked at room temperature using a texture analyzer. A 10×80 mm film strip was placed on a 50 mm measuring fixture and tested at a speed of 50 mm / min and a tensile force of 5 kN. TS and EAB were calculated as follows:

[0096]

[0097] Where F represents the tensile strength (N); X represents the sample thickness (mm); W is the sample width (mm); ΔL represents the increase in distance at break (mm); and L represents the initial length between the sample clamps (mm).

[0098] 9. Biodegradability of membrane

[0099] The biodegradation rate of SCG-based composite films was qualitatively measured in the collected soil. TTI film samples (2×2 cm) were cut and buried in the soil and sprayed with water every day until complete degradation. The biodegradation rate of the film was calculated using the following formula:

[0100]

[0101] Where M0 represents the original weight of the sample and M1 represents the weight after biodegradation. 2.6.12.

[0102] 10. Data processing

[0103] The various properties of the film were measured at least five times, and the data were expressed as mean ± standard deviation. The film data were statistically analyzed using statistical software SPSS16.0. One-way analysis of variance (ANOVA) was used to determine the differences between factors and levels. Duncan's test was used to determine the main differences between samples (p < 0.05).

[0104] The results of the indicators mentioned are as follows:

[0105] 1. Color changes of TTI at different temperatures

[0106] Figure 1 As shown in AE, there was no obvious change in the visual observation of comparative example 1 during storage, and the color of the film changed to varying degrees after adding amaranth leaf extract ( Figure 1 AE). For example, the TTI films of Comparative Example 1 and Examples 1 to 3 were placed at -10°C, and the color did not change within 15 days. Until 15-20 days, the TTI films of Examples 1 to 3 showed slight changes ( Figure 1 A). At 4°C, Examples 1 and 2 showed significant color changes within 2 days. The film of Example 3 changed to dark red due to the large amount of Amaranth leaf extract added and the high content of anthocyanins. Figure 1 B). At 15°C, Examples 1 and 2 changed from reddish purple to light brown within 12 hours and changed to yellow within 24 hours, while Example 3 changed to brown within 24 hours and changed to yellow within 48 hours ( Figure 1 C). At 25°C, Examples 1 and 2 changed from reddish purple to light brown within 3 hours and yellow within 12 hours, while Example 3 changed from brown within 9 hours and yellow within 12 hours ( Figure 1 D). At 35°C, Examples 1 and 2 turned light brown within 3 hours, turned yellow within 6 hours, and Example 3 turned yellow within 9 hours ( Figure 1 E).

[0107] The results in Table 1 also support the changes in the color (ΔE) values ​​of the TTI films at all temperatures. The color changes of the TTI films at all temperatures can be observed with the naked eye. These total color differences (ΔE) are critical for food manufacturers, distributors, and consumers to accurately determine the condition of food products. These results show that the TTI film with the addition of amaranth leaf extract can effectively indicate the cumulative changes in temperature and time.

[0108] Table 1. ΔE values ​​of TTI films changing with time at different temperatures

[0109]

[0110]

[0111] The values ​​are expressed as mean ± standard deviation (n = 5). Different letters in the same row indicate significant differences (p < 0.05).

[0112] 2. Irreversibility of TTI film color change

[0113] Irreversibility is an important characteristic of TTI films. The color change of irreversibility test is as follows Figure 1 F. After placing the film at -10°C for 1 day, it is easy to observe with the naked eye that the film that has changed color at different temperatures will not return to its original color or any other color. Comparison of the chromaticity values ​​in Table 1 and Table 3 also supports this conclusion. It can be confirmed that the color change of this TTI film is irreversible and will not return to its original color after the temperature is lowered again.

[0114] Table 2. Color change ΔE values ​​of TTI films after storage at -10°C for 1 day.

[0115]

[0116] Values ​​are expressed as mean ± standard deviation (n = 5).

[0117] 3. Storage stability test at -15℃

[0118] This study evaluated the color change of TTI indicator films after 10 and 20 days at -15°C. This is shown in Table 3. After 10 days, the color of the TTI film remained stable and did not change significantly during storage. After 20 days, slight differences appeared in the chromaticity values ​​(increase in L* and b* values, decrease in a* and ΔE values). This shows that the prepared TTI film can be stably stored at -15°C for at least 10 days.

[0119] Table 3. Colorimetric values ​​of TTI films stored at -15°C for 20 days.

[0120]

[0121]

[0122] Values ​​are expressed as mean ± standard deviation (n = 5).

[0123] 4. Activation energy of TTI film

[0124] like Figure 2As shown, the Arrhenius equation should be used to calculate the apparent activation energy of the total color difference change, and the activation energies of the films in Examples 1, 2 and 3 are calculated to be 115 kJ / mol, 49 kJ / mol and 78 kJ / mol, respectively. If the difference in activation energy values ​​between food system corruption and TTI is within the range of ±25 kJ / mol, TTI can be used to monitor the shelf life and quality of the food in the supply chain. Therefore, the indicator film of Example 1 can be used in food systems with activation energies in the range of 90-140 kJ / mol. Examples 2 and 3 can be used in food systems with activation energies in the range of 24-74 kJ / mol and 53-103 kJ / mol. The activation energy of beef TVB-N was measured to be 42 kJ / mol, while the activation energy of the film of Example 2 was 49 kJ / mol, indicating that the film of Example 2 can be used to track the TVB-N content of beef.

