Preparation method and application of intelligent developing antibacterial antioxidant multifunctional fresh-keeping film

Abstract

The invention discloses a preparation method and application of an intelligent developing antibacterial antioxidant multifunctional fresh-keeping film, and belongs to the field of food packaging materials, and the intelligent developing antibacterial antioxidant multifunctional fresh-keeping film is mainly prepared from water-soluble starch, polyvinyl alcohol, glycerol, water, pitaya anthocyanin and epsilon-polylysine hydrochloride according to a specific ratio. And after fully stirring and uniformly mixing, casting the mixed solution on an acrylic plate, and naturally drying to form the film. The prepared film shows excellent mechanical properties through the molecular synergistic effects of hydrogen bond crosslinking, electrostatic interaction and the like among the components. The synergistic effect of the antibacterial property and the antioxidant property of the preservative can better reduce the quality deterioration of food and prolong the shelf life of the food, and especially can significantly inhibit the quality deterioration and the edible safety problem caused by the proliferation and metabolism of putrefying bacteria and pathogenic bacteria in the cold-chain circulation of chilled meat. And the intelligent color development characteristic can monitor the freshness of food such as chilled meat in real time.

Description

Technical Field

[0001] This invention patent belongs to the field of food packaging materials, and specifically relates to a preparation method and application of an intelligent color-changing antibacterial, antioxidant and fresh-keeping film. Background Art

[0002] Chilled fresh meat retains relatively complete nutrients and sensory characteristics such as tender texture of fresh meat. However, the existing fresh-keeping method combining 0-4°C cold chain with traditional plastic packaging has limited fresh-keeping effect on meat, and it is difficult to prevent the quality deterioration caused by bacterial proliferation and enzymatic oxidation during the fresh-keeping process, resulting in loss of meat nutritional value, food waste and outbreaks of foodborne diseases. In addition, traditional physical and chemical detection, microbial detection and new spectral technology and other detection methods all have time or space limitations and cannot detect samples non-destructively and quickly. Therefore, the development of a functional active packaging film with both active fresh-keeping and real-time intelligent monitoring functions can be the key to breaking through the bottleneck of cold chain fresh-keeping technology. Summary of the Invention

[0003] The purpose of the present invention is to provide a preparation method and application of an intelligent color-changing antibacterial, antioxidant and multi-functional fresh-keeping film. The present invention innovatively uses a composite system of pitaya anthocyanin, ε-polylysine hydrochloride and polyvinyl alcohol-starch to prepare a fresh-keeping film with antibacterial, antioxidant and intelligent display of spoilage degree. The film exhibits excellent mechanical properties through optimizing the synergistic effect of components, its unique pH response characteristics can real-time indicate the spoilage degree of chilled fresh meat, and at the same time has broad-spectrum antibacterial and significant antioxidant functions, realizing visual monitoring of freshness while ensuring food safety.

[0004] Technical Solution: The present invention provides a preparation method of an intelligent color-changing antibacterial, antioxidant and multi-functional fresh-keeping film, including the following steps:

[0005] Step 1: Prepare a polyvinyl alcohol (PVA) mother liquor containing glycerol, a water-soluble starch solution, a pitaya anthocyanin solution and an ε-polylysine hydrochloride solution;

[0006] Step 2: Add the water-soluble starch solution to the polyvinyl alcohol (PVA) mother liquor to prepare a PVA-starch mixed solution;

[0007] Step 3: Add the pitaya anthocyanin solution and the ε-polylysine hydrochloride solution to the PVA-starch mixed solution, fully stir, and then form a film by solution casting on a flat device and naturally dry it.

[0008] Further, in the above Step 1:

[0009] (1) The preparation method of the polyvinyl alcohol (PVA) mother liquor is as follows:

[0010] Add PVA to deionized water, stir magnetically in a water bath at 60 °C, and add glycerol as a plasticizer during stirring to obtain a PVA mother liquor.

[0011] (2) The preparation method of the aqueous starch solution is as follows:

[0012] Add water-soluble starch to deionized water, stir magnetically in a water bath at 60 °C to obtain an aqueous starch solution.

[0013] (3) The preparation method of the pitaya anthocyanin solution is as follows:

[0014] Dissolve pitaya anthocyanin with deionized water, stir magnetically at room temperature until completely dissolved to obtain a pitaya anthocyanin solution, and place the solution in a brown bottle to avoid light.

[0015] (4) The preparation method of the ε-polylysine hydrochloride solution is as follows:

[0016] Dissolve ε-polylysine hydrochloride with deionized water, stir magnetically at room temperature until completely dissolved to obtain an ε-polylysine hydrochloride solution.

[0017] Further, in step two, the preparation method of the PVA-starch mixed solution is as follows:

[0018] Place the PVA mother liquor in a water bath at 60 °C and stir magnetically. While stirring, add the aqueous starch solution and continue to stir magnetically in the water bath for 2 h to finally prepare a PVA-starch mixed solution.

[0019] Further, in step three, stir the PVA-starch mixed solution magnetically in a water bath at 60 °C. While stirring, add the pitaya anthocyanin solution and the ε-polylysine hydrochloride solution, and then continue to stir for 20 minutes and then ultrasonically treat the blend solution for 5 minutes. In the obtained mixed solution:

[0020] The mass ratio of polyvinyl alcohol, water-soluble starch, pitaya anthocyanin, and ε-polylysine hydrochloride is 20:20:(5 - 10):(5 - 10), and the concentration of the polyvinyl alcohol is 20 g / L.

[0021] Further, the mass ratio of polyvinyl alcohol, water-soluble starch, pitaya anthocyanin, and ε-polylysine hydrochloride is 20:20:5:10.

[0022] Compared with the prior art, the present invention has the following significant advantages:

[0023] (1) The process is simple, efficient: The present invention adopts a one-step blending-casting forming process, which shortens the production cycle compared with the traditional multi-layer composite process.

[0024] (2) Excellent performance of the film-forming solution: The mixed solution after optimizing the proportion in the present invention has both ideal leveling property and coating adaptability.

[0025] (3) Outstanding comprehensive performance of the product: The film prepared by the method of the present invention not only maintains high light transmittance and mechanical strength, but also realizes the triple-functional integration of color development sensitivity, antibacterial efficiency and antioxidant activity through the molecular synergy of pitaya anthocyanin and ε-polylysine.

[0026] The present invention innovatively adds ε-polylysine hydrochloride with strong antibacterial efficacy and anthocyanin with indicating function into the film based on polyvinyl alcohol, and constructs an intelligent color-developing antibacterial and antioxidant multifunctional fresh-keeping film based on pitaya anthocyanin / ε-polylysine hydrochloride / polyvinyl alcohol-starch through intermolecular hydrogen bonds and electrostatic interactions, and significantly improves the physical and chemical properties of the film. The film added with ε-polylysine hydrochloride has significantly enhanced bactericidal property, and the addition of a certain concentration of anthocyanin makes the film have color-changing indication and antioxidant effects. The combination of its antibacterial property and antioxidant property can better extend the shelf life of chilled fresh meat, and the film can show different color changes in different pH environments, and it can be intuitively known whether the packaged food has started to deteriorate. This intelligent packaging technology integrating real-time monitoring and active fresh-keeping provides an innovative solution for the quality guarantee of fresh food. Description of the Drawings

[0027] Figure 1 Shows the color change of the freeze-dried powder solution in Example 1 within the range of pH 3 - 15;

[0028] Figure 2 Shows the colors of different films in Example 3;

[0029] Figure 3 Shows the L*, a*, b* and ΔE values of different films in Example 3;

[0030] Figure 4 Shows the light transmittance of different films in Example 3;

