Natural polysaccharide composite preparation method of edible film

Abstract

The invention relates to the field of edible films, and particularly discloses a natural polysaccharide composite preparation method of an edible film, which comprises the following steps: step 1, preparation of an acidic polysaccharide solution: dissolving citrus low-ester amidated pectin in hot water with the temperature of 60-80 DEG C, stirring until the citrus low-ester amidated pectin is completely dissolved, and cooling to room temperature to obtain a pectin solution with the mass concentration of 2-4%; step two, preparation of a cationic polysaccharide solution: dissolving chitosan in an acetic acid aqueous solution with the volume fraction of 1%, and stirring until the chitosan is completely dissolved to obtain a chitosan solution with the mass concentration of 1.5-2.5%; natural cationic polysaccharide and specific anionic polysaccharide are subjected to electrostatic compounding in a controllable acid environment, and a natural polyphenol compound is introduced as a multifunctional cross-linking agent, so that a composite matrix with a compact three-dimensional network structure and biological activity is successfully constructed; the method effectively overcomes the technical bottlenecks that the mechanical property and the ductility of the traditional edible film are difficult to consider and the function is single.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of edible film, in particular to a natural polysaccharide composite preparation method of edible film. BACKGROUND

[0002] Edible film is a kind of thin layer material formed by the intermolecular interaction of edible natural macromolecular materials, which shows broad application prospects in food preservation, drug coating and other fields. The basic film-forming materials mainly include natural substances such as polysaccharides, proteins and lipids.

[0003] Among them, polysaccharide materials (such as chitosan, pectin, sodium alginate, etc.) are concerned due to their good film-forming property, biocompatibility and biodegradability. In particular, chitosan, as the only alkaline polysaccharide with positive charge in nature, has natural antibacterial activity and good film-forming ability due to the amino groups on its molecular chain. It is often used as the basic matrix of edible film. Another kind of anionic polysaccharide, pectin, can not only form electrostatic interaction with cationic substances, but also form hydrogen bond network, which affects the structure and performance of the film.

[0004] At present, the edible film prepared by single polysaccharide material often has obvious limitations. For example, pure chitosan film usually has high mechanical strength but brittle and hard texture, and its antibacterial activity is difficult to effectively release in dry solid state; and some anionic polysaccharide films may have insufficient mechanical strength.

[0005] In order to improve the performance, researchers often use physical blending or chemical crosslinking method. However, simple physical blending often cannot realize uniform dispersion and stable combination of components, which may lead to uneven material performance; while using chemical crosslinking agent can improve the mechanical properties, but it may introduce non-edible synthetic chemicals, which violates the original intention of "pure natural" of edible film, and has food safety hidden danger. SUMMARY

[0006] In order to solve the above technical problems, the present application provides a natural polysaccharide composite preparation method of edible film, to solve the problem that the existing technology excessively relies on chemical crosslinking agent and the mechanical properties and functional activity of film-forming material are difficult to be considered.

[0007] A natural polysaccharide composite preparation method of edible film, comprising:

[0008] Step one, preparation of acid polysaccharide solution:

[0009] Dissolve the citrus low-ester amidated pectin in hot water with a temperature of 60-80℃, stir until completely dissolved, and cool to room temperature to obtain a pectin solution with a mass concentration of 2%-4%;

[0010] Step two, preparation of cationic polysaccharide solution:

[0011] Dissolve chitosan in 1% acetic acid aqueous solution, stir until completely dissolved, to obtain a chitosan solution with a mass concentration of 1.5%-2.5%;

[0012] Step three, functional crosslinker addition:

[0013] Dissolve rosmarinic acid in a small amount of ethanol, then add it to the chitosan solution obtained in step two, stir until uniform, to obtain a mixed solution A; the amount of rosmarinic acid added is 10%-30% of the mass of chitosan;

[0014] Step four, composite blending:

