Chitosan-based preservative film with pH colorimetric function as well as preparation method and application of chitosan-based preservative film

Abstract

The invention discloses a chitosan-based preservative film with a pH colorimetric function as well as a preparation method and application of the chitosan-based preservative film. The chitosan-based preservative film is prepared from the following preparation raw materials: chitosan, cellulose nanofibers, a deep eutectic solvent and a beet red-zinc ion complex. Wherein the chitosan enables the film to have antibacterial property; the cellulose nanofiber can enhance the tensile strength and toughness and reduce the water vapor / oxygen permeation coefficient; the beet red-zinc ion complex has the characteristic of more stable color development, and the color difference response delta E to pH 2-12 is greater than or equal to 8. The chitosan-based preservative film disclosed by the invention can be applied to food fresh-keeping packaging and food freshness monitoring; the preservative film disclosed by the invention can be used for effectively prolonging the service life of fruits and vegetables while ensuring the food safety; in food freshness monitoring application, the preservative film changes from low-pH-value orange to high-pH-value yellow according to the change of acid-base conditions, and the preservative film can be used as a pH colorimetric sensing film for freshness monitoring.

Description

Technical Field

[0001] The invention belongs to the technical field of fresh-keeping materials, and in particular relates to a chitosan-based fresh-keeping film with pH colorimetric function, and a preparation method and application thereof. Background Art

[0002] With the improvement of modern living standards, consumers have increasingly stringent requirements for food freshness. To maintain food freshness, traditional petrochemical-based preservatives, such as polyethylene (PE), polyvinyl chloride (PVC), and polyvinylidene chloride (PVDC), are widely used. These materials, due to their low cost and ease of use, occupy a key position in the food preservation field. However, they generally suffer from issues such as non-degradability, high safety risks, poor air permeability, and the tendency to cause white pollution. Furthermore, these materials may release potentially carcinogenic substances.

[0003] Chitosan is a natural biopolymer synthesized by deacetylation of chitin. Due to its excellent biocompatibility, degradability, and film-forming ability, it is considered an environmentally friendly preservative. Its degradation products are safe and harmless to the human body. Furthermore, chitosan itself is an organic polymer antimicrobial agent that exhibits significant inhibitory effects against bacteria, mold, and fungi. It is also non-toxic, odorless, and acid- and alkali-resistant. The film it forms is selectively permeable to oxygen and carbon dioxide, effectively regulating the gas composition of the preservation microenvironment and inhibiting aerobic respiration and metabolism in fruits and vegetables, thereby maintaining a certain preservation effect on fruits, vegetables, and meat.

[0004] However, chitosan, as a hydrophilic polysaccharide, has problems such as low mechanical strength and excessive hydrophilicity when it is formed into a film alone, which limits its application in the field of food packaging and is not conducive to maintaining the integrity of food packaging. At the same time, its heat resistance is not high and the duration of its antibacterial effect is also short. In the prior art, the modification of chitosan films is mostly achieved by adding inorganic nanomaterials (such as titanium dioxide) or synthetic plasticizers (such as glycerol), but these methods may introduce toxicity or reduce the biocompatibility of the film. In addition, various natural pigment indicators are used in the prior art to prepare smart packaging, but the stability and color sensitivity of natural pigment indicators in the chitosan system still need to be optimized. Therefore, it is of great application value to develop a chitosan-based fresh-keeping film with antibacterial effect, high mechanical strength and sensitive pH indication function. Summary of the Invention

[0005] To overcome the problems of the prior art, one object of the present invention is to provide a chitosan-based cling film. A second object of the present invention is to provide a method for preparing the chitosan-based cling film. A third object of the present invention is to provide applications of the chitosan-based cling film. A fourth object of the present invention is to provide a method for preserving food.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is:

[0007] A first aspect of the present invention provides a chitosan-based fresh-keeping film, comprising the following preparation raw materials: chitosan, cellulose nanofibers, a deep eutectic solvent, and a beetroot red-zinc ion complex.

