Preservative film based on modified starch and packaging film capable of intelligently indicating freshness

Abstract

The invention provides a preservative film based on modified starch, which is prepared by the following steps: 1) dispersing starch in water to enable the concentration of the obtained solution to be 2-4% (w / v), and adding 0.5-1% (w / v) of glycerol into the solution; 2) preserving the heat of the obtained mixed solution at 90-98 DEG C for 25-40 minutes; and 3) spreading and drying the starch paste. The invention further provides the packaging film capable of intelligently indicating the freshness. According to the preservative film based on the modified starch, the amylopectin side chain is prolonged through the glucoamylase, and the B3 chain content is increased. The structural modification significantly improves the degree of order of the internal structure of the starch film, and further enhances the mechanical strength and barrier property of the starch film. The intelligent indication composite film prepared by the invention shows a remarkable colorimetric response characteristic in a pH range of 2-11, and the color of the intelligent indication composite film is in gradient transition from bright red to purple to yellowish brown along with the change of the pH value.

Description

TECHNICAL FIELD

[0001] The present invention belongs to the technical field of films made of starch, and in particular to a preservative film and a packaging film capable of indicating the freshness of food. BACKGROUND

[0002] Biodegradable films can be a sustainable alternative to petroleum-based packaging materials, which can effectively alleviate the environmental pollution problem caused by traditional plastic packaging. Such films are usually made of natural compounds, such as proteins, lipids, polysaccharides or their mixtures. Among them, starch is an ideal raw material for large-scale production of biodegradable materials. During the preparation of starch films, a series of changes occur in starch molecules: when heated, amylose and amylopectin molecules are dissolved, and starch granules are disintegrated; then water molecules combine with starch through hydrogen bonds to form a paste, which gels and forms a network structure after cooling; during drying and water balance, the starch molecular chain rearranges to form an ordered structure. Studies by Tang (Tang J, Zou F X, Guo L, Wang N, Zhang H X, Cui B, Liu X X. The relationship between linear chain length distributions of amylopectin and the functional properties of the debranched starch-based films [J]. Carbohydrate Polymers, 2022, 279: 119012.) et al. showed that the hydrophobicity of debranched starch-based films is positively correlated with the apparent amylose content, because high amylose content helps starch films form a dense structure and reduce intermolecular gaps. Pure starch films have a large surface energy and strong hydrophilicity due to the presence of hydroxyl polar groups. Chemical modification (such as esterification, crosslinking, etc.) can effectively regulate the content of hydroxyl groups in starch molecules or introduce hydrophobic groups, thereby significantly reducing the hydrophilicity of pure starch films. For example, Liu et al. (Liu Q, Gao L, Qin Y, Ji N, Dai L, Xiong L, Sun Q J. Incorporation of oxidized debranched starch / chitosan nanoparticles for enhanced hydrophobicity of corn starch films [J]. Food Packaging and Shelf Life, 2023, 35: 101032.) found that the WCA of pure corn starch films was only 19.4°, while adding 3% oxidized debranched starch / chitosan nanoparticles (SC-NPs) increased the WCA to more than 70°, indicating that SC-NPs can effectively enhance hydrophobicity. However, the comprehensive improvement of the mechanical properties, thermal stability, barrier properties, etc. of starch films still needs more in-depth and extensive research.

[0003] In recent years, intelligent packaging technology has rapidly developed to improve food safety and quality monitoring. This technology can directly reflect the changes in food freshness by monitoring the internal environmental parameters of the packaging in real time, and has the advantages of rapid response, high sensitivity, etc., and has become a new solution for visual monitoring of food quality. Fresh food such as meat and seafood will continuously release acidic or alkaline substances during storage, and its freshness is usually detected by pH-responsive indicator film. Common natural biopolymers such as starch, chitosan, cellulose, pectin, etc. have been proven to be able to effectively immobilize natural pigments due to their good biocompatibility.

[0004] In existing food packaging and preservation technology, volatile alkaline substances released by meat / seafood spoilage are generally monitored by pH indicator film, but traditional indicator film also has the problem of strong hydrophilicity and easy swelling and inactivation. Commercial PE preservative film has no freshness indication function and is not biodegradable; the existing intelligent film has poor structural stability in a humid environment, resulting in color response distortion. In the existing technology, anthocyanins are used to indicate the freshness of food, for example, patent application CN117586564A provides an anthocyanin intelligent preservative film and a preparation method thereof; but the strong interaction between the hydrophilic polymer (such as starch) in the packaging structure and the water molecules leads to the destruction of the film structure, and the anthocyanins are easily migrated and degraded in a humid environment (ΔE fluctuates by 2.36 in 30 days), resulting in poor preservation performance. SUMMARY

[0005] In view of the deficiencies of the existing technology, the first object of the present application is to provide a modified starch-based preservative film to improve the mechanical strength and barrier properties of the starch film, thereby expanding its application field.

[0006] The second object of the present application is to provide an intelligent freshness-indicating packaging film composed of the preservative film and an intelligent indication composite film, which has the advantages of low hydrophilicity and high stability.

[0007] The technical solution for achieving the above-mentioned objects of the present application is as follows: A modified starch-based preservative film is prepared by the following steps: 1) Disperse starch in water to obtain a solution with a concentration of 2-4%, w / v (according to the ratio of 2-4 grams of starch dispersed in 100 mL of water), and add 0.5-1% (w / v) of glycerol to the solution, wherein the starch is obtained by enzyme modification; 2) Heat the mixed solution obtained in step 1) at 90-98°C for 25-40 min, 3) Spread the starch paste obtained in step 2) to a flat level of 1-5 mm, and dry.

[0008] Step 3) After spreading, the starch film is obtained by drying. The height of the liquid level is controlled to obtain a film with a thickness of 0.07-0.1 mm. The operator can use other known methods to achieve this film thickness, for example, 20 g of starch paste is transferred to a circular container with a diameter of 9 cm, and after drying, the thickness of the film is between 0.084-0.086 mm.

[0009] In the formula, the starch is one of pumpkin starch (PP), potato starch (PT), wheat starch (WH), cassava starch (CS), and pea starch (PE), and the enzyme is glucoamylase.