[0015] Slowly add the pectin solution obtained in step one to the mixed solution A obtained in step three under magnetic stirring, control the dry mass ratio of pectin to chitosan to be 1:1 to 1:2; after the addition is complete, adjust the pH of the mixed system to 4.5-5.5, and continue the reaction at 50-60°C for 60-120 minutes, to obtain a uniform, viscous composite sol;

[0016] Step five, degassing and casting:

[0017] After the composite sol obtained in step four is degassed under vacuum, it is cast onto a polytetrafluoroethylene flat plate;

[0018] Step six, drying to form a film:

[0019] Place the cast flat plate in a constant temperature and humidity box with a temperature of 35-45°C and a relative humidity of 50%-60%, dry for 18-24 hours, then carefully remove the film, to obtain the edible film.

[0020] Preferably, the degree of esterification of the citrus low-ester amidated pectin in step one is ≤40%, and the degree of amidation is ≥15%.

[0021] Preferably, in step four, the pH of the mixed system is accurately adjusted to 5.0 using a citric acid-sodium phosphate buffer system.

[0022] Preferably, 0.5%-1% of nanocellulose whiskers with a length of 100-500 nm and a diameter of 10-50 nm are also added to the composite sol obtained in step four, based on the total mass of the composite sol.

[0023] An edible film prepared by the above method, the tensile strength of the film is not less than 35 MPa, the elongation at break is not less than 25%, and the antibacterial rate against Escherichia coli and Staphylococcus aureus is greater than 90%.

[0024] Preferably, the light transmittance of the film at a wavelength of 500 nm is ≥85%, and the haze is ≤10%.

[0025] Preferably, the film has a slow-release antioxidant activity, and when placed in a simulated food system, its DPPH free radical scavenging rate retention rate is still above 50% within 14 days.

[0026] The application of the edible film as described above as a fresh-keeping packaging material for fresh-cut fruits, pastries and meat products.

[0027] Compared with the prior art, the application has the following beneficial effects:

[0028] By using natural cationic polysaccharides and specific anionic polysaccharides to electrostatically compound in a controllable acidic environment, and introducing natural polyphenol compounds as a multifunctional crosslinking agent, a composite matrix with a dense three-dimensional network structure and biological activity is successfully constructed.

[0029] The method effectively overcomes the technical bottleneck that the mechanical properties and ductility of traditional edible films are difficult to balance and the functions are single, and realizes synchronous significant improvement of the mechanical strength, flexibility, optical transparency and inherent antibacterial and antioxidant activity of the film without relying on synthetic additives.

[0030] The film obtained by the method not only has good processing film-forming property and stability, but also realizes the organic unification of active preservation function and material intrinsic performance, and provides a new solution for developing high-performance all-natural edible packaging materials. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The preparation method flowchart of the application is shown. DETAILED DESCRIPTION

[0032] The technical solutions in the embodiments of the application will be clearly and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the application.

[0033] As shown in the following: Figure 1

[0034] Embodiment 1

[0035] 1. Raw material ratio:

[0036] Citrus low-ester amidated pectin: 2.0 g (esterification degree 35%, amidation degree 18%);

[0037] Chitosan: 1.5 g (deacetylation degree ≥ 90%);

[0038] ​Rosmarinic acid: 0.15 g (10% of the mass of chitosan);

[0039] Anhydrous ethanol: 5 mL (for dissolving rosmarinic acid);

[0040] 1% (v / v) aqueous acetic acid: diluted to 100 mL (for dissolving chitosan);

[0041] Deionized water: diluted to 100 mL (for dissolving pectin);

[0042] Nanocellulose whiskers: not added;

[0043] pH adjusting buffer: 0.1 M citric acid-0.2 M disodium hydrogen phosphate buffer, adjusted to pH 4.5;

[0044] 2. Preparation steps:

[0045] Step one, weigh 2.0 g of citrus low-ester amidated pectin, add to a magnetic stirring water bath containing about 80 mL of water at a temperature of 60 °C, and stir at a speed of 500 rpm until completely dissolved. Stop heating and cool to room temperature (about 25 °C), dilute to 100 mL with deionized water, and obtain a pectin solution with a mass concentration of 2%, ready for use.