[0031] Figure 5 Shows the SEM surface observation images of different films in Example 3;

[0032] Figure 6 Shows the FTIR analysis of different films in Example 3;

[0033] Figure 7 Shows the X-ray diffraction patterns of different films in Example 3;

[0034] Figure 8 Shows the thermogravimetric analysis of different films in Example 3;

[0035] Figure 9DPPH radical scavenging ability of different thin films in Example 3;

[0036] Figure 10 Color change diagram of the response of the film to volatile trimethylamine in Example 3;

[0037] Figure 11 Color and color difference (△E) changes of the SP-2A1B film in Example 4;

[0038] Figure 12 Effect of the SP-2A1B film on the total viable count (TVC) of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP2A1B film;

[0039] Figure 13 Effect of the SP-2A1B film on the pH of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0040] Figure 14 Effect of the SP-2A1B film on the total volatile basic nitrogen (TVB-N) of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0041] Figure 15 Effect of the SP-2A1B film on the thiobarbituric acid reactive substances (TBARS) of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0042] Figure 16 Effect of the SP-2A1B film on the degree of proteolysis of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0043] Figure 17 Effect of the SP2A1B film on the weight loss rate of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0044] Figure 18 Effect of the SP-2A1B film on the cooking rate of chilled fresh meat. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP2A1B film;

[0045] Figure 19 Effect of the SP-2A1B film on the protein content of chilled fresh meat in Example 4. The control group was chilled fresh meat without treatment, and the experimental group was chilled fresh meat wrapped with the fresh-keeping SP-2A1B film;

[0046] Figure 20For the effect of the SP-2A1B film on the microstructure of chilled meat in Example 4, the control group was untreated chilled meat, and the experimental group was chilled meat wrapped with the fresh-keeping SP2A1B film;

[0047] Figure 21 For the effect of the SP-2A1B film on the sensory score of chilled meat in Example 4, the control group was untreated chilled meat, and the experimental group was chilled meat wrapped with the fresh-keeping SP2A1B film. Specific implementation mode

[0048] The technical solution of the present invention will be described in detail below through examples, but the protection scope of the present invention is not limited to the described examples.

[0049] Based on PVA matrix composite with water-soluble starch, ε-polylysine hydrochloride and pitaya pigment, the present invention constructs a film with both intelligent real-time color display, broad-spectrum antibacterial and superior antioxidant properties. By compounding functional substances such as antibacterial agents and antioxidants with polymer substrates, a bioactive packaging system is constructed, which can effectively inhibit the growth and reproduction of microorganisms and oxidation reactions in food, and can also realize the real-time monitoring of food quality through an intelligent response mechanism, thereby extending the shelf life of fresh food.

[0050] Polyvinyl alcohol (PVA), as a water-soluble polymer material with excellent mechanical properties and biocompatibility, its outstanding film-forming property and biodegradability make it highly favored in the packaging field. However, the melting temperature of PVA approaching the decomposition temperature due to the strong hydrogen bond interaction between molecular chains makes it difficult to be widely produced by conventional thermoplastic processing, seriously limiting the scope of its industrial application. Since both starch and PVA are hydrophilic materials, after mixing, they can form a dense network structure relying on the hydrogen bond interaction between hydroxyl groups. The mechanical properties, light transmittance, gas barrier properties, etc. of the formed starch-PVA composite are greatly improved. At the same time, this bio-based composite material has both degradability and processing adaptability, and has been widely used in fields such as food packaging, agriculture, and biomedicine, becoming an ideal choice to replace traditional petroleum-based packaging materials. ε-Polylysine hydrochloride, as a natural antibacterial peptide, has broad-spectrum antibacterial properties and good thermal stability. Therefore, applying ε-polylysine hydrochloride to the chilled meat food packaging system and evaluating its antibacterial effect on spoilage bacteria and pathogenic bacteria can provide a scientific theoretical basis for the industrial production of high-efficiency antibacterial packaging. Red pitaya is rich in betacyanin, and its unique pH response characteristic (turning yellow when encountering alkalinity) makes it possible to be used as an intelligent indicator to monitor the freshness of meat.

[0051] Unless otherwise specified, the reagents and methods used in the following examples were prepared and operated according to conventional methods; the reagents and raw materials used, except for pitaya pigment, were all commercially available. The indicator bacteria used in the experiments of the present invention, such as Pseudomonas lactic K-8, Pseudomonas kretschmeri C-16, Salmonella enterica L1, Salmonella typhimurium N20, Bacillus subtilis C2, Bacillus halmapalus N1, Staphylococcus aureus N1, Staphylococcus aureus N2, etc., were isolated from meat products by conventional methods and were all wild-type strains provided by Yangzhou University.

[0052] Example 1: Extraction of pitaya anthocyanins and determination of the properties of anthocyanins

[0053] (1) Extraction of red pitaya pigment and preparation of pigment freeze-dried powder (FPRE)

[0054] The pitaya peel was dried in an oven at 50 °C until constant weight. After drying, the raw material was crushed, passed through a 40-mesh sieve, and stored in a brown bottle in the dark. Then, the anthocyanin powder was mixed with 75% ethanol aqueous solution at a ratio of 1:10 g / mL, and extracted in a water bath for 3 h. The extract was filtered by suction, and ethanol was removed using a rotary evaporator to obtain an anthocyanin concentrate. The anthocyanin concentrate was loaded onto a macroporous adsorption resin chromatographic column, allowed to stand for 2 h, washed with distilled water to remove water-soluble impurities such as sugars, and then eluted with 70% ethanol solution to elute anthocyanins until the column was colorless. The eluate was concentrated under reduced pressure and then freeze-dried to obtain red pitaya anthocyanin freeze-dried powder.

[0055] (2) Determination of the total amount of betalains in the freeze-dried powder (FPRE)

[0056] The content of betanin in the pigment freeze-dried powder was determined by ultraviolet spectrophotometry. Prepare an FPRE aqueous solution of 3 - 10 g / L, scan it at a UV wavelength of 536 nm, and the content of betanin was calculated by the following formula:

[0057] Content of betanin [% or mg betanin / g FPRE] = (A × Mw) / kbc

[0058] Wherein, A is the absorbance value of the solution at 536 nm (the wavelength of the maximum absorption peak); Mw is the relative molecular mass of betanin (550.46 g / mol); k is the molar absorption coefficient of betanin (65000 L / mol×cm -1 ); b is the optical path length (1 cm); c is the concentration of the sample solution (g / L).

[0059] Analyze the content of betacyanin in the freeze-dried powder of red pitaya pulp pigment obtained. Using ultraviolet spectrophotometry, scan at an ultraviolet wavelength of 536 nm and substitute it into the Lambert-Beer law for calculation to obtain the result. Table 1 shows the maximum absorbance values of freeze-dried powder solutions with different concentrations at 536 nm, and the total amount of betacyanin pigment is calculated to be 1.215 mg / g according to the formula.

[0060] Table 1 Absorbance values of freeze-dried powder solutions with different concentrations at 536 nm

[0061]

[0062]

[0063] (3) Response of freeze-dried powder aqueous solution to pH change

[0064] Prepare 13 portions of 8 g / L FPRE aqueous solution, and prepare NaOH solutions and acetic acid (CH3COOH) solutions with different concentrations. Adjust the pH of the FPRE aqueous solution to 3.0 - 15.0 with the acid-base solutions. Record the color changes to judge the response of the pigment freeze-dried powder solution to pH change.