[0010] Further, the starch is pumpkin starch modified by glucoamylase, and the modification operation is as follows: S1: pumpkin starch is dispersed in a buffer solution with a pH of 5-7 at a mass ratio of 1-2%, heated in a boiling water bath for 30-40 minutes, and cooled to obtain a starch solution; S2: glucoamylase is added at an enzyme / substrate ratio of 10-40 U / g based on the dry basis of the starch; S3: reaction at a temperature of 50-60°C for 1-6 hours.

[0011] More preferably, glucoamylase is added at an enzyme / substrate ratio of 20-30 U / g based on the dry basis of the starch; and in step S3, the reaction is carried out at a temperature of 50-60°C for 5 hours.

[0012] The application also provides a packaging film for intelligently indicating freshness, which is composed of the preservative film and an intelligent indication composite film. The intelligent indication film indicates the freshness of food through color change and is attached to the inner side of the preservative film (the inner side is the side of the food). The food includes meat and seafood.

[0013] The intelligent indication film as a label for indicating freshness can have a size of 0.3-3 cm x 0.3-3 cm.

[0014] The intelligent indication film changes from fresh red to purple to yellow-brown, indicating the change of TVB-N value, and turns yellow-brown when it indicates spoilage.

[0015] In the formula, the starch is one of pumpkin starch (PP), potato starch (PT), wheat starch (WH), cassava starch (CS), and pea starch (PE), and the enzyme is glucoamylase. 1) pumpkin starch is dispersed in water to obtain a solution with a concentration of 2-4%, w / v, and 0.5-1% (w / v) glycerol is added to the solution, 2) the mixed solution obtained in step 1) is incubated at 90-98°C for 25-40 min, After the starch paste solution is cooled to room temperature, 0.5-2.0%, w / v of anthocyanins (ACNs) is added to the starch paste; 3) Spread the starch paste obtained in step 2) to a height of 1-5 mm, and dry.

[0016] After adding the anthocyanins in step 2), preferably stir at a speed of 300-900 rpm for 15-40 min to ensure that the starch paste solution and the ACNs are uniformly mixed. In step 3), 20 g of the mixed solution can be transferred to a polystyrene culture dish with a diameter of 9 cm to a height of 1-5 mm, and then dried at 35°C for 10 h to obtain a pumpkin starch / anthocyanin starch composite film.

[0017] Further, after obtaining the pumpkin starch / anthocyanin starch composite film by drying, polydimethylsiloxane is coated on the surface of the film.

[0018] The operation of coating the polydimethylsiloxane is as follows: Prepare a polydimethylsiloxane (PDMS) solution in a ratio of 0.2-0.4 g of polydimethylsiloxane: 0.02-0.04 g of curing agent: 10 mL of ethyl acetate, and ultrasonically treat for 3-10 min. For example, 0.3 g of polydimethylsiloxane (PDMS) and 0.03 g of curing agent (both reagents can use commercially available polydimethylsiloxane main agent and curing agent) are dissolved in 10 mL of ethyl acetate. Ultrasonically treat the solution for 5 min.

[0019] Use an airbrush to uniformly spray the polydimethylsiloxane (PDMS) solution onto the surface of the pumpkin starch / anthocyanin starch film.

[0020] As a preferred technical solution of the present application, the spraying parameters are set as follows: pressure 0.2-0.5 MPa, flow rate 3-6 mL / min, distance between the airbrush and the pumpkin starch / anthocyanin starch film 100-200 mm, moving speed 2-4 cm / s, and spraying angle 80-100°.

[0021] More preferably, the spraying parameters are as follows: pressure 0.3 MPa, flow rate 5 mL / min, distance between the airbrush and the starch film 140-160 mm, moving speed 2-3 cm / s, and spraying angle 85-95°. Finally, the pumpkin starch / anthocyanin starch composite film coated with PDMS is cured at 80°C for 2 h.

[0022] The present application has the following advantages: (1) The present application successfully prepares the glucoamylase with enzyme activity of 102.43 U / mg and purity of 90% or more by gene synthesis, plasmid introduction, protein expression, enzyme separation and purification. The internal structure order degree analysis finds that the double helix order degree (DO) and double helix degree (DD) of the modified pumpkin starch film PPF-5 are significantly improved; the double helix content increases from 16.26% to 25.82%; and the relative crystallinity increases from 9.46% to 15.77%. It is shown that the glucoamylase modification treatment improves the short-range order degree and long-range order degree of the starch film.

[0023] (2) The modified starch-based preservative film provided by the present application prolongs the side chain of amylopectin by using glucoamylase, and improves the content of B3 chain. This structural modification significantly improves the order degree of the internal structure of the starch film, and further enhances the mechanical strength and barrier properties thereof.

[0024] (3) The intelligent indicating composite film prepared by the present application has good compatibility between the pumpkin starch and anthocyanin. The surface and cross-section micro-morphology graphs of the pumpkin starch / anthocyanin / PDMS composite film are uniform and smooth, and have no pores and particle aggregation. The introduction of anthocyanin improves the hydrophobicity of the composite film. After being coated with PDMS, the water contact angle of the composite film is more than 100°, reaching the standard of hydrophobic materials. Anthocyanin and PDMS can both improve the thermal stability of the starch film. The PDMS coating can significantly reduce the water vapor transmission rate of the composite film. The composite film shows significant colorimetric response characteristics in the pH 2-11 range, and the color thereof changes from fresh red (acidic) to purple (neutral) and then to yellow brown (alkaline) in a gradient transition. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The figure shows the water contact angle of the starch film. In the figure: PPF (A), PTF (B), CSF (C), PEF (D), WHF (E).

[0026] Figure 2 The figure is the Fourier transform infrared spectrum (FTIR) of the five kinds of starch films.

[0027] Figure 3 The figure shows the TG curve and DTG curve of the modified starch film.

[0028] Figure 4 The figure is the surface and cross-section micro-morphology of the modified starch film. In the figure: (a) PPF, (b) PPF-1, (c) PPF-3, (d) PPF-5.

[0029] Figure 5 The figure shows the color change (A) and ultraviolet spectrum (B) of the ACNs solution under the condition of pH 2-11.