[0046] Step two, weigh 1.5 g of chitosan, add to 97.5 mL of 1% aqueous acetic acid, and stir at a speed of 400 rpm at 25 °C for 4 hours until completely dissolved, to obtain a clear chitosan solution with a mass concentration of 1.5%.

[0047] Step three, weigh 0.15 g of rosmarinic acid, dissolve in 5 mL of anhydrous ethanol, and slowly add dropwise to the entire chitosan solution prepared in step two under stirring for 30 minutes, to obtain a mixed solution A.

[0048] Step four, slowly add dropwise the entire pectin solution (100 mL) prepared in step one to mixed solution A under magnetic stirring (300 rpm) for 10 minutes. After the addition is complete, accurately adjust the pH of the mixed system to 4.5 with the previously prepared citric acid-disodium hydrogen phosphate buffer. Transfer the reaction cup to a water bath at 50 °C, and continue to react at a speed of 300 rpm for 60 minutes, to obtain a uniform, viscous composite sol.

[0049] Step five, place the obtained composite sol in a vacuum dryer, and deaerate at -0.09 MPa for 20 minutes to remove the gas bubbles entrained in the solution. Carefully flow cast the deaerated sol on a polytetrafluoroethylene flat plate with a size of 20 cm x 20 cm, and control the flow casting thickness to be 2 mm.

[0050] Step six, place the casted flat plate in a constant temperature and humidity chamber, dry at 40°C, 55% relative humidity for 4 hours in the first stage; reduce the temperature to 25°C, continue to dry for 14 hours to constant weight in the second stage. Carefully remove the film from the flat plate, and the target edible film is obtained.

[0051] 3. Film properties:

[0052] Appearance: light yellow transparent, uniform film.

[0053] Thickness: 45 ± 5 μm.

[0054] Tensile strength: 36.2 MPa.

[0055] Elongation at break: 26.8%.

[0056] Antibacterial property (bacteriostatic rate): 91.5% for E. coli, 93.2% for S. aureus.

[0057] Optical properties: 500 nm light transmittance 85.5%, haze 9.8%.

[0058] Antioxidant property (14-day DPPH retention rate): 51.3%.

[0059] Example 2

[0060] 1. Raw material ratio:

[0061] Citrus low-ester amidated pectin: 3.0 g (esterification degree 38%, amidation degree 20%);

[0062] Chitosan: 2.0 g (deacetylation degree ≥ 90%);

[0063] Rosmarinic acid: 0.40 g (20% of the mass of chitosan);

[0064] Anhydrous ethanol: 8 mL;

[0065] 1% (v / v) acetic acid aqueous solution: constant volume to 100 mL;

[0066] Deionized water: constant volume to 100 mL;

[0067] Nanocellulose whiskers: 0.15 g (0.75% of the total mass of the composite sol, length 200-300 nm, diameter 20-30 nm);

[0068] pH adjusting buffer: adjust pH to 5.0;

[0069] 2. Preparation steps:

[0070] Step one: same as example 1, prepare a 3% pectin solution.

[0071] Step two: Prepare 2% chitosan solution as in Example 1.

[0072] Step three: Weigh 0.40 g of rosmarinic acid, dissolve in 8 mL of ethanol, and then add to the chitosan solution, stirring for 30 minutes to obtain mixture A.

[0073] Step four: Add all of the pectin solution dropwise to mixture A. Adjust the pH to 5.0 with buffer solution. React at 55°C in a water bath at a rotation speed of 300 rpm for 90 minutes. At 60 minutes, add 0.15 g of nanocellulose whiskers and continue to react for the remaining 30 minutes.

[0074] Step five: Defoam and then cast as in Example 1.

