Pectin-Nano-Lignin-Citrus Essential Oil Composite Coating Solution, Its Preparation Method and Application

Abstract

This invention discloses a pectin-nanolignin-citrus essential oil composite coating liquid, its preparation method, and its application. The preparation method includes the following steps: first, a nanolignin suspension is prepared; then, citrus essential oil is added and homogenized to obtain a nanolignin Pickering emulsion; finally, a pectin solution is added and homogenized again to obtain the pectin-nanolignin-citrus essential oil composite coating liquid. When used for food preservation, the pectin-nanolignin-citrus essential oil composite coating liquid of this invention forms a composite coating with excellent broad-spectrum antibacterial, antioxidant, UV-resistant, and long-term essential oil release properties. It also enhances physical properties such as mechanical properties, water vapor / oxygen barrier properties, and hydrophobicity, significantly extending the shelf life of climacteric fruits (kiwifruit) and non-climacteric fruits (strawberries), demonstrating great application potential in the field of fruit preservation.

Description

Technical Field

[0001] This invention belongs to the field of food packaging technology, specifically relating to a pectin-nanolignin-citrus essential oil composite coating liquid, its preparation method, and its application. Background Technology

[0002] Fresh fruits and other agricultural products are particularly perishable, leading to resource waste, economic losses, and potential threats to human health. Good packaging solutions can prevent food spoilage, but currently used petrochemical plastic packaging is difficult to degrade and lacks antibacterial properties. Currently, the development of biodegradable, natural polysaccharide-based multifunctional coatings for food preservation has received widespread attention. Among these, natural polymers such as pectin are widely used due to their renewable, degradable, non-toxic, and film-forming properties. However, coatings prepared from pure pectin have almost no antibacterial activity, making their effective application in fruit preservation difficult. To overcome these shortcomings, a common approach is to modify the pectin matrix with antibacterial agents, such as essential oils, metal oxides / nanoparticles, and polyphenols. While these methods can achieve the desired antibacterial activity in pectin coatings, the following problems remain: a) Physically mixed antibacterial agents exhibit significant aggregation and low stability, making them prone to oxidation or volatilization during coating preparation and storage; that is, most antibacterial coatings reported to date have difficulty maintaining long-term antibacterial effects; b) The elusive compatibility issues between these antibacterial agents and the polymer matrix often lead to a significant decrease in the mechanical properties of the coating, and may even cause partial structural collapse, making it difficult for the coating film to form stably; c) The process of modifying the pectin matrix with these antibacterial agents may also introduce potential biotoxicity, which further limits their application in food-related fields. Therefore, improving the compatibility and stability between antibacterial agents and the pectin matrix, and ensuring that the coating achieves a balance between good mechanical properties and antibacterial activity, is meaningful and challenging.

[0003] Lignin possesses advantages such as biodegradability, UV resistance, antioxidant properties, and antibacterial properties. However, due to its inherent rigid structure, lignin exhibits very poor compatibility and dispersibility in polymer matrices, which severely affects the stability and mechanical properties of polymer materials. Citrus essential oil, derived from citrus peel, possesses highly effective antioxidant and antibacterial properties; however, it is incompatible with pectin solutions, making it difficult to use directly in the production of antibacterial pectin coatings. Furthermore, due to its unstable nature, it is easily deactivated during the coating production process. Therefore, overcoming these challenges and obtaining a pectin-nanolignin-citrus essential oil composite coating solution with good compatibility and stability is of great significance for the widespread application of pectin-based coatings. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a pectin-nanolignin-citrus essential oil composite coating liquid with good compatibility and stability, its preparation method and application.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution.

[0006] A method for preparing a pectin-nanolignin-citrus essential oil composite coating liquid includes the following steps:

[0007] (1) Dissolve lignin in acetone to obtain a mixed solution;

[0008] (2) The mixed solution obtained in step (1) is subjected to ultrasonic treatment. After the treatment is completed, it is added to water and subjected to rotary evaporation to obtain a nano-lignin suspension.

[0009] (3) Mix the nano-lignin suspension obtained in step (2) with citrus essential oil and homogenize it to obtain nano-lignin Pickering emulsion;

[0010] (4) Mix the nano-lignin Pickering emulsion and pectin solution obtained in step (3) and homogenize them to obtain a pectin-nano-lignin-citrus essential oil composite coating liquid.

[0011] In the above preparation method, preferably, in step (3), the volume ratio of the nano-lignin suspension to citrus essential oil is 5-9:1-9, and the mass-volume percentage of lignin in the nano-lignin suspension is 0.25%-2%.

[0012] In the above preparation method, preferably, in step (4), the volume of the nano-lignin Pickering emulsion is 5% to 50% of the volume of the pectin solution, and the mass-volume percentage of pectin in the pectin solution is 1% to 2%.

[0013] In the above preparation method, preferably, the volume ratio of the nano-lignin suspension to citrus essential oil is 5-9:1-5, and the volume of the nano-lignin Pickering emulsion is 5%-25% of the volume of the pectin solution.

