Biodegradable food packaging films, their preparation methods and applications

Abstract

This invention discloses a biodegradable food packaging film, its preparation method, and its applications. Bacterial cellulose is covalently cross-linked with guanidine salts to obtain the covalently cross-linked compound OBC-PHGH. Gallic acid is then added to the cross-linking solution to obtain a guanidine salt-functionalized bacterial cellulose-based film, OBC-PHGH / GA. The food packaging film material prepared by this invention can prevent weight loss of strawberries during storage and extend the shelf life of strawberries from 2 days to 5 days. Therefore, this method is expected to have wide applications in green food packaging.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of food packaging, and relates to a bacterial cellulose-based degradable food packaging film with antibacterial and antioxidant properties, a preparation method thereof and application thereof in food packaging. BACKGROUND

[0002] Food safety and food waste problems have caused serious threats to human health, economy and environment. Most foods, such as fruits, vegetables and meat, are susceptible to microbial contamination, which can cause serious quality loss, economic loss, and even foodborne diseases. Based on the above problems, there is a great demand for the development of food packaging materials to protect food from pathogenic microorganisms, mechanical damage, and prolong the shelf life of fresh food. At present, most traditional food packaging materials are petroleum-based polymers, which can protect food, are simple to make and have low cost. However, they cannot effectively prevent microbial contamination, are not biodegradable, and cannot be recycled, which can cause serious ecological, economic, environmental and health problems. Therefore, it is urgent to develop new food packaging materials to improve the problems existing in traditional packaging materials.

[0003] Bacterial cellulose (BC) is a natural nanocellulose obtained by microbial fermentation, which has a natural three-dimensional network structure, high mechanical strength, high transparency, high biological safety, biodegradability and other advantages. These advantages make bacterial cellulose receive more and more attention in food packaging and biomedical applications. However, although bacterial cellulose is an ideal food packaging material, it lacks antibacterial and antioxidant functions, which hinders the potential application of BC in food packaging. Based on this, functional bacterial cellulose materials have attracted widespread attention and have become an important strategy in the field of food packaging. At present, physical doping methods are mainly used to modify bacterial cellulose, but it is difficult to obtain uniformly modified BC composites by physical doping methods, and functional components may be lost during washing of the composite material. Therefore, it is urgent to develop new strategies for functional bacterial cellulose materials.

[0004] Guanyl is imine urea or called carbamid amidine, guanyl salt can derive a variety of guanyl derivatives, and is widely concerned due to its strong antibacterial performance, and has broad application prospects in the fields of medicine, packaging, food, agriculture and the like. Guanyl salt (PHGH) can inactivate bacteria by destroying cell membranes. Polyphenol is a group of naturally occurring phytochemicals, which exist in large quantities in fruits, vegetables, green tea and other natural products, and has the characteristics of antioxidant, antibacterial and ultraviolet protection. Among the numerous natural phenolic compounds present in tea leaves, grapes, berries and other fruits, and grape wine, gallic acid (GA) is a low-cost compound with strong antioxidant and antibacterial properties. At present, gallic acid has been widely used in the preparation of active food packaging materials. SUMMARY

[0005] TECHNICAL PROBLEMS SOLVED: The present application provides a degradable guanidine salt functionalized bacterial cellulose-based active food packaging film with high efficiency antibacterial and antioxidant properties, as well as a preparation method and application thereof, to solve the problems of poor antibacterial effect, lack of antioxidant performance, and non-biodegradability of some existing food packaging film materials.

[0006] TECHNICAL SCHEME: A preparation method of a food packaging film, wherein bacterial cellulose and guanidine salt are covalently cross-linked together to obtain a covalent cross-linked material OBC-PHGH, and gallic acid is added in the cross-linking solution to obtain a guanidine salt functionalized bacterial cellulose-based film OBC-PHGH / GA.

[0007] The above bacterial cellulose is obtained from a culture solution of Xylella fastidiosa.