[0075] Step six: After two-stage drying (40°C / 4h + 25°C / 16h), remove the film as in Example 1.

[0076] 3. Film properties:

[0077] Appearance: Light yellow transparent, very uniform film, with a tougher texture.

[0078] Thickness: 48 ± 3 μm.

[0079] Tensile strength: 45.7 MPa.

[0080] Elongation at break: 28.5%.

[0081] Antibacterial properties: 95.8% for E. coli and 96.5% for S. aureus.

[0082] Optical properties: 88.2% light transmittance at 500 nm and 8.1% haze.

[0083] Antioxidant properties: 58.9% DPPH retention rate after 14 days.

[0084] Example 3

[0085] 1. Raw material ratio:

[0086] Citrus low-ester amidated pectin: 4.0 g (esterification degree 40%, amidation degree 22%);

[0087] Chitosan: 2.5 g (deacetylation degree ≥ 95%);

[0088] Rosmarinic acid: 0.75 g (30% of the mass of chitosan);

[0089] Anhydrous ethanol: 10 mL;

[0090] 1% (v / v) aqueous acetic acid solution: make up to 100 mL;

[0091] Deionized water: bring the volume up to 100 mL;

[0092] Nanocellulose whiskers: 0.24g (1.0% of the total mass of the composite sol, length 400-500nm, diameter 40-50nm);

[0093] pH adjustment buffer: Adjust the pH to 5.5;

[0094] 2. Preparation steps:

[0095] Step 1: Same as in Example 1, prepare a 4% pectin solution (dissolution temperature is 80℃).

[0096] Step 2: Same as in Example 1, prepare a 2.5% chitosan solution.

[0097] Step 3: Weigh 0.75g of rosmarinic acid, dissolve it in 10mL of ethanol, add it to the chitosan solution, and stir for 40 minutes to ensure thorough mixing, to obtain mixture A.

[0098] Step 4: Add all the pectin solution dropwise to mixture A. Adjust the pH to 5.5 with buffer solution. React in a 60°C water bath at 400 rpm for 120 minutes. After 90 minutes of reaction, add 0.24 g of nanocellulose whiskers and continue the reaction for the remaining 30 minutes.

[0099] Step 5: Same as Example 1, defoam for 30 minutes, then cast.

[0100] Step 6: Place the cast plate at 35℃ and 60% relative humidity to dry for 6 hours, then at 25℃ for 18 hours until constant weight, and then peel off the film.

[0101] 3. Thin film properties:

[0102] Appearance: Amber-colored, transparent, dense and tough film.

[0103] Thickness: 52±4μm.

[0104] Tensile strength: 50.1 MPa.

[0105] Elongation at break: 30.1%.

[0106] Antibacterial activity: 98.0% against Escherichia coli and 98.5% against Staphylococcus aureus.

[0107] Optical performance: 86.0% transmittance at 500nm, 9.5% haze.

[0108] Antioxidant properties: DPPH retention rate of 65.5% after 14 days.

[0109] Example 4

[0110] 1. Raw material ratio:

[0111] Citrus low-ester amidated pectin: 2.5 g (esterification degree 30%, amidation degree 20%);

[0112] Chitosan: 2.0 g (deacetylation degree ≥ 92%);

[0113] Rosmarinic acid: 0.30 g (15% of the mass of chitosan);

[0114] Anhydrous ethanol: 6 mL;

[0115] 1% (v / v) aqueous acetic acid solution: constant volume to 100 mL;

[0116] Deionized water: constant volume to 100 mL;

[0117] Nanocellulose whiskers: not added;

[0118] pH adjusting buffer: adjust pH to 5.2;

[0119] 2. Preparation steps:

[0120] Step one: weigh 2.5 g of citrus low-ester amidated pectin, dissolve in about 80 mL of hot water at 70°C, stir to dissolve, cool and constant volume to 100 mL to obtain a 2.5% pectin solution.