[0014] In the above preparation method, preferably, in step (2), the specific process of ultrasonic treatment is as follows: ultrasonic treatment for 5 seconds, stop for 6 to 10 seconds, and repeat the cycle; the power of ultrasonic treatment is 200W to 500W, and the time of ultrasonic treatment is 5 to 30 minutes; the specific process after ultrasonic treatment is as follows: under stirring conditions, the treated mixed solution is added to water; the volume ratio of the mixed solution to water is 1:8 to 15, and the temperature of rotary evaporation treatment is 45℃ to 60℃.

[0015] In the above preparation method, preferably, in step (1), the mass-to-volume ratio of lignin to acetone is 1g:15mL~25mL, the lignin is alkali lignin, and the mixed solution is further treated as follows before use: the mixed solution is centrifuged at 6500rpm for 5min.

[0016] In step (3), the specific process of homogenization is as follows: stirring at a speed of 10000 rpm to 14000 rpm for 2 min to 5 min;

[0017] In step (4), the specific process of homogenization is as follows: stirring at a speed of 10000 rpm to 14000 rpm for 2 min to 5 min.

[0018] As a general technical concept, the present invention also provides a pectin-nanolignin-citrus essential oil composite coating liquid prepared by the above preparation method.

[0019] As a general technical concept, the present invention also provides an application of the above-mentioned pectin-nanolignin-citrus essential oil composite coating liquid in food preservation.

[0020] The above-mentioned application, preferably, includes the following steps: adding glycerin to a pectin-nanolignin-citrus essential oil composite coating solution, stirring, vacuuming, spraying onto the substrate surface, and drying to form a pectin-nanolignin-citrus essential oil composite coating film.

[0021] In the above application, preferably, the mass of the glycerol is 15% to 40% of the mass of the pectin solution in the pectin-nanolignin-citrus essential oil composite coating liquid, the stirring speed is 100 rpm to 250 rpm, the vacuum degree of the vacuum is -0.1 MPa, the substrate is fruit, and the fruit includes at least one of climacteric and non-climacteric fruits, the climacteric fruit includes kiwifruit, and the non-climacteric fruit includes strawberry.

[0022] Compared with the prior art, the advantages of the present invention are as follows:

[0023] (1) This invention provides a method for preparing a pectin-nanolignin-citrus essential oil composite coating liquid. First, lignin is dissolved in acetone and subjected to ultrasonic treatment. Then, it is added to water and subjected to rotary evaporation to obtain a nanolignin suspension. That is, lignin is nano-sized by ultrasonic-assisted antisolvent precipitation to obtain nanolignin. Citrus essential oil is added and homogenized to obtain a nanolignin Pickering emulsion. Pectin solution is added and homogenized again. On the one hand, the pectin solution can fill the gaps between the nanolignin (Pickering emulsion dispersion phase) and tightly connect them together through hydrogen bonds to form a stable barrier structure, thereby achieving long-term stability of the nanolignin Pickering emulsion in the pectin solution. On the other hand, the pectin embedded in the nanolignin Pickering emulsion plays a bridging role, enabling it to be perfectly compatible with the pectin solution, filling and connecting the gaps between the pectin molecular chains. The nanolignin Pickering emulsion and the pectin solution form an interlocking structure at the interface, which promotes each other and ultimately combines to form a pectin-nanolignin-citrus essential oil composite coating liquid with good compatibility and stability. The raw materials used in the preparation method of this invention are all from organic processing waste of agricultural products. Pectin and citrus essential oil come from citrus peel, and lignin comes from sugarcane bagasse or sawdust. They are not only easy to biodegrade, but also promote the full utilization and development of agricultural processing waste, realize the efficient management and high-value application of organic solid waste, and have the advantages of high efficiency, simplicity, safety, green and environmental protection.

[0024] (2) This invention also provides an application of a pectin-nanolignin-citrus essential oil composite coating liquid in food preservation. Glycerin is added to the pectin-nanolignin-citrus essential oil composite coating liquid and sprayed onto the substrate surface to form a dense and stable pectin-nanolignin-citrus essential oil composite coating film. This coating film has reliable biocompatibility and can be safely applied in the field of food preservation. The pectin-nanolignin-citrus essential oil composite coating film of this invention has excellent broad-spectrum antibacterial, antioxidant, UV-resistant, and long-term essential oil release properties. At the same time, it also enhances its mechanical properties, water vapor / oxygen barrier properties, hydrophobicity, and other physical properties, achieving a perfect balance between the physical properties and long-term biological activity of the composite coating film. When the pectin-nanolignin-citrus essential oil composite coating liquid of this invention is used on fruits, it can prevent ripening, dehydration, browning, and spoilage of climacteric fruits (kiwifruit) and non-climacteric fruits (strawberries), thereby extending their shelf life. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the application of pectin-nanolignin-citrus essential oil composite coating liquid in food preservation in Example 2 of the present invention.

[0026] Figure 2This is a microstructure diagram of alkali lignin and nano lignin in Example 1 of the present invention.

