Preparation method and application of tea residue carbon dot composite film

Abstract

The invention discloses a preparation method of a tea residue carbon dot composite film, which comprises the following steps: taking tea residue as a raw material, and carrying out hydrothermal reaction, purification and drying to obtain CDs; the preparation method comprises the following steps: respectively dissolving CDs, polyvinyl alcohol and fish gelatin in deionized water to prepare a CDs solution and a polyvinyl alcohol / fish gelatin solution; adding glycerol and hydrochloric acid which are 5-30% of the mass fraction of the polyvinyl alcohol / fish gelatin into the polyvinyl alcohol / fish gelatin solution, uniformly mixing, and then adding the CDs solution to obtain a composite membrane solution; and uniformly spreading the composite membrane liquid, and drying at constant temperature to obtain the tea residue carbon dot composite membrane. The invention further discloses the tea residue carbon dot composite film prepared by the method and application of the tea residue carbon dot composite film. The raw materials are cheap and easy to obtain, the process is simple, green and environment-friendly, and the prepared CDs / polyvinyl alcohol / fish gelatin composite film has a very good antibacterial effect on staphylococcus aureus and escherichia coli, has relatively high pH sensitivity, can monitor the freshness of food, has a relatively strong ultraviolet blocking effect, and can protect packaged food from light degradation.

Description

Technical Field

[0001] The invention belongs to the technical field of food preservation, and specifically relates to a preparation method and application of a tea residue carbon dot composite film. Background Art

[0002] Food packaging is an important part of the food industry. Currently, most food packaging is made of petroleum-based polymers because they are cheap, easy to process, and can be mass-produced. However, the use of such polymers has brought about serious environmental problems such as fossil resource depletion and global warming. Bio-based polymer composites have attracted widespread attention in the field of food packaging due to their biodegradability and non-toxicity. However, existing bio-based polymer composite packaging materials cannot simultaneously take into account the dual functions of freshness indication and antibacterial preservation.

[0003] Tea residues are rich in gallic acid, catechin compounds and other phenolic substances, which have various biological activities. However, the extraction of polyphenols requires the use of a large amount of organic reagents, and the extracts are easily degraded by factors such as light, heat, and oxygen, which limits their application in the field of food packaging. Summary of the invention

[0004] Based on this, the present invention provides a method for preparing a tea residue carbon dot composite membrane and its application. The present invention composites CDs with antibacterial activity and pH responsiveness with polyvinyl alcohol / fish gelatin to prepare a new composite membrane.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for preparing a tea residue carbon dot composite film comprises the following steps:

[0007] S1. Using tea residue as raw material, CDs are obtained through hydrothermal reaction, purification and drying;

[0008] S2. The CDs, polyvinyl alcohol and fish gelatin obtained in step S1 were dissolved in deionized water to prepare a CDs solution and a polyvinyl alcohol / fish gelatin solution having a concentration of 50 to 80 mg / ml, respectively;

[0009] S3. Add glycerol and hydrochloric acid with a mass fraction of 5 to 30% of polyvinyl alcohol / fish gelatin to the polyvinyl alcohol / fish gelatin solution obtained in step S2, mix well and then add the CDs solution obtained in step S2 to obtain a composite membrane solution;

[0010] S4. The composite film liquid obtained in step S3 is evenly spread and dried at a constant temperature to obtain a tea residue carbon dot composite film.

[0011] Preferably, the tea residues described in step S1 can be any one or more of green tea, white tea, yellow tea, oolong tea, black tea and dark tea.

[0012] Preferably, the tea residue in step S1 is dissolved in the aqueous solution at a solid-liquid ratio of 1:3 to 1:5.

[0013] Preferably, the heating time of the hydrothermal reaction in step S1 is 4 to 12 hours.

[0014] Preferably, the purification method described in step S1 is any one or more of filtration, centrifugation, dialysis, column chromatography and the like.

[0015] Preferably, the mass fraction of the CDs solution in step S2 is 0.25-1%.

[0016] The tea residue carbon dot composite film is prepared by the above method.

[0017] The application of the tea residue carbon dot composite film is to prepare food preservative materials.

[0018] The effective effects of the present invention are as follows:

[0019] 1. The raw materials for the preparation of the present invention are cheap and readily available, the preparation process is simple and convenient, and it is green and environmentally friendly.