[0121] Step two: weigh 2.0 g of chitosan, dissolve in 1% acetic acid solution, constant volume to 100 mL, stir until completely dissolved to obtain a 2.0% chitosan solution.

[0122] Step three: weigh 0.30 g of rosmarinic acid, dissolve in 6 mL of ethanol, then add to the chitosan solution, stir for 30 minutes to obtain a mixed solution A.

[0123] Step four: slowly add the pectin solution to the mixed solution A. Accurately adjust the pH to 5.2 with the buffer. React in a 55°C water bath at a speed of 300 rpm for 80 minutes to obtain a uniform composite sol.

[0124] Step five: after vacuum degassing of the composite sol (-0.085 MPa, 25 minutes), cast on a polytetrafluoroethylene plate.

[0125] Step six: after drying the cast flat plate at 38°C and 50% relative humidity for 20 hours, remove the film.

[0126] 3. Film performance:

[0127] Appearance: light amber color, very high transparency, uniform texture.

[0128] Thickness: 40 ± 2 μm.

[0129] Tensile strength: 38.5 MPa.

[0130] Elongation at break: 27.2%.

[0131] Antibacterial activity: 93.0% against Escherichia coli and 94.5% against Staphylococcus aureus.

[0132] Optical performance: 90.5% transmittance at 500nm, 6.8% haze.

[0133] Antioxidant properties: DPPH retention rate of 55.0% after 14 days.

[0134] Example 5

[0135] 1. Raw material ratio:

[0136] Citrus low-ester amidated pectin: 3.5g (esterification degree 35%, amidation degree 18%);

[0137] Chitosan: 2.2g (acetylation degree ≥93%);

[0138] Rosmarinic acid: 0.55g (25% of chitosan mass);

[0139] Anhydrous ethanol: 9 mL;

[0140] 1% (v / v) acetic acid aqueous solution: bring to a final volume of 100 mL;

[0141] Deionized water: bring the volume to 100 mL;

[0142] Nanocellulose whiskers: 0.20g (~0.8% of the total mass of the composite sol, length 150-250nm, diameter 15-25nm);

[0143] pH adjustment buffer: Adjust pH to 4.8;

[0144] 2. Preparation steps:

[0145] Step 1: Weigh 3.5g of citrus low-ester amidated pectin, dissolve it in hot water at 75℃, stir to dissolve, and make up to 100mL to obtain a 3.5% pectin solution.

[0146] Step 2: Weigh 2.2g of chitosan, dissolve it and bring the volume to 100mL to obtain a 2.2% chitosan solution.

[0147] Step 3: Weigh 0.55g of rosmarinic acid, dissolve it in 9mL of ethanol, add it to the chitosan solution, and stir for 35 minutes to obtain mixture A.

[0148] Step four: The pectin solution was added dropwise into the mixed solution A. The pH was adjusted to 4.8 with buffer solution. The reaction was carried out in a 58 °C water bath for 100 minutes. After 70 minutes of reaction, 0.20 g of nanocellulose whiskers were added and the reaction was continued for 30 minutes.

[0149] Step five: The composite sol was degassed under vacuum (-0.095 MPa, 22 minutes) and then cast.

[0150] Step six: The cast flat plate was dried at 42 °C, 55% relative humidity for 22 hours and then the film was peeled off.

[0151] 3. Film properties:

[0152] Appearance: yellow transparent, very tough texture.

[0153] Thickness: 50 ± 3 μm.

[0154] Tensile strength: 48.9 MPa.

[0155] Elongation at break: 26.5%.

[0156] Antibacterial property: E. coli 96.8%, S. aureus 97.5%.

[0157] Optical property: 500 nm transmittance 87.0%, haze 8.5%.

[0158] Antioxidant property: DPPH retention rate after 14 days 62.1%.