[0027] Figure 3 This is a particle size distribution diagram of nano-lignin in Example 1 of the present invention.

[0028] Figure 4 This study examines the stability of different nano-lignin Pickering emulsions and pectin-nano-lignin-citrus essential oil composite coating solutions in Example 1 of this invention.

[0029] Figure 5 This is a time-particle size variation diagram of different nano-lignin Pickering emulsions and pectin-nano-lignin-citrus essential oil composite coating liquids in Example 1 of the present invention.

[0030] Figure 6 The mechanical properties of the composite coatings prepared by different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention are shown in the figure.

[0031] Figure 7 The graph shows the water vapor permeability and oxygen permeability of the composite coatings prepared by different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention.

[0032] Figure 8 This is a graph showing the antioxidant capacity of composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of the present invention.

[0033] Figure 9 This is a diagram illustrating the biocompatibility of the composite coating prepared from the pectin-nanolignin-citrus essential oil composite coating liquid (50% NLCPE) in Example 1 of this invention.

[0034] Figure 10 This is a dynamic water contact angle diagram of the composite coatings prepared by different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of the present invention.

[0035] Figure 11 This is a slow-release curve of the composite coating prepared by different pectin-nanolignin-citrus essential oil composite coating liquids in Example 1 of the present invention.

[0036] Figure 12 This is a UV-Vis transmittance diagram of the composite coatings prepared by different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of the present invention.

[0037] Figure 13 The image shows the antibacterial performance of the composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention.

[0038] Figure 14This image shows the preservation effect of the pectin-nanolignin-citrus essential oil composite coating liquid on kiwifruit and strawberries in Example 2 of the present invention.

[0039] Figure 15 This is a graph showing the changes in hardness and soluble solids content of kiwifruit when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of the present invention.

[0040] Figure 16 This is a graph showing the changes in hardness and soluble solids content of strawberries when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of the present invention.

[0041] Figure 17 This is a graph showing the weight changes of kiwifruit and strawberries when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of the present invention. Detailed Implementation

[0042] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention. All materials and instruments used in the following embodiments are commercially available.

[0043] Example 1:

[0044] A method for preparing a pectin-nanolignin-citrus essential oil composite coating liquid of the present invention includes the following steps:

[0045] (1) Dissolve alkali lignin completely in acetone. After complete dissolution, centrifuge at 6500 rpm for 5 min to remove any undissolved substances and obtain a mixed solution. The mass-to-volume ratio of alkali lignin to acetone is 1 g: 20 mL.

[0046] (2) The above mixed solution was subjected to ultrasonic treatment, i.e., ultrasonic treatment for 5 seconds, stopping for 6 seconds, and repeated. The ultrasonic power was 200W and the ultrasonic treatment time was 5 minutes. Then, the treated mixed solution was slowly poured into water under stirring conditions within 1 minute. Then, rotary evaporation was performed at 50°C to remove acetone and excess water, resulting in a nano-lignin suspension. The volume ratio of the mixed solution to water was 1:10. The mass-volume percentage of nano-lignin in the nano-lignin suspension was 0.5%, and the particle size distribution of the nano-lignin was concentrated at around 19.80 nm.

[0047] (3) The above-mentioned nano-lignin suspension was mixed with citrus essential oil and homogenized, that is, stirred at 12000 rpm for 2 minutes to form a nano-lignin Pickering emulsion, denoted as NLCPE. The volume ratio of nano-lignin suspension to citrus essential oil was 9:1.

[0048] (4) The above-mentioned nano-lignin Pickering emulsion was mixed with the pectin solution and homogenized, i.e., stirred at 12000 rpm for 2 minutes to form a pectin-nano-lignin stabilized citrus essential oil Pickering emulsion, i.e., a pectin-nano-lignin-citrus essential oil composite coating liquid, denoted as 15% NLCPE. The pectin in the pectin solution had a pectin mass-volume percentage of 2%, and the volume of the nano-lignin Pickering emulsion was 15% of the volume of the pectin solution.

[0049] In this embodiment, pectin-nanolignin-citrus essential oil composite coating solutions with different mass-volume percentage contents of nanolignin suspensions were also prepared. The preparation method of the pectin-nanolignin-citrus essential oil composite coating solution in Example 1 is basically the same, except that in step (2), the volume ratio of the mixed solution to water is 1:10, and the mass-volume percentage content of nanolignin in the nanolignin suspension is 0.25%.

[0050] In this embodiment, pectin-nanolignin-citrus essential oil composite coating solutions with different amounts of nano-lignin Pickering emulsion were also prepared. The preparation method of the pectin-nanolignin-citrus essential oil composite coating solution in Example 1 is basically the same, except that in step (4), the volume of nano-lignin Pickering emulsion is 5%, 25%, and 50% of the volume of pectin solution; the pectin-nanolignin-citrus essential oil composite coating solutions obtained are respectively denoted as 5% NLCPE, 25% NLCPE, and 50% NLCPE.

