pH-responsive food spoilage indicator film based on carboxymethyl chitosan sodium alginate and preparation method thereof

Abstract

This invention belongs to the field of food packaging materials technology and discloses a pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate, and its preparation method. This invention provides a smart packaging film that integrates pH-responsive color change and active antibacterial functions within a single film layer. The film uses carboxymethyl chitosan and sodium alginate as the composite film-forming substrate, and simultaneously loads anthocyanins as a pH indicator and magnolol as a natural antibacterial agent. This invention is prepared by a casting method, which is simple. The film exhibits sensitive pH-responsive color change capability for amines produced by food spoilage, enabling real-time visual early warning of the spoilage process; simultaneously, magnolol and carboxymethyl chitosan synergistically endow the film with broad-spectrum antibacterial activity, actively extending the shelf life of fresh food. All components of this invention are derived from natural sources and have biodegradable capabilities, providing a simple, functionally integrated, green, and safe smart packaging solution for fresh food.

Description

Technical Field

[0001] This invention belongs to the field of food packaging materials technology, and relates to a pH-responsive food spoilage indicator film based on carboxymethyl chitosan sodium alginate and its preparation method. Specifically, it relates to a natural polysaccharide-based composite film with both pH-responsive color change and active antibacterial function and its preparation method. Background Technology

[0002] Globally, food spoilage causes hundreds of billions of dollars in economic losses annually, with microbial contamination being a core factor leading to food deterioration. Taking meat, fish, and fruits and vegetables as examples, the organic acids (such as lactic acid and acetic acid), amines (such as trimethylamine and cadaverine), and sulfides (such as hydrogen sulfide) produced by microbial metabolism during spoilage significantly alter the pH value of the environment inside the packaging. For instance, the pH value of spoiled fish can drop from an initial 6.8-7.2 to 5.5-6.0, accompanied by an increase in TVB-N (volatile basic nitrogen); the pH value of rotting strawberries can rise above 4.5, while releasing volatile substances such as acetaldehyde. Traditional packaging materials can only passively block the external environment and cannot reflect the internal state of food in real time, making it difficult for consumers to accurately judge the freshness of food by appearance or smell, thus increasing food safety risks.

[0003] Spoilage of fresh food during storage and transportation is a key factor leading to foodborne illnesses and resource waste. Amines produced during food spoilage can increase the pH of the packaging microenvironment; therefore, developing pH-responsive smart packaging based on this characteristic has become a research hotspot.

[0004] The existing technology has the following shortcomings: (1) Packaging with pH indication function often adopts a double / multi-layer structure, which is complicated in preparation process and there are hidden dangers in the interlayer bonding force.

[0005] (2) Most smart packaging has a single function and it is difficult to efficiently integrate "real-time visual monitoring" with "active antibacterial preservation".

[0006] (3) Packaging films based on natural polysaccharides generally suffer from insufficient mechanical properties and poor water resistance, which limits their practical application.

[0007] (4) The functional components are not compatible with the substrate, which can easily lead to indicator leakage and antibacterial agent release.

[0008] Therefore, there is a need to develop a green and intelligent packaging material that is simple in structure, easy to prepare, has balanced performance, and integrates multiple preservation functions. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of existing smart packaging materials, which have complex structures and limited functions, and to provide a simple single-layer multifunctional composite film with both pH-responsive color change and active antibacterial functions, as well as its preparation method.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate is disclosed. The composite film has a homogeneous and dense single-layer structure with a continuous and integral cross-section, without any visible delamination interfaces and no penetrating pores. Carboxymethyl chitosan and sodium alginate are used as the composite film-forming substrates, and anthocyanins are simultaneously loaded into the single-layer structure as a pH-responsive indicator and magnolol as a natural antibacterial agent. The thickness of the composite film is 20–50 μm.

[0011] Furthermore, the mass ratio of carboxymethyl chitosan to sodium alginate is 6.5:2.5 to 7.5:3.5.