[0159] Example 6

[0160] 1. Raw material ratio:

[0161] Citrus low-ester amidated pectin: 2.8 g (esterification degree 40%, amidation degree 15%);

[0162] Chitosan: 1.8 g (deacetylation degree ≥ 90%);

[0163] Rosmarinic acid: 0.36 g (20% of the mass of chitosan);

[0164] Anhydrous ethanol: 7 mL;

[0165] 1% (v / v) acetic acid aqueous solution: constant volume to 100 mL;

[0166] Deionized water: constant volume to 100 mL;

[0167] Nanocellulose whiskers: not added;

[0168] pH adjustment buffer: adjust pH to 5.5;

[0169] 2. Preparation steps:

[0170] Step one: Weigh 2.8 g of citrus low-ester amidated pectin, dissolve in hot water at 65°C, stir to dissolve, and dilute to 100 mL to obtain a 2.8% pectin solution.

[0171] Step two: Weigh 1.8 g of chitosan, dissolve and dilute to 100 mL to obtain a 1.8% chitosan solution.

[0172] Step three: Weigh 0.36 g of rosemary acid, dissolve in 7 mL of ethanol, and then add to the chitosan solution, stir for 30 minutes to obtain a mixed solution A.

[0173] Step four: Add the pectin solution to the mixed solution A. Adjust the pH to 5.5 with a buffer solution. React in a 52°C water bath at a speed of 250 rpm for 110 minutes to obtain a uniform composite sol.

[0174] Step five: After vacuum degassing of the composite sol (-0.08 MPa, 28 minutes), cast.

[0175] Step six: After drying the cast flat plate at 35°C and 60% relative humidity for 24 hours, remove the film.

[0176] 3. Film performance:

[0177] Appearance: light yellow, very flexible, good transparency.

[0178] Thickness: 42 ± 2 μm.

[0179] Tensile strength: 36.8 MPa.

[0180] Elongation at break: 31.5%.

[0181] Antibacterial properties: 92.2% for E. coli, 93.8% for S. aureus.

[0182] Optical properties: 500 nm transmittance 88.8%, haze 7.9%.

[0183] Antioxidant properties: DPPH retention rate 66.8% after 14 days.

[0184] Experimental Example 1

[0185] 1. Purpose of the experiment: to verify the cross-linking effect of citrus pectin, chitosan, and rosemary acid, the reinforcing effect of nanocellulose whiskers, and the synergistic contribution of the best pH environment to the final film performance (mechanical properties, antibacterial properties, antioxidant properties, optical properties).

[0186] 2. Reference group: the formulation and process of Example 2.

[0187] 3. Design of Comparative Examples: A total of 4 comparative examples (Comparative Example 1 to Comparative Example 4) were designed, each of which only changed one variable in the reference group.

[0188] Comparative Example 1 (No Pectin Control): To demonstrate that the presence of anionic polysaccharide is essential for the formation of the electrostatic complex matrix.

[0189] Changes: In Step 4, replace the pectin solution with an equal volume of water.

[0190] Comparative Example 2 (No Rosmarinic Acid Control): To demonstrate that rosmarinic acid not only provides antioxidant properties, but also significantly improves the mechanical properties of the film as a functional crosslinker.

[0191] Changes: Step 3 was omitted, and no rosmarinic acid ethanol solution was added to the chitosan solution.

[0192] Comparative Example 3 (No Nano-cellulose Whisker Control): To demonstrate the nano-enhancing effect of nano-cellulose whiskers.

[0193] Changes: In Step 4, no nano-cellulose whiskers were added.

[0194] Comparative Example 4 (Non-optimal pH Control): To demonstrate that the optimal pH environment is crucial for the formation of a stable, uniform complex.

[0195] Changes: In Step 4, the pH of the mixed system was adjusted to 6.5 (far above the optimal range).

[0196] 4. Performance Test Methods:

[0197] Mechanical Properties: According to the standard ASTM D882, the tensile strength (TS) and elongation at break (EAB) of the film were determined using a universal material testing machine.