[0051] In this embodiment, pectin-nanolignin-citrus essential oil composite coating liquids with different pectin solution concentrations were also prepared. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid in Example 1 is basically the same, except that in step (4), the mass volume percentage of pectin in the pectin solution is 1.5%.

[0052] In this embodiment, pectin-nanolignin-citrus essential oil composite coating liquids with different homogenization speeds and times were also prepared. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid in Example 1 is basically the same, except that in steps (3) and (4), the specific process of homogenization is: stirring at 14000 rpm for 4 minutes.

[0053] In this embodiment, pectin-nanolignin-citrus essential oil composite coating liquids with different ratios of nanolignin suspension to citrus essential oil were also prepared. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid in Example 1 is basically the same, except that in step (3), the volume ratio of nanolignin suspension to citrus essential oil is 1∶9, 3∶7, 5∶5, and 7∶3, respectively.

[0054] In this embodiment, pectin-nanolignin-citrus essential oil composite coating liquids with different ultrasonic treatment methods were also prepared. The preparation methods of pectin-nanolignin-citrus essential oil composite coating liquids in Example 1 are basically the same, except that: in step (2), the specific process of ultrasonic treatment is: ultrasonic treatment for 5s, stop for 10s, and repeat the cycle; the power of ultrasonic treatment is 250W, and the time of ultrasonic treatment is 10min.

[0055] In this embodiment, pectin-nanolignin-citrus essential oil composite coating liquid with different ultrasonic treatment times was also prepared. The preparation method of pectin-nanolignin-citrus essential oil composite coating liquid in Example 1 is basically the same, except that the ultrasonic treatment time in step (2) is 30 min.

[0056] Comparative Example 1:

[0057] A pectin-nanolignin-citrus essential oil composite coating liquid is prepared in a manner that is basically the same as the preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid in Example 1. The only difference is that in step (2), ultrasonic treatment is not performed, but magnetic stirring is used to achieve uniformity.

[0058] In Comparative Example 1, the mixed solution of alkali lignin and acetone was not subjected to ultrasonic treatment, resulting in larger particle size of nano-lignin in the subsequent suspension and a tendency for partial aggregation. This has an adverse effect on the stability of the properties of the pectin-nano-lignin-citrus essential oil composite coating liquid, thus affecting its long-term stability.

[0059] Example 2:

[0060] Application of a pectin-nanolignin-citrus essential oil composite coating liquid in food preservation, such as Figure 1 As shown, it includes the following steps:

[0061] Glycerin was added dropwise to the pectin-nanolignin-citrus essential oil composite coating solution (15% NLCPE, 5% NLCPE, 25% NLCPE, 50% NLCPE) prepared in Example 1, wherein the mass of glycerin was 30% of the mass of the pectin solution in the pectin-nanolignin-citrus essential oil composite coating solution. The mixture was magnetically stirred at a speed of 100 rpm to 250 rpm, and then vacuumed to -0.1 MPa to remove air bubbles. The mixture was then sprayed onto the surface of kiwifruit and strawberries using a squeeze sprayer. After natural drying, a coating film was formed.

[0062] Control group (0% NLCPE): Pectin solution was sprayed onto the surface of fruit, and the pectin content in the solution was 2% by mass and volume, forming a coating film.

[0063] (I) Structure of Nanolignin

[0064] Morphological properties: Morphological characteristics of alkali lignin and nano-lignin obtained by scanning electron microscopy and transmission electron microscopy.

[0065] Particle size distribution of nano-lignin: The particle size distribution of nano-lignin suspension was measured using dynamic light scattering method.

[0066] Figure 2 This is a microstructure diagram of alkali lignin and nano lignin in Example 1 of the present invention. Figure 2 In the diagram, (a) represents lignin, and (b) and (c) represent nano-lignin. Figure 3 This is a particle size distribution diagram of the nano-lignin in Example 1 of the present invention. From... Figure 2 It can be seen that (a) the original alkali lignin has an aggregated morphology and a large size; while after treatment with ultrasound-assisted antisolvent precipitation, the alkali lignin is nano-sized, and the nano-lignin exhibits a spherical shape. Combined with... Figure 3 It can be seen that the particle size of nano-lignin is mainly concentrated around 19.80 nm.

[0067] (II) Stability of Nano-Lignin Pickering Emulsion in Pectin Solution

[0068] The stability of nano-lignin Pickering emulsions prepared with different ratios of nano-lignin suspensions and citrus essential oils and their stability in pectin solution (i.e., pectin-nano-lignin-citrus essential oil composite coating liquid) was observed. Their stability was determined by tracking changes in their apparent morphology and droplet size over 21 days through photography.

[0069] Using nano-lignin as the dispersed phase and citrus essential oil as the continuous phase, nano-lignin Pickering emulsions with different proportions were prepared. The preparation method was basically the same as that of the nano-lignin Pickering emulsion in Example 1, except that in step (3), the volume ratio of nano-lignin suspension to citrus essential oil was 1:9, 3:7, 5:5, and 7:3, respectively.