[0020] 2. The CDs / polyvinyl alcohol / fish gelatin composite film prepared by the present invention has a good antibacterial effect on Staphylococcus aureus and Escherichia coli, and has good prospects for application in the field of food safety packaging.

[0021] 3. The CDs / polyvinyl alcohol / fish gelatin composite film prepared by the present invention has high pH sensitivity and can monitor the freshness of food.

[0022] 4. The CDs / polyvinyl alcohol / fish gelatin composite film prepared by the present invention has a strong UV blocking effect and can protect packaged food from UV-induced photodegradation. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described below in conjunction with embodiments with reference to the accompanying drawings.

[0024] In the accompanying drawings of the present invention, polyvinyl alcohol is abbreviated as PVA, and fish gelatin is abbreviated as FG.

[0025] Figure 1 Images were taken for CDs / PVA / FG composite films.

[0026] Figure 2 This is the antibacterial result image of CDs / PVA / FG composite membrane liquid.

[0027] Figure 3 Images of CDs / PVA / FG under different pH conditions.

[0028] Figure 4 This is the mechanical properties diagram of CDs / PVA / FG composite film.

[0029] Figure 5 This is the UV-visible transmission spectrum of CDs / PVA / FG composite film. DETAILED DESCRIPTION

[0030] In order to better understand the present invention, the present invention is further described in detail below in conjunction with the embodiments and drawings. However, those skilled in the art understand that the following embodiments are not limitations on the scope of protection of the present invention, and any changes and modifications made on the basis of the present invention are within the scope of protection of the present invention.

[0031] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0032] Unless otherwise specified, the materials and reagents used in the following examples can be obtained from commercial sources.

[0033] In the embodiments of the present invention, polyvinyl alcohol is referred to as PVA, and fish gelatin is referred to as FG.

[0034] Test method:

[0035] Antibacterial performance test

[0036] 1. Antibacterial performance test of carbon dots: The obtained CDs were prepared into solutions with concentrations of 0.0625, 0.125, 0.25, 0.5, 1 and 2 mg / mL, and cultured with Gram-positive bacteria Staphylococcus aureus (S. aureus) and Gram-negative bacteria Escherichia coli (E. coli) for 24 hours respectively. The CDs concentration with no obvious bacterial growth was recorded as the minimum inhibitory concentration;

[0037] 2. Antibacterial performance test of composite film: The antibacterial activity of CDs / PVA / FA film was tested by Oxford cup method. Bacterial suspension (10 7 CFU / mL), coated in solid culture medium, maintained at 37°C for 24h, specifically including:

[0038] (1) Staphylococcus aureus and Escherichia coli were used as test strains, and the bacterial suspension was added into the prepared liquid culture medium to activate the strains.

[0039] (2) When the colony grows to 10 9 CFU / mL, and dilute the two bacterial suspensions to 10 7 CFU / mL. Take 100 μL of the diluted bacterial solution and spread it evenly on the surface of the nutrient solution agar plate, take 100 μL of the membrane solution of a certain concentration and add it to the Oxford cup, incubate at 37°C for 24 hours, and observe the bacterial growth.

[0040] pH sensitivity test of CDs / PVA / FG composite membrane

[0041] To determine the pH response of the CDs / PVA / FG composite films, they were exposed to different buffer solutions (pH 3, 5, 7, 9, 11, and 12) for 30 min. The color changes of the films were photographed.

[0042] Mechanical properties test of CDs / PVA / FG composite film

[0043] The composite film was cut into rectangular strips of 10×40 mm and tested using a texture analyzer. The test parameters were: initial distance of the clamps was 20 mm, and the stretching speed was 1 mm / s.

[0044] UV blocking performance test of CDs / PVA / FG composite film

[0045] The UV-visible transmittance (200-800nm) of the composite films was measured using a UV-visible spectrophotometer. The transmittance spectra were recorded to determine the UV shielding and transparency of the films. The transparency of the films was determined by measuring the transmittance (%) at 660nm. UV rays can be divided into three forms: UVA-A (320-400nm), UV-B (280-320nm), and UV-C (200-280nm)

[0046] Example 1 Preparation of CDs / PVA / FG composite film

[0047] A method for preparing a tea residue carbon dot composite film comprises the following steps:

[0048] S1. The tea residue was dissolved in an aqueous solution at a solid-liquid ratio of 1:5, and a hydrothermal reaction was performed to generate a solid-liquid mixture having carbon dots. The heating time of the hydrothermal reaction was 8 h.