[0008] The cellulose nanofiber (CNF) is a chain-like structure composed of 30 to 40 cellulose molecules arranged in bundles. Its width is approximately 4 nanometers, making it ultra-fine. The fiber has a crystallinity of over 70% and exhibits excellent physical properties. The application of cellulose nanofibers in plastic wrap can improve the mechanical strength of the film.

[0009] The structural formula of the beetroot red is shown in formula (1):

[0010]

[0011] The present invention complexes beetroot red with zinc ions, and the color development stability of the obtained complex is enhanced.

[0012] Preferably, the deep eutectic solvent comprises a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is selected from at least one of acetylcholine and betaine, and the hydrogen bond donor is selected from at least one of methanesulfonic acid, acetamide, polyethylene glycol, citric acid, ethanol and ethylene glycol; the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is (1-3):1.

[0013] The hydrogen bond acceptor of the present invention does not use choline chloride groups, thereby avoiding residual chloride ions and having the significance of being green and environmentally friendly.

[0014] More preferably, the preparation method of the deep eutectic solvent comprises the following steps: heating the hydrogen bond donor and the hydrogen bond acceptor at 60-80° C. and stirring for 4-7 hours until a uniform transparent liquid is formed, and then cooling to obtain the deep eutectic solvent.

[0015] More preferably, the pH value of the deep eutectic solvent is 3.5-4.0.

[0016] Preferably, the preparation method of the beetroot red-zinc ion complex comprises the following steps: carrying out a complex reaction between beetroot red and a zinc salt in a solvent to obtain the beetroot red-zinc ion complex.

[0017] More preferably, the zinc salt is ZnCl2 and / or ZnSO4.

[0018] More preferably, the mass ratio of beetroot red to zinc salt is 1:(0.5-2).

[0019] More preferably, the solvent is an ethanol-water solvent.

[0020] More preferably, the complexation reaction is carried out by stirring for 3-6 hours in a dark condition.

[0021] Preferably, the preparation raw materials further include nano titanium dioxide.

[0022] Preferably, the preparation raw materials further include acetic acid water solvent.

[0023] More preferably, the concentration of acetic acid in the acetic acid aqueous solvent is 1%.

[0024] Preferably, the mass ratio of the chitosan to the beetroot red-zinc ion complex is 1:(0.01-0.3).

[0025] Preferably, the mass ratio of chitosan to deep eutectic solvent is 1:(1-4).

[0026] Preferably, the thickness of the chitosan-based fresh-keeping film is 10 to 100 μm.

[0027] The second aspect of the present invention provides a method for preparing the chitosan-based fresh-keeping film according to the first aspect, comprising the following steps:

[0028] Chitosan solution, cellulose nanofiber dispersion, low eutectic solvent and beet red-zinc ion complex are mixed to prepare a film-forming solution; the film-forming solution is cast or coated on a substrate, and the chitosan-based fresh-keeping film is prepared after drying.

[0029] Preferably, the chitosan solution is an acetic acid solution containing chitosan;

[0030] Preferably, the ratio of chitosan to solvent in the chitosan solution is 1 g: (50-200) mL.

[0031] Preferably, the method comprises the following steps: ultrasonically mixing a chitosan solution and a cellulose nanofiber dispersion, and then sequentially adding a low eutectic solvent and a betaine-zinc ion complex and stirring and mixing them to obtain a film-forming solution.

[0032] More preferably, the ultrasonic mixing time is 20 to 40 minutes.

[0033] Preferably, the substrate is a food surface or a plastic sheet surface.

[0034] Preferably, the drying temperature is 20-80°C.

[0035] The third aspect of the present invention provides the use of the chitosan-based fresh-keeping film described in the first aspect in any of the following aspects:

[0036] a) Food freshness-keeping packaging;

[0037] b) Food freshness monitoring.

[0038] Preferably, the food freshness detection is specifically as follows: the color of the chitosan-based fresh-keeping film changes from red-orange at low pH values ​​to yellow at high pH values ​​according to the acid-base conditions of the food.

[0039] Preferably, the food types are dairy products, meat or fruits and vegetables.