[0070] Figure 4 This study examines the stability of different nano-lignin Pickering emulsions and pectin-nano-lignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 5 This is a time-particle size variation diagram of different nano-lignin Pickering emulsions and pectin-nano-lignin-citrus essential oil composite coating liquids in Example 1 of the present invention. Figure 4 , Figure 5In the NLCPE group, different nano-lignin Pickering emulsions were used, with 1 / 9, 3 / 7, 5 / 5, 7 / 3, and 9 / 1 representing volume ratios of nano-lignin suspension to citrus essential oil of 1:9, 3:7, 5:5, 7:3, and 9:1, respectively. The P-NLCPE group consisted of different pectin-nano-lignin-citrus essential oil composite coatings, with 1 / 9, 3 / 7, 5 / 5, 7 / 3, and 9 / 1 representing volume ratios of nano-lignin suspension to citrus essential oil of 1:9, 3:7, 5:5, 7:3, and 9:1, respectively. Figure 4 , Figure 5 It can be seen that when the volume ratio of nano-lignin suspension to citrus essential oil is 1:9 and 3:7, the amount of nano-lignin suspension dispersed phase is too small to stabilize the high proportion of oil phase (citrus essential oil), making it difficult or impossible for nano-lignin suspension and citrus essential oil to form an emulsion. After 21 days, all nano-lignin Pickering emulsions in the NLCPE group showed obvious phase separation and demulsification, and the droplet size change rate of the emulsion was large during storage, indicating poor stability. This shows that nano-lignin can stabilize the oil phase to a certain extent to form Pickering emulsion, but its efficiency and stability are relatively poor. We further investigated the stability of the nano-lignin Pickering emulsion in pectin. In the P-NLCPE group, when the volume ratio of nano-lignin suspension to citrus essential oil was 5:5, 7:3, and 9:1, the nano-lignin Pickering emulsion exhibited perfect stability in the pectin matrix. Specifically, no phase separation occurred, and the droplet size remained stable. This is because pectin, as a water-soluble linear macromolecule, can effectively fill the gaps between the dispersed nano-lignin and interact with it to form a co-stable pectin-lignin complex, thereby further promoting the formation and stabilization of the Pickering emulsion. It is evident that the pectin matrix can enhance the long-term stability of the nano-lignin Pickering emulsion, resulting in materials with long-lasting bioactivity. More importantly, the interlocking structure of the nano-lignin Pickering emulsion and part of the pectin matrix is ​​particularly beneficial for promoting perfect compatibility with pectin, effectively avoiding problems such as membrane phase separation, rupture, or film formation difficulties caused by incompatibility. Ultimately, this yields a pectin-based coating with high mechanical properties and long-lasting antibacterial activity.

[0071] (III) Performance testing of composite coatings (the effect of NLCPE on the performance of pectin coatings)

[0072] To investigate the positive contribution of nano-lignin Pickering emulsion to improving the coating performance of pectin, the mechanical properties, hydrophobicity, gas barrier properties, antibacterial properties, antioxidant properties, long-term essential oil release properties, and UV resistance of the pectin-nano-lignin-citrus essential oil composite coating were carefully measured.

[0073] a) The tensile strength (TS) and elongation at break (EAB) of the coating (150 mm × 50 mm) were tested using a tensile testing machine. b) The surface water contact angle of the coating was measured using a water contact angle meter, and the water droplet profile data were fitted using the Laplace-Young equation. c) The coating was cut into circles with a diameter of 80-100 mm, and the water vapor and oxygen permeability of the coating were measured using a water vapor permeability meter and a pressure gas permeability meter. d) The transmission spectrum of the coating (5 cm × 1.5 cm) in the wavelength range of 200-800 nm was measured using a UV-Vis spectrophotometer to identify its UV resistance. e) The DPPH free radical scavenging rate and total antioxidant capacity of the coating were determined using a Micro DPPH free radical scavenging ability kit and a Micro T-AOC kit to confirm its antioxidant activity. f) The antibacterial activity of the coating was tested with Staphylococcus aureus, Escherichia coli, and Botrytis cinerea. For bacteria, the coating was mixed with bacterial culture medium (10... 6 Mix (CFU / mL) and incubate at 190 rpm and 37°C for 1 hour. Then, evenly disperse 200 μL of the diluted culture medium on an agar plate and incubate at 37°C for 12 hours. The control group was treated the same way, but no coated sample was added to the culture medium. For fungi, Botrytis cinerea suspension (10 CFU / mL) was added to PDA medium. 6 (CFU / mL) and spread evenly. Cut the coating into circles, place them in the center of the culture medium, and incubate at 27°C for 7 days. g) The release of effective volatile components in the coating was determined using a GC-2010Plus gas chromatography-flame ionization detector equipped with an Rtx-5 silica capillary column (30m × 0.25mm × 0.25μm). Limonene is the main active component of citrus essential oils, so its peak area was used for quantitative analysis of the essential oils. The release rate of citrus essential oils could be estimated by the ratio of the peak area of ​​the main volatiles in the coating to the peak area of ​​the original coating solution during the test. Measurements were performed at days 0, 7, 14, and 21. h) Biotoxicity of the composite coating: The effect of the sample on the viability of GES-1 cells was determined using the CCK8 method. GES-1 cells were stained for live / dead staining and observed using a laser confocal scanning microscope.