[0065] The results are shown as Figure 1 shown. The color of the freeze-dried powder aqueous solution will vary with the change of environmental pH. As the pH changes from 3 to 15, the solution color changes from red to purple and finally to yellow. When pH > 14, the solution color turns yellow because the betacyanin molecules hydrolyze into betaxanthin under alkaline conditions.

[0066] Example 2: Preparation of fresh-keeping film

[0067] Step 1. Prepare the basic solution.

[0068] (1) Preparation of polyvinyl alcohol mother liquor: Accurately weigh 4 g of PVA and place it in a beaker, add 100 mL of deionized water, place the beaker in a thermostatic heating magnetic stirrer with a temperature of 60 °C, adjust the rotation speed to 50 rpm, and add 1 mL of glycerol as a plasticizer while stirring magnetically in the water bath. After complete dissolution, finally obtain the PVA mother liquor.

[0069] (2) Preparation of aqueous starch solution: Accurately weigh 4 g of water-soluble starch and place it in a beaker, add 100 mL of deionized water, place the beaker in a thermostatic heating magnetic stirrer with a temperature of 60 °C, adjust the rotation speed to 50 rpm, and finally obtain the aqueous starch solution.

[0070] (3) Preparation of PVA-starch mixed solution: Place 50 mL of PVA mother liquor in a thermostatic heating magnetic stirrer with a collecting head at 60 °C, adjust the rotation speed to 50 rpm, and while stirring magnetically in a water bath, add 50 mL of aqueous starch solution to the PVA mother liquor, and stir magnetically in the water bath for 2 h to finally obtain the PVA-starch mixed solution.

[0071] (4) Preparation of pitaya anthocyanin solution: Dissolve two portions of 0.5 g and 1 g of pitaya anthocyanin with 1 - 3 mL respectively, stir magnetically at room temperature until completely dissolved, adjust the rotation speed to 30 rpm to obtain the pitaya anthocyanin solution, and place the solution in a brown bottle to avoid light.

[0072] (5) The preparation method of ε-polylysine hydrochloride solution is as follows: Dissolve 0.5 g and 1 g of ε-polylysine hydrochloride with 1 - 3 mL of water respectively, stir magnetically at room temperature until completely dissolved, adjust the rotation speed to 30 rpm to obtain the ε-polylysine hydrochloride solution, and place the solution in a conical flask for standby.

[0073] Step 2: Prepare polyvinyl alcohol-starch film, pitaya anthocyanin / polyvinyl alcohol-starch film, ε-polylysine hydrochloride / polyvinyl alcohol-starch film, and pitaya anthocyanin / ε-polylysine hydrochloride / polyvinyl alcohol-starch film.

[0074] (1) Preparation of polyvinyl alcohol-starch film:

[0075] After magnetically stirring the 100 mL PVA-starch mixed solution in a water bath for 20 minutes, ultrasonically treat the blend solution for 5 minutes to remove the bubbles in the solution, and finally obtain the polyvinyl alcohol-starch mother liquor. Cast the solution onto a 10 cm × 10 cm acrylic plate and naturally dry to obtain the film, denoted as SP. After peeling off the film, put them into bags for standby.

[0076] (2) Preparation of ε-polylysine hydrochloride / polyvinyl alcohol-starch film: Place 100 mL of PVA-starch mixed solution in a thermostatic heating magnetic stirrer with a collecting head at 60 °C, adjust the rotation speed to 50 rpm, and while stirring magnetically in a water bath, add the prepared ε-polylysine hydrochloride solution containing 0.5 g of ε-polylysine hydrochloride to it. After magnetically stirring in the water bath for 20 minutes, ultrasonically treat the blend solution for 5 minutes to remove the bubbles in the solution, and finally obtain the pitaya anthocyanin / polyvinyl alcohol-starch mother liquor. Cast the solution onto a 10 cm × 10 cm acrylic plate and naturally dry to obtain the film, denoted as SPA. After peeling off the film, put them into bags for standby.

[0077] (2) Preparation of pitaya anthocyanin / polyvinyl alcohol-starch film: Place 100 mL of PVA-starch mixed solution in a thermostatic heating magnetic stirrer with a heating jacket at 60 °C, adjust the rotation speed to 50 rpm, and while stirring magnetically in a water bath, add the prepared pitaya anthocyanin solution containing 0.5 g of pitaya anthocyanin. After stirring magnetically in the water bath for 20 minutes, ultrasonically treat the blended solution for 5 minutes to remove the bubbles in the solution. Finally, prepare the pitaya anthocyanin / polyvinyl alcohol-starch mother liquor, cast the solution on a 10 cm × 10 cm acrylic plate, and dry it naturally to obtain the film denoted as SPB. After peeling off the film, put them into bags for standby.

[0078] (4) Preparation of pitaya anthocyanin / ε-polylysine hydrochloride / polyvinyl alcohol-starch film: Place 100 mL of PVA-starch mixed solution in a thermostatic heating magnetic stirrer with a heating jacket at 60 °C, adjust the rotation speed to 50 rpm, and while stirring magnetically in a water bath, add the prepared pitaya anthocyanin solution and ε-polylysine hydrochloride solution. The mass ratios of pitaya anthocyanin and ε-polylysine hydrochloride are 1:2, 1:1, and 2:1 respectively. After stirring magnetically in the water bath for 20 minutes, ultrasonically treat the blended solution for 5 minutes to remove the bubbles in the solution. Finally, prepare the pitaya anthocyanin / ε-polylysine hydrochloride / polyvinyl alcohol-starch mixed solutions with different ratios. Cast the solution on a 10 cm × 10 cm acrylic plate, and dry it naturally to obtain the film. After peeling off the film, put them into bags for standby. When adding 1 g of ε-polylysine hydrochloride and 1 g of pitaya anthocyanin, the obtained film is denoted as SP-1A1B; when adding 0.5 g of ε-polylysine hydrochloride and 1 g of pitaya anthocyanin, the obtained film is denoted as SP-1A2B; when adding 1 g of ε-polylysine hydrochloride and 0.5 g of pitaya anthocyanin, the obtained film is denoted as SP-2A1B.

[0079] The specific formulations of different films in this example are as follows:

[0080] Table 2 Formulations for preparing different films

[0081]

[0082]

[0083] Example 3: Structural characterization of the film prepared in Example 2

[0084] 1. Color of the film

[0085] The color of the film is measured using a color difference meter. L* represents the lightness, and a* and b* are two components of chromaticity: Taking the standard white board as a reference, the total color difference ΔE is calculated according to the following formula.

[0086]

[0087] The results are as Figure 2 and Figure 3 shown. The SP and SP-A films have no color, while the films with added betanin are purple. Generally speaking, the addition of FPRE reduces the film brightness (L* value) and yellowness (b* value), but increases the redness (a* value) and total color difference (ΔE).

[0088] 2. Determination of film thickness and mechanical properties

[0089] The film thickness is measured using a digital display high-precision micrometer. Randomly select three points on the film for measurement, and take the average of the three measurement values as the film thickness, with the unit of mm. Calculate the tensile strength and elongation at break of the film according to the corresponding formula. Cut the film into strips of 20 mm × 10 mm. The parameters of the texture analyzer are set as the initial distance of the mechanical head is 15 mm and the moving speed is 60 mm / min. Determine the elongation at break (EB) and tensile strength (TS) of the film. Each group of films is measured 3 times repeatedly. Calculate according to the following formula:

[0090] TS = F / (w×d)

[0091] where, TS is the tensile strength, MPa; F is the maximum tensile force borne by the specimen at break, N; w is the width of the film, mm; d is the thickness of the film, mm.

[0092] EB = (L1 - L0)×100%

[0093] where, EB is the elongation at break, %; L1 is the tensile length of the film at break, mm; L0 is the initial clamping distance, mm.