[0030] Figure 6 Water vapor transmission rate (WVP) results for PP / ACNs and PP / ACNs / PDMS at different ACN contents (0.5, 1.0, 1.5).

[0031] Figure 7 The morphology of PP / ACNs and PP / ACNs / PDMS composite films is shown.

[0032] Figures 8 to 10 Colorimetric analysis of PP / ACNs / PDMS solutions with different ACNs addition amounts at pH 2-11. Figure 8 Color response of PP / ACNs / PDMS-0.5 composite film. Figure 9 Color response of PP / ACNs / PDMS-1.0 composite film. Figure 10 Color response of PP / ACNs / PDMS-1.5 composite film.

[0033] Figure 11 : Color change of composite film over preservation time.

[0034] Figure 12 Application of PP / ACNs / PDMS composite membrane in monitoring the freshness of Litopenaeus vannamei, where B: pH change of Litopenaeus vannamei with preservation time, C: TVB-N change of Litopenaeus vannamei with preservation time, D1 and D2: color change of composite membrane in experimental group, and D3 and D4: color change of composite membrane in control group. Detailed Implementation

[0035] The following examples are used to illustrate the present invention, but are not intended to limit the scope of the invention.

[0036] In this specific embodiment, the water contact angle (WCA) of the starch film was measured using a contact angle meter (OCA50, Dataphysics); thermogravimetric analysis (TGA) was performed on the film samples using a thermogravimetric analyzer (DTG-60, Shimadzu, Japan). The tensile strength (TS) and elongation at break (EB) of the film samples were measured using a texture analyzer (CT3, Brookfield Materials, USA). The water vapor transmission rate (WVP) of the starch film was tested using a cup weighing method. The thickness of the starch film was measured using a digital micrometer (accuracy 0.001 mm). Ultraviolet-visible absorption spectroscopy analysis of the film samples was performed in the wavelength range of 200-800 nm using a UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan), and the opacity was expressed as the ratio of absorbance at 600 nm to film thickness.

[0037] High-performance anion exchange chromatography (HPAEC) and sec method were used to determine the chain length distribution of starch and amylopectin. The sec method is suitable for determining the chain length distribution (CLD) of amylose (DP>100), while the HPAEC method has higher analytical precision for the chain length distribution of short amylopectin (DP<100).

[0038] Unless otherwise specified, all methods used in this instruction manual are existing techniques in the field. All raw materials used are commercially available.

[0039] Example 1: The Cucurbita maxima used in this project was purchased from Xinjiang Uygur Autonomous Region. Potatoes, cassava, peas, and wheat starch were purchased from Beijing Gusong Trade Co., Ltd. PP, PT, CS, PE, and WH represent pumpkin, potato, cassava, peas, and wheat starch, respectively.

[0040] Preparation of pumpkin starch: Cut the peeled pumpkin into small pieces. Using a 1:2 ratio of pumpkin flesh to distilled water, add the pumpkin pieces and water to a blender and blend on high speed until a smooth paste is formed. Filter the mixture through cheesecloth and collect the filtrate. Wash the remaining residue on the cheesecloth repeatedly with distilled water and filter again, repeating this process three times. Let the collected filtrate stand for 24 hours to allow the starch to completely precipitate. Pour off the supernatant and wash the starch at least four times with distilled water until the supernatant becomes clear, transparent, and colorless. Dry the starch, grind it, and pass it through a 100-mesh sieve.

[0041] The composition and physicochemical properties of the five starches used in the test were tested, and the results are shown in Table 1. The moisture content of pumpkin (PP), potato (PT), cassava (CS), pea (PE), and wheat (WH) starches ranged from 9.40% to 13.05%.

[0042] Table 1: Composition of Starch

[0043] Note: Different letters indicate significant differences. p <0.05) The Mw values ​​for the five starches are listed in Table 2, with CS (5.13 × 10⁻⁶) as an example. 7 g / mol) and WH (4.83×10 7 The Mw value was the highest at (g / mol) and there was no significant difference. PT (4.25×10) 7 g / mol) and PE (4.43×10 7 The Mw value (g / mol) is relatively small, and PP (2.49×10) 7The Mw value was the lowest for PP (g / mol). The amylopectin content of PP (9.21 × 10⁻⁶ g / mol) was also the lowest. 7 (g / mol) and the molecular weight of amylose (1.32×10) 7 (g / mol) is also the smallest among the five starches. WH and PE have the largest molecular weights of amylopectin, at 11.39 × 10⁻⁶ g / mol. 7 g / mol and 11.31×10 7 g / mol. PT and CS have the largest amylose molecular weights, at 2.12 × 10⁻⁶ g / mol. 7 g / mol and 2.02×10 7 g / mol. As shown in Table 2, the molecular weight of amylopectin in all five starches is much larger than that of amylose. This is because amylose is a linear polymer with almost no branches, while amylopectin molecules have a highly branched structure.

[0044] Table 2 Molecular weight distribution of five starches

[0045] Based on their degree of polymerization, amylopectin can be divided into four parts: A chain (DP 6-12), B1 chain (DP 13-24), B2 chain (DP 25-36), and B3 chain (DP>36). The average chain lengths of the five starches ranged from DP 19.58 to 21.54, showing significant differences. Among them, PP had the longest average chain length (21.54 ± 0.22). a The high B3 chain content of PP is related to its high B3 chain content. PP has a B3 chain content of 12.84%, the highest among the five starches. Studies have shown that the chain length of amylose has a significant impact on its physicochemical properties. Medium chain length (DP ~1100) can significantly reduce starch digestibility, while excessively long chain lengths can inhibit the formation of double helix structures due to steric hindrance.

[0046] Example 2 Preparation of starch film Pumpkin starch was prepared according to the method in Example 1. PPF, PTF, CSF, PEF, and WHF represent pumpkin, potato, cassava, pea, and wheat starch films, respectively. The film preparation is as follows: 1) Preparation of starch film-forming solution (3.0%, w / v): Pumpkin starch (PP), potato starch (PT), wheat starch (WH), tapioca starch (CS), and pea starch (PE) were dispersed separately in distilled water. 0.9% (w / v) of glycerol (equivalent to 30% of the dry weight of the starch) was added to each solution.

