Antibacterial, antioxidant and degradable composite film for litchi preservation and preparation method of antibacterial, antioxidant and degradable composite film

Abstract

The invention discloses an antibacterial antioxidant degradable composite film for litchi preservation and a preparation method thereof, and mainly relates to the field of fruit and vegetable preservation. The adhesive is prepared by mixing polyvinyl alcohol (PVA), fumed silica (FS), polyethylene glycol 400 (PEG 400), melatonin (MT) and eugenol (EG). The method further comprises the following steps: S1, preparing a PVA solution; s2, adding PEG (Polyethylene Glycol) 400 into S1, and uniformly stirring; s3, weighing a certain amount of FS, MT and EG, and dissolving the FS, MT and EG in absolute ethyl alcohol; s4, adding the mixed solution in the step S3 into the mixed solution in the step S2, and homogenizing; and S5, carrying out defoaming, curtain coating and drying treatment on the solution treated in the step S4 to finally obtain the PVA / FS / MT / EG composite film. The method has the advantages that the method can be used as a physical barrier, color deterioration, browning and rotting of the Feizixiao litchis can be actively and effectively inhibited under normal-temperature and low-temperature conditions through the synergistic effect of active ingredients in the film, and the shelf life of the collected Feizixiao litchis can be well prolonged.

Description

Technical Field

[0001] This invention relates to the field of fruit and vegetable preservation, specifically to an antibacterial, antioxidant, and biodegradable composite film for lychee preservation and its preparation method. Background Technology

[0002] 'Feizixiao' lychee is a specialty tropical fruit of southern China, renowned for its large size, thick flesh, small seed, and sweet taste, making it a favorite among consumers. However, lychee trees experience vigorous physiological activity after harvest, making the peel highly susceptible to dehydration and browning, as well as rotting and spoilage, leading to a rapid decline in their commercial value. Especially under normal temperature conditions, the shelf life of lychees is typically only 2-3 days, severely limiting their storage, transportation, and sales.

[0003] Currently, common methods for preserving lychees include low-temperature storage, modified atmosphere packaging, and chemical preservative treatment. However, these methods suffer from high costs, complex operations, and potential food safety hazards. In recent years, biodegradable film packaging technology has shown broad application potential in the field of fruit and vegetable preservation due to its green, safe, and efficient characteristics. However, ordinary biodegradable films mainly provide physical barriers and lack active antibacterial and antioxidant biological activities, making it difficult to specifically inhibit the rapid quality deterioration of lychees after harvest. Summary of the Invention

[0004] The purpose of this invention is to provide an antibacterial, antioxidant, and biodegradable composite film for lychee preservation and its preparation method. It can not only act as a physical barrier, but also actively and effectively inhibit the color deterioration, browning, and rotting of 'Feizixiao' lychee under normal and low temperature conditions through the synergistic effect of the active ingredients in the film, thus greatly extending its post-harvest shelf life.

[0005] To achieve the above objectives, the present invention employs the following technical solution: The antibacterial, antioxidant, and biodegradable composite film used for lychee preservation is made by mixing polyvinyl alcohol (PVA), fumed silica (FS), polyethylene glycol 400 (PEG 400), melatonin (MT), and eugenol (EG).

[0006] A method for preparing an antibacterial, antioxidant, and biodegradable composite film for lychee preservation includes the following steps: S1. Weigh 3g of PVA and dissolve it in 100mL of distilled water. Stir magnetically at 90-100°C for 1.5-2.5 hours to prepare a 3% m / v PVA solution, and then cool it to room temperature. S2. Add PEG 400 to the solution from step S1 and stir until homogeneous; S3. Weigh a certain amount of FS, MT, and EG and dissolve them in anhydrous ethanol, then mix thoroughly. S4. Add the mixed solution from step S3 to the mixed solution from step S2 and homogenize it. S5. The solution processed in step S4 is subjected to degassing, casting, and drying to finally obtain the PVA / FS / MT / EG composite film.

[0007] Furthermore, step S5 specifically includes: After the solution treated in step S4 is allowed to stand to remove bubbles, it is quantitatively poured into a circular polyethylene plate mold and then transferred to a 60°C forced-air drying oven to dry for 12 hours to form a uniform PVA / FS / MT / EG composite film.