[0094] As shown in Table 3 of the results, the addition of FPRE and ε-polylysine hydrochloride has little effect on the film thickness. The thickness of the composite film is uniform, the difference between groups is small, and the thickness value is stable at 0.05 mm. It is worth noting that the SP-A film is significantly thicker than the SP-B film, indicating that after adding FPRE, the internal structure of the film is denser. The mechanical strength and flexibility of packaging films are usually reflected by TS and EB respectively. As shown in Table 3, the TS and EB of the SP film are 18.50 MPa and 62.95% respectively. Adding FPRE and ε-polylysine hydrochloride improves the mechanical strength of the polyvinyl alcohol-starch film.

[0095] 3. Moisture content, water vapor transmission coefficient and swelling ratio of the film

[0096] Weigh a film with a mass of M1 (g) and put it into a forced-air drying oven at a temperature of 105 °C. After drying to a constant weight, take it out and weigh the mass as M2 (g). Conduct 3 parallel experiments, and calculate the moisture content (MC) according to the formula.

[0097] MC(%) = (M1 - M2) / M1

[0098] The moisture content of the SP film is 28.12%. The WVP value of the SP film is the highest because starch, PVA, and glycerol contain hydrophilic hydroxyl groups. The moisture content of the film with added FPRE and ε-polylysine hydrochloride is lower than that of the SP film, probably because the amino groups in ε-polylysine hydrochloride and the phenolic hydroxyl groups in the anthocyanin extract form more hydrogen bonds with the hydroxyl groups of the film-forming matrix, making the internal structure of the composite film more stable and inhibiting the cross-linking with water molecules, resulting in a significant reduction in moisture content.

[0099] Wrap the film in a 50 mL centrifuge tube containing 20 mL of distilled water and place it in a desiccator containing silica gel at 22 °C. Weigh it three times every 1 day to measure three parallel samples, and calculate the water vapor permeability coefficient (WVP) according to the formula.

[0100] WVP = (Δm × x) / (A × ΔP × t)

[0101] Where x is the film thickness, in mm; A is the effective permeation area of the film, in m 2 ; Δm is the mass of water permeated, in g; t is the time interval, in s; ΔP is the pressure difference across the film, ΔP = 3179 Pa (22 °C).

[0102] The water vapor permeability coefficient should be as small as possible to block the exchange of water vapor between food and the outside world. The water vapor permeability coefficient of the SP film is the largest, which is 2.66×10 -9 g / (m·s·Pa). The water vapor permeability rate of the film with added FPRE and ε-polylysine hydrochloride is lower than that of the SP film, indicating that the dense structure of the composite film can reduce the water vapor permeability rate.

[0103] The swelling rate is measured by immersing the film sample (2 cm × 2 cm) in distilled water at room temperature for 30 minutes. After 30 minutes, wipe off the surface water with filter paper and immediately weigh the swollen film sample three times to measure three parallel samples. The swelling rate of the film is calculated by the following formula:

[0104] Swelling rate (%) = (W2 - W1) / W1

[0105] Where W2 is the weight of the swollen film sample and W1 is the initial weight of the film sample.

[0106] Swelling degree is an important property of packaging films. The swelling degree of the film is affected by the hydrophilicity of the film, the pH value of the medium, and the environmental temperature. As shown in Table 3, the swelling rate of the SP film is the highest (175.95%), because both starch and PVA are hydrophilic polymers and are prone to water absorption. Adding FPRE significantly reduces the swelling rate of the film. The main reason for the decrease in the film swelling rate is that the structure between anthocyanin and the film matrix is tighter, and the intermolecular interaction is enhanced. It is worth noting that the swelling rate of the SP-B film (152.93%) is lower than that of the SP-A film (165.49%), which is because the intermolecular interaction between FPRE and the film matrix is stronger.

[0107] Table 3 Mechanical properties and waterproof properties of different films

[0108]

[0109] 4. Light transmittance properties of the film

[0110] The light transmittance of the film is used to characterize the light transmittance performance of the film. Specifically, the composite film is cut into a size of 4 cm × 1 cm, and the film is directly pasted into a cuvette, scanned in the wavelength range of 300 nm to 800 nm, and the ABS (absorbance) value of the film placed inside the quartz cuvette is put in and its light transmittance is calculated according to the formula.

[0111] T = (1 - A) × 100%

[0112] The results are as Figure 4 shown. When the film-packaged food is exposed to ultraviolet and visible light, it is easily oxidized, resulting in nutrient loss, color change, and peculiar smell. Therefore, the light transmittance is particularly important for the packaging film. The transparency of the film is further characterized by its ultraviolet-visible light transmittance. The SP film has a relatively high light transmittance in both the ultraviolet and visible light ranges because there are few chromophores in this film. Since anthocyanin (such as C = C and C = O) contains rich chromophores, adding FPRE can effectively reduce the ultraviolet-visible light transmittance of the film. Among all the films, the SPA1B1 film has the highest light transmittance, probably because the structure between this ratio of FPRE and ε-polylysine hydrochloride and the film matrix is not tight enough, and the following microstructure also confirms this view. The improvement of the ultraviolet-visible light barrier ability of the film is beneficial to protecting foods rich in proteins and lipids from light-induced oxidation.

[0113] 5. Structural characterization of the film

[0114] Observation was carried out using a GeminiSEM 4800 scanning electron microscope at 2 kV. Fourier transform infrared (FT-IR) spectroscopy tests were performed in the attenuated total reflection mode on a Varian 670 spectrometer, and the recording range was from 400 to 4000 cm -1 , with a resolution of 2 cm-1 , the number of scans was 32 times. X-ray diffraction (XRD) tests were performed on a D8 Advance diffractometer to determine the crystal properties of the films using Ni-filtered CuKα radiation at 40 kV voltage and 40 mA current, and the recording range was from 5 to 40. The results are as Figures 5 - 7 shown.

[0115] Figure 5 Figure shows the SEM surface observations of different films. Among them, (a) represents SP, (b) represents SP-A, (c) represents SP-B, (d) represents SP-1A1B, (e) represents SP-1A2B, and (f) represents SP-2A1B. The microstructure results show that due to the certain incompatibility between starch and PVA, the surface of the SP film is rough and phase-separated. Adding FPRE and ε-polylysine hydrochloride has an impact on the microstructure of the SP film, while in contrast, the pitaya extract makes the surface of the film a little smoother. This indicates that FPRE has better compatibility with the film matrix than ε-polylysine hydrochloride. By adding different ratios of FPRE and ε-polylysine hydrochloride, it is found that the surface is smoother when the ratio of FPRE to ε-polylysine hydrochloride is 1:2, while cracks appear when the ratio is 1:1.

[0116] Figure 6 FTIR analysis of different films. The infrared spectrum results show that adding FPRE and ε-polylysine hydrochloride does not significantly change the Fourier transform infrared spectrum of the film, indicating that neither of them reacts chemically with the film matrix.

[0117] Figure 7 X-ray diffraction patterns of different films. The crystallinity of the films can reflect the compatibility and intermolecular interactions between different film components, and the structural characteristics of the films are determined by XRD analysis. The SP film has a sharp diffraction band at 19.8°, corresponding to the characteristic crystallization band of PVA. Adding FPRE or FPRE and ε-polylysine hydrochloride will to a certain extent reduce the intensity of the crystallization band of the film, which is due to the intermolecular interactions promoting the dispersion of PVA and interfering with the rearrangement and crystallization of PVA.

[0118] 6. Thermal stability of the films

[0119] Thermogravimetric analysis (TGA) was carried out on a Pyris1 instrument from 30 to 800 °C. The film samples (2 mg) were tested at a rate of 20 mL / min in a nitrogen atmosphere.