[0049] S2. After the reaction is completed, centrifuge at a centrifugal force of 10000g for 30 minutes to separate and remove macromolecular substances and insoluble precipitates, and collect the supernatant; then dialyze the obtained preliminary carbon dot aqueous solution through a 1000Da dialysis bag to obtain a light brown clear solution; finally freeze-dry for 2 days, and collect the obtained carbon dot powder, i.e., the carbon dots with antibacterial activity and pH response;

[0050] S3. Add 5g PVA and 5g FG into 100mL deionized water respectively, and stir magnetically at 95℃ and 50℃ for 60min until the solution becomes clear and transparent; wait for the solution to cool to room temperature; mix the PVA and FG solutions in a ratio of 1:1, add glycerol and hydrochloric acid accounting for 20% of the dry weight of PVA and FG for cross-linking; after magnetic stirring, add CDs to the solution so that the carbon dot concentrations are 0.25%, 0.5%, 0.75%, and 1%, respectively, stir for 15min, and mix well to obtain a composite membrane liquid; take 25mL CDs / PVA / FG solution and evenly spread it in a culture dish, dry it at room temperature for 48h, and obtain the composite membrane with antibacterial activity and pH response.

[0051] Comparative Example 1 A method for preparing a thin film 1

[0052] 5 g FG was added into 100 mL deionized water and magnetically stirred at 50° C. for 60 min until the solution was clear and transparent; the solution was cooled to room temperature; glycerol accounting for 20% of the dry weight of FG was then added to obtain a composite membrane liquid; the composite membrane liquid was evenly spread and dried at a constant temperature to obtain a FG film.

[0053] Comparative Example 2 A method for preparing a thin film

[0054] 5 g PVA and 5 g FG were added into 100 mL deionized water respectively, and magnetically stirred at 95°C and 50°C for 60 min until the solution was clear and transparent; the solution was cooled to room temperature; the PVA and FG solutions were then mixed in a 1:1 ratio, and glycerol and hydrochloric acid accounting for 20% of the dry weight of PVA and gelatin were added for cross-linking; a composite membrane liquid was obtained; the composite membrane liquid was evenly spread and dried at a constant temperature to obtain a PVA / FG composite membrane.

[0055] Comparative Example 3 Preparation of a high concentration CDs / PVA / FG composite film

[0056] 5 g PVA and 5 g FG were added to 100 mL deionized water respectively, and magnetically stirred at 95°C and 50°C for 60 min until the solution was clear and transparent; the solution was cooled to room temperature; the PVA and FG solutions were mixed in a 1:1 ratio, and glycerol and hydrochloric acid accounting for 20% of the dry weight of PVA and FG were added for cross-linking; CDs were added to the solution after magnetic stirring so that the carbon dot concentrations were 1.5% and 2%, respectively, and stirred for 15 min to obtain a composite membrane solution; 25 mL of CDs / PVA / FG solution was evenly spread on a culture dish and dried at room temperature for 48 h to obtain a high-concentration CDs composite membrane.

[0057] The obtained bioactive composite film was photographed, and the results are as follows:

[0058] Figure 1This is a picture of CDs / PVA / FG composite film. It can be seen that with the increase of carbon dot concentration, the composite film gradually changes from colorless to light brown, but all films are transparent, and the addition of carbon dots does not change their transparent properties.

[0059] Figure 2 This is the picture of the antibacterial results of the composite membrane liquid. It can be seen that the PVA / FG composite membrane liquid has no antibacterial effect on S. aureus and E. coil. When the addition amount of CDs is 0.75%, the inhibition zone diameters of the composite membrane on S. aureus and E. coil are 22.3 mm and 16.8 mm, respectively, which indicates that the composite membrane with the addition of CDs has excellent antibacterial activity.

[0060] Table 1 shows the antibacterial activity of CDs against different bacteria. It can be seen that CDs have a certain degree of antibacterial activity against foodborne pathogens, among which S. aureus is the most sensitive, with a minimum inhibitory concentration of 0.125 mg / mL, and the minimum inhibitory concentration of E. coil is 1 mg / mL.