[0008] Compared with the prior art, the beneficial effects of the present invention are as follows: The antibacterial, antioxidant, and biodegradable composite film for lychee preservation of this invention not only acts as a physical barrier but also actively and effectively inhibits color deterioration, browning, and rotting of 'Feizixiao' lychees under both room temperature and low temperature conditions through the synergistic effect of the active ingredients melatonin and eugenol in the film. This results in better lychee preservation and significantly extends their post-harvest shelf life. Furthermore, the materials used in this invention are all safe components commonly found in the food industry, and the preparation process is simple. This invention provides a green, safe, and efficient new solution for post-harvest lychee preservation and has broad prospects for industrial application. Attached Figure Description

[0009] Appendix Figure 1 This is the ultraviolet spectrum of the antibacterial, antioxidant, and biodegradable composite film of the present invention.

[0010] Appendix Figure 2 This is the FTIR spectrum of the antibacterial, antioxidant, and biodegradable composite film of the present invention.

[0011] Appendix Figure 3 This is the XRD pattern of the antibacterial, antioxidant, and biodegradable composite film of the present invention.

[0012] Appendix Figure 4 This is a SEM image of the antibacterial, antioxidant, and biodegradable composite film of the present invention.

[0013] Appendix Figure 5 This is a comparative diagram showing the effects of different film packaging on the quality of litchi fruit under normal temperature storage conditions.

[0014] Appendix Figure 6 The different film packaging of this invention affects the color of lychee fruit at room temperature. L * (A) Chromaticity a * (B) Chromaticity b * (C) Data graph showing the impact of soluble solids content (D), browning index (E), and decay index (F).

[0015] Appendix Figure 7 This is a comparative diagram showing the effects of different film packaging on the quality of litchi fruit under low-temperature storage conditions, according to the present invention.

[0016] Appendix Figure 8 The different film packaging of this invention affects the color of low-temperature litchi fruit. L * (A) a * (B) b * (C) and browning index (D) impact data plot. Detailed Implementation

[0017] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined in this application.

[0018] Polyvinyl alcohol (PVA) possesses excellent film-forming properties, biocompatibility, and biodegradability, making it an ideal film substrate. Fumed silica (FS), as a nano-reinforcing material, can improve the mechanical properties and barrier properties of the film; melatonin (MT), as a natural hormone, has anti-aging and antioxidant activities; and eugenol (EG) is a plant-derived antibacterial component. Combining PVA with FS, MT, and EG can construct an active packaging system that combines physical barrier and physiological regulatory functions.

[0019] Example 1: An antibacterial, antioxidant, and biodegradable composite film for lychee preservation, made from a mixture of polyvinyl alcohol (PVA), fumed silica (FS), polyethylene glycol 400 (PEG 400), melatonin (MT), and eugenol (EG). The PVA has a saponification degree of 98-99% and a molecular weight of 105,000. The FS has a molecular weight of 60.08 and a specific surface area of ​​260 m². 2 / g, particle size 7-40 nm. Polyethylene glycol 400 (PEG 400), pH 4.0-7.0. Melatonin (MT), analytical grade, purity 99%. Eugenol (EG), purity 98.5%, molecular weight 164.2.

[0020] The preparation steps of the above-mentioned antibacterial, antioxidant, and biodegradable composite film for lychee preservation are as follows: S1. Weigh 3g of PVA and dissolve it in 100mL of distilled water. Stir magnetically at 95°C for 2 hours to prepare a 3% m / v PVA solution, and then cool it to room temperature. S2. Add 0.18g PEG 400 to the solution from step S1 and stir for 1 hour; Traditional PVA film preparation only goes up to this step; based on this, the present invention: S3. Weigh 0.015g of FS, 0.006g of MT, and 0.3g of EG and dissolve them together in 10 mL of anhydrous ethanol. Mix well. S4. Slowly add the mixed solution from step S3 to the mixed solution from step S2, and homogenize it at 13,000 rpm for 5 minutes using a high-speed dispersion homogenizer to ensure that the nanoparticles and active ingredients are uniformly dispersed in the film-forming solution. S5. After the solution treated in step S4 is allowed to stand to remove bubbles, it is quantitatively poured into a circular polyethylene plate mold and then transferred to a 60°C forced-air drying oven to dry for 12 hours, finally obtaining a PVA / FS / MT / EG composite film (hereinafter referred to as PSM-EG10 film).