[0008] The preparation method of the food packaging film comprises the following steps according to the following proportions: (1) 1 g of bacterial cellulose film is suspended in 1 L of deionized water, the pH value of the solution is adjusted to not less than 10 with NaOH, then 2,2,6,6-tetramethylpiperidine oxide (TEMPO), sodium bromide (NaBr) and sodium hypochlorite (NaClO) are sequentially added, and stirred at room temperature for 1 h, and after dispersion, an oxidized bacterial cellulose dispersion (OBC) with a concentration of 0.65 wt.% is obtained; (2) the above oxidized bacterial cellulose (OBC), 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC·HCl) and N-hydroxysuccinimide (NHS) are dispersed in ionized water and reacted at room temperature and 100-1000 rpm for 1-3 h, and guanidine salt is added and reacted at room temperature and 100-1000 rpm overnight; (3) gallic acid is mixed with the above bacterial cellulose-guanidine salt grafting dispersion, then suction filtration is performed, and after drying, an OBC-PHGH / GA film is obtained.

[0009] Preferably, the reaction time under the above 100-1000 rpm condition is preferably 1 h.

[0010] Preferably, the above guanidine salt is replaced by an antibacterial substance with an amino group, and the antibacterial substance is an antibacterial peptide, chitosan or an aminoglycoside antibiotic.

[0011] Preferably, the addition amount of the above gallic acid is not more than 1 wt.% of the reaction system.

[0012] Preferably, the addition amount of the above gallic acid is 0.2 wt.%.

[0013] The food packaging film prepared by the above method.

[0014] The application of the above food packaging film in the preparation of antibacterial, antioxidant and food packaging.

[0015] Beneficial effects: 1. The bacterial cellulose obtained by the present application is derived from microorganisms, which have fast growth cycle, cheap and easily available raw materials, high product yield, and production is not limited by natural conditions such as geography, climate, season, etc. 2. The food packaging film obtained by the present application has good biodegradability, can be degraded by enzymes to produce sugar, and the produced sugar can be further used to produce more value-added products, and has reusability; at the same time, environmental pollution after use is avoided. 3. The food packaging film material prepared by the present application has excellent antioxidant property and antibacterial property, and can kill 100% of Escherichia coli and 99.9% of Staphylococcus aureus. 4. The food packaging film material prepared by the present application can prevent weight loss of strawberries during storage, and prolongs the shelf life of strawberries from 2 days to 5 days. Therefore, the method is expected to be widely used in green food packaging. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is the electrogram of OBC-PHGH / GA in Example 1.

[0017] Figure 2 It is the mechanical property result diagram of OBC-PHGH / GA.

[0018] Figure 3 It is the DPPH free radical and AABTS free radical scavenging rate result diagram of OBC-PHGH / GA.

[0019] Figure 4 It is the anti-Escherichia coli and Staphylococcus aureus result diagram of OBC-PHGH / GA.

[0020] Figure 5 It is the degradation result diagram of commercial preservative film.

[0021] Figure 6 It is the degradation result diagram of OBC-PHGH / GA.

[0022] Figure 7 It is the strawberry fresh-keeping effect diagram of the non-packaging group.

[0023] Figure 8 It is the strawberry fresh-keeping effect diagram of the OBC-PHGH / GA treatment group.

[0024] Figure 9 It is the weight loss rate curve diagram of strawberries in the storage process of the non-packaging group and the OBC-PHGH / GA treatment group.

[0025] Figure 10 It is the bacterial colony diagram of strawberries in the non-packaging group after storage.

[0026] Figure 11The figure shows the fungal colony of the non-packaged strawberry group after storage.

[0027] Figure 12 The figure shows the bacterial colony of the OBC-PHGH / GA treated strawberry group after storage.

[0028] Figure 13 The figure shows the fungal colony of the OBC-PHGH / GA treated strawberry group after storage. DETAILED DESCRIPTION

[0029] Strain activation method: the E. coli and S. aureus strains preserved in the-80℃ refrigerator were inoculated into 10mL LB broth at a 3% inoculation amount, and the strains were cultured at 37℃ for 12h to activate the strains.