[0040] A fourth aspect of the present invention provides a method for preserving food, comprising the following steps: coating the surface of the food with the chitosan-based preservative film described in the first aspect;

[0041] Alternatively, the film-forming solution in the preparation method of the second aspect is applied to the surface of the food, and the chitosan-based fresh-keeping film is formed on the surface of the food after drying.

[0042] The beneficial effects of the present invention are:

[0043] The present invention provides a chitosan-based fresh-keeping film, comprising the following raw materials: chitosan, cellulose nanofibers, a deep eutectic solvent, and a beetroot red-zinc ion (Anth-Zn) complex. The chitosan can impart antibacterial properties to the film. The cellulose nanofibers are commonly used in battery separators in the prior art. The present invention innovatively applies cellulose nanofibers to the chitosan film, thereby enhancing its tensile strength and toughness and reducing its water vapor / oxygen permeability coefficient. Specific performances include: tensile strength ≥20 MPa, elongation at break ≥50%, and water vapor permeability ≤400 g / m 2 day, oxygen transmission rate ≤50cm 3 / m 2 ·day; Anth-Zn complex has a more stable color development compared to natural pigment indicators, with a color difference response ΔE ≥ 8 for pH 2-12.

[0044] The chitosan-based cling film of the present invention can be used in food packaging and food freshness monitoring. In food packaging, the cling film of the present invention can effectively extend the lifespan of fruits and vegetables while ensuring food safety. In food freshness monitoring applications, the cling film changes color from orange at low pH values ​​to yellow at high pH values, depending on the acid-base conditions. Therefore, when food spoils, increased microbial growth and the production of biogenic amines cause changes in its acid-base environment, allowing the cling film to function as a pH colorimetric sensor for freshness monitoring. Thus, the chitosan-based cling film of the present invention has both freshness preservation and freshness monitoring functions, showing great potential for use in food packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 The tensile strength test results of chitosan-based cling film with different CNF addition amounts are shown;

[0046] Figure 2 The elongation at break test results of chitosan-based cling film with different CNF addition amounts;

[0047] Figure 3 The water vapor transmission coefficient test results of chitosan-based cling film with different CNF addition amounts;

[0048] Figure 4 The oxygen permeability coefficient test results of chitosan-based cling film with different CNF addition amounts are shown below;

[0049] Figure 5 Schematic diagram of the color change of chitosan-based plastic wrap in different pH buffers;

[0050] Figure 6 The stability test results of chitosan-based cling film with and without nano-TiO2 under ultraviolet light are shown;

[0051] Figure 7 The ΔE value change curve and color change diagram of chitosan-based plastic wrap in milk rancidity detection;

[0052] Figure 8 Comparison of spoilage between the treated group and the blank group after 7 days of storage. DETAILED DESCRIPTION

[0053] The present invention is further described in detail below through specific examples. Unless otherwise specified, the raw materials used in the following examples can be obtained from conventional commercial sources or prepared and isolated by simple synthesis; unless otherwise specified, the processes used are conventional processes in the art.

[0054] Example 1

[0055] This embodiment provides a chitosan-based fresh-keeping film, and the preparation steps of the chitosan-based fresh-keeping film are as follows:

[0056] S1. Preparation of deep eutectic solvent (DES):

[0057] Citric acid and betaine were mixed in a molar ratio of 1:2, with a total mass of 50 g, and placed in a reactor; the mixture was melted at 70°C with magnetic stirring at 500 rpm for 5 hours until a colorless transparent liquid was formed; the mixture was cooled to room temperature to obtain a deep eutectic solvent (DES), which was stored in a dry, light-proof container until further use; the obtained DES was transparent and viscous, free of impurity precipitation, and had a pH of 3.5-4.0, making it suitable for subsequent film-forming solution preparation.

[0058] S2. Preparation of beet red-zinc ion complex:

[0059] Beetroot red and ZnCl2 were dissolved in 50 mL of ethanol-water mixed solvent in a mass ratio of 1:0.8, stirred in the dark for 4 hours, and centrifuged and dried to obtain beetroot red-zinc ion complex (Anth-Zn).