[0120] The results are as Figure 8As shown, it can be seen from the figure that each curve begins to have a slight weight loss around 100 °C, which may be due to the volatilization of moisture or other volatile substances in the sample. Between approximately 200 °C and 300 °C, each curve begins to decline significantly, indicating that the main thermal decomposition or mass loss process of the sample begins within this temperature range. At 600 °C, the residual weight of the composite film with both FPRE and ε-polylysine hydrochloride increased is relatively high, while the residual weight of the composite film with only FPRE or ε-polylysine hydrochloride increased is relatively low, which indicates that the composite film with both FPRE and ε-polylysine hydrochloride may form more stable residues or have better thermal stability at high temperatures. And between 300 °C and 400 °C, the slope of the composite film with only FPRE or ε-polylysine hydrochloride increased is relatively large, indicating that the mass loss rate is relatively fast and the thermal stability is relatively poor in this temperature range; while the slope of the composite film with both FPRE and ε-polylysine hydrochloride increased is relatively small, indicating that its mass loss rate is slower and the thermal stability is better. This shows that the addition of FPRE and ε-polylysine hydrochloride improves the stability of the film.

[0121] 7. Determination of Film Functional Properties

[0122] (1) Antioxidant Activity

[0123] The film was cut into 20 mg specimens respectively and the film specimens were soaked in 10 mL of ethanol solution for 24 h for use. Accurately weigh 9.85 mg of DPPH and make up the volume to 250 mL with ethanol in a volumetric flask to prepare a 0.1 mM DPPH ethanol solution. Mix 2 mL of the film extract solution with 2 mL of the DPPH ethanol solution, and the control group is the mixture of 2 mL of ethanol and 2 mL of the DPPH ethanol solution. Shake the mixture well and let it stand in the dark for 30 min. The blank sample is ethanol, and then the absorbance value is measured using a UV-visible spectrophotometer under the condition of 517 nm. The experiment was repeated three times in parallel, and the experimental results were expressed as the mean ± standard deviation. The antioxidant activity of the film was calculated by the following formula:

[0124] DPPH radical scavenging rate (%) = (A0 - A1) / A0

[0125] Where, A1 is the absorbance measured for the sample; A0 is the absorbance measured for the blank.

[0126] The results are as Figure 9As shown, DPPH is a stable free radical with strong oxidizing properties, centered around nitrogen, and is a commonly used method for detecting the antioxidant capacity of substances. It can be seen from the figure that after the addition of FPRE, the antioxidant performance of the packaging film also increases accordingly. This is because anthocyanin is a type of flavonoid polyphenol substance, and the phenolic hydroxyl groups and aromatic ring structures carried by this type of substance can prevent free radical chain reactions by forming phenoxy rings, playing an antioxidant role. Moreover, when the ratio of FPRE to ε-polylysine hydrochloride is 1:1, the antioxidant effect is the best.

[0127] (2) Antibacterial activity

[0128] Place the film slices (6 mm in diameter) on a soy casein agar plate inoculated with 0.1 mL of bacteria at 106 CFU / mL, and culture the bacteria at 37 °C for 16 h to evaluate the antibacterial activity of the film against foodborne spoilage or pathogenic bacteria. Record the formed inhibition zone. Repeat the experiment three times for each film.

[0129] Pseudomonas, Staphylococcus aureus, Salmonella, and Bacillus are common spoilage or pathogenic bacteria in chilled meat. The growth and reproduction of these microorganisms are the main reasons for the decline in the freshness and spoilage of chilled meat. Therefore, these four bacteria are selected to test and evaluate the antibacterial performance of the packaging film. The results show that ε-polylysine hydrochloride has significant antibacterial properties. This is because the polycationic ε-polylysine hydrochloride mainly exerts its antibacterial effect through two ways: destroying the cell membrane function by antibacterial peptides and changing the fluidity and permeability of the cell membrane through electrostatic interaction, even causing the phospholipid bilayer of the cell membrane to bend and perforate, resulting in the rupture of the membrane structure and the damage of membrane integrity, thereby destroying the cell membrane function and interfering with normal metabolism by affecting intracellular biochemical reactions. When FPRE is increased alone, there is no antibacterial effect, and when the ratio of ε-polylysine hydrochloride to FPRE is 1:1, the antibacterial effect is the best.

[0130] Table 4 Comparison of antibacterial abilities of different films

[0131]

[0132]

[0133] (3) Ammonia-sensing performance

[0134] Based on the previous experimental results, taking physical properties, functional properties, etc. as indicators, the optimal addition amount for preparing the composite film is determined as the ratio of ε-polylysine hydrochloride to pitaya anthocyanin being 2:1. Place 80 mL of trimethylamine in a 250 mL conical flask, and suspend the film (SP-2A1B, 20 mm × 20 mm) 5 cm above the liquid surface. Use a scanner to collect the image information of the film every 15 min for a total of 45 min.

[0135] The results are as follows Figure 10 As shown, over time, the concentration of volatile trimethylamine in the container increases, the alkalinity of the film exposed to its surface enhances, and the maximum absorbance of the film gradually decreases. At the same time, the color changes from red to purple and finally to yellow. This color change may be consistent with the reason for the color change of betacyanin at different pH values.

[0136] Example 4: Application of the thin film in pork

[0137] Based on the previous experimental results, taking physical properties, functional properties, etc. as indicators, the optimal addition amount for preparing the composite film is determined as the ratio of ε-polylysine hydrochloride to FPRE being 2:1, and the actual effect of the composite film (SP-2A1B) is investigated.

[0138] 1. Treatment of pork samples

[0139] Step (1): Fresh pork was purchased from the local market. The meat samples and the thin film need to be exposed to ultraviolet light for 30 min in advance for sterilization treatment.

[0140] Step (2): The treatment for indication is as follows: In a laminar flow hood, the pork was prepared into cubes of about 5 g using a disinfected chopping board and kitchen knife. It was placed in a transparent square box, and the lids of each box were tightened. An indicator film was attached to the lid. In addition, one group was a blank control group without any treatment. The treated pork was stored in an incubator at 4°C for 8 d, and the spoilage indicators of the pork were measured every 2 d.

[0141] Step (3): The treatment for preservation is as follows: In a laminar flow hood, the pork was cut into 20 g with a disinfected chopping board and kitchen knife and then put into a pre-prepared sterile bag. One group was wrapped with a 10 mm × 10 mm thin film, and the four corners were folded to ensure that the thin film completely wrapped it (experimental group), and the other group was not treated at all (control group). The treated pork was stored in an incubator at 4°C for 8 d, and the spoilage indicators of the pork were measured every 2 d.

[0142] In the following tests, the color analysis of the thin film used the thin film treated in the manner of step (2), and the remaining tests used the thin film treated in the manner of step (3).

[0143] 2. Color analysis of the thin film

[0144] Every 2 d, the color change parameters of the film were measured with a color difference meter, the color difference ΔE of the film was calculated, and the image information of the indicator film was photographed and recorded. The results are as follows Figure 11 shown.

[0145] 3. Change in total viable count (TVC) of pork

[0146] Total bacterial count is an indicator used to determine the degree of bacterial contamination and hygienic quality of food. The detection method refers to GB 4789.2—2016 National Food Safety Standard - Determination of Total Bacterial Count in Food Microbiological Examination.