[0198] Antibacterial Properties: Using the film covering method (modified agar plate diffusion method), the inhibition zone diameter or inhibition rate (%) against E. coli and S. aureus was determined.

[0199] Antioxidant Properties: Using the DPPH free radical scavenging method, the DPPH free radical scavenging rate of fresh film and after being placed in a simulated food system (such as 40°C olive oil) for 14 days was determined, and the activity retention rate was calculated.

[0200] Optical Properties: Using a UV-Vis spectrophotometer and a haze meter, the light transmittance (%) and haze (%) of the film at a wavelength of 500 nm were determined.

[0201] Experimental Table

[0202] Table 1: Summary of Performance Test Results for Each Group of Films

[0203]

[0204] Note: "-" means that the performance cannot be effectively measured due to the inability to form a film or complete opacity.

[0205] Analysis of experimental results

[0206] Analysis of Comparative Example 1 (without pectin):

[0207] Results: It was not possible to form a complete and uniform film, and the solution dried into brittle fragments. The mechanical properties were completely lost, the antibacterial properties and antioxidant properties were greatly reduced, and it was completely opaque.

[0208] Analysis: This demonstrates that the electrostatic complexation of citrus pectin as an anionic polysaccharide with cationic chitosan is the basis for film formation. Without pectin, chitosan acetic acid solution can form a film after drying, but the structure is dense, brittle and hard, and a uniform network structure cannot be formed. Its weak antibacterial properties are completely derived from chitosan itself, but lack the synergistic effect of pectin, and the effect is poor. This comparison highlights the necessity of "complexation".

[0209] Analysis of Comparative Example 2 (without rosmarinic acid):

[0210] Results: The tensile strength of the film decreased significantly by 38% (from 45.7 MPa to 28.3 MPa), but the flexibility (EAB) increased slightly. The antibacterial properties changed little, but the retention rate of antioxidant activity decreased sharply (from 58.9% to 15.2%).

[0211] Analysis: This shows that the core role of rosmarinic acid has two aspects: one is to act as a natural crosslinking agent, and its polyphenol structure forms a large number of hydrogen bonds with biological macromolecules, significantly enhancing the mechanical strength of the film; the second is to act as a source of antioxidant activity, providing long-lasting and slow-release antioxidant capacity. Without it, the film can be formed, but the performance is mediocre, especially the mechanical strength and long-term antioxidant properties are far inferior to the reference group.

[0212] Analysis of Comparative Example 3 (without nanocellulose whiskers):

[0213] Results: The tensile strength of the film decreased by about 16% (from 45.7 MPa to 38.5 MPa), and other properties such as antibacterial properties and optical properties were not significantly different from the reference group.

[0214] Analysis: This demonstrates that nanocellulose whiskers mainly play a reinforcing and toughening role. Its nanoscale and high aspect ratio allow it to be effectively dispersed in the polymer matrix, transmitting stress and thus significantly improving the mechanical properties of the material. This comparison verifies the unexpected reinforcing effect brought by the addition of nanocellulose whiskers.

[0215] Analysis of Comparative Example 4 (non-optimal pH):

[0216] Results: The film appearance is turbid with flocculation, the light transmittance sharply decreases, the haze increases, and the mechanical properties are extremely poor.

[0217] Analysis: pH value is the key to control the electrostatic complexation of protein / polysaccharide. At pH 6.5, the amino group of chitosan is deprotonated, the positive charge is weakened, while the carboxyl group of pectin is ionized, the negative charge is enhanced, the electrostatic attraction between the two is weakened, and a stable and uniform complex cannot be formed, leading to macroscopic phase separation and flocculation. This result proves the necessity of strictly controlling the pH in the acidic range (4.5-5.5), which is one of the key cores of whether the preparation method can be successful.

[0218] Conclusion: Through comparison with the reference group (Example 2), the four comparative examples strongly prove that:

[0219] Citrus pectin and chitosan are necessary components to form the basis of edible films.