[0047] 2) Place the mixed solution in a 95°C water bath and heat for 30 minutes under magnetic stirring.

[0048] 3) After the starch has completely gelatinized, accurately weigh 20 g of starch paste and transfer it to a polystyrene petri dish with a diameter of 9 cm. Place the petri dish in a 35℃ oven to dry for 10 hours, and finally store the obtained starch film in an environment of 25℃ and 43±3% relative humidity.

[0049] The five types of starch films obtained were tested.

[0050] Water contact angle (WCA) is a key parameter characterizing the hydrophilicity / hydrophobicity of thin film surfaces. See also Figure 1 The WCA value of the prepared starch films was obtained by calculating the average contact angle on both sides, ranging from 47.1° to 112°. The thickness and transparency data of the five starch films (PPF, PTF, CSF, PEF, and WHF) are shown in Table 3. Opacity is inversely proportional to transmittance; the higher the value, the lower the transparency of the starch film. In Table 3, PPF has the highest tensile strength (14.29 MPa).

[0051] Table 3. Thermal stability, mechanical properties, opacity, and thickness of starch films.

[0052] The water vapor barrier properties of starch films directly determine their ability to regulate moisture migration between food and the environment, which is crucial for extending food shelf life. The table below shows the water vapor permeability. As shown in Table 4, all starch films have low WVP values, ranging from 4.16 to 4.42 × 10⁻⁶. −10 gm −1 s −1 Pa −1 There were no significant differences among the samples (p>0.05).

[0053] Table 4. Water vapor transmission rate of starch film

[0054] Figure 2 Fourier transform infrared (FTIR) spectra of five starch films. 3427 cm⁻¹ −1 The absorption peak at 2930 cm⁻¹ corresponds to the stretching vibration of -OH. −1 The absorption peak at 1638 cm⁻¹ is due to the stretching vibration of the CH bond. −1 The absorption peak at 800-1200 cm⁻¹ reflects the presence of water molecules. −1 The spectral region includes stretching vibrations of CC, C-OH, and CH2. Short-range order analysis of starch membranes can be performed using FTIR. Located at approximately 1080 cm2 −1 and 924 cm −1 The absorption peak at 1022 cm⁻¹ represents the crystal structure, while the absorption peak at 1022 cm⁻¹ represents the crystal structure. −1The absorption peak at that point corresponds to the amorphous region. (1080 / 1022 cm⁻¹) −1 The ratio of absorption peaks represents the degree of double helix order (DO) of the starch film, 924 / 1022 cm⁻¹ −1 The ratio reflects the degree of double helix (DD). Data analysis showed that the DD values ​​of the five starch films ranged from 0.771 to 0.793, and the DO values ​​ranged from 0.288 to 0.318. PPF had the highest DD and DO values, at 0.793 and 0.318, respectively. The NMR analysis results in this study were cross-validated with the XRD and FTIR data, confirming that both PPF and PEF samples (with PPF showing a more significant difference) exhibited high short-range and long-range ordered structural characteristics.

[0055] Pumpkin starch PP film has the highest mechanical strength and internal structural order, so it was chosen to prepare modified starch.

[0056] Example 3 Preparation of enzyme-modified pumpkin starch film The molecular weight of starch has been shown to affect its physicochemical properties. Researchers have used enzymatic hydrolysis to break down amylopectin into low molecular weight fragments. By reducing the molecular weight, they not only significantly reduced the recrystallization tendency of starch molecules but also limited crystal growth, thereby achieving effective regulation of the physicochemical properties of starch. In selecting the enzyme, we chose glucosylamylase, which meets national food safety standards, and based on previous studies, determined that the enzyme / substrate ratio should be 10–40 U / g.

[0057] Preparation of glucose-amylase-modified pumpkin starch: S1: Disperse pumpkin starch (PP, 1% w / w) in 100 mL buffer (pH 5.0 citrate buffer) and heat in a boiling water bath with shaking (120 rpm) for 30 minutes.

[0058] S2: After the starch paste has cooled to room temperature, add glucosyl amylase at an enzyme / substrate ratio of 25 U / g based on dry starch.

[0059] S3: Under magnetic stirring at 300 r / min, set the reaction temperature to 55℃ and carry out the enzymatic reaction for the specified time (1 hour, 3 hours, and 5 hours). After the reaction is completed, terminate the reaction by autoclaving at 121℃ for 15 minutes.

[0060] The resulting modified pumpkin starch paste was immediately treated with liquid nitrogen and freeze-dried. The freeze-dried modified pumpkin starch was then ground and sieved (100 mesh) for later use. Based on the modification time, the modified starches were named PP-1, PP-3, and PP-5, respectively. The preparation method of the modified pumpkin starch film was the same as in Example 2. The starch films prepared from the original pumpkin starch PP and the modified pumpkin starches PP-1, PP-3, and PP-5 were named PPF, PPF-1, PPF-3, and PPF-5, respectively.

[0061] Experimental Example 1: Performance Study of Glucosylamylase-Modified Pumpkin Starch Film Table 5 shows the amylopectin chain length distribution of glucoamylase-modified starch. Combined with the chain length distribution diagram obtained from the starch chain length determination (SEC method), it can be seen that the enzyme modification treatment did not change the basic chain length distribution pattern of starch, with B1 chain remaining the most abundant amylopectin chain. Combined with HPAEC-PAD quantitative analysis, it was shown that the B3 chain content significantly increased from 12.8% to 18.1%. As shown in Table 4-2, the relative contents of amylopectin A, B1, and B3 chains in samples treated with enzymes for 1 h (PP-1) and 3 h (PP-3) were not significantly different from those in the unmodified PP sample. Compared with PP, the A chain content of PP-5 significantly decreased to 19.91%, the contents of B1 and B2 did not change significantly, and the B3 chain content significantly increased by 5.27%. Furthermore, the average chain length of PP-5 increased from 21.54 DP before modification to 23.56 DP. In summary, enzyme modification treatment can significantly increase the content of B3 chains and extend the length of amylopectin chains.