[0147] The results are as Figure 12 shown. The total bacterial count reflects the degree of microbial contamination of food. The relevant national standards stipulate the relationship between the freshness of chilled meat and the total microbial colony count: first-class freshness ≤ 4 lg CFU / g; second-class freshness is 4 - 6 lg CFU / g; spoiled meat ≥ 6 lg CFU / g. The total bacterial counts of both groups of chilled fresh pork increased with the extension of refrigeration time. The initial TVC of chilled fresh pork was 2.37 lg CFU / g, and the meat quality was in a fresh state. Within 0 - 6 days of storage, the total bacterial count increased. At 6 days of storage, the TVC of the control group reached 5.98 lg CFU / g, and that of the experimental group reached 5.21 lg CFU / g, and the meat quality was in a sub-fresh state. At 8 days of storage, the TVC of the control group increased rapidly to 6.57 lg CFU / g, indicating that the meat had spoiled, while the TVC of the experimental group was 5.64 lg CFU / g, still in a sub-fresh state.

[0148] 4. Determination of Pork Chromaticity

[0149] After cutting the chilled fresh pork samples of the control group and the experimental group into small pieces, immediately use a CR-400 full-automatic color difference meter to measure the fresh meat color indicators during refrigeration, and record them as L* (light-dark), a* (red-green), and b* (yellow-blue). Each result is in triplicate (n = 3).

[0150]

[0151] The L* value represents brightness, with higher L* values indicating greater gloss. Table 5 shows the changes in glossiness of fresh pork during storage in each treatment group. The glossiness of the four groups of fresh meat gradually decreased with increasing storage time, with the rate of decline in the experimental group significantly less than that of the control group. This suggests that the plastic wrap acts as a protective barrier for the fresh meat, reducing its surface contact with air and inhibiting the growth and proliferation of spoilage bacteria, thereby reducing oxidative discoloration. The a* value represents redness, which is primarily determined by the color and oxidation state of myoglobin in the meat. As shown in Table 5, the redness values of the control group varied significantly over the entire refrigerated storage period from 0 to 8 days, while the experimental group showed less variation compared to the control group. This suggests that the use of this composite film in preserving fresh meat creates a barrier on the surface of the meat, prolonging color stability and slightly slowing the degree of oxidation. The b* value represents yellowness; high b* values indicate increased oxidation and bacterial growth. Table 5 shows the changes in yellowness of the fresh pork in each treatment group during storage. It can be seen that the yellowness values of the two groups of fresh pork gradually increased with the extension of storage time, and the yellowness value growth rate of the experimental group was significantly lower than that of the control group. This shows that the plastic wrap can protect the color of fresh pork.

[0152] Table 5 Chromaticity changes of fresh meat during storage days

[0153] Number of days L*(Control group) L*(Experimental group) a*(Control group) a*(Experimental group) b*(Control group) b*(Experimental group) 0d <![CDATA[54.66 a > <![CDATA[54.66 a > <![CDATA[15.62 a > <![CDATA[15.62 a > <![CDATA[6.20 a > <![CDATA[6.20 a > 2d <![CDATA[53.14 b > <![CDATA[53.95 b > <![CDATA[13.93 b > <![CDATA[14.00 b > <![CDATA[6.80 a > <![CDATA[5.58 ab > 4d <![CDATA[51.16 c > <![CDATA[52.45 c > <![CDATA[12.41 c > <![CDATA[13.36 bc > <![CDATA[8.97 a > <![CDATA[5.74 bc > 6d <![CDATA[48.80 d > <![CDATA[51.90 c > <![CDATA[11.75 cd > <![CDATA[13.68 bc > <![CDATA[9.42 b > <![CDATA[7.00 c > 8d <![CDATA[47.35 e > <![CDATA[49.58 d > <![CDATA[11.06 d > <![CDATA[12.76 c > <![CDATA[9.99 b > <![CDATA[7.70 c >

[0154] 5. Determination of pH value of pork

[0155] Refer to GB 5009.237-2016 "National Food Safety Standard - Determination of pH Value of Food". pH value can be used as an indicator to evaluate the freshness of meat. The relevant national standards stipulate the relationship between meat freshness and pH: first-level freshness pH 5.80-6.20; second-level freshness pH 6.30-6.60; spoiled meat pH 6.70 or above. Figure 13 As shown, the pH values of both groups of samples initially decreased and then increased over time during storage. This may be because during the initial storage period of chilled meat, muscle glycogen undergoes anaerobic fermentation, producing lactic acid, which lowers the pH of the meat. Subsequently, microbial activity degrades proteins and amino acids, producing amines, which gradually increase the pH of the meat. The initial pH of chilled pork was 5.91 on day 0. By day 8, the pH of the experimental group samples reached 6.27, while the pH of the control group had reached 6.67, approaching spoilage. These results indicate that treating chilled meat with cling film can extend its shelf life.

[0156] 6. Determination of Volatile Basic Nitrogen in Pork

[0157] TVB-N is one of the commonly used indicators for evaluating the freshness of meat. The detection method refers to GB 5009.228—2016 "National Food Safety Standard - Determination of Volatile Basic Nitrogen in Foods". The TVB-N value can be used as an indicator to evaluate the freshness of meat. The relationship between the freshness of meat and the content of volatile basic nitrogen is stipulated in relevant national standards: for first-class fresh meat, TVB-N ≤ 15.0 mg / 100 g; for second-class fresh meat, 15.0 mg / 100 g < TVB-N ≤ 25.0 mg / 100 g; for spoiled meat, TVB-N > 25.0 mg / 100 g. The results are as Figure 14 shown. The TVB-N values of the two groups of chilled fresh pork both showed an upward trend. Due to the increase in alkaline substances produced by the decomposition of proteins in meat by bacteria, the TVB-N value increased. At day 0, the initial TVB-N value of the chilled fresh pork was 5.74 mg / 100 g. In the control group, it rose to 21.82 mg / 100 g after 6 days of storage, being in the sub-fresh level, and reached as high as 25.28 mg / 100 g after 8 days of storage, indicating that the meat quality had deteriorated. The generation and accumulation of alkaline nitrogen compounds would lead to the increase of TVB-N. While the TVB-N value of the experimental group was 21.70 mg / 100 g, and after being treated with fresh-keeping film, the rising speed of the TVB-N value was slower than that of the control group. The test results showed that using fresh-keeping film to treat chilled fresh pork could achieve good fresh-keeping effects.

[0158] 7. Determination of Thiobarbituric Acid Reactive Substances (TBARS) in Pork

[0159] The effect of the packaging film on the lipid oxidation during the storage of pork was evaluated by measuring the amount of 2-thiobarbituric acid reactive substances (TBARS). 5 g of samples at each sampling time for each group (2 g of the surface and 3 g of the interior) were homogenized with 25 mL of trichloroacetic acid solution and centrifuged (12,000×g, 5 minutes, 4 °C). 5 mL of the filtrate was taken and the same volume of 0.02 mol of 2-thiobarbituric acid solution was added. After heating in a water bath at 90 °C for 40 min and then cooling, it was centrifuged at 3000 r / min for 10 min, and the supernatant was taken. After adding 5 mL of chloroform and standing for stratification, the supernatant was taken, and the absorbances were measured at wavelengths of 532 nm and 600 nm respectively.