[0074] Pectin and pectin-nanolignin-citrus essential oil composite coating liquid were poured into a 15cm×25cm×0.5cm glass plate using a casting method. After natural drying at room temperature, pectin coating and composite coating were obtained for structural characterization and functional property determination. Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 11 , Figure 120% NLCPE represents pure pectin coating; 5%, 15%, 25%, and 25% represent composite coatings prepared from pectin-nanolignin-citrus essential oil composite coating solutions when the volume of the nano-lignin Pickering emulsion is 5%, 15%, 25%, and 50% of the volume of the pectin solution, respectively.

[0075] Figure 6 This is a graph showing the mechanical properties of the composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 6 It can be seen that the tensile strength of the composite coating significantly increases with the increase of nano-lignin Pickering emulsion; however, when the content of nano-lignin Pickering emulsion exceeds 15%, the tensile strength of the composite coating begins to decrease; when the content of nano-lignin Pickering emulsion is 50%, the tensile strength of the composite coating is lower than that of the pure pectin composite coating. Similarly, the elongation at break of the composite coating significantly increases with the increase of nano-lignin Pickering emulsion, reaching its highest value when the content of nano-lignin Pickering emulsion is 15%; after the content of nano-lignin Pickering emulsion exceeds 15%, the elongation at break of the composite coating gradually decreases. Therefore, the composite coating exhibits excellent mechanical properties when the volume of nano-lignin Pickering emulsion accounts for 5% to 25% of the pectin solution volume; particularly, the composite coating exhibits optimal mechanical properties when the volume of nano-lignin Pickering emulsion accounts for 15% of the pectin solution volume.

[0076] Figure 7 This is a graph showing the water vapor permeability and oxygen permeability of composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 7 It can be seen that the addition of nano-lignin Pickering emulsion can significantly reduce the water vapor permeability and oxygen permeability of pectin coating film (p<0.05), indicating that it can enhance the water vapor / oxygen barrier properties of pectin matrix coating film.

[0077] Figure 8 This is a graph showing the antioxidant capacity of composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 8 It can be seen that the pure pectin coating has a low DPPH free radical scavenging ability; in contrast, the composite coating prepared from the pectin-nanolignin-citrus essential oil composite coating solution has a significantly higher DPPH free radical scavenging rate (p<0.05), showing better antioxidant activity. Figure 8(b) It can be seen that the total antioxidant capacity of the composite coating also shows the same trend, indicating that the nano-lignin Pickering emulsion enhances the antioxidant activity of the pectin matrix coating. Furthermore, the antioxidant activity of the composite coating is concentration-dependent on the nano-lignin Pickering emulsion, suggesting that citrus essential oil in the nano-lignin Pickering emulsion is the main factor enhancing its antioxidant activity. This antioxidant capacity of the composite coating will provide more opportunities to reduce food oxidation and spoilage.

[0078] The effect of the composite coating on GES-1 cell viability was determined using the CCK8 method to assess its biosafety. Since all coatings used the same matrix and differed due to varying concentrations of nano-lignin Pickering emulsion, the composite coating with the highest emulsion content (50%) was selected for the experiment. Specifically, a composite coating prepared using a pectin-nano-lignin-citrus essential oil composite coating solution (50% NLCPE) was used as the material. Different amounts of the composite coating were mixed with bacterial culture medium, i.e., different ratios of composite coating to bacterial culture medium were used in the experiment.

[0079] Figure 9 This image shows the biocompatibility of the composite coating prepared from the pectin-nanoligin-citrus essential oil composite coating solution (50% NLCPE) in Example 1 of this invention. Figure 9 It can be seen that when different amounts of the composite coating were used as the culture medium, the cells maintained a high survival rate (>85%) within 24h and 48h, indicating that they have no biotoxicity. This is because all materials used in the composite coating solution, including pectin, citrus essential oil, and alkali lignin, are derived from edible food processing waste. Therefore, the pectin-nanolignin-citrus essential oil composite coating solution of the present invention has good biosafety and can shine in the field of food preservation.

[0080] Figure 10 This is a dynamic water contact angle diagram of the composite coatings prepared by different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of the present invention. Figure 10 In the images, (a) is a pure pectin coating, (b) is a 5% NLCPE composite coating, (c) is a 15% NLCPE composite coating, (d) is a 25% NLCPE composite coating, and (e) is a 50% NLCPE composite coating. Figure 10It can be seen that the instantaneous water contact angle of the pure pectin coating is 64.55°, exhibiting hydrophilic properties. With the increase of the nano-lignin Pickering emulsion content, the water contact angle of the composite coating significantly increases, reaching a maximum of 96.68°, completing the transition from hydrophilic to hydrophobic. Furthermore, dynamic water contact angle analysis shows that even after 300 seconds, the composite coating containing the nano-lignin Pickering emulsion still maintains a large contact angle. Therefore, the nano-lignin Pickering emulsion can significantly improve the hydrophobicity of the pectin-based coating, meaning that the composite film prepared by the pectin-nano-lignin-citrus essential oil composite coating solution of this invention has good hydrophobicity.