[0062] Internal structural order analysis revealed that the double helix order (DO) and double helix degree (DD) of the modified pumpkin starch film PPF-5 were significantly improved; the double helix content increased from 16.26% to 25.82%; and the relative crystallinity increased from 9.46% to 15.77%. This indicates that glucoamylase modification treatment improved the short-range and long-range order of the starch film.

[0063] This embodiment uses glucosylamylase to extend the side chains of amylopectin and increase the B3 chain content. This structural modification significantly improves the orderliness of the internal structure of the starch film, thereby enhancing its mechanical strength and barrier properties. This experiment achieves improved performance of starch-based preservative films by enzymatically regulating the starch chain structure.

[0064] Table 5. Molecular weight and amylopectin chain length distribution of modified starch Figure 3The images show the TG and DTG curves of the modified starch film. Similar to the original pumpkin starch film, the modified starch film also exhibits three stages of thermal weight loss, corresponding to water evaporation, glycerol degradation, and depolymerization of starch macromolecules, respectively. Significant differences exist in the TG and DTG curves of the four starch films, indicating that the thermal stability of the modified pumpkin starch film has changed. The differences in thermal stability among the samples are mainly reflected in the third thermal degradation stage, which corresponds to the depolymerization process of starch macromolecules. TG analysis results show that the order of thermal degradation initiation of the modified starch film is: PPF-5 > PPF-3 > PPF-1 > PPF.

[0065] Table 6 shows the tensile properties of the original starch film PPF and three enzyme-modified starch films (PPF-1, PPF-3, and PPF-5), including two key parameters: tensile strength (TS) and elongation at break (EB). The tensile strength of PPF was 14.29 MPa, while the TS values ​​of the modified starch films PPF-1 and PPF-3 were 15.69 MPa and 16.02 MPa, respectively. Compared to the other three starch films, PPF-5 exhibited the strongest mechanical strength, with a TS value of 23.21 MPa, a 62.4% increase over PPF. The difference in TS values ​​between PPF-1 and PPF-3 was not significant, but both showed statistically significant improvements compared to PPF. The elongation at break results showed that the EB value of PPF was 45.46%. The EB values ​​of PPF-1 and PPF-3 were 40.5% and 40.75%, respectively, with no statistically significant difference from PPF. The EB value of PPF-5 decreased significantly to 26.75%, a decrease of 41.2% compared to PPF. After 5 h of enzyme modification treatment, the tensile strength of the starch film increased significantly, which may be because the enzyme modification treatment further increased the content of B3 chains in pumpkin starch.

[0066] Table 6 Mechanical properties of modified starch film

[0067] Water vapor transmission rate analysis of modified pumpkin starch film: This study measured the water vapor transmission rate (WVP) of four starch film samples. The results showed that the WVP value of unmodified PPF was 4.21 × 10⁻¹. 0 gm⁻¹ s⁻¹ Pa⁻¹. The WVP values ​​of the modified samples PPF-1 and PPF-3 were 4.08 and 4.12 × 10⁻¹, respectively. 0 gm⁻¹ s⁻¹ Pa⁻¹. PPF-5 has the lowest WVP value, at 3.33 × 10⁻¹. 0 gm⁻¹ s⁻¹ Pa⁻¹ showed a significant decrease compared to PPF. This indicates that enzyme modification treatment for 1 h and 3 h did not significantly improve the WVP of the starch film, but the WVP of the starch film was significantly improved when the treatment time was extended to 5 h.

[0068] Morphology analysis of modified pumpkin starch film: Figure 4 The images show the scanning electron microscope (SEM) morphology analysis of the modified pumpkin starch films, with ad representing a cross-sectional view of the starch film. As can be seen from the figures, both the original pumpkin starch film (PPF) and the modified starch films (PPF-1, PPF-3, and PPF-5) exhibit a smooth, dense morphology without pores, cracks, or bubbles. This dense structure helps improve the mechanical and barrier properties of the starch film. However, compared to PPF, the cross-sectional morphology of the modified starch films (PPF-1, PPF-3, and PPF-5) changes significantly: all modified samples exhibit an uneven, rough structure with visible insoluble particles dispersed within.

[0069] The results of this embodiment show that the modified starch film PPF-5 treated with glucoamylase exhibits significantly improved mechanical and barrier properties, making it a higher quality and more environmentally friendly food preservation film.

[0070] This study incorporated anthocyanins into a pumpkin starch matrix and prepared a pumpkin starch / anthocyanin / PDMS smart indicator film through PDMS surface modification. The effects of different anthocyanin addition amounts on the composite film structure, barrier properties, mechanical properties, thermal stability, and color stability were evaluated, along with its application potential in monitoring the freshness of Litopenaeus vannamei.

[0071] First, the color response of anthocyanin (ACN) solutions to different pH values ​​was tested: 2 g of ACNs were dissolved in 1000 mL of deionized water, divided into 10 equal portions, and the pH values ​​were adjusted with 0.2 M HCl and 0.2 M NaOH respectively, so that the pH gradient was distributed in the range of 2 to 11. All tests were performed in the wavelength range of 300-700 nm. Figure 5 As shown, as the pH value increased from 2.0 to 11.0, the color of the ACNs solution changed sequentially from orange-red to purple, and finally to yellowish-brown. The color change from purple to yellowish-brown was more significant than the change from orange-red to purple, indicating that ACNs are suitable for monitoring protein-rich foods, as these foods produce alkaline gases during protein decomposition.

[0072] Example 4: Development and application of pumpkin starch / anthocyanin / PDMS indicator membrane Preparation of Pumpkin Starch / Anthocyanin / PDMS Smart Indicator Composite Membrane 1) Disperse 3 g of pumpkin starch in 100 mL of distilled water, and then add 0.9% (w / v) of glycerol (equivalent to 30% of the dry weight of starch) to the solution. 2) Heat the starch paste in a water bath with a magnetic stirrer at 95°C for 30 minutes with continuous stirring to ensure complete gelatinization. After the starch paste solution has cooled to room temperature, add different amounts (0.5, 1.0, and 1.5%, w / v, i.e., 0.5~1.5 g to 100 mL of starch paste) of ACNs to the starch paste and stir magnetically at 600 rpm for 30 minutes to ensure that the starch paste solution and ACNs are mixed evenly.