[0061] Table 1 Antibacterial activity of CDs against different bacteria

[0062] Bacterial species G+ / - MIC (mg / mL) S. aureus + 0.125 E. coli - 1

[0063] Figure 3 The following are pictures of CDs / PVA / FG composite membranes under different pH conditions. It can be seen that the composite membrane is sensitive to pH changes. When pH>7, the color of the composite membrane changes to reddish brown; when pH<7, the color of the composite membrane is yellow-green. Therefore, the color change of the composite membrane can be used as a pH indicator and monitor the freshness of food.

[0064] Figure 4The mechanical test results of CDs / PVA / FG composite films show that the mechanical properties of the FG film in comparative example 1 are the worst, with TS and EAB% of 23.12±1.51Mpa and 16.58±0.87%, respectively. The TS of the PVA / FG composite film in comparative example 2 is 27.53±2.07MPa, and the EAB% is 78.67±1.52. The results show that the addition of PVA improves the mechanical properties of the film. When CDs are added to the PVA / FG composite film, the TS (MPa) value and EAB% increase. The results show that adding a lower amount of CDs has a significant effect on the composite film. Therefore, only incorporating 0.25-0.5% of CDs can improve the TS and EAB% of the film, up to 32.51±4.52Mpa and 118.15±2.52%. When the CDs concentration in the composite film exceeds 1%, its mechanical properties decrease. The addition of CDs to PVA / FG composite films provided plasticizer and surfactant functions to the composite films, thereby improving their flexibility. In addition, when higher concentrations of CDs (1-2%) were added, the films became harder and less flexible.

[0065] Figure 5 The test results of the UV protection effect of CDs / PVA / FG composite film show that the UV blocking performance of PVA / FG composite film is insufficient. With the incorporation of CDs into the composite film, it was observed that the transmittance of high wavelength (>600nm) visible light decreased insignificantly. On the other hand, a large amount of light blocking was observed in the low wavelength (400-600nm) region. When 0.25% CDs was incorporated, more than 80% of the light in the UV region was blocked, that is, 83.1% in the UV-A region, 99.0% and 100% in the UV-B and UV-C regions. It shows that the composite film of the present invention has UV blocking effect and can protect packaged food from UV-induced photodegradation.

[0066] Although the specific implementation modes of the present invention are described above, those skilled in the art should understand that the specific implementation modes described are only illustrative and are not intended to limit the scope of the present invention. Equivalent modifications and changes made by those skilled in the art in accordance with the spirit of the present invention should be included in the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a tea residue carbon dot composite film, characterized in that: The steps include: S1. Using tea residue as raw material, CDs are obtained through hydrothermal reaction, purification and drying; S2. The CDs, polyvinyl alcohol and fish gelatin obtained in step S1 were dissolved in deionized water to prepare a CDs solution and a polyvinyl alcohol / fish gelatin solution having a concentration of 50 to 80 mg / ml, respectively; S3. Add glycerol and hydrochloric acid with a mass fraction of 5 to 30% of polyvinyl alcohol / fish gelatin to the polyvinyl alcohol / fish gelatin solution obtained in step S2, mix well and then add the CDs solution obtained in step S2 to obtain a composite membrane solution; S4. The composite membrane liquid obtained in step S3 is evenly spread and dried at a constant temperature to obtain a tea residue carbon dot composite membrane.

2. The method according to claim 1, characterized in that The tea residues described in step S1 can be any one or more of green tea, white tea, yellow tea, oolong tea, black tea, and dark tea.

3. The method according to claim 1, characterized in that The tea residues in step S1 are dissolved in an aqueous solution at a material-liquid ratio of 1:3 to 1:

5.

4. The method according to claim 1, characterized in that: The heating time of the hydrothermal reaction in step S1 is 4 to 12 hours.

5. The method according to claim 1, characterized in that The purification method described in step S1 is any one or more of filtration, centrifugation, dialysis, column chromatography and the like.

6. The method according to claim 1, characterized in that The mass fraction of the CDs solution in step S2 is 0.25-1%.

7. A tea residue carbon dot composite film prepared by the method according to any one of claims 1 to 6.

8. The use of the tea residue carbon dot composite film according to claim 7, characterized in that: The tea residue carbon dot composite film is used for preparing food preservation materials.

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