[0060] S3. Preparation of film-forming solution:

[0061] Chitosan solution preparation: 3 g chitosan (CS) was dissolved in 100 mL 1% acetic acid solution and magnetically stirred until completely dissolved to form a 3 wt% CS solution;

[0062] CNF dispersion addition: 1.0 g of cellulose nanofibers (CNF, mass concentration 1.0 wt%) was added to the CS solution;

[0063] DES and Anth-Zn were added: 5 g of DES prepared in Example 1 and 0.3 g of beetroot red-zinc ion complex (mass concentration 0.3 wt%) were added in sequence;

[0064] Mixing and stirring: Magnetic stirring was performed at 400 rpm at room temperature for 2 hours to obtain a uniform, bubble-free film-forming solution.

[0065] S4. Preparation of chitosan-based fresh-keeping film:

[0066] The film-forming liquid was evenly cast onto the surface of the polytetrafluoroethylene substrate, dried in an oven at 50° C. for 6 hours, and peeled off to obtain a chitosan-based fresh-keeping film with a thickness of 50 μm.

[0067] Example 2

[0068] This embodiment provides a chitosan-based fresh-keeping film, and its preparation method is similar to that of Example 1, except that, in the preparation of the film-forming solution of S3, the added amount of the cellulose nanofibers is 0.5 g.

[0069] Example 3

[0070] This embodiment provides a chitosan-based fresh-keeping film, and its preparation method is similar to that of Example 1, except that, in the preparation of the film-forming solution of S3, the amount of cellulose nanofibers added is 2 g.

[0071] Example 4

[0072] This embodiment provides a chitosan-based fresh-keeping film, and its preparation method is similar to that of Example 1, except that, in the preparation of the film-forming solution of S3, the added amount of the cellulose nanofibers is 3 g.

[0073] Example 5

[0074] This embodiment provides a chitosan-based fresh-keeping film, the preparation method of which is similar to that of Example 1, except that nano-titanium dioxide is added. The preparation steps are as follows:

[0075] S1 and S2 are the same as in Example 1.

[0076] S3. Preparation of film-forming solution:

[0077] Chitosan solution preparation: 3 g chitosan (CS) was dissolved in 100 mL 1% acetic acid solution and magnetically stirred until completely dissolved to form a 3 wt% CS solution;

[0078] Addition of CNF: 1.0 g of cellulose nanofibers (CNF, mass concentration 1.0 wt%) was added to the CS solution and ultrasonicated for 30 min;

[0079] DES, TiO2, and Anth-Zn were added: 5 g of DES prepared in Example 1, 0.3 g of beetroot red-zinc ion complex (mass concentration 0.3 wt%), and 0.2 g of nano-titanium dioxide (TiO2, 0.2 wt%, particle size 10-25 nm) were added in sequence;

[0080] Mixing and stirring: Magnetic stirring was performed at 400 rpm at room temperature for 2 hours to obtain a uniform, bubble-free film-forming solution.

[0081] S4 is the same as Example 1.

[0082] Comparative Example 1

[0083] This comparative example provides a chitosan-based fresh-keeping film, the preparation method of which is similar to that of Example 1, except that beetroot red is used instead of beetroot red-zinc ion complex. The specific preparation steps are as follows:

[0084] S1 is the same as Example 1, and S2 does not make any improvement to beetroot red.

[0085] S3. Preparation of film-forming solution:

[0086] Chitosan solution preparation: 3 g chitosan (CS) was dissolved in 100 mL 1% acetic acid solution and magnetically stirred until completely dissolved to form a 3 wt% CS solution;

[0087] Addition of CNF: 1.0 g of cellulose nanofibers (CNF, mass concentration 1.0 wt%) was added to the CS solution and ultrasonicated for 30 min;

[0088] DES, TiO2, and Anth-Zn were added: 5 g of DES prepared in Example 1 and 0.3 g of beetroot red (mass concentration 0.3 wt%) were added in sequence;

[0089] Mixing and stirring: Magnetic stirring was performed at 400 rpm at room temperature for 2 hours to obtain a uniform, bubble-free film-forming solution.

[0090] S4 is the same as Example 1.