[0073] 3) Transfer 20 g of the mixed solution to a polystyrene petri dish with a diameter of 9 cm and dry at 35℃ for 10 h to obtain a pumpkin starch / anthocyanin starch composite film. According to the different anthocyanin contents, the pumpkin starch / anthocyanin starch composite films are named PP / ACNs-0.5, PP / ACNs-1.0 and PP / ACNs-1.5, respectively.

[0074] 4) Dissolve 0.3 g of polydimethoxysiloxane (PDMS) and 0.03 g of curing agent in 10 mL of ethyl acetate. Sonicate the solution for 5 minutes. Then, uniformly spray the solution onto the surface of the pumpkin starch / anthocyanin starch film using a spray gun. Spraying parameters were set as follows: pressure 0.3 MPa, flow rate 5 mL / min, with precise control over the distance between the spray gun and the starch film (approximately 150 mm), the moving speed (approximately 3 cm / s), and the spraying angle (approximately 90°). Finally, the PDMS-coated pumpkin starch / anthocyanin starch composite film was cured at 80℃ for 2 hours. The pumpkin starch / anthocyanin / PDMS smart indicator composite films were named PP / ACNs / PDMS-0.5, PP / ACNs / PDMS-1.0, and PP / ACNs / PDMS-1.5, respectively. All samples were stored at 25℃ and 43±3% relative humidity before analysis.

[0075] Experimental Example 3: Performance Testing of Smart Indicator Composite Film The water contact angle (WCA) value of the pumpkin starch / anthocyanin starch composite membrane was calculated by averaging the contact angles of both sides. The WCA values ​​of PP / ACNs-0.5, PP / ACNs-1.0, and PP / ACNs-1.5 were found to be 57.8°, 68.1°, and 72.7°, respectively. The WCA value of the composite membrane increased with increasing ACNs content, indicating that the addition of ACNs helps improve the hydrophobicity of the composite membrane.

[0076] The water contact angle of the pumpkin starch / anthocyanin / PDMS smart indicator composite membrane was measured, revealing that PDMS coating significantly improved the hydrophobicity of the membrane. The water contact angles (WCA) of PP / ACNs / PDMS-0.5, PP / ACNs / PDMS-1.0, and PP / ACNs / PDMS-1.5 were 106.2°, 106.8°, and 105.7°, respectively, all meeting the standard for hydrophobic materials (WCA > 90°). This indicates that coating the composite membrane with PDMS can modify it from a hydrophilic material to a hydrophobic material.

[0077] PP / ACNs / PDMS composite film thickness: The thickness range of all samples was 0.082-0.098 mm. In the PP / ACNs composite film, the thickness gradually increased as the ACNs content increased from 0.5% to 1.5%, with PP / ACNs-1.0 having a thickness of 0.085 mm and PP / ACNs / PDMS-1.0 having a thickness of 0.095 mm; among them, PP / ACNs-1.5 had the largest thickness (0.098 mm).

[0078] Thermal stability: The initial degradation temperatures of PP / ACNs-0.5, PP / ACNs-1.0, and PP / ACNs-1.5 were 199℃, 202℃, and 203℃, respectively. The initial degradation temperatures of PP / ACNs / PDMS-0.5, PP / ACNs / PDMS-1.0, and PP / ACNs / PDMS-1.5 were 212℃, 213℃, and 215℃, respectively. With the addition of ACNs, the initial degradation temperature continuously increased; furthermore, the initial degradation temperature of PP / ACNs / PDMS was higher than that of PP / ACNs, indicating that PDMS can improve the thermal stability of the composite film.

[0079] Mechanical properties of PP / ACNs / PDMS composite membranes: Table 7 shows the mechanical properties of PP / ACNs and PP / ACNs / PDMS composite membranes. Among the PP / ACNs composite membranes, the PP / ACNs-1.0 sample exhibited the highest value (14.03 MPa). The TS value of the PP / ACNs / PDMS composite membrane varied in the range of 13.19-14.34 MPa but there was no significant difference (p>0.05), with the highest value being PP / ACNs / PDMS-1.0 (14.34 MPa).

[0080] Table 7 Mechanical properties of PP / ANS / PDMS composite films

[0081] Water vapor transmission rate of PP / ACNs / PDMS composite membrane: See Figure 6The water vapor transmission rate (WVP) tests of PP / ACNs and PP / ACNs / PDMS at different ACN contents (0.5, 1.0, 1.5) showed that the WVP value of the composite membrane first decreased and then increased with the increase of ACN content. When the ACN content increased from 0.5% to 1.0%, the WVP increased from 3.69 × 10⁻¹ 0 gm⁻¹ s⁻¹ Pa⁻¹ decreased to 3.34×10⁻¹ 0 gm⁻¹ s⁻¹ Pa⁻¹; However, when the ACNs addition was further increased to 1.5%, WVP actually rose to 3.83 × 10⁻¹ 0 gm⁻¹ s⁻¹ Pa⁻¹. A similar trend was observed in PP / ACNs / PDMS composite membranes. The WVP values ​​of PP / ACNs / PDMS composite membranes ranged from 2.80 to 3.10 × 10⁻¹. 0 In the gm⁻¹ s⁻¹ Pa⁻¹ range, the overall water vapor transmission rate was significantly lower than that of PP / ACNs, with the PP / ACNs / PDMS-1.0 sample exhibiting the lowest water vapor transmission rate (2.80 × 10⁻¹). 0 gm⁻¹ s⁻¹ Pa⁻¹). This indicates that coating with PDMS can significantly reduce the WVP value and improve the hydrophobicity of the composite membrane.

[0082] Microstructure of PP / ACNs / PDMS composite films: The morphology of PP / ACNs and PP / ACNs / PDMS composite films is as follows... Figure 7 As shown in the figure, all samples were translucent, and the color of the composite film changed from pink to dark red with increasing ACN content. AF in the figure is a scanning electron microscope (SEM) image of the composite film surface. All samples had a uniform and smooth surface, without delamination, pores, or cracks. This is attributed to the complete gelatinization of starch and the plasticizing effect of glycerol, while the gelatinized starch exhibits good compatibility with ACNs. When the ACN addition increased from 0.5% to 1.5%, the surface of the PP / ACNs remained smooth, and the ACNs were uniformly dispersed in the starch matrix without aggregation or phase separation. Figure 7 The area marked by the red box in the df shows a thin layered structure, which is presumably a micron-sized PDMS coating layer formed by spraying.