[0160] TBARS value (mg / kg) = {(A532 - A600) / 155} × 0.1 × 72.6 × 1000

[0161] In the formula, 155 is the molar extinction coefficient; A532 and A600 are the absorbances at wavelengths of 532 nm and 600 nm respectively

[0162] The TBARS value is generally used to measure the degree of lipid oxidation in meat. The higher the degree of lipid oxidation in fresh meat, the higher the TBARS value. Generally, if the TBARS value is greater than 1 mg / kg, it is considered that the meat product has deteriorated. The results are as Figure 15As shown, during the refrigeration process, with the extension of refrigeration time, the TBARS values of both the control group and the experimental group showed an upward trend. The initial TBARS was 0.27 mg / kg, and the pork was fresh. As the storage time increased, the TBARS increased significantly. On the 6th day, it was 1.01 mg / kg, exceeding the threshold, indicating that the pork had developed an off-odor. Due to the addition of the fresh-keeping film on the surface of the pork, the oxidation rate was slowed down, resulting in a slow increase in the TBARS value of the experimental group and maintaining a fresh state during storage, indicating that the composite biological preservative can effectively alleviate the oxidative decomposition of fat in chilled fresh pork.

[0163] 8. Determination of the texture of pork

[0164] The texture analyzer was used in the TPA mode to measure the meat samples. Parameter settings: The detection probe was selected as P / 36R; the speeds before, during, and after the measurement were all 1.0 mm / s; the test time interval was 5 s; the trigger force was 5 g; the data acquisition rate was 400 PPS; the strain was 75%. The hardness and elasticity of the meat samples were measured.

[0165] The results are shown in Table 6. Texture characteristics generally include hardness, elasticity, adhesiveness, etc., which are important edible quality indicators of meat products. The initial texture indicators of the pork were hardness 150, elasticity 3.23, and gumminess 35.3, indicating good initial texture characteristics of the chilled fresh pork. During the storage period, the hardness and elasticity of the two groups of meat samples generally showed a downward trend, which may be because the main structural proteins affecting the texture of pork, such as myosin, actin, and myogen, were decomposed and affected by microorganisms. While the gumminess generally showed an upward trend, which may be due to the denaturation of proteins in the chilled fresh pork with the extension of storage time, resulting in a decrease in the water-holding capacity of the pork and a decrease in the binding force between cells, thus leading to a decrease in adhesiveness. In addition, the change ranges of hardness, elasticity, and gumminess in the experimental group were smaller than those in the control group, and the elasticity of the experimental group was significantly higher than that of the control group, and the gumminess was significantly lower than that of the control group. Therefore, the fresh-keeping film has a positive effect on maintaining the texture characteristics of chilled fresh pork.

[0166] Table 6 Changes in texture characteristics of chilled fresh meat during storage

[0167]

[0168] 9. Determination of the degree of protein hydrolysis in pork

[0169] The degree of protein hydrolysis in the meat was analyzed by SDS-PAGE method. After taking the samples, the samples were boiled in boiling water for 5 min and then analyzed. Among them, the conditions of SDS-PAGE were: the concentration of the separating gel was 12%, and the concentration of the stacking gel was 4%. After the electrophoresis was completed, it was stained with 1% Coomassie Brilliant Blue and then decolorized with the decolorizing solution (methanol: glacial acetic acid: water = 1:1:8) on a shaker until the background was clear.

[0170] The results are as follows Figure 16 as shown Figure 6 In Figure 6 , note: Lane 1: Marker; Lane 2: Untreated chilled fresh meat at 0 h; Lane 3: Chilled fresh meat after 2 days without treatment; Lane 4: Chilled fresh meat after 2 days treated with plastic wrap; Lane 5: Chilled fresh meat after 4 days without treatment; Lane 6: Chilled fresh meat after 4 days treated with plastic wrap; Lane 7: Chilled fresh meat after 6 days with treatment; Lane 8: Chilled fresh meat after 6 days treated with plastic wrap; Lane 9: Chilled fresh meat after 8 days with treatment; Lane 8: Chilled fresh meat after 8 days treated with plastic wrap. It can be seen from the figure that the degree of proteolysis of chilled fresh meat at 0 d is the lowest, and the degree of proteolysis in other lanes is moderate, showing different degrees of band diffusion. With the increase of storage time of chilled fresh meat, obvious proteolysis phenomenon begins on the 6th day. However, after the chilled fresh meat is treated with plastic wrap under low temperature conditions, it can be clearly seen that the degree of proteolysis in the chilled fresh meat decreases. The above results prove the role of the plastic wrap prepared in this study in preserving and fresh-keeping chilled fresh meat.

[0171] 10. Weight loss rate of pork

[0172] The initial mass of pork is denoted as m1. After storage for the corresponding time, take it out, dry the water on the surface of the pork, and record the weight as m2. The weight loss rate is calculated according to the following formula

[0173] Weight loss rate = {(m1 - m2) / m1} × 100%

[0174] The results are as follows Figure 17 as shown. During the refrigeration process, the weight loss rate of chilled fresh pork increases with the extension of refrigeration time. The initial juice loss rate of the control group is 1.70%, and the weight loss rate of the control group is 5.51% at 8 days, while the weight loss rate of the experimental group is 4.11%, and the increasing amplitude is smaller than that of the control group. It may be that the fresh-keeping film forms a protective barrier on the surface of chilled fresh pork, reducing the juice loss on the meat surface.

[0175] 11. Cooking rate of pork

[0176] Cut the chilled fresh meat into pieces about 3 cm × 3 cm × 6 cm, weigh the mass m1 respectively, put them into a cooking bag, evacuate the air and seal it, heat it in a water bath at 85 °C for 30 min, take it out and cool it to room temperature at normal temperature, and then weigh the mass m2.

[0177] Cooking loss rate = {(m1 - m2) / m1} × 100%

[0178] The water loss rate of muscle reflects the level of water-holding capacity of muscle. The higher the water loss rate, the smaller the water-holding capacity, the worse the water-holding property, and the worse its edible quality. Figure 18Shown is the effect of the fresh-keeping film on the cooking loss rate of chilled fresh pork. As can be seen from the figure, during the refrigeration process, the cooking loss rate of chilled fresh pork generally shows an upward trend. The initial cooking loss rate is 25.68%, the control group reaches 34.31% after 8 days, and the experimental group reaches 32.23% after 8 days. After using the fresh-keeping film, the increase in the cooking loss rate is slower than that of the control group, indicating that the fresh-keeping film slows down the increase in pH value and the water loss rate.

[0179] 12. Determination of the protein content of pork

[0180] The protein quantification was determined using a kit purchased from Nanjing Jiancheng Bioengineering Institute according to the instructions provided by the supplier. The results are as Figure 9 shown. As can be seen from Figure 19 it, during the refrigeration process of chilled fresh pork, the protein content of both groups shows an upward trend and tends to be stable in the later stage. The initial protein concentration is 0.67 μg / mL, the control group reaches 2.58 μg / mL after 8 days, and the experimental group reaches 2.13 μg / mL after 8 days, with a slower increase rate compared to the control group. The larger the value, the more the myofibrillar protein is degraded, the more severely damaged it is, and the more incomplete the internal structure is. The myofibrillar protein of chilled donkey meat after using the fresh-keeping film degrades slowly, and the degree of damage is lighter than that of the control group at 8 days.

[0181] 13. Microstructure of pork

[0182] The meat samples were cut into small pieces of 3×3×3 mm perpendicular to the muscle fiber direction and soaked overnight (4°C, 24 h) in 2.5% glutaraldehyde (pH 6.8). First, soak and wash 3 times with phosphate buffer at pH 6.8, 10 min each time. Then dehydrate with 30%, 50%, 70%, 80%, 90%, 95% ethanol respectively, and dehydrate with absolute ethanol twice, 10 min each time, for a total of 3 times. Finally, defat with amyl acetate for 1 h, and perform displacement with absolute ethanol:tert-butanol = 3:1, 1:1, 1:3 and tert-butanol for 15 min each. Observe with a scanning electron microscope after vacuum freeze-drying. The results are as Figure 20 shown, where (a) represents the microstructure diagram of pork on the slaughter day, (b) represents the microstructure diagram of pork stored for 4 days after being wrapped with SP2A1B film, (c) represents the microstructure diagram of pork stored for 4 days without being wrapped with fresh-keeping film, (d) represents the microstructure diagram of pork stored for 8 days after being wrapped with SP2A1B film, and (e) represents the microstructure diagram of pork stored for 8 days without being wrapped with fresh-keeping film (magnification 500 times).