[0091] Mechanical properties test of plastic wrap

[0092] The tensile strength, elongation at break, water vapor permeability, and oxygen permeability of the chitosan-based cling films prepared in Examples 1-4 were measured using the following method:

[0093] 1. Mechanical properties test

[0094] The test was performed using an automatic tensile tester. Figure 1 The tensile strength test results of chitosan-based cling film with different CNF addition amounts are shown; Figure 2 The elongation at break test results of chitosan-based cling film with different CNF addition amounts.

[0095] 2. Water vapor transmission rate test

[0096] Cut the film into discs with a diameter of 6-7 cm and tightly mount them on a small vial cup (5 cm diameter) filled with dry, anhydrous CaCl2. Cover the top of the vial with the film and secure it with wax. Place the cup in a storage box filled with a saturated solution (RH 75%) at 25 ± 1°C and weigh it after 24 hours.

[0097] The WVP calculation formula of the film is as follows (I):

[0098] WVP=w / t×(l / A·ΔP) formula (Ⅰ);

[0099] w / t is the linear regression of mass and time, l is the thickness of the film, A is the area of ​​the film, and ΔP is the partial pressure difference across the film at 25°C (3169 Pa).

[0100] Figure 3 The water vapor permeability coefficient test results of chitosan-based cling film with different CNF addition amounts.

[0101] 3. Oxygen transmission rate test

[0102] The test was performed using an automatic oxygen permeability meter. Figure 4 The oxygen permeability coefficient test results of chitosan-based cling film with different CNF addition amounts.

[0103] The results are ranked by Figures 1-4As shown in the figure, the tensile strength and elongation at break of chitosan cling film increase with the increase of CNF. When the CNF addition amount is 3%, the tensile strength is ≥20MPa and the elongation at break is ≥50%. The water vapor permeability and oxygen permeability of chitosan cling film decrease with the increase of CNF. When the CNF addition amount is 3%, the water vapor permeability is ≤400g / m 2 day; oxygen transmission rate ≤50cm 3 / m 2 ·day.

[0104] Cling film color performance test

[0105] The ΔE value test method is to record the color change of the film with a digital camera and calculate the corresponding ΔE value. The corresponding L*, a, and b parameters are recorded. The color difference of the film is calculated according to the following formula (II). The stability of the film is determined by the color change of the film.

[0106]

[0107] Where L* represents lightness, a* represents red / green values, and b* represents blue / yellow values. ΔE describes the total color difference; larger ΔE indicates greater color difference.

[0108] 1. Color rendering difference

[0109] The film of Example 1 was placed in different pH buffers (pH 3 to 12). Figure 5 Schematic diagram of the color change of chitosan-based plastic wrap in different pH buffers. Depending on the acid-base conditions, the plastic wrap changes from orange at low pH to yellow at high pH.

[0110] 2. Color stability

[0111] The chitosan-based fresh-keeping films prepared in Example 1 and Example 5 were placed under ultraviolet light (365 nm, intensity 10 mW / cm 2 ) for 60 hours, and the ΔE value was measured every 12 hours. Figure 6 The stability test results of chitosan-based cling film with and without nano-TiO2 under UV light:

[0112] The group without TiO2 addition: ΔE value decreased from the initial 12.3 to 5.1, with a decay rate of 58.5%;

[0113] Adding TiO2 group: the ΔE value dropped from the initial 12.6 to 10.5, the decay rate was 16.7, and the color stability was improved by more than 50%.

[0114] Application Example 1

[0115] The cling film of Example 1 and Comparative Example 1 is used as a freshness indicator film in food freshness detection. The specific application method is as follows:

[0116] The color change of the membrane indicates the rancidity of milk or yogurt at 4-50°C. Pour 20 ml of milk into a sterilized Petri dish and let it sit for 60 hours. Every 12 hours, remove a portion of the milk and place the colorimetric membrane in the milk. Wait 10 minutes, then use a digital camera to record the color change and calculate the corresponding ΔE value. Figure 7 The ΔE value change curve and color change diagram of the chitosan-based cling film in milk rancidity detection are shown. The color of the pH colorimetric film of Example 1 and Comparative Example 1 changes from yellow to red. The ΔE of Example 1 is larger than that of Comparative Example 1, which is more conducive to intuitively judging the early spoilage of dairy products based on the color change of the freshness indicator film.