[0083] Color stability and colorimetric response of PP / ACNs / PDMS composite film: Previous experiments showed that PDMS coating improves the hydrophobicity and water-blocking ability of the composite film without affecting its tensile strength or damaging its structure. Therefore, the next step will be to evaluate whether the PP / ACNs / PDMS composite film can be used for monitoring food freshness, and to measure its color stability and colorimetric response. Colorimetry is typically characterized by three parameters: L*, a*, and b*. The color stability of the film can be assessed by observing its color difference during storage, characterized by the ΔE value. Results are shown in [link to results]. Figures 8 to 10 See Table 8.

[0084] Table 8 shows the color stability of the PP / ANNs / PDMS composite film prepared in Example 4.

[0085] Table 8 Color stability of PP / ANS / PDMS composite film In PP / ACNs / PDMS-0.5, the L* value increased from 43.50 to 70.71 between pH 2 and 7, indicating a continuous increase in the brightness of the composite membrane. However, between pH 8 and 11, the L* value decreased from 69.91 to 57.46, because anthocyanins degrade in an alkaline environment to produce yellowish-brown products, leading to a decrease in the brightness of the composite membrane. Furthermore, within the pH range of 2-11, the a* value of the composite membrane decreased from 46.46 to 0.36, indicating that the red color gradually disappeared as the pH increased. The b* value was positive between pH 2 and 4 and negative between pH 5 and 11, indicating that the composite membrane turned blue. The composite membrane had the highest ΔE value at pH 2. The color responses of PP / ACNs / PDMS-1.0 and PP / ACNs / PDMS-1.5 to different pH values ​​were similar to those of PP / ACNs / PDMS-0.5 but showed differences, which stemmed from the different amounts of ACNs added. At pH 11, the L*, a*, and ΔE* values ​​of PP / ACNs / PDMS-0.5 were 57.46, 0.36, and 31.67, respectively. The L*, a*, and ΔE* values ​​of PP / ACNs / PDMS-1.0 were 55.50, 4.32, and 33.89, respectively; and the L*, a*, and ΔE* values ​​of PP / ACNs / PDMS-1.5 were 44.14, 5.28, and 45.25, respectively. This indicates that with increasing ACNs content, the composite membrane's brightness decreases, the red color intensifies, and the color difference increases.

[0086] The above experiments show that PP / ACNs / PDMS-1.0 exhibits good colorimetric response and the best color stability in solutions with pH values ​​ranging from 2 to 11. Therefore, PP / ACNs / PDMS-1.0 was selected to monitor the freshness of Litopenaeus vannamei during storage at 4°C.

[0087] Example 5: Application of PP / ACNs / PDMS composite film in the preservation of Litopenaeus vannamei shrimp Based on the results of Example 3, the modified starch film PPF-5 treated with glucosylamylase exhibited significantly improved mechanical and barrier properties. Therefore, this chapter selects PPF-5 as the packaging material for Litopenaeus vannamei and uses commercial polyethylene (PE) preservation film as a control to study its effect on the preservation of Litopenaeus vannamei.

[0088] Live Litopenaeus vannamei shrimp were purchased commercially. Before the experiment, the shrimp were placed in crushed ice for 10 minutes until they died suddenly. A plastic wrap was prepared using modified pumpkin starch, following the same method as PPF-5 in Example 3.

[0089] PP / ACNs / PDMS-1.0 (1 cm × 1 cm) was selected for monitoring the freshness of Litopenaeus vannamei. The cut composite film was attached to the inside of a petri dish (9 cm in diameter), avoiding direct contact with the shrimp. The experimental group's petri dishes were wrapped with modified pumpkin starch film (PPF-5), while the control group was wrapped with polyethylene (PE) plastic wrap. Both were stored at 4℃ for 8 days. Color changes of the composite film were recorded using a colorimeter.

[0090] Quality analysis of Litopenaeus vannamei: pH value: During the 8-day storage period, it was measured every 2 days starting from day 0.

[0091] Volatile basic nitrogen (TVB-N) was determined using the Kjeldahl method.

[0092] During spoilage, protein-rich seafood decomposes due to enzymatic hydrolysis and microbial proliferation, producing volatile basic nitrogen (TVB-N), including ammonia, dimethylamine, and trimethylamine. The OH⁻ groups released by TVB-N cause an increase in pH within the packaging environment. Therefore, TVB-N and pH are often used as indicators to assess the degree of spoilage in shrimp. According to the Chinese national standard GB / T9959.2-2008, a TVB-N content exceeding 20 mg / 100g indicates spoilage of the meat.

[0093] like Figure 11 As shown, the experimental group used modified pumpkin starch film PPF-5 to encapsulate Litopenaeus vannamei, while polyethylene plastic wrap served as the control group. During an 8-day storage experiment at 4℃, the pH changes of the Litopenaeus vannamei were as follows: Figure 12 As shown in Figure B, in the PPF-5 group, the pH value of Litopenaeus vannamei increased continuously with the extension of storage time. The initial pH value of Litopenaeus vannamei was 6.43, reaching a maximum of 7.87 on day 8. The pH value change trend of the control group was consistent with that of the experimental group. On days 2, 6, and 8, the pH value of the experimental group was higher than that of the control group. Figure 12As shown in Figure C, the initial TVB-N value in the experimental group was 5.43 mg / 100g. The TVB-N value increased over time, reaching 18.43 mg / 100g on day 6 and 23.92 mg / 100g on day 8. The control group showed consistent results, with an initial TVB-N value of 5.84, reaching 18.42 mg / 100g on day 6 and 23.15 mg / 100g on day 8.