[0021] Example 2: Characterization data of an antibacterial, antioxidant, and biodegradable composite film for lychee preservation: The ultraviolet shielding capability of the thin film is crucial for delaying the photo-oxidative deterioration of fruits and vegetables. (See attached image) Figure 1 As shown, pure PVA films exhibit high transmittance in the ultraviolet region (200-400 nm), indicating that they possess almost no ultraviolet shielding function. In contrast, the PSM-EG10 composite film of this invention demonstrates extremely strong absorption capacity in the ultraviolet region, especially in the 100-280 nm (UVC) and 280-400 nm (UVB / UVA) bands, with a sharp decrease in transmittance. This clearly proves that the active ingredients MT and EG in the PSM-EG10 composite film are successfully loaded and play a crucial role in ultraviolet shielding. MT, as a highly efficient antioxidant, with its indole ring structure, together with the benzene ring structure of EG, forms a conjugated system that can effectively absorb the energy of ultraviolet photons, thereby preventing ultraviolet light from penetrating the film and causing photosensitive browning reactions and degradation of nutrients such as vitamins in the litchi peel. This characteristic provides a key theoretical basis for the composite film to effectively delay the color deterioration of litchi peel even under transparent packaging conditions.

[0022] FTIR spectroscopy is used to analyze the molecular interactions between components within a thin film. (See attached image.) Figure 2 As shown, pure PVA film at 3305 cm⁻¹ -1 A broad and strong absorption peak is observed at this point, attributed to the stretching vibration of OH, indicating a strong hydrogen bond network between PVA molecular chains. In the spectrum of the PSM-EG10 composite film, the intensity of this OH peak is significantly reduced and a slight blue shift occurs. Meanwhile, characteristic PVA peaks such as the CH stretching vibration (2907 cm⁻¹) are also observed. -1) and CO stretching vibration (1088 cm) -1 The intensity of CO also generally decreased, and the CO peak position shifted to 1086 cm⁻¹. -1 .

[0023] These changes clearly demonstrate that the introduction of FS, MT, and EG successfully achieved molecular-level interactions with the PVA matrix. In particular, the active groups in EG and MT formed new hydrogen bonds with the hydroxyl groups of PVA, thereby disrupting and reorganizing the original dense hydrogen bond network of PVA. This molecular-level miscibility is the basis for the uniform dispersion of the active ingredients and their sustained functionality, while also altering the free volume of the film and affecting its permeability.

[0024] The microcrystalline structure of the thin film was analyzed by XRD. (See attached image.) Figure 3 As shown, the pure PVA film exhibits a typical semi-crystalline diffuse peak at approximately 23°. Notably, the diffraction pattern of the PSM-EG10 composite film shows significant changes: firstly, new diffraction peaks appear in the low-angle region (decreasing 2θ), with significantly enhanced crystallinity; secondly, the intensity of the original characteristic peaks of PVA weakens. FS nanoparticles act as nucleation sites, while the intercalation of EG and MT molecules within the PVA chains jointly promotes the rearrangement of the composite system, forming a new crystalline phase with larger interlayer spacing and higher crystallinity. This denser microstructure effectively enhances the mechanical strength of the film and provides a structural basis for improving its barrier properties against small molecules such as water vapor and oxygen.

[0025] The cross-sectional morphology of the thin film was observed using SEM. (See attached image.) Figure 4 In the above description, (A) represents PVA, and (B) represents a PSM-EG10 composite film. The pure PVA film has a relatively smooth and uniform cross-section. In contrast, the cross-section of the PSM-EG10 composite film exhibits a rougher morphology, with uniformly distributed nanoscale particles and protruding structures, and no obvious phase separation. This demonstrates that the active substances in the PSM-EG10 composite film achieve a good dispersion state in the PVA matrix, and that each active component has good compatibility with the matrix. This uniform composite structure is a prerequisite for ensuring the stability of the film's mechanical properties and its long-lasting function. Uneven dispersion can easily become defect points, reducing the integrity of the film.