[0117] Application Example 2

[0118] The application of biocomposite coating in the preservation of blueberries, the specific preservation method is as follows:

[0119] 1. Apply the film-forming solution of S3 in Example 1 to the surface of fresh blueberries by dip coating to ensure complete coverage;

[0120] 2. Place in a ventilated drying oven at 30°C and dry for 2 hours to form a uniform and transparent biocomposite coating;

[0121] 3. Store the coated blueberries and the uncoated control group at 25°C and 60% humidity and observe their spoilage. This is referred to as the treated group. The blank group remains untreated.

[0122] The following decay rate was calculated as the number of decayed fruits divided by the total number of fruits; weight loss was calculated as (initial weight - weight after treatment) / initial weight; Figure 8 The figure shows a comparison of the decay of the treated group and the blank group after 7 days of storage. After 7 days of storage, the decay rate of the coated blueberries was only 20% (64% for the uncoated group), indicating that the plastic wrap of the present invention can effectively extend the life of fruits and vegetables; the weight loss rate was reduced to 7.8% (20.4% for the uncoated group), indicating that the plastic wrap of the present invention has excellent water vapor barrier properties and effectively delays water loss.

[0123] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A chitosan-based fresh-keeping film, characterized in that: The preparation method comprises the following raw materials: chitosan, cellulose nanofiber, low eutectic solvent and beet red-zinc ion complex.

2. The chitosan-based fresh-keeping film according to claim 1, wherein The deep eutectic solvent includes a hydrogen bond acceptor and a hydrogen bond donor; the hydrogen bond acceptor is selected from at least one of acetylcholine, betaine and 1-ethylammonium chloride, and the hydrogen bond donor is selected from at least one of methanesulfonic acid, acetamide, polyethylene glycol, citric acid, ethanol and ethylene glycol; the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is (1-3):

1.

3. The chitosan-based fresh-keeping film according to claim 1, wherein The preparation method of the beetroot red-zinc ion complex comprises the following steps: carrying out a complex reaction between beetroot red and a zinc salt in a solvent to obtain the beetroot red-zinc ion complex.

4. The chitosan-based fresh-keeping film according to claim 1, wherein The preparation raw materials also include nano titanium dioxide.

5. The chitosan-based fresh-keeping film according to claim 1, wherein The mass ratio of the chitosan to the beetroot red-zinc ion complex is 1:(0.01-0.3).

6. The method for preparing the chitosan-based fresh-keeping film according to any one of claims 1 to 5, characterized in that: The steps include: Chitosan solution, cellulose nanofiber dispersion, low eutectic solvent and beet red-zinc ion complex are mixed to prepare a film-forming solution; the film-forming solution is cast or coated on a substrate, and the chitosan-based fresh-keeping film is prepared after drying.

7. The method for preparing the chitosan-based fresh-keeping film according to claim 6, wherein The chitosan solution is an acetic acid solution containing chitosan; And / or, the ratio of chitosan to solvent in the chitosan solution is 1 g: (50-200) mL.

8. The method for preparing the chitosan-based fresh-keeping film according to claim 6, wherein The method specifically comprises the following steps: ultrasonically mixing a chitosan solution and a cellulose nanofiber dispersion, and then sequentially adding a low eutectic solvent and a beetroot red-zinc ion complex and stirring and mixing them to obtain a membrane solution.

9. Use of the chitosan-based fresh-keeping film according to any one of claims 1 to 5 in any of the following aspects: a) Food freshness-keeping packaging; b) Food freshness monitoring.

10. A method for preserving food, characterized in that: The method comprises the following steps: coating the chitosan-based preservative film according to any one of claims 1 to 5 on the surface of food; Alternatively, the film-forming solution in the preparation method according to any one of claims 6 to 8 is applied to the surface of food, and the chitosan-based fresh-keeping film is formed on the surface of the food after drying.

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