[0183] As can be seen from the figure, immediately after slaughter, the muscle presents a typical complete sarcomere structure, with tightly arranged myofibrils, extremely small cell gaps, an intact sarcolemma, and no signs of protein hydrolysis or microbial colonization. Compared with the samples without plastic wrap treatment, the muscle tissues of the two groups of samples wrapped with plastic wrap are more complete, the muscle fibers are clear and neat, and there is no obvious separation of connective tissue. This indicates that plastic wrap can basically maintain the characteristics of the muscle tissue of chilled fresh meat and reduce the dispersion of muscle tissue.

[0184] 14. Sensory evaluation of pork

[0185] It was determined with reference to GB / T22210-2008 "Sensory Evaluation Specification for Meat and Meat Products". The sensory evaluation was carried out by a sensory evaluation panel consisting of 5 males and 5 females, and the chilled fresh pork was scored separately in terms of color, odor, texture, elasticity and overall acceptability. Each index is divided into 5 grades (fresh, relatively fresh, average, spoiled and severely spoiled), with a full score of 10 points. Those below 6 points are regarded as unqualified, the total score is 50 points, and those below 30 points are regarded as inedible. The specific scoring criteria are shown in the following table.

[0186] Table 7 Changes in sensory scores of chilled fresh meat during storage

[0187]

[0188]

[0189] Sensory evaluation is a method for grading the freshness of chilled fresh meat by evaluating quality indicators such as color, odor and texture. The results of the sensory evaluation of the color, odor, texture, elasticity and overall acceptability of chilled fresh pork treated with plastic wrap during storage are shown in Figure 21 . As can be seen from the figure, fresh pork has good flavor and color, elastic muscle, good texture, high sensory scores and high edible value. As the storage time prolongs, the scores of each index of the two groups of samples decrease, and during the whole storage period, the scores of each index of the treated group samples are higher than those of the control group. On the 8th day of storage, the sensory scores of the pork samples in the control group dropped to about 3. Specifically, the color became darker, the elasticity became worse, the texture became loose, the surface was sticky, and the edible quality decreased significantly. While the muscle color of the treated group was dark red and the muscle fiber lines were basically clear. Therefore, the treatment with plastic wrap can effectively slow down the decline of sensory quality during the storage of pork.

[0190] The film prepared by the method of the present invention exhibits excellent mechanical properties through molecular synergistic effects such as hydrogen bond crosslinking and electrostatic interactions among its components. The synergistic effect of its antibacterial and antioxidant properties can better reduce the deterioration of food quality and extend the shelf life of food. In particular, it can significantly inhibit the quality deterioration and food safety problems caused by the proliferation and metabolism of spoilage bacteria and pathogenic bacteria during the cold chain circulation of chilled meat. Moreover, its intelligent color display characteristics can monitor the freshness of foods such as chilled meat in real time. The film is simple to prepare and easy to produce on a large scale, and is convenient to use. Its unique color display indication, broad-spectrum antibacterial and highly efficient antioxidant triple-functional synergistic system can achieve the purpose of real-time detection of food freshness and active preservation, showing superior performance and wide application scenarios in the fields of fresh meat, aquatic products and ready-to-eat food packaging.

[0191] As described above, although the present invention has been shown and described with reference to specific preferred embodiments, it should not be construed as a limitation of the present invention itself. Various changes in form and detail may be made without departing from the spirit and scope of the present invention.

Claims

1. A preparation method of an intelligent color-changing antibacterial, antioxidant and multifunctional fresh-keeping film, characterized in that, It includes the following steps: Step 1: Prepare a polyvinyl alcohol mother liquor containing glycerol, a water-soluble starch solution, a pitaya anthocyanin solution, and an ε-polylysine hydrochloride solution; Step 2: Add the water-soluble starch solution to the polyvinyl alcohol mother liquor to prepare a PVA-starch mixed solution; Step 3: Add the pitaya anthocyanin solution and the ε-polylysine hydrochloride solution to the PVA-starch mixed solution, fully stir, and then cast the mixed solution on a flat plate device and naturally dry it to form a film.

2. The preparation method according to claim 1, wherein In Step 1: (1) The preparation method of the polyvinyl alcohol mother liquor is as follows: Add PVA to deionized water, magnetically stir in a 60°C water bath, and add glycerol as a plasticizer during stirring to obtain a PVA mother liquor; (2) The preparation method of the aqueous starch solution is as follows: Add water-soluble starch to deionized water, magnetically stir in a 60°C water bath to obtain an aqueous starch solution; (3) The preparation method of the pitaya anthocyanin solution is as follows: Dissolve pitaya anthocyanin with deionized water, magnetically stir at room temperature until completely dissolved to obtain a pitaya anthocyanin solution, and place the solution in a brown bottle to avoid light; (4) The preparation method of the ε-polylysine hydrochloride solution is as follows: Dissolve ε-polylysine hydrochloride with deionized water, magnetically stir at room temperature until completely dissolved to obtain an ε-polylysine hydrochloride solution.

3. The preparation method according to claim 2, characterized in that, The preparation method of pitaya anthocyanin is: Select the peel of red-fleshed pitaya, place the raw material in an oven at 50°C and dry it to constant weight; after drying, crush the raw material, pass it through a 40-mesh sieve and store it in a brown bottle in the dark; then mix the crushed raw material with 75% ethanol aqueous solution at a ratio of 1g:10mL, extract it in a water bath for 3h; filter the extract, use a rotary evaporator to remove ethanol to obtain an anthocyanin concentrate; load the anthocyanin concentrate onto a macroporous adsorption resin chromatographic column, let it stand for 2h, wash water-soluble impurities such as sugars with distilled water, and then elute anthocyanin with 70% ethanol solution until the inside of the chromatographic column is colorless. Concentrate the eluate under reduced pressure and then perform freeze-drying to obtain a freeze-dried powder of red-fleshed pitaya anthocyanin.

4. The preparation method according to claim 1, wherein In Step 2, the preparation method of the PVA-starch mixed solution is as follows: Place the PVA mother liquor in a 60°C water bath and magnetically stir. While stirring, add the aqueous starch solution and continue magnetic stirring in the water bath for 2h to finally prepare a PVA-starch mixed solution.

5. The preparation method according to claim 1, characterized in that, In Step 3, magnetically stir the PVA-starch mixed solution in a 60°C water bath. While stirring, add the pitaya anthocyanin solution and the ε-polylysine hydrochloride solution, then continue stirring for 20 minutes and ultrasonically treat the mixed solution for 5 minutes.

6. The preparation method according to claim 1, characterized in that, In the mixed solution obtained in Step 3: The mass ratio of polyvinyl alcohol, water-soluble starch, pitaya anthocyanin, ε-polylysine hydrochloride, and glycerol is 20:20:(5-10):(5-10):6, and the concentration of the polyvinyl alcohol is 20g / L.

7. The preparation method according to claim 6, characterized in that In the mixed solution of Step 3, the mass ratio of polyvinyl alcohol, water-soluble starch, pitaya anthocyanin, ε-polylysine hydrochloride, and glycerol is 20:20:5:10:

6.

8. Use of the fresh-keeping film prepared by the preparation method according to any one of claims 1-7 in fresh chilled meat preservation.

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