[0030] Method for determining viable bacterial count: the dilution plate method was used to determine the viable bacterial count, and the specific steps were as follows: 0.1mL of sample liquid was taken by a pipette and diluted with sterile physiological saline to a dilution of 10 -1 , 10 -2 , 10 -3 , 10 -4 , 10 -5 , 10 -6 , 10 -7 , 10 -8 , 10 -9 , 10 -10 , 10 -11 , 10 -12 . The dilution of 10 -4 to 10 -12 was coated on the LB medium, and each dilution was repeated three times, and then placed in a 37℃ incubator for 24h before counting.

[0031] The present application covalently crosslinks bacterial cellulose and guanidine salt together to obtain covalent crosslinking material OBC-PHGH, and adds gallic acid in the crosslinking solution, and then performs suction filtration to obtain guanidine salt functionalized bacterial cellulose based film (OBC-PHGH / GA), which is used for food packaging. The obtained OBC-PHGH film shows good antibacterial effect, can effectively prolong the shelf life of strawberries, and is easy to be degraded. Therefore, it can not only meet the application in many fields such as food, medicine and cosmetics, but also provides a train of thought for the development of new food packaging film.

[0032] Example 1

[0033] Synthesis of bacterial cellulose (BC)

[0034] The Acetobacter xylinum was inoculated into the culture medium at a concentration of 7wt.%-10wt.%, and statically cultured in a petri dish for 5 days; bacterial cellulose membrane (BC) was obtained and purified by a mixed solution of 0.3wt.% NaOH and 0.09wt.% H2O2, and then treated in a constant-temperature water bath at 80°C for 1-2h. After the treatment, the solution was cooled and washed with water until neutral.

[0035] Synthesis of bacterial cellulose-guanidinium (OBC-PHGH)

[0036] 1g of bacterial cellulose membrane was suspended in 1L of deionized water, and the pH value of the solution was adjusted to greater than 10 with NaOH. Then the bacterial cellulose membrane suspension was mixed with 31.25mg TEMPO, 1.03g NaBr, and 37.2mL NaClO, and stirred at room temperature for 1h. The obtained oxidized bacterial cellulose was washed with deionized water until neutral to remove residual small molecules, thereby obtaining oxidized BC.

[0037] The obtained oxidized BC was dispersed into a 0.65wt.% dispersion liquid with a disperser, and the pH value of the solution was adjusted to less than 6 with HCl. Then 30mL of oxidized bacterial cellulose dispersion liquid was mixed with 400μL of EDC-HCl solution (14mg / mL), stirred at room temperature for 10min, 400μL of NHS solution (13.4mg / mL) was added, and incubated at room temperature at a speed of 90rpm for 1h. 31.2mg of guanidinium was added, and the mixture was magnetically stirred at room temperature for 24h. The obtained solution was dialyzed (MW=14kDa) with deionized water for 72h to remove residual guanidinium, thereby obtaining bacterial cellulose-guanidinium crosslinker.

[0038] Synthesis of gallic acid-loaded bacterial cellulose-guanidinium membrane (OBC-PHGH / GA)

[0039] To the above-obtained OBC-PHGH dispersion liquid, 0.05wt.%, 0.1wt.%, and 0.2wt.% of gallic acid solution was added respectively, and the mixture was filtered into a film under a pressure of-0.1MPa. After the water was completely removed, the mixture was pressed with an iron block (2.3kg) at room temperature for 48h to obtain OBC-PHGH / GA film.

[0040] Morphology observation of the synthesized OBC-PHGH / GA

[0041] The above-obtained OBC-PHGH / GA was subjected to cold field emission scanning electron microscope observation, and the results are shown in Figure 1 OBC-PHGH / GA has a good 3D network structure, and the fibers are tightly combined.

[0042] Water vapor transmission performance of OBC-PHGH / GA

[0043] The water vapor transmission performance of OBC-PHGH / GA films was tested using a water vapor transmission rate tester (PERME-W3 / 060, Labthink Instrument Co., Ltd., China). The results showed that the water vapor transmission rate of OBC-PHGH / GA films was 17.11 g / m2day. 2 ·day.

[0044] Mechanical properties of OBC-PHGH / GA

[0045] The mechanical properties of OBC-PHGH / GA films were analyzed using a universal tensile testing machine (Shimadzu, AGS-X, Japan), and the results are shown in Figure 2 The results showed that the breaking stress of OBC-PHGH / GA increased with increasing GA content. When the GA content was 0.2 wt.%, the breaking stress of OBC-PHGH / GA was the largest, reaching 55.81 MPa, and the elongation at break was 3.60%. Therefore, OBC-PHGH / GA had good mechanical properties.

[0046] Antioxidant capacity of OBC-PHGH / GA

[0047] (1) DPPH free radical scavenging experiment

[0048] OBC-PHGH / GA films of 3 cm x 3 cm were immersed in 3 mL of DPPH methanol solution (5 mM) and reacted in the dark at room temperature for 30 min. The absorbance value of the mixed solution at 517 nm was measured using a UV spectrophotometer, and DPPH methanol solution was used as a control. The percentage of DPPH free radical scavenging was calculated using the following equation:

[0049] DPPH free radical scavenging % = (control absorbance - sample absorbance) / (control absorbance) x 100% Equation (1)

[0050] (2) ABTS + free radical scavenging experiment

[0051] ABTS solution (7 mM) was mixed with an equal volume of potassium persulfate solution (2.45 mM) at 4°C in the dark for 12-16 h to form ABTS free radical (ABTS +The stock solution was prepared and then diluted with ethanol until the absorbance at 734 nm was 0.7 ± 0.02, thus obtaining the ABTS working solution. Similar to the DPPH radical scavenging method, a 3 cm × 3 cm OBC-PHGH / GA film was immersed in 3 mL of ABTS working solution and reacted at room temperature in the dark for 5 min. The absorbance of the mixed solution at 734 nm was measured using a UV spectrophotometer. Using the ABTS working solution as a control, the percentage of ABTS radical scavenging was calculated using the following equation:

[0052] ABTS free radical scavenging rate % = (control absorbance - sample absorbance) / (control absorbance) × 100% Equation (2)

[0053] The results are as follows Figure 3 As shown, the antioxidant properties of the OBC-PHGH / GA film increase with increasing GA content. The OBC-PHGH / GA film exhibits the best antioxidant properties when the GA content is 0.2 wt.%. It also shows resistance to DPPH free radicals and ABTS. + The free radical scavenging rates reached 92.1% and 99.9%, respectively. Therefore, the OBC-PHGH / GA composite film with a GA content of 0.2 wt.% is a highly promising antioxidant packaging material.

[0054] Antibacterial properties of OBC-PHGH / GA

[0055] Using *Escherichia coli* and *Staphylococcus aureus* as representatives of Gram-negative and Gram-positive bacteria, respectively, the antibacterial activity of the samples was calculated. 100 μL of *Escherichia coli* and 10 μL of *Staphylococcus aureus* were added to each sample. 8 After adding CFU / ml drops to the sample surface and incubating at 37°C for 24 h, the sample was thoroughly vortexed and washed with sterile physiological saline. The washing solution was then diluted to an appropriate factor and spread onto LB medium. Each dilution was performed in triplicate. The samples were incubated at 37°C for 24 h before counting. The results are as follows: Figure 4 As shown, OBC-PHGH / GA achieved kill rates of 100% and 99.9% against Escherichia coli and Staphylococcus aureus, respectively.

[0056] Degradability of OBC-PHGH / GA

[0057] 45 mg of OBC-PHGH / GA membrane was cut into small pieces (0.5 cm × 0.5 cm) and placed in 27 mL of HAc / NaAc buffer (50 mm, pH = 4.8). Then, 3 mL of crude cellulase complex was added to the buffer, and the mixture was incubated at 50 °C and 200 rpm for 12 h. Commercial plastic wrap was used as a control group to observe the degradation of the OBC-PHGH / GA membrane. The results are as follows:Figure 6 As shown, OBC-PHGH / GA was almost completely degraded after 6h cellulase treatment, while the commercial cling film was still in square pieces, indicating that the commercial cling film could not be degraded.

[0058] Test of OBC-PHGH / GA on strawberry preservation effect

[0059] Strawberries with consistent maturity and no damage or mildew were selected as experimental raw materials and stored in plastic containers. The prepared OBC-PHGH / GA composite film was covered on the above plastic containers, and the control group was not covered with any film. Store at 25°C for 5 days. The samples were weighed and photographed every 24h. The weight loss rate was the ratio of the weight loss of the strawberries to the total weight of the strawberries.

[0060] Take 4g of strawberries on the 5th day and mix with 2mL of physiological saline homogenate. Dilute the strawberry homogenate to the appropriate multiple, and spread 100μL of the diluted solution on LB agar plates and PDA agar plates, respectively. After incubation at 37°C for 24-48h, take photos and observe. As shown in the figure, on the 5th day of storage, the weight loss of the strawberries in the OBC-PHGH / GA group was 1.56g, which was much lower than that of the unpackaged group (3.67g). The unpackaged strawberries had obvious rot and microbial damage on the surface after 5 days of storage. Figure 7 ) While the strawberries in the OBC-PHGH / GA group were in good color after 5 days of storage, and there was no rot and microbial loss on the surface. Figure 8 ) The control group LB plate ( Figure 9 ) and PDA plate had a large number of bacterial colonies ( Figure 10 ) In the OBC-PHGH / GA group, no bacterial colonies were found ( Figure 11 ), only a few fungal colonies ( Figure 12 ) were found. This shows that the OBC-PHGH / GA film can effectively inhibit microbial contamination and prolong the preservation time of strawberries.

[0061] The above examples are only to illustrate the technical concept and characteristics of the present application, the purpose is to enable those skilled in the art to understand the content of the present application and to implement it, and cannot limit the protection scope of the present application. Any equivalent transformation or modification made in accordance with the spirit and essence of the present application shall be covered within the protection scope of the present application.

Claims

1. A method for producing a food packaging film, characterized by, According to the following proportions, the steps include: (1) 1 g of bacterial cellulose film is suspended in 1 L of deionized water, the pH value of the solution is adjusted to no less than 10 with NaOH, then 2,2,6,6-tetramethylpiperidine oxide, sodium bromide and sodium hypochlorite are added in sequence, and stirred at room temperature for 1 h, and a 0.65 wt.% oxidized bacterial cellulose dispersion is obtained after dispersion; (2) the above oxidized bacterial cellulose, 1-ethyl-(3-dimethylaminopropyl) carbodiimide hydrochloride and N-hydroxysuccinimide are dispersed in deionized water, and reacted at room temperature under the condition of 100-1000 rpm for 1-3 h, and guanidine salt is added and reacted at room temperature under the condition of 100-1000 rpm overnight; (3) gallic acid is mixed with the above bacterial cellulose-guanidine salt grafted dispersion, then suction filtration is performed, and OBC-PHGH / GA film is obtained after drying.

2. The method for preparing the food packaging film according to claim 1, characterized in that, The bacterial cellulose is obtained from a culture solution of Acetobacter xylinum.

3. The method for preparing the food packaging film according to claim 1, characterized in that, Before the guanidine salt is added, the reaction is carried out under the condition of 100-1000 rpm for 1 h.

4. The method for preparing the food packaging film according to claim 1, characterized in that, The guanidine salt is replaced by an amino-containing antibacterial substance, which is an antibacterial peptide, chitosan or an aminoglycoside antibiotic.

5. The method for preparing the food packaging film according to claim 1, characterized in that, The addition amount of the gallic acid is no more than 1 wt.% of the reaction system.

6. The method of claim 5, wherein the film is prepared by coextruding the first and second layers. The addition amount of the gallic acid is 0.2 wt.%.

7. The food packaging film prepared by the method of any one of claims 1-6.

8. The use of the food packaging film of claim 7 in the preparation of antibacterial, antioxidant and food packaging.

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