[0220] The addition of rosemary acid brings a double effect: significantly enhancing the mechanical properties and providing long-term antioxidant properties, far exceeding the expectations as a simple additive.

[0221] Nanocellulose whiskers as reinforcing agents make a significant contribution to improving the mechanical properties of the film.

[0222] The pH value during the preparation process is a key process parameter to control the success or failure of the reaction.

[0223] The embodiments of the present application are given for the purpose of illustration and description, although embodiments of the present application have been shown and described above, it is understood that the above embodiments are exemplary and cannot be understood as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above embodiments within the scope of the present application, which should be included in the protection scope of the present application.

Claims

1. A process for the preparation of a natural polysaccharide composite of edible film, characterized by, The application relates to a preparation method of an edible film. Step one: preparation of an acid polysaccharide solution Citrus low-ester amidated pectin is dissolved in hot water with a temperature of 60-80 DEG C, and stirring is conducted until complete dissolution; after cooling to room temperature, a pectin solution with a mass concentration of 2%-4% is obtained; Step two: preparation of a cationic polysaccharide solution Chitosan is dissolved in 1% acetic acid aqueous solution, and stirring is conducted until complete dissolution; a chitosan solution with a mass concentration of 1.5%-2.5% is obtained; Step three: addition of a functional crosslinking agent Rosmarinic acid is dissolved in a small amount of ethanol, and then added into the chitosan solution obtained in step two; uniform stirring is conducted to obtain a mixed solution A; the addition amount of the rosmarinic acid is 10%-30% of the mass of the chitosan; Step four: compound blending Under magnetic stirring, the pectin solution obtained in step one is slowly added into the mixed solution A obtained in step three; the dry mass ratio of the pectin to the chitosan is controlled to be 1:1 to 1:2; after the addition is completed, the pH value of the mixed system is adjusted to 4.5-5.5, and the reaction is continuously conducted at 50-60 DEG C for 60-120 minutes to obtain a uniform and viscous compound sol; Step five: defoaming and casting The compound sol obtained in step four is defoamed under vacuum, and then cast on a polytetrafluoroethylene flat plate; Step six: drying to form a film The cast flat plate is placed in a constant-temperature and constant-humidity box with a temperature of 35-45 DEG C and a relative humidity of 50%-60%, and dried for 18-24 hours; after the film is carefully peeled off, the edible film is obtained.

2. The process for the preparation of natural polysaccharide composite of edible film as claimed in claim 1, wherein, The esterification degree of the citrus low-ester amidated pectin in step one is less than or equal to 40%, and the amidation degree is greater than or equal to 15%.

3. The process for the preparation of natural polysaccharide composite of edible film as claimed in claim 1, wherein, In step four, the pH value of the mixed system is accurately adjusted to 5.0 by using a citric acid-sodium phosphate buffer system.

4. The process for the preparation of natural polysaccharide composite of edible film as claimed in claim 1, wherein, In the compound sol obtained in step four, 0.5%-1% of nanocellulose whiskers with a length of 100-500 nm and a diameter of 10-50 nm are further added, and the total mass of the compound sol is 0.5%-1%.

5. The edible film prepared by the method of any one of claims 1-4, wherein the film has a thickness of about 0.1 to about 0.5 mm. The tensile strength of the film is not less than 35 MPa, the elongation at break is not less than 25%, and the antibacterial rates of the film on escherichia coli and staphylococcus aureus are both greater than 90%.

6. The edible film of claim 5 wherein the film is substantially free of water. The light transmittance of the film at a wavelength of 500 nm is greater than or equal to 85%, and the haze is less than or equal to 10%.

7. The edible film of claim 5 wherein the film is substantially free of water. The film has a slow-release antioxidant activity; when placed in a simulated food system, the retention rate of the DPPH free radical scavenging rate of the film in 14 days is still greater than 50%.

8. Application of the edible film in claim 5-7 as fresh-cut fruit, cake and meat product preservative packaging materials.

Images