[0125] 5. Microstructure of TTI film

[0126] The microscopic surface morphology of different composite films (in micrometers) is as follows: Figure 3 As shown. The surface and cross section of the film in Comparative Example 1 are relatively smooth, defect-free, uniform and have a dense network structure. This means that polyvinyl alcohol and citric acid are mixed uniformly and show good compatibility. The smoothness of the surface or cross section of the film in Examples 1 and 2 is better than that in Comparative Example 1. The roughness of the film in Example 3 is slightly increased. This shows that the addition amount of Amaranth leaf extract in the range of 0.5-0.75g / 100mL has the effect of improving the overall smoothness of the film.

[0127] 6. Light blocking properties

[0128] Opacity is a key characteristic that represents the film's ability to act as a light barrier, since light is detrimental to food preservation to a certain extent. Figure 4 The results in B show that the addition of amaranth leaf extract significantly reduced the light transmittance and improved the light blocking ability of the film.

[0129] 7. Mechanical properties and thickness

[0130] The thickness of all TTI films ranged from 0.20 to 0.22 mm, and the introduction of Amaranth leaf extract did not significantly change the film thickness ( Figure 4 A). Compared with comparative example 1, the introduction of amaranth leaf extract can significantly improve the tensile strength of the TTI film ( Figure 4 C) and elongation at break ( Figure 4 D). This indicates that the addition of amaranth leaf extract can not only impart TTI function to the film but also significantly improve the mechanical properties of the film and enhance its mechanical strength.

[0131] 8. Biodegradability of membranes

[0132] In the material selection process, a factor that needs to be considered in particular is biodegradability. Compared with comparative example 1, the addition of amaranth leaf extract significantly improved the biodegradability of the film, and showed a concentration-dependent effect. After one week, the weight loss of the film in comparative example 1 was about 15%, while the weight loss of the film with the addition of amaranth leaf extract could reach 19%-24%. After three weeks, the weight loss of the film in comparative example 1 was 43%, while the weight loss of the film with the addition of amaranth leaf extract reached 47%-57% ( Figure 5 ). The results showed that the introduction of Amaranth leaf extract significantly enhanced the biodegradability of the film and improved the environmental friendliness of the film.

[0133] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still modify the technical solutions described in the aforementioned embodiments or replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0134] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a time-temperature indicating film using amaranth leaf extract as a raw material, characterized in that: The method comprises the following steps: 1) Take amaranth leaves, homogenize the leaf tissue in ethanol, and soak overnight; 2) filtering the extract after overnight to remove coarse particles, centrifuging, filtering to obtain the supernatant, removing ethanol by rotary evaporation, and freeze-drying the solution to obtain the amaranth leaf extract; 3) Mix the hydrochloric acid aqueous solution and ethylene glycol and stir evenly; 4) adding polyvinyl alcohol and citric acid to the mixed solution obtained in step 3), stirring until fully mixed, then heating and stirring to allow the polyvinyl alcohol and citric acid to fully react, and then keeping the temperature and standing until the bubbles on the top of the solution are removed; preferably, the stirring time is not less than 3 hours; 5) cooling the mixed system obtained in step 4) to room temperature, adding the amaranth leaf extract to the solution, and stirring to uniformly disperse the amaranth leaf extract in the solution; 6) storing the solution obtained in step 5) under refrigeration conditions below -15°C for not less than 3 hours; 7) The refrigerated solution obtained in step 6) is freeze-dried to obtain the time-temperature indicating film.

2. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: Step 1) The concentration of the ethanol solution is 95%, and the soaking process after homogenization is carried out at 4°C.

3. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: The centrifugation process in step 2) is carried out at 4°C.

4. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: In step 3), the pH of the hydrochloric acid aqueous solution is 1.5, and the volume ratio of the hydrochloric acid solution to ethylene glycol is 4:

1.

5. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: In step 4), the amount of polyvinyl alcohol added is 12 g / 100 mL, and the concentration of citric acid is 1 g / 100 mL; in step 4), the heating process needs to be heated to 90° C., with a rotation speed of 250 rpm and a time of 2 hours.

6. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: In step 5), the amount of the Amaranth leaf extract added is 0.2-1 g / 100 mL, and the stirring process in step 5) is carried out at 4°C.

7. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: Step 6) The solution is stored at a temperature below -80°C.

8. The method for preparing a time-temperature indicating film using an amaranth leaf extract as a raw material according to claim 1, characterized in that: Step 7) The freeze-drying condition is that the vacuum degree is controlled below 0.1 m bar after precooling.

9. A time-temperature indicating film made of amaranth leaf extract, characterized in that: The time-temperature indicating film is prepared by the preparation method according to any one of claims 1 to 8; Preferably, the time-temperature indicating film is cross-linked by ethylene glycol and citric acid to form a skeleton, and the amaranth leaf extract is used as the time-temperature indicator; Further preferably, the mass ratio of amaranth leaf extract, citric acid and polyvinyl alcohol as raw materials is 1-2:2:

24.

10. Use of the time-temperature indicating film according to claim 9 in the preparation of shelf life prediction of food, preferably, the food is a physical object that is sensitive to temperature and / or time, and the time-temperature indicating film plays an indicating role by directly observing its color change or measuring its total color difference value change. When exposed to an environment of 4-15°C, the color of the time-temperature indicating film changes from purple-red to khaki within 0.5-8 days, and when exposed to an environment of 25-35°C, the color of the time-temperature indicating film changes from purple-red to khaki within 3-12 hours.

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