[0094] During an 8-day storage period, the intelligent indicator composite film PP / ACNs / PDMS-1.0 exhibited a color change sequentially from bright red to purple to brown, demonstrating good indicator sensitivity. Detailed color parameters can be found [link to relevant documentation]. Figure 12 Days 1 and 2. On day 8, the TVB-N value exceeded 20 mg / 100g, indicating spoilage of the sample, corresponding to a brown color in PP / ACNs / PDMS-1.0. This demonstrates that PP / ACNs / PDMS-1.0 can accurately monitor changes in the freshness of Litopenaeus vannamei. The above study shows that the shelf life of Litopenaeus vannamei in both the experimental and control groups is approximately 6 days, indicating that the modified pumpkin starch film PPF-5 has the same preservation effect as polyethylene preservation film.

[0095] Experimental results show that the introduction of anthocyanins improves the hydrophobicity of the composite membrane. After PDMS coating, the water contact angle of the composite membrane exceeds 100°, meeting the definition standard for hydrophobic materials. Due to the physical filling effect of anthocyanins, the thickness of the composite membrane gradually increases as the anthocyanin content increases from 0.5% to 1.5%. Both anthocyanins and PDMS can improve the thermal stability of the starch membrane. The addition of anthocyanins did not affect the tensile strength of the composite membrane, but due to the interaction of water molecules in the starch matrix, it significantly reduced the elongation at break of the composite membrane.

[0096] The pumpkin starch / anthocyanin / PDMS smart indicator composite membrane exhibits significant colorimetric response characteristics within a pH range of 2-11, with its color changing in a gradient from bright red (acidic) to purple (neutral) and then to yellowish-brown (alkaline) with pH changes. The color change of the pumpkin starch / anthocyanin / PDMS smart indicator composite membrane shows a good correlation with freshness: it is bright red when fresh (day 0), turns purple on day 3 (when quality begins to decline), and turns brown by day 8 (when spoilage occurs). This composite membrane demonstrates excellent indicator sensitivity and can accurately reflect changes in the freshness of Litopenaeus vannamei. Example 6

[0097] This embodiment provides a smart freshness indicator packaging film, which consists of a modified starch-based preservation film and a smart indicator composite film. The smart indicator film indicates the freshness of food by changing the film color. It is about 1cm × 1cm in size and is attached to the inside of the preservation film. The food includes meat and seafood.

[0098] The starch used to prepare the plastic wrap was glucoamylase-modified pumpkin starch, prepared according to the method in Example 3, wherein the enzymatic reaction time in step S3 was 5 hours. The steps for making the plastic wrap from the modified pumpkin starch were the same as in Example 2.

[0099] The preparation method of the intelligent indicator composite film is the same as in Example 4, wherein in step 2), 1.0% w / v of ACNs is added to the starch paste.

[0100] Although the present invention has been described above through embodiments, those skilled in the art should understand that any improvements and modifications made to the present invention without departing from its spirit and essence should fall within the protection scope of the present invention.

Claims

1. A modified starch-based cling film, characterized in that, Prepared by the following steps: 1) dispersing starch modified by enzyme in water to make the concentration of the solution 2-4%, w / v, adding 0.5-1% (w / v) glycerol in the solution, the starch is one of pumpkin starch, potato starch, wheat starch, cassava starch and pea starch; 2) keeping the mixed solution of step 1) at 90-98 ℃ for 25-40 min, 3) spreading the starch paste of step 2) to make the height of the liquid level 1-5 mm, drying.

2. The modified starch-based cling film according to claim 1, wherein, The starch is pumpkin starch modified by glucoamylase, the modification operation is:

3. The modified starch-based cling film of claim 1, wherein, S1: dispersing pumpkin starch in a buffer solution with pH 5-7 at a mass ratio of 1-2%, heating in a boiling water bath for 30-40 min, cooling to obtain a starch solution; S2: adding glucoamylase at an enzyme / substrate ratio of 10-40 U / g based on the dry starch basis; S3: reacting at a temperature of 30-60 ℃ for 1-6 hours. In step S1, the pumpkin starch is dispersed in a buffer solution with pH 5-7 at a mass ratio of 1-2%, heated in a boiling water bath for 30-40 min, and cooled to obtain a starch solution; the pumpkin starch is obtained by chopping pumpkin, adding water to make a slurry with a mass ratio of pumpkin to water of 1:1-3, filtering, and drying the precipitate.

4. The modified starch-based cling film according to claim 3, wherein, Glucoamylase is added at an enzyme / substrate ratio of 20-30 U / g (based on the dry starch basis); in step S3, the reaction is carried out at a temperature of 50-60 ℃ for 5 hours.

5. The modified starch-based cling film according to claim 3, wherein, The food freshness indicating film is prepared by the following steps:

6. A smart freshness indicating packaging film characterized in that, 1) dispersing pumpkin starch in water to make the concentration of the solution 2-4%, w / v, adding 0.5-1%, w / v glycerol in the solution, 7. The smart freshness-indicating packaging film according to claim 6, characterized in that, 2) keeping the mixed solution of step 1) at 90-98 ℃ for 25-40 min, after the starch paste solution is cooled to room temperature, adding 0.5-2.0%, w / v anthocyanin to the starch paste; 3) spreading the starch paste of step 2) to make the height of the liquid level 1-5 mm, drying. After obtaining the pumpkin starch / anthocyanin starch composite film by drying, polydimethylsiloxane is coated on the surface of the film. The operation of coating polydimethylsiloxane is:

8. The intelligent freshness indicating packaging film according to claim 7, characterized in that, Configuring a polydimethylsiloxane solution at a ratio of 0.2-0.4 g polydimethylsiloxane: 0.02-0.04 g curing agent: 10 mL ethyl acetate, ultrasonic treatment for 3-10 min; 9. The intelligent freshness indicating packaging film according to claim 8, characterized in that, Uniformly spraying the polydimethylsiloxane solution onto the surface of the pumpkin starch / anthocyanin starch film with an airbrush. The spraying parameters are set as follows: pressure 0.2-0.5 MPa, flow rate 3-6 mL / min, distance between the airbrush and the pumpkin starch / anthocyanin starch film 100-200 mm, moving speed 2-4 cm / s, spraying angle 80-100°. ​ 10. The smart freshness-indicating packaging film according to claim 9, characterized in that, ​

Images