[0026] The test results of the mechanical and barrier properties of the film are shown in the table below, revealing the unique effect of the synergistic interaction between the components of PSM-EG10 and the PVA matrix. film Thickness (μm) Tensile strength (MPa) Elongation at break (%) Young's modulus (MPa) <![CDATA[Water vapor permeability WVP (g·m -1 s -1 ·Pa -1 )]]> PVA 61.50 ± 1.77 50.18 ± 0.96 286.71 ± 3.42 17.528 ± 0.51 <![CDATA[(1.73 ± 0.21)×10 9 ]]> PSM-EG10 78.00 ± 2.88 45.55 ± 3.49 306.19 ± 6.76 14.93 ± 1.24 <![CDATA[(2.22 ± 0.33)×10 13 ]]>

[0027] Table 1. Physicochemical properties of thin films As shown in Table 1, compared with pure PVA film, the elongation at break of PSM-EG10 composite film is significantly improved, while the thickness is also increased. This indicates that the flexibility and toughness of the composite film are significantly improved. This phenomenon is mainly attributed to the internal plasticizing effect of small molecule active substances such as eugenol (EG): they intersect between PVA molecular chains, weakening the excessively strong hydrogen bonding of PVA to a certain extent (which is consistent with the FTIR results), increasing the mobility of molecular chain segments, and thus enabling the material to undergo greater plastic deformation during stretching.

[0028] In terms of strength, the tensile strength and Young's modulus of the PSM-EG10 composite film decreased only slightly. This result is highly valuable, demonstrating an excellent balance between the reinforcing effect of fumed silica (FS) nanoparticles and the plasticizing effect of EG. FS, as a rigid filler, effectively compensates for the strength loss that might result from plasticizing by achieving good interfacial bonding with the PVA matrix (as shown in SEM) and inducing the formation of a more ordered composite structure (as shown in XRD), enabling the film to maintain good structural strength while achieving high toughness.

[0029] The most crucial aspect lies in its barrier properties. Despite a significant increase in film thickness, the water vapor transmission rate (WVP) of the PSM-EG10 composite film actually decreased significantly. This constitutes a prominent technical highlight of this invention. Because WVP is an intrinsic property of the material, its reduction signifies a qualitative improvement in the film material's water-blocking ability. This phenomenon demonstrates that the composite of FS, MT, and EG introduces a deep barrier mechanism that transcends a simple "thickness enhancement" effect. FS nanoparticles create tortuous penetration paths within the PVA matrix, greatly extending the diffusion channels for water molecules; simultaneously, the interactions between the components induce the formation of a more compact and crystalline microstructure (as shown by XRD). These two factors synergistically enhance the barrier properties of the film bulk material.

[0030] In summary, the composite modification strategy of this invention successfully prepared a functional film with high toughness, high strength, and inherent water resistance. Its performance advantages do not simply stem from increased thickness, but rather from a fundamental optimization of the film's microstructure. This comprehensive effect, achieved through the synergistic action of multiple components, which enhances the film's toughness while maintaining its strength, and fundamentally reduces its water vapor permeability, is one of the core innovations of this invention. It provides an ideal packaging material for lychee preservation, possessing both excellent physical barrier and physiological regulatory functions, and perfectly explains, from a "structure-performance" perspective, its ability to effectively prevent fruit dehydration and browning during storage experiments.

[0031] Example 3: Lychee Preservation Experiment 1. Fruit preparation: The 'Feizixiao' lychee varieties were transported to the laboratory on the day of harvest. Fruits of uniform size, without mechanical damage, and free from pests and diseases were selected as experimental materials. The selected lychee fruits were disinfected by immersing them in a 0.05% sodium hypochlorite solution for 30 seconds, then rinsed three times with running water, and finally air-dried at room temperature.