[0081] Figure 11 This is a sustained-release curve of the composite coating prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of the present invention. Figure 11 It can be seen that during storage, the content of effective volatile components in all tested composite coatings gradually decreased over time, exhibiting a slow release characteristic. Specifically, in the initial release phase, the maximum concentration gradient between the active volatile components in the composite coating and the external medium caused them to diffuse from high to low concentrations, resulting in a significant release on the first day (19.14%-32.79%). In the following period, the essential oils in the composite coating began to migrate slowly and continuously outwards. By day 21, sufficient essential oils remained in the composite coating (up to 37.87%), indicating that the essential oils were still being released slowly. It is foreseeable that the composite coating will continue to release essential oils for an even longer period in the future, providing more opportunities for long-term bioactivity. Therefore, this slow-release characteristic of essential oils exhibited by the composite coating indicates that the coating formed by the pectin-nanolignin-citrus essential oil composite coating liquid can maintain antioxidant and antibacterial properties throughout the entire shelf life of the fruit.

[0082] Figure 12 This is a UV-Vis transmittance diagram of the composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 12 It can be seen that the pure pectin coating has a high transmittance in the ultraviolet region (400-200nm), indicating that it has obvious defects in ultraviolet blocking. However, the addition of nano-lignin Pickering emulsion reduces the transmittance of the composite coating in the ultraviolet region, that is, it almost completely blocks the ultraviolet region in the spectrum, thus exhibiting excellent anti-ultraviolet performance.

[0083] Figure 13 This image shows the antibacterial performance of composite coatings prepared from different pectin-nanolignin-citrus essential oil composite coating solutions in Example 1 of this invention. Figure 13It can be seen that pure pectin coatings have almost no antibacterial activity against bacteria and fungi, and may even promote microbial growth to some extent. In contrast, culture media containing the composite coating show almost no bacterial or fungal growth, indicating that the composite coating has the effect of killing or inhibiting microbial growth. Even with the addition of a small amount of nano-lignin Pickering emulsion, the composite coating still exhibits excellent antibacterial properties, demonstrating the effectiveness and efficiency of nano-lignin Pickering emulsion in preparing active antibacterial coatings. Therefore, based on the excellent antibacterial properties of the composite coating solution, the pectin-nano-lignin-citrus essential oil composite coating solution of this invention is particularly suitable for food preservation and antiseptic applications.

[0084] (iv) The preservative effect of the composite coating liquid

[0085] Because the pectin-nanoligin-citrus essential oil composite coating solution has shear-thinning properties, it was evenly sprayed onto the fruit surface using a squeeze sprayer. After the coating solution on the fruit surface dried naturally, another round of spraying and drying was performed. Subsequently, the fruit was stored indoors, and its surface changes were recorded using a camera. To assess changes in fruit quality, changes in the fruit's weight loss rate, firmness, and soluble solids content during storage were monitored.

[0086] Figure 14 This image shows the preservation effect of the pectin-nanolignin-citrus essential oil composite coating liquid on kiwifruit and strawberries in Example 2 of this invention. The fruits were placed in a high temperature and high humidity environment (30±4℃, 57±5% RH) for the experiment. Figure 14 In the table, a1 and b1 are untreated, a2 and b2 have 0% NLCPE, a3 and b3 have 5% NLCPE, a4 and b4 have 15% NLCPE, a5 and b5 have 25% NLCPE, and a6 and b6 have 50% NLCPE. Figure 14 It can be seen that the untreated kiwifruit and strawberries had a shorter storage period. Kiwifruit showed signs of spoilage, softening, or dehydration on day 7 and strawberries on day 2. The pure pectin coating also extended the shelf life of kiwifruit to some extent, which may be related to its barrier properties. After 17 days of storage for kiwifruit and 3 days for strawberries, the fruits in the untreated group and the pure pectin coating group showed severe spoilage. Compared with the untreated group and the pure pectin coating group, the kiwifruit treated with the pectin-nanolignin-citrus essential oil composite coating liquid of this invention maintained its original appearance for the first 16 days and the strawberries for the first 5 days, without any spoilage or dehydration. The quality of kiwifruit began to decline on day 17 and strawberries on day 6. In other words, compared with untreated or 0% NLCPE treatment, the shelf life of fruits (kiwifruit and strawberries) can be extended by 2.43-3.00 times, which indicates that the shelf life of fruits has been significantly extended.