[0012] Furthermore, the anthocyanin content is 8–12 wt% based on the dry total mass of the composite membrane.

[0013] Furthermore, based on the total dry weight of the composite membrane, the content of magnolol is 0.1–0.5 wt%.

[0014] Furthermore, the synergistic effect of magnolol and carboxymethyl chitosan resulted in a composite film with an antibacterial rate of over 85% against foodborne pathogens.

[0015] A method for preparing a pH-responsive food spoilage indicator film based on carboxymethyl chitosan sodium alginate includes the following steps: (1) Prepare aqueous solutions of carboxymethyl chitosan and sodium alginate separately; (2) Mix the two solutions obtained in step (1) according to the above mass ratio and stir to obtain a blended adhesive solution; (3) Add anthocyanins and magnolol to the blended solution obtained in step (2) at the same time, stir and disperse to obtain a functionalized film-forming solution; (4) The functionalized membrane solution obtained in step (3) is allowed to stand to degas, then cast into a film once, dried and then removed to obtain a single-layer composite membrane with both indicator and antibacterial functions.

[0016] Preferably, the anthocyanin content in step (3) is 0.2% (w / v), and the magnolol content is 2% (w / w).

[0017] Preferably, the drying temperature in step (4) is 40~45℃ and the drying time is 10~14 hours.

[0018] The application of a pH-responsive food spoilage indicator film based on carboxymethyl chitosan sodium alginate in the preparation of food packaging materials. The composite film simultaneously performs pH-responsive color-changing indication function and active antibacterial and preservation function.

[0019] Preferably, the food is any one of fresh meat, aquatic products, and fruit.

[0020] Compared with the prior art, the beneficial effects of the present invention include: 1. Single-layer structure achieves dual-function integration Unlike existing technologies that require complex designs with double / multi-layer structures to achieve functional partitioning, this invention utilizes a specific ratio of CS / SA composite network to simultaneously and stably load anthocyanins (indicator) and magnolol (antibacterial) within a single membrane layer, achieving integrated "indicator-antibacterial" function. This design simplifies the preparation process and realizes the integration of "indicator-antibacterial" dual functions.

[0021] 2. Synergistic effect mechanism of natural ingredients Magnolol and carboxymethyl chitosan exhibit synergistic antibacterial effects; the carboxymethyl chitosan / sodium alginate composite network provides a stable loading environment for anthocyanins, while alkaline substances produced by putrefaction can disturb the polyelectrolyte balance, triggering a more sensitive color change response.

[0022] 3. All-natural ingredients and controllability All components are derived from natural renewable resources, the preparation process requires no organic solvents, and the product is biodegradable. By adjusting the CS / SA ratio and the loading of functional ingredients, it can be flexibly adapted to the packaging needs of fresh foods. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1: Optimization of the carboxymethyl chitosan / sodium alginate substrate ratio (providing a foundation for dual functionality) Prepare a 1% carboxymethyl chitosan aqueous solution and a 1% sodium alginate aqueous solution, and mix them according to the mass ratio of carboxymethyl chitosan to sodium alginate of 9:1, 7:3, 5:5, 3:7, and 1:9. Stir magnetically for 2 hours, degas ultrasonically for 30 minutes, cast on a plate, and dry at 45°C for 12 hours.

[0026] Mechanical tests showed that the membrane surface was smooth and flat when the ratio of carboxymethyl chitosan to sodium alginate was 7:3, with a tensile strength of 9.76 MPa and an elongation at break of 67.20%, exhibiting the best overall performance. This ratio provides a good mechanical platform for the subsequent loading of bifunctional components, ensuring the practical application performance of the functional membrane.

[0027] Example 2: Preparation and Characterization of a Single-Layer Bifunctional Smart Membrane Based on a ratio of 7:3 for carboxymethyl chitosan and sodium alginate, anthocyanins and magnolol were added and dispersed by stirring. The subsequent film-forming process was the same as in the previous method to obtain a single-layer bifunctional smart membrane.