[0032] 2. Room Temperature Storage Experiment: All litchi fruits were placed in polypropylene containers with a cylindrical cross-section, measuring 15 cm in upper diameter, 10 cm in lower diameter, and 8 cm in height. Each container contained 11 litchi fruits. The container openings were sealed using PE film, PVA film, and PSM-EG10 composite film, respectively. Unsealed containers served as the control group. Each treatment was performed in triplicate (i.e., three independent containers). All samples were stored at 25 ± 1°C and 85-90% RH for 8 days. Samples were taken on day 0 (at the beginning of storage) and day 8 (at the end of storage) to determine various quality indicators.

[0033] 3. Low-temperature storage experiment: The packaging and grouping methods were exactly the same as those for the room-temperature storage experiment. The packaged samples were placed in a low-temperature incubator and stored for 28 days at 4 ± 0.5°C and 85-90% RH. The parameters were measured on days 0, 7, 14, 21, and 28 of the storage period.

[0034] Measurement method: 1. The color of the fruit peel was measured using a Minolta-Konica CR400 colorimeter, and the results were recorded. L * , a * , b * value.

[0035] 2. The soluble solids content of the pulp was determined using a handheld refractometer, and the unit is expressed as %.

[0036] 3. Browning Index (BI): Graded based on the percentage of brown spots on the fruit surface relative to the total peel area: Grade 0 (0%), Grade 1 (1-25%), Grade 2 (26-50%), Grade 3 (51-75%), Grade 4 (76-100%). Calculated using the formula DI = Σ(Browning Grade × Number of Fruits in that Grade) / (Highest Grade × Total Number of Fruits) × 100%.

[0037] 4. Decay Index (DI): Graded based on the percentage of moldy area on the fruit peel: Grade 0 (0%), Grade 1 (1-25%), Grade 2 (26-50%), Grade 3 (51-75%), Grade 4 (76-100%). Calculated using the formula DI = Σ(decay grade × number of fruits in that grade) / (highest grade × total number of fruits) × 100%.

[0038] 1. Preservation effect at room temperature: During an 8-day period of storage at room temperature, the quality of litchi fruits in the control group showed significant deterioration. (See attached image) Figure 5 As shown, the fruit in group CK had completely browned and severely moldy peels at the end of storage, and had completely lost its commercial value. In contrast, the fruit packaged with the PSM-EG10 composite film of this invention could effectively delay browning and significantly inhibit mold growth. Most of the fruit maintained a bright red appearance without mold spots, which directly proves the preservation effect of this invention.

[0039] Specific physicochemical data are attached. Figure 6 As shown, this provides strong evidence for the above observations. Analysis of the fruit peel color indicates that the CK fruit had a higher brightness (…). L * The value decreased significantly from the initial state to 27.86 ± 0.12 on day 8, and the redness ( a * The value also dropped sharply to 13.04 ± 0.31, while the yellowness ( b * The decrease in the color index (p<0.01) confirmed a comprehensive deterioration in fruit color. The PSM-EG10 treatment significantly (p<0.01) delayed this process, resulting in a decrease in the color of the fruit at the end of storage. L * The value (33.73 ± 0.19) and a * The values ​​(19.32 ± 0.86) were significantly higher than those in other treatment groups, and b * The value (17.97 ± 0.18) remained the highest, indicating that this treatment was most effective in maintaining the bright commercial appearance of the litchi peel.

[0040] In terms of intrinsic flavor, the PSM-EG10 treatment also showed the best preservation effect. Its fruit soluble solids content (17.97 ± 0.18%) was significantly higher than that of the PE film group (14.82 ± 0.15%) and the PVA film group (15.23 ± 0.25%) after storage (p<0.05), which proves that the composite film can more effectively slow down the sugar consumption caused by post-harvest metabolism of fruit and better maintain the inherent flavor of lychee.

[0041] More importantly, the PSM-EG10 composite film treatment demonstrated outstanding performance in controlling browning and decay, which are crucial for determining marketability. The browning index of the CK group reached 1.00 ± 0.01 (i.e., complete browning) on ​​day 8, while the browning index of the PSM-EG10 treatment group was effectively suppressed to a low level of 0.51 ± 0.03; simultaneously, its decay index (0.33 ± 0.01) was also significantly lower than that of the CK group (0.89 ± 0.02) (p<0.01). This collectively indicates that the PSM-EG10 film, through its excellent physical barrier and the synergistic antibacterial and antioxidant effects of its active ingredients (MT and EG), achieves comprehensive protection of litchi quality.