[0087] Figure 15 This is a graph showing the changes in hardness and soluble solids content of kiwifruit when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of the present invention. Figure 16 This is a graph showing the changes in hardness and soluble solids content of strawberries when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of the present invention. Figure 17 This is a graph showing the weight changes of kiwifruit and strawberries when the pectin-nanolignin-citrus essential oil composite coating liquid was used for preservation in Example 2 of this invention. (Combined with...) Figure 15 , 16 As shown in Figures 17 and 18, the fruits in both the untreated group and the pure pectin-coated group exhibited severe softening, nutrient loss, and dehydration, consistent with the appearance captured in the digital photographs. In contrast, the fruits treated with the pectin-nanolignin-citrus essential oil composite coating solution showed minimal changes in firmness, TSS content, and weight, indicating that the coating formed by the pectin-nanolignin-citrus essential oil composite coating solution effectively maintains the quality of the fruit throughout storage.

[0088] In summary, the pectin-nanoligin-citrus essential oil composite coating liquid of the present invention, when used for food preservation, not only has higher water vapor / oxygen barrier properties, which helps reduce the respiration of fruits and delay the ripening process, but also has excellent antioxidant and antibacterial activities, which can effectively prevent enzymatic browning, oxidative deterioration and rot on the surface of fruits. It has great potential in food preservation and food safety applications.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make many possible variations and modifications to the technical solutions of the present invention using the methods and techniques disclosed above, or modify them into equivalent embodiments with equivalent changes, without departing from the spirit and technical essence of the present invention. Therefore, any simple modifications, equivalent substitutions, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solutions of the present invention shall still fall within the protection scope of the technical solutions of the present invention.

Claims

1. A method for preparing a pectin-nanolignin-citrus essential oil composite coating liquid, characterized in that, Includes the following steps: (1) Dissolve lignin in acetone to obtain a mixed solution; (2) The mixed solution obtained in step (1) is subjected to ultrasonic treatment. After the treatment is completed, it is added to water and subjected to rotary evaporation to obtain a nano-lignin suspension. (3) Mix the nano-lignin suspension obtained in step (2) with citrus essential oil and homogenize it to obtain nano-lignin Pickering emulsion; (4) Mix the nano-lignin Pickering emulsion and pectin solution obtained in step (3) and homogenize them to obtain a pectin-nano-lignin-citrus essential oil composite coating liquid.

2. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid according to claim 1, characterized in that, In step (3), the volume ratio of the nano-lignin suspension to citrus essential oil is 5-9:1-9, and the mass-volume percentage of lignin in the nano-lignin suspension is 0.25%-2%.

3. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid according to claim 2, characterized in that, In step (4), the volume of the nano-lignin Pickering emulsion is 5% to 50% of the volume of the pectin solution, and the mass-volume percentage of pectin in the pectin solution is 1% to 2%.

4. The preparation method of the pectin-nanolignin-citrus essential oil composite coating liquid according to claim 3, characterized in that, The volume ratio of the nano-lignin suspension to citrus essential oil is 5-9:1-5, and the volume of the nano-lignin Pickering emulsion is 5%-25% of the volume of the pectin solution.

5. The method for preparing the pectin-nanolignin-citrus essential oil composite coating liquid according to any one of claims 1 to 4, characterized in that, In step (2), the specific process of ultrasonic treatment is as follows: ultrasonic treatment for 5 seconds, stop for 6 to 10 seconds, and repeat the cycle; the power of ultrasonic treatment is 200W to 500W, and the time of ultrasonic treatment is 5min to 30min; the specific process after ultrasonic treatment is as follows: under stirring conditions, the treated mixed solution is added to water; the volume ratio of the mixed solution to water is 1:8 to 15, and the temperature of rotary evaporation treatment is 45℃ to 60℃.

6. The method for preparing the pectin-nanolignin-citrus essential oil composite coating liquid according to any one of claims 1 to 4, characterized in that, In step (1), the mass-to-volume ratio of lignin to acetone is 1g:15mL~25mL, the lignin is alkali lignin, and the mixed solution is further treated as follows before use: the mixed solution is centrifuged at 6500rpm for 5min. In step (3), the specific process of homogenization is as follows: stirring at a speed of 10000 rpm to 14000 rpm for 2 min to 5 min; In step (4), the specific process of homogenization is as follows: stirring at a speed of 10000 rpm to 14000 rpm for 2 min to 5 min.

7. A pectin-nanolignin-citrus essential oil composite coating liquid prepared by any one of claims 1 to 6.

8. The application of the pectin-nanolignin-citrus essential oil composite coating liquid as described in claim 7 in food preservation.

9. The application according to claim 8, characterized in that, Includes the following steps: Glycerin was added to the pectin-nanolignin-citrus essential oil composite coating solution, stirred, vacuumed, and sprayed onto the substrate surface. After drying, a pectin-nanolignin-citrus essential oil composite coating was formed.

10. The application according to claim 9, characterized in that, The mass of the glycerol is 15% to 40% of the mass of the pectin solution in the pectin-nanolignin-citrus essential oil composite coating liquid. The stirring speed is 100 rpm to 250 rpm. The vacuum degree of the vacuum is -0.1 MPa. The substrate is fruit, which includes at least one of climacteric and non-climacteric fruits. The climacteric fruit includes kiwifruit, and the non-climacteric fruit includes strawberry.

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