[0028] Performance testing: (1) Indicator function: It changes from red to blue-green in pH 3~9 buffer solution, and the color change is sensitive and can be distinguished by the naked eye; (2) Antibacterial function: The antibacterial rate against Escherichia coli and Staphylococcus aureus is >90%, and it has the ability to actively inhibit bacteria and preserve freshness; (3) Mechanical properties: tensile strength 9.21 MPa, elongation at break 63.8%, meeting the requirements for use as fresh produce packaging materials; (4) Antioxidant properties: The higher the anthocyanin content, the higher the DPPH free radical scavenging rate and the more sensitive the response to ammonia.

[0029] Example 3: Application Verification of Pork Preservation (Synergistic Effect of Dual Functions) Fresh pork was divided into two groups: the experimental group was wrapped with the single-layer dual-function intelligent film of this invention, and the control group was wrapped with ordinary PE preservation film and stored at 4°C for 7 days.

[0030] Results: (1) Indicator function verification: The color of the membrane in the experimental group gradually changed from purple-red to blue-green with storage time. ΔE was significantly positively correlated with TVB-N content, realizing a visual early warning of the spoilage process; (2) Antibacterial function verification: The total number of colonies in the pork in the experimental group increased significantly slower than that in the control group, and the shelf life was extended by about 30%, proving that magnolol played an active antibacterial role.

[0031] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate, characterized in that: The composite membrane is a homogeneous and dense single-layer structure with a continuous and integral cross-section, without any visible delamination interfaces and no penetrating pores inside. Carboxymethyl chitosan and sodium alginate are used as the composite film-forming substrates, and anthocyanins are simultaneously loaded into the single-layer structure as a pH response indicator and magnolol as a natural antibacterial agent. The thickness of the composite membrane is 20–50 μm.

2. The pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate according to claim 1, characterized in that: The mass ratio of carboxymethyl chitosan to sodium alginate is 6.5:2.5 to 7.5:3.

5.

3. The pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate according to claim 1 or 2, characterized in that: The anthocyanin content is 8-12 wt% based on the dry total mass of the composite membrane.

4. The pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate according to claim 1 or 2, characterized in that: The content of magnolol is 0.1 to 0.5 wt% based on the total dry weight of the composite membrane.

5. The pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate according to claim 1 or 2, characterized in that: The synergistic effect of magnolol and carboxymethyl chitosan results in a composite film with an inhibition rate of more than 85% against foodborne pathogens.

6. A method for preparing a pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate as described in any one of claims 1-5, characterized in that, Includes the following steps: (1) Prepare aqueous solutions of carboxymethyl chitosan and sodium alginate separately; (2) The two solutions obtained in step (1) are mixed according to the mass ratio described in claim 1 and stirred to obtain a blended adhesive solution; (3) Add anthocyanins and magnolol to the blended solution obtained in step (2) at the same time, stir and disperse to obtain a functionalized film-forming solution; (4) The functionalized membrane solution obtained in step (3) is allowed to stand to degas, then cast into a film once, dried and then removed to obtain a single-layer composite membrane with both indicator and antibacterial functions.

7. The preparation method according to claim 6, characterized in that: The anthocyanin content in step (3) is 0.2% (w / v), and the magnolol content is 2% (w / w).

8. The preparation method according to claim 6, characterized in that: The drying temperature in step (4) is 40~45℃ and the drying time is 10~14 hours.

9. The application of a pH-responsive food spoilage indicator film based on carboxymethyl chitosan and sodium alginate as described in any one of claims 1-5 in the preparation of food packaging materials, characterized in that: The composite membrane simultaneously functions as a pH-responsive color indicator and an active antibacterial and preservation device.

10. The application according to claim 9, characterized in that: The food mentioned can be any one of fresh meat, aquatic products, and fruit.