[0042] 2. Low-temperature storage and preservation effect In a 28-day low-temperature storage experiment, the PSM-EG10 composite film demonstrated long-lasting and stable preservation advantages. (See attached image) Figure 7 As shown, during the entire storage period, the browning degree of the control group fruits continued to worsen over time; although the PE film and PVA film groups showed improvement, significant browning still occurred in the later stages; only the PSM-EG10 treatment group fruits maintained relatively good appearance quality from beginning to end.

[0043] The quantitative analysis results are attached. Figure 8 As shown, this is highly consistent with the above observations. The trend in peel color change indicates that the CK fruit... L * , a * , b * The values ​​decreased rapidly throughout the storage period. In contrast, the PSM-EG10 treatment significantly (p<0.01) slowed the rate of decline of all chromaticity indices. By day 28 of storage, the PSM-EG10 group... L * Value (30.81 ± 0.33) a * Value (23.30 ± 0.68) and b * The values ​​(16.83 ± 0.35) were significantly higher than those of the CK group (p<0.01), indicating that this treatment can most effectively maintain the fullness, redness and stability of the litchi peel color in a long-term low-temperature environment.

[0044] In terms of inhibiting browning, the PSM-EG10 treatment also showed the best effect. On day 7 of storage, the browning index of the PSM-EG10 group (0.11 ± 0.01) was already significantly lower than that of the CK group (0.77 ± 0.01) (p<0.01). Although the browning index of all treatment groups increased with prolonged storage, the value of the PSM-EG10 group remained the lowest throughout the 28-day storage period, and was still significantly lower than that of the PE film group (0.73 ± 0.02) and the PVA film group (0.80 ± 0.05) at the end of storage (0.67 ± 0.01). This fully demonstrates that the PSM-EG10 film can effectively and long-term inhibit the enzymatic browning reaction of litchi peel during low-temperature storage, providing a reliable technical guarantee for its long-term cold chain storage and transportation.

[0045] In summary, the experimental results demonstrate that the PSM-EG10 composite film of this invention not only acts as a physical barrier but also, through the synergistic effect of its active ingredients (MT and EG), actively and effectively inhibits color deterioration, browning, and decay of 'Feizixiao' lychees under both room and low temperature conditions, significantly extending their post-harvest shelf life. The materials used in this invention are all safe ingredients commonly found in the food industry, and the preparation process is simple. This invention provides a green, safe, and efficient new solution for post-harvest preservation of lychees, with broad prospects for industrial application.

Claims

1. An antibacterial, antioxidant, and biodegradable composite film for preserving lychees, characterized in that: It is made by mixing polyvinyl alcohol (PVA), fumed silica (FS), polyethylene glycol 400 (PEG 400), melatonin (MT), and eugenol (EG).

2. The method for preparing the antibacterial, antioxidant, and biodegradable composite film for lychee preservation as described in claim 1, characterized in that: Includes the following steps: S1. Weigh 3g of PVA and dissolve it in 100mL of distilled water. Stir magnetically at 90-100°C for 1.5-2.5 hours to prepare a 3% m / v PVA solution, and then cool it to room temperature. S2. Add PEG 400 to the solution from step S1 and stir until homogeneous; S3. Weigh a certain amount of FS, MT, and EG and dissolve them in anhydrous ethanol, then mix thoroughly. S4. Add the mixed solution from step S3 to the mixed solution from step S2 and homogenize it. S5. The solution processed in step S4 is subjected to degassing, casting, and drying to finally obtain the PVA / FS / MT / EG composite film.

3. The method for preparing an antibacterial, antioxidant, and biodegradable composite film for lychee preservation according to claim 2, characterized in that: Step S5 specifically involves: After the solution treated in step S4 is allowed to stand to remove bubbles, it is quantitatively poured into a circular polyethylene plate mold and then transferred to a 60°C forced-air drying oven to dry for 12 hours to form a uniform PVA / FS / MT